[Federal Register Volume 75, Number 119 (Tuesday, June 22, 2010)]
[Rules and Regulations]
[Pages 35520-35603]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2010-13947]
[[Page 35519]]
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Part II
Environmental Protection Agency
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40 CFR Parts 50, 53, and 58
Primary National Ambient Air Quality Standard for Sulfur Dioxide; Final
Rule
Federal Register / Vol. 75 , No. 119 / Tuesday, June 22, 2010 / Rules
and Regulations
[[Page 35520]]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Parts 50, 53, and 58
[EPA-HQ-OAR-2007-0352; 9160-4]
RIN 2060-A048
Primary National Ambient Air Quality Standard for Sulfur Dioxide
AGENCY: Environmental Protection Agency (EPA).
ACTION: Final rule.
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SUMMARY: Based on its review of the air quality criteria for oxides of
sulfur and the primary national ambient air quality standard (NAAQS)
for oxides of sulfur as measured by sulfur dioxide (SO2),
EPA is revising the primary SO2 NAAQS to provide requisite
protection of public health with an adequate margin of safety.
Specifically, EPA is establishing a new 1-hour SO2 standard
at a level of 75 parts per billion (ppb), based on the 3-year average
of the annual 99th percentile of 1-hour daily maximum concentrations.
The EPA is also revoking both the existing 24-hour and annual primary
SO2 standards.
DATES: This final rule is effective on August 23, 2010.
ADDRESSES: EPA has established a docket for this action under Docket ID
No. EPA-HQ-OAR-2007-0352. All documents in the docket are listed on the
http://www.regulations.gov Web site. Although listed in the index, some
information is not publicly available, e.g., confidential business
information or other information whose disclosure is restricted by
statute. Certain other material, such as copyrighted material, will be
publicly available only in hard copy form. Publicly available docket
materials are available either electronically through http://www.regulations.gov or in hard copy at the Air and Radiation Docket and
Information Center, EPA/DC, EPA West, Room 3334, 1301 Constitution
Ave., NW., Washington, DC. The Public Reading Room is open from 8:30
a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The
telephone number for the Public Reading Room is (202) 566-1744 and the
telephone number for the Air and Radiation Docket and Information
Center is (202) 566-1742.
FOR FURTHER INFORMATION CONTACT: Dr. Michael J. Stewart, Health and
Environmental Impacts Division, Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, Mail code C504-06,
Research Triangle Park, NC 27711; telephone: 919-541-7524; fax: 919-
541-0237; e-mail: [email protected].
SUPPLEMENTARY INFORMATION:
Table of Contents
The following topics are discussed in this preamble:
I. Background
A. Summary of Revisions to the SO2 Primary NAAQS
B. Statutory Requirements
C. Related SO2 Control Programs
D. History of Reviews of the Primary NAAQS for Sulfur Oxides
E. Summary of Proposed Revisions to the SO2 Primary
NAAQS
F. Organization and Approach to Final SO2 Primary
NAAQS Decisions
II. Rationale for Decisions on the Primary Standards
A. Characterization of SO2 Air Quality
1. Anthropogenic Sources and Current Patterns of SO2
Air Quality
2. SO2 Monitoring
B. Health Effects Information
1. Short-Term (5-Minute to 24-Hour) SO2 Exposure and
Respiratory Morbidity Effects
a. Adversity of Short-Term Respiratory Morbidity Effects
2. Health Effects and Long-Term Exposures to SO2
3. SO2-Related Impacts on Public Health
C. Human Exposure and Health Risk Characterization
D. Approach for Determining Whether To Retain or Revise the
Current Standards
E. Adequacy of the Current Standards
1. Rationale for Proposed Decision
2. Comments on the Adequacy of the Current Standards
a. Comments on EPA's Interpretation of the Epidemiologic
Evidence
b. Comments on EPA's Interpretation of the Controlled Human
Exposure Evidence
c. Comments on EPA's Characterization of SO2-
Associated Exposures and Health Risks
3. Conclusions Regarding the Adequacy of the Current 24-Hour and
Annual Standards
F. Conclusions on the Elements of a New Short-Term Standard
1. Indicator
a. Rationale for Proposed Decision
b. Comments on Indicator
c. Conclusions on Indicator
2. Averaging Time
a. Rationale for Proposed Decision
b. Comments on Averaging Time
c. Conclusions on Averaging Time
3. Form
a. Rationale for Proposed Decision
b. Comments on Form
c. Conclusions on Form
4. Level
a. Rationale for Proposed Decision
b. Comments on Level
c. Conclusions on Level
5. Retaining or Revoking the Current 24-Hour and Annual
Standards
a. Rationale for Proposed Decision
b. Comments on Retaining or Revoking the Current 24-Hour and
Annual Standards
c. Conclusions on Retaining or Revoking the Current 24-Hour and
Annual Standards
G. Summary of Decisions on Primary Standards
III. Overview of the Approach for Monitoring and Implementation
IV. Amendments to Ambient Monitoring and Reporting Requirements
A. Monitoring Methods
1. Requirements for SO2 Federal Reference Method
(FRM)
a. Proposed Ultraviolet Fluorescence SO2 FRM and
Implementation
b. Public Comments
c. Conclusions on Ultraviolet Fluorescence SO2 FRM
and Implementation
2. Requirements for Automated SO2 Methods
a. Proposed Performance Specifications for Automated Methods
b. Public Comments
c. Conclusions for Performance Specifications for SO2
Automated Methods
B. Network Design
1. Approach for Network Design
a. Proposed Approach for Network Design
b. Alternative Network Design
c. Public Comments
2. Modeling Ambient SO2 Concentrations
3. Monitoring Objectives
a. Proposed Monitoring Objectives
b. Public Comments
c. Conclusions on Monitoring Objectives
4. Final Monitoring Network Design
5. Population Weighted Emissions Index
a. Proposed Use of the Population Weighted Emissions Index
b. Public Comments
c. Conclusions on the Use of the Population Weighted Emissions
Index
6. Regional Administrator Authority
a. Proposed Regional Administrator Authority
b. Public Comments
c. Conclusions on Regional Administrator Authority
7. Monitoring Network Implementation
a. Proposed Monitoring Network Implementation
b. Public Comments
c. Conclusions on Monitoring Network Implementation
C. Data Reporting
1. Proposed Data Reporting
2. Public Comments
3. Conclusions on Data Reporting
V. Initial Designation of Areas for the 1-Hour SO2 NAAQS
A. Clean Air Act Requirements
1. Approach Described in Proposal
2. Public Comments
B. Expected Designations Process
VI. Clean Air Act Implementation Requirements
A. How This Rule Applies to Tribes
B. Nonattainment Area Attainment Dates
1. Attaining the NAAQS
2. Consequences of a Nonattainment Area Failing To Attain by the
Statutory Attainment Date
C. Section 110(a)(1) and (2) NAAQS Maintenance/Infrastructure
Requirements
1. Section 110(a)(1)-(2) Submission
[[Page 35521]]
D. Attainment Planning Requirements
1. SO2 Nonattainment Area SIP Requirements
2. New Source Review and Prevention of Significant Deterioration
Requirements
3. General Conformity
E. Transition From the Existing SO2 NAAQS to a
Revised SO2 NAAQS
VII. Appendix T--Interpretation of the Primary NAAQS for Oxides of
Sulfur and Revisions to the Exceptional Events Rule
A. Interpretation of the NAAQS for Oxides of Sulfur
1. Proposed Interpretation of the Standard
2. Comments on Interpretation of the Standard
3. Conclusions on Interpretation of the Standard
B. Exceptional Events Information Submission Schedule
VIII. Communication of Public Health Information
IX. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children From
Environmental Health & Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
I. National Technology Transfer and Advancement Act
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
References
I. Background
A. Summary of Revisions to the SO2 Primary NAAQS
Based on its review of the air quality criteria for oxides of
sulfur and the primary national ambient air quality standard (NAAQS)
for oxides of sulfur as measured by sulfur dioxide (SO2),
EPA is making revisions to the primary SO2 NAAQS so the
standards are requisite to protect public health with an adequate
margin of safety, as appropriate under section 109 of the Clean Air Act
(Act or CAA). Specifically, EPA is replacing the current 24-hour and
annual standards with a new short-term standard based on the 3-year
average of the 99th percentile of the yearly distribution of 1-hour
daily maximum SO2 concentrations. EPA is setting the level
of this new standard at 75 ppb. EPA is adding data handling conventions
for SO2 by adding provisions for this new 1-hour primary
standard. EPA is also establishing requirements for an SO2
monitoring network. These new provisions require monitors in areas
where there is an increased coincidence of population and
SO2 emissions. EPA is also making conforming changes to the
Air Quality Index (AQI).
B. Statutory Requirements
Two sections of the Clean Air Act (Act or CAA) govern the
establishment and revision of National Ambient Air Quality Standards
NAAQS. Section 108 of the Act directs the Administrator to identify and
list air pollutants that meet certain criteria, including that the air
pollutant ``in his judgment, cause[s] or contribute[s] to air pollution
which may reasonably be anticipated to endanger public health and
welfare'' and ``the presence of which in the ambient air results from
numerous or diverse mobile or stationary sources.'' CAA section
108(a)(1)(A) and (B). For those air pollutants listed, section 108
requires the Administrator to issue air quality criteria that
``accurately reflect the latest scientific knowledge useful in
indicating the kind and extent of all identifiable effects on public
health or welfare which may be expected from the presence of [a]
pollutant in ambient air * * *'' Section 108(a)(2).
Section 109(a) of the Act directs the Administrator to promulgate
``primary'' and ``secondary'' NAAQS for pollutants for which air
quality criteria have been issued. Section 109(b)(1) defines a primary
standard as one ``the attainment and maintenance of which in the
judgment of the Administrator, based on [the air quality] criteria and
allowing an adequate margin of safety, are requisite to protect the
public health.'' \1\ Section 109(b)(1). A secondary standard, in turn,
must ``specify a level of air quality the attainment and maintenance of
which, in the judgment of the Administrator, based on [the air quality]
criteria, is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of such
pollutant in the ambient air.'' \2\ Section 109(b)(2) This rule
concerns exclusively the primary NAAQS for oxides of sulfur.
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level * * * which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group.'' S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970). See also American
Lung Ass'n v. EPA, 134 F. 3d 388, 389 (DC Cir. 1998) (``NAAQS must
protect not only average healthy individuals, but also `sensitive
citizens'--children, for example, or people with asthma, emphysema,
or other conditions rendering them particularly vulnerable to air
pollution. If a pollutant adversely affects the health of these
sensitive individuals, EPA must strengthen the entire national
standard.''); Coalition of Battery Recyclers Ass'n v. EPA, No. 09-
1011 (DC Cir. May 14, 2010) slip op. at 7 (same).
\2\ EPA is currently conducting a separate review of the
secondary SO2 NAAQS jointly with a review of the
secondary NO2 NAAQS (see http://www.epa.gov/ttn/naaqs/standards/no2so2sec/index.html for more information).
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The requirement that primary standards include an adequate margin
of safety is intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It is also intended to provide a reasonable degree
of protection against hazards that research has not yet identified.
Lead Industries Association v. EPA, 647 F.2d 1130, 1154 (DC Cir 1980),
cert. denied, 449 U.S. 1042 (1980); American Petroleum Institute v.
Costle, 665 F.2d 1176, 1186 (DC Cir. 1981), cert. denied, 455 U.S. 1034
(1982). Both kinds of uncertainties are components of the risk
associated with pollution at levels below those at which human health
effects can be said to occur with reasonable scientific certainty.
Thus, in selecting primary standards that include an adequate margin of
safety, the Administrator is seeking not only to prevent pollution
levels that have been demonstrated to be harmful but also to prevent
lower pollutant levels that may pose an unacceptable risk of harm, even
if the risk is not precisely identified as to nature or degree. The CAA
does not require the Administrator to establish a primary NAAQS at a
zero-risk level or at background concentration levels, see Lead
Industries Association v. EPA, 647 F.2d at 1156 n. 51, but rather at a
level that reduces risk sufficiently so as to protect public health
with an adequate margin of safety.
In addressing the requirement for a margin of safety, EPA considers
such factors as the nature and severity of the health effects involved,
the size of the at-risk population(s), and the kind and degree of the
uncertainties that must be addressed. The selection of any particular
approach to providing an adequate margin of safety is a policy choice
left specifically to the Administrator's judgment. Lead Industries
Association v. EPA, 647 F.2d at 1161-62.
In setting standards that are ``requisite'' to protect public
health and welfare, as provided in section 109(b), EPA's task is to
establish standards that are neither more nor less stringent than
necessary for these purposes. In so doing, EPA may not consider the
costs of implementing the standards. Whitman v. American Trucking
[[Page 35522]]
Associations, 531 U.S. 457, 471, 475-76 (2001).
Section 109(d)(1) of the Act requires the Administrator to
periodically undertake a thorough review of the air quality criteria
published under section 108 and the NAAQS and to revise the criteria
and standards as may be appropriate. The Act also requires the
Administrator to appoint an independent scientific review committee
composed of seven members, including at least one member of the
National Academy of Sciences, one physician, and one person
representing State air pollution control agencies, to review the air
quality criteria and NAAQS and to ``recommend to the Administrator any
new * * * standards and revisions of existing criteria and standards as
may be appropriate under section 108 and subsection (b) of this
section.'' CAA section 109(d)(2). This independent review function is
performed by the Clean Air Scientific Advisory Committee (CASAC) of
EPA's Science Advisory Board.
C. Related SO2 Control Programs
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once EPA has established
them. Under section 110 of the Act, and related provisions, States are
to submit, for EPA approval, State implementation plans (SIPs) that
provide for the attainment and maintenance of such standards through
control programs directed to sources of the pollutants involved. The
States, in conjunction with EPA, also administer the prevention of
significant deterioration program that covers these pollutants. See CAA
sections 160-169. In addition, Federal programs provide for nationwide
reductions in emissions of these and other air pollutants through the
Federal motor vehicle and motor vehicle fuel control program under
title II of the Act (CAA sections 202-250) which involves controls for
emissions from all moving sources and controls for the fuels used by
these sources; new source performance standards under section 111; and
title IV of the Act (CAA sections 402-416), which specifically provides
for major reductions in SO2 emissions. EPA has also
promulgated the Clean Air Interstate Rule (CAIR) to require additional
SO2 emission reductions needed in the eastern half of the
United States to address emissions which contribute significantly to
nonattainment with, or interfere with maintenance of, the PM NAAQS by
downwind States in the CAIR region. This rule was remanded by the DC
Circuit, and although it remains in effect, EPA is reevaluating it
pursuant to the court remand.
Currently, there are several areas designated as being in
nonattainment of the primary SO2 NAAQS (see section VI).
Moreover, as a result of this final rule, additional areas could be
classified as non-attainment. Certain States would then be required to
develop SIPs that identify and implement specific air pollution control
measures to reduce ambient SO2 concentrations to attain and
maintain the revised SO2 NAAQS, most likely by requiring air
pollution controls on sources that emit oxides of sulfur
(SOx).
D. History of Reviews of the Primary NAAQS for Sulfur Oxides
On April 30, 1971, the EPA promulgated primary SO2 NAAQS
(36 FR 8187). These primary standards, which were based on the findings
outlined in the original 1969 Air Quality Criteria for Sulfur Oxides,
were set at 0.14 parts per million (ppm) averaged over a 24-hour
period, not to be exceeded more than once per year, and 0.030 ppm
annual arithmetic mean. In 1982, EPA published the Air Quality Criteria
for Particulate Matter and Sulfur Oxides (EPA, 1982) along with an
addendum of newly published controlled human exposure studies, which
updated the scientific criteria upon which the initial standards were
based (EPA, 1982). In 1986, EPA published a second addendum presenting
newly available evidence from epidemiologic and controlled human
exposure studies (EPA, 1986). In 1988, EPA published a proposed
decision not to revise the existing standards (53 FR 14926) (April 26,
1988). However, EPA specifically requested public comment on the
alternative of revising the current standards and adding a new 1-hour
primary standard of 0.4 ppm (400 ppb) to protect asthmatics against 5-
10 minute peak SO2 concentrations.
As a result of public comments on the 1988 proposal and other post-
proposal developments, EPA published a second proposal on November 15,
1994 (59 FR 58958). The 1994 re-proposal was based in part on a
supplement to the second addendum of the criteria document, which
evaluated new findings on 5-10 minute SO2 exposures in
asthmatics (EPA, 1994a; EPA, 1994b). As in the 1988 proposal, EPA
proposed to retain the existing 24-hour and annual standards. EPA also
solicited comment on three regulatory alternatives to further reduce
the health risk posed by exposure to high 5-minute peaks of
SO2 if additional protection were judged to be necessary.
The three alternatives were: (1) Revising the existing primary
SO2 NAAQS by adding a new 5-minute standard of 0.6 ppm (600
ppb) SO2; (2) establishing a new regulatory program under
section 303 of the Act to supplement protection provided by the
existing NAAQS, with a trigger level of 0.6 ppm (600 ppb)
SO2, one expected exceedance; and (3) augmenting
implementation of existing standards by focusing on those sources or
source types likely to produce high 5-minute peak concentrations of
SO2.
On May 22, 1996, EPA announced its final decision not to revise the
NAAQS for SOx (61 FR 25566). EPA found that asthmatics--a
susceptible population group--could be exposed to short-term
SO2 bursts resulting in repeated `exposure events' such that
tens or hundreds of thousands of asthmatics could be exposed annually
to lung function effects ``distinctly exceeding * * * [the] typical
daily variation in lung function'' that asthmatics routinely
experience, and found further that repeated occurrences should be
regarded as significant from a public health standpoint. 61 FR at
25572, 25573. Nonetheless, the agency concluded that ``the likelihood
that asthmatic individuals will be exposed * * * is very low when
viewed from a national perspective'', that ``5-minute peak
SO[2] levels do not pose a broad public health problem when
viewed from a national perspective'', and that ``short-term peak
concentrations of SO[2] do not constitute the type of
ubiquitous public health problem for which establishing a NAAQS would
be appropriate.'' Id. at 25575. EPA concluded, therefore, that it would
not revise the existing standards or add a standard to specifically
address 5-minute exposures. EPA also announced an intention to propose
guidance, under section 303 of the Act, to assist States in responding
to short-term peaks of SO2 and later initiated a rulemaking
to do so (62 FR 210 (Jan. 2, 1997).
The American Lung Association and the Environmental Defense Fund
challenged EPA's decision not to establish a 5-minute standard. On
January 30, 1998, the Court of Appeals for the District of Columbia
Circuit found that EPA had failed to adequately explain its
determination that no revision to the SO2 NAAQS was
appropriate and remanded the determination back to EPA for further
explanation. American Lung Ass'n v. EPA, 134 F. 3d 388 (DC Cir. 1998).
Specifically, the court held that EPA had failed to adequately explain
the basis for its conclusion that short-term SO2 exposures
to asthmatics do not constitute a public health problem,
[[Page 35523]]
noting that the agency had failed to explain the link between its
finding that repeated short-term exposures were significant, and that
there would be tens to hundreds of thousands of such exposures annually
to a susceptible subpopulation. 134 F. 3d at 392. The court also
rejected the explanation that short-term SO2 bursts were
``localized, infrequent, and site-specific'' as a rational basis for
the conclusion that no public health problem existed for purposes of
section 109: ``[N]othing in the Final Decision explains why
`localized', `site-specific', or even `infrequent' events might
nevertheless create a public health problem, particularly since, in
some sense, all pollution is local and site-specific * * *''. Id. The
court accordingly remanded the case to EPA to adequately explain its
determination or otherwise take action in accordance with the opinion.
In response, EPA has collected and analyzed additional air quality data
focused on 5-minute concentrations of SO2. These air quality
analyses conducted since the last review helped inform the current
review, which (among other things) address the issues raised in the
court's remand of the Agency's last decision.
EPA formally initiated the current review of the air quality
criteria for oxides of sulfur and the SO2 primary NAAQS on
May 15, 2006 (71 FR 28023) with a general call for information. EPA's
draft Integrated Review Plan for the Primary National Ambient Air
Quality Standards for Sulfur Dioxide (EPA, 2007a) was made available in
April 2007 for public comment and was discussed by the CASAC via a
publicly accessible teleconference on May 11, 2007. As noted in that
plan, SOX includes multiple gaseous (e.g., SO3)
and particulate (e.g., sulfate) species. Because the health effects
associated with particulate species of SOX have been
considered within the context of the health effects of ambient
particles in the Agency's review of the NAAQS for particulate matter
(PM), the current review of the primary SO2 NAAQS is focused
on the gaseous species of SOX and does not consider health
effects directly associated with particulate species.
The first draft of the Integrated Science Assessment for Oxides of
Sulfur-Health Criteria (ISA) and the Sulfur Dioxide Health Assessment
Plan: Scope and Methods for Exposure and Risk Assessment (EPA, 2007b)
were reviewed by CASAC at a public meeting held on December 5-6, 2007.
Based on comments received from CASAC and from the public, EPA
developed the second draft of the ISA and the first draft of the Risk
and Exposure Assessment to Support the Review of the SO2
Primary National Ambient Air Quality Standard (Risk and Exposure
Assessment (REA)). These documents were reviewed by CASAC at a public
meeting held on July 30-31, 2008. Based on comments received from CASAC
and the public at this meeting, EPA released the final ISA in September
of 2008 (EPA, 2008a; henceforth referred to as ISA). In addition,
comments received were considered in developing the second draft of the
REA. Importantly, the second draft of the REA contained a draft staff
policy assessment that considered the evidence presented in the final
ISA and the air quality, exposure, and risk characterization results
presented in the second draft REA, as they related to the adequacy of
the current SO2 NAAQS and potential alternative primary
SO2 standards. This document was reviewed by CASAC at a
public meeting held on April 16-17, 2009. In preparing the final REA
report, which included the final staff policy assessment, EPA
considered comments received from CASAC and the public at and
subsequent to that meeting. The final REA containing the final staff
policy assessment was completed in August 2009 (EPA 2009a; henceforth
referred to as REA)).
On December 8, 2009 EPA published its proposed revisions to the
primary SO2 NAAQS. 74 FR 64810 presented a number of
conclusions, findings, and determinations proposed by the
Administrator. EPA invited general, specific, and/or technical comments
on all issues involved with this proposal, including all such proposed
judgments, conclusions, findings, and determinations. EPA invited
specific comment on the level, or range of levels, appropriate for such
a standard, as well as on the rationale that would support that level
or range of levels. These comments were carefully considered by the
Administrator as she made her final decisions, as described in this
notice, on the primary SO2 NAAQS
The schedule for completion of this review is governed by a
judicial order resolving a lawsuit filed in September 2005, concerning
the timing of the current review. Center for Biologic Diversity v.
Johnson (Civ. No. 05-1814) (D.D.C. 2007). The order that now governs
this review, entered by the court in August 2007 and amended in
December 2008, provides that the Administrator will sign, for
publication, a final rulemaking concerning the review of the primary
SO2 NAAQS no later than June 2, 2010.
E. Summary of Proposed Revisions to the SO2 Primary NAAQS
For the reasons discussed in the preamble of the proposal for the
SO2 primary NAAQS, EPA proposed to make revisions to the
primary SO2 NAAQS (and to add SO2 data handling
conventions) so the standards provide requisite protection of public
health with an adequate margin of safety. Specifically, EPA proposed to
replace the current 24-hour and annual standards with a new short-term
SO2 standard. EPA proposed that this new short-term standard
would be based on the 3-year average of the 99th percentile (or 4th
highest) of the yearly distribution of 1-hour daily maximum
SO2 concentrations. EPA proposed to set the level of this
new 1-hour standard within the range of 50 to 100 ppb and solicited
comment on standard levels as high as 150 ppb. EPA also proposed to
establish requirements for an SO2 monitoring network at
locations where maximum SO2 concentrations are expected to
occur and to add a new Federal Reference Method (FRM) for measuring
SO2 in the ambient air. Finally, EPA proposed to make
corresponding changes to the Air Quality Index for SO2.
F. Organization and Approach to Final SO2 Primary NAAQS Decisions
This action presents the Administrator's final decisions regarding
the need to revise the current SO2 primary NAAQS, and what
those revisions should be. Revisions to the primary NAAQS for
SO2, and the rationale supporting those revisions, are
described below in section II.
An overview of the approach for monitoring and implementation is
presented in section III. Requirements for the SO2 ambient
monitoring network and for a new, additional FRM for measuring
SO2 in the ambient air are described in section IV. EPA's
current plans for designations and for implementing the revised
SO2 primary NAAQS are discussed in sections V and VI
respectively. Related requirements for data completeness, data
handling, data reporting, rounding conventions, and exceptional events
are described in section VII. Communication of public health
information through the AQI is discussed in section VIII. A recitation
of statutory authority and a discussion of those executive order
reviews which are relevant are provided in section IX.
Today's final decisions are based on a thorough review in the ISA
of scientific information on known and potential human health effects
associated with exposure to SO2 in the
[[Page 35524]]
air. These final decisions also take into account: (1) Assessments in
the REA of the most policy-relevant information in the ISA as well as
quantitative exposure and risk analyses based on that information; (2)
CASAC Panel advice and recommendations, as reflected in its letters to
the Administrator and its public discussions of the ISA and REA; (3)
public comments received during the development of the ISA and REA; and
(4) public comments received on EPA's notice of proposed rulemaking.
II. Rationale for Decisions on the Primary Standards
This section presents the rationale for the Administrator's
decision to revise the existing SO2 primary standards by
replacing the current 24-hour and annual standards with a new 1-hour
SO2 standard at a level of 75 ppb, based on the 3-year
average of the annual 99th percentile of 1-hour daily maximum
concentrations. As discussed more fully below, this rationale takes
into account: (1) Judgments and conclusions presented in the ISA and
the REA; (2) CASAC advice and recommendations as reflected in the CASAC
panel's discussions of drafts of the ISA and REA at public meetings, in
separate written comments, and in letters to the Administrator
(Henderson 2008a; Henderson 2008b; Samet, 2009); (3) public comments
received at CASAC meetings during the development of the ISA and the
REA; and (4) public comments received on the notice of proposed
rulemaking.
In reaching this decision, EPA has drawn upon an integrative
synthesis of the entire body of evidence on human health effects
associated with the presence of SO2 in the ambient air, and
upon the results of the quantitative exposure and risk assessments
reflecting this evidence. As discussed below, this body of evidence
addresses a broad range of health endpoints associated with exposure to
SO2 in the ambient air. In considering this entire body of
evidence, EPA chose to focus most on those health endpoints for which
the ISA found the strongest evidence of an association with
SO2 (see section II.B below). Thus, the rationale for this
final decision on the SO2 NAAQS focused primarily on
respiratory morbidity following short-term (5-minutes to 24-hours)
exposure to SO2, for which the ISA found a causal
relationship.
As discussed below, a substantial amount of new research has been
conducted since EPA's last review of the SO2 NAAQS, with
important new information coming from epidemiologic studies in
particular. In addition to the substantial amount of new epidemiologic
research, the ISA considered a limited number of new controlled human
exposure studies and re-evaluated key older controlled human exposure
studies. In evaluating both the new and key older controlled human
exposure studies, the ISA utilized updated guidelines published by the
American Thoracic Society (ATS) on what constitutes an adverse effect
of air pollution (see ISA, section 3.1.3; p. 3-4). Importantly, all
controlled human exposure and epidemiologic studies evaluated in the
ISA have undergone intensive scrutiny through multiple layers of peer
review and opportunities for public review and comment. Thus, the
review of this information has been extensive and deliberate.
After a background discussion of the principal emitting sources and
current patterns of SO2 air quality and a description of the
current SO2 monitoring network from which those air quality
patterns are obtained (section II.A), the remainder of this section
discusses the Administrator's rationale for her final decisions on the
primary standards. Section II.B includes an overview of the scientific
evidence related to the respiratory effects associated with ambient
SO2 exposure. This overview includes a discussion of the at-
risk populations considered in the ISA. Section II.C summarizes the key
approaches taken by EPA to assess exposures and health risks associated
with exposure to ambient SO2. Section II.D summarizes the
approach that was used in the current review of the SO2
NAAQS with regard to consideration of the scientific evidence and the
air quality, exposure, and risk-based results related to the adequacy
of the current standards and potential alternative standards. Sections
II.E and II.F discuss, respectively, the Administrator's decisions
regarding the adequacy of the current standards and the elements of a
new short-term standard, taking into consideration public comments on
the proposed decisions. Section II.G summarizes the Administrator's
decisions with regard to the SO2 primary NAAQS.
A. Characterization of SO2 Air Quality
1. Anthropogenic Sources and Current Patterns of SO2 Air
Quality
Anthropogenic SO2 emissions originate chiefly from point
sources, with fossil fuel combustion at electric utilities (~66%) and
other industrial facilities (~29%) accounting for the majority of total
emissions (ISA, section 2.1). Other anthropogenic sources of
SO2 include both the extraction of metal from ore as well as
the burning of high sulfur-containing fuels by locomotives, large
ships, and equipment utilizing diesel engines. SO2 emissions
and ambient concentrations follow a strong east to west gradient due to
the large numbers of coal-fired electric generating units in the Ohio
River Valley and upper Southeast regions. In the 12 Consolidated
Metropolitan Statistical Areas (CMSAs) that had at least four
SO2 regulatory monitors from 2003-2005, 24-hour average
concentrations in the continental U.S. ranged from a reported low of ~1
ppb in Riverside, CA and San Francisco, CA to a high of ~12 ppb in
Pittsburgh, PA and Steubenville, OH (ISA, section 2.5.1). In addition,
outside or inside all CMSAs from 2003-2005, the annual average
SO2 concentration was 4 ppb (ISA, Table 2-8). However,
spikes in hourly concentrations occurred. The mean 1-hour maximum
concentration outside or inside CMSAs was 13 ppb, with a maximum value
of greater than 600 ppb outside CMSAs and greater than 700 ppb inside
CMSAs (ISA, Table 2-8).
Temporal and spatial patterns of 5-minute peaks of SO2
are also important given that controlled human exposure studies have
demonstrated that exposure to these peaks can result in adverse
respiratory effects in exercising asthmatics (see section II.B below).
For those monitors which voluntarily reported 5-minute block average
data,\3\ when maximum 5-minute concentrations were reported, the
absolute highest concentration over the ten-year period exceeded 4000
ppb, but for all individual monitors, the 99th percentile was below 200
ppb (ISA, section 2.5.2 Table 2-10). Median concentrations from these
monitors reporting 5-minute data ranged from 1 ppb to 8 ppb, and the
average for each maximum 5-minute level ranged from 3 ppb to 17 ppb.
Delaware, Pennsylvania, Louisiana, and West Virginia had mean values
for maximum 5-minute data exceeding 10 ppb. Among aggregated within-
State data for the 16 monitors from which all 5-minute average
intervals were reported, the median values ranged from 1 ppb to 5 ppb,
and the means ranged from 3 ppb to 11 ppb (ISA, section 2.5.2 at 2-43).
The highest reported concentration was 921 ppb, but the 99th percentile
values
[[Page 35525]]
for aggregated within-State data were all below 90 ppb (id).
---------------------------------------------------------------------------
\3\ A small number of sites, 98 total from 1997 to 2007 of the
approximately 500 SO2 monitors, and not the same sites in
all years, voluntarily reported 5-minute block average data to AQS
(ISA, section 2.5.2). Of these, 16 reported all twelve 5-minute
averages in each hour for at least part of the time between 1997 and
2007. The remainder reported only the maximum 5-minute average in
each hour.
---------------------------------------------------------------------------
2. SO2 Monitoring
Although EPA established the SO2 standards in 1971,
uniform minimum monitoring network requirements for SO2
monitoring were only adopted in May 1979. From the time of the
implementation of the 1979 monitoring rule through 2008, the
SO2 monitoring network has steadily decreased in size from
approximately 1496 sites in 1980 to the approximately 488 sites
operating in 2008. At present, except for SO2 monitoring
required at National Core Monitoring Stations (NCore stations), there
are no minimum monitoring requirements for SO2 in 40 CFR
part 58 Appendix D, other than a requirement for EPA Regional
Administrator approval before removing any existing monitors and a
requirement that any ongoing SO2 monitoring must have at
least one monitor sited to measure the maximum concentration of
SO2 in that area. EPA removed the specific minimum
monitoring requirements for SO2 in the 2006 monitoring rule
revisions, except for monitoring at NCore stations, based on the fact
that there were no SO2 nonattainment areas at that time,
coupled with trends showing an increasing gap between national average
SO2 concentrations and the current 24-hour and annual
standards. The rule was also intended to provide State, local, and
Tribal air monitoring agencies flexibility in meeting perceived higher
priority monitoring needs for other pollutants, or to implement the new
multi-pollutant sites (NCore network) required by the 2006 rule
revisions (71 FR 61236, (October 6, 2006)). More information on
SO2 monitoring can be found in section IV.
B. Health Effects Information
The ISA concluded that there was sufficient evidence to infer a
``causal relationship'' between respiratory morbidity and short-term
(5-minutes to 24-hours) exposure to SO2 (ISA, section 5.2).
Importantly, we note that a ``causal relationship'' is the strongest
finding the ISA can make.\4\ This conclusion was based on the
consistency, coherence, and plausibility of findings observed in
controlled human exposure studies of 5-10 minutes, epidemiologic
studies mostly using 1-hour daily maximum and 24-hour average
SO2 concentrations, and animal toxicological studies using
exposures of minutes to hours (ISA, section 5.2). This evidence is
briefly summarized below and discussed in more detail in the proposal
(see sections II.B.1 to II.B.5, see 74 FR at 64815-821). We also note
that the ISA judged evidence of an association between SO2
exposure and other health categories to be less convincing; other
associations were judged to be suggestive but not sufficient to infer a
causal relationship (i.e., short-term exposure to SO2 and
mortality) or inadequate to infer the presence or absence of a causal
relationship (i.e., short-term exposure to SO2 and
cardiovascular morbidity, and long-term exposure to SO2 and
respiratory morbidity, other morbidity, and mortality). Key conclusions
from the ISA are described in greater detail in Table 5-3 of the ISA.
---------------------------------------------------------------------------
\4\ A causal relationship is based on ``[e]vidence [that] is
sufficient to conclude that there is a causal relationship between
relevant pollutant exposures and the health outcome. That is, a
positive association has been observed between the pollutant and the
outcome in studies in which chance, bias, and confounding could be
ruled out with reasonable confidence. Evidence includes, for
example, controlled human exposure studies; or observational studies
that cannot be explain by plausible alternatives or are supported by
other lines of evidence (e.g. animal studies or mechanism of action
information). Evidence includes replicated and consistent high-
quality studies by multiple investigators.'' ISA Table 1-2, at 1-11.
---------------------------------------------------------------------------
1. Short-Term (5-minute to 24-hour) SO2 Exposure and
Respiratory Morbidity Effects
The ISA examined numerous controlled human exposure studies and
found that moderate or greater decrements in lung function (i.e.,
[gteqt] 15% decline in Forced Expiratory Volume (FEV1) and/
or [gteqt] 100% increase in specific airway resistance (sRaw)) occur in
some exercising asthmatics exposed to SO2 concentrations as
low as 200-300 ppb for 5-10 minutes. The ISA also found that among
asthmatics, both the percentage of individuals affected, and the
severity of the response increased with increasing SO2
concentrations. That is, at 5-10 minute concentrations ranging from
200-300 ppb, the lowest levels tested in free breathing chamber
studies, approximately 5-30% percent of exercising asthmatics
experienced moderate or greater decrements in lung function (ISA, Table
3-1). At concentrations of 400-600 ppb, moderate or greater decrements
in lung function occurred in approximately 20-60% of exercising
asthmatics, and compared to exposures at 200-300 ppb, a larger
percentage of asthmatics experienced severe decrements in lung function
(i.e., [gteqt] 20% decrease in FEV1 and/or [gteqt] 200%
increase in sRaw; ISA, Table 3-1). Moreover, at SO2
concentrations [gteqt] 400 ppb (5-10 minute exposures), moderate or
greater decrements in lung function were often statistically
significant at the group mean level and frequently accompanied by
respiratory symptoms. Id.
The ISA also found that in locations meeting the current
SO2 NAAQS, numerous epidemiologic studies reported positive
associations between ambient SO2 concentrations and
respiratory symptoms in children, as well as emergency department
visits and hospitalizations for all respiratory causes and asthma
across multiple age groups. Moreover, the ISA concluded that these
epidemiologic studies were consistent and coherent. This evidence was
consistent in that associations were reported in studies conducted in
numerous locations and with a variety of methodological approaches
(ISA, section 5.2; p. 5-5). It was coherent in that respiratory symptom
results from epidemiologic studies of short-term (predominantly 1-hour
daily maximum or 24-hour average) SO2 concentrations were
generally in agreement with respiratory symptom results from controlled
human exposure studies of 5-10 minutes. These results were also
coherent in that the respiratory effects observed in controlled human
exposure studies of 5-10 minutes further provided a basis for a
progression of respiratory morbidity that could lead to the increased
emergency department visits and hospital admissions observed in
epidemiologic studies (ISA, section 5.2; p. 5-5). In addition, the ISA
found that when evaluated as a whole, SO2 effect estimates
in multi-pollutant models generally remained positive and relatively
unchanged when co-pollutants were included. Therefore, although
recognizing the uncertainties associated with separating the effects of
SO2 from those of co-occurring pollutants, the ISA concluded
that ``the limited available evidence indicates that the effect of
SO2 on respiratory health outcomes appears to be generally
robust and independent of the effects of gaseous co-pollutants,
including NO2 and O3, as well as particulate co-
pollutants, particularly PM2.5'' (ISA, section 5.3; p. 5-9).
The ISA also found that the respiratory effects of SO2
were consistent with the mode of action as it is currently understood
from animal toxicological and controlled human exposure studies (ISA,
section 5.2; p. 5-2). The immediate effect of SO2 on the
respiratory system is bronchoconstriction. This response is mediated by
chemosensitive receptors in the tracheobronchial tree. Activation of
these receptors triggers central nervous system reflexes that result in
[[Page 35526]]
bronchoconstriction and respiratory symptoms that are often followed by
rapid shallow breathing (id). The ISA noted that asthmatics are likely
more sensitive to the respiratory effects of SO2 due to pre-
existing inflammation associated with the disease. For example, pre-
existing inflammation may lead to enhanced release of inflammatory
mediators, and/or enhanced sensitization of the chemosensitive
receptors (id).
Taken together, the ISA concluded that the controlled human
exposure, epidemiologic, and toxicological evidence supported its
determination of a causal relationship between respiratory morbidity
and short-term (5-minutes to 24-hours) exposure to SO2.
a. Adversity of Short-Term Respiratory Morbidity Effects
As discussed more fully in the proposal (section II.B.1.c, 74 FR at
64817) and in section II.E.2.b below, based on: (1) American Thoracic
Society (ATS) guidelines; (2) advice and recommendations from CASAC
(see specific consensus CASAC comments in sections II.E.2.b and
II.F.4.b below); and (3) conclusions from previous NAAQS reviews, EPA
found that 5-10 minute exposures to SO2 concentrations at
least as low as 200 ppb can result in adverse health effects in some
asthmatics (i.e., 5-30% of the tested individuals in controlled human
exposure studies of 200-300 ppb). As just mentioned, at SO2
concentrations >= 400 ppb, controlled human exposure studies have
reported decrements in lung function that are often statistically
significant at the group mean level, and that are frequently
accompanied by respiratory symptoms. Being mindful that the ATS
guidelines specifically indicate decrements in lung function with
accompanying respiratory symptoms as being adverse (see proposal
section II.B.1.c, 74 FR at 64817 and section II.E.2.b below), exposure
to 5-10 minute SO2 concentrations >= 400 ppb can result in
health effects that are clearly adverse.
The ATS also indicated that exposure to air pollution that
increases the risk of an adverse effect to a population is adverse,
even though it may not increase the risk of any individual to an
unacceptable level (ATS 2000; see proposal section II.B.1.c, 74 FR at
64817). As an example, ATS states:
A population of children with asthma could have a distribution
of lung function such that no individual child has a level
associated with significant impairment. Exposure to air pollution
could shift the distribution toward lower levels without bringing
any individual child to a level that is associated with clinically
relevant consequences. Individuals within the population would,
however, have diminished reserve function and are at potentially
increased risk if affected by another agent, e.g., a viral
infection. Assuming that the relationship between the risk factor
and the disease is causal, the committee considered that such a
shift in the risk factor distribution, and hence the risk profile of
the exposed population, should be considered adverse, even in the
absence of the immediate occurrence of frank illness (ATS 2000, p.
668).
As mentioned above, the ISA reported that exposure to
SO2 concentrations as low as 200-300 ppb for 5-10 minutes
results in approximately 5-30% of exercising asthmatics experiencing
moderate or greater decrements in lung function (defined in terms of a
>= 15% decline in FEV1 or 100% increase in sRaw; ISA, Table
3-1). Even though these results were not statistically significant at
the group mean level, in light of EPA's interpretation of how to apply
the ATS guidelines for defining an adverse effect, as described above,
the REA found that these results could reasonably indicate an
SO2-induced shift in these lung function measurements for
this subset of the population. As a result, an appreciable percentage
of exercising asthmatics exposed to SO2 concentrations as
low as 200 ppb would be expected to have diminished reserve lung
function and would be expected to be at greater risk if affected by
another respiratory agent, for example, viral infection. Importantly,
as explained immediately above, diminished reserve lung function in a
population that is attributable to air pollution is considered an
adverse effect under ATS guidance. In addition to the 2000 ATS
guidelines, the REA was also mindful of previous CASAC recommendations
(Henderson 2006) and NAAQS review conclusions (EPA 2006, EPA 2007d)
indicating that moderate decrements in lung function can be clinically
significant in some asthmatics (discussed in detail below, see section
II.E.2.b). The REA further considered that subjects participating in
these controlled human exposure studies do not include severe
asthmatics and that it was reasonable to presume that persons with more
severe asthma than the study participants would have a more serious
health effect from short-term exposure to 200 ppb SO2.\5\
Taken together, the REA concluded that exposure to SO2
concentrations at least as low as 200 ppb can result in adverse health
effects in asthmatics and that this conclusion was in agreement with
consensus CASAC comments and recommendations expressed during the
current SO2 NAAQS review (see sections II.E.2.b and II.F.4.b
below).
---------------------------------------------------------------------------
\5\ We also note that very young children were not included in
the controlled human exposure studies and this absence of data on
what is likely to be a sensitive life stage is a source of
uncertainty for children's susceptibility to SO2.
---------------------------------------------------------------------------
In addition to the controlled human exposure evidence,
epidemiologic studies also indicate that adverse respiratory morbidity
effects are associated with SO2 (REA, section 4.3). As
mentioned above, in reaching the conclusion of a causal relationship
between respiratory morbidity and short-term SO2 exposure,
the ISA generally found positive associations between ambient
SO2 concentrations and emergency department visits and
hospitalizations for all respiratory causes and asthma. Notably,
emergency department visits, hospitalizations, episodic respiratory
illness, and aggravation of respiratory diseases (e.g. asthma)
attributable to air pollution are considered adverse health effects
under ATS guidelines.
2. Health Effects and Long-Term Exposures to SO2
There were numerous studies published since the last review
examining possible associations between long-term SO2
exposure and mortality and morbidity (respiratory morbidity,
carcinogenesis, adverse prenatal and neonatal outcomes) endpoints.
However, the ISA concluded that the evidence relating long-term (weeks
to years) SO2 exposure to adverse health effects was
``inadequate to infer the presence or absence of a causal
relationship'' (ISA, Table 5-3). That is, the ISA found the long-term
health evidence to be of insufficient quantity, quality, consistency,
or statistical power to make a determination as to whether
SO2 was truly associated with these health outcomes (ISA,
Table 1-2).
3. SO2-Related Impacts on Public Health
Interindividual variation in human responses to air pollutants
indicates that some populations are at increased risk for the
detrimental effects of ambient exposure to SO2. The NAAQS
are intended to provide an adequate margin of safety for both the
general population and susceptible populations that are potentially at
increased risk for health effects in response to exposure to ambient
air pollution (see footnote 1 above). To facilitate the identification
of populations at increased risk for SO2-related health
effects, studies have identified factors that contribute to the
susceptibility of individuals to SO2. Susceptible
individuals are broadly defined as those with a greater
[[Page 35527]]
likelihood of an adverse outcome given a specific exposure in
comparison with the general population (American Lung Association,
2001). The susceptibility of an individual to SO2 can
encompass a multitude of factors which represent normal developmental
phases or life stages (e.g., age) or biologic attributes (e.g.,
gender); however, other factors (e.g., socioeconomic status (SES)) may
influence the manifestation of disease and also increase an
individual's susceptibility (American Lung Association, 2001). In
addition, populations may be at increased risk to SO2 due to
an increase in their exposure during certain life stages (e.g.,
childhood or old age) or as a result of external factors (e.g., SES)
that contribute to an individual being disproportionately exposed to
higher concentrations than the general population.\6\ It should be
noted that in some cases specific populations may be affected by
multiple susceptibility factors. For example, a population that is
characterized as having low SES may have less access to healthcare
resulting in the manifestation of a disease, which increases their
susceptibility to SO2, while they may also reside in a
location that results in disproportionately high exposure to
SO2.
---------------------------------------------------------------------------
\6\ This aspect of susceptibility is referred to as
vulnerability in the proposal and in the ISA.
---------------------------------------------------------------------------
To examine whether SO2 differentially affects certain
populations, stratified analyses are often conducted in epidemiologic
investigations to identify the presence or absence of effect
modification. A thorough evaluation of potential effect modifiers may
help identify susceptible populations that are at increased risk to
SO2 exposure. These analyses are based on the proper
identification of confounders and subsequent adjustment for them in
statistical models, which helps separate a spurious from a true causal
association. Although the design of toxicological and human clinical
studies does not allow for an extensive examination of effect
modifiers, the use of animal models of disease and the study of
individuals with underlying disease or genetic polymorphisms do allow
for comparisons between subgroups. Therefore, the results from these
studies, combined with those results obtained through stratified
analyses in epidemiologic studies, contribute to the overall weight of
evidence for the increased susceptibility of specific populations to
SO2. Those populations identified in the ISA to be
potentially at greater risk of experiencing an adverse health effect
from SO2 were described in detail in the proposal (section
II.B.5) and include: (1) Those with pre-existing respiratory disease;
(2) children and older adults; (3) persons who spend increased time
outdoors or at elevated ventilation rates; (4) persons with lower SES;
and (5) persons with certain genetic factors.
As discussed in the proposal (section II.B.5.g, 74 FR at 64821),
large proportions of the U.S. population are likely to be at increased
risk of experiencing SO2-related health effects. In the
United States, approximately 7% of adults and 9% of children have been
diagnosed with asthma. Notably, the prevalence and severity of asthma
is higher among certain ethnic or racial groups such as Puerto Ricans,
American Indians, Alaskan Natives, and African Americans (EPA 2008b).
Furthermore, a higher prevalence of asthma among persons of lower SES
and an excess burden of asthma hospitalizations and mortality in
minority and inner-city communities have been observed (EPA, 2008b). In
addition, population groups based on age comprise substantial segments
of individuals that may be potentially at risk for SO2-
related health impacts. Based on U.S. census data from 2000, about 72.3
million (26%) of the U.S. population are under 18 years of age, 18.3
million (7.4%) are under 5 years of age, and 35 million (12%) are 65
years of age or older. There is also concern for the large segment of
the population that is potentially at risk to SO2-related
health effects because of increased time spent outdoors at elevated
ventilation rates (those who work or play outdoors). Overall, the
considerable size of the population groups at risk indicates that
exposure to ambient SO2 could have a significant impact on
public health in the United States.
C. Human Exposure and Health Risk Characterization
To put judgments about SO2-associated health effects
into a broader public health context, EPA has drawn upon the results of
the quantitative exposure and risk assessments. Judgments reflecting
the nature of the evidence and the overall weight of the evidence are
taken into consideration in these quantitative exposure and risk
assessments. These assessments include estimates of the likelihood that
asthmatic children at moderate or greater exertion (e.g. while
exercising) in St. Louis or Greene County, Missouri would experience
SO2 exposures of potential concern. In addition, these
analyses include an estimate of the number and percent of exposed
asthmatic children in these locations likely to experience
SO2-induced lung function responses (i.e., moderate or
greater decrements in lung function defined in terms of sRaw or
FEV1) under varying air quality scenarios (i.e., current air
quality and air quality simulated to just meet the current or potential
alternative standards). These assessments also characterize the kind
and degree of uncertainties inherent in such estimates.
As previously mentioned, the ISA concluded that the evidence for an
association between respiratory morbidity and short-term SO2
exposure was ``sufficient to infer a causal relationship'' (ISA,
section 5.2) and that the ``definitive evidence'' for this conclusion
was from the results of 5-10 minute controlled human exposure studies
demonstrating decrements in lung function and/or respiratory symptoms
in exercising asthmatics (ISA, section 5.2). Accordingly, the air
quality and exposure analyses and their associated risk
characterizations focused on 5-minute concentrations of SO2
in excess of potential health effect benchmark values derived from the
controlled human exposure literature (see proposal section II.C.1, 74
FR at 64821, and REA, section 6.2). These benchmark levels are not
potential standards, but rather are SO2 exposure
concentrations which represent ``exposures of potential concern'' which
are used in these analyses to estimate potential exposures and risks
associated with 5-minute concentrations of SO2. The REA
considered 5-minute benchmark levels of 100, 200, 300, and 400 ppb in
these analyses, but especially noted exceedances or exposures with
respect to the 200 and 400 ppb 5-minute benchmark levels. These
benchmark levels were highlighted because (1) 400 ppb represents the
lowest concentration in free-breathing controlled human exposure
studies where moderate or greater lung function decrements occurred
which were often statistically significant at the group mean level and
were frequently accompanied by respiratory symptoms; and (2) 200 ppb is
the lowest level at which moderate or greater decrements in lung
function in free-breathing controlled human exposure studies were found
in some individuals, although these lung function changes were not
statistically significant at the group mean level. Notably, 200 ppb is
also the lowest level that has been tested in free-breathing controlled
human exposure studies (REA, section 4.2.2).\7\
---------------------------------------------------------------------------
\7\ The ISA cites one chamber study with intermittent exercise
where healthy and asthmatic children were exposed to 100 ppb
SO2 in a mixture with ozone and sulfuric acid. The ISA
notes that compared to exposure to filtered air, exposure to the
pollutant mix did not result in statistically significant changes in
lung function or respiratory symptoms (ISA, section 3.1.3.4).
---------------------------------------------------------------------------
[[Page 35528]]
The REA utilized three approaches to characterize health risks. In
the first approach, for each air quality scenario, statistically
estimated 5-minute SO2 concentrations \8\ and measured
ambient 5-minute SO2 concentrations were compared to the 5-
minute potential health effect benchmark levels discussed above (REA,
chapter 7). This air quality analysis included all available ambient
monitoring data as well as a more detailed analysis in 40 counties. The
air quality analysis was considered a broad characterization of
national air quality and human exposures that might be associated with
these 5-minute SO2 concentrations. An advantage of the air
quality analysis is its relative simplicity; however, there is
uncertainty associated with the assumption that SO2 air
quality can serve as an adequate surrogate for total exposure to
ambient SO2. Actual exposures might be influenced by factors
not considered by this approach, including small-scale spatial
variability in ambient SO2 concentrations (which might not
be represented by the current fixed-site ambient monitoring network)
and spatial/temporal variability in human activity patterns. A more
detailed overview of the air quality analysis and its associated
limitations and uncertainties is provided in the proposal (see sections
II.C.2, 74 FR at 64822 and II.C.3, 74 FR at 64823, respectively) and
the air quality analysis is thoroughly described in the REA (chapter
7).
---------------------------------------------------------------------------
\8\ Benchmark values derived from the controlled human exposure
literature were associated with a 5-minute averaging time. However,
as noted in footnote 3 above, only 98 ambient monitors located in 13
States from 1997-2007 reported measured 5-minute SO2
concentrations since such monitoring is not required (see section
II.A.2 and section IV). In contrast, 809 monitors in 48 States, DC,
Puerto Rico, and the Virgin Islands reported 1-hour SO2
concentrations over a similar time period. Therefore, to broaden
analyses to areas where measured 5-minute SO2
concentrations were not available, the REA utilized a statistical
relationship to estimate the highest 5-minute level in an hour,
given a reported 1-hour average SO2 concentration (REA,
section 6.4). Then, similar to measured 5-minute SO2
concentrations, statistically estimated 5-minute SO2
concentrations were compared to 5-minute potential health effect
benchmark values (REA, chapters 7 and 8, respectively).
---------------------------------------------------------------------------
In the second approach, an inhalation exposure model was used to
generate more realistic estimates of personal exposures in asthmatics
(REA, chapter 8). This analysis estimated temporally and spatially
variable microenvironmental 5-minute SO2 concentrations and
simulated asthmatics' contact with these pollutant concentrations while
at moderate or greater exertion (i.e., while at elevated ventilation
rates). The approach was designed to estimate exposures that are not
necessarily represented by the existing ambient monitoring data and to
better represent the physiological conditions corresponding with the
respiratory effects reported in controlled human exposure studies.
AERMOD, an EPA dispersion model, was used to estimate 1-hour ambient
SO2 concentrations using emissions estimates from
stationary, non-point, and where applicable, port sources. The Air
Pollutants Exposure (APEX) model, an EPA human exposure model, was then
used to estimate population exposures using the estimated hourly census
block level SO2 concentrations. From the 1-hour census block
concentrations, 5-minute maximum SO2 concentrations within
each hour were estimated by APEX (REA, section 8.7.1) using the
statistical relationship mentioned above in footnote 8. Estimated
exposures to 5-minute SO2 levels were then compared to the
5-minute potential health effect benchmark levels discussed above. This
approach to assessing exposures was more resource intensive than using
ambient levels as an indicator of exposure; therefore, the final REA
included the analysis of two locations: St. Louis and Greene County,
MO. Although the geographic scope of this analysis was limited, the
approach provided estimates of SO2 exposures in asthmatics
and asthmatic children in St. Louis and Greene Counties, and thus
served to complement the broader air quality characterization. A more
detailed overview of this exposure analysis and its associated
limitations and uncertainties is provided in the proposal (see sections
II.C.2, 74 FR at 64822 and II.C.3, 74 FR at 64823, respectively) and
the exposure analysis is thoroughly described in the REA (chapter 8).
The third approach was a quantitative risk assessment. This
approach combined results from the exposure analysis (i.e., the number
of exposed total asthmatics or asthmatic children while at moderate or
greater exertion) with exposure-response functions derived from
individual level data from controlled human exposure studies (see ISA,
Table 3-1 and Johns (2009) \9\) to estimate the percentage and number
of exposed asthmatics and asthmatic children in St. Louis and Greene
County likely to experience a moderate or greater lung function
response (i.e., decrements in lung function defined in terms of
FEV1 and sRaw) under the air quality scenarios mentioned
above (REA, chapter 9). A more detailed overview of this analysis and
its associated limitations and uncertainties is provided in the
proposal (see sections II.C.2, 74 FR at 64822 and II.C.3, 74 FR at
64823, respectively) and the quantitative risk analysis is thoroughly
described in the REA (chapter 9).
---------------------------------------------------------------------------
\9\ EPA recently conducted a complete quality assurance review
of all individual subject data. The results of this review did not
substantively change any of the entries in ISA, Table 3-1, and did
not in any way affect the conclusions of the ISA (see Johns and
Simmons, 2009).
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Notably, for the reasons described in the REA (REA, section 10.3.3)
and the proposal (see section II.E.1.b, 74 FR at 64827), when
considering the St. Louis and Greene County exposure and risk results
as they relate to the adequacy of the current standards, the REA
concluded that the St. Louis results were more informative in terms of
ascertaining the extent to which the current standards protect against
health effects linked to the various benchmarks (linked in turn to 5-
minute SO2 exposures). The results in fact suggested that
the current standards may not adequately protect public health (REA,
section 10.3.3, p. 364). Moreover, the REA judged that the exposure and
risk estimates for the St. Louis study area provided useful insights
into exposures and risks for other urban areas in the U.S. with similar
population and SO2 emissions densities (id.). For similar
reasons, the St. Louis results were more informative for ascertaining
the adequacy of the potential alternative standards under
consideration.
Key results of the air quality, exposure, and risk analyses were
presented in the policy assessment chapter of the REA (chapter 10) and
summarized in the proposal (see Tables 2-4 in the preamble to the
proposed rule). In considering these results, the proposal noted that
these analyses support that 5-minute SO2 exposures,
reasonably judged important from a public health perspective, were
associated with air quality adjusted upward to simulate just meeting
the current standards (see proposal, section II.E.1.c, 74 FR at 64828).
Moreover, these results indicated that 99th percentile 1-hour daily
maximum standard levels in the range of 50-100 ppb could substantially
limit exposures of asthmatic children at moderate or greater exertion
from 5-minute SO2 concentrations >=400 ppb, and appreciably
limit exposures of these children from 5-minute SO2
concentrations >=200 ppb (REA, p. 392-393). Results of these analyses
also indicated that a 1-hour standard at 150
[[Page 35529]]
ppb could still substantially limit exposures of asthmatic children at
moderate or greater exertion from 5-minute SO2
concentrations >=400 ppb, but would provide these children appreciably
less protection from exposure to 5-minute SO2 concentrations
>=200 ppb (REA, p. 395-396).
D. Approach for Determining Whether To Retain or Revise the Current
Standards
EPA notes that the final decision on retaining or revising the
current primary SO2 standards is a public health policy
judgment to be made by the Administrator. This judgment has been
informed by a recognition that the available health effects evidence
reflects a continuum consisting of ambient levels of SO2 at
which scientists generally agree that health effects are likely to
occur, through lower levels at which the likelihood and magnitude of
the response become increasingly uncertain. The Administrator's final
decisions draw upon scientific information and analyses related to
health effects, population exposures and risks; judgments about the
appropriate response to the range of uncertainties that are inherent in
the scientific evidence and analyses; and comments received from CASAC
and the public.
To evaluate whether the current primary SO2 standards
are adequate or whether revisions are appropriate, EPA has used an
approach in this review described in chapter 10 of the REA which builds
upon the approaches used in reviews of other criteria pollutants,
including the most recent reviews of the NO2, Pb,
O3, and PM NAAQS (EPA, 2008c; EPA, 2007c; EPA, 2007d; EPA,
2005), and reflects the latest body of evidence and information that is
currently available, as reflected by the ISA. As in other recent
reviews, EPA considered the implications of placing more or less weight
or emphasis on different aspects of the scientific evidence and the
exposure-/risk-based information, recognizing that the weight to be
given to various elements of the evidence and exposure/risk information
is part of the public health policy judgments that the Administrator
will make in reaching decisions on the standard.
A series of general questions framed this approach to considering
the scientific evidence and exposure-/risk-based information. First,
EPA's consideration of the scientific evidence and exposure/risk
information with regard to the adequacy of the current standards has
been framed by the following questions:
To what extent does evidence that has become available
since the last review reinforce or call into question evidence for
SO2-associated effects that were identified in the last
review?
To what extent has evidence for different health
effects and/or susceptible populations become available since the
last review?
To what extent have uncertainties identified in the
last review been reduced and/or have new uncertainties emerged?
To what extent does evidence and exposure-/risk-based
information that has become available since the last review
reinforce or call into question any of the basic elements
(indicator, averaging time, form, and level) of the current
standard?
To the extent that the available evidence and exposure-/risk-based
information suggests it may be appropriate to consider revision of the
current standards, EPA considers that evidence and information with
regard to its support for consideration of a standard that is either
more or less stringent than the current standards. This evaluation is
framed by the following questions:
Is there evidence that associations, especially causal
or likely causal associations, extend to ambient SO2
concentrations as low as, or lower than, the concentrations that
have previously been associated with health effects? If so, what are
the important uncertainties associated with that evidence?
Are exposures above benchmark levels and/or health
risks estimated to occur in areas that meet the current standard? If
so, are the estimated exposures and health risks important from a
public health perspective? What are the important uncertainties
associated with the estimated risks?
To the extent that there is support for consideration of a revised
standard, EPA then considers the specific elements of the standard
(indicator, averaging time, form, and level) within the context of the
currently available information. In so doing, the Agency addresses the
following questions regarding the elements of the standard:
Does the evidence provide support for considering a
different indicator for gaseous SOX?
Does the evidence provide support for considering
different, or additional averaging times?
What ranges of levels and forms of alternative
standards are supported by the evidence, and what are the associated
uncertainties and limitations?
To what extent do specific averaging times, levels, and
forms of alternative standards reduce the estimated exposures above
benchmark levels and risks attributable to exposure to ambient
SO2, and what are the uncertainties associated with the
estimated exposure and risk reductions?
The questions outlined above have been addressed in the REA. The
following sections present considerations regarding the adequacy of the
current standards and conclusions on the elements of a new short-term
standard in terms of indicator, averaging time, form, and level.
E. Adequacy of the Current Standards
This section discusses considerations related to the decision as to
whether the current 24-hour and annual SO2 primary NAAQS are
requisite to protect public health with an adequate margin of safety.
Specifically, section II.E.1 provides an overview of the rationale
supporting the Administrator's proposal that the current standards do
not provide adequate public health protection; section II.E.2 discusses
public comments received on the adequacy of the current standards; and
section II.E.3 discusses the Administrator's final decision on whether
the current SO2 primary NAAQS is requisite to protect public
health with an adequate margin of safety, as required by sections
109(d) and (b) of the Act.
1. Rationale for Proposed Decision
In the proposal, the Administrator initially concluded that the
current 24-hour and annual SO2 NAAQS were not adequate to
protect public health with an adequate margin of safety (see section
II.E.4, 74 FR at 64829). In reaching this conclusion, she considered
the: (1) Scientific evidence and conclusions in the ISA; (2) exposure
and risk information presented in the REA; (3) conclusions of the
policy assessment chapter of the REA; and (4) views expressed by CASAC.
These considerations are discussed in detail in the proposal (see
section II.E., 74 FR at 64826) and are summarized in this section.
In the proposal the Administrator noted the following in
considering the adequacy of the current 24-hour and annual primary
SO2 standards:
The conclusion of the ISA that the results of controlled
human exposure and epidemiologic studies form a plausible and coherent
data set that supports a causal relationship between short-term (5-
minutes to 24-hours) SO2 exposures and adverse respiratory
effects, and that the epidemiologic evidence (buttressed by the
clinical evidence) indicates that the effects seen in the epidemiologic
studies are attributable to exposure to SO2 (ISA, section
5.2).
The conclusion of the ISA that ``[i]n the epidemiologic
studies, respiratory effects were observed in areas where the maximum
ambient 24-h avg SO2 concentration was below the current 24-
[[Page 35530]]
h avg NAAQS level * * *.'' (ISA, section 5.2, p. 5-2.) and so would
occur at ambient SO2 concentrations that are present in
locations meeting the current 24-hour NAAQS.
These respiratory effects also occurred in areas with
annual air quality levels considerably lower than those allowed by the
current annual standard, indicating that the current annual standard is
also not providing protection against short-term health effects
reported in epidemiologic studies (ISA, section 5.2).
Analyses in the REA supporting that 5-minute exposures,
reasonably judged important from a public health perspective (i.e.,
respiratory effects judged to be adverse to the health of asthmatics,
see sections II.B.1.c above, and II.E.2.b below), were associated with
air quality adjusted upward to simulate just meeting the current 24-
hour and annual standards.
CASAC advice ``that the current 24-hour and annual
standards are not adequate to protect public health, especially in
relation to short term exposures to SO2 (5-10 minutes) by
exercising asthmatics'' (Samet, 2009, p. 15).
Based on these considerations (discussed in more detail in the
proposal, see sections II.E.1 and II.E.2), the Administrator proposed
that the current 24-hour and annual SO2 standards are not
requisite to protect public health with an adequate margin of safety
against adverse respiratory effects associated with short-term (5-
minute to 24-hour) SO2 exposures. In considering approaches
to revising the current standards, the Administrator initially
concluded it appropriate to consider setting a new 1-hour standard. The
Administrator noted that a 1-hour standard would likely provide
increased public health protection, especially for members of at-risk
groups, from the respiratory effects described in both epidemiologic
and controlled human exposure studies.
2. Comments on the Adequacy of the Current Standards
This section discusses public comments on the proposal that either
supported or opposed the Administrator's proposed decision to revise
the current SO2 primary NAAQS. Comments on the adequacy of
the current standards that focused on the scientific and/or the
exposure/risk basis for the Administrator's proposed conclusions are
discussed in sections II.E.2.a-II.E.2.c. Comments on the epidemiologic
evidence are considered in section II.E.2.a. Comments on the controlled
human exposure evidence are considered in section II.E.2.b. Comments on
human exposure and health risk assessments are considered in section
II.E.2.c. To the extent these comments on the evidence and information
are also used to justify commenters' conclusions on decisions related
to indicator, averaging time, form, or level, they are noted as well in
the appropriate sections below (II.F.1-II.F.4, respectively). The
summaries of comments, and responses thereto, presented below are not
exclusive: other comments and responses are being included in the
Response to Comment (RTC) Document which is part of the record for this
rulemaking (EPA, 2010).
Many public commenters agreed with the proposal that based on the
available information, the current SO2 standards are not
requisite to protect public health with an adequate margin of safety
and that revisions to the standards are therefore appropriate. Among
those calling for revisions to the standards were environmental groups
(e.g., Sierra Club, WEACT for Environmental Justice, Center for
Biological Diversity, (CBD) Environmental Defense Fund (EDF), Natural
Resources Defense Council (NRDC)); medical/public health organizations
(e.g., American Lung Association (ALA), American Thoracic Society
(ATS)); State environmental organizations (e.g., National Association
of Clean Air Agencies (NACAA), Northeast States for Coordinated Air Use
Management (NESCAUM); State environmental agencies (e.g., such agencies
in DE, IA, IL, MI, NY, NM, OH, PA, TX, VT); the Fond du Lac Band of
Lake Superior Chippewa (Fond du Lac) Tribe, local groups (e.g.,
Houston-Galveston Area Council, Alexandria Department of Transportation
and Environmental Services) and most individual commenters (~13,000).
These commenters generally concluded that the current SO2
standards need to be revised and that a more stringent standard is
needed to protect the health of susceptible population groups. In
supporting the need to adopt a more stringent NAAQS for SO2,
these commenters often referenced the conclusions of CASAC, as well as
evidence and information presented in the proposal. As such, the
rationale offered by these commenters was consistent with that
presented in the proposal to support the Administrator's proposed
decision to revise the current SO2 NAAQS.
Most industry commenters (e.g., Utility Air Regulatory Group
(UARG), American Petroleum Institute (API), Arizona Public Service,
National Petrochemical & Refiners Association (NPRA), Montana-Dakota
Utilities Co., Dominion Resources, Council of Industrial Boiler Owners
(CIBO), Edison Electric Institute (EEI), Duke Energy, National Mining
Association (NMA)); and some organizations (e.g., Texas Association of
Business, The Annapolis Center for Science-Based Public Policy
(ACSBPP), South Carolina Chamber of Commerce) opposed the proposed
revisions to the SO2 primary NAAQS. In supporting their
views, industry commenters generally concluded that EPA did not
appropriately consider uncertainties associated with the epidemiologic
and controlled human exposure evidence.
More specifically, with respect to the epidemiologic studies, many
of these commenters concluded that results of these studies are
confounded by co-pollutants and thus too uncertain to determine whether
SO2 is truly associated with the health outcomes being
measured (e.g., hospital admissions; Federal Register see below). With
respect to the controlled human exposure studies, many commenters were
critical of the 5-minute benchmark levels that were derived from these
studies and subsequently used by EPA in the air quality, exposure, and
risk analyses. These groups were particularly concerned about the
Administrator's reliance on the 200 ppb 5-minute benchmark level in
assessing the adequacy of the current and potential alternative
standards. In general, many industry groups maintained that adverse
respiratory effects did not occur following 5-10 minute SO2
exposures < 400 ppb (e.g., API, EEI, CIBO) and some groups stated that
even at SO2 concentrations >= 400 ppb, reported effects may
not be of clinical concern, and thus are likely not adverse (e.g.,
UARG). Many industry groups (e.g., API, UARG) also disagreed with EPA's
(and CASAC's) conclusions that severe asthmatics were not included in
these controlled human exposure studies, and that severe asthmatics
would likely have a more pronounced response to SO2
exposures at a given level, or would respond to even lower levels of
SO2.
In responding to these specific comments, we note that the
Administrator relied in the proposal on the evidence, information, and
judgments contained in the ISA and the REA (including the policy
assessment chapter), as well as on the advice of CASAC. In considering
the evidence, information, and judgments of the ISA and the REA, the
Agency notes that these documents have been reviewed and discussed
extensively by CASAC at multiple public meetings (see above, section
I.D) and in their letters to the
[[Page 35531]]
EPA Administrator. Thus, it is important to note that CASAC generally
accepted the key findings and conclusions presented in both the ISA and
REA (see Henderson 2008a, Henderson 2008b, and Samet, 2009).
a. Comments on EPA's Interpretation of the Epidemiologic Evidence
Many industry groups (e.g., API, UARG, American Chemistry Council
(ACC), Dominion Resources, ExxonMobil, Progress Energy, CIBO, The
Fertilizer Institute, EEI, Dow Chemical Company (Dow), MeadWestvaco
Corporation (MWV), (NMA) and some organizations (e.g., ACSBPP)
commented that, given the presence of numerous co-pollutants in the
air, the epidemiologic studies do not support the contention that
SO2 itself is causing health effects. For example, UARG
stated: ``The epidemiological evidence cannot determine that
SO2 is a cause of or a contributor to hospital admissions
(``HA''), emergency department (``ED'') visits or respiratory symptoms,
the effects cited in the Proposed Rule.''
Although EPA has recognized that multiple factors can contribute to
the etiology of respiratory disease and that more than one air
pollutant could independently impact respiratory health, we continue to
judge, as discussed in the ISA, that the available evidence supports
the conclusion that there is an independent effect of SO2 on
respiratory morbidity. In reaching this judgment, we recognize that a
major methodological issue affecting SO2 epidemiologic
studies concerns the evaluation of the extent to which other air
pollutants, particular PM2.5,\10\ may confound or modify
SO2-related effect estimates. The use of multi-pollutant
regression models is a common approach for evaluating potential
confounding by co-pollutants in epidemiologic studies. It is therefore
important to note that when the ISA evaluated U.S. and international
epidemiologic studies employing multi-pollutant models, SO2
effect estimates generally remained positive and relatively unchanged
when co-pollutants, including PM, were included (see ISA, p. 5-5).
Therefore, although recognizing the uncertainties associated with
separating the effects of SO2 from those of co-occurring
pollutants, the ISA concluded that the limited available evidence
indicates that the effect of SO2 on respiratory health
outcomes appears to be generally robust and independent of the effects
of gaseous co-pollutants, including NO2 and O3,
as well as particulate co-pollutants, particularly PM2.5
(ISA, section 5.2; p. 5-9).
---------------------------------------------------------------------------
\10\ As noted in the proposal (see sections II.D.1, 74 FR at
64824-64825 and II.F.4.a, 74 FR at 64835), there is special
sensitivity in this review in disentangling SO2-related
effects from PM-related effects (especially sulfate PM).
---------------------------------------------------------------------------
In considering questions of confounding and causation, the
epidemiologic studies should not be considered in a vacuum. As
emphasized by the ISA, and endorsed by CASAC, controlled human exposure
studies provide support for the plausibility of the associations
reported in epidemiologic studies (ISA, section 5-5; Henderson 2008a;
Henderson 2008b). These controlled human exposure studies exposed
exercising asthmatics to 5-10 minute peaks of SO2 and
reported decrements in lung function and/or respiratory symptoms in up
to 60% of these individuals (depending on exposure concentration; see
ISA, Table 5-3; p. 5-11). Thus, these experimental study results
provide strong support for an independent contribution of
SO2 to the respiratory health effects reported in
epidemiologic studies: ``The effects of SO2 on respiratory
symptoms, lung function, and airway inflammation observed in the human
clinical studies using peak exposures further provides a basis for a
progression of respiratory morbidity resulting in increased emergency
department visits and hospital admissions. Collectively, these findings
provide biological plausibility for the observed association between
ambient SO2 levels and emergency department visits and
hospitalizations for all respiratory diseases and asthma, notably in
children and older adults. * * *'' (ISA, section 5.2 at p. 5-5). Thus,
EPA is not relying solely on the epidemiologic studies to evaluate
whether associations reported in these studies (e.g., associations with
emergency department visits) are likely the result of ambient
SO2 exposure.
b. Comments on EPA's Interpretation of the Controlled Human Exposure
Evidence
Many industry groups (e.g., API, ACC, Progress Energy, EEI, CIBO)
commented that adverse health effects do not occur following 5-10
minute SO2 exposures < 400 ppb. In addition, some groups
(e.g., UARG) commented that adverse respiratory effects do not occur in
exercising asthmatics following SO2 exposures below 600 ppb.
The disagreement is not whether effects occur in exercising asthmatics
at these exposure levels and exposure durations. Rather, the issue is
whether the effects experienced can properly be regarded as adverse. In
general, these groups conclude that EPA's judgment of adverse health
effects at SO2 exposure levels below 600 or 400 ppb is
inappropriately based on an unsound interpretation of ATS guidelines.
More specifically, these groups generally contend that decrements in
lung function without accompanying respiratory symptoms are not adverse
effects of SO2 exposure, and that decrements in lung
function in a percentage of exercising asthmatics does not represent a
shift in lung function at the population level. Some of these groups
also contend that EPA followed the advice of individual CASAC members,
rather than consensus CASAC written comments on the ISA and REA when
concluding respiratory effects associated with SO2 exposures
below 600 or 400 ppb are adverse. Furthermore, some groups contend that
effects below 400 ppb should not be considered adverse because compared
to the number of asthmatics experiencing decrements in lung function,
there were similar numbers of asthmatics experiencing increases in lung
function. EPA disagrees with these comments, and believes that the
clinical evidence also supports the conclusion that the current
standards are not requisite to protect public health with and adequate
margin of safety.
The Agency disagrees that adverse respiratory effects do not occur
in exercising asthmatics following 5-10 minute SO2 exposures
ranging from 400-600 ppb. As previously mentioned, at SO2
concentrations ranging from 400-600 ppb, moderate or greater decrements
in lung function occur in approximately 20-60% of exercising asthmatics
(again, defined in terms of a >= 15% decline in FEV1 or 100%
increase in sRaw; ISA, Table 3-1). Moreover, at concentrations >= 400
ppb, decrements in lung function are often statistically significant at
the group mean level, and are frequently accompanied by respiratory
symptoms (ISA, Table 5-1). ATS guidelines on what constitutes an
adverse health effect of air pollution clearly state that reversible
loss of lung function in combination with the presence of symptoms
should be considered adverse (ATS 1985, 2000). Moderate or greater
decrements in lung function accompanied by respiratory symptoms fit
this description. Thus, the Agency's conclusion of adverse health
effects associated with SO2 concentrations >= 400 ppb is
consistent with ATS guidelines.
The Agency also disagrees with industry commenters regarding the
adversity of the respiratory effects seen in exercising asthmatics
following 5-10 minute SO2 exposures ranging from 200-300
ppb. As mentioned above (section II.B.1), and discussed more
[[Page 35532]]
fully in the proposal (see section II.B.3, 74 FR at 64819), the ISA
reported that exposure to SO2 concentrations as low as 200-
300 ppb for 5-10 minutes results in approximately 5-30% of exercising
asthmatics experiencing moderate or greater decrements in lung
function. In 2000, the ATS updated its guidelines on ``what constitutes
an adverse health effect of air pollution.'' These guidelines indicated
that exposure to air pollution that increases the risk of an adverse
effect to the entire population is adverse, even though it may not
increase the risk of any individual to an unacceptable level (ATS
2000). For example, ATS notes that a population of asthmatics could
have a distribution of lung function such that no individual has a
level associated with significant impairment. Exposure to air pollution
could shift the distribution to lower levels that still do not bring
any individual to a level that is associated with clinically relevant
effects. However, this would be considered adverse because individuals
within the population would have diminished reserve function, and
therefore would be at increased risk if affected by another agent (ATS
2000).
Considering the 2000 ATS guidelines, the results of the clinical
studies conducted at 200-300 ppb were reasonably interpreted by EPA to
indicate an SO2-induced shift in these lung function
measurements for a subset of this population. That is, an appreciable
percentage of this population of exercising asthmatics would be
expected to experience moderate or greater decrements in lung function
in response to SO2 concentrations as low as 200 ppb, and
thus would be expected to have diminished reserve lung function. As a
result, this sub-population would be at greater risk of a more severe
response if affected by another respiratory agent (e.g., viral
infection, or O3).
EPA is also mindful of CASAC comments on this issue following the
second draft ISA. The second draft ISA placed relatively little weight
on health effects associated with SO2 exposures at 200-300
ppb. CASAC strongly disagreed with this characterization of the health
evidence. Their consensus letter following the second draft ISA states:
Our major concern is the conclusions in the ISA regarding the
weight of the evidence for health effects for short-term exposure to
low levels of SO2. Although the ISA presents evidence
from both clinical and epidemiological studies that indicate health
effects occur at 0.2 ppm or lower, the final chapter emphasizes
health effects at 0.4 ppm and above * * * CASAC believes the
clinical and epidemiological evidence warrants stronger conclusions
in the ISA regarding the available evidence of health effects at 0.2
ppm or lower concentrations of SO2. The selection of a
lower bound concentration for health effects is very important
because the ISA sets the stage for EPA's risk assessment decisions.
In its draft Risk and Exposure Assessment (REA) to Support the
Review of the SO2 Primary National Ambient Air Quality Standards
(July 2008), EPA chose a range of 0.4 ppm-0.6 ppm SO2
concentrations for its benchmark analysis. As CASAC will emphasize
in a forthcoming letter on the REA, we recommend that a lower bound
be set at least as low as 0.2 ppm. (Henderson 2008a)
EPA also notes the similar CASAC comments on the first draft of the
REA. The consensus CASAC letter following the 1st draft REA states:
The CASAC believes strongly that the weight of clinical and
epidemiology evidence indicates there are detectable clinically
relevant health effects in sensitive subpopulations down to a level
at least as low as 0.2 ppm SO2. These sensitive
subpopulations represent a substantial segment of the at-risk
population. (Henderson 2008b; p. 1)
See Coalition of Battery Recyclers Association v. EPA, No. 09-1011 (DC
Cir., May 14, 2010), slip opinion at 9, holding that it was reasonable
for EPA to conclude that a two IQ point mean population loss is an
adverse effect based in part on CASAC advice that such a decrement is
significant. CASAC's strong advice regarding the adversity of effects
at the 200 ppb level similarly supports EPA's conclusion that the
observed lung decrements are adverse.
In addition to the considerations described above, we also note the
following key points:
In the current SO2 NAAQS review, clinicians on
the CASAC Panel advised that moderate or greater decrements in lung
function can be clinically significant in some individuals with
respiratory disease.\11\
---------------------------------------------------------------------------
\11\ See hearing transcripts from EPA Clean Air Scientific
Advisory Committee (CASAC), July 30-31 2008, Sulfur Oxides-Health
Criteria (part 3 of 4) pages 211-213). These transcripts can be
found in Docket ID No. EPA-HQ-ORD-2006-0260. Available at http://www.regulations.gov.
---------------------------------------------------------------------------
In the last O3 NAAQS review, CASAC indicated
that moderate decrements in lung function can be clinically significant
in some asthmatics (Henderson 2006), and that in the context of
standard setting, a focus on the lower end of the range of moderate
functional responses is most appropriate for estimating potentially
adverse lung function decrements in people with lung disease (e.g.,
asthma; see 73 FR at 16463).
In the last O3 NAAQS review, the Criteria
Document and the Staff Paper indicated that for many people with lung
disease (e.g., asthma), even moderate decrements in lung function or
respiratory symptoms would likely interfere with normal activities and
result in additional and more frequent use of medication (EPA 2006, EPA
2007d).
Subjects participating controlled human exposure studies
do not include severe asthmatics, and it is reasonable to presume that
persons with more severe asthma than the study participants would have
a more serious health effect from short-term exposure to 200 ppb
SO2.
Considering these key points along with the ATS guidelines and
consensus CASAC comments on the draft ISA and REA described above, we
reasonably conclude that 5-10 minute exposures to SO2
concentrations at least as low as 200 ppb can result in adverse health
effects in exercising asthmatics.
In addition, as noted above some groups (e.g., API) contend that
effects below 400 ppb should not be considered adverse because compared
to the number of asthmatics experiencing decrements in lung function,
there were similar numbers of asthmatics experiencing increases in lung
function.
The commenters correctly point out that at the lowest concentration
tested in free-breathing chamber studies (200 ppb), there are a similar
number of asthmatics experiencing a moderate or greater decrease in
lung function (i.e., >= 100 increase in sRaw or >= 15 decrease in
FEV1) and experiencing what might be called a moderate
improvement in lung function (i.e., >= 100 decrease in sRaw or >= 15
increase in FEV1). This observation is consistent with data
presented in Figures 4-2 and 4-3 of the ISA showing essentially no
SO2 -induced change in lung function at 200 ppb when
averaged across asthmatics participating in the three Lin et al.,
controlled human exposure studies. However, these figures also
demonstrate that asthmatics who are sensitive to SO2 at a
higher concentration (600 ppb) experience, on average, a greater
decrement in lung function at lower concentrations, including 200 ppb,
when compared with all subjects combined. Therefore, while some
asthmatics are relatively insensitive to SO2-induced
respiratory effects even at concentrations >= 600 ppb, there is clear
empirical evidence that others experience significant
bronchoconstriction following exposures to both relatively high (600
ppb) and low (200 ppb) SO2 concentrations. Among these
SO2-sensitive asthmatics, Figures 4-2 and 4-3 of the ISA
show a clear increase in
[[Page 35533]]
bronchoconstriction with increasing SO2 concentrations from
200-400 ppb. Given this clear relationship of exposure and effect at
all levels in the sensitive asthmatics (i.e. those who experienced
significant decrements in lung function at the highest exposure
concentration used (600 ppb)), EPA does not accept the commenter's
premise that controlled human exposure studies do not demonstrate
adverse effects in some asthmatics at 5-10 minute levels below 400 ppb.
In addition to disagreeing with EPA's proposed finding of adverse
health effects following 5- 10 minute SO2 exposures as low
as 200 ppb, many industry groups (e.g., API, UARG, ACC, ExxonMobil)
also disagreed with EPA that severe asthmatics were not included in
controlled human exposure studies. That is, these groups contend that
EPA is incorrect in assuming that severe asthmatics would likely have a
more pronounced response to SO2 exposures at a given level,
or would respond to even lower levels of SO2 and that this
should be taken into account when judging the adequacy of the current
standards. As support for their assertion, multiple industry groups
cite controlled human exposure studies in the ISA stating that they
included ``severe asthmatics'' and also cite a study by Linn et al.
(1987) which concluded that among asthmatics, responses to
SO2 exposure are not dependent on the clinical severity of
asthma and that ``the subjects with the highest risk [of temporary
respiratory disturbances from ambient SO2] can be identified
only by actually measuring their responses to SO2''.
We disagree with the assertion that severe asthmatics have been
evaluated in 5-10 minute controlled human exposure studies. Although
studies cited in the ISA referred to a group of subjects as ``moderate/
severe'' asthmatics, these individuals had well-controlled asthma, were
able to withhold medication, were not dependent on corticosteroids, and
were able to engage in moderate to heavy levels of exercise. By today's
standards, these individuals would clearly be classified as moderate
asthmatics. EPA therefore concludes that persons with asthma that is
more severe than moderate asthma, as that term is currently understood,
were not included in the controlled human exposure studies (and
understandably so, for ethical reasons).
In addition, EPA agrees with the commenters that there is little
evidence from controlled human exposure studies to suggest that the
respiratory effects of SO2 differ between mild and moderate
asthmatics (see Linn et al., 1987). However, this may very well be due,
at least in part, to persistence of medication among the moderate
asthmatic subjects. More importantly, the moderate asthmatics began the
exposure with compromised lung function relative to the mild
asthmatics. Therefore, similar functional declines from different
baselines between mild and moderate asthmatics would clearly not have
the same physiological importance. CASAC specifically addressed the
issue of asthma severity in a letter to the Administrator: ``For
ethical reasons severe asthmatics were not part of these clinical
studies, but it is not unreasonable to presume that they would have
responded to even a greater degree (Henderson 2008a; p. v).'' It is
also important to note that in addition to the strict health-specific
inclusion and exclusion criteria for a given controlled human exposure
study, many asthmatics who might otherwise be able to participate
choose not to participate because of anxiety related to what they
viewed as potential adverse health risks. EPA concludes that it is
appropriate to assume, as CASAC suggested, that persons with more
severe asthma would respond to an even greater degree than the moderate
asthmatics in the clinical studies.
c. Comments on EPA's Characterization of SO2-Associated
Exposures and Health Risks
Several commenters discussed the analyses of SO2-
associated exposures and health risks presented in the REA. As in past
reviews (EPA 2005, 2007c, 2007d), EPA has estimated risks associated
with the current standards to inform judgments on the public health
risks that could exist under different standard options. Some industry
commenters (e.g., API, UARG, Lignite Energy Council (LEC), Jackson
Walker, ASARCO, the National Rural Electric Cooperative Association)
concluded that when considering the adequacy of the current standards,
the Administrator should consider exposures and risks associated with
actual SO2 air quality rather than air quality allowed by
the current NAAQS. They consequently challenged the relevance and
appropriateness of EPA's use of SO2 concentrations that have
been simulated to just meet the current standards in assessing the
adequacy of the current standards.
In addition to the objections noted above, we note that UARG
generally concluded that the results of EPA's quantitative risk
assessment are fundamentally flawed in that they substantially
overestimate risks associated with the various air quality scenarios.
UARG contends that this is because EPA did not use proper exposure-
response functions in estimating risks associated with SO2
exposure. Moreover, UARG contends EPA further overestimates risk
because of the use of 50 ppb exposure bins in estimating the number of
occurrences of an adverse lung function response (see below).
With respect to comments that when considering the adequacy of the
current standards, the Administrator should consider exposures and
risks associated with actual SO2 air quality rather than
that simulated to just meet the current standards, these commenters
generally concluded: (1) It is more relevant to assess exposures and
risks associated with actual SO2 air quality since adjusting
air quality to just meet the current standards require large
adjustments to air quality that are highly uncertain; and (2) NAAQS are
intended to address actual, rather than highly improbable, risks to
human health. In addition, these groups generally concluded that
exposure and risk estimates presented in the REA suggest relatively
little health risk associated with current levels of SO2,
and thus, there is no need to revise the current SO2
standards.
We disagree with these commenters that exposure- and risk-related
considerations in the NAAQS reviews should rely only on actual air
quality, and that EPA therefore improperly adjusted air quality in its
risk and exposure analyses to simulate air quality allowed by the
current primary SO2 NAAQS. EPA is required to review whether
the present standards--not present air quality--are requisite to
protect public health with an adequate margin of safety. Section
109(b)(1). In making this determination it is relevant to consider
exposures and risks which could be permissible under the current
standards. See American Trucking Associations v. EPA, 283 F.3d 355, 370
(DC Cir. 2002) (existence of evidence showing adverse effects occurring
at levels allowed by the current standards justifies finding that it is
appropriate to revise the existing NAAQS). Consequently, it is at the
very least reasonable for EPA, in its REA, to make air quality
adjustments to estimate SO2-related exposures and health
risks that could exist in areas that just meet the present standards.
Thus, although we acknowledge that exposure and health risk estimates
associated with current ambient concentrations are substantially
smaller than those associated with air quality adjusted to just meet
the current standards, we also note that this is
[[Page 35534]]
irrelevant to the question of whether the current standards are
requisite to protect public health with an margin of safety.
In both of these cases, EPA is not trying to evaluate whether areas
would or would not be in attainment of the current standards. Those are
issues that are addressed during the implementation of the NAAQS.
Instead, in this rulemaking EPA is evaluating what NAAQS would be
appropriate under section 109(b)(1), by evaluating the impact on or
risks to public health from air quality that is at the level of the
current standards, as well as evaluating air quality that is at the
level of various alternative standards. EPA uses this information to
inform the decision on what NAAQS would be requisite to protect public
health with an adequate margin of safety.
If EPA determines that the current standards require revision, EPA
is further required to determine what revisions are appropriate in
light of the requirement that primary NAAQS be requisite to protect
public health with an adequate margin of safety. Section 109(d)(1). It
is thus similarly reasonable for EPA to make air quality adjustments to
simulate different potential alternative standards to provide
information on exposures and risks under these potential alternative
standards.\12\
---------------------------------------------------------------------------
\12\ In conducting these analyses, EPA is not trying to evaluate
whether areas would or would not be in attainment of the current
standards. Again, those issues are addressed during the
implementation of the NAAQS.
---------------------------------------------------------------------------
We agree that there are uncertainties inherent in making air
quality adjustments. These uncertainties are discussed thoroughly in
the REA (REA, sections 6.5 and 7.4.2.5). For example, the REA noted the
following regarding adjustment of SO2 concentrations:
This procedure for adjusting either the ambient concentrations
(i.e., in the air quality characterization) or health effect
benchmark levels (i.e., in the exposure assessment) was necessary to
provide insight into the degree of exposure and risk which would be
associated with an increase in ambient SO2 levels such
that the levels were just at the current standards in the areas
analyzed. Staff recognizes that it is extremely unlikely that
SO2 concentrations in any of the selected areas where
concentrations have been adjusted would rise to meet the current
NAAQS and that there is considerable uncertainty associated with the
simulation of conditions that would just meet the current standards.
Nevertheless, this procedure was necessary to assess the ability of
the current standards, not current ambient SO2
concentrations, to protect public health (REA, section 6.5; p. 64)
These air quality adjustments are not meant to imply an expectation
that SO2 concentrations will increase broadly across the
United States or in any given area. Rather, as just noted above, they
are meant to estimate SO2-related exposures and health risks
if air quality were at the level of the current and potential
alternative standards. Such estimates can inform decisions on whether
the current standards, or particular potential alternative standards,
provide the requisite protection of public health.
As mentioned above, UARG generally concluded that under all air
quality scenarios, the results of EPA's quantitative risk assessment
(the third of the analyses conducted in the REA (chapter 9), see
section II.C above) are substantially overestimated because EPA did not
use proper methods to estimate the parameters of the exposure-response
functions used in its analyses. UARG contends this is because many of
the subjects in the controlled human exposure studies from which EPA's
exposure-response functions were derived (see REA, Table 9-3) were
exposed to more than one SO2 concentration, yet EPA treated
each exposure event as being independent (e.g., if the same subject was
exposed to 200 and 300 ppb SO2, EPA considered these as
representing two independent exposure events). UARG contends that
observations from the same subject exposed to different SO2
concentrations are not independent observations and should not be
treated as such. Notably, when UARG derived their own exposure-response
functions taking into account that observations from the same subject
exposed to different SO2 concentrations are not independent
of each other, they estimated appreciably less risk than that estimated
by EPA.
There are a variety of techniques and/or assumptions that can be
used to fit individual subject data from the controlled human exposure
studies (see REA, Table 9-3) to exposure-response curves. Moreover, any
technique or assumption utilized will have inherent uncertainties. EPA
discussed the uncertainties associated with our quantitative risk
assessment in detail in the REA (REA, section 9.4); we also gave an
overview of key uncertainties in the proposal (see section II.C.3, 74
FR at 64824). The approach used to estimate the exposure-response
functions was not first introduced in the SO2 risk
assessment, it was previously recommended to EPA by an applied
statistician serving on the O3 CASAC Panel and used in the
O3 risk assessment (which had individual controlled human
exposure data similar to that in the current SO2 NAAQS
review; see EPA 2007d and EPA 2007e). Importantly, this approach
allowed EPA to use all the available individual subject data. Moreover,
an inspection of the estimated exposure-response curve and the
underlying data suggest that any biases in the parameter estimates are
likely to be slight (see EPA 2010, section II.C). Consequently, EPA
does not accept UARG's view that the methodology used in EPA's
quantitative risk assessment was inappropriate.
We further note that UARG's exposure-response functions do not fit
the underlying controlled human exposure data (the proportions of
subjects who responded at each exposure level) nearly as well as the
exposure-response functions estimated using EPA's approach. We believe
this could be due to the methodology used in UARG's reanalysis of the
individual-level data from the controlled human exposure studies used
in the quantitative risk assessment. UARG attempted to estimate
subject-specific exposure-response functions, and to use the results of
these estimates to obtain estimates of the two parameters in the
population-level exposure-response functions. As described in more
detail in section II.C of the RTC document (EPA 2010), EPA does not
believe there are sufficient data to properly estimate the parameters
of subject-specific exposure-response functions. More specifically,
UARG chose a three-parameter quadratic function for the subject-
specific exposure-response functions. However, none of the subjects had
more than three exposures, and many had only one or two. EPA believes
that this information is particularly limited for estimating these
subject-specific exposure-response functions, especially given that a
large percentage of the total number of subjects had fewer exposures
than the number of parameters UARG was attempting to estimate (i.e.,
UARG estimated three parameters in its exposure-response functions, but
over fifty percent of subjects only had one or two exposures). It
appears that UARG's population-level exposure-response function
estimates depended on these subject-specific exposure-response function
estimates and thus could explain why UARG's estimated population-level
exposure-response functions do not fit the underlying controlled human
exposure data nearly as well as the approach used by EPA. A more
detailed response to this comment can be found in section II.C of the
RTC document (EPA 2010).
As mentioned above, UARG also concluded that EPA further
overestimates the total number of occurrences of an adverse lung
function response (i.e., total number of
[[Page 35535]]
occurrences of increases in sRaw >= 100 or 200% and/or declines in
FEV1 >= 15 or 20%) in its quantitative risk assessment. More
specifically, UARG concluded that the use of 50 ppb bins, combined with
assigning all exposures within a bin the probability of an adverse lung
function response at the midpoint of that bin (e.g., all exposures from
0-50 ppb were assigned the probability of an adverse lung function
response occurring at 25 ppb), resulted in a substantial overestimate
of the total number of occurrences of lung function responses in
asthmatics at moderate or greater exertion. UARG generally concludes
that this is because the vast majority of exposures of asthmatics at
moderate or greater exertion are occurring below the midpoint of the 0-
50 ppb exposure bin (i.e., most exposures are occurring below 25 ppb),
yet EPA is assigning these very low SO2 exposures the higher
probability of a lung function response associated with the midpoint of
the 0-50 ppb exposure bin. UARG contends that this results in a
substantial overestimation of the total number of occurrences of lung
function response in asthmatics and asthmatic children at moderate or
greater exertion. UARG further notes that this methodological concern
was raised in its comments on the second draft REA, but EPA failed to
address this issue and relied heavily on this metric in the proposal
with respect to the adequacy of the current and potential alternative
standards. EPA's response to this comment is discussed below and in
more detail in section II.C of the RTC document (EPA 2010).
EPA generally agrees with UARG's technical comments that there is a
substantial overestimation of the total occurrences of lung function
responses because of the binning issues described above. However, we
strongly disagree that: (1) This issue was not acknowledged in the
final REA; and (2) the metric of total occurrences was relied on
heavily in the policy assessment chapter of the REA (REA, chapter 10)
and in the Administrator's rationale with respect to the adequacy of
the current and potential alternative standards. First, EPA did respond
to this concern in the final REA. More specifically, page 344 of the
final REA states:
As noted in public comments on the 2nd draft SO2 REA,
the assignment of response probability to the midpoint of the
exposure bin combined with the lack of more finely divided intervals
in this range can lead to significant overestimation of risks based
on total occurrences of a defined lung function response. This is
because the distribution of population exposures for occurrences is
not evenly distributed across the bin, but rather is more heavily
weighted toward the lower range of the bin. Thus, combining all
exposures estimated to occur in the lowest bin with a response
probability assigned to the midpoint of the bin results in a
significant overestimate of the risk. Therefore, staff places less
weight on the estimated number of occurrences of lung function
responses.
Thus, as noted in the final REA, less weight was placed on this
metric in the quantitative risk assessment chapter (REA, chapter 9),
and importantly, no weight was placed on this metric in either the
policy assessment chapter of the REA (REA, chapter 10) or in the
Administrator's rationale sections of the proposal preamble. Rather,
the policy assessment chapter of the REA and the Administrator's
rationale at the proposal considered the percent of exposed asthmatic
children at moderate or greater exertion estimated to have at least one
defined lung function response per year in St. Louis. Importantly, this
metric is not appreciably affected by the binning issue raised in
UARG's comments. As stated on page 344-345 of the final REA:
This overestimation of total occurrences does not impact the
risk metric expressed as incidence or percent incidence of a defined
lung function response 1 or more times per year because the bulk of
the exposures contributing to these risk metrics are not skewed
toward the lower range of the reported exposure bins.\13\
---------------------------------------------------------------------------
\13\ Although in St. Louis, the percent of exposed asthmatic
children at moderate or greater exertion estimated to have at least
one defined lung function response per year was not appreciably
affected, it was found that for this same metric, the already very
low risk estimates in Greene County became appreciably lower when
the binning issue discussed above was considered. However, as noted
above in section II.C and discussed in more detail in the REA (REA,
section 10.3.3) and the proposal (see section II.E.b, 74 FR at
64827), the St. Louis exposure and risk results were found to be
more informative in addressing the adequacy of the current and
potential alternative standards. Moreover, while the Administrator's
rationale in the proposal relied minimally on the St. Louis
quantitative risk results (see above), she importantly placed no
weight on any metric from the Greene County quantitative risk
assessment.
Finally, it is important to note that the Administrator's rationale
in the proposal regarding the adequacy of the current and potential
alternative standards in general placed only limited reliance on the
results of the quantitative risk assessment in St. Louis, with no
reliance on the estimates of total occurrences. Rather, in addition to
the substantial weight that she placed on the scientific evidence as
described in the ISA, the Administrator placed relatively more weight
on the results of the St. Louis exposure analysis. For example, in
discussing the adequacy of the current standards, the proposal states:
``The Administrator especially notes the results of the St. Louis
exposure analysis which, as summarized above, indicates that
substantial percentages of asthmatic children at moderate or greater
exertion would be exposed, at least once annually, to air quality
exceeding the 400 and 200 ppb benchmarks'' (see 74 FR at 64829). We
note that results of the quantitative risk assessment in St. Louis,
with respect to the percent of asthmatic children estimated to have at
least one lung function response per year (using EPA's exposure-
response functions), supports the Administrator's overall conclusions
in the proposal regarding the adequacy of the current and potential
alternative standards.
3. Conclusions Regarding the Adequacy of the Current 24-Hour and Annual
Standards
In reviewing the adequacy of the current standards, the
Administrator has considered the scientific evidence assessed in the
ISA, the exposure and risk results presented in the REA, the
conclusions of the policy assessment chapter of the REA, and comments
from CASAC and the public. These considerations are described below.
As in the proposal, the Administrator accepts and agrees with the
ISA's conclusion that the results of controlled human exposure and
epidemiologic studies form a plausible and coherent data set that
supports a causal relationship between short-term (5 minutes to 24
hours) SO2 exposures and adverse respiratory effects. The
Administrator acknowledges that there are uncertainties associated with
the epidemiologic evidence (e.g., potential confounding by co-
pollutants). However, she agrees that the epidemiologic evidence,
supported by the controlled human exposure evidence, generally
indicates that the effects seen in these studies are attributable to
exposure to SO2, rather than co-pollutants, most notably
PM2.5. She also accepts and agrees with the conclusion of
the ISA that ``[i]n the epidemiologic studies, respiratory effects were
observed in areas where the maximum ambient 24-h avg SO2
concentration was below the current 24-h avg NAAQS level. * * *'' (ISA,
section 5.2, p. 5-2) and so would occur at ambient SO2
concentrations that are present in locations meeting the current 24-
hour NAAQS. The Administrator also notes that these effects occurred in
areas with annual air quality levels considerably lower than those
allowed by the current annual standard, indicating that the annual
standard also
[[Page 35536]]
is not providing protection against such effects. Existence of
epidemiologic studies showing adverse effects occurring at levels
allowed by the current standards is an accepted justification for
finding that it is appropriate to revise the existing standards. See,
e.g. American Trucking Associations v. EPA, 283 F. 3d at 370; see also
American Farm Bureau v. EPA, 559 F. 3d.512, 521-23 (DC Cir. 2009)
(effects associated with short-term exposure seen in areas with ambient
concentrations lower than long-term standard, so that without further
explanation, standard does not adequately protect against short-term
exposures).
With respect to the controlled human exposure studies, the
Administrator judges that effects following 5-10 minute SO2
exposures >= 400 ppb and >= 200 ppb can result in adverse health
effects to asthmatics. This judgment is based on ATS guidelines,
explicit CASAC consensus written advice and recommendations, and
judgments made by EPA in previous NAAQS reviews. Thus, similar to the
proposal, she notes analyses in the REA supporting that 5-minute
exposures >= 400 ppb and >= 200 ppb were associated with air quality
adjusted upward to simulate just meeting the current standards. The
Administrator especially notes the results of the St. Louis exposure
analysis which, as summarized in the proposal (see section II.E.1.b and
Table 3, see 74 FR at 64841), indicates that substantial percentages of
asthmatic children at moderate or greater exertion would be exposed, at
least once annually, to air quality exceeding the 400 and 200 ppb 5-
minute benchmarks given air quality simulated to just meet the current
standards. The Administrator judged these 5-minute exposures to be
significant from a public health perspective due to their estimated
frequency: Approximately 24% of child asthmatics at moderate or greater
exertion in St. Louis are estimated to be exposed at least once per
year to air quality exceeding the 5-minute 400 ppb benchmark, a level
associated with lung function decrements in the presence of respiratory
symptoms. Additionally, approximately 73% of child asthmatics in St.
Louis at moderate or greater exertion would be expected to be exposed
at least once per year to air quality exceeding the 5-minute 200 ppb
benchmark. This health evidence and risk-based information underlie
CASAC's conclusion that the current SO2 standards do not
adequately protect public health. As discussed in the proposal, CASAC
stated: ``the current 24-hour and annual standards are not adequate to
protect public health, especially in relation to short-term exposures
to SO2 (5-10 minutes) by exercising asthmatics'' (Samet,
2009, p. 15). The Administrator agrees with this conclusion.
In considering approaches to revising the current standards, the
Administrator concludes that it is appropriate to set a new standard,
that such standard must provide requisite protection with an adequate
margin of safety to a susceptible population (i.e., asthmatics at
elevated ventilation), and that the standard must afford protection
from short-term exposures to SO2 in order to prevent the
adverse health effects reported in both the controlled human exposure
and epidemiologic studies. The Administrator notes that a 1-hour
standard could provide increased public health protection, especially
for members of at-risk groups, from health effects described in both
controlled human exposure and epidemiologic studies, and hence, health
effects associated with 5-minute to 24-hour exposures to
SO2.\14\ As discussed in section II.F.5 below, given the
degree of protection afforded by such a standard, it may be appropriate
to replace, and not retain, the current 24-hour and annual standards in
conjunction with setting a new short-term standard.
---------------------------------------------------------------------------
\14\ We also note that such a standard would, among other
things, address the deficiency in the current NAAQS which occasioned
the remand of that standard for failing to adequately explain the
absence of protection from short-term SO2 bursts which
could cause adverse health effects in hundreds of thousands of
heavily breathing asthmatics. American Lung Ass'n v. EPA, 134 F. 3d
at 392-93.
---------------------------------------------------------------------------
F. Conclusions on the Elements of a New Short-Term Standard
In considering a revised SO2 primary NAAQS, the
Administrator notes the need to protect at-risk populations from: (1)
1-hour daily maximum and 24-hour average exposures to SO2
that could cause the types of respiratory morbidity effects reported in
epidemiologic studies; and (2) 5-10 minute SO2 exposure
concentrations reported in controlled human exposure studies to result
in moderate or greater decrements in lung function and/or respiratory
symptoms. Considerations with regard to potential alternative standards
and the specific conclusions of the Administrator are discussed in the
following sections in terms of indicator, averaging time, form, and
level (sections II.F.1 to II.F.4 below).
1. Indicator
a. Rationale for Proposed Decision
In the last review, EPA focused on SO2 as the most
appropriate indicator for ambient SOX. In making a decision
in the current review on the most appropriate indicator, the
Administrator has considered the conclusions of the ISA and REA as well
as the views expressed by CASAC and the public. The REA noted that,
although the presence of gaseous SOX species other than
SO2 has been recognized, no alternative to SO2
has been advanced as being a more appropriate surrogate for ambient
gaseous SOX. Controlled human exposure studies and animal
toxicology studies provide specific evidence for health effects
following exposure to SO2. Epidemiologic studies also
typically report levels of SO2, as opposed to other gaseous
SOX. Because emissions that lead to the formation of
SO2 generally also lead to the formation of other
SOX oxidation products, measures leading to reductions in
population exposures to SO2 can generally be expected to
lead to reductions in population exposures to other gaseous
SOX. Therefore, as noted in the proposal, meeting an
SO2 standard that protects the public health can also be
expected to provide protection against potential health effects that
may be independently associated with other gaseous SOX even
though such effects are not discernable from currently available
studies indexed by SO2 alone. See American Petroleum
Institute v. EPA, 665 F, 2d 1176, 1186 (DC Cir. 1981) (reasonable for
EPA to use ozone as the indicator for all photochemical oxidants even
though health information on the other photochemical oxidants is
unknown; regulating ozone alone is reasonable since it presents a
``predictable danger'' and in doing so EPA did not abandon its
responsibility to regulate other photochemical oxidants encompassed by
the determination that photochemical oxidants as a class may be
reasonably anticipated to endanger public health or welfare). Given
these key points, the REA concluded that the available evidence
supports the retention of SO2 as the indicator in the
current review (REA, section 10.5.1). Consistent with this conclusion,
CASAC stated in a letter to the EPA Administrator that: ``for
indicator, SO2 is clearly the preferred choice'' (Samet
2009, p. 14).
b. Comments on Indicator
A small number of commenters directly addressed the issue of the
indicator for the standard. These
[[Page 35537]]
commenters generally endorsed the proposal to continue to use
SO2 as the indicator for ambient SOX.
c. Conclusions on Indicator
Based on the available information discussed above, and consistent
with the views of CASAC and other commenters, the Administrator
concludes that it is appropriate to continue to use SO2 as
the indicator for a standard that is intended to address effects
associated with exposure to SO2, alone or in combination
with other gaseous SOX. In so doing, the Administrator
recognizes that measures leading to reductions in population exposures
to SO2 will also reduce population exposures to other oxides
of sulfur.
2. Averaging Time
This section discusses considerations related to the averaging time
of the SO2 primary NAAQS. Specifically, this section
summarizes the rationale for the Administrator's proposed decision
regarding averaging time (II.F.2.a below; see section II.F.2 of the
proposal for more detail at 74 FR 64832-64833), discusses public
comments and EPA responses related to averaging time (II.F.2.b), and
presents the Administrator's final conclusions regarding averaging time
(II.F.2.c). Notably, public comments and the Administrator's
conclusions on whether to retain or revoke the current 24-hour and/or
annual standards given a new 1-hour standard are discussed in section
II.F.5.
a. Rationale for Proposed Decision
In considering the most appropriate averaging time for the
SO2 primary NAAQS, the Administrator noted in the proposal
the conclusions and judgments made in the ISA about the available
scientific evidence, air quality correlations discussed in the REA,
conclusions of the policy assessment chapter of the REA, and CASAC
recommendations (section II.F.2 in the proposal). Specifically, she
noted the following:
The REA conclusion that an appropriate averaging time
should focus protection on SO2 exposures from 5-minutes to
24-hours (REA, section, 10.5.2).
Air quality, exposure, and risk analyses from the REA
indicating it is likely a 1-hour standard--with the appropriate form
and level--can substantially reduce 5-10 minute peaks of SO2
shown in controlled human exposure studies to result in respiratory
symptoms and/or decrements in lung function in exercising asthmatics
(i.e. 5-minute SO2 concentrations >= 200 and 400 ppb).
Air quality analyses indicating that a 1-hour standard--
with the appropriate form and level--can substantially reduce the upper
end of the distribution of SO2 levels more likely to be
associated with adverse respiratory effects (see section II.F.3 below);
that is: (1) 99th percentile 1-hour daily maximum air quality
concentrations in U.S. cities where positive effect estimates in
epidemiologic studies of hospital admissions and emergency department
visits for all respiratory causes and asthma were observed; and (2)
99th percentile 24-hour average air quality concentrations found in
U.S. cities where emergency department visit and hospitalization
studies (for all respiratory causes and asthma) reported statistically
significant associations in multi-pollutant models with PM.
The REA conclusion that a 5-minute averaging time is
undesirable because it would result in significant and unnecessary
instability due to the likelihood that locations would frequently shift
in and out of attainment--thereby reducing public health protection by
disrupting an area's ongoing implementation plans and associated
control programs.
CASAC statement addressing whether a 1-hour averaging time
can adequately control 5-10 minute peak exposures and whether there
should be a 5-minute averaging time. CASAC stated that the REA's
rationale for a one-hour standard was ``convincing'' (Samet 2009, p.
16), and that ``a one-hour standard is the preferred averaging time''
(Samet 2009, p. 15).
CASAC's statement that they were ``in agreement with
having a short-term standard and finds that the REA supports a 1-hour
standard as protective of public health'' (Samet 2009, p. 1).
b. Comments on Averaging Time
A large number of public commenters also endorsed the establishment
of a new standard with a 1-hour averaging time (although some groups'
support hinged on the accompanying level). These included a number of
State organizations (e.g., NACAA, NESCAUM); State environmental
agencies (e.g., such agencies in IA, IL, NY, MI, NM, OH, PA, TX, VT);
public health and environmental organizations (e.g., ALA, ATS, New York
Department of Health (NYDOH), Sierra Club, EDF); the Fond du Lac Tribe;
local groups (e.g., Houston-Galveston Area Council, New York City); and
almost all of the individual commenters (13,000). The supporting
rationales offered by these commenters often acknowledged the
recommendations of CASAC and the Administrator's rationale as discussed
in the proposal.
Though many industry commenters did not support the proposed
revisions to the SO2 primary NAAQS (as discussed above in
section II.E.2), a few of these groups did express that if a short-term
standard were to be set, a 1-hour averaging time could be appropriate,
depending on the level and form selected (e.g., ExxonMobil, Kean
Miller). Other industry commenters (e.g., ASARCO, RIO Tinto Alcan,
Association of Battery Recyclers (ABR)) and the South Dakota Department
of Environment and Natural Resources (SD DENR) expressed that EPA
should have considered longer averaging times (e.g., 3 hours). In
addition, although health and environmental groups were supportive of
setting a new 1-hour standard to protect against short-term exposures
to SO2 (again, depending on the level of the 1-hour standard
selected), these groups also commented that a 5-minute standard to
protect susceptible populations from health effects associated with 5-
minute peaks of SO2 would be optimal (e.g., ALA, ATS, Sierra
Club, EDF). These comments, and EPA's responses, are discussed in more
detail below.
As discussed above, industry commenters who disagreed with setting
a new 1-hour standard generally based this conclusion on their
interpretation of the scientific evidence and their conclusion that
this evidence does not support the proposed revisions to the current
SO2 NAAQS. EPA's responses to these commenters were
presented above in section II.E.2.a and II.E.2.b.
Also noted above, some industry commenters (e.g., ASARCO, RIO Tinto
Alcan, ABR) and the SD DENR expressed that EPA should have considered
longer averaging times (e.g., 3-hour, 8-hour, 24-hour). In general,
these groups concluded that a standard with a longer averaging time
could potentially provide the same public health protection as a 1-hour
standard, while also providing a more stable regulatory target. For
example, in its comments, the SD DENR states: ``DENR recommends EPA
evaluate a 3-hour or 8-hour standard to determine if these averaging
periods are also protective of the public health. If they are, EPA
should propose a 3-hour or 8-hour sulfur dioxide standard instead of a
1-hour standard. A longer averaging period would smooth out the
variability of the upper range measurements and provide a more stable
standard.'' Similarly, Rio Tinto Alcan stated in its comments: ``the
short-term averaging period defined by EPA (i.e., 5 minutes
[[Page 35538]]
to 24 hours) is not limited to only 5-minute, 1-hour and 24-hour
averaging periods. EPA could explain in more detail why these three
averaging periods were examined when considering appropriate averaging
periods to limit short-term peaks of SO2 * * * a longer term
average could provide additional stability to the standard while at the
same time effectively protecting public health.''
Although we agree that alternative averaging times could
potentially provide similar public health protection (assuming an
appropriate form and level), we believe that a 1-hour averaging time is
reasonably justified by the scientific evidence presented in the ISA
and by the air quality information presented in the REA. As described
in detail in the proposal (see section II.F.2), the controlled human
exposure evidence presented in the ISA provided support for an
averaging time that protects against 5-10 minute peak SO2
exposures (REA, section 10.5.2, pp. 371-372), and results from
epidemiologic studies most directly provided support for both 1-hour
and 24-hour averaging times (REA, section 10.5.2, p. 372). Thus, we
found it most reasonable to consider these averaging times for a
revised SO2 NAAQS given that there is very little basis in
the health evidence presented in the ISA to consider other averaging
times (e.g., 3-hour or 8-hour). In so doing, we first noted the
likelihood that averaging times of 1 and 24 hours could provide
protection against 5-minute peak SO2 exposures. As described
in detail in the proposal (see section II.F.2, 74 FR at 64830-64833),
it was initially concluded that a 1-hour averaging time, rather than a
24-hour averaging time, would be more appropriate for limiting 5-minute
peaks of SO2. Similarly, we concluded that a 1-hour
standard, given the appropriate form and level, could likely limit 99th
percentile 24-hour average air quality concentrations found in U.S.
locations where emergency department visit and hospitalization studies
(for all respiratory causes and asthma) observed statistically
significant associations in multi-pollutant models with PM (i.e., 99th
percentile 24-hour average SO2 concentration >= 36 ppb).
Taken together, we reasonably concluded that a 1-hour standard, with an
appropriate form and level, can provide adequate protection against the
range of health outcomes associated with averaging times from 5 minutes
to 24 hours (proposal section II.F.2 and REA, section 10.5.2.3). We
also note that our conclusion is in agreement with CASAC comments on
the second draft REA. CASAC stated that they were ``in agreement with
having a short-term standard and finds that the REA supports a one-hour
standard as protective of public health'' (Samet 2009, p. 1). In
addition, as discussed in more detail below in section II.F.3, we found
that a 1-hour standard in combination with the selected form, will
provide a stable regulatory target.
As noted above, although health and environmental groups were
supportive of setting a new 1-hour standard to protect against short-
term exposures to SO2 (again, depending on the level of the
1-hour standard selected), these groups generally commented that a 5-
minute standard to protect against health effects associated with 5-
minute peaks would be optimal (e.g., ALA, Sierra Club, EDF). For
example, in their combined comments ALA, EDF, NRDC, and Sierra Club
(ALA et al.,) stated: ``We need a short-term SO2 standard,
optimally a 5-minute standard, to protect against bursts of pollution
that can result from start-up, shutdown, upset, malfunction, downwash,
complex terrain, atmospheric inversion conditions, and other
situations'' and that ``EPA has over emphasized a concern about the
stability of a 5-minute standard * * * The record does not show that
any alleged instability of a 5-minute standard has any relevance to
whether such a standard is requisite to protect public health.''
We agree that there needs to be a short-term standard to protect
against 5-minute peaks of SO2. However, we do not believe
setting a 5-minute standard to be the best way of accomplishing that
objective. As in past NAAQS reviews, EPA properly considered the
stability of the design of pollution control programs in its review of
the elements of a NAAQS, since more stable programs are more effective,
and hence result in enhanced public safety. American Trucking
Associations v. EPA, 283 F. 3d at 375 (choice of 98th percentile form
for 24-hour PM NAAQS, which allows a number of high exposure days per
year to escape regulation under the NAAQS, justifiable as ``promot[ing]
development of more `effective [pollution] control programs' '', since
such programs would otherwise be ``less `stable'--and hence * * * less
effective--than programs designed to address longer-term average
conditions'', and there are other means (viz. emergency episode plans)
to control those high exposure days). In this review, there were
legitimate concerns about the stability of a standard using a 5-minute
averaging time. Specifically, there was concern that compared to longer
averaging times (e.g., 1-hour, 24-hour), year-to-year variation in 5-
minute SO2 concentrations were likely to be substantially
more temporally and spatially diverse. Thus, it is more likely that
locations would frequently shift in and out of attainment thereby
reducing public health protection by disrupting an area's ongoing
implementation plans and associated control programs. Consequently, the
REA concluded that a 5-minute averaging time would not provide a stable
regulatory target and therefore would not be the preferred approach to
provide adequate public health protection. A 1-hour averaging time does
not have these drawbacks. As noted in the REA and the proposal (see
proposal sections II.F.2.a and II.F.2.c), air quality, exposure, and
risk analyses support that a 1-hour averaging time, given an
appropriate form and level can adequately limit 5-minute SO2
exposures and provide a more stable regulatory target than setting a 5-
minute standard. More specifically, based on the air quality and
exposure analyses presented in chapters 7 and 8 of the REA, there is
also a strong likelihood that a 99th percentile 1-hour daily maximum
standard will limit 5-10 minute peaks of SO2 shown in
controlled human exposure studies to result in decrements in lung
function and/or respiratory symptoms in exercising asthmatics (see
especially REA Tables 7-11 to 7-14 and Figure 8-19).
We also note that a 1-hour standard to protect against 5-minute
exposures is in agreement with CASAC advice and recommendations. That
is, CASAC stated that they were ``in agreement with having a short-term
standard and finds that the REA supports a 1-hour standard as
protective of public health'' (Samet 2009, p. 1). Similarly, in a CASAC
statement addressing whether a 1-hour averaging time can adequately
control 5-10 minute peak exposures and whether there should be a 5-
minute averaging time, CASAC stated that the REA had presented a
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and
that ``a one-hour standard is the preferred averaging time'' (Samet
2009, p. 15).
c. Conclusions on Averaging Time
In considering the most appropriate averaging time(s) for the
SO2 primary NAAQS, the Administrator notes the conclusions
and judgments made in the ISA about the available scientific evidence,
air quality considerations from the REA, CASAC advice and
recommendations, and public comments received. Based on these
considerations, the Administrator concludes that a new standard based
on
[[Page 35539]]
1-hour daily maximum SO2 concentrations will provide
increased protection against effects associated with short-term (5
minutes to 24 hours) exposures. The rationale for this decision is
described below.
Similar to the proposal (see section II.F.2.c), the Administrator
first agrees with the REA's conclusion that the standard should focus
protection on short-term SO2 exposures from 5 minutes to 24
hours. As noted above, CASAC's strong recommendation supports this
approach as well.\15\ The Administrator further agrees that the
standard must provide requisite protection from 5-10 minute exposure
events, but believes that this can be provided without having a
standard with a 5-minute averaging time. The Administrator agrees with
the REA conclusion that it is likely a 1-hour standard--with the
appropriate form and level--can substantially reduce 5-10 minute peaks
of SO2 shown in controlled human exposure studies to result
in respiratory symptoms and/or decrements in lung function in
exercising asthmatics. The Administrator further believes that a 5-
minute averaging time would result in significant and unnecessary
instability and is undesirable for that reason. The Administrator also
notes the statements from CASAC mentioned above addressing whether a 1-
hour averaging time can adequately control 5-10 minute peak exposures
and whether there should be a 5-minute averaging time. As noted above,
addressing this question, CASAC stated that the REA had presented a
``convincing rationale'' (Samet 2009, p. 16) for a 1-hour standard, and
that ``a one-hour standard is the preferred averaging time'' (Samet
2009, p. 15).
---------------------------------------------------------------------------
\15\ As noted above, such a standard also satisfactorily
addresses the issue raised by the reviewing court in the litigation
that followed the last review of the SO2 NAAQS: Why was
no protection afforded in the standard for a susceptible
subpopulation known to experience repeated adverse effects from
exposure to 5-10 minute SO2 bursts. American Lung Ass'n,
134 F. 3d at 392-93.
---------------------------------------------------------------------------
Second, as in the proposal the Administrator agrees that a 1-hour
averaging time (again, with the appropriate form and level) would
provide protection against the range of health outcomes associated with
averaging times of 1 hour to 24 hours. Specifically, the Administrator
finds that a 1-hour standard can substantially reduce the upper end of
the distribution of SO2 levels more likely to be associated
with adverse respiratory effects (see discussion on Form, section
II.F.3); that is: (1) 99th percentile 1-hour daily maximum
SO2 air quality concentrations in U.S. locations where
positive SO2 effect estimates were reported in epidemiologic
studies of emergency department visits and hospital admissions for all
respiratory causes and asthma; and (2) 99th percentile 24-hour average
SO2 air quality concentrations found in U.S. locations where
emergency department visit and hospital admission studies using multi-
pollutant models with PM reported statistically significant
associations (for all respiratory causes or asthma) with ambient
SO2 (see REA, section 10.5.2.2 and proposal section II.F.2,
74 FR at 64831). Finally, the Administrator again notes that
establishing a new 1-hour averaging time is in agreement with CASAC
recommendations. As noted above, CASAC stated that they were ``in
agreement with having a short-term standard and finds that the REA
supports a one-hour standard as protective of public health'' (Samet
2009, p. 1). Moreover, CASAC agreed with the REA that a ``one-hour
standard is the preferred averaging time'' (Samet 2009, p.15).
3. Form
This section discusses considerations related to the form of the 1-
hour SO2 primary NAAQS. Specifically, this section
summarizes the rationale for the Administrator's proposed decision
regarding form (II.F.3.a; see proposal section II.F.3, 74 FR at 64833-
64834 of the proposal for more detail), discusses comments related to
form (II.F.3.b), and presents the Administrator's final conclusions
regarding form (II.F.3.c).
a. Rationale for Proposed Decision
In considering the most appropriate form for the SO2
primary NAAQS, the Administrator noted in the proposal the conclusions
and judgments made in the ISA about available scientific evidence, air
quality information discussed in the REA, conclusions of the policy
assessment chapter of the REA, and CASAC recommendations (see section
II.F.3, 74 FR at 64833-64834 in the proposal). Specifically, the
proposal referenced the following:
Information in the ISA that suggested that adverse
respiratory effects are more likely to occur at the upper end of the
distribution of ambient SO2 concentrations. That is, the ISA
describes a few studies that reported an increase in SO2-
related respiratory health effects at the upper end of the distribution
of SO2 concentrations (ISA, section 5.3, p. 5-9).
The REA conclusion that a concentration-based form
averaged over three years would better reflect the continuum of health
risks posed by increasing SO2 concentrations (i.e. the
percentage of asthmatics affected and the severity of the response
increases with increasing SO2 concentrations; REA, section
10.5.3) by giving proportionally greater weight to years when 1-hour
daily maximum SO2 concentrations are well above the level of
the standard, than just above the level of the standard.
Analyses in the REA that suggested for a given
SO2 standard level, a 99th percentile form is appreciably
more effective at limiting 5-minute peak SO2 concentrations
than a 98th percentile form (REA, section 10.5.3 and REA, Figures 7-27
and 7-28).
Analyses in the REA indicating that over the last 10 years
and for the vast majority of the sites examined, there appears to be
little difference in 98th and 99th percentile design value stability
(REA, section 10.5.3).
The REA conclusion that taken together, the evidence and
air quality information indicate that consideration should be given
primarily to a 1-hour daily maximum standard with a 99th percentile or
4th highest daily maximum form (REA, section 10.5.3.3).
CASAC indications that: ``there is adequate information to
justify the use of a concentration-based form averaged over 3 years''
(Samet 2009, p. 16).
CASAC recommendations that when evaluating 98th vs. 99th
percentile forms, EPA should consider the number of days per year 98th
vs. 99th percentile forms would allow SO2 concentrations to
exceed the selected standard level. Similarly, CASAC recommendations to
consider the number of exceedences of 5-minute benchmarks given 98th
vs. 99th percentile forms at a given standard level (Samet 2009).
b. Comments on Form
Most all State organizations and agencies (e.g., NAACA, NESCAUM and
agencies in FL, NM, PA, SC, TX, VT) supported a 99th percentile or 4th
highest form. Similarly, public health (e.g., ALA, ATS) and
environmental organizations (e.g., CBD, WEACT for Environmental
Justice) and the Alexandria Department of Transportation and
Environmental Services preferred either a 99th percentile or a more
stringent form (e.g., no exceedence) to further limit the occurrence of
SO2 concentrations that exceed the standard level in
locations that attain the standard. In contrast, many industry groups
(e.g., UARG, NAM, LEC, RRI Energy, AirQuality Research & Logistics
(AQRL)), and the SD DENR conditionally supported a
[[Page 35540]]
98th percentile form if EPA were to set a 1-hour standard.\16\ EPA
responses to specific comments on the form of the standard can be found
below and in the RTC document (EPA 2010).
---------------------------------------------------------------------------
\16\ EPA did not propose or seek comment on a 98th percentile
form or a more restrictive form (e.g., an exceedence based form).
EPA also considered a 4th highest form, which is generally
equivalent to the 99th percentile. However, a percentile based form
is preferred since it results in a sampling from the same part of
the annual distribution of 1-hour daily maximum SO2
concentrations regardless of the number of 1-hour daily maximum
concentrations reported in a given year for a particular location.
---------------------------------------------------------------------------
As mentioned above, a number of industry groups and the SD DENR
preferred a 98th percentile form. In general, their preference for a
98th percentile form was based on their conclusion that a form based on
the 98th percentile would be more stable than a form based on the 99th
percentile, and that a 98th percentile form is consistent with the
forms selected in recent NAAQS reviews (i.e. PM2.5 and
NO2). For example AQRL stated: ``The Administrator should
reconsider her proposal and choose instead the 98th percentile (or
equivalent nth highest value) form of the standard for the added
reliability and stability it offers in determining compliance or
progress towards attainment. This approach has been promulgated for
recent revisions of the PM2.5 and NO2 standards
and this consistency should be maintained with SO2.''
We agree with the commenters that it is important that a 1-hour
standard have a form that is reasonably stable, but we disagree that a
98th percentile form is significantly more stable than a 99th
percentile form. We note that the REA discussed analyses (also briefly
described in the proposal; see section II.F.3, 74 FR at 64834)
comparing trends in 98th and 99th percentile design values from 54
sites located in the 40 counties selected for the detailed air quality
analysis (REA section 10.5.3 and Thompson, 2009). These results
suggested that at the vast majority of sites, there would have been
similar changes in 98th and 99th percentile design values over the last
ten years (i.e. based on evaluating overlapping three year intervals
over the last ten years; see REA, Figure 10-1 and Thompson, 2009). As
part of this analysis, all of the design values over this ten year
period for all 54 sites were aggregated and the standard deviation
calculated (REA, Figure 10-2 and Thompson, 2009). Results demonstrated
similar standard deviations--i.e. similar stability--based on
aggregated 98th or aggregated 99th percentile design values over the
ten year period (see REA, Figure 10-2 and Thompson 2009). Thus, we
believe that in most locations, there will not be a substantial
difference in stability between 98th and 99th percentile forms.
We also disagree with the commenters that the forms of NAAQS
standards should be consistent across different NAAQS pollutants. This
is almost like advocating consistent levels or averaging times for
different NAAQS pollutants. Each pollutant is manifestly different from
another, and the decision as to an appropriate standard for each, and
appropriate elements (including form) of each standard and the
interaction of these elements, necessarily is fact-specific. Cf. Sierra
Club v. EPA, 353 F. 3d 976, 986 (DC Cir. 2004) (``This court has
adopted an `every tub on its own bottom' approach to EPA's setting of
standards pursuant to the CAA, under which the adequacy of the
underlying justification offered by the agency is the pertinent
factor--not what the agency did on a different record concerning a
different industry'') (Roberts J.). There is thus no basis to say a
priori that any element of one NAAQS should be consistent with another,
although if all other things are equal, selecting stable forms for each
NAAQS is a legitimate objective.
A 99th percentile form, rather than a 98th percentile form, is also
needed for the standard to provide requisite public health protection.
In this review of the primary SO2 NAAQS, we considered
information in the ISA suggesting that adverse respiratory effects are
more likely to occur at the upper end of the distribution of ambient
SO2 concentrations. That is, the ISA described a few studies
that reported an increase in SO2-related respiratory health
effects at the upper end of the distribution of SO2
concentrations (i.e., above 90th percentile SO2
concentrations; ISA, section 5.3, p. 5-9). Moreover, we considered the
extent to which different percentile forms, given the same standard
level, limit 5-minute concentrations of SO2 above benchmark
levels. As noted above in section II.F.3.a, and in more detail in the
proposal (see section II.F.3.a, 74 FR at 64834), air quality analyses
presented in the REA suggested that at a given SO2 standard
level, a 99th percentile form is appreciably more effective at limiting
5-minute peak SO2 concentrations than a 98th percentile form
(REA, section 10.5.3, and REA, Figures 7-27 and 7-28). Taken together
with the analyses suggesting that 98th and 99th percentile forms have
similar stabilities, we reasonably concluded that a 99th percentile
form was most appropriate for a 1-hour SO2 standard.
As mentioned above, a number of health and environmental groups
supported a 99th percentile form, but expressed that they would prefer
a more restrictive form, such as a no-exceedence based form. In
addition, the Alexandria Department of Transportation and Environmental
Services only recommended a no, or one exceedence based form. In
general, these groups concluded that a more restrictive form would
further limit the: (1) Number of days an area could exceed the standard
level and still attain the standard; and (2) the occurrence of 5-minute
peaks of SO2 above benchmark levels.
It is important that the particular form selected for a 1-hour
daily maximum standard reflect the nature of the health risks posed by
increasing SO2 concentrations. The REA and proposal (see
section II.F.3, 74 FR at 64833) noted that the form of the standard
should reflect results from controlled human exposure studies
demonstrating that the percentage of asthmatics affected, and the
severity of the respiratory response (i.e. decrements in lung function,
respiratory symptoms) increases as SO2 concentrations
increase. Taking this into consideration, EPA staff concluded that a
concentration-based form, averaged over three years, is more
appropriate than an exceedance-based form (REA, section 10.5.3). This
is because a concentration-based form averaged over three years gives
proportionally greater weight to years when 1-hour daily maximum
SO2 concentrations are well above the level of the standard,
as it gives to years when 1-hour daily maximum SO2
concentrations are just above the level of the standard. In contrast,
an expected exceedance form gives the same weight to years when 1-hour
daily maximum SO2 concentrations are just above the level of
the standard as it gives to years when 1-hour daily maximum
SO2 concentrations are well above the level of the standard.
Therefore, we concluded that a concentration-based form, averaged over
three years (which also increases the stability of the standard) better
reflects the continuum of health risks posed by increasing
SO2 concentrations (i.e. the percentage of asthmatics
affected and the severity of the response increases with increasing
SO2 concentrations; REA, section 10.5.3). Moreover, we note
that analyses in the REA indicate that in most locations analyzed, a
99th percentile form would correspond to the 4th highest daily maximum
concentration in a year, and that the 99th percentile, combined with
the standard level
[[Page 35541]]
selected, will substantially limit 5-minute peaks of SO2
above the 200 ppb and higher benchmark levels (see below, section
II.F.4). Finally, we note that a concentration based form is in
agreement with CASAC advice that: ``there is adequate information to
justify the use of a concentration-based form averaged over 3 years''
(Samet 2009, p. 16).
c. Conclusions on Form
The Administrator agrees that the form of the standard should
reflect the health evidence presented in the ISA indicating that the
percentage of asthmatics affected and the severity of the response
increases with increasing SO2 concentrations. The
Administrator also agrees that it is reasonable to consider the
standard's stability as part of consideration of the form of the
standard. Thus, the Administrator agrees that the standard should use a
concentration-based form averaged over three years in order to give due
weight to years when 1-hour SO2 concentrations are well
above the level of the standard, than to years when 1-hour
SO2 concentrations are just above the level of the standard.
She also notes that a concentration-based form averaged over 3 years
would likely be appreciably more stable than a no-exceedence based
form.
In selecting a specific concentration based form, the Administrator
first notes that a few epidemiologic studies described in the ISA
reported an increase in SO2-related respiratory health
effects at the upper end of the distribution of ambient SO2
concentrations (i.e., above 90th percentile SO2
concentrations; see ISA, section 5.3, p. 5-9). The Administrator notes
further that numerous controlled human exposure studies have reported
decrements in lung function and/or respiratory symptoms in exercising
asthmatics exposed to peak 5-10 minute SO2 concentrations.
The Administrator therefore concludes that the form of a new 1-hour
standard should be especially focused on limiting the upper end of the
distribution of ambient SO2 concentrations (i.e., above 90th
percentile SO2 concentrations) in order to provide
protection with an adequate margin of safety against effects reported
in both epidemiologic and controlled human exposure studies.
In further considering specific concentration based forms, the
Administrator notes as outlined above in section II.F.3.b, and
discussed in more detail in the REA (REA, section 10.5.3) and proposal
(see section II.F.3, 74 FR at 64834), that a 99th percentile form is
likely to be appreciably more effective at limiting 5-minute benchmark
exposures of concern compared to a 98th percentile form. Taken together
with the considerations just discussed above, the Administrator has
selected a 99th percentile form, averaged over 3 years. The
Administrator concludes that a 99th percentile form, given the level
selected (see section II.F.4 immediately below), will limit both the
upper end of the distribution of ambient SO2 concentrations
reported in some epidemiologic studies to be associated with increased
risk of SO2-related respiratory morbidity effects (e.g.,
emergency department visits), as well as 5-minute peak SO2
concentrations resulting in decrements in lung function and/or
respiratory symptoms in exercising asthmatics participating in
controlled human exposure studies.
4. Level
As discussed below and in more detail in the proposal (section
II.F.4, 74 FR at 64834), the Administrator proposed to set a 1-hour
standard with a 99th percentile form (averaged over three years), with
a level in the range of 50 to 100 ppb. The Administrator also solicited
comment on standard levels greater than 100 ppb up to 150 ppb. This
section summarizes the rationale for the Administrator's proposed range
of standard levels (II.F.3.a), discusses comments related to the range
of standard levels (II.F.3.b), and presents the Administrator's final
conclusions regarding the level of a new 1-hour SO2 standard
(II.F.3.c).
a. Rationale for Proposed Decision
In assessing the level of a 1-hour standard with a 99th percentile
form (averaged over three years), the Administrator considered the
broad range of scientific evidence assessed in the ISA, including the
epidemiologic studies and controlled human exposure studies, as well as
the results of air quality, exposure, and risk analyses presented in
the REA. In light of this body of evidence and analyses, the
Administrator found it is necessary to provide increased public health
protection for at-risk populations against an array of adverse
respiratory health effects related to short-term (i.e., 5 minutes to 24
hours) exposures to ambient SO2. In considering the most
appropriate way to provide this protection, the Administrator was
mindful of the extent to which the available evidence and analyses
could inform a decision on the level of a standard. The Administrator's
proposed decisions on level, as discussed in detail in the proposal
(see section II.F.4.e), are outlined below.
Given the above considerations, the Administrator proposed to set a
level for a new 99th percentile 1-hour daily maximum primary
SO2 standard within the range from 50 to 100 ppb and took
comment on levels above 100 ppb, up to 150 ppb. In reaching this
proposed decision, the Administrator considered: (1) The evidence-based
considerations from the final ISA and the final REA; (2) the results of
the air quality, exposure, and risk assessments discussed above and in
the final REA; (3) CASAC advice and recommendations on both the ISA and
REA discussed above and provided in CASAC's letters to the
Administrator; and (4) public comments received on the first and second
drafts of the ISA and REA. In considering what level of a 1-hour
SO2 standard is requisite to protect public health with an
adequate margin of safety, the Administrator was mindful that this
choice requires judgments based on an interpretation of the evidence
and other information that neither overstates nor understates the
strength and limitations of that evidence and information.
As noted above, the Administrator selected an upper end of a range
of levels to propose at 100 ppb. The selection of this level focused on
the results of the controlled human exposure studies and is primarily
based on the results of the air quality and exposure analyses which
suggest that a 1-hour standard should be at or below 100 ppb to
appreciably limit 5-minute SO2 benchmark concentrations >=
200 ppb (see proposal Tables 2-4, and proposal sections II.F.4.a and
II.F.4.b). That is, as described in the proposal (see section
II.F.4.e), the 40-county air quality analysis estimates that a 100 ppb
1-hour standard would allow at most 2 days per year on average when
estimated 5-minute daily maximum SO2 concentrations exceed
the 400 ppb benchmark, and at most 13 days per year on average when 5-
minute daily maximum SO2 concentrations exceed the 200 ppb
benchmark (see proposal Table 2). Furthermore, given a simulated 1-hour
100 ppb standard level, most counties in the air quality analysis were
estimated to experience 0 days per year on average when 5-minute daily
maximum SO2 concentrations exceed the 400 ppb benchmark and
<= 3 days per year on average when 5-minute daily maximum
SO2 concentrations were estimated to exceed the 200 ppb
benchmark (see REA, Tables 7-14 and 7-12). The Administrator also noted
that the St. Louis exposure analysis indicated that a 1-hour standard
at
[[Page 35542]]
100 ppb would still be estimated to protect > 99% of asthmatic children
at moderate or greater exertion from experiencing at least one 5-minute
SO2 exposure >= 400 ppb per year, and about 97% of these
children from exposures >= 200 ppb. In contrast, as described in the
proposal (see section II.F.4.b), the St. Louis exposure analysis
estimated that a 1-hour standard at 150 ppb would likely only protect
about 88% of asthmatic children at moderate or greater exertion from
experiencing at least one 5-minute SO2 exposure >= 200 ppb
per year.
As noted above and described in detail in the proposal (see section
II.F.4.e), the Administrator selected 50 ppb as the lower end of a
range of levels to propose, which is consistent with CASAC's advice.
The selection of this level focused in part on the U.S. epidemiologic
evidence described in detail in the proposal (see sections II.B.2,
II.F.4.a, and II.F.4.e). With respect to these epidemiologic studies,
seven of ten U.S. emergency department visit and hospital admission
studies reporting generally positive associations with ambient
SO2 were conducted in locations where 99th percentile 1-hour
daily maximum SO2 levels were about 75-150 ppb, and three of
these studies observed statistically significant positive associations
between ambient SO2 and respiratory-related emergency
department visits and hospitalizations in multi-pollutant models with
PM (NYDOH (2006), Ito et al., (2007), and Schwartz et. al, (1995)).
Thus, the Administrator noted that a 99th percentile 1-hour daily
maximum standard set at a level of 50 ppb is well below the 99th
percentile 1-hour daily maximum SO2 concentrations reported
in locations where these three studies were conducted (i.e. well below
99th percentile 1-hour daily maximum SO2 levels of 78-150
ppb seen in NYDOH (2006), Ito et al., (2007), and Schwartz et. al,
(1995)). Finally, the Administrator noted that two epidemiologic
studies reported generally positive associations between ambient
SO2 and emergency department visits in cities when 99th
percentile 1-hour daily maximum SO2 concentrations were
approximately 50 ppb, but did not consider that evidence strong enough
to propose setting a standard level lower than 50 ppb.
In considering the results of the air quality and exposure
analyses, the Administrator also noted that the 40-county air quality
analysis estimates that a 99th percentile 1-hour daily maximum standard
set at a level of 50 ppb would result in zero days per year when
estimated 5-minute SO2 concentrations exceed the 400 ppb 5-
minute benchmark level and at most 2 days per year when modeled 5-
minute SO2 concentrations exceed the 200 ppb 5-minute
benchmark level (see proposal section II.F.4.b and proposal Table 2).
In addition, the St. Louis exposure analysis estimates that a 99th
percentile 1-hour daily maximum standard set at a level of 50 ppb would
likely protect > 99% of asthmatic children at moderate or greater
exertion from experiencing at least one 5-minute exposure both >= 400
and 200 ppb per year (see proposal section II.F.4.b and
Table 3). In addition, although not directly analyzed in the REA, the
proposal (section II.F.4.b) noted that a 1-hour daily maximum standard
at a level of 75 ppb would be bound by the exposure estimates from air
quality adjusted to just meet 99th percentile 1-hour daily maximum
standards at 50 and 100 ppb. Thus, a 1-hour daily maximum standard at a
level of 75 ppb would be estimated to protect > 99% of asthmatic
children at moderate or greater exertion in St. Louis from experiencing
at least one exposure >= 400 ppb per year, and about 97% to > 99% of
these children from experiencing at least one exposure >= 200 ppb per
year.
The Administrator thus proposed to set the level of a new 1-hour
standard that would protect public health with an adequate margin of
safety between 50 ppb and 100 ppb. In so doing, the Administrator
relied on reported findings from both epidemiologic and controlled
human exposure studies, as well as the results of air quality and
exposure analyses. The Administrator noted that the lower end of the
proposed range was consistent with CASAC advice that there is clearly
sufficient evidence for consideration of standard levels starting at 50
ppb (Samet 2009, p. 16). With respect to the upper end of the proposed
range, the Administrator noted that CASAC concluded that standards up
to 150 ppb ``could be justified under some interpretations of weight of
evidence, uncertainties, and policy choices regarding margin of
safety'' (id.), although the letter did not provide any indication of
what interpretations, uncertainties, or policy choices might support
selection of a level as high as 150 ppb.
In light of the range of levels included in CASAC's advice, the
Administrator also solicited comment on setting a standard level above
100 ppb and up to 150 ppb. In so doing, the Administrator recognized
that there are uncertainties with the scientific evidence, such as
attributing effects reported in epidemiologic studies specifically to
SO2 given the presence of co-occurring pollutants,
especially PM, and the uncertainties associated with using ambient
SO2 concentrations as a surrogate for exposure. However, the
Administrator noted that compared to the proposed range of 50-100 ppb,
a standard level as high as 150 ppb would not comparably limit 5-minute
SO2 exposures >= 200 ppb. That is, she noted that the St.
Louis exposure analysis estimated that a 150 ppb standard would protect
approximately 88% of asthmatic children at moderate or greater exertion
from experiencing at least one SO2 exposure >= 200 ppb per
year (compared to > 99% and approximately 97% given standards at 50 and
100 ppb respectively; see proposal Table 3 at 74FR at 64841).
b. Comments on Level
Most State and local agencies and organizations that commented on
this issue expressed support for setting the level of a 1-hour
SO2 standard somewhere within the proposed range of 50 to
100 ppb. More specifically, State environmental organizations (i.e.,
NACAA and NESCAUM); State environmental agencies (e.g., such agencies
in DE, IL, MI, NY, NM, PA, VT), the Fond du Lac Tribe, and local groups
(e.g., NYDOH, City of Houston, New York City, Houston-Galveston Area
Council) supported a level of a 1-hour SO2 standard in the
range of 50 to 100 ppb. In addition, State environmental agencies in IA
and TX specifically supported a standard level of 100 ppb. In general,
these groups cited the conclusions of CASAC and the Administrator's
rationale as stated in the proposal as a basis for their
recommendations, though State environmental agencies in IA and TX
generally recommended placing more weight on the controlled human
exposure evidence rather than on the epidemiology.
A number of environmental and medical/public health organizations
(e.g., ALA, ATS, EDF, Sierra Club, WEACT for Environmental Justice,
NRDC, CBD) and some local organizations (e.g., Alexandria Department of
Transportation and Environmental Services, and Harris County (TX)
Public Health & Environmental Services) supported setting a standard
level at or near 50 ppb. This recommendation was typically based on the
commenters' interpretation of the controlled human exposure and
epidemiologic evidence, as described below.
With regard to the controlled human exposure evidence, health and
environmental groups generally concluded that a 1-hour SO2
standard
[[Page 35543]]
no higher than 50 ppb is needed to protect against 5-minute
SO2 benchmark exposures as low as 100 ppb identified from
mouthpiece exposure studies, rather than the 200 ppb 5-minute
SO2 benchmark identified from ``free breathing'' controlled
human exposure studies. More specifically, ALA et al., stated:
In its analysis of data from chamber studies in the ISA and in
the REA, EPA focuses on studies of ``free breathing'' exposure. In
doing so, EPA improperly and arbitrarily downplays important
evidence that reported increased airway resistance, a measure of
bronchoconstriction, in subjects with mild asthma at concentrations
of 100 ppb. Regrettably, EPA does not rely on the mouthpiece studies
in formulating its proposed standards * * * In downplaying the
mouthpiece studies, EPA ignores the large segment of people who rely
on oral or oronasal breathing some or all of the time.
The Administrator disagrees with the assertion that results from
mouthpiece studies were improperly downplayed. These studies are
discussed in the ISA, REA, and proposed rule as demonstrating
respiratory effects of SO2 at concentrations of 100 ppb, the
lowest concentration tested using a mouthpiece exposure system.
Nonetheless, these mouthpiece studies are not a reasonable proxy for
actual exposure. In these studies, SO2 is delivered directly
through the mouth, typically in conjunction with nasal occlusion. This
allows a greater fraction of the inhaled SO2 to reach the
tracheobronchial airways. Although we agree with commenters that some
individuals do breathe oronasally both while at rest and during
exercise, nasal ventilation still constitutes a significant percentage
of total ventilation. The consequence is that individuals exposed to
SO2 through a mouthpiece are likely to experience greater
respiratory effects from a given SO2 exposure than they
would in real life. Thus, as noted in the REA (REA, section 6.2) and in
the proposal preamble (see section II.B.1.b), these mouthpiece studies
only provide very limited evidence of decrements in lung function
following exposure to 100 ppb SO2. Therefore, the
Administrator did not place great weight on these mouthpiece studies
when considering the appropriate level of a 1-hour SO2
standard.
In addition to their interpretation of the controlled human
exposure evidence, health and environmental groups (e.g., ALA, ATS,
EDF, NRDC, Sierra Club, CBD) and the Alexandria Department of
Transportation and Environmental Services generally concluded that the
epidemiologic evidence indicates that a standard no higher than 50 ppb
is required to protect public health. For example, it its comments the
CBD stated:
Epidemiologic studies referenced in the Proposed Rule showed
positive, and in many cases statistically significant, relationships
between ambient SO2 concentrations and hospital
admissions where 99th percentile 1-hour concentrations ranged from
50-460 ppb. Of these studies, two showed positive and sometimes
statistically significant relationships in single-pollutant models
at 50 ppb, and three studies showed statistically significant
correlations at 78-150 ppb in multi-pollutant models. These three
multipollutant studies, moreover, ``lend[] strong support * * * to
the conclusion that SO2 effects are generally
independent'' of those of co-pollutants like particulate matter.
Giving these studies their proper weight, and allowing for an
adequate margin of safety, EPA should set a one-hour NAAQS at a
level no higher than the lowest concentration at which positive,
adverse relationships have been demonstrated: 50 ppb (note that
footnotes were omitted).
The Administrator agrees that the epidemiologic studies referenced
in the proposal need to be considered in judging the appropriate level
for a new 99th percentile 1-hour SO2 standard. However, she
disagrees that when considered in total, these studies strongly support
an SO2 standard no higher than 50 ppb. The Administrator
notes that selecting a standard level of 50 ppb would place
considerable weight on the two U.S. emergency department visit studies
conducted in locations where 99th percentile 1-hour SO2
concentrations were approximately 50 ppb (i.e., Wilson et al., (2005)
in Portland, ME and Jaffe et al., (2003) in Columbus, OH). However, the
Administrator does not find this appropriate given that, importantly,
neither of these studies evaluated the potential for confounding by co-
pollutants through the use of multipollutant models and thus, left
unaddressed the issue of whether the effects seen in the studies were
partially or totally attributable to exposure to sulfate PM. In
addition, the Administrator notes that the overall results reported in
these studies are mixed. It is important to note that mixed results do
not automatically disqualify studies from being used as part of the
evidence base for setting levels in NAAQS reviews. However, in this
review the Administrator judges that the lack of mutipollutant model
evaluation for potential confounding by PM in two locations with the
lowest SO2 levels combined with the presence of mixed
emergency department visit results renders these two studies
inappropriate to serve as the primary basis for the selection of the
level of the SO2 NAAQS. As an additional matter, the
suggestion in some of the comments that EPA should necessarily base the
level of a NAAQS on the lowest level seen in epidemiologic studies has
been rejected repeatedly. See, e.g. American Petroleum Inst. v. EPA,
665 F. 2d at 1187 (``In so arguing NRDC essentially ignores the mixed
results of the medical studies evident in the record, choosing instead
to rely only on the studies that favor its position. The Administrator,
however, was required to take into account all the relevant studies
revealed in the record. Because he did so in a rational manner, we will
not overrule his judgment as to the margin of safety.'') Thus, although
the Administrator finds that these two studies provide limited evidence
of emergency department visits in cities where 99th percentile 1-hour
daily maximum SO2 concentrations are approximately 50 ppb,
she also concludes that these studies do not provide enough evidence to
warrant a standard at this level.
As discussed above in section, II.E.2, a number of industry groups
(e.g., ACC, UARG) did not support setting a new 1-hour SO2
standard. However, several of these groups (e.g., UARG, API) and the SC
Chamber of Commerce concluded that, if EPA does choose to set a new 1-
hour standard, the level of that standard should be >= 150 ppb. In
addition, State environmental agencies in SD (SD DENR) and OH
recommended standard levels at 150 ppb. As a basis for this
recommendation, these groups generally emphasized uncertainties in the
scientific evidence. Specifically, as discussed in more detail above
(section II.E.2.a), these commenters typically concluded that the
available epidemiologic studies do not support the conclusion that
SO2 causes the reported health effects. This was based on
their assertion that the presence of co-pollutants in the ambient air
precludes the identification of a specific SO2 contribution
to reported effects. Thus, these groups generally concluded that weight
should not be placed on the cluster of three epidemiologic studies
reporting statistically significant effects in multipollutant models
with PM (i.e., NYDOH 2006; Ito 2007; and Schwartz 1995). That is, these
groups contend that these studies do not demonstrate an independent
effect of SO2. In addition, as noted in section II.E.2.b,
many of these groups also disagreed with the Agency's judgment that
adverse respiratory effects occur following 5-minute exposures to
SO2 concentrations as low as 200 ppb. These comments and
EPA's responses are discussed below
[[Page 35544]]
and in section II of the RTC document (EPA 2010).
As described in more detail in section II.E.2.a, we agree that the
interpretation of SO2 epidemiologic studies is complicated
by the fact that SO2 is but one component of a complex
mixture of pollutants present in the ambient air. However, the ISA
concluded that when U.S. and international epidemiologic literature is
evaluated as a whole, SO2 effect estimates generally
remained positive and relatively unchanged in multi-pollutant models
with gaseous or particulate co-pollutants. Thus, although recognizing
the uncertainties associated with separating the effects of
SO2 from those of co-occurring pollutants, the ISA concluded
that the limited available evidence from studies employing multi-
pollutant models indicates that the effect of SO2 on
respiratory health outcomes appears to be generally robust and
independent of the effects of gaseous co-pollutants, including
NO2 and O3, as well as particulate co-pollutants,
particularly PM2.5 (ISA, section 5.2; p. 5-9).
In addition, as described in detail above in section II.E.2.a, the
ISA emphasized that controlled human exposure studies provide support
for the plausibility of the associations reported in epidemiologic
studies. The ISA noted that the results of controlled human exposure
and epidemiologic studies form a plausible and coherent data set that
supports a causal relationship between short-term (5-minutes to 24-
hours) SO2 exposures and adverse respiratory effects, and
that the epidemiologic evidence (buttressed by the clinical evidence)
indicates that the effects seen in the epidemiologic studies are
attributable to exposure to SO2 (ISA, section 5.2). The ISA
in fact made the strongest finding possible regarding causality:
``[e]valuation of the health evidence, with consideration of issues
related to atmospheric sciences, exposure assessment, and dosimetry,
led to the conclusion that there is a causal relationship between
respiratory morbidity and short-term exposure to SO2. This
conclusion is supported by the consistency, coherence, and plausibility
of findings observed in the human clinical, epidemiologic, and animal
toxicological studies.'' ISA p. 5-2 (emphasis original).
As mentioned above, many groups dispute the ISA conclusion that
taken together, results from U.S. and international epidemiologic
studies employing multipollutant models indicate that SO2
has an independent effect on the respiratory health outcomes reported
in these studies. Thus, these groups contend that the Administrator
should not place weight on epidemiologic studies and their associated
air quality information in general, and more specifically, the
Administrator should not place weight on air quality information from
the three U.S. epidemiologic studies reporting statistically
significant effects in multipollutant models with PM (i.e., NYDOH 2006;
Ito 2007; and Schwartz 1995). Specific comments on these three
epidemiologic studies reporting statistically significant effects in
multi-pollutant models with PM, and EPA responses are presented below
and in the RTC document (EPA 2010).
Industry groups (e.g., API) had several comments with respect to
the study conducted by the NYDOH (NYDOH, 2006). First, these groups
generally concluded that the results of this study are mixed. That is,
while SO2 effect estimates were positive and statistically
significant even in multipollutant models with PM2.5 or
NO2 in the Bronx, SO2 effect estimates were
actually negative in Manhattan in both single and multipollutant
models. These groups also contend that this report was not peer-
reviewed and that the authors of this study indicated that high
correlations among pollutants in the Bronx made it difficult to
confidently identify which pollutants are actually increasing risks.
For these reasons, industry groups generally concluded that this study
should not be relied upon by the Administrator.
We acknowledge that the results of the NYDOH analysis are mixed
when comparing the Bronx and Manhattan study areas. However, we
disagree that the presence of mixed results renders this study
unreliable. We note that the mixed results reported in this study are
likely to reflect greater statistical power for identifying effects in
the Bronx, where the average daily emergency department visits differed
substantially from those in Manhattan. Specifically, daily asthma
emergency department visits were six times higher in the Bronx study
area (43 per day) than in the Manhattan study area (7.2 per day). Thus,
the more prominent effects in the Bronx likely at least partially
reflect greater statistical power for identifying effects there. To put
these numbers in perspective, the crude daily rates of asthma emergency
department visits can be estimated by dividing the daily asthma counts
by the population. The mean daily crude rates of asthma emergency
department visits were over eight-fold higher in the Bronx study area
(16.9 per 100,000 persons) than in the Manhattan area (2.02 per 100,000
persons). Population age structures were quite different in the two
communities, with larger proportions of younger persons in the Bronx
versus Manhattan. There are likely additional differences in population
structures of the two communities, including differences in SES, race/
ethnicity, and access to primary asthma care. These differences in the
two communities may explain the differences in the results, and do not
prevent EPA from legitimately relying on this study.
As mentioned above, these groups also contend that the NYDOH
epidemiologic study should not be relied upon because it was not peer-
reviewed. We disagree with this assertion. The NYDOH study was subject
to multiple peer-review processes. This included reviews by the Agency
for Toxic Substances and Disease Registry (ATSDR), EPA, and CASAC.
Finally, as also mentioned above, these groups contend that the
NYDOH epidemiologic study is unreliable because the study authors
indicated that high correlations among pollutants in the Bronx make it
difficult to confidently identify which pollutants are actually
increasing risks. In response, we note that high correlations among
ambient air pollutant concentrations are not specific to the NYDOH
study, and may contribute to uncertainty in the interpretation of many
epidemiologic studies of air pollution. The approach most commonly
utilized to disentangle the effects of correlated pollutants in air
pollution epidemiology is the copollutant model. The NYDOH uses
copollutant models and finds that the results for SO2 remain
significant in models considering the simultaneous effects of
NO2, O3, and PM2.5. This indicates an
independent effect of SO2 on the asthma emergency department
visits reported in this study.
With respect to Ito et al., (2007), industry groups generally
commented that since the SO2 effect estimate did not remain
statistically significant in multipollutant models with NO2,
this study does not indicate an independent effect of SO2 on
emergency department visits in the NYC study area. API specifically
commented:
The RR for an increase of 6 ppb SO2 was statistically
significant (1.20; 95% CI: 1.13, 1.28) and remained so when
PM2.5, O3, or CO was included in the model,
but became nonsignificant when NO2 was included in the
model (RR not provided, 95% CI: 0.9, 1.1). Because associations with
SO2 could be attributable to NO2, this study
cannot be used to assess the effects of SO2 on health
effects with small incremental increases in exposure.
[[Page 35545]]
We disagree with the commenters. We believe that this study does
demonstrate an independent effect of SO2 on emergency
department visits in NYC. We note that evidence from controlled human
exposure studies has demonstrated effects of NO2 (EPA,
2008b) and SO2 independently on respiratory morbidity. Since
each of these criteria pollutants has an independent effect on the
respiratory system, it is logical that each may be responsible for an
increase in emergency department visits for asthma in epidemiologic
studies. In addition, the authors note that the attenuation of the
SO2 effect estimate when NO2 is included in the
model is ``consistent with the result of monitor-to-monitor
correlations, suggesting that NO2 has less exposure error
than CO or SO2 in this data set.'' Thus, it appears as
though the high spatial heterogeneity of SO2 compared to
NO2, leading to increased exposure error, may be causing the
attenuation of the SO2 effect estimate when NO2
is included in the model in this study--not that the effects seen in
the study are attributable to NO2. Overall, the results from
this study are consistent with the SO2 effect on respiratory
emergency department visits and hospital admissions across studies and
are coherent with the respiratory effects observed in controlled human
exposure studies. This study thus provides persuasive evidence of an
independent effect of short-term SO2 exposure on respiratory
morbidity.
With respect to Schwartz et al., (1995), industry groups generally
commented that the results of this study are mixed, and therefore
should not be considered by the Administrator. More specifically, these
commenters noted that although the results in New Haven remained
statistically significant in the presence of PM10, the
SO2 effect estimate in Tacoma was reduced and no longer
statistically significant in the presence of PM10.
Commenters also noted that in both cities, the SO2 effect
estimate was reduced and no longer statistically significant in the
presence of O3.
We disagree that the results of this study of hospital admissions
should not be considered by the Administrator. As noted by the
commenters, this study was conducted in two cities, New Haven, CT and
Tacoma, WA. These cities were chosen because they differ in several
important aspects and the author expected the results from the two
cities to be different due to the inherent nature of the study design
and study locations. ``New Haven has almost twice the mean
SO2 concentration of Tacoma, almost two and a half times the
SO2 concentration in the peak winter season, and a much
larger summer ozone peak than Tacoma (Schwartz 1995).'' Since the study
was designed to examine the differences in these two cities, the fact
that the results differed in the two cities does not invalidate those
results. In addition, EPA considers the SO2 effect to be
robust to inclusion of O3 in New Haven. The central effect
estimate for SO2 changed from 1.03 to 1.02 after the
addition of O3 as a copollutant and likely lost statistical
significance due to a greater than 40% reduction in the number of days
included because O3 was only measured during the warm
months. This reduction likely led to model instability and a loss of
statistical significance. To be consistent with how results of other
studies were interpreted in the ISA, and as supported by the CASAC, the
effect of SO2 is considered robust to the inclusion of
O3 in New Haven.
In addition to generally concluding that the epidemiology is too
uncertain to demonstrate that SO2 has an independent effect
on the respiratory effects reported in those studies, many industry
groups (e.g., API, ACC, Progress Energy, EEI, CIBO) also commented that
adverse health effects do not occur following 5-10 minute
SO2 exposures < 400 ppb in controlled human exposure studies
(an issue also discussed above in section II.E.2.b). Thus, these groups
generally maintained that the level of a 1-hour standard should not
take into account limiting 5-minute peaks as low as 200 ppb. From this
argument, many of these groups further maintained that 1-hour standard
levels >= 150 ppb are requisite to protect public health with an
adequate margin of safety.
As first discussed in section II.E.2.b above, we disagree with the
commenters that adverse respiratory effects do not occur following 5-
minute SO2 exposures as low as 200 ppb. The ISA reported
that exposure to SO2 concentrations as low as 200-300 ppb
for 5-10 minutes results in approximately 5-30% of exercising
asthmatics experiencing moderate or greater decrements in lung function
(defined in terms of a >= 15% decline in FEV1 or 100%
increase in sRaw; ISA, Table 3-1). Considering the 2000 ATS guidelines
described in section II.E.2.b, we determined that these results could
reasonably indicate an SO2-induced shift in these lung
function measurements for this sub-population. Under this scenario, an
appreciable percentage of exercising asthmatics exposed to
SO2 concentrations as low as 200 ppb would likely have
diminished reserve lung function and thus would likely be at greater
risk if affected by another respiratory agent (e.g., viral infection).
Importantly, diminished reserve lung function in a population that is
attributable to air pollution is considered an adverse effect under ATS
guidance.\17\ Also noted in section II.E.2.b, we were mindful of
CASAC's pointed comments. The second draft ISA placed relatively little
weight on health effects associated with SO2 exposures at
200-300 ppb. CASAC strongly disagreed with this characterization of the
health evidence. Their consensus letter following the second draft ISA
states:
---------------------------------------------------------------------------
\17\ See Coalition of Battery Recyclers Association v. EPA, No.
09-1011 (DC Cir., May 14, 2010), slip opinion at 9, holding that it
was reasonable for EPA to conclude that a two IQ point mean
population loss is an adverse effect based in part on consideration
of comments from the American Academy of Pediatrics that such a loss
should be prevented.
Our major concern is the conclusions in the ISA regarding the
weight of the evidence for health effects for short-term exposure to
low levels of SO2. Although the ISA presents evidence
from both clinical and epidemiological studies that indicate health
effects occur at 0.2 ppm or lower, the final chapter emphasizes
health effects at 0.4 ppm and above * * * CASAC believes the
clinical and epidemiological evidence warrants stronger conclusions
in the ISA regarding the available evidence of health effects at 0.2
ppm or lower concentrations of SO2. The selection of a
lower bound concentration for health effects is very important
because the ISA sets the stage for EPA's risk assessment decisions.
In its draft Risk and Exposure Assessment (REA) to Support the
Review of the SO2 Primary National Ambient Air Quality Standards
(July 2008), EPA chose a range of 0.4 ppm--0.6 ppm SO2
concentrations for its benchmark analysis. As CASAC will emphasize
in a forthcoming letter on the REA, we recommend that a lower bound
---------------------------------------------------------------------------
be set at least as low as 0.2 ppm (Henderson 2008a).
Similarly, we were also mindful of CASAC comments on the first
draft of the REA. The consensus CASAC letter following the 1st draft
REA states:
The CASAC believes strongly that the weight of clinical and
epidemiology evidence indicates there are detectable clinically
relevant health effects in sensitive subpopulations down to a level
at least as low as 0.2 ppm SO2. These sensitive
subpopulations represent a substantial segment of the at-risk
population (Henderson 2008b).
As noted in section II.E.2.b, we were also mindful of: (1) Previous
CASAC recommendations (Henderson 2006) and NAAQS review conclusions
(EPA 2006, EPA 2007d) indicating that moderate decrements in lung
function can be clinically significant in some asthmatics (see section
II.E.2.b for more detail) and
[[Page 35546]]
(2) controlled human exposure studies not including severe asthmatics
and thus, that it is reasonable to assume that persons with more severe
asthma than the study participants would have a more serious health
effect from short-term exposure to 200 ppb SO2. CASAC echoed
this concern in its comments on the policy assessment chapter of the
REA:
Chapter 10 should better address uncertainty in identifying
alternative NAAQS for SO2. In particular, the
uncertainties discussed in the health risk characterization should
be considered in specifying a NAAQS that provides adequate margin of
safety. One particular source of uncertainty needing acknowledgment
is the characteristics of persons included in the clinical studies.
The draft REA acknowledges that clinical studies are unlikely to
have included severe asthmatics that are likely to be potentially at
greater risk than those persons included in the clinical studies
(Samet 2009; p. 15).
Taken together, the Administrator concluded that exposure to
SO2 concentrations as low as 200 ppb can result in adverse
health effects in asthmatics. Consequently the Administrator also
concluded that a 1-hour standard of 150 ppb is not requisite to protect
public health with an adequate margin of safety, even with a 99th
percentile form. This conclusion takes into account the St. Louis
exposure analysis estimating that only 88% of asthmatic children at
moderate or greater exertion would be protected from at least one 5-
minute SO2 exposure >= 200 ppb per year at a 1-hour standard
level of 150 ppb, and appropriate weight placed on the epidemiologic
evidence (see section II.F.4.c for a discussion of the epidemiologic
evidence with respect to level).
c. Conclusions on Standard Level
Having carefully considered the public comments on the appropriate
level for a 1-hour SO2 standard, as discussed above, the
Administrator believes the fundamental conclusions reached in the ISA
and REA remain valid. In considering the level at which the 1-hour
primary SO2 standard should be set, the Administrator
continues to place primary emphasis on the body of controlled human
exposure and epidemiologic evidence assessed in the ISA, as summarized
above in section II.B. In addition, the Administrator continues to view
the results of exposure and risk analyses, discussed above in section
II.C, as providing supporting information for her decision.
In considering the level of a 1-hour SO2 standard, the
Administrator notes that there is no bright line clearly mandating the
choice of level within the reasonable range proposed. Rather, the
choice of what is appropriate within this reasonable range is a public
health policy judgment entrusted to the Administrator. This judgment
must include consideration of the strengths and limitations of the
evidence and the appropriate inferences to be drawn from the evidence
and the exposure and risk assessments. These considerations and the
Administrator's final decision with regard to the level of a new 1-hour
SO2 standard are discussed below.
In considering the controlled human exposure studies, the
Administrator notes that these studies provide the most direct evidence
of respiratory effects from exposure to SO2. These studies
exposed groups of exercising asthmatics to defined concentrations of
SO2 for 5-10 minutes and found adverse respiratory effects.
As noted above (see section II.C), SO2 exposure levels which
resulted in respiratory effects in these studies were considered 5-
minute benchmark exposures of potential concern in the analyses found
in the REA. With respect to this evidence, the Administrator notes the
following key points:
Exposure of exercising asthmatics to 5-10 minute
SO2 concentrations >= 400 ppb results in moderate or greater
decrements in lung function (in terms of FEV1 or sRaw) in
20-60% of tested individuals in these studies. Moreover, these
decrements in lung function are often statistically significant at the
group mean level and are frequently accompanied by respiratory
symptoms.\18\ Based on ATS guidelines, exposure to SO2
concentrations >= 400 ppb clearly result in adverse respiratory effects
(i.e., decrements in lung function in the presence of respiratory
symptoms). Therefore, the Administrator has concluded it appropriate to
place weight on the 400 ppb 5-minute SO2 benchmark
concentration of concern.
---------------------------------------------------------------------------
\18\ The ISA concluded that collective evidence from key
controlled human exposure studies considered in the previous review,
along with a limited number of new controlled human exposure
studies, consistently indicates that with elevated ventilation rates
a large percentage of asthmatic individuals tested in a given
chamber study (up to 60%, depending on the study) experience
moderate or greater decrements in lung function, frequently
accompanied by respiratory symptoms, following peak exposures to
SO2 at concentrations of 0.4-0.6 ppm. (ISA, p. 3-9).
---------------------------------------------------------------------------
Exposure of exercising asthmatics to 5-10 minute
SO2 concentrations at 200-300 ppb results in moderate or
greater decrements in lung function in 5-30% of the tested individuals
in these studies. The Administrator notes that although these
decrements in lung function have not been shown to be statistically
significant at the group mean level, or to be frequently accompanied by
respiratory symptoms, she considers effects associated with exposures
as low as 200 ppb to be adverse in light of CASAC advice, similar
conclusions in prior NAAQS reviews, and the ATS guidelines described in
detail above (see section II.E.2.b and II.F.4.b). Therefore, she has
concluded it appropriate to place weight on the 200 ppb 5-minute
benchmark concentration.
There is very limited evidence from two mouthpiece
exposure studies suggesting respiratory effects in exercising
asthmatics following SO2 exposures at 100 ppb. However,
given the uncertainties and potential unrepresentativeness associated
with mouthpiece studies (see section II.F.4.b above), the Administrator
found it appropriate not to place weight on this 5-minute
SO2 benchmark concentration.
The Administrator also considered the results of the air quality,
exposure, and risk analyses, as they serve to estimate the extent to
which a given 1-hour standard limits the 5-minute benchmark
concentrations of concern identified from controlled human exposure
studies (see REA chapters 7-9, proposal section II.F.4.b, and proposal
Tables 2-4). In considering these results as they relate to limiting 5-
minute SO2 benchmark concentrations >= 200 and 400 ppb, the
Administrator notes the following key points:
The 40-county air quality analysis estimates that a 100
ppb 1-hour daily maximum standard would allow at most 2 days per year
on average in any county when estimated 5-minute daily maximum
SO2 concentrations exceed the 400 ppb benchmark, and at most
13 days per year on average when 5-minute daily maximum SO2
concentrations exceed the 200 ppb benchmark (see proposal, Table 2, 74
FR at 64840). Furthermore, given a simulated 1-hour 100 ppb standard
level, most of the counties in that air quality analysis were estimated
to experience 0 days per year on average when 5-minute daily maximum
SO2 concentrations exceed the 400 ppb benchmark and <= 3
days per year on average when 5-minute daily maximum SO2
concentrations were estimated to exceed the 200 ppb benchmark (see REA,
Tables 7-14 and 7-12).
The St. Louis exposure analysis estimates that a 99th
percentile 1-hour daily maximum standard at a level of 100 ppb would
likely protect > 99% of asthmatic children in that city at moderate or
greater exertion from experiencing at least one 5-minute exposure >=
400 ppb per year, and
[[Page 35547]]
approximately 97% of those asthmatic children at moderate or greater
exertion from experiencing at least one exposure >= 200 ppb per year
(see proposal, section II.F.4.b).
The St. Louis risk assessment estimates that a 99th
percentile 1-hour standard level at 100 ppb would likely protect about
97-98% of exposed asthmatic children in that city at moderate or
greater exertion from experiencing at least one moderate or greater
lung function response (defined as a >= 100% increase in sRaw; see
proposal, section II.F.4.b).
Given the above considerations, the Administrator concludes that a
1-hour standard at a level of 100 ppb would appropriately limit 5-
minute SO2 benchmark concentrations >= 200 or 400 ppb.
Moreover, although the Administrator acknowledges that the air quality
and exposure analyses mentioned above suggest that a 50 ppb standard
may somewhat further limit 5-minute SO2 concentrations/
exposures in excess of the 200 ppb benchmark (see proposal section
II.F.4.b), she does not believe this information alone warrants a
standard level lower than 100 ppb. More specifically, although she
considers the health effects resulting from 5-minute SO2
exposures as low as 200 ppb to be adverse, she also recognizes that
such effects are appreciably less severe than those at SO2
concentrations >= 400 ppb. Thus, she concludes that there is little
difference in limiting 5-minute concentrations/exposures >= 400 ppb
given 1-hour standard levels in the range of 50 to 100 ppb.
In considering the epidemiologic evidence with regard to level, the
Administrator notes that there have been more than 50 peer reviewed
epidemiologic studies published worldwide evaluating SO2
(ISA, Tables 5-4 and 5-5). These studies have generally reported
positive, although not always statistically significant associations
between more serious health outcomes (i.e. respiratory-related
emergency department visits and hospitalizations) and ambient
SO2 concentrations and have generally included populations
potentially at increased risk for SO2-related respiratory
effects (e.g, children, older adults, and those with pre-existing
respiratory disease). The Administrator finds that in assessing the
extent to which these studies and their associated air quality
information can inform the level of a new 99th percentile 1-hour daily
maximum standard for the U.S., air quality information from the U.S.
and Canada is most relevant since these areas have similar monitor
network designs and patterns of air quality. However, as described in
proposal section II.F.4.a, SO2 concentrations reported for
Canadian studies were not directly comparable to those reported for
U.S. studies due to use of different monitoring protocols in those
studies. Thus, the Administrator focused on 99th percentile air quality
information from U.S. studies for informing potential 1-hour standard
levels. She concludes that this information provides evidence of
associations between ambient SO2 and emergency department
visits and hospital admissions in U.S. cities with particular 99th
percentile 1-hour SO2 levels, and thus provides information
that is particularly relevant for setting the level of a 1-hour
SO2 standard. With regard to these studies she notes the
following key points:
Ten studies (some conducted in multiple locations)
reported mostly positive, and sometimes statistically significant,
associations between ambient SO2 concentrations and
emergency department visit and hospital admissions in locations where
99th percentile 1-hour daily maximum SO2 levels ranged from
approximately 50-460 ppb.
Within this broader range of SO2
concentrations, there is a cluster of three epidemiologic studies
between 78-150 ppb (for the 99th percentile of the 1-hour
SO2 concentrations) where the SO2 effect estimate
remained positive and statistically significant in multi-pollutant
models with PM (NYDOH (2006), Ito et al., (2007), and Schwartz et al.,
(1995)). Notably, although statistical significance in multi-pollutant
models is an important consideration, it is not the only consideration
when relying on such epidemiologic evidence.\19\ However, as noted
earlier, there is special sensitivity in this review in disentangling
PM-related effects (especially sulfate PM) from SO2-related
effects in interpreting the epidemiologic studies; thus, these studies
are of particular relevance here, lending strong support both to the
conclusion that SO2 effects are generally independent of PM
(ISA, section 5.2) and that these independent adverse effects of
SO2 have occurred in cities with 1-hour daily maximum, 99th
percentile concentrations in the range of 78-150 ppb. Nor did EPA find
the comments criticizing these studies persuasive, as explained above
in section II.F.4.b and in the RTC document (EPA 2010). The
Administrator therefore judges it appropriate to place substantial
weight on this cluster of three U.S. epidemiologic studies in selecting
a standard level, as they are a group of studies that reported positive
and statistically significant associations between ambient
SO2 and emergency department visits or hospital admissions
even when potential confounding by PM was considered.
---------------------------------------------------------------------------
\19\ For example, as noted in the proposal (proposal, section
II.F.4, 74 FR at 64835) evidence of a pattern of results from a
group of studies that find effect estimates similar in direction and
magnitude would warrant consideration of and reliance on such
studies even if the studies did not all report statistically
significant associations in single- or multi-pollutant models. The
SO2 epidemiologic studies fit this pattern, and are
buttressed further by the results of the clinical studies. ISA,
section 5.2.
---------------------------------------------------------------------------
The Administrator agrees with the finding in the ISA that
the controlled human exposure evidence lends biological plausibility to
the effects reported in epidemiologic studies (ISA, p. 5-9).
There is limited evidence from two epidemiologic studies
employing single pollutant models that found generally positive
associations between ambient SO2 and emergency department
visits in locations where 99th percentile 1-hour SO2
concentrations were approximately 50 ppb (see proposal, Figures 1 and
2). However, considering that the results of these studies were mixed,
and importantly, that neither of these two studies evaluated the
potential for confounding by co-pollutants through the use of
multipollutant models (particularly with PM), the Administrator judges
it appropriate to place limited weight on these studies.
With regard to the cluster of three studies conducted in
the Bronx (NYDOH 2006), NYC (Ito et al., 2007), and New Haven (Scwartz
et al., 1995), there is a degree of uncertainty as to whether the 99th
percentile 1-hour daily maximum SO2 concentrations reported
from monitors in these three study areas reflect the highest 99th
percentile 1-hour daily maximum SO2 concentration. Our
limited qualitative analysis suggests that 99th percentile 1-hour daily
maximum SO2 concentrations reported by monitors in these
study areas are reasonable approximations for the absolute highest 99th
percentile 1-hour daily maximum SO2 concentration that can
occur across the entire area in these studies (including the areas
where monitors were not located) (see Brode, 2010). However, although a
reasonable approximation, it is still likely that these monitored
concentrations are somewhat lower than the absolute highest 99th
percentile 1-hour daily maximum SO2 concentrations occurring
across these epidemiologic study areas.
[[Page 35548]]
Weighing all of this evidence, the Administrator concludes that the
epidemiologic studies provide strong support for setting a standard
that limits the 99th percentile of the distribution of 1-hour daily
maximum SO2 concentrations to 75 ppb. This judgment takes
into account the strong determinations in the ISA (and endorsed by
CASAC), based on a much broader body of evidence, that there is a
causal association between exposure to SO2 and the types of
respiratory morbidity effects reported in these studies. The
Administrator further judges that it is not necessary based on existing
epidemiologic evidence, to set a standard below 75 ppb. That is, the
Administrator concludes that a standard level of 75 ppb is sufficiently
below the SO2 levels in three cities where epidemiologic
studies found statistically significant effects in multipollutant
models with PM (i.e., 78, 82, and 150 ppb) to provide an adequate
margin of safety given the uncertainty as to whether monitors in these
study locations reflected the highest 1-hour daily maximum
SO2 concentration across the entire study area. Thus, a
standard set at a level of 75 ppb is likely further below the 99th
percentile 1-hour daily maximum concentrations in these three study
areas than the bare comparison of levels would otherwise indicate.
Finally, the Administrator again notes that epidemiologic evidence
below 75 ppb is more uncertain because studies below 75 ppb did not
evaluate potential confounding of results in multipollutant models, and
because these studies reported mixed results.
Given the above considerations and the comments received on the
proposal, the Administrator determines that the appropriate judgment,
based on the entire body of evidence and information available in this
review, and the related uncertainties, is a standard level of 75 ppb.
She concludes that such a standard, with a 1-hour averaging time and
99th percentile form, will provide a significant increase in public
health protection compared to the current standards and would be
expected to protect against the respiratory effects that have been
linked with SO2 exposures in both controlled human exposure
and epidemiologic studies. Specifically, she concludes that such a
standard will limit 1-hour exposures at and above 75 ppb for those in
susceptible populations that are at-risk of experiencing adverse health
effects from short-term exposure to SO2. Such a standard
will also maintain SO2 concentrations below those in
locations where key U.S. epidemiologic studies have reported that
ambient SO2 is associated with clearly adverse respiratory
health effects, as indicated by increased hospital admissions and
emergency department visits. She also notes that a 1-hour standard at a
level of 75 ppb is expected to substantially limit asthmatics' exposure
to 5-10 minute SO2 concentrations >= 200 ppb, thereby
substantially limiting the adverse health effects associated with such
exposures. Finally, the Administrator notes that a standard level of 75
ppb is consistent with the consensus recommendation of CASAC.
In setting the standard level at 75 ppb rather than at a lower
level, the Administrator notes that a 1-hour standard with a level
lower than 75 ppb would only result in significant further public
health protection if, in fact, there is a continuum of serious, adverse
health risks caused by exposure to SO2 concentrations below
75 ppb. Based on the available evidence, the Administrator does not
believe that such assumptions are warranted. Taking into account the
uncertainties that remain in interpreting the evidence from available
controlled human exposure and epidemiologic studies, the Administrator
notes that the likelihood of obtaining benefits to public health with a
standard set below 75 ppb decreases, while the likelihood of requiring
reductions in ambient concentrations that go beyond those that are
needed to protect public health increases.
Therefore, the Administrator judges that a 1-hour SO2
standard at 75 ppb is sufficient to protect public health with an
adequate margin of safety. This includes protection with an adequate
margin of safety for susceptible populations at increased risk for
adverse respiratory effects from short-term exposures to SO2
for which the evidence supports a causal relationship with
SO2 exposures. The Administrator does not believe that a
lower standard level is needed to provide this degree of protection.
These conclusions by the Administrator appropriately consider the
requirement for a standard that is neither more nor less stringent than
necessary for this purpose and recognizes that the CAA does not require
that primary NAAQS be set at a zero-risk level or to protect the most
susceptible individual, but rather at a level that reduces risk
sufficiently so as to protect the public health with an adequate margin
of safety.
5. Retaining or Revoking the Current 24-Hour and Annual Standards
This section discusses considerations related to retaining or
revoking the current 24-hour and annual SO2 primary NAAQS.
Specifically, this section summarizes the rationale for the
Administrator's proposed decision regarding whether to retain or revoke
the current standards (section II.F.5.a), discusses public comments
related to whether to retain or revoke the current standards
(II.F.5.b), and presents the Administrator's final conclusions
regarding whether to retain or revoke the current standards (II.F.5.c).
a. Rationale for Proposed Decision
As noted in the proposal (see section II.F.5), the REA recognized
that the particular level selected for a new 99th percentile 1-hour
daily maximum standard would have implications for deciding whether to
retain or revoke the current 24-hour and annual standards. That is,
with respect to SO2-induced respiratory morbidity, the lower
the level selected for a 99th percentile 1-hour daily maximum standard,
the less additional public health protection the current standards
would be expected to provide. CASAC expressed a similar view following
their review of the 2nd draft REA: ``Assuming that EPA adopts a one
hour standard in the range suggested, and if there is evidence showing
that the short-term standard provides equivalent protection of public
health in the long-term as the annual standard, the panel is supportive
of the REA discussion of discontinuing the annual standard'' (Samet
2009, p. 15). With regard to the current 24-hour standard, CASAC was
generally supportive of using the air quality analyses in the REA as a
means of determining whether the current 24-hour standard was needed in
addition to a new 1-hour standard to protect public health. CASAC
stated: ``The evidence presented [in REA Table 10-3] was convincing
that some of the alternative one-hour standards could also adequately
protect against exceedances of the current 24-hour standard'' (Samet
2009, p. 15).
In accordance with the REA findings and CASAC recommendations
mentioned above, the Administrator noted that 1-hour standards in the
range of 50-100 ppb would have the effect of maintaining 24-hour and
annual SO2 concentrations generally well below the levels of
the current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and
REA Appendix Tables D-3 to D-6). Thus, if a new 99th percentile 1-hour
daily maximum standard was set in the proposed range of 50-100 ppb,
then the Administrator proposed to revoke the current 24-hour and
annual standards. However, as noted in the proposal, if a standard was
set at a level >100 ppb and
[[Page 35549]]
up to 150 ppb, then the Administrator indicated that she would retain
the existing 24-hour standard, recognizing that a 99th percentile 1-
hour daily maximum standard at 150 ppb would not have the effect of
maintaining 24-hour average SO2 concentrations below the
level of the current 24-hour standard in all locations analyzed (see
REA Appendix Table D-4). Under this scenario, the Administrator would
still revoke the current annual standard recognizing: (1) 99th
percentile 1-hour daily maximum standards in the range of 50-150 ppb
would maintain annual average SO2 concentrations below the
level of the current annual standard (see REA Table 10-4 and REA
Appendix tables D-5 and D-6); and (2) the lack of sufficient evidence
linking long-term SO2 exposure to adverse health effects.
b. Comments on Retaining or Revoking the Current 24-Hour and Annual
Standards
As noted above, most industry groups were opposed to the proposed
revisions to the SO2 NAAQS. However, some of these groups
noted that if a 1-hour standard was adopted, then they would support
revoking the current 24-hour and annual standards. State agencies
generally supported revoking the current standards if a 1-hour standard
was set in the proposed range, although NAACA, NESCAUM, and VT, while
supportive of revoking the existing standards, also suggested that EPA
explore setting a new 24-hour standard to minimize the potential that
multiple hours within a day would exceed a 1-hour standard (see RTC
document (EPA 2010), section IV). Groups which supported revoking the
current 24-hour and annual standards (if a 1-hour standard was set in
the proposed ranged) generally referenced the Administrator's rationale
and CASAC advice described in the proposal (see section II.F.5).
Public health (e.g., ALA, ATS) and environmental organizations
(e.g., CBD, WEACT for Environmental Justice) were generally opposed to
revoking the current 24-hour and annual standards. These groups
generally concluded that the 24-hour standard should be revised while
the annual standard should be retained. In support of this position,
ALA et al., cited air quality information from the REA indicating that
if air quality was simulated to just meet a 99th percentile 1-hour
daily maximum standard in the proposed range of 50-100 ppb, then in
some locations analyzed, 99th percentile 24-hour average SO2
concentrations would be above concentrations (i.e., above 99th
percentile 24-hour average concentrations) in cities where U.S.
emergency department visit and hospital admission studies reported
positive associations with SO2. In addition, many of these
groups were opposed to revoking the current annual standard. In
general, these groups concluded that given the uncertainties associated
with SO2 exposure and long-term health effects, EPA should
err on the side of being health protective and retain the existing
annual standard. EPA responses to comments on whether the current
standards should be retained or revoked are presented below as well as
in section IV of the RTC document (EPA 2010).
As stated in the REA and proposal, 99th percentile 24-hour average
SO2 concentrations in cities where U.S. emergency department
visit and hospital admission studies (for all respiratory causes and
asthma; identified from Table 5-5 of the ISA) were conducted ranged
from 16 ppb to 115 ppb (Thompson and Stewart, 2009). Moreover, as
stated in the REA and proposal (see section II.F.2), effect estimates
that remained statistically significant in multi-pollutant models with
PM were found in cities with 99th percentile 24-hour average
SO2 concentrations ranging from approximately 36 ppb to 64
ppb. In its comments, ALA et al., stated (based on the air quality
information in REA Appendix Table D-2) ``with a 1-hour 50 ppb 99th
percentile standard, 7 counties would experience a 99th percentile 24-
hour concentration of 16 ppb or greater, the range found to be harmful
in epidemiological studies. With an hourly standard of 100 ppb, 24 of
30 counties would have 99th percentile 24-hour concentrations above 16
ppb, with 1 county exceeding 36 ppb.'' Thus, these commenters generally
maintained that a lowered 24-hour standard is needed to protect against
these 24-hour SO2 concentrations.
We disagree that a lowered 24-hour standard is needed to protect
against 24-hour average SO2 concentrations of concern
identified from cities where U.S. emergency department visit and
hospital admission studies were conducted. As noted in detail in the
REA, there is uncertainty as to whether the health effects reported in
epidemiologic studies using 24-hour average SO2
concentrations are in fact due to 24-hour average SO2
exposures (REA, section 10.5.2). That is, when describing epidemiologic
studies observing positive associations between ambient SO2
and respiratory symptoms, the ISA stated ``that it is possible that
these associations are determined in large part by peak exposures
within a 24-hour period'' (ISA, section 5.2 at p. 5-5). Similarly, the
ISA stated that: ``The effects of SO2 on respiratory
symptoms, lung function, and airway inflammation observed in the human
clinical studies using peak exposures further provides a basis for a
progression of respiratory morbidity resulting in increased emergency
department visits and hospital admissions'' and makes the associations
observed in the epidemiologic studies ``biologica[lly] plausib[le]''
(id.). In contrast, evidence from controlled human exposure studies of
5-10 minutes and epidemiologic studies using 1-hour daily maximum
SO2 concentrations provided appreciably stronger evidence of
respiratory morbidity effects following SO2 exposures <= 1-
hour.
Given that respiratory morbidity effects following SO2
exposure may be most related to averaging times <=1-hour, EPA found it
most reasonable to consider the extent to which a 1-hour averaging
time, given an appropriate form and level (which as discussed above,
also substantially limits 5-minute benchmark exposures of concern; see
sections II.F.2 and II.F.4), limited 99th percentile 24-hour average
concentrations of SO2 in locations where emergency
department visit/hospitalization studies reported that the
SO2 effect estimate remained statistically significant in
multi-pollutant models with PM (i.e., locations with 99th percentile
24-hour average SO2 concentrations >=36 ppb). Considering
this, we note that ALA et al., identified only one county with 99th
percentile 24-hour average SO2 concentrations >=36 ppb given
a 99th percentile 1-hour daily maximum standard at 100 ppb, and no
counties >=36 ppb given a 99th percentile 1-hour daily maximum standard
at 50 ppb. Thus, given a 99th percentile 1-hour daily maximum standard
level at 75 ppb (i.e., the form and level selected for a new 1-hour
SO2 standard), it is possible that no county in the ALA et
al., analysis would have had a 99th percentile 24-hour average
SO2 concentration >=36 ppb.
With regard to the annual standard, we also disagree that this
standard needs to be retained. First, the ISA found that ``[t]he
evidence linking short-term SO2 exposure and cardiovascular
effects, and morbidity and mortality with long-term exposures to
SO2 is inadequate to infer a causal relationship.'' ISA, p.
5-10. Thus, an annual standard is unnecessary to prevent long-term
health effects. The remaining issue is whether such a standard provides
further protection
[[Page 35550]]
against short-term effects, given the new one hour standard. We
conclude that it does not. As noted in the proposal, our air quality
information indicates that 1-hour standard levels in the range of 50-
100 ppb are estimated to generally keep annual SO2
concentrations well below the level of the current annual standard.
CASAC agreed. The panel stated: ``Assuming that EPA adopts a one hour
standard in the range suggested, and if there is evidence showing that
the short-term standard provides equivalent protection of public health
in the long-term as the annual standard, the panel is supportive of the
REA discussion of discontinuing the annual standard'' (Samet 2009, p.
15). Taken together, this information indicates that retaining the
annual standard would add no additional public health protection.
c. Administrator's Conclusions on Retaining or Revoking the Current 24-
Hour and Annual Standards
In accordance with the REA findings and CASAC recommendations
mentioned above, the Administrator concludes that a 1-hour standard at
level of 75 ppb would have the effect of maintaining 24-hour and annual
SO2 concentrations generally well below the levels of the
current 24-hour and annual NAAQS (see REA Tables 10-3 and 10-4 and REA
Appendix Tables D-3 to D-6). She also concludes that, as noted above in
section II.F.2, a 1-hour standard at 75 ppb will likely limit 99th
percentile 24-hour SO2 concentrations in U.S. locations
where emergency department visit and hospital admission studies
reported statistically significant associations in multi-pollutant
models with PM. Finally, she notes the lack of sufficient health
evidence to support an annual standard to protect against health
effects associated with long-term SO2 exposure. Taken
together, the Administrator concludes it appropriate to revoke the
current 24-hour and annual standards.
G. Summary of Decisions on the Primary Standards
For the reasons discussed above, and taking into account
information and assessments presented in the ISA and REA as well as the
advice and recommendations of CASAC, the Administrator concludes that
the current 24-hour and annual primary standards are not requisite to
protect public health with an adequate margin of safety. The
Administrator also concludes that establishing a new 1-hour standard
will appropriately protect public health with an adequate margin of
safety, and specifically will afford requisite increased protection for
asthmatics and other at-risk populations against an array of adverse
respiratory health effects related to short-term (5 minutes to 24
hours) SO2 exposure. These effects include decrements in
lung function (defined in terms of sRaw and FEV1), increases
in respiratory symptoms, and related serious indicators of respiratory
morbidity including emergency department visits and hospital admissions
for respiratory causes.
Specifically, the Administrator is establishing a new short-term
primary SO2 standard with a 1-hour (daily maximum) averaging
time and a form defined as the 3-year average of the 99th percentile of
the yearly distribution of 1-hour daily maximum SO2
concentrations, and a level of 75 ppb. In addition to setting a new 1-
hour standard at 75 ppb, the Administrator is revoking the current 24-
hour and annual standards recognizing that a 1-hour standard set at 75
ppb will have the effect of generally maintaining 24-hour and annual
SO2 concentrations well below the levels of the current 24-
hour and annual standards.
III. Overview of the Approach for Monitoring and Implementation
We received several comments regarding the approaches discussed in
the proposal for monitoring and modeling for comparison to the proposed
new 1-hour SO2 NAAQS, designations of areas as either
attaining or not attaining the NAAQS, and implementation of the new
NAAQS in State implementation plans (SIPs) that would ensure ultimate
attainment of the new NAAQS in transitioning from the annual and 24-
hour NAAQS in a timely manner. These comments raised fundamental
questions regarding our contemplated approaches in all three areas, and
caused us to re-examine them and review their consistency with past
practice under the SO2 NAAQS implementation program. After
conducting that review, and in response to the public comments we are
revising our general anticipated approach toward implementation of the
new 1-hour NAAQS. This revised approach would better address: (1) The
unique source-specific impacts of SO2 emissions; (2) the
special challenges SO2 emissions present in terms of
monitoring short-term SO2 levels for comparison with the
NAAQS in many situations; (3) the superior utility that modeling offers
for assessing SO2 concentrations; and (4) the most
appropriate method for ensuring that areas attain and maintain the new
1-hour SO2 NAAQS in a manner that is as expeditious as
practicable, taking into account the potential for substantial
SO2 emissions reductions from forthcoming national and
regional rules that are currently underway.
Below, we provide an overview of our revised approach to
monitoring, and of our expected approaches to designations of areas,
and implementation of the NAAQS. Due to the unique challenges presented
by SO2, we do not expect that the anticipated approaches
discussed below would be necessarily transferable to other NAAQS
pollutant situations. For NAAQS pollutants other than SO2,
air quality monitoring is more appropriate for determining whether all
areas are attaining the NAAQS, and there is comparatively less
dependence upon conducting refined modeling. Each of these subjects
(i.e., our revised approach to monitoring, and our expected approaches
to designations of areas, and implementation of the NAAQS) is further
addressed later in the preamble, in sections IV, V and VI,
respectively. Where specific public comments on the proposal are
addressed and responded to, further details of the specific revised
approaches are explained. In many respects, both the overview
discussion below and the subsequent more detailed discussions explain
our expected and intended future action in implementing the new 1-hour
NAAQS--in other words, they constitute guidance, rather than final
agency action--and it is possible that our approaches may continue to
evolve as we, States, and other stakeholders proceed with actual
implementation. In other respects, such as in the final regulatory
provisions regarding the promulgated monitoring network, we are
explaining EPA's final conclusions regarding what is required by this
rule. We expect to issue further guidance regarding implementation,
particularly concerning issues that may arise regarding the application
of refined dispersion modeling under this revised approach for
monitoring and implementation, and issues that States and other
stakeholders may also ask us to address as we proceed toward various
stages of ensuring attainment. EPA intends to solicit public comment
prior to finalizing this guidance.
The main necessary elements of implementing the new 1-hour NAAQS
are: (1) An approach for assessing ambient concentrations to determine
compliance with the NAAQS; (2) a process for using these assessments to
designate areas relative to the new standard; and (3) the development
of State plans that include control measures sufficient for ensuring
the NAAQS is attained everywhere as expeditiously as possible, which we
[[Page 35551]]
believe should be no later than 2017. EPA's revised anticipated
approach to determining compliance with the new SO2 NAAQS is
consistent with our historical approach to SO2 designations
and implementation through permits and emissions limitations, which
involves the combined use of monitoring and modeling. The emphasis we
would place on monitoring and modeling, compared with each other, under
the revised expected approach is therefore significantly different than
that in the approach discussed in the proposal, which was less in line
with our historical practice for SO2, as the public comments
highlighted.
In the SO2 NAAQS proposal, we recommended a monitoring-
focused approach for comparison to the new NAAQS, featuring a two-
pronged monitoring network design. This included monitors in certain
CBSAs based on a combination of population and SO2 emissions
coupled with additional monitors within a State based on that State's
contribution to national SO2 emissions. The resulting
proposed network would have required approximately 348 monitors
nationwide to be sited at the locations of maximum concentration.
Numerous State and local government commenters expressed concerns
regarding the burdens of implementing the proposed monitoring network
and the sufficiency of its scope for purposes of identifying
violations. These commenters contended that our proposed monitoring
network was too small and insufficient to cover the range of
SO2 sources, and yet too burdensome and expensive to expand
to an adequate scale. Some of these commenters (the City of Alexandria,
and the States of Delaware, North Carolina and Pennsylvania) suggested
using modeling to determine the scope of monitoring requirements, or
favored modeling over monitoring to determine compliance with the
NAAQS.
Partly in response to these comments, and after reconsidering the
proposal's monitoring-focused approach in light of EPA's historical
approach to SO2 NAAQS implementation and area designations
decisions, we intend to use a hybrid analytic approach that would
combine the use of monitoring and modeling to assess compliance with
the new 1-hour SO2 NAAQS. We believe that some type of
hybrid approach is more consistent with our historical approach and
longstanding guidance toward SO2 than what we originally
proposed. In addition, we believe that for a short-term 1-hour standard
it is more technically appropriate, efficient, and effective to use
modeling as the principle means of assessing compliance for medium to
larger sources, and to rely more on monitoring for groups of smaller
sources and sources not as conducive to modeling. We discuss the
details of the final revised monitoring network requirements in section
IV later in the preamble, but note here the relationship that the
revised approach toward monitoring and modeling--taken partly in
response to the public comments mentioned above--has to the other two
general subject areas in implementation for which we are providing
guidance, namely initial area designations and development of
substantive implementation plans that ensure timely attainment and
maintenance of the NAAQS. Our ultimate intention is to place greater
emphasis on modeling than did the proposed rule as the most technically
appropriate, efficient, and readily available method for assessing
short-term ambient SO2 concentrations in areas with large
point sources. This projected change in approach would necessarily
result in a lesser emphasis on the less appropriate, more expensive,
and slower to establish monitoring tool than did the proposed rule.
Therefore, the minimum requirements for the SO2 monitoring
network in this final rule are of a smaller scale than proposed, and we
do not expect monitoring to become the primary method by which ambient
concentrations are compared to the new 1-hour SO2 NAAQS.
Instead, in areas without currently operating monitors but with
sources that might have the potential to cause or contribute to
violations of the NAAQS, we anticipate that the identification of NAAQS
violations and compliance with the 1-hour SO2 NAAQS would
primarily be done through refined, source-oriented air quality
dispersion modeling analyses, supplemented with a new, limited network
of ambient air quality monitors. Historically, we have favored
dispersion modeling to support SO2 NAAQS compliance
determinations for areas with sources that have the potential to cause
an SO2 NAAQS violation, and we have explained that for an
area to be designated as ``attainment,'' dispersion modeling regarding
such sources needs to show the absence of violations even if monitoring
does not show a violation. This has been our general position
throughout the history of implementation of the SO2 NAAQS
program. See, e.g., ``Air Quality Control Regions, Criteria, and
Control techniques; Attainment Status Designations,'' 43 FR 40412,
40415-16 (Sept. 11, 1978); ``Air Quality Control Regions, Criteria, and
Control Techniques,'' 43 FR 45993, 46000-02 (Oct. 5, 1978); ``Air
Quality Implementation Plans: State Implementation Plans; General
Preamble,'' 57 FR 13498, 13545, 13547-48 (Apr. 16, 1992); ``Approval
and Promulgation of State Implementation Plans; Call for Sulfur Dioxide
SIP Revisions for Billings/Laurel, MT,'' 58 FR 41430 (Aug. 4, 1993);
``Designation of Areas for Air Quality Planning Purposes; Ohio,'' 59 FR
12886, 12887 (Mar. 18, 1994); ``Ambient Air Quality Standards, National
and Implementation Plans for Sulfur Oxides (Sulfur Dioxide),'' 60 FR
12492, 12494-95 (Mar. 7, 1995); ``Air Quality Implementation Plans;
Approval and Promulgation: Various States: Montana,'' 67 FR 22167,
22170-71, 22183-887 (May 2, 2002).
Compared to other NAAQS pollutants, we would not consider ambient
air quality monitoring alone to be the most appropriate means of
determining whether all areas are attaining a short-term SO2
NAAQS. Due to the generally localized impacts of SO2, we
have not historically considered monitoring alone to be an adequate,
nor the most appropriate, tool to identify all maximum concentrations
of SO2. In the case of SO2, we further believe
that monitoring is not the most cost-efficient method for identifying
all areas of maximum concentrations. However, for some situations
monitoring is well suited, and we therefore will require it to some
extent, as further explained in section IV of the preamble. For
example, monitoring may appropriately be relied upon to assess
compliance with the NAAQS by groups of smaller sources and sources that
may not be as conducive to modeling as are larger SO2
sources.
States will need to make any adjustments to the existing monitoring
network to ensure that monitors meeting today's network design
regulations for the new 1-hour NAAQS are sited and operational by
January 1, 2013. We also expect to provide additional guidance
regarding the application of refined dispersion modeling under this
revised expected approach for implementation of the new SO2
standard. Appendix A to the Guideline on Air Quality Models (Appendix W
of 40 CFR part 51), Summaries of Preferred Air Quality Models, provides
``key features of refined air quality models preferred for specific
regulatory applications'' (see Appendix A to Appendix W of Part 51 at
A.0(1)). Refined dispersion modeling, following our current Guideline
on Air Quality Models with appropriate flexibility for use in
implementation, is anticipated to better reflect and account
[[Page 35552]]
for source-specific SO2 impacts than the more limited
monitoring-focused proposal. As noted above, EPA intends to solicit
public comment prior to finalizing this guidance.
Based on a revised, hybrid approach, we expect to implement the new
SO2 standard in the following manner. In accordance with CAA
section 107(d), EPA must designate areas as ``attainment,''
``nonattainment'' or ``unclassifiable'' for the new 1-hour
SO2 NAAQS by June 2012 (i.e., two years following
promulgation of the new NAAQS).\20\ State Governors are required to
submit their initial area designation recommendations to EPA no later
than June 2011. We expect that EPA's final area designation decisions
in 2012 would be based principally on data reported from SO2
monitors currently in place today, and any refined modeling the State
chooses to conduct specifically for initial area designations.\21\ For
these initial designations, we would expect to designate an area
``nonattainment'' if either monitoring data or appropriate refined
modeling results show a violation. Any area that has monitoring and
appropriate modeling data showing no violations we would expect to
designate as ``attainment.'' \22\ All other areas, absent monitoring
data and air quality modeling results showing no violations, we would
expect to initially designate as ``unclassifiable,'' as required by the
Clean Air Act. The expected presumptive boundary for any area
designated ``nonattainment'' would be the county boundary associated
with the violation unless additional information provided to EPA
demonstrates otherwise, as has been our general approach for other
NAAQS pollutants. Any area initially designated ``nonattainment'' or
``unclassifiable'' could request redesignation to ``attainment'' after
an assessment based on air quality modeling, conducted in accordance
with the new guidance, and available monitoring data indicates that the
standard has been met, as well as meeting all other requirements of the
CAA for redesignation to attainment.
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\20\ EPA is authorized by the Clean Air Act to take up to 3
years to complete the initial area designations in the event that
insufficient information is available to complete the designations
within 2 years.
\21\ Since three complete years of data from any newly sited
monitors meeting the new monitoring network design criteria are not
expected to be obtained until the end of 2015, any newly sited
monitors will not play a role in EPA's initial area designations.
\22\ EPA anticipates making the determination of when monitoring
alone is ``appropriate'' for a specific area on a case-by-case
basis, informed by that area's factual record, as part of the
designations process. EPA would expect to address this issue for
such areas by examining the historic treatment of the area with
respect to prior SO2 designations as well as whether the
area is one in which monitoring would be the more technically
appropriate tool for determining compliance with the new
SO2 NAAQS. An example of a situation in which monitoring
may be the more preferred approach is a shipping port (non-point
source or ``area'' source) that is not in close proximity to other
significant stationary SO2 sources.
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This anticipated approach toward initial area designations is a
change from the approach discussed in the proposal, and logically
follows from our general change in approach to the use and utility of
monitoring versus modeling for determining short-term SO2
ambient concentrations. As public commenters pointed out, establishment
and implementation of the proposed monitoring network would have been
both too limited and too late to inform initial area designations, and
the expense and burden of accelerating it and expanding it would have
been severe for State implementing agencies. Given the time needed to
establish monitors, it is not realistic to expect either such an
expanded monitoring network or even the more reasonable limited network
of the final rule to be the chief tool for informing initial
designations.
That means that some other approach is needed to inform initial
designations of areas and other implementation decisions under the new
SO2 NAAQS. In addition to using any valid data generated by
existing monitors, refined dispersion modeling may inform designation
and implementation decisions regarding sources that may have the
potential to cause or contribute to a NAAQS violation. In order for
modeling to be done on the scale sufficient to identify all areas that
might violate the new 1-hour standard, EPA anticipates issuing guidance
that addresses a variety of issues, such as how to identify and
appropriately assess the air quality impacts of small SO2
sources (e.g., those emitting less than 100 tons of SO2 per
year) that may potentially cause or contribute to a violation of the
new SO2 NAAQS. EPA expects that it will take more time for
EPA to issue that guidance than is available in order to use it for the
initial round of attainment designations. In addition to any smaller
sources that might cause or contribute to NAAQS violations, States
would need to model approximately 2000 larger sources across the
country (i.e., sources that emit greater than 100 tons per year and are
collectively responsible for about 99% of all SO2 emissions
from point sources in the U.S.) to determine whether areas are
attaining or not attaining the 1-hour standard. While these sources
emitting 100 or more tons of SO2 per year represent the
significant fraction of the total emissions from point sources in the
U.S., smaller sources also have the potential to violate the new
SO2 NAAQS.
After receiving EPA's forthcoming modeling guidance, States might
initially focus modeling assessments on these larger sources that have
been subject to permitting requirements and are generally better
characterized than smaller sources. But even this effort would entail a
substantial burden on States, under a compressed timeline following
EPA's issuance of further modeling guidance. Consequently, EPA does not
believe that for this new 1-hour SO2 NAAQS it would be
realistic or appropriate to expect States to complete such modeling and
incorporate the results in initial designation recommendations, which
under CAA section 107(d)(1)(A) must be submitted to EPA within 1 year
of the promulgation of the 1-hour standard.
The remaining issue, then, is how to most appropriately use a
modified hybrid approach, and its constituent modeling and monitoring
tools, in the implementation plan development process in order to
ensure expeditious attainment and maintenance of the NAAQS. Under the
CAA, all States must develop and submit to EPA State implementation
plans (SIPs) to attain and maintain the new 1-hour SO2
NAAQS. CAA section 110(a)(1) requires States, regardless of designation
status, to adopt SIPs that provide for implementation, maintenance and
enforcement of each primary NAAQS. Traditionally, for areas that were
designated ``attainment'' or ``unclassifiable'', we accepted State
submissions of prevention of significant deterioration (PSD) permitting
programs and other ``infrastructure'' SIP elements contained in CAA
section 110(a)(2) as being sufficient to satisfy the section 110(a)(1)
SIP submission requirement. However, due to our recognition here that
monitoring is not generally the most appropriate or effective tool for
assessing compliance with the new 1-hour SO2 NAAQS, that
additional guidance from EPA on conducting refined modeling for the new
1-hour NAAQS is anticipated to support our expected implementation
approach, and that considerable time and resources may be needed to
fully identify and properly characterize all SO2 sources
(including those emitting less than 100 tons of SO2 per
year) that may potentially cause or contribute to a violation of the
new SO2 NAAQS, we also had to assess how and when to best
use modeling as the primary method in implementation.
[[Page 35553]]
The approach that EPA expects to take, which is described in
sections V and VI of the preamble, is consistent with the language of
the Clean Air Act and would accommodate the time needed for an accurate
assessment of ambient air quality levels for the 1-hour SO2
standard. Section 107(d)(1) requires areas to be designated
``attainment'' if they meet the standard, ``nonattainment'' if they do
not meet the standard or contribute to a nearby violation, or
``unclassifiable'' if they cannot be designated on the basis of
available information. EPA's expected approach would enable us to make
the appropriate designation decision required by the CAA, based on the
record of information that will be before EPA regarding each area.
Areas would be designated ``nonattainment'' if either available
monitoring data or modeling shows that a violation exists, or
``attainment'' if both available monitoring data and modeling indicate
the area is attaining. All other areas would be designated
``unclassifiable,'' as required by section 107(d)(1)(A).
We currently anticipate that our projected post-designation
implementation approach would look to robust CAA section 110(a)(1)
SIPs, which have sometimes been previously referred to as
``maintenance'' or ``infrastructure'' SIPs but for the new
SO2 NAAQS would serve as substantive ``attainment'' SIPs.
Our current thinking is that, to be approved by EPA, such plans would
need to provide for attainment and maintenance of the new 1-hour
SO2 NAAQS as expeditiously as practicable, which we expect
to be no later than five years after initial designation (or
approximately August 2017) in all areas of the State, including any
area initially designated ``nonattainment,'' and also including any
area designated ``unclassifiable'' that has SO2 sources with
the potential to cause or contribute to a violation of the NAAQS. The
CAA establishes deadlines for States to submit these plans to EPA.\23\
State plans that address areas designated as ``nonattainment'' (i.e.,
``nonattainment area SIPs'') are due within 18 months from the
effective date of the designation, under CAA section 192. EPA
anticipates that this deadline would be February 2014. State plans
addressing all other areas (i.e., ``maintenance SIPs'') are due within
3 years following the promulgation of the new NAAQS, or June 2013,
under CAA section 110(a)(1).
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\23\ The schedule for State plans addressing areas designated
``nonattainment'' is governed by CAA section 191. The schedule for
State plans for all other areas, including areas designated
``unclassifiable'' and ``attainment,'' is governed by CAA section
110(a)(1).
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Section 110(a)(1), unlike section 192, does not specify a maximum
deadline by which States are required to show they have met the
requirements to implement, maintain, and enforce a NAAQS. EPA believes,
however, that August 2017 is the latest date by which areas should show
they have achieved attainment and maintenance of the standard because
this deadline is the same as would be required for areas designated
nonattainment in June 2012. It is therefore presumptively reasonable as
it is identical to the period Congress provided for nonattainment areas
to reach attainment. Moreover, EPA notes that the maintenance SIPs will
be due in June 2013, rather than in February 2014, giving States and
sources at least as much time between SIP development and submission
and the date by which attainment should be achieved as they would have
had the area been designated nonattainment in 2012. These section
110(a)(1) SIPs would be able to rely on modeling reflecting any
SO2 reductions that we expect to result before the
attainment date from compliance with the rules EPA expects to
promulgate before 2013, (including technology-based standards under CAA
section 112(d) for certain source categories emitting large amounts of
SO2 such as Electric Generating Units and industrial
boilers, and revised rules establishing further limits on
SO2 emitted by sources in upwind States which contribute
significantly to downstream States' inability to attain or maintain the
PM2.5 NAAS (the so-called Clean Air Interstate Replacement rule)).
Thus, we intend that a State's section 110(a)(1) SIP may account for
projected emissions reductions, including any from national and
regional rules that are promulgated before these SIP submissions,
provided that those reductions occur under a schedule that ensures
attainment as expeditiously as practicable. We expect that date to be
no later than 5 years from the date of initial designation or August
2017.
Under this anticipated approach, attainment SIPs for nonattainment
areas would have to include enforceable emissions limitations,
timetables for compliance, and appropriate testing/reporting to assure
compliance, and demonstrate attainment through air quality modeling for
all sources contributing to monitored and modeled violations, or that
have the potential to cause or contribute to a violation of the NAAQS.
The SIPs under section 110(a)(1) would need to demonstrate through
refined air quality modeling that any source or group of sources that
have the potential to cause or contribute to a violation of the NAAQS
are, or will be, sufficiently controlled to ensure timely attainment
and maintenance of the NAAQS. We would expect this to include any
individual sources with the potential to emit 100 or more tons per year
of SO2, and other sources that may also cause or contribute
to violations of the new SO2 NAAQS. We expect to develop
guidance for the States' use on how best to identify and assess the
impact of sources that may have this potential. As mentioned
previously, we intend to provide an opportunity for notice and comment
on this guidance before finalizing it.
EPA again notes that it anticipates several forthcoming national
and regional rules, such as the pending Industrial Boilers MACT
standard under CAA section 112(d), that are likely to require
significant reductions in SO2 emissions over the next
several years. A limited qualitative assessment based on the results of
preliminary modeling of some sample facilities indicates that well
controlled sources should meet the new SO2 NAAQS (see Brode
2010b). Exceptions could include unique sources with specific
characteristics that contribute to higher ambient impacts (short stack
heights, complex terrain, etc.). These national and regional rules are
expected to lead to SO2 reductions that will help achieve
compliance with the new SO2 NAAQS prior to 2017. If, upon
EPA review of submitted SIPs that rely upon those reductions or other
local controls, it appears that States will nevertheless fail to attain
the NAAQS as expeditiously as practicable (and no later than August
2017), the Clean Air Act provides authorities for EPA to solve such
failure, including, as appropriate, disapproving submitted SIPs, re-
designating unclassifiable areas to nonattainment, issuing SIP calls,
and promulgating FIPs.
For the reasons discussed above, EPA has determined that it is
appropriate and efficient to principally use modeling to assess
compliance for medium to larger sources, and to rely more on monitoring
for groups of smaller sources and sources not as conducive to modeling.
EPA's revised monitoring network requirements have been developed to be
consistent with this approach. However, EPA is still considering how
monitoring and modeling data would be used together in specific
situations to define attainment and nonattainment boundaries and under
what circumstances it may be appropriate to rely on monitoring data
alone to make attainment determinations. EPA intends
[[Page 35554]]
to address these issues as it develops implementation guidance.
In light of the new approach that EPA intends to take with respect
to implementation of the SO2 NAAQS, EPA intends to solicit
public comment on guidance regarding modeling, and also solicit public
comment on additional implementation planning guidance, including the
content of the maintenance plans required under section 110(a)(1) of
the Clean Air Act. EPA also notes that State monitoring plans and the
SIP submissions that States will make will also be subject to public
notice and comment.
IV. Amendments to Ambient Monitoring and Reporting Requirements
In this section of the preamble, we describe the proposal, the
public comments that we received on the proposed monitoring and
reporting requirements, and the final requirements for the
SO2 monitoring network. We are modifying our proposed
approach to the amount of monitoring to require following consideration
of public comments and a review of our historic practice in assessing
compliance with the SO2 NAAQS. As we explain above in
section III, we will use a hybrid approach that combines monitoring and
modeling, using each of these analytic tools where they are most
appropriate and effective. This approach and its requirements are
intended to support the revised SO2 NAAQS, described in
section II above. For a short-term 1-hour standard, dispersion modeling
of stationary sources will generally be more technically appropriate,
efficient, and effective because it takes into account fairly
infrequent combinations of meteorological and source operating
conditions that can contribute to peak ground-level concentrations of
SO2. Even an expansive monitoring network could fail to
identify all such locations. Consequently, we have revised the scope of
the monitoring network, reflecting a modified and expanded set of
objectives. This section also describes and explains the final
requirements for the new SO2 Federal Reference Method (FRM),
and the SO2 network design, monitoring objectives, data
reporting, and data quality objectives that support the revised primary
SO2 NAAQS.
A. Monitoring Methods
1. Requirements for SO2 Federal Reference Method (FRM)
The proposal to promulgate an automated SO2 FRM was
based on a need to update the cumbersome existing manual wet-chemistry
(pararosaniline) method to a continuous-type automated method that can
readily provide 1-hour SO2 measurement capability. See 74 FR
at 64846-849. The following paragraphs provide background, rationale,
and the final changes to the automated SO2 Federal Reference
Method (FRM) and to the associated performance specifications for
automated SO2 analyzers.
a. Proposed Ultraviolet Fluorescence SO2 FRM and Its
Implementation
FRMs, set forth in several appendices to 40 CFR Part 50, serve (1)
To provide a specified methodology for definitively measuring
concentrations of ambient air pollutants for comparison to the NAAQS in
Part 50, and (2) to provide a standard of comparison for determining
equivalency of alternative pollutant measurement methods that can be
used in lieu of the FRM for such monitoring.
The FRM for measuring SO2 in the ambient air was
promulgated on April 30, 1971 in conjunction with the first primary
SO2 NAAQS (36 FR 8196). This SO2 FRM is specified
in Appendix A of Part 50 and identified as the pararosaniline manual
method. See generally 74 FR at 64846. In the interim, EPA has
designated many SO2 methods as equivalent methods (FEMs),
most of which are based on the ultraviolet fluorescence (UVF) measuring
technique. Id. In fact, virtually all SO2 monitoring data
are now obtained with FEMs that use the UVF technique.
In light of this, EPA proposed to establish a new automated
SO2 FRM based on UVF--the same measurement technique
employed by FEM analyzers now in widespread use by most State and local
monitoring agencies and having the measurement capability needed to
implement the proposed 1-hour SO2 NAAQS. FRM analyzers using
this UVF technique can provide the needed detection limits, precision,
and accuracy and fulfill other purposes of an FRM, including use as an
appropriate standard of reference for testing and designation of new
FEM analyzers. At proposal, EPA specified the new method in
performance-based form, describing a generic reference measurement
principle and associated calibration procedure in a new Appendix A-1 to
40 CFR Part 50. Associated performance requirements applicable to
candidate automated SO2 analyzers (both FRMs and FEMs) were
proposed in 40 CFR Part 53.
EPA also proposed retaining the existing manual pararosaniline FRM
for SO2. Although EPA recognized that the existing method is
cumbersome for one-hour measurements, it is capable of making
measurements of 1 hour or even 30 minute periods. 74 FR at 64846; see
also Part 50 Appendix A at 1.1 (``[t]he method is applicable to the
measurement of ambient SO2 concentrations using sampling
periods ranging from 30 minutes to 24 hours''). Supersession of the
existing manual FRM, as defined in Sec. 53.16, would require not only
withdrawal of that existing FRM but also the cancellation of the
designations of all existing SO2 FEMs. Loss of the use of
these FEM analyzers would leave State and local monitoring agencies
with no approved SO2 monitors until new FRM and FEM
analyzers could be designated under the new FRM. The resulting costs
and disruptions to monitoring agencies is unnecessary because the
current SO2 FEMs readily and accurately measure (and report)
one-hour ambient measurements. See 74 FR at 64847. Accordingly, EPA
concluded that supersession of the existing FRM was not warranted,
given the costs and disruptions which would occur to State monitoring
programs and the limited benefits from such an action given the
suitability of the in-use FEMs. Id. at 68646; see also section
53.16(b)(1) stating that in exercising its discretion as to whether to
proceed with supersession of an FRM, EPA will consider the benefits (in
terms of requirements and purposes of the Act) from specifying a new
reference method, potential economic consequences of such supersession
for State and local monitoring agencies, and disruption to State and
local air quality monitoring programs. Instead, EPA proposed to add the
new UVF FRM while retaining the existing FRM for some period of time to
support the continued approval of existing SO2 FEM
analyzers.
b. Public Comments on the Proposed FRM and Implementation
EPA received comments from State and local groups (e.g., City of
Houston, Houston-Galveston Area Council, KY, NC, NY, PA, SC, SD, and
WI) and industry (e.g., AirQuality Research and Logistics (AQRL),
Consumers Energy, ExxonMobil, Montana Sulfur and Chemical Company, Inc.
(MSCC), and the Utility Air Regulatory Group (UARG)), all generally
supporting EPA's proposal to adopt the proposed automated UVF as an
FRM. For example, South Dakota supported adding the UVF SO2
method as an additional FRM and stated that this method is currently
being used in the network and will reduce the cost of implementing the
new monitoring
[[Page 35555]]
requirements for this rule. The UARG stated that the proposal to
specify a different FRM to judge compliance is entirely reasonable, and
UARG generally supported the proposed specifications for a new FRM but
maintained that the current FRM could not be used along with a new FRM.
ExxonMobil stated that it supports ``* * * EPA allowing monitoring
agencies to choose mobile monitoring that meets monitoring quality
requirements.'' AQRL stated that ``EPA is correct in choosing to
designate [promulgate] a new (automated) FRM for measurement of
SO2.''
EPA did not receive any public comments opposing the proposed
automated UVF SO2 FRM but did receive a few technical
comments on specific provisions of the method. EPA proposed use of an
inlet line particle filter as a requirement for new UVF SO2
FRM analyzers, believing that use of a particle filter is advantageous
to prevent interference, malfunction, or damage to the analyzer from
particles in the sampled air. The State of Missouri questioned this
requirement, noting that such a filter can sometimes cause problems and
that filter requirements for other FRM and FEM analyzers have been
analyzer-specific depending on the manufacturer's stipulation. EPA
believes, however, that for new SO2 FRM analyzers, the
benefits and uniformity provided by a mandatory filter requirement
outweigh possible disadvantages of such a filter.
Missouri also suggested that the language of proposed Sections
4.1.1 and 4.1.2 regarding calibration system flow rate requirements
were somewhat confusing, and that the high (50-100 ppm) concentration
requirement for the calibration standard specified in Section 4.1.6.1
is sometimes a problem. In response to these comments, the language of
Sections 4.1.1 and 4.1.2 has been clarified, and the concentration of
the standard specified in Section 4.1.6.1 has been reduced to 10 ppm.
EPA received a number of comments from States (e.g., NC, NYSDEC,
PA, SC, and SD) that supported the EPA proposed plan of temporary
retention of the existing wet-chemistry pararosaniline FRM and for FEMs
approved based on that method. For example, Pennsylvania stated
``[t]his methodology should enable State and local agencies to continue
using their existing monitoring equipment and [thereby] avoid large
capital fund outlays for samplers and ultimately avoid any delays in
collecting data that would be comparable to the proposed new primary
sulfur dioxide NAAQS.'' North Carolina requested ``* * * that the EPA
maintain the current reference method for at least an additional 10
years.'' Wisconsin and the Center for Biological Diversity (CBD)
suggested expeditiously phasing out the existing manual SO2
FRM.
In contrast, however, EPA also received comments from industry that
opposed the retention of the existing pararosaniline FRM while
promulgating a new automated UVF FRM. In particular, UARG stated ``* *
* having two FRMs specified for a given NAAQS--is not viable,''
pointing out that there is only one FRM for each NAAQS under the
present standards, a result UARG appears to believe is legally
mandated.
EPA disagrees with this comment. First, there is nothing in the Act
that mandates a single FRM for each NAAQS. Section 109 of the Act, in
fact, does not address this issue at all. Second, as noted previously,
there are sound policy reasons for not withdrawing the existing FRM at
this time. Therefore, EPA sees no legal or other obstacle in adding a
new automated UVF FRM while retaining the existing manual FRM.
UARG further maintained that EPA provided no support for its
statement that the existing FEMs, which constitute the bulk of the
existing SO2 monitoring network, are adequate for the
current and proposed new SO2 NAAQS. UARG also stated that
``although the FEMs may be adequate for many other purposes, they may
only be used to judge compliance with the 1-hour NAAQS if they are
shown to qualify as FRMs or FEMs under the new FRM definition.''
EPA disagrees with this comment also. In answer to UARG's second
point, it is not necessary that these existing FEMs be re-designated as
FRMs pursuant to the new automated FRM to continue their approved use.
There is no legal impediment to such continued use, since they are (and
will continue to be) FEMs approved based on an FRM that adequately
measures one-hour ambient SO2 concentrations. Nor is there
any technical impediment to the continued use of these FEMs, given that
they are automated continuous monitoring methods capable of measuring
SO2 concentrations ranging from a few minutes to a 1-hour
period. The existing FEMs in the network use the same UVF technology as
the proposed (and now final) automated FRM and have been reporting 1-
hour monitoring data for decades. These FRMs have been tested against
the test and performance requirements of Part 53, which are designed
specifically to test such continuous methods. Further, the proposed
SO2 method performance specifications for the standard
measurement range were derived from data submitted in FEM applications
for analyzers that were subsequently designated as FEMs. Therefore,
these FEMs are technically and legally sound to judge compliance with
the one-hour NAAQS.
EPA has clarified the regulatory text so that the rules state
unambiguously that both SO2 FRMs apply to the new one-hour
standard (as well as to the 24-hour and annual standards so long as
they are retained), as do all presently-designated FEMs.
c. Conclusions on Ultraviolet Fluorescence SO2 FRM and
Implementation
We are finalizing the proposed new automated SO2 FRM,
which is based on UVF technology, with the following minor technical
changes: The language of Sections 4.1.1 and 4.1.2 has been clarified,
and the minimum concentration of the calibration standard specified in
Section 4.1.6.1 has been reduced to 10 ppm. The new FRM is codified as
Appendix A-1 to 40 CFR Part 50 and titled ``Reference Measurement
Principle and Calibration Procedure for the Measurement of Sulfur
Dioxide in the Atmosphere (Ultraviolet Fluorescence Method).'' EPA is
retaining the previously existing manual pararosaniline SO2
FRM for the time being and re-codifying it as Appendix A-2 to 40 CFR
Part 50. However, EPA plans to rescind this manual FRM at a future time
when new SO2 FRM analyzers have adequately permeated State
monitoring networks.
2. Requirements for Automated SO2 Methods
a. Performance Specifications for Automated Methods
In association with the proposal to adopt a new automated FRM, EPA
proposed to update the performance-based designation requirements for
FEM SO2 analyzers currently specified in 40 CFR Part 53. As
noted in the proposal preamble (74 at 64846), these requirements were
established in the 1970's, based primarily on the wet-chemical
measurement technology available at that time. Those initial
requirements have become significantly outdated and need to be modified
to match current technology, particularly because they would apply to
new SO2 FRM analyzers under the proposed new FRM. The better
instrumental performance available with the proposed new UVF FRM
technique allows the performance requirements in Part 53 to be made
more stringent for
[[Page 35556]]
both FRM and FEM SO2 analyzers. Updating these performance
requirements is needed to ensure that, going forward, all new
SO2 monitors will have improved performance.
EPA solicited comments on the proposed new performance requirements
for automated SO2 methods that were included in Table B-1
(Performance Specifications for Automated Methods) of Part 53. We
proposed revised performance specifications for noise, lower detectable
limit, interference equivalent, zero drift, span drift, lag time, rise
time, fall time, and precision. EPA proposed to reduce the allowable
noise limit from 5 to 1 ppb, the lower detectable limit from 10 to 2
ppb, the interference equivalent limits from 20 ppb to
5 ppb for each interferent, and from 60 ppb to 20 ppb for
the total of all interferents, the zero drift limit from 20
to 4 ppb, the lag time limit from 20 to 2 minutes, both
rise and fall time limits from 15 to 2 minutes, and the precision
limits from 15 ppb to 2 percent of the upper range limit. EPA further
proposed to eliminate the requirements for span drift at 20% of the
upper range limit. In addition, to address the need for more sensitive,
lower measurement ranges for SO2 analyzers, EPA proposed a
separate set of performance requirements that would apply specifically
to narrower measurement ranges, i.e. ranges extending from zero to
concentrations less than 0.5 ppm. Other minor changes were proposed in
the wording of a few sections of Part 53 Subparts A and B, including
provision for alternate data recording devices in Sec. 53.21 to
supplement the older language relating specifically to strip chart
recorders.
b. Public Comments
EPA received a number of comments from industry (AQRL and UARG) and
from the multi-State organization NESCAUM regarding the proposed
interferent limit requirements listed in Table B-1. UARG submitted
comments supportive of all the proposed requirements for the new UVF
SO2 FRM, except for the proposed total interferent limits of
20 ppb. UARG acknowledged that EPA proposed to reduce the total
interferent level substantially from 60 ppb to 20 ppb, but maintained
that the proposed level of 20 ppb is still too high because it amounts
to 20%-40% of the levels being considered for the NAAQS (50-150 ppb).
AQRL recommended limiting ``* * * each interferent to no more than
3 ppb and total interference to no more than 12 ppb.''
NESCAUM recommended tightening the nitric oxide (NO) interference limit
from 100:1 to 300:1 (i.e., one third of the proposed value of 5 ppb). NESCAUM states that the proposed interferent value of
5 ppb results in substantial NO interference at sites with
low SO2 levels in urban areas.
EPA revisited the issue of the interferent equivalent limit for
SO2 analyzers in context of the above comments and
reconsidered what is reasonably feasible with current technology. We
reviewed the current instrument specifications and test data submitted
for numerous SO2 FEM applications. We also took into account
that the test concentrations of most of these interferents are
substantially higher than the concentrations normally observed in
ambient air. EPA considered lowering the testing concentrations of
these interferents, which would have correspondingly lowered the
interferent equivalent for each analyte. However, EPA took a more
conservative approach and retained the existing test concentrations for
H2S, NO2, NO, O3, m-xylene, and water
vapor. Based on this review, we found that it is not feasible to
further lower the limit requirement for these interferents below 5 ppb. However, in response to the NESCAUM comment, EPA
determined that the interferent equivalent limit requirement for NO
interference could be reduced to 3 ppb (166:1) for the new,
lower measurement range to reduce possible NO interference at sites
with low SO2 levels in urban area.
In regard to the total limit for all interferent equivalents for
SO2 analyzers, EPA notes that many of the interferents for
which testing is required (specified in Table B-3 of Part 53) would
likely react with each other and would thus not co-exist in ambient air
at the specified test concentrations. Therefore, EPA determined that
the limit requirement for total interference equivalent can be
eliminated, and Table B-1 now reflects this change.
EPA received comment from AQRL on the existing span drift
requirement for SO2 analyzers specified in Table B-1. AQRL
recommended lowering the span drift requirement at 80% URL to 2.5%,
stating that ``ambient air monitors in the 21st century should be able
to hold span drift to no more than 2.5% under the
conditions specified in EPA testing * * *.'' Based on information from
FEM testing laboratories and manufacturers' data (EPA, 2009c), EPA
largely agrees with this comment and concludes that the span drift
requirement at 80% can be lowered to 3%. Table B-1 has been
changed to include this revised limit.
EPA received comment from the State of Wisconsin suggesting that
the proposed revised provisions of section 53.21 (Test conditions) be
further changed to more specifically recognize use of digital recorders
for obtaining test results rather than maintaining the tie to analog
strip chart recorder technology. EPA acknowledges that industry has
moved away from strip chart recording technology to digital data
recording. However, the proposed language of Sec. 53.21 calls for a
graphic representation of analyzer responses to test concentrations to
facilitate visual examination of test results and allows any
``alternative measurement data recording device'' as long as it can
provide such a graphic representation. Describing the analog strip
chart recorder in this section provides an appropriate model to help
define the type of graphic representation needed for the Part 53 tests.
EPA believes that the proposed language of Sec. 53.21 is adequately
broad to permit digital or other types of data recording devices.
c. Conclusions for Performance Specifications for SO2
Automated Methods
Based on typical performance capabilities of current UVF analyzers
and manufacturers' actual testing data, we are keeping the limit for
each interference equivalent for SO2 analyzers at 5 ppb. However, we are lowering the interference equivalent
requirement for NO to 3 ppb for the lower measurement
range. A footnote denoting this specific requirement is being added to
Table B-1. We are eliminating the total interference equivalent
requirement for SO2 analyzers, and Table B-1 is being
revised to incorporate this change.
The 24-hour span drift at 80% of the upper range limit for
SO2 analyzers is being lowered to 3% in Table B-
1 to be in line with current technology. Also, unrelated to
SO2, a typographical error for the noise requirement for CO
analyzers is being corrected to 0.5 ppm in Table B-1.
Finally, information on generation and verification of test
concentrations for naphthalene was inadvertently omitted from Table B-
2, Test Atmospheres, even though it was added as a required interferent
test in our proposal. Therefore, we are adding that information for
naphthalene. Also in Table B-2, we are correcting the verification
information for nitric oxide.
B. Network Design
Ambient SO2 monitoring data are collected by State,
local, and Tribal monitoring agencies (``monitoring agencies'') in
accordance with the monitoring requirements contained in
[[Page 35557]]
40 CFR parts 50, 53, and 58. A monitoring network is generally designed
to measure, report, and provide related information on air quality data
as described in 40 CFR Part 58. To ensure that the data from the
network is accurate and reliable, the monitors in the network must meet
a number of requirements including the use of monitoring methods that
EPA has approved as Federal Reference Methods (FRMs) or Federal
Equivalent Methods (FEMs) (discussed in some detail above in section
IV.A), focusing on particular monitoring objectives, and following
specific siting criteria, data reporting, quality assurance and data
handling rules or procedures.
With the revision to the SO2 NAAQS, which establishes a
new 1-hour averaging period intended to limit short-term exposures that
may occur anywhere in an area, EPA evaluated the existing network to
determine if it was adequate to support the revised SO2
NAAQS. A significant fact for ambient SO2 concentrations is
that stationary sources are the predominant emission sources of
SO2 and the peak, maximum SO2 concentrations that
may occur are most likely to occur nearer the parent stationary source,
as noted in the ISA (ISA, 2-1), section II.A.1 above, and in section
IV.B.1 below. According to the 2005 National Emissions Inventory, there
are 32,288 sources (facilities) emitting SO2, of which 1,928
are emitting 100 tons per year (tpy) or more. In the proposal (74 FR
64851), EPA had anticipated requiring 348 source-oriented monitors in
the network design based on a population and emissions metric and a
State's emissions contribution to the National Emissions Inventory
(NEI). In response to this proposal, EPA received numerous comments
arguing that the required number of monitors in the network would be
too small. Other commenters argued that expanding the monitoring to an
adequate scale would impose a large burden and expense on the States.
Some commenters referred to SO2 modeling in their
submissions as an addition or alternative to monitoring. Consequently,
as part of developing a balanced response to these comments, we
revisited how we had historically dealt with SO2 for various
purposes including designations and implementation through permitting
and emissions limitations. As explained in section III, this has been
realized through a combined monitoring and modeling approach. As set
out below, and in sections III, VI, and VII, our ultimate intention is
to utilize a combined monitoring and modeling approach, a hybrid
analytic approach, to assess compliance with the revised SO2
NAAQS.
As a result of this contemplated hybrid analytic approach, the
minimum number of monitors required in the network through this
rulemaking is reduced to approximately 163 monitors from the
approximated 348 monitors that were proposed. This section of the
preamble includes a discussion of the proposal, the comments received,
and the details of and the rationale for the final changes to the
SO2 network design requirements.
1. Approach for Network Design
a. Proposed Approach for Network Design
To fully support the proposed revision to the SO2 NAAQS,
EPA indicated the need to identify where short-term, peak ground-level
concentrations--i.e., concentrations from 5 minutes to one hour (or
potentially up to 24 hours)--may occur. Given that large stationary
sources are the predominant source of emissions, monitoring short-term,
peak ground-level concentrations would require monitors to be sited to
assess impacts of individual or groups of sources and therefore be
source-oriented in nature. As a result, under a monitoring-focused
approach, EPA proposed a two-pronged monitoring network of all source-
oriented monitors. However, due to the multiple variables that affect
ground level SO2 concentrations from individual or groups of
sources, including stack heights, emission velocities, stack diameters,
terrain, and meteorology, EPA could not specify a source specific
threshold, algorithm, or metric by which to require monitoring. The
design of the proposed network represented a primarily monitoring-
focused approach to assess compliance with the primary SO2
NAAQS.
In preparation for the SO2 NAAQS proposal, EPA conducted
an analysis of the approximately 488 SO2 monitoring sites
operating during calendar year 2008 (Watkins and Thompson, 2009). This
analysis indicated that approximately ~ 35% of the monitoring network
was addressing locations of maximum (highest) concentrations, likely
linked to a specific source or group of sources. Meanwhile, just under
half (~ 46%) of the sites were reported to be for the assessment of
concentrations for general population exposure. These data allowed EPA
to conclude that the network \24\ was not properly focused to support
the revised NAAQS (under the assumption that source-oriented monitoring
data would be the primary tool for assessing compliance with the
NAAQS). As a result, EPA proposed a two-pronged monitoring network (74
FR 64850), based on the premise of a monitoring-focused approach, with
minimum requirements for: (1) Monitors in urban areas where there is a
higher coincidence of population and emissions, utilizing a Population
Weighted Emissions Index (PWEI), and (2) monitors in States based on
each State's contributions to the national SO2 emissions
inventory. In addition, all the monitors in the network would be sited
at locations of expected maximum hourly concentrations and therefore
likely be source-oriented. This two-pronged network would have resulted
in a minimum of approximately 348 monitors nationwide \25\ providing
data for comparison with the 1-hour standard and supporting its
implementation.
---------------------------------------------------------------------------
\24\ Prior to this rulemaking there were no minimum monitoring
requirements, except for those required at the multi-pollutant
National Core (NCore) monitoring sites. The monitoring rule
promulgated in 2006 (71 FR 61236) removed minimum monitoring
requirements (except for those NCore stations). This change was
largely driven by the fact that there was no longer an
SO2 nonattainment problem under the then-existing
standards. However, this logic does not apply to the revised primary
SO2 NAAQS.
\25\ Required monitor estimates were based on 2008 Census
estimates and the 2005 National Emissions Inventory.
---------------------------------------------------------------------------
Under the first prong of the network design, EPA proposed that the
ambient SO2 monitoring network account for SO2
exposure by requiring monitors in locations where population and
emissions may lead to higher potential for population exposure to peak
hourly SO2 concentrations. In order to do this, EPA
developed a Population Weighted Emissions Index (PWEI) that uses
population and emissions inventory data at the CBSA \26\ level to
assign required monitoring for a given CBSA (with population and
emissions being obvious relevant factors in prioritizing numbers of
required monitors). The PWEI for a particular CBSA was proposed to be
calculated by multiplying the population (using the latest Census
Bureau estimates) of a CBSA by the total amount of SO2
emissions in that CBSA. The CBSA SO2 emission value would be
in tons per year, and calculated by aggregating the county level
emissions for each county in a CBSA. We would then divide the resulting
product of CBSA population and CBSA SO2 emissions by
1,000,000 to provide a PWEI value, the units of
[[Page 35558]]
which would be millions of people-tons per year.
---------------------------------------------------------------------------
\26\ CBSAs are defined by the U.S. Census Bureau, and are
comprised of both Metropolitan Statistical Areas and Micropolitan
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------
We proposed that the first prong of the SO2 network
design require monitors in CBSAs, according to the following criteria.
For any CBSA with a calculated PWEI value equal to or greater than
1,000,000, a minimum of three SO2 monitors would be required
within that CBSA. For any CBSA with a calculated PWEI value equal to or
greater than 10,000, but less than 1,000,000, a minimum of two
SO2 monitors would be required within that CBSA. For any
CBSA with a calculated PWEI value equal to or greater than 5,000, but
less than 10,000, a minimum of one SO2 monitor would be
required within that CBSA. EPA estimated that the proposed criteria
would have resulted in 231 required sites in 131 CBSAs.
Under the second prong of the network design, EPA proposed to
require a monitor or monitors in each State, allocated by State-level
SO2 emissions. This prong of the network design was intended
to allow a portion of the overall required monitors to be placed where
needed, independent of the first prong of the network design, inside or
outside of CBSAs. EPA proposed to require monitors, using State
boundaries as the geographic unit for allocation purposes, in
proportion to a State's SO2 emissions, i.e., a State with
higher emissions would have been required to have a proportionally
higher number of monitors. The proposed percent contribution of
individual States would have been based on the most recent NEI, with
SO2 emissions being aggregated by State. The number of
required monitors per State would correspond to every one percent
(after rounding) of each State's contribution to the national
SO2 inventory. EPA also proposed that each State have at
least one monitor required as part of this second prong, even if a
particular State contributes less than 0.5% of the total anthropogenic
national emissions inventory. As a result, the proposed second prong
would have required approximately 117 monitoring sites based on State-
level SO2 emissions in the most recent NEI, which at the
time of the proposal, was the 2005 NEI.
EPA also stated in the proposal that the multi-pollutant National
Core (NCore) monitoring sites would not have counted towards meeting
the proposed monitoring requirements. However, data from the NCore
would be compared to the NAAQS even though NAAQS comparisons are not
the sole objective of NCore monitors. The monitoring rule promulgated
in 2006 (71 FR 61236) and codified at 40 CFR Part 58 and its Appendices
established the NCore multi-pollutant network requirement to support
integrated air quality management data needs. In particular, NCore
sites are intended to provide long-term data for air quality trends
analysis, model evaluation, and, for urban sites, tracking metropolitan
air quality statistics. To do this, NCore sites are required to measure
various pollutants, including SO2, but they are not source
oriented monitoring sites, and therefore are not likely to be the
location of maximum expected concentration in an area. NCore sites are
intended to provide data representing concentrations at the broader
neighborhood and urban spatial scales. These reasons were the rationale
justifying why SO2 monitors at NCore stations would not have
been part of the minimum monitors required under the proposed network.
b. Alternative Network Design
EPA also solicited comment on an alternative network design,
including alternative methods to determine the minimum number of
monitors per State (74 FR 64854). EPA requested comment on whether a
screening approach for assessing the likelihood of a NAAQS exceedance
could be developed and serve as a basis for determining the number and
location of required monitors. In particular, EPA requested comment on
whether it should utilize existing screening tools such as AERSCREEN or
SCREEN3, which use parameters such as effective stack height and
emissions levels to identify facilities with the potential to cause an
exceedance of the proposed standard. For that set of sources, EPA could
then require States to conduct more refined modeling (using the
American Meteorological Society (AMS)/EPA Regulatory Model (AERMOD)) to
determine locations where monitoring should be conducted. Any screening
or refined modeling would likely be carried out by States by using EPA
recommended models and techniques referenced by 40 CFR Part 51,
Appendix W, which provides guidance on air quality modeling. Such
screening or refined modeling uses facility emission tonnage, stack
heights, stack diameters, emission temperatures, emission velocities,
and accounts for local terrain and meteorology in determining where
expected maximum hourly concentrations may occur. In using this
approach, EPA would then require States to locate monitors at the point
of maximum concentration around sources identified as likely causing
NAAQS exceedances. EPA also noted that this alternative approach would
not distinctly use population as a factor for where monitors should be
placed.
c. Public Comments
EPA received many comments on the proposed network design and the
alternative network design approaches. Based on comments that were
clear enough on the issue, EPA believes the commenters' positions on
the network design approach generally fell into one of three
categories: (1) Those who supported the two-prong approach, but
suggested some modification to it, (2) those who supported the
alternative network design, and (3) those who suggested other concepts
for the network instead of the two approaches EPA presented in the
proposal.
The commenters who generally supported the two-prong network
design, but suggested some modification included some State and local
air agencies (e.g. NACAA and nine other State groups or agency
commenters) and industry groups (e.g. AQRL, ACC, and eight other
commenters). Of this group, some of the State and local air agencies
specifically commented on how EPA should modify one or both of the
prongs of the proposed network design. Some particular individual
suggestions will be addressed here and those comments not addressed
here will be addressed in the response to comment document. However,
one recurring suggestion from the State and local agency commenters in
this group was that the network design leads to some duplicative and/or
unneeded monitoring, and therefore they requested that EPA include a
provision to ``waive'' the monitoring network design requirements in
situations where minimum monitoring requirements appear duplicative or
unnecessary. In particular, NACAA stated that it ``* * * is concerned
that the two pronged approach in the proposed regulation will lead to
duplicative monitoring in some areas and require monitors in areas
where monitors are not needed. EPA recognizes the potential for
duplicative monitoring, but the proposal does not permit the removal of
duplicative monitors.'' This NACAA comment was echoed by some of the
other States who commented on the proposed approach (e.g. AK, FL, IL,
NC, SC, and WI). The industry commenters were also generally supportive
of the two-prong approach, with some making general suggestions to
modify the network design. For example, AQRL stated that the ``* * *
network design proposal seems to provide the flexibility for States and
the EPA regions to work together to arrive at the adequate monitoring
network.'' AQRL also
[[Page 35559]]
suggests that ``a State/local area should have the option to shutdown
or relocate any site mandated [by monitoring requirements] if measured
design values at the site are less than 75% of the selected standard
level.'' Multiple industry commenters (e.g. API, LEC, and RRI Energy)
expressed concern that the proposed network design had no monitoring
required specifically to measure background concentrations of
SO2. Dow Chemical suggested that EPA maintain some of the
existing monitors that characterize population exposure and other non-
source oriented sites for trends analysis.
Those commenters who did not support the proposed network design,
and instead generally supported the concepts of the alternative network
design, include public health and environmental groups (e.g. ALA, CBD,
EDF, EJ, NRDC, and SC) and the States of Delaware and Iowa. In
particular, ALA, EDF, NRDC, and SC stated ``* * * the proposed 348
monitors are a grossly inadequate number to detect peak concentrations
from the nearly 2,000 major sources that emit more than 100 tons per
year of sulfur dioxide * * *'' and that ``it is most appropriate to use
screening tools to site all the monitors in the areas of highest
expected concentration * * *'' The Center for Biological Diversity,
with regard to the proposed network design, stated that ``* * * a
number of communities with very significant SO2 emissions
will not have any monitoring stations at all * * *'' Further, the State
of Iowa claimed that ``the proposed design of the SO2
ambient monitoring network provides insufficient assurances that the
public is protected from the health effects of SO2
exposure,'' and suggested that ``* * * the final rule contain
provisions that require monitors to be sited only at locations where
dispersion modeling indicates that the NAAQS is violated.''
Commenters also suggested other concepts for the monitoring network
design in lieu of the approaches discussed in the proposal. NESCAUM,
NYSDEC, and PADEP, all suggested using an emissions-only approach to
trigger required monitoring instead of using the PWEI to require
monitors in an area. For example, NYSDEC suggests that the proposed
approach, using the PWEI, is ``* * * not more predictive than using
emissions data alone.'' NYSDEC went on to recommend that monitors be
required in CBSAs with aggregated emissions of 50,000 tons per year or
more and that ambient monitoring be considered for point sources with
20,000 tons per year. PADEP made several suggestions on network design,
including monitoring in any CBSA ``where there is a sulfur dioxide
source or combination of sources within 50 miles emitting a total of at
least 20,000 tons of SO2 per year * * *''
Among all three groups of commenters discussed above, there was a
subset of commenters who specifically mentioned using modeling in some
form. Modeling was a component of the alternative network design, where
monitors would be required based on screening models and possibly
refined modeling of individual sources. EPA also expected that under
the proposed approach, many States would use modeling as a quantitative
analysis tool to site required monitors. Finally, source modeling is a
critical element for PSD and facility permitting. In their comments,
NESCAUM recommended that EPA allow modeling to be used in conjunction
with monitoring data to better determine nonattainment areas. North
Carolina advocated that EPA require SO2 sources, without
specifying a threshold size for sources, to perform modeling to
demonstrate that fence-line (ambient) air does not exceed the NAAQS due
to that particular source's emissions. North Carolina went on to
suggest that if a source's modeling showed an exceedance of the NAAQS,
the source could ``then be required to reduce emissions from the stack,
install continuous emissions monitoring (CEM) in the stack itself, or
require a fence-line monitor at the target facility.'' North Carolina
also stated, in the context of discussing its own PSD program, that
``the costs for modeling are small compared to the costs for
monitoring.'' Sierra Club stated that EPA should ``* * * employ modern
computer models to determine whether areas should be designated
nonattainment because they do not meet the NAAQS in areas where there
is no monitor.'' From these comments, EPA gathers that some public
commenters find modeling a useful tool and support the use of modeling
to ascertain ambient concentrations of SO2.
2. Modeling Ambient SO2 Concentrations
EPA considered the various and sometimes competing concerns raised
by the commenters including duplicative monitoring, lack of adequate
number of monitors, insufficient flexibility, the monitoring burden,
and the modeling suggestions. EPA considered its historic practice and
the analytic tools available to arrive at a balanced approach that took
into account these concerns. In the past, EPA used a combination of
modeling and monitoring for SO2 during permitting,
designations, and re-designations in recognition of the fact that a
single monitoring site is generally not adequate to fully characterize
ambient concentrations, including the maximum ground level
concentrations, which exist around stationary SO2 sources.
With representative and appropriate meteorological and other input
data, refined dispersion models are able to characterize air quality
impacts from the modeled sources across the domain of interest on an
hourly basis with a high degree of spatial resolution, overcoming the
limitations of an approach based solely on monitoring. By simulating
plume dispersion on an hourly basis across a grid of receptor
locations, dispersion models are able to estimate the detailed spatial
gradients of ambient concentrations resulting from SO2
emission sources across a full range of meteorological and source
operating conditions. The 1-hour NAAQS is intended to provide
protection against short-term (5 minute to 24 hour) peak exposures,
whether they result from typical meteorological conditions or not.
Because ambient monitors are in fixed locations and a single monitor
can only represent impacts which occur at the location of the monitor,
a single monitor cannot identify all instances of peak ground-level
concentrations if, for example, different wind directions on various
days cause peak ground-level concentrations in different areas that do
not overlap. The uncertainty associated with this limitation is much
higher for an hourly standard than a long-term standard due to the
higher degree of spatial and temporal variability associated with peak
hourly impacts (discussed in ISA chapters 2.4 and 2.5). This limitation
of ambient monitoring may be true even if the source-oriented ambient
monitor was sited with the aid of modeling data, since the model is
less reliable at predicting the precise location of maximum impacts
than at predicting the distribution of impacts across the full modeling
domain, and no single monitor can be sited in a way to always measure
the peak ground-level SO2 concentrations that may be
occurring in the area around a source.
EPA's Guideline on Air Quality Models, Appendix W to 40 CFR Part
51, provides recommendations on modeling techniques and guidance for
estimating pollutant concentrations in order to assess control
strategies and determine emission limits. These recommendations were
originally published in April 1978 and were incorporated by reference
in the PSD regulations, 40 CFR sections 51.166 and
[[Page 35560]]
52.21 in June 1978 (43 FR 26382). The purpose of Appendix is to promote
consistency in the use of modeling within the air quality management
process. Appendix W is periodically revised to ensure that new model
developments or expanded regulatory requirements are incorporated. The
most recent revision to Appendix W was published on November 9, 2005
(70 FR 68218), wherein EPA adopted AERMOD as the preferred dispersion
model for a wide range of regulatory applications in all types of
terrain. AERMOD is a steady-state plume dispersion model that employs
hourly sequential preprocessed meteorological data to simulate
transport and dispersion from multiple point, area, or volume sources
for averaging times from one hour to multiple years, based on an
advanced characterization of the atmospheric boundary layer. AERMOD
also accounts for building wake effects (i.e., downwash) on plume
dispersion. To support the promulgation of AERMOD as the preferred
model for near-field dispersion (50 km or less), EPA evaluated the
performance of the model across a total of 17 field study data bases
(Perry, et al., 2005; EPA, 2003), including several field studies based
on model-to-monitor comparisons of SO2 concentrations from
operating power plants.
EPA anticipates that additional guidance for States may be needed
to clarify how to conduct dispersion modeling under Appendix W to
support the implementation of the new 1-hour SO2 NAAQS.
Although AERMOD is identified as the preferred model under Appendix W
for a wide range of applications and will be appropriate for most
modeling applications to support the new SO2 NAAQS, Appendix
W allows flexibility to consider the use of alternative models on a
case-by-case basis when an adequate demonstration can be made that the
alternative model performs better than, or is more appropriate than,
the preferred model for a particular application.
In conclusion, EPA believes that a hybrid analytic approach that
uses a combination of modeling and monitoring information addresses the
varying and competing concerns expressed by the commenters. Modeling
large emission sources, along with smaller sources with the potential
to violate the NAAQS, deals effectively with the concern that the
monitoring network is not large enough to account for all sources that
could have high ambient SO2 concentrations. EPA believes
that more SO2 sources will ultimately be directly addressed
through modeling alone versus the number of sources which would have
been monitored under the proposed network design (which proposed a
minimum of 348 monitors). Because modeling provides a technically
appropriate and efficient method to identify locations of maximum
concentrations attributable to the major stationary SO2
sources, in the final network design (discussed below in section
IV.B.4), EPA is not requiring that monitors must be in locations of
expected maximum concentration, and thus, typically source-oriented.
Instead, monitors required under the final network design now can
address multiple monitoring objectives (discussed in IV.B.3 below),
with fewer number of monitors required overall than the number
estimated in the proposal. The flexibility that States now have, where
relatively fewer required monitors may be sited to meet multiple
objectives, effectively addresses concerns about duplicative monitoring
and the need for waivers, the need for measuring background
concentrations, and that emissions data rather than the PWEI could be
more predictive of high ambient SO2 concentrations as a
basis on which to require monitoring. The comments that suggested the
use of modeling, along with an examination of past practice, resulted
in the change to a hybrid approach where we use both modeling and
monitoring to assess ambient SO2 concentrations.
3. Monitoring Objectives
Because EPA contemplates an ultimate approach that combines both
monitoring and modeling, the monitor objectives of the final network
design are now broadened to include assessment of source impacts,
highest concentration, population exposure, general background
concentrations, SO2 transport, and long-term trends. The
following paragraphs provide background, rationale, and details for the
final changes to monitoring objectives.
a. Proposed Monitoring Objectives
EPA proposed that all minimally required monitoring sites in the
proposed two-prong network design be sited at locations of expected
maximum 1-hour concentrations, which would also likely discern 5-minute
peaks. EPA noted that in general, such locations would be close to
larger emitting sources (in tons per year) and/or areas of relatively
high emissions densities where multiple sources may be contributing to
peak ground-level concentrations. As a result, the proposed monitoring
network would have been comprised primarily of source-oriented
monitors. EPA also proposed that when selecting monitoring sites from
among a pool of candidate locations (which would be source-oriented
under the proposed network design), States prioritize these sites based
on where the maximum expected hourly concentrations would occur in
greater proximity to populations. EPA solicited general comments on the
role of population exposure in the site selection process.
b. Public Comments
Commenters discussed a variety of issues on the subject of
monitoring objectives including the importance of considering
population exposure, the need for flexibility in monitor placement,
monitoring for background concentrations, monitoring for long term
trends analysis, and characterizing potential long-range transport of
SO2.
EPA received many comments from States (e.g., NACAA, DE, IL, IN,
MO, SD, WI), the public health group ATS, and industry (e.g., AQRL,
Consumers Energy, Dominion, Dow, EPRI, ExxonMobil, Montana Sulfur and
Chemical, NPRA, Portland Cement, Rio Tinto, and UARG) suggesting that
required monitors account for, or be focused on, population exposure.
EPA also received many comments from States (e.g., NACAA, NESCAUM, FL,
IL, IN, IA, MI, OH, SC, and WI) and industry (e.g., API, Dow, and
TxOGA) asking for more flexibility in (source-oriented) monitor
placement with regard to both the target source and the physical
location of a monitor relative to that source. For example NACAA stated
that ``for source oriented monitors, placement at the point of 1-hour
maximum concentration must be realistic and flexible. EPA must allow
agencies to determine the most scientifically defensible location,
while taking into account potential exposures and access to locations
with adequate siting.'' Wisconsin stated that ``* * * monitor siting
should be balanced toward population-based monitors with a preference
toward maximum exposure.'' Wisconsin added that ``* * * placing
monitors at the maximum downwind location does not necessarily result
in effective protection of public health.''
EPA received a number of comments on background monitoring \27\
from industry (API, LEC, and RRI Energy) and from the State of South
Carolina. API stated that ``because the monitors provide background
concentrations
[[Page 35561]]
needed to model impacts of new sources or sources undergoing major
modification in addition to providing data for judging compliance with
the NAAQS, it is important that some monitors be sited in a manner
suitable for assessing this background.'' API went on to state that ``*
* * EPA should encourage States to site an appropriate number of area-
wide monitors for use in establishing ambient background levels of
SO2.'' South Carolina states that ``to better support the
monitoring objectives, in particular those improving our understanding
and context for the source oriented monitoring data, the monitoring
requirements must include the ability for States to address the needs
for area and regional background concentration measurements.''
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\27\ Background monitoring can be considered to be
representative of ambient concentrations upwind of (and therefore
not typically influenced by) a geographic area such as an urban
area, or of an individual or group of emission sources.
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A number of commenters, including States (e.g., Missouri, NESCAUM,
Ohio, and South Carolina), citizens (Valley Watch at the Atlanta public
hearing), the CBD, and Dow, commented on SO2 transport and
related cross-boundary monitoring. Dow stated that ``SO2
distribution has long been known as an interstate issue with the vast
majority of SO2 sources being power plants and other fossil
fuel combustion facilities. These facilities are more likely to impact
distant areas than local areas and the resultant ground-level
concentrations are often minimal.'' Ohio stated that, under the
proposed approach, ``* * * it is likely that OH, WV, KY, and IN will
find sources along the Ohio River which could result in monitors being
located across the river from each other.'' In such situations, Ohio
asserts that ``States are capable of working with our neighbors to
determine which State would be in the best position to site and operate
a monitor.''
c. Conclusions on Monitoring Objectives
A hybrid analytical approach, as noted above in section III and
IV.B.1 would ultimately make the most appropriate use of available
tools such as modeling and monitoring. Thus, unlike under the proposal,
the monitoring network will not have to be focused solely at locations
of expected maximum concentration relative to an SO2 source
given the anticipated adoption of a hybrid analytical approach. The
final network design is intended to be flexible to meet multiple
monitoring objectives, most of which were identified in the public
comments. Ambient monitoring networks are generally designed to meet
three primary monitoring objectives, as listed in 40 CFR Part 58
Appendix D, Section 1, including: (1) Providing air pollution data to
the general public in a timely manner, (2) support compliance with
ambient air quality standards and emissions strategy development, and
(3) support air pollution research studies (which includes health
studies and research). In order to support these air quality management
objectives, monitoring networks can have a variety of monitoring sites
that can be sited, as necessary, to characterize (a) emission sources
(i.e., source-oriented monitoring), (b) the highest concentration in an
area, (c) population exposure, (d) general background concentrations,
(e) regional transport, and (f) welfare-based impact.
In light of the approach described in section III and further in
IV.B.1 above, EPA is finalizing an SO2 network design, with
broadened objectives, which EPA believes will address the concerns
noted in the public comments above, particularly those regarding siting
flexibility, population exposure, cross-boundary impacts, and the need
for the network to address multiple monitoring objectives. The final
network design requires that any SO2 monitors required in a
particular CBSA as determined based on PWEI values, discussed below in
section IV.B.4, shall satisfy the minimum monitoring requirements if
they are sited at locations where they can meet any one or more of the
following objectives (see Part 58 Appendix D section 4.4.2 as added by
today's final rule):
(1) Source-Oriented Monitoring: This is accomplished with a monitor
sited to determine the impact of significant sources or source
categories on air quality. In some situations, such monitoring sites
may also be classified as high concentration sites (discussed below).
Examples of source-oriented monitors include those sited to capture or
assess peak ground-level concentrations from one or more major
SO2 sources, or those sited in an area with multiple smaller
sources with overlapping plumes.
(2) Highest Concentration: This is assessed by a monitor sited to
measure the highest concentrations expected to occur in the area
covered by the network. Such a location may, or may not, also be
considered a source-oriented location (discussed above). Depending on
the case, this location is representative of the highest concentration
occurring across a relatively homogeneous area with spatial scales
typically ranging from tens of meters up to four kilometers.\28\
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\28\ Spatial scales are defined in 40 CFR Part 58 Appendix D,
section 1. Each scale is a description of the physical dimensions of
an air parcel nearest a monitoring site throughout which pollutant
concentrations are reasonably similar.
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(3) Population Exposure: This is assessed by a monitor sited to
measure typical concentrations in areas of (relatively) high population
density. Some examples are a monitor placed in an area of elevated or
high SO2 concentrations that also has a high population
density, an area that might be included in public health studies, or in
areas with vulnerable and susceptible populations.
(4) General Background: This is assessed by placing a monitor in an
area to determine general background concentrations. Such locations
might be considered to be representative of ambient concentrations
upwind of (and therefore not typically influenced by) a geographic area
such as an urban area, or of an individual or group of emission
sources. EPA notes that although a required monitor is allowed to be
sited to assess background concentrations, the required monitor is not
allowed to be sited outside of the parent CBSA (whose PWEI value
triggered required monitoring, discussed in section IV.B.4 and IV.B.5).
If a State believes that there is a need to conduct background
monitoring outside of CBSAs with required monitoring, EPA notes that
States always have the prerogative to conduct monitoring above the
minimum requirements in any location the State believes is appropriate.
(5) Regional Transport: This is assessed by placing a monitor in a
location to determine the extent of regional pollutant transport. Such
locations could be either upwind or downwind of urban areas,
characterizing the entry or exit of the pollutant in a region,
respectively. EPA notes that although a required monitor is allowed to
be sited to assess regional transport, the required monitor is not
allowed to be sited outside of the parent CBSA (whose PWEI value
triggered required monitoring, discussed in section IV.B.4 and IV.B.5).
If a State believes that there is a need to conduct background
monitoring outside of CBSAs with required monitoring, EPA notes that
States always have the prerogative to conduct monitoring above the
minimum requirements in any location the State believes is appropriate.
In regard to the public comments expressing concerns on the issue
of cross-boundary transport, i.e., a source on one side of a political
boundary contributes to peak ground-level concentrations on the other
side of that boundary, EPA will allow a required monitor to be placed
outside of the parent CBSA (whose PWEI value triggered monitoring,
discussed in section IV.B.4 and IV.B.5) under one
[[Page 35562]]
particular condition. A source-oriented monitor may be sited outside of
the parent CBSA, whose PWEI value triggered required monitoring, if
that monitor is characterizing the location of expected maximum
concentration of a source inside that parent CBSA. If a State chooses
to exercise this flexibility in source-oriented monitor siting, the
State must provide clear rationale for their choice in their annual
monitoring plan, which is subject to EPA regional approval. If the
source-oriented monitor is to be placed in another State, such as the
example provided by the State of Ohio in the public comments above, the
two States are responsible for collaboration on the location and
operation of that monitoring site.
Further, due to the broadened objectives of the final network
design, EPA also is finalizing the provision that an NCore
SO2 monitor within a CBSA (where a CBSAs PWEI value
triggered required monitoring) can be counted towards meeting the
minimum monitoring requirements in this rulemaking (discussed in
section IV.B.4) because they can meet some of the expanded objectives
of the network. NCore sites are intended to provide long-term data for
air quality trends analysis, model evaluation, and, for urban sites,
tracking metropolitan air quality statistics, and therefore are
appropriate to allow to count towards minimum monitoring requirements
under the revised monitoring scheme.
Finally, EPA strongly encourages State and local air agencies to
consider using required monitoring, as appropriate, to characterize
those sources which are not as conducive to dispersion modeling and to
assess population exposure. Sources that are not conducive to
dispersion modeling include (1) sources classified as non-point sources
(a.k.a. ``area-sources'') such as shipping ports, (2) a source situated
in an area of complex terrain and/or situated in a complex
meteorological regime, and (3) locations that have multiple, relatively
small sources with overlapping plumes.
4. Final Monitoring Network Design
The use of a hybrid analytic approach (discussed above in section
III and IV.B.1) makes it unnecessary for the final monitoring network
design to be distinctly focused on monitoring locations of expected
maximum concentration (and thus be primarily source-oriented), as
discussed in section IV.B.3 above. Instead, with the dual use of
modeling and monitoring for designations, the final monitoring network
is designed to provide flexibility for required monitors to address the
multiple monitoring objectives just discussed in the preceding section.
This flexibility in monitoring objectives is in response, in part, to
the many public comments received from States (e.g., NACAA and six
other States), industry (API, EPRI, UARG, and eight other groups), and
from the American Thoracic Society (ATS), urging EPA to ensure that
some or all of the required monitors be sited and suited to
characterize population exposure and, from many of these same
commenters, to allow flexibility in implementing the siting
requirements for the monitors. Under a hybrid approach, and the
different monitoring objectives resulting thereof, the final monitoring
network design also does not need to be a two-prong approach like the
one proposed. Therefore, EPA is adopting a modified version of the
first prong of the proposed network design, which will use PWEI values
to require monitors in certain CBSAs where there is increased
coincidence of population and SO2 emissions. There is no
second prong in the final network design by which monitors are required
based on a State's individual contribution to the national
anthropogenic SO2 inventory, as was proposed.
The final monitoring network design requires monitoring in CBSAs
based on calculated PWEI values, where a PWEI shall be calculated (as
discussed in section IV.B.5 below) for each CBSA. For any CBSA with a
calculated PWEI value equal to or greater than 1,000,000, a minimum of
three SO2 monitors are required within that CBSA. This
requirement remains the same as proposed. For any CBSA with a
calculated PWEI value equal to or greater than 100,000, but less than
1,000,000, a minimum of two SO2 monitors are required within
that CBSA. For any CBSA with a calculated PWEI value equal to or
greater than 5,000, but less than 100,000, a minimum of one
SO2 monitor is required within that CBSA. EPA has adjusted
the thresholds for requiring one or two monitors in a CBSA and the
rationale for this adjustment is explained more fully below in section
IV.B.5. As just explained in section III.B.3, these monitors shall be
sited to meet one or more of a number of monitoring site objectives,
including the assessment of source impacts, highest concentrations,
population exposure, general background, and regional transport. EPA
believes that the monitors required within these PWEI breakpoints
provide a reasonable minimum number of monitors in a CBSA, where there
is a relatively increased coincidence of population and SO2
emissions and therefore increased potential for exposures, because we
are directly accounting for both population and emissions that exist in
individual CBSAs.\29\ EPA estimates that these minimum monitoring
criteria (based on 2008 population and 2005 NEI data) require 163
monitors within 131 CBSAs. EPA also intends for SO2 monitors
at NCore stations to satisfy these minimum monitoring requirements.
Based on analysis of proposed and approved NCore sites (as of April
2010), all of which are scheduled to be operational no later than
January 1, 2011, EPA estimates that 52 of the total 80 SO2
monitors at NCore stations are within the 131 CBSAs that have required
monitors based on their PWEI values. As a result, EPA estimates that
between these minimum monitoring requirements and the NCore network,
there will be at least 191 SO2 monitors operating across the
country.
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\29\ The rationale for finalizing the use of the PWEI and the
number of monitors required through its application are discussed in
section III.B.4.
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5. Population Weighted Emissions Index
In the proposal, EPA had introduced a metric based on population
and emissions as a basis for locating monitors in the network. EPA
anticipated that this metric would characterize the potential for
exposure based on the proximity of source emissions to populations. The
following paragraphs provide background, rationale, and details for the
final changes of the calculation and use of the Population Weighted
Emissions Index in determining minimum monitoring requirements.
a. Proposed Use of the Population Weighted Emissions Index
In the proposed network design approach, which utilized a two-prong
network design, EPA created the Population Weighted Emissions Index
(PWEI) in an attempt to focus monitoring resource where there was a
higher proximity of population and SO2 emissions. In effect,
areas with higher PWEI values have higher potential for population
exposure to short-term SO2 emissions. EPA proposed that the
PWEI be calculated using population and emissions inventory data at the
Core Based Statistical Area (CBSA) \30\ level to assign required
monitoring for a given CBSA, with population and emissions being the
relevant factors. To calculate the PWEI for a particular CBSA, using
[[Page 35563]]
the latest Census Bureau estimates, the population of a CBSA must be
multiplied by the total amount of SO2 emissions in that
CBSA. The CBSA emission value is in tons per year (using the latest
available National Emissions Inventory [NEI] data), and is calculated
by aggregating the county level emissions for each county in a CBSA. We
then divide the resulting product of CBSA population and CBSA
SO2 emissions by 1,000,000 to provide a PWEI value in more
manageable units of millions of people-tons per year.
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\30\ CBSAs are defined by the U.S. Census Bureau, and are
comprised of both Metropolitan Statistical Areas and Micropolitan
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------
With the change in the approach discussed in section III and
section IV.B.1 above, and considering the final monitoring network
design discussed in IV.B.4 above, the use of the PWEI from that which
was proposed also changes. The following paragraphs discuss some of the
public comments received on the general use and calculation of the
PWEI; other comments that focused on the detailed application of the
PWEI as proposed will be addressed in the response to comments document
since our approach in applying the PWEI has changed.
b. Public Comments
EPA received a number of comments from State and local groups
(e.g., NACAA and eight others) and industry (e.g., AQRL, ACC, and eight
others) who generally agreed with the two-pronged network design
concept which had the PWEI as a component. More specifically, some
State commenters (e.g. NACAA, AK, FL, IL, NC, SC, and WI) expressed
concern that the PWEI (along with the second prong of the proposed
network design) created monitoring requirements that were
``duplicative'' and also called for monitors in areas where they were
not needed. Even amongst some of the commenters who generally agreed
with the PWEI concept, some provided examples of where the PWEI
appeared to be duplicative in its proposed application. One example was
provided by the State of Florida, ``in the case of Homosassa Springs,
the [proposed network design] requires two monitors [in that CBSA as a
result of the proposed use of the PWEI]. The driving source is the
Crystal River Power Plant, with emissions in 2008 of over 85,000 tons
per year of SO2. The next largest source in the CBSA has
emissions of roughly two tons per year.'' EPA believes that Florida is
asserting that the one large source disproportionately drove the PWEI
too high for that particular CBSA and only one monitor was actually
needed. EPA notes that these particular comments on duplicative
monitoring were made under the premise that all proposed required
monitors would be sited in locations of expected maximum concentration,
and therefore would be source-oriented in nature. As a result, these
commenters believed it was necessary that a waiver provision be
included if they could show that the required number of monitors was
too many, as in Florida's example.
As discussed in section IV.B.4 above, a hybrid approach results in
a final network design with a reduced number of required monitors from
the number proposed, a different application of the PWEI, and provides
flexibility in meeting additional monitoring objectives for the
required monitors, making the need for a waiver from the minimally
required monitors unnecessary. If a CBSA is required to have multiple
monitors now, those monitors are not specifically required to be
located near sources where maximum concentrations of SO2 are
expected to occur. Instead, they can be sited at different locations to
fulfill a variety of objectives, although, as noted in secion IV.B.3
above, EPA is strongly encouraging States to consider monitoring near
sources not conducive to dispersion modeling and for characterization
of population exposures.
EPA received comments from Michigan, South Carolina, and CBD
requesting clarification on the logic behind the proposed PWEI
thresholds, or breakpoints, by which three, two, one, or no monitors
would be required in a given CBSA. In addition, some States (e.g., MI,
MO, SC, and WI) and industry (e.g., LCA, LMOGA, and LPPA) suggested
specific adjustments to the proposed application of the PWEI. For
example, Michigan suggested that the required monitor breakpoint values
be adjusted to the ``natural breakpoints in the overall distribution''.
South Carolina suggested EPA identify a way to normalize the PWEI
stating the PWEI would be more appropriate ``* * * if it used a value
that better addressed difference in area, population distribution, land
use, number, types of sources, etc.''
In the proposed network design, EPA selected the PWEI values, or
breakpoints, to require one or more monitors based on the overall
distribution of PWEI values across all CBSAs. Based on U.S. Census
Bureau data (http://www.census.gov), there are approximately 939 CBSAs
in the country. EPA proposed and now requires that a PWEI value be
calculated for each of these CBSAs to determine if monitoring is
required in that CBSA. Based on 2008 census estimates and the 2005 NEI,
the average CBSA PWEI value is 21,900 while the median value is only
121. This indicates that a relatively small number of CBSAs with high
PWEI values are driving the very upper end of the PWEI distribution.
The proposed breakpoint where one monitor was required in a CBSA was a
PWEI value of 5,000. EPA estimated that 131 out of 939 CBSAs (~14%)
have a PWEI value of 5,000 or more. Further, these 131 CBSAs occupy
~98% of the sum of PWEI values across all 939 CBSAs, where high PWEI
values indicate increased coincidence in population and SO2
emissions. Within this group of CBSAs with PWEI values of 5,000 or
more, EPA considered the relative amounts of population, emissions, and
general frequency of occurrence of relatively larger SO2
sources (such as those that emit 100 tons per year or more) in
selecting the breakpoints to require two and three monitors in a CBSA
for the proposed network design. These considerations were made in an
effort to apply a nationally applicable process by which to require a
minimum number of monitors for an area, which all were to be sited in
locations of expected maximum concentration, and therefore likely
source-oriented monitors. In regard to the comments suggesting
modification to the calculation or to normalize the PWEI, EPA believes
that the proposed calculation, under a hybrid analytical approach, is
still most appropriate. Under a hybrid analytical approach, States have
the flexibility to move monitoring resources where needed within CBSAs
that have a high coincidence of population and emissions instead of
only being able to site monitors to characterize sources. States have
the option to consider additional factors such as those listed in South
Carolina's comments above in further identifying where required
monitoring may be most appropriate in their areas with required
monitoring.
Several States (e.g. NESCAUM, NYSDEC, and PADEP) suggested
abandoning the PWEI concept altogether and instead using some form of
emissions-only approach to require monitors. For example, NESCAUM, who
generally supported a ``hot-spot'' monitoring approach, suggested that
the PWEI be abandoned and EPA instead ``* * * adopt an emissions-only
approach, resulting in fewer CBSA monitors. We [NESCAUM] suggest a
threshold of 50,000 tpy CBSA SO2 emissions to trigger the
first CBSA monitor and a second CBSA monitor required when emissions
exceed 200,000 tpy.'' NESCAUM states that the proposed use of the PWEI
``* * * can
[[Page 35564]]
result in multiple monitors in large cities that have relatively small
CBSA SO2 emissions, or no monitor in a CBSA with large
emissions.'' NYSDEC suggests that the proposed approach, using the
PWEI, is ``* * * not more predictive than using emissions data alone.''
NYSDEC went on to suggest that monitors be required in CBSAs with
aggregated emissions of 50,000 tons per year or more and that ambient
monitoring be considered for point sources with 20,000 tons per year.
PADEP made several suggestions on network design, with one that
suggested monitoring in any CBSA ``where there is a sulfur dioxide
source or combination of sources within 50 miles emitting a total of at
least 20,000 tons of SO2 per year * * *''
EPA reviewed emissions and 2005 NEI data and compared the
suggestions provided by NESCAUM and NYSDEC to the requirement of the
final network design. Under NESCAUM's suggested design, EPA estimates
there would be 75 required monitors in 65 CBSAs. Of these 65 CBSAs, 6
CBSAs that are not covered by the final network design would be
included; however, 72 CBSAs that will have monitors under the final
network design would otherwise not have monitors under NESCAUM's
design. EPA believes that the exclusion of those 72 CBSAs would lead to
too sparse a network to adequately meet the monitoring objectives of
the network. Under NYSDEC's suggested network design, EPA estimates
that there would be a minimum of 65 monitors in the same 65 CBSAs of
the NESCAUM suggested design. Further, if States ensured that monitors
were placed near all sources emitting 20,000 tons per year (as NYSDEC
suggested should be ``considered'' for monitoring), there could be an
additional 69 monitors.\31\ EPA believes that the final network design
as discussed above in section IV.B.4, with the increased flexibility
for monitors to meet multiple monitoring objectives (discussed in
IV.B.3 above) including, among others, characterization of source
impacts or population exposure, is better served using PWEI values to
require monitors because it explicitly accounts for population to
require and distribute monitors as compared to an emissions-only
approach. If there is reason for concern that other CBSAs or areas not
included in the final network design, such as the six CBSAs that were
included in the NESCAUM and NYSDEC suggested network designs noted
above, warrant monitoring resources, States or the EPA Regional
Administrator may take action to require monitoring in such areas. The
authority of an EPA Regional Administrator to require additional
monitoring above the minimum requirements is discussed in section
IV.B.6 below.
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\31\ In simulating NYSDEC's suggested network design, EPA
assumed that no CBSA would have more than one monitor. According to
the 2005 NEI, there are 162 sources emitting 20,000 tpy or more a
year. 93 of those sources are estimated to be inside CBSAs that have
emissions of 50,000 tpy, leaving approximately 62 sources that would
need a monitor to satisfy NYSDEC's suggested network design.
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EPA received a number of comments from States (e.g., IA, NESCAUM,
NC, NYSDEC, SC, and WI) and industry (e.g., CE, Dominion, EEI, LCA,
LMOGA, LPPA, and UARG) raising concern over the way the PWEI is
calculated. Specifically, many commenters in this group indicated that
they believed that the 2005 NEI would be used in an exclusive or
permanent fashion to calculate the PWEI, and that updated NEI data
would not be used. For example, NESCAUM states that ``EPA should not
require States to rely solely on EPA's inventories [for calculating the
PWEI], such as the National Emissions Inventory (NEI), as they do not
always have the updated information that is necessary for such
regulatory decisions.'' Wisconsin ``* * * believes that States should
be allowed to use their own annual point source inventories instead of
EPA's National Emissions Inventory (NEI) for evaluating emission
sources. Wisconsin's point inventory is updated annually and has a
reporting threshold of five tons per year for SO2, making it
more sensitive to changes in facility operations than the NEI, which is
updated triennially.'' UARG stated that their ``primary concern with
this network design is its reliance on old emissions data. For electric
utilities which report their SO2 emissions to EPA annually,
the use of more recent data would be appropriate.''
EPA does not intend for relatively old emissions data to be used in
calculating the PWEI values for individual CBSAs. As was detailed in
the proposed regulatory text for 40 CFR Part 58 Appendix D (74 FR
64880), EPA stated that ``The PWEI shall be calculated by multiplying
the population of each CBSA, using the most current census data, by the
total amount of SO2 in tons per year emitted within the CBSA
area, using an aggregate of the most recent county level emissions data
available in the National Emissions Inventory for each county in each
CBSA.'' Although commenters suggested that there may be other resources
from which emissions data may be obtained, particularly at the
individual State level, the NEI is comprised of emissions data which is
collected by EPA from the States themselves. The Air Emissions
Reporting Requirements (40 CFR Part 51), by which EPA sets out how
States are to report their emission inventories, was recently revised
in December of 2008. That rulemaking was intended to provide enhanced
options to States for emissions data collection and exchange and unify
reporting dates for various categories of inventories. EPA notes that
the NEI is updated in full every three years and the 2008 NEI is
scheduled to be available by January 2011. States will have submitted
their data by May 31, 2010, before this rule is promulgated and
published, and EPA will provide comment on these submittals during the
summer of 2010. States will have an opportunity to revise their 2008
data submissions in the fall of 2010. In the triennial update, both
point and nonpoint data are required to be submitted by States and are
included in the inventory. Further, States are required to submit
emissions data annually for all sources emitting 2,500 tons per year or
more of SO2 as well as for sources emitting other pollutants
in excess of thresholds set for those pollutants. In all point source
submittals to the NEI, States are also allowed to submit emissions data
for sources of any emissions level, but are not required to do so.
Starting with the 2009 NEI, the annual and triennial State NEI
submittals will be due one year after the end of the emissions year.
States have an additional opportunity to revise their submittals based
on EPA comment in the spring of the following year, with EPA publishing
the inventory no later than 6 months after the inventory submittal
dates (18 months after the end of the emissions year). This approach
and schedule is accelerated over past NEI schedules and has been
designed as part of the development of the new Emission Inventory
System (EIS). Rather than representing old emissions data, the NEI
available through EIS represents a timely and appropriate source of
emissions data.
EPA believes that the process by which the NEI will be updated
(through use of the EIS) will be adjusted in a manner that will allow
for more frequent insertion of State supplied emissions data, allowing
for a more up-to-date inventory. EPA takes this opportunity to
encourage States to supply all of their available emissions information
to the NEI as soon as practicable. Therefore, EPA believes that the NEI
is an appropriate and nationally representative source of emissions
data by which PWEI calculations may be
[[Page 35565]]
made. PWEI calculations for all CBSAs will use the same year of data at
any given time, and States, local agencies, and Tribes will have
uniform opportunity for revising their emissions data for this purpose.
EPA again encourages States to view the NEI submittals as their
opportunity to submit their best available SO2 and other
inventory data with the knowledge that it will be used for the purpose
of PWEI values.
c. Conclusions on the Use of the Population Weighted Emissions Index
In the final network design, EPA has determined that it is
appropriate to use PWEI values as the mechanism by which to require
monitors in certain CBSAs, similar to its use in the first prong of the
proposed two-prong network design. EPA believes that using the PWEI
metric to inform where monitoring is required is more appropriate for
the SO2 network design than utilizing a population-only or
emissions-only type of approach, because it takes into account not just
one factor, i.e., only population or only emissions, but instead takes
into account the exposure from SO2 emissions to groups of
people who are in greater proximity to such emissions.
In the final rule, EPA is retaining the requirement to calculate
the PWEI by multiplying the population of each CBSA, using the most
current census data/estimates from the U.S. Census bureau, by the total
amount of SO2 in tons per year emitted within the CBSA area,
using an aggregate of county level emissions data available in the most
recent published version of the National Emissions Inventory for each
county in each CBSA. The resulting product shall be divided by one
million, providing a PWEI value, the units of which are million
persons-tons per year. For any CBSA with a calculated PWEI value equal
to or greater than 1,000,000, a minimum of three SO2
monitors are required within that CBSA. For any CBSA with a calculated
PWEI value equal to or greater than 100,000, but less than 1,000,000, a
minimum of two SO2 monitors are required within that CBSA.
For any CBSA with a calculated PWEI value equal to or greater than
5,000, but less than 100,000, a minimum of one SO2 monitor
is required within that CBSA. EPA believes that the monitors required
within these breakpoints provide a reasonable minimum number of
monitors in a CBSA that considers the combination of population and
emissions that exist in a CBSA. These criteria (based on 2008
population and 2005 NEI data) are estimated to require 163 monitors
within 131 CBSAs.
EPA has changed the PWEI breakpoint in the final rule at which two
monitors are required in a CBSA to 100,000 from the breakpoint of
10,000 in the proposed network design based on multiple considerations.
First, EPA changed the breakpoint because of a hybrid analytic approach
and attendant changes in monitoring objectives (see section IV.B.3),
with the result being that the monitoring network is no longer intended
to be comprised primarily of source-oriented monitors that are sited at
locations of expected maximum concentration. This change in objective
of the network design allows fewer monitors to provide the necessary
amount of ambient monitoring data EPA to meet the multiple monitoring
objectives. Second, the breakpoint of 100,000 occurs near a ``natural''
breakpoint in the PWEI distribution, a consideration that Michigan
suggested, where the estimated 28 CBSAs with PWEI values of 100,000 or
more occupy ~87% of the sum of PWEI values across all 939 CBSAs.
Finally, EPA considered commenters' assertion that the first prong of
the proposed network design created duplicative monitoring in certain
CBSAs. This duplicative monitoring is especially recognized in some
CBSAs with relatively small populations and somewhat large emissions
which are dominated by a single source (such as the Homosassa Springs,
FL example discussed above). Raising the second breakpoint helps to
alleviate some of the duplicative monitoring that many of the State
commenters noted.
EPA therefore is keeping the first and third breakpoints, which
require one monitor in a CBSA having a PWEI value of 5,000 and three
monitors in a CBSA having a PWEI value of 1,000,000. EPA believes
maintaining these breakpoints along with the revised 100,000 PWEI
breakpoint, will (1) ensure that highly populated areas will be
monitored for ambient SO2 concentrations even if the
emissions in that area are moderate, which is appropriate given the
fact that the greater population creates increased potential for
exposure to those moderate emissions, and (2) that those areas with
higher emissions or emission densities, with moderate or modest
populations will be monitored because those increased emissions are
likely to have a significant impact on nearby populations.
6. Regional Administrator Authority
The following paragraphs provide background, rationale, and details
for the final changes to Regional Administrator authority to use
discretion in requiring additional SO2 monitors beyond the
minimum network requirements.
a. Proposed Regional Administrator Authority
EPA proposed that the Regional Administrators will have discretion
to require monitoring above the minimum requirements, as necessary, to
address situations where the minimum monitoring requirements are not
sufficient to meet monitoring objectives. EPA recognized that the
minimum required monitors in the proposed two-pronged network design
were based on indicators that may not have always provided spatial
coverage for all the areas that have SO2 sources. Although
the network design and the objectives of the network design have
changed from those that were proposed because of our contemplated use
of a hybrid analytical approach, EPA believes it is still important for
Regional Administrators to have the discretion, and authority, to
require monitoring above the minimum requirements. Providing the RAs
with this discretion will allow them to fill any identified gaps in
meeting the monitoring objectives of the network.
b. Public Comments
Some commenters (e.g., LCA, LMOGA, LPPA, and South Carolina)
expressed concerns with the proposed provision authorizing the Regional
Administrator to require additional monitoring above the minimum
requirements. The LCA, LMOGA, and LPPA stated that ``the EPA's proposal
to allow the Regional Administrator discretion to require a State to
add additional monitors is flawed in that it provides unfettered
discretion. Criteria should be added * * * that limit such discretion
and require the Regional Administrator to consider certain objective
factors when determining whether to require any additional ambient
SO2 monitors to the network.'' South Carolina stated that
``the Regional Administrators should not have the discretion to require
monitoring above the requirements described in [the proposal for] Part
58 and its Appendices. State monitoring organizations must be given
discretion to decide the appropriate use of resources to meet uniform
monitoring requirements. Additional monitoring requirements should not
be imposed without concurrence of the monitoring organization and
additional funding that completely supports the additional costs.''
[[Page 35566]]
c. Conclusions on Regional Administrator Authority
The authority of Regional Administrators to require additional
monitoring above the minimum required is not unique to the
SO2 NAAQS. For example, Regional Administrators have the
authority to use their discretion to require additional NO2
or Pb monitors (40 CFR Part 58 Appendix D section 4.3.4 and 4.5,
respectively) and to work with State and local air agencies in
designing and/or maintaining an appropriate ozone monitoring network
(40 CFR Part 58 Appendix D section 4.1). EPA believes that the
nationally applicable final network design, although somewhat dictated
by local factors (population and emissions), may not account for all
locations where monitors should be sited, including where potentially
high concentrations of SO2 may be occurring. Examples
include locations that have the potential to violate or contribute to
violations of the NAAQS, areas that might have high concentrations of
SO2 that are not characterized by modeling or have sources
that are not conducive to modeling, and locations with susceptible and
vulnerable populations. As a result, EPA believes it is important for
Regional Administrators to have the authority to address possible gaps
in the minimally required monitoring network, especially near sources
or areas that are not conducive to modeling by granting them authority
to require monitoring above the minimum requirements. However, in
response to public comments, EPA notes that Regional Administrators
would use this authority in collaboration with State agencies to design
and/or maintain the most appropriate SO2 monitoring network
to meet the needs of a given area. For all the situations where the
Regional Administrators may require additional monitoring, it is
expected that the Regional Administrators will work on a case-by-case
basis with State or local air agencies. Further, any monitor required
through the Regional Administrator and selected by the State agency, or
any new monitor proposed by the State itself, is not done so with
unfettered discretion, since any such action would be included in the
Annual Monitoring Network Plan per Sec. 58.10, which must be made
available for public inspection or comment, and approval by the EPA
Regional Administrator.
Therefore, EPA is finalizing the proposal that Regional
Administrators may use their authority to require monitoring above the
minimum requirements, as necessary, in any area, to address situations
where the minimally required monitoring network is not sufficient to
meet monitoring objectives. In all cases in which a Regional
Administrator may consider the need for additional monitoring, it is
expected that the Regional Administrators will work with the State or
local air agencies to evaluate evidence or needs to determine if a
particular area may warrant additional monitoring.
7. Monitoring Network Implementation
The following paragraphs provide background, rationale, and details
for the final approach for the monitoring network implementation.
a. Proposed Monitoring Network Implementation
EPA proposed that State and, where appropriate, local air
monitoring agencies submit a plan for deploying SO2 monitors
in accordance with the proposed requirements discussed above by July 1,
2011. EPA also proposed that the SO2 network be physically
established no later than January 1, 2013. EPA also proposed that the
number of sites required to operate as a result of the Population
Weighted Emissions Index (PWEI) values calculated for each CBSA be
reviewed and revised for each CBSA through the 5-year network
assessment cycle required in Sec. 58.10.
b. Public Comments
EPA received comments from the ALA, EDF, NRDC, and SC that
supported ``* * * a more accelerated deployment of new monitoring than
the 2013 target date proposed by EPA. The sooner monitors are in place,
the sooner the public will experience the health benefits of the new
standard.'' However, EPA received comment from States (e.g., IA, MI,
NC, SC and WI), industry (e.g., LCA, LMOGA, and LPPA) and public health
and environmental groups (e.g., ALA, EDF, NRDC, and SC) expressing
concern with the proposed deployment schedule of the proposed
SO2 network in that it was too fast or needed to be phased
in. The States of Iowa, South Carolina, and Wisconsin suggested that
EPA allow the proposed network to deploy on a phased schedule. For
example, South Carolina recommended a ``phased implementation with
largest source/highest probability population exposure areas designated
for implementation in 2013 (some proportion of the highest PWEI
monitors) and establishment of the remaining PWEI and the State level
emissions triggered monitoring required the following year.''
Meanwhile, the States of Michigan and North Carolina, along with the
industry commenters LCA, LMOGA, and LPPA, suggested EPA reconsider
implementation dates in light of the multiple rulemakings that impose
mandates on States that have and will be occurring in the future. For
example, North Carolina stated that ``EPA must keep in mind that it is
simultaneously revising numerous ambient standards and associated
monitoring requirements. EPA seems to view each of these proposals as
independent actions; but the State and local agencies must consider the
cumulative impact of EPA's various regulatory actions on their ability
to comply.'' North Carolina goes on to say that ``EPA must allow States
the flexibility to prioritize among the new requirements to get
community based monitors in place first and to establish the others as
funding and personnel resources allow.''
EPA believes that with the use of a hybrid analytical approach, the
concerns raised by States and industry commenters suggesting a phased
or delayed implementation are addressed because the final network
minimum design requirements result in fewer monitors being required
than in the proposed network design. EPA's analysis of the existing
network had indicated that a substantial number of monitors were not
sited at locations of maximum concentrations. These monitors would have
had to be re-located to count towards minimum monitoring requirements
under the proposed monitoring-focused approach. Under a combined
modeling and monitoring approach, the required monitors can be used to
satisfy multiple monitoring objectives and therefore, many of the
monitors in the existing network will satisfy the requirements in the
final network design, eliminating any need for a phased or delayed
network implementation. In regard to the suggestion by public health
and environmental groups to speed up implementation, EPA notes that
under a hybrid analytical approach much of the existing network will
fulfill minimum monitoring requirements, and an accelerated schedule is
not necessary; the network implementation date provides a balance
between ensuring the minimally required network is fully in place in a
reasonable amount of time and providing States adequate time to fulfill
all the requirements in this rulemaking.\32\
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\32\ Moreover, as explained in section IV.A, the existing FEM
monitors in operation may continue to be used to monitor compliance
with the NAAQS.
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EPA received comment on the frequency by which the minimally
[[Page 35567]]
required network will be reviewed and possibly adjusted based on
updated population and emissions inventories. The State commenters
listed above, and some others including NACAA, indicated that they
believed that the proposal for reviewing the SO2 network
every five years was intended to be a separate review from the required
5-year network assessments required in Sec. 58.10(d). NACAA stated
``EPA proposes that the SO2 monitoring network be evaluated
every five years. This is an unnecessary duplication of effort in light
of the current requirements for the annual network plan and five year
network review.'' NACAA went on to say that ``the current requirements
[in Sec. 58.10] should be regarded as the primary source of monitoring
network information for all NAAQS pollutant monitoring, regardless of
the pollutant.''
EPA concurs with NACAA's statements that the existing requirements
for network assessment are an appropriate primary source of monitoring
network information. In the proposal, EPA did not intend for a required
5-year review of the SO2 network to be an additional effort
on top of the existing required network assessments but instead to be
included as part of the 5-year assessment in Sec. 58.10(d). EPA notes
that CBSA populations and emissions inventories change over time,
suggesting a need for periodic review of the monitoring network. At the
same time, EPA recognizes the advantages of a stable monitoring
network. However, after considering comments, EPA is not finalizing the
proposed language for 40 CFR Part 58 Appendix D, section 4.4.3(2) which
simply referenced back to Sec. 58.10. This proposed text it is not
needed and appears to simply cause confusion. EPA asserts that the
existing requirements in Sec. 58.10 provide a sufficient and
appropriate mechanism for network updates and assessment.
c. Conclusions on Monitoring Network Implementation
Based on the public comments, and due to the contemplated use of a
hybrid analytical approach, EPA is finalizing, as was proposed, that
State and, where appropriate, local air monitoring agencies submit a
plan for deploying SO2 monitors in accordance with the
proposed requirements presented below by July 1, 2011. Minimally
required SO2 monitors shall be physically established no
later than January 1, 2013.
C. Data Reporting
The following paragraphs provide background, rationale, and details
for monitor data reporting requirements.
a. Proposed Data Reporting
Controlled human exposure studies indicate that exposures to peaks
of SO2 on the order of 5 to 10 minutes result in moderate or
greater decrements in lung function and/or respiratory symptoms in
exercising asthmatics (section II.B.1 above, ISA section 5.2, REA
section 7.2.3, and REA section 10.3.3.2). As a result, the 1-hour
standard is intended to protect against short term exposures, including
exposures on the order of 5 minutes up to 24 hours, as is discussed in
section II.F.2 above. Therefore, in support of the revised NAAQS and
its intent, EPA proposed that State and local agencies shall report to
AQS the maximum 5-minute block average of the twelve 5-minute block
averages of SO2 for each hour. This 5-minute block reporting
requirement is in addition to the existing requirement to report the 1-
hour average. In addition, EPA solicited comment on the advantages and
disadvantages (including associated resource burdens) of alternatively
requiring State and local agencies to report all twelve 5-minute
SO2 values for each hour or the maximum 5-minute
concentration in an hour based on a moving 5-minute averaging period
rather than time block averaging.
EPA also proposed Data Quality Objectives (DQOs) for the
SO2 network. DQOs generally specify the tolerable levels for
potential decision error used as a basis for establishing the quality
and quantity of data needed to support the objectives of the monitors.
EPA proposed the goal for acceptable measurement uncertainty for
SO2 methods to be defined as an upper 90 percent confidence
limit for the coefficient of variation (CV) of 15 percent for precision
and as an upper 95 percent confidence limit for the absolute bias of 15
percent for bias.
b. Public Comments
EPA received many comments on the reporting of 5-minute data
values. The comments generally fell into one of the following
categories: \33\ (1) Those State, public health, and environmental
groups who supported the proposed requirement to report the maximum 5-
minute block average of the twelve 5-minute block averages of
SO2 for each hour (e.g., Missouri, NESCAUM, North Carolina,
ALA, EJ, EDF, NRDC, and SC), (2) those State, public health, and
environmental groups who supported the reporting of all twelve 5-minute
averages of each hour (e.g., Kentucky, NYSDEC, AQRL, ALA, ATS, CBD, EJ,
EDF, NRDC, and SC), (3) those State, public health, and environmental
groups who supported reporting the maximum 5-minute concentration in an
hour based on a moving 5-minute average (e.g., South Dakota, ALA, CBD,
EJ, EDF, NRDC, and SC), and (4) those State and industry groups who did
not support the reporting of any 5-minute data (e.g., Iowa, South
Carolina, LEC, and RRI Energy).
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\33\ Note that some commenters supported more than one form of
reported 5-minute data.
---------------------------------------------------------------------------
Public health and environmental groups (e.g. ALA, CBD, EJ, EDF,
NRDC, and SC) supported an approach where 5-minute data must be
reported. However, these commenters were flexible in their position and
supported multiple forms or types of 5-minute data reporting. The ALA,
EJ, EDF, NRDC, and SC stated that ``we support the proposed requirement
for State and local monitoring agencies to report both hourly average
and maximum 5-minute averages out of the twelve 5-minute block averages
of SO2 for each hour.'' They also expressed a preference for
alternative 5-minute data reporting stating that they ``strongly prefer
that States be required to report the peak 5-minute concentrations of
SO2 based on a rolling average.'' Similarly, CBD stated that
``* * * EPA should require that State and local agencies report all 12
five-minute SO2 values for each hour in addition to 1-hour
averages. Where possible, EPA also should require reporting of rolling
five-minute averages rather than block data * * *''
Missouri generally supported the proposed requirement to report the
maximum 5-minute average in the hour, saying ``it is not a problem to
report both the hourly average and the maximum 5-minute block
average.'' Nevertheless, Missouri went on to note constraints, stating
that ``* * * [their] data logger and associated software do not have
the capability to report all twelve 5-minute SO2 values for
each hour'' and that they ``* * * could not do this without software
being developed for this purpose and it could be time intensive to
validate this data.''
Kentucky did not support the proposal to report the maximum 5-
minute data block in the hour because of the limitations in their data
acquisition systems. They explained that ``the data acquisition system
used by the [State] does not have the capability to automatically
report the maximum 5-minute block of data from an hour concentration.
[State] personnel would have to manually determine that
[[Page 35568]]
value and then manually enter that data into AQS.'' Kentucky goes on to
suggest that ``the only feasible option for the [State] to submit 5-
minute data to AQS would be to submit all twelve 5-minute blocks of
data for each hour to AQS.''
South Dakota stated that its ``* * * preference would be to report
the maximum 5-minute average for each hour calculated using a 5-minute
rolling average.'' South Dakota goes on to state that ``* * * while
doubling the work required to validate data and load the data into AQS,
the additional data should help determine if the selected standard
concentration level has achieved the necessary reduction in high
concentration 5-minute levels and provide the necessary data for
further study of health impacts * * *''
South Carolina stated that it ``* * * does not support mandatory
reporting of 5-minute averages in addition to the 1-hour average
required for comparison to the standard. The validation and reporting
of 5-minute averages imposes a significant additional burden on the
reporting organization and its Quality System.'' Iowa, who also did not
support any form of 5-minute data reporting stated that ``the five-
minute data is not used to determine compliance with the NAAQS, and
represents ancillary data,'' and that ``validating and uploading the
five-minute data will take at least as much staff time as generating
the hourly data used for compliance.'' As a result, Iowa states that
``if EPA determines that five-minute data is needed, we recommend that
EPA require the maximum five-minute average in each hour, rather than
all twelve five-minute averages, in order to reduce the burden
associated with generation of the ancillary data set.''
With regard to the proposed DQOs, EPA received comments from some
States (e.g., Kentucky, North Carolina, NYSDEC, and South Carolina)
providing general support for the goals for acceptable measurement
uncertainty for precision and bias. North Carolina stated that the ``*
* * precision and bias measurement uncertainty criteria should emulate
those that have been established for other recent NAAQS and NCore
pollutants.'' NYSDEC stated that ``the proposal does not seem
unreasonable, however these statistics are now expressed in terms of
confidence limits: Precision--90% confidence of a CV of 15% and Bias--
95% confidence of a CV of 15%.'' NYSDEC raises concern that ``* * * the
results are now dependent on the number of audits performed. This is
highly variable because some agencies run automatic audits every night,
[while] others use the old standard of once every 2 weeks.''
In regard to comments on the proposed DQOs, EPA notes that the
precision and bias estimation technique on which NYSDEC comments were
focused were proposed and adopted in the monitoring rule promulgated on
October 6, 2006 and EPA did not intend to reopen those requirements for
comment. Moreover, SO2 precision and bias estimates have
been performed in this manner for the past four years and there have
been no adverse effects on data quality at the minimum required level
of performance checks every two weeks. The statistics for the precision
and bias estimates and the DQO goals are based on the accumulation of
the one-point precision checks aggregated at the frequencies required
in CFR which is every two weeks. Any organization performing more
frequent checks (such as every night) would accumulate more data for
the precision and bias estimates, have higher confidence in the data,
and would have less potential for outliers or higher than normal values
effecting the precision and bias estimate. In addition, monitoring
organizations running precision checks every 24 hours would be more
able to control data quality to meet the DQO goals than organizations
running the check every two weeks.
c. Conclusions on Data Reporting
EPA received a fairly diverse set of comments on the
appropriateness of reporting 5-minute data and in what particular
format it may be provided in. EPA has considered the comments by the
States regarding validation of potentially 13 data values per hour
(instead of 1 or 2) and some States' lack of data acquisition capacity
or processing capability to report any particular type of 5-minute
value. EPA believes that in light of these comments, adopting a
requirement for continuous SO2 analyzers to report all
twelve 5-minute values or a rolling 5-minute value does not appear to
provide enough added value for the potential increased burden on
States, such as increased staff time dedicated to data processing and
QA, or in improving or adjusting data acquisition capabilities.
However, EPA also believes that obtaining some form of 5-minute data is
appropriate because such data have been critical to this NAAQS review,
and are anticipated to be of high value to inform future health studies
and, subsequently, future SO2 NAAQS reviews.\34\ Indeed, as
noted earlier, it was EPA's failure to adequately explain the absence
of protection from elevated short-term (5- to 10-minute exposure)
SO2 concentrations for heavily breathing asthmatics that
occasioned the remand of the 1996 SO2 primary NAAQS
(American Lung Association, 134 F.3d at 392). This belief is supported
further by the expectation that a significant portion of the monitors
operating to satisfy the final monitoring network design will likely be
sited for population exposures, which have traditionally provided
ambient data that is often utilized by epidemiologic health studies.
Therefore, EPA is finalizing the requirement that State and local air
agencies operating continuous SO2 analyzers shall report the
maximum 5-minute block average out of the twelve 5-minute block
averages in each hour, for each hour of the day, and that State and
local air agencies operating any type of SO2 analyzer shall
report the integrated 1-hour average value, as was proposed. EPA
encourages States capable of reporting all twelve 5-minute data blocks
in an hour to report such data to AQS. AQS is currently set-up to take
the 5-minute maximum value in an hour under parameter code 42406 and
can take all twelve 5-minute values under parameter code 42401 (with a
duration code of H). EPA notes that if a State were to choose to submit
all twelve 5-minute blocks in the hour, by default, they would be
submitting the maximum 5-minute data block within that hour, although
they have not singled out that particular value. Since the 5-minute
data is not directly being used for comparison to the NAAQS, EPA
believes that any State electing to submit all twelve 5-minute values
is still satisfying the intent of having the maximum 5-minute value
reported. Therefore, if a State chooses to submit all twelve 5-minute
values in an hour, they will be considered to be satisfying the data
reporting requirement of submitting the maximum 5-minute value in an
hour, and they do not have to separately report the maximum 5-minute
value from within that set of data values to AQS under parameter code
42406.
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\34\ The REA assessed exposure and risks associated with 5-
minute SO2 concentrations above 5-minute health effect
benchmark levels derived from controlled human exposure studies. In
the analyses, the REA noted that very few State and local agencies
report ambient 5-minute SO2 data (REA, section 10.3.3.2)
and that the lack of 5-minute data necessitated the use of
statistically estimated 5-minute SO2 data in order to
expand the geographic scope of the exposure and risk analyses (REA,
section 7.2.3).
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EPA proposed new regulation text for 40 CFR Part 58 Appendix C,
which would have added section 2.1.2 that would have required any
SO2 FRM or
[[Page 35569]]
FEM used for making NAAQS decisions to be capable of providing both 1-
hour and 5-minute averaged concentration data. EPA is not finalizing
this proposed language, as the manual wet-chemistry pararosaniline
reference method cannot provide 5-minute data. Therefore, the proposed
language is inappropriate. However, both the UVF FEM and the new UVF
FRM continuous methods are capable of providing 5-minute averaged data.
As a result, the language in 58.12(g) and 58.16(g) requiring 5-minute
SO2 data has been adjusted to appropriately specify that
only those States operating continuous FRM or FEMs are required to
report the maximum 5-minute data value for each hour.
With regard to acceptable measurement uncertainties, EPA reviewed
summary data for each Primary Quality Assurance Organization (PQAO) in
the 2008 Data Quality Indicator Report on SO2 data within
the 2008 Criteria Pollutant Quality Indicator Summary Report for AQS
Data (http://www.epa.gov/ttn/amtic/qareport.html). Of the 100 PQAOs in
the report, none of those organizations had summary CV or bias values
exceeding 10 percent. Thus, EPA believes that the SO2
network can and does easily attain measurement uncertainty criteria
more stringent than the finalized goal values and the monitoring
required under the final network design should be able to maintain this
level of performance. Therefore, in consideration of comments and
existing quality assurance data, EPA is changing the final goals from
those which were proposed for acceptable measurement uncertainty for
SO2 methods to be defined for precision as an upper 90
percent confidence limit for the coefficient of variation (CV) of 10
percent and for bias as an upper 95 percent confidence limit for the
absolute bias of 10 percent.
V. Initial Designation of Areas for the 1-Hour SO2 NAAQS
This section of the preamble further addresses the process under
which EPA intends to identify whether areas of the country attain or do
not attain or are ``unclassifiable'' regarding the new 1-hour
SO2 NAAQS. After EPA establishes a new NAAQS, the CAA
directs States and EPA to take this first step, known as the ``initial
area designations,'' in ensuring that the NAAQS is ultimately attained.
We are revising our discussion of an expected approach toward
issuing initial area designations in response to comments we received
on the proposed rule's treatment of monitoring and modeling (both
generally and in the specific context of designations), and to make the
expected process more consistent with our historical approach to
implementing the SO2 NAAQS. A revised anticipated approach
for issuing designations logically follows from our revised hybrid
approach to monitoring and modeling as discussed above in sections III
and IV. It would also affect a revised expected implementation approach
that we later discuss in section VI. 1. Designations.
a. Clean Air Act Requirements
The CAA requires EPA and the States to take steps to ensure that
the new NAAQS are met following promulgation. The first step is for EPA
to identify whether areas of the country meet, do not meet, or cannot
yet be classified as either meeting or not meeting the new NAAQS.
Section 107(d)(1)(A) provides that, ``By such date as the Administrator
may reasonably require, but not later than 1 year after promulgation of
a new or revised NAAQS for any pollutant under section 109, the
Governor of each State shall * * * submit to the Administrator a list
of all areas (or portions thereof) in the State'' that should be
designated as nonattainment, attainment, or unclassifiable for the new
NAAQS. 42 U.S.C. 7407(d)(1)(A)(i)-(iii). Section 107(d)(1)(B)(i)
further provides, ``Upon promulgation or revision of a NAAQS, the
Administrator shall promulgate the designations of all areas (or
portions thereof) * * * as expeditiously as practicable, but in no case
later than 2 years from the date of promulgation. Such period may be
extended for up to one year in the event the Administrator has
insufficient information to promulgate the designations within 2
years.'' 42 U.S.C. 7407(d)(1)(B)(i).
Under CAA section 107(d)(1)(B)(ii), no later than 120 days prior to
promulgating designations, EPA is required to notify States of any
intended modifications to their boundaries as EPA may deem necessary,
and States will have an opportunity to comment on EPA's tentative
decision. Whether or not a State provides a recommendation, the EPA
must promulgate the designation that it deems appropriate. 42 U.S.C.
7407(d)(1)(B)(ii).
Accordingly, since the new 1-hour SO2 NAAQS is being
promulgated today, Governors should submit their initial SO2
designation recommendations to EPA no later than June 2, 2011. If the
Administrator intends to modify any State's boundary recommendation,
the EPA will notify the Governor no later than 120 days prior to
designations or, February 2012. States that believe the Administrator's
modification is inappropriate will have an opportunity to demonstrate
why they believe their recommendation is more appropriate before
designations are finalized in June 2012.
For initial designations that will be finalized in June 2012,
States should use monitoring data from the existing SO2
network for the years 2008-2010, as well as any refined SO2
dispersion modeling (see Appendix W to 40 CFR Part 51) for sources that
may have the potential to cause or contribute to a NAAQS violation,
provided that it is recent and available. EPA will then issue
designations based on the record of information for that area. Under
our anticipated approach, an area that has monitoring data or refined
modeling results showing a violation of the NAAQS would be designated
as ``nonattainment.'' An area that has both monitoring data and
appropriate modeling results showing no violations would be designated
as ``attainment.'' All other areas, including those with SO2
monitors showing no violations but without modeling showing no
violations, would be designated as ``unclassifiable.'' Areas with no
SO2 monitors at all i.e., ``rest of State,'' would be
designated as ``unclassifiable'' as well.
b. Approach Described in Proposal
In the proposed rule's preamble, we explained that we had proposed
a new SO2 ambient monitoring network, with new monitors
expected to be deployed no later than January 2013. We also explained
that we expected compliance with the new NAAQS to be determined based
on 3 years of complete, quality assured, certified monitoring data. We
further explained that we did not expect newly-cited monitors for the
proposed network to generate sufficient monitoring data for us to use
in determining whether areas complied with the new NAAQS by the
statutory deadline to complete initial designations. Therefore, we
explained, we intended to complete designations by June 2012 based on 3
years of complete, quality assured, certified air quality monitoring
data as generated from the current monitoring network.
Consequently, we discussed our expectations to base initial
designations on air quality data from the years 2008-2010 or 2009-2011,
from SO2 monitors operating at current locations, which we
expected to continue through 2011. While those monitors are generally
sited to measure 24-hour and annual average SO2
concentrations, we noted that they all report hourly data, and we
estimated that at least one third of those monitors might meet the
proposed network
[[Page 35570]]
design requirements and not need to be moved. We explained that if any
monitor in the current network indicated a violation of the new 1-hour
NAAQS, we would intend to designate the area as ``nonattainment.'' We
further explained that if a monitor did not indicate a violation, our
designation decision for the area would be made on a case-by-case
basis, with one possibility being a designation of ``unclassifiable.''
We also explained that while the CAA section 107 designation
provisions specifically address States, we intended to follow the same
process for Tribes to the extent practicable, pursuant to CAA section
301(d), 42 U.S.C. 7601(d), and the Tribal Authority Rule, 40 CFR part
49.
c. Comments
Several commenters stated that the EPA did not provide
nonattainment boundary guidance in the proposed rule and argued that
guidance should be developed. Commenters also stated that EPA should
consider boundaries that are less than the Core Based Statistical Area
(CBSA), and perhaps even smaller than the county boundary (State of
Michigan, Sierra Club).
In response, we note that the CAA requires that the EPA designate
as ``nonattainment'' any area that does not meet (or contributes to an
area that does not meet) the NAAQS. 42 U.S.C. 7407(d)(1)(A)(i). States
with monitored or modeled SO2 violations will need to
recommend an appropriate nonattainment boundary that both includes
sources contributing to that violation, as well as informs the public
of the extent of the violation. For purposes of determining
nonattainment boundaries, the EPA expects to consider the county line
as the presumptive boundary for SO2. This would be
consistent with our approach under other NAAQS. States recommending
less-than-countywide nonattainment boundaries should provide additional
information along with their recommendation, demonstrating why a
smaller area is more appropriate, as we have advised for other NAAQS.
If States request it, EPA may develop additional guidance on the
factors that States should consider when determining nonattainment
boundaries.
In addition, as further discussed in section IV.B above, in the
SO2 NAAQS proposal, we proposed a monitoring-focused
approach for comparison to the new NAAQS. The proposed network would
have required approximately 348 monitors nationwide to be sited at the
locations of maximum concentration. Numerous State and local government
commenters expressed concerns regarding the perceived burdens of
implementing the proposed monitoring network and the sufficiency of its
scope for purposes of identifying violations. Some of these commenters
(the City of Alexandria, and the States of Delaware, North Carolina and
Pennsylvania) suggested using modeling to determine the scope of
monitoring requirements, or favored modeling over monitoring to
determine compliance with the NAAQS. Partly in response to these
comments, and after reconsidering the proposal's monitoring-focused
approach, specifically regarding how we have historically implemented
SO2 designations, we now anticipate taking a revised
approach toward designations, using a hybrid analytic approach that
combines the use of monitoring and available modeling to assess
compliance with the new 1-hour SO2 NAAQS. We discuss a
revised expected approach toward designations below, and further
discuss in section VI how we expect a hybrid approach to affect other
implementation activities.
d. Expected Designations Process
As discussed in sections III and IV of this preamble, in response
to the comments and after reviewing our historical SO2
implementation practice, we intend to use a hybrid analytic approach
for assessing compliance with the new 1-hour SO2 NAAQS for
initial designations. We also believe that a hybrid approach is more
consistent with our historical approach and longstanding guidance
toward SO2 NAAQS designations and implementation than what
we originally proposed. Technically, for a short-term 1-hour standard,
it is more appropriate and efficient to principally use modeling to
assess compliance for medium to larger sources, and to rely more on
monitoring for groups of smaller sources and sources not as conducive
to modeling.
In cases where there is complete air quality data from FRM and FEM
SO2 monitors, that data would be considered by EPA in
designating areas as either ``attainment'' or ``nonattainment'' for the
new SO2 NAAQS. See Appendix T to Part 50 section 3b. In
addition, in cases where a State submits air quality modeling data that
are consistent with our current guidance or our expected revisions
thereto, and which indicates that an area is attaining the standard or
violating the standard, these data may support recommendations of
``attainment'' or ``nonattainment.'' As explained in section IV above,
we would not consider monitoring alone to be an adequate, nor the most
accurate, tool to identify all areas of maximum concentrations of
SO2. In the case of SO2, we further believe that
monitoring is not the most cost-efficient method for identifying all
areas of maximum concentrations.
Due to the necessarily limited spatial coverage provided by any
monitoring regime, and the strong source-oriented nature of
SO2 ambient impacts, we recognize that using this more
traditional approach in designations, would be more likely to identify
a greater number of potential instances of nonattainment, if areas were
to immediately conduct modeling of current source emissions, as
compared to the approach we discussed in the proposed rule. As
discussed in section III, forthcoming national and regional rules, such
as the pending Industrial Boilers ``Maximum Achievable Control
Technology'' (MACT) standard under CAA section 112(d), are likely to
result in significant SO2 emissions reductions in the next
three to four years. A limited qualitative assessment of preliminary
modeling of some sample facilities that would be covered by those rules
indicates that well-controlled facilities should meet the new
SO2 NAAQS. However, there are some exceptions. These
exceptions include unique sources with specific source characteristics
that contribute to higher ambient impacts (short stack heights, complex
terrain, etc.).
Again as described in section III, in order for States to conduct
modeling on a large scale for the new 1-hour NAAQS, EPA expects
additional guidance would be needed to clarify how to conduct
dispersion modeling under Appendix W to support the implementation of
the new 1-hour SO2 NAAQS, and how to identify and
appropriately assess the air quality impacts of sources that
potentially may cause or contribute to violations of the NAAQS. Our
anticipated modeling guidance will provide for refined modeling that
will better reflect and account for source-specific impacts by
following our current Guideline on Air Quality Models, Appendix W to 40
CFR Part 51, with appropriate flexibility for use in implementation.
EPA intends to solicit public comment on this modeling guidance. We
expect it will take some time for EPA to issue this guidance, and
believe that given the timing and substantial burden of having to model
several hundred sources, it would not be realistic or appropriate to
expect States to complete such modeling and incorporate the results in
designation recommendations for the new 1-hour SO2 NAAQS
that, under CAA section
[[Page 35571]]
107(d), are due to EPA within 1 year of the promulgation of the NAAQS.
Consequently, we expect that in most instances, Governors will
submit designation recommendations of ``unclassifiable'' rather than
conduct large-scale refined modeling of sources in advance of receiving
our anticipated guidance. The absence of monitoring data showing
violations for most areas, combined with the paucity of refined
modeling of sources that have the potential to cause or contribute to
violations of the NAAQS, will likely result in informational records
that are insufficient to support initial designations of either
``attainment'' or ``nonattainment.'' Under the Clean Air Act, in such a
situation EPA is required to issue a designation for the area as
``unclassifiable.'' However, we do not expect this result to delay
expeditious attainment and maintenance of the new NAAQS, or to cause
inappropriate, indefinite uncertainty regarding whether or not sources
cause or contribute to NAAQS violations.
As described more fully in section III above and in section VI
below, EPA's expected implementation approach would rely on the CAA
section 110(a)(1) SIP obligation to ensure that all areas of the
country attain and maintain the NAAQS on a timely basis even if they
are designated ``unclassifiable'' initially. This SIP is due under CAA
section 110(a)(1) within 3 years after promulgation of the new NAAQS,
and does not depend upon EPA designating an area ``nonattainment''
based on recently monitored or modeled SO2 levels. This
period of time would allow States to use EPA's anticipated guidance on
modeling for the new 1-hour SO2 NAAQS, as well as account
for SO2 reduction levels at individual sources that are
anticipated to result from promulgated national and regional rules to
show attainment.
Once areas have both appropriate monitoring data (if required) and
modeling data as appropriate, consistent with the new guidance, showing
no violations of the SO2 NAAQS, and have met other
applicable requirements of CAA section 107(d)(3), the Agency would
consider re-designating them from ``unclassifiable'' or
``nonattainment'' to ``attainment'' under CAA section 107(d)(3).
VI. Clean Air Act Implementation Requirements
This section of the preamble discusses the CAA requirements that
States and emissions sources would need to address when implementing
the new 1-hour SO2 NAAQS based on the structure outlined in
the CAA and existing rules. The EPA believes that existing guidance
documents and regulations will be useful in helping States and sources
to implement the new SO2 NAAQS, but we also expect to
develop additional guidance on modeling for the new one-hour standard
and on developing SIPs under Section 110(a)(1) of the CAA.\35\ In light
of the new approach that EPA intends to take with respect to
implementation of the SO2 NAAQS, EPA intends to solicit
public comment on guidance regarding modeling, and also solicit public
comment on additional implementation planning guidance, including the
content of the maintenance plans required under section 110(a)(1) of
the Clean Air Act. EPA also notes that State monitoring plans and the
SIP submissions that States will make will also be subject to public
notice and comment.''
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\35\ See SO2 Guideline Document, Office of Air
Quality Planning and Standards, Research Triangle Park, NC 27711,
EPA-452/R-94-008, February 1994.
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In this section, we also further discuss how EPA's modified
expected approaches toward monitoring and modeling and toward initial
designations under the new SO2 NAAQS (compared to how the
proposed rule discussed addressing these issues) are anticipated to
affect the types of SIP submissions States will need to provide to EPA
and the timing of EPA's actions on those submissions leading up to
attainment and maintenance of the new SO2 NAAQS. In section
IV above, we discuss the final amendments to the ambient monitoring and
reporting requirements, and explain how in response to comments
received on the proposal and after revisiting our historical practice
in assessing compliance with prior SO2 NAAQS, we have
revised both the scope of the revised monitoring network and our
expectations on how monitoring will be used in conjunction with
modeling in assessing compliance and designating areas. In section V
above, we discuss how we have revised our expected approach for issuing
designations for the new 1-hour SO2 NAAQS, and similarly
explain how, in response to comments and after reviewing our historical
approach, we have modified our expectations as discussed in the
proposal for how and when monitoring and modeling will be used for
designations. In this section VI, we describe in more detail how and
when we expect States to demonstrate attainment, implementation,
maintenance and enforcement of the new one-hour SO2 NAAQS.
The CAA assigns important roles to EPA, States and Tribal
governments to achieve the NAAQS. States have the primary
responsibility for developing and implementing State implementation
plans (SIPs) that contain State measures necessary to achieve the air
quality standards in each area once EPA has established the NAAQS. EPA
provides assistance to States and Tribes by providing technical tools,
assistance, and guidance, including information on the potential
control measures that may assist in helping areas attain the standards.
Under section 110 of the CAA, 42 U.S.C. 7410, and related
provisions, States are directed to submit, for EPA approval, SIPs that
provide for the attainment, implementation, maintenance, and
enforcement of such standards through control programs directed at
sources of SO2 emissions. See CAA sections 110(a), and 191-
192, 42 U.S.C. 7410(a) and 7514-7514a. If a State fails to adopt and
implement the required SIPs by the time periods provided in the CAA,
EPA has the responsibility under the CAA to adopt a Federal
implementation plan (FIP) to ensure that areas attain the NAAQS in an
expeditious manner. The States, in conjunction with EPA, also
administer the prevention of significant deterioration (PSD) program
for SO2. See sections 160-169 of the CAA, 42 U.S.C. 7470-
7479. In addition, Federal programs provide for nationwide reductions
in emissions of SO2 and other air pollutants under Title II
of the Act, 42 U.S.C. 7521-7574. These programs involve limits on the
sulfur content of the fuel used by automobiles, trucks, buses,
motorcycles, non-road engines and equipment, marine vessels and
locomotives. Emissions reductions for SO2 are also obtained
from implementation of the new source performance standards (NSPS) for
stationary sources under sections 111 and 129 of the CAA, 42 U.S.C.
7411 and 7429; and the national emission standards for hazardous air
pollutants (NESHAP) for stationary sources under section 112 of the
CAA, 42 U.S.C. 7412 (such reductions resulting due to control of
hazardous air pollutants (HAP) such as hydrogen chloride (HCl) under
those rules). Title IV of the CAA, sections 402-416, 42 U.S.C. 7651a-
7651o, specifically provides for major reductions in SO2
emissions. EPA has also promulgated the Clean Air Interstate Rule
(CAIR) to define additional SO2 emission reductions needed
in the Eastern United States to eliminate significant contribution of
upwind States to downwind States'
[[Page 35572]]
nonattainment, or inability to maintain, the PM2.5 NAAQS
pursuant to CAA section 110(a)(2)(D), 42 U.S.C. 7410(a)(2)(D), a rule
which EPA is reevaluating pursuant to court remand.
A. How This Rule Applies to Tribes
CAA section 301(d) authorizes EPA to treat eligible Indian Tribes
in the same manner as States under the CAA and requires EPA to
promulgate regulations specifying the provisions of the statute for
which such treatment is appropriate. EPA has promulgated these
regulations--known as the Tribal Authority Rule or TAR--at 40 CFR Part
49. See 63 FR 7254 (February 12, 1998). The TAR establishes the process
for Indian Tribes to seek treatment-as-a-State eligibility and sets
forth the CAA functions for which such treatment will be available.
Under the TAR, eligible Tribes may seek approval for all CAA and
regulatory purposes other than a small number of functions enumerated
at section 49.4. Implementation plans under section 110 are included
within the scope of CAA functions for which eligible Tribes may obtain
approval. Section 110(o) also specifically describes Tribal roles in
submitting implementation plans. Eligible Indian Tribes may thus submit
implementation plans covering their reservations and other areas under
their jurisdiction.
The CAA and TAR do not, however, direct Tribes to apply for
treatment as a State or implement any CAA program. In promulgating the
TAR EPA explicitly determined that it was not appropriate to treat
Tribes similarly to States for purposes of, among other things,
specific plan submittal and implementation deadlines for NAAQS-related
requirements. 40 CFR 49.4(a). In addition, where Tribes do seek
approval of CAA programs, including section 110 implementation plans,
the TAR provides flexibility and allows them to submit partial program
elements, so long as such elements are reasonably severable--i.e.,
``not integrally related to program elements that are not included in
the plan submittal, and are consistent with applicable statutory and
regulatory requirements.'' 40 CFR 49.7.
To date, very few Tribes have sought treatment as a State for
purposes of section 110 implementation plans. However, some Tribes may
be interested in pursuing such plans to implement today's proposed
standard, once it is promulgated.
1. Approach Described in the Proposal
In the proposed rule preamble, EPA described the various roles and
requirements States would address in implementing the proposed NAAQS.
Such references to States generally included eligible Indian Tribes to
the extent consistent with the flexibility provided to Tribes under the
TAR. Where Tribes do not seek treatment as a State for section 110
implementation plans, we explained that EPA under its discretionary
authority will promulgate FIPs as ``necessary or appropriate to protect
air quality.'' 40 CFR 49.11(a). EPA also noted that some Tribes operate
air quality monitoring networks in their areas. We explained that for
such monitors to be used to measure attainment with the proposed
revised primary NAAQS for SO2, the criteria and procedures
identified in the proposed rule would apply.
2. Current Approach
EPA did not receive any comments on this issue. However, as
discussed elsewhere in this preamble, the final rule reflects in
several respects modified expected approaches regarding the use of
monitoring and modeling, the manner in which we expect to issue
designations under the new SO2 NAAQS, and the types of SIP
submissions we expect would be needed to show attainment,
implementation, maintenance and enforcement of the new NAAQS. Those
changes in expected approach would, as appropriate, also apply to how
we address data and any other submissions from Tribes for purposes of
the new SO2 NAAQS.
B. Nonattainment Area Attainment Dates
The latest date by which an area designated as nonattainment is
required to attain the SO2 NAAQS is determined from the
effective date of the nonattainment designation for the affected area.
For areas designated nonattainment for the revised SO2
NAAQS, SIPs must provide for attainment of the NAAQS as expeditiously
as practicable, but no later than 5 years from the effective date of
the nonattainment designation for the area. See section 192(a) of the
CAA, 42 U.S.C. 7651a(a). The EPA expects to determine whether an area
has demonstrated attainment of the new SO2 NAAQS by
evaluating air quality monitoring and modeling data consistent with 40
CFR part 50, Appendix T and 40 CFR part 51, Appendix W. (Note that this
differs from how we explained we would expect to make such
determinations in the proposed rule, where we only mentioned monitoring
as supplying the data we would evaluate. This expanded and changed
discussion reflects the contemplated changes in our overall approaches
to using monitoring and modeling, expectations for issuing
designations, and expectations for reviewing SIP submissions showing
attainment, implementation, maintenance, and enforcement of the new
SO2 NAAQS.)
1. Attaining the NAAQS
a. Approach Described in the Proposal
In the proposal preamble, we set forth the basic five conditions
provided under section 107(d)(3)(E) of the CAA, 42 U.S.C. 7407(d)(3)(E)
that a nonattainment area must meet in order to be redesignated as
attainment:
EPA must have determined that the area has met the
SO2 NAAQS;
EPA has fully approved the State's implementation plan;
The improvement in air quality in the affected area is due
to permanent and enforceable reductions in emissions;
EPA has fully approved a maintenance plan for the area;
and
The State(s) containing the area have met all applicable
requirements under section 110 and part D.
b. Current Approach
EPA did not receive any comments on this aspect of the preamble of
the proposal. However, in light of the fact that in the final rule, in
response to other comments and consistent with historic practice, we
are revising our proposed anticipated approaches to the overall use of
monitoring and modeling and our expected approaches to issuing initial
designations and reviewing SIP submissions, it follows that the way in
which a nonattainment area seeks redesignation as an attainment area
would also be affected by the final rule's overall changed approaches.
For example, for EPA to determine that a nonattainment area has met the
SO2 NAAQS, we anticipate that the area would need to not
only provide any monitoring data showing such compliance (and there
would need to be an absence of monitoring data showing otherwise), but
modeling where appropriate, consistent with modeling guidance that we
plan to issue, would also need to show that the area is attaining and
maintaining the NAAQS.
2. Consequences of a Nonattainment Area Failing To Attain by the
Statutory Attainment Date
a. Approach Described in the Proposal
We explained in the proposal that any SO2 nonattainment
area that fails to attain by its statutory attainment date would be
subject to the requirements of sections 179(c) and (d) of the CAA, 42
[[Page 35573]]
U.S.C. 7509(c) and (d). EPA is required to make a finding of failure to
attain no later than 6 months after the specified attainment date and
publish a notice in the Federal Register. The State would then need to
submit an implementation plan revision no later than one year following
the effective date of the Federal Register notice making the
determination of the area's failure to attain. This submission must
demonstrate that the standard will be attained as expeditiously as
practicable, but no later than 5 years from the effective date of EPA's
finding that the area failed to attain. In addition, section 179(d)(2)
provides that the SIP revision must include any specific additional
measures as may be reasonably prescribed by EPA, including ``all
measures that can be feasibly implemented in the area in light of
technological achievability, costs, and any nonair quality and other
air quality-related health and environmental impacts.''
b. Current Approach
EPA did not receive any comments on this aspect of the discussion
in the preamble to the proposal. However, due to the changes in the
final rule's discussion of the overall expected approaches to
monitoring and modeling, designations and EPA review of SIP
submissions, it follows that the implementation of CAA sections 179(c)
and (d) would also be affected by those changes. For example, under the
anticipated approach, a nonattainment area's initial demonstration of
attainment would need to show through modeling consistent with modeling
guidance that we plan to issue, that the area attains and maintains the
new SO2 NAAQS. If the area fails to attain on time, any
remedial implementation plan submission would also need to show, where
appropriate, through modeling consistent with modeling guidance that we
plan to issue, that the area attains and maintains the new
SO2 NAAQS.
C. Section 110(a)(1) and (2) NAAQS Maintenance/Infrastructure
Requirements
We are significantly revising our expected approaches to the use of
monitoring and modeling, expected issuance of initial designations, and
EPA review of SIP submissions. This change in anticipated approach has
particular relevance for how States would meet their statutory
obligations under CAA section 110(a) to implement, maintain and enforce
the new SO2 NAAQS. In short, under such an approach, all
areas, whether designated as attainment, nonattainment, or
unclassifiable, would need to submit SIPs under CAA section 110(a) that
show that they are attaining and maintaining the 1-hour SO2
NAAQS as expeditiously as practicable through permanent and enforceable
measures. In other words, the duty to show maintenance of the
SO2 NAAQS would not be limited to areas that are initially
designated as nonattainment, but instead would apply regardless of
designation. As has been expected historically, areas initially
designated attainment for SO2 are expected to submit to EPA
the infrastructure elements of the 110(a) SIP, including the PSD
program. Historically, EPA has determined this to be sufficient to
demonstrate maintenance absent other available information to suggest
the area would have difficulty maintaining the NAAQS.
As required by CAA section 192, nonattainment areas must
demonstrate attainment as expeditiously as practicable, and no later
than 5 years after designation (which would be August 2017). Under a
hybrid approach as we have discussed earlier in sections III, IV, and V
of this preamble, EPA believes that August 2017 would be the latest
point that could be as expeditiously as practicable for attainment and
unclassifiable areas as well, and EPA anticipates establishing this
date through future rulemaking actions on individual SIPs.
As noted in earlier sections of this preamble, in the
SO2 NAAQS proposal, we recommended a monitoring-focused
approach for comparison to the NAAQS. We received public comments that
contended our proposed monitoring network was too small and
insufficient to assess the hundreds of areas that might violate the new
SO2 NAAQS and yet too burdensome and expensive to expand to
an adequate scale. Some commenters, especially State air agencies,
recommended the use of modeling either to determine potential
nonattainment areas or to identify areas subject to monitoring
requirements. Because SO2 is primarily a localized
pollutant, modeling is the the most appropriate tool to accurately
predict SO2 impacts from large sources, EPA has used it in
the past to determine SO2 attainment status, and it can be
performed more quickly and less costly than monitoring. Consequently,
as part of developing a balanced response to the numerous comments we
received on modeling and monitoring, we expect to use a hybrid analytic
approach that combines the use of monitoring and modeling to assess
compliance with respect to the new SO2 NAAQS.
A hybrid analytic approach for assessing compliance with the new
SO2 NAAQS would make the most appropriate use of available
tools and be more consistent with our historical approach than was what
we originally proposed. For a short-term 1-hour standard, it is more
accurate and efficient to use modeling to assess medium to larger
sources and to rely on monitoring for groups of smaller sources and
sources not as conducive to modeling.
We expect that States would initially focus performance of
attainment demonstration modeling on larger sources (e.g., those
= 100 tons per year (tpy) of SO2), and that
States would also identify and eventually conduct refined modeling of
any other sources that may be anticipated to cause or contribute to a
violation to determine compliance with the new SO2 NAAQS. As
discussed in Section III, EPA anticipates providing additional guidance
to States to clarify how to conduct dispersion modeling under Appendix
W to support the implementation of the new 1-hour SO2 NAAQS.
Prior to issuing this guidance, EPA intends to solicit public comment.
Since determining compliance with the SO2 NAAQS will
likely be a uniquely source-driven analysis, EPA explored options to
ensure that the SO2 designations process realistically
accounts for anticipated SO2 reductions at those sources
that we expect will be achieved by current and pending national and
regional rules. To ensure that all areas of the country attain the
NAAQS on a timely basis, while accommodating modeling that is both
informed by anticipated modeling guidance and accounts for those
anticipated SO2 reductions, EPA's intention is to emphasize
the CAA section 110(a)(1) requirement that all States submit a SIP that
shows implementation, maintenance and enforcement of the NAAQS. This
SIP would be due under CAA section 110(a)(1) within 3 years after
promulgation of the new NAAQS, and would not depend upon EPA
designating an area nonattainment based on recently monitored or
modeled SO2 levels. In addition, like an attainment SIP
required for a designated nonattainment area under CAA section 192, to
show attainment this SIP can account for controlled SO2
levels at individual sources that will be achieved after submission of
the SIP but before the demonstrated attainment date. EPA intends to
implement this approach in a way that ensures expeditious attainment of
the NAAQS, under a schedule that we explain more fully below.
[[Page 35574]]
1. Section 110(a)(1)-(2) Submission
a. Approach Described in the Proposal
In the preamble to the proposal, we explained that section
110(a)(2) of the CAA directs all States to develop and maintain a solid
air quality management infrastructure, including enforceable emission
limitations, an ambient monitoring program, an enforcement program, air
quality modeling capabilities, and adequate personnel, resources, and
legal authority. Section 110(a)(2)(D) also requires State plans to
prohibit emissions from within the State which contribute significantly
to nonattainment or maintenance areas in any other State, or which
interfere with programs under part C of the CAA to prevent significant
deterioration of air quality or to achieve reasonable progress toward
the national visibility goal for Federal class I areas (national parks
and wilderness areas).
Under sections 110(a)(1) and (2) of the CAA, all States are
directed to submit SIPs to EPA which demonstrate that basic program
elements have been addressed within 3 years of the promulgation of any
new or revised NAAQS. Subsections (A) through (M) of section 110(a)(2)
set forth the elements that a State's program must contain in the
SIP.\36\ The proposed rule listed section 110(a)(2) NAAQS
implementation requirements as the following:
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\36\ In the proposed rule preamble, we explained that two
elements identified in section 110(a)(2) were not listed in our
summary because, as EPA interprets the CAA, SIPs incorporating any
necessary local nonattainment area controls would not be due within
3 years, but rather are generally due at the time the nonattainment
area planning requirements are due. See 74 FR 64860 at n. 39. These
elements are: (1) Emission limits and other control measures,
section 110(a)(2)(A), and (2) Provisions for meeting part D, section
110(a)(2)(I), which requires areas designated as nonattainment to
meet the applicable nonattainment planning requirements of part D,
title I of the CAA. To implement our revised intended approach in
the final rule, however, it would be necessary for States to
include, if relied upon to show attainment and maintenance of the
new SO2 NAAQS, any necessary emission limits and other
control measures under section 110(a)(2)(A).
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Ambient air quality monitoring/data system: Section
110(a)(2)(B) requires SIPs to provide for setting up and operating
ambient air quality monitors, collecting and analyzing data and making
these data available to EPA upon request.
Program for enforcement of control measures: Section
110(a)(2)(C) requires SIPs to include a program providing for
enforcement of SIP measures and the regulation and permitting of new/
modified sources.
Interstate transport: Section 110(a)(2)(D) requires SIPs
to include provisions prohibiting any source or other type of emissions
activity in the State from contributing significantly to nonattainment
or interfering with maintenance of the NAAQS in another State, or from
interfering with measures required to prevent significant deterioration
of air quality or to protect visibility.
Adequate resources: Section 110(a)(2)(E) directs States to
provide assurances of adequate funding, personnel and legal authority
to implement their SIPs.
Stationary source monitoring system: Section 110(a)(2)(F)
directs States to establish a system to monitor emissions from
stationary sources and to submit periodic emissions reports to EPA.
Emergency power: Section 110(a)(2)(G) directs States to
include contingency plans, and adequate authority to implement them,
for emergency episodes in their SIPs.
Provisions for SIP revision due to NAAQS changes or
findings of inadequacies: Section 110(a)(2)(H) directs States to
provide for revisions of their SIPs in response to changes in the
NAAQS, availability of improved methods for attaining the NAAQS, or in
response to an EPA finding that the SIP is inadequate.
Consultation with local and Federal government officials:
Section 110(a)(2)(J) directs States to meet applicable local and
Federal government consultation requirements when developing SIPs and
reviewing preconstruction permits.
Public notification of NAAQS exceedances: Section
110(a)(2)(J) directs States to adopt measures to notify the public of
instances or areas in which a NAAQS is exceeded.
PSD and visibility protection: Section 110(a)(2)(J) also
directs States to adopt emissions imitations, and such other measures,
as may be necessary to prevent significant deterioration of air quality
in attainment areas and protect visibility in Federal Class I areas in
accordance with the requirements of CAA Title I, part C.
Air quality modeling/data: Section 110(a)(2)(K) requires
that SIPs provide for performing air quality modeling for predicting
effects on air quality of emissions of any NAAQS pollutant and
submission of data to EPA upon request.
Permitting fees: Section 110(a)(2)(L) requires the SIP to
include requirements for each major stationary source to pay permitting
fees to cover the cost of reviewing, approving, implementing and
enforcing a permit.
Consultation/participation by affected local government:
Section 110(a)(2)(M) directs States to provide for consultation and
participation by local political subdivisions affected by the SIP.
b. Final
EPA did not receive any comments on this aspect of the approached
explained in the proposal preamble. However, in light of the modified
approach discussed above, EPA is providing additional guidance
concerning the CAA section 110(a)(1) maintenance plan requirement as a
part of this discussion so that States will have sufficient information
to meet this requirement with a SIP submittal three years after
promulgation of the NAAQS. Section 110(a)(1) of the CAA states that
each State, after reasonable notice and public hearing, is required to
adopt and to submit to EPA, within 3 years after promulgation of any
new or revised NAAQS for any pollutant, a SIP which provides for the
implementation, maintenance, and enforcement of any new or revised
NAAQS in each area of the State. As stated previously, in light of the
new approach that EPA intends to take with respect to implementation of
the SO2 NAAQS, EPA intends to solicit public comment on
guidance regarding modeling, and also solicit public comment on
additional implementation planning guidance, including the content of
the maintenance plans required under section 110(a)(1) of the Clean Air
Act.
EPA expects that most areas of the country would be designated as
unclassifiable for the 1-hour NAAQS for SO2, due to a lack
of both monitoring and modeling information concerning the attainment
status of areas, in advance of States conducting further refined
modeling according to our anticipated guidance. For areas that are
designated unclassifiable, States are required to submit section
110(a)(1) plans to demonstrate implementation, maintenance and
enforcement of the new SO2 NAAQS. As previously explained in
section III of the preamble, in order to meet the requirements of
section 110(a)(1) and to ensure timely attainment of the NAAQS on a
schedule that is as expeditious as would be required if an area had
been designated nonattainment, EPA's current expectation is that States
would submit SIPs which provide for attainment, implementation,
maintenance, and enforcement of the 1-hour SO2 NAAQS in all
areas as expeditiously as practicable, which EPA believes in these
cases would be no later than 5 years from the effective date of the
area's designation. The section 110(a)(1) maintenance plan would also
need to contain the following elements: (1) An
[[Page 35575]]
attainment emissions inventory, (2) a control strategy, as appropriate,
(3) a maintenance demonstration, using an EPA approved air quality
model as appropriate, (4) a contingency plan, and (5) a plan for
verification of continued attainment of the standard. Attainment areas
that appear to have difficulty maintaining attainment may also have to
submit some of these elements. These elements are now explained in
detail.
(1) Attainment Emissions Inventory
The State should develop an accurate attainment emissions inventory
to identify the level of emissions in the area which is sufficient to
attain the 1-hour SO2 NAAQS. This inventory should be
consistent with EPA's most recent guidance on emissions inventories
currently available, and should include the emissions for the time
period associated with the modeling and monitoring data showing
attainment. Major source size thresholds for SO2 are
currently listed as 100 ton/yr, however, in cases where sources,
individually, or collectively, that are below this level may
potentially cause or contribute to a violation of the standard, these
sources should also be included in the emissions inventory for the
affected area. EPA notes that, unlike any monitoring or modeling data
used in the initial designations context, which would be limited to
current emissions levels, this estimate under a hybrid approach we
expect to use for the new SO2 NAAQS would be able to rely on
modeled controlled emissions levels at sources achieved by enforceable
national, regional or local rules that will be in place within the
timeframe for demonstrating attainment. This is because demonstrations
of attainment and maintenance of a NAAQS, unlike designations, are
necessarily projections regarding future and continuing levels of
ambient air pollution concentrations given that the statutory deadlines
for their submission are in advance of the required achievement of
attainment and maintenance. See, e.g., CAA sections 191(a) and 192(a).
(2) Maintenance Demonstration
The key element of a section 110(a)(1) maintenance plan is a
demonstration using, as appropriate, refined SO2 dispersion
modeling (see Appendix W to 40 CFR Part 51) which provides an
indication of how the area will attain and maintain the 1-hour
SO2 NAAQS as expeditiously as practicable, which EPA
believes would be within the 5 year period following the designation of
the area. For SO2 the State may generally demonstrate
maintenance of the NAAQS by using refined dispersion modeling to show
that the future mix of sources and emission rates in an area will not
cause a violation of the 1-hour SO2 NAAQS. As a result of
applying the control strategy, EPA anticipates that additional guidance
for States may be needed to clarify how to conduct dispersion modeling
under Appendix W to support the implementation of the new 1-hour
SO2 NAAQS.
As explained above in IV.B, EPA believes that for SO2
attainment and maintenance demonstrations, monitoring data alone is
generally not adequate to characterize fully short-term ambient
concentrations around major stationary sources of SO2, and
as a result may not capture the maximum SO2 impacts. With
representative and appropriate meteorological and other input data,
refined dispersion models are able to characterize air quality impacts
from the modeled sources across the domain of interest on an hourly
basis with a high degree of spatial resolution, overcoming the
limitations of an approach based solely on monitoring. By simulating
plume dispersion on an hourly basis across a grid of receptor
locations, dispersion models are able to estimate the detailed spatial
gradients of ambient concentrations resulting from SO2
emission sources across a full range of meteorological and source
operating conditions. To capture such results on a monitor would
normally require a prohibitively expansive air quality monitoring
network. Further, as we have observed in prior actions (see., e.g., 43
FR 45993, 45997, 46000-03 (Oct. 5, 1978)), monitoring data would not be
adequate to demonstrate attainment if sources are using stacks with
heights that are greater than good engineering practice (GEP), or other
prohibited dispersion techniques, as section 123 prohibits credit in an
attainment demonstration for any such practices.
Refined dispersion modeling for the section 110(a)(1) maintenance
plan is expected to follow EPA's Guideline on Air Quality Models,
Appendix W to 40 CFR Part 51, which provides recommendations on
modeling techniques and guidance for estimating pollutant
concentrations in order to assess control strategies and determine
emission limits. These recommendations were originally published in
April 1978 and were incorporated by reference in the PSD regulations,
40 CFR sections 51.166 and 52.21 in June 1978 (43 FR 26382-26388). The
purpose of Appendix W is to promote consistency in the use of modeling
within the air quality management process. Appendix W is periodically
revised to ensure that new model developments or expanded regulatory
requirements are incorporated. The most recent revision to Appendix W
was published on November 9, 2005 (70 FR 68218), wherein EPA adopted
AERMOD as the preferred dispersion model for a wide range of regulatory
applications in all types of terrain. To support the promulgation of
AERMOD as the preferred model, EPA evaluated the performance of the
model across a total of 17 field study data bases (Perry, et al., 2005;
EPA, 2003), including several field studies based on model-to-monitor
comparisons of SO2 concentrations from operating power
plants. AERMOD is a steady-state plume dispersion model that employs
hourly sequential preprocessed meteorological data to simulate
transport and dispersion from multiple point, area, or volume sources
for averaging times from one hour to multiple years, based on an
advanced characterization of the atmospheric boundary layer. AERMOD
also accounts for building wake effects (i.e., downwash) on plume
dispersion.
As stated previously, EPA anticipates that additional guidance for
States, Tribal, and local governments is needed to clarify how to
conduct refined dispersion modeling under Appendix W to support the
implementation of the new 1-hour SO2 NAAQS. EPA intends to
solicit public comment on guidance regarding modeling. Although AERMOD
is identified as the preferred model under Appendix W for a wide range
of applications and will be appropriate for most modeling applications
to support the new SO2 NAAQS, Appendix W allows flexibility
to consider the use of alternative models on a case-by-case basis when
an adequate demonstration can be made that the alternative model
performs better than, or is more appropriate than, the preferred model
for a particular application.
(3) Control Strategy
The EPA believes that in order to meet the implementation,
maintenance and enforcement plan requirements of section 110(a)(1) for
the new SO2 NAAQS, States should consider all control
measures that are reasonable to implement in light of the attainment
and maintenance needs for the affected area(s). The EPA believes that
where additional controls are necessary it would be appropriate for the
level of controls in these areas to be similar to that required in
areas that are designated as nonattainment for SO2. These
controls would provide for the attainment and maintenance of the
SO2 1-hour standard as expeditiously as
[[Page 35576]]
practicable. EPA believes that expeditious attainment in these areas
will be within 5 years of the effective date of designation of an area.
This approach would allow States to take into consideration emission
reductions that we expect to be achieved from the implementation of
future controls from national control measures as well as regional and
local control measures that will be in place by the anticipated
attainment date and are projected to help achieve attainment and
maintenance of the standard. It would also reduce the risk of such
areas failing to meet the NAAQS as expeditiously as nonattainment areas
must meet it.
(4) Contingency Plan
The contingency plan is considered to be an enforceable part of the
section 110(a)(1) plan and should ensure that there are appropriate
contingency measures which can be implemented as expeditiously as
practicable once they are triggered. The contingency plan should
clearly identify the measures to be adopted, provide a schedule and
procedures for adoption and implementation, and provide a specific time
limit for actions by the State.
The EPA believes that in this case the contingency measures
implemented under the contingency plan requirement for the section
110(a)(1) plan in unclassifiable areas under a revised approach for
SO2 should closely resemble the contingency measures
required under section 172(c)(9) of the CAA. Section 172(c)(9) of the
CAA defines contingency measures as measures in the SIP which are to be
implemented in the event that an area fails to attain the NAAQS, or
fails to meet the reasonable further progress (RFP) requirement, by the
applicable attainment date for the area. Contingency measures become
effective without further action by the State or EPA, upon
determination by EPA that the area (1) failed to attain the NAAQS by
the applicable attainment date, or (2) fail to meet RFP. These
contingency measures should consist of other available control measures
that are not included in the control strategy for the SIP.
The EPA interprets the contingency measure provision as primarily
directed at general control programs which can be undertaken on an
area-wide basis. Since SO2 control measures are based on
what is directly and quantifiably necessary to attain the
SO2 NAAQS, it would be unlikely for an area to implement the
necessary emissions control yet fail to attain the NAAQS. Therefore,
for SO2 programs, EPA believes that State agencies should
have a comprehensive program to identify sources of violations of the
SO2 NAAQS and undertake an aggressive follow-up for
compliance and enforcement, including expedited procedures for
establishing enforceable consent agreements pending the adoption of
revised SIPs.
Such an approach toward minimum contingency measures for
SO2 would not preclude a State from requiring additional
contingency measures that are enforceable and appropriate for a
particular source or source category. A contingency measure for an
SO2 SIP might be a consent agreement between the State and
EPA to reduce emissions from a source further in the event that the
contingency measures are triggered. Alternatively, a source might adopt
a contingency measure such as switching to low sulfur coal or reducing
load until more permanent measures can be put into place to correct the
problem. In either case, the contingency measure should be a fully
adopted provision in the SIP in order for it to become effective at the
time that EPA determines that the area either fails to attain the NAAQS
or fails to meet RFP.
As a necessary part of the section 110(a)(1) plan, the State should
also identify specific indicators, or triggers, which will be used to
determine when the contingency measures need to be implemented. The
identification of triggers would allow a State an opportunity to take
early action to address potential violations of the NAAQS before they
occur. By taking early action, States may be able to prevent any actual
violations of the NAAQS, and therefore, reduce the need on the part of
EPA to start the process to re-designate the areas as nonattainment. An
example of a trigger would be monitored or modeled violations of the
NAAQS. The EPA will review what constitutes an approvable contingency
plan on a case-by-case basis.
(5) Verification of Continued Attainment
The submittal should provide an indication of how the State will
track the progress of the section 110(a)(1) plan. This is necessary due
to the fact that the emissions projections made for the attainment and
maintenance demonstrations depend on assumptions of point, area, and
mobile source growth. One option for tracking the progress of the
attainment and maintenance demonstrations, provided here as an example,
would be for the State to update periodically the emissions inventory.
The attainment and maintenance demonstration should project maintenance
during the five year period following the designations for the 1-hour
SO2 NAAQS, not simply that the area will be in attainment in
the fifth year.
States should develop interim emission projection years to show a
trend analysis for attainment and maintenance of the standard. These
emission projections can also be used as triggers for implementing
contingency measures. The EPA recognizes that it would be difficult and
time consuming to develop projections for each year of the 5 year
period. Therefore, the number of interim projection years should
reflect whatever information exists regarding the potential for
increases in emissions in the intervening years. For instance, if there
is a high probability that emissions will increase to such an extent as
to jeopardize continued maintenance of the standard even temporarily
over the intervening years, the number of interim projection periods
should be sufficient to document that such increases will not interfere
with maintenance of the 1-hour SO2 NAAQS.
When modeling for the attainment and maintenance demonstrations,
one option for tracking progress would also be for the State to
reevaluate periodically the modeling assumptions and data input. Such
reevaluation, for example, could address any delays in source
compliance with national, regional or local rules for which the State
had previously modeled timely SO2 reductions. In any event,
the State should monitor the indicators for triggering the contingency
measures on a regular basis.
EPA recognizes that the approach discussed above for SO2
SIPs submitted under CAA section 110(a)(1)-(2) is significantly
different from the one outlined in the proposal, and from what we have
applied in the context of other criteria pollutants. However, EPA
anticipates using a revised approach under section 110(a)(1)-(2) as
part of an overall revised hybrid monitoring and modeling approach in
response to comments on the proposed monitoring-focused approach to
implementation of the new SO2 NAAQS. We believe that such an
approach would best account for the unique source-specific and
localized impacts inherent to SO2, and would be the most
reasonable way to ensure that all areas of the United States timely
attain and maintain the new NAAQS, while at the same time avoiding
inappropriately requiring immediate refined modeling of all sources
without appropriate EPA guidance. This would also allow attainment
demonstrations to account
[[Page 35577]]
for expected substantial SO2 reductions that will occur well
in advance of the attainment deadline. Of course, for such a unique
SO2 approach to work, it would be imperative for all areas
to timely submit, and for EPA to able to approve, adequate attainment,
implementation, maintenance and enforcement SIPs that show attainment
as expeditiously as practicable, and no later than 5 years following
initial designations. Only by applying such a timeframe to the section
110(a)(1) SIP approach we are adopting for SO2 could the
approach be a reasonable one. To that end, EPA would not intend to
approve SIPs that do not meet this schedule, and would take necessary
and appropriate actions in response to any submission that would result
in unacceptable delay of attainment. Such actions may include, but are
not limited to, any combination of SIP disapproval, redesignation to
nonattainment, and promulgation of a Federal implementation plan (FIP).
Any future action establishing an attainment deadline will be completed
through notice-and-comment rulemaking on individual SIP submissions.
The timeline below shows how we expect the several steps from
promulgation of the new NAAQS through attainment should proceed,
whether areas are designated nonattainment or unclassifiable, assuming
timely action at each step:
June 2010: EPA issues new SO2 NAAQS, which
starts periods within which CAA section 107 initial area designations
must occur and CAA section 110(a)(1)-(2) SIPs must be submitted.
June 2011: States submit initial area designations
recommendations, based on available monitoring data, and on any refined
modeling performed in advance of submitting CAA section 110(a)(1)-(2)
SIPs.
June 2012: EPA issues initial area designations. Any
monitored or modeled violations would trigger nonattainment
designations. (Per below, States designated nonattainment would submit
nonattainment SIPs by February 2014, relying on refined modeling that
demonstrates attainment by no later than August 2017.) States would be
designated attainment if they submit both monitoring and modeling
showing adequate evidence of no violations. All other cases would be
initially designated as unclassifiable.
June 2013: States submit CAA section 110(a)(1)-(2) SIPs.
SIPs would rely on refined modeling and any required monitoring that
demonstrates attainment and maintenance of the new SO2 NAAQS
as expeditiously as practicable, and no later than August 2017. For
areas within the State designated attainment and unclassifiable, the
section 110(a) SIP must contain any additional Federally enforceable
control measures necessary to ensure attainment and maintenance of the
NAAQS. (Control measures to be implemented in designated nonattainment
areas are due later as part of the nonattainment SIP in February 2014.)
February 2014: Any initially designated nonattainment
areas submit CAA section 191-192 SIPs showing attainment no later than
August 2017.
June 2014: EPA approves or disapproves submitted CAA
section 110(a)(1)-(2) SIPs. For attainment and unclassifiable areas,
EPA's action would be based on adequacy of States' modeling (and any
required monitoring) showing attainment as expeditiously as
practicable, and no later than August 2017, in partial reliance on
SO2 reductions from national and regional standards that are
achieved by the attainment date. EPA would also have discretion to re-
designate areas based on these SIPs, including to nonattainment if SIPs
are inadequate, as well as promulgate FIPs.
February 2015: EPA approves or disapproves CAA section
191-192 attainment SIPs submitted by areas initially designated as
nonattainment, with similar remedies as discussed above if SIPs are
deficient.
June 2016: CAA section 110(c) deadline by which EPA must
issue a FIP for any area whose section 110(a)(1) SIP is disapproved in
June 2014.
February 2017: CAA section 110(c) deadline by which EPA
must issue a FIP for a nonattainment area whose section 192 SIP is
disapproved in February 2015.
August 2017: Expected date by which all areas, regardless of
classification, achieve attainment, implementation, maintenance and
enforcement of the new SO2 NAAQS.
D. Attainment Planning Requirements
1. SO2 Nonattainment Area SIP Requirements
a. Approach Described in the Proposal
We explained in the preamble to the proposal that any State
containing an area designated as nonattainment with respect to the
SO2 NAAQS would need to develop for submission to EPA a SIP
meeting the requirements of part D, Title I, of the CAA, providing for
attainment by the applicable statutory attainment date. See sections
191(a) and 192(a) of the CAA. As indicated in section 191(a), all
components of the SO2 part D SIP must be submitted within 18
months of the effective date of an area's designation as nonattainment.
Section 172 of the CAA addresses the general requirements for areas
designated as nonattainment. Section 172(c) directs States with
nonattainment areas to submit a SIP which contains an attainment
demonstration showing that the affected area will attain the standard
by the applicable statutory attainment date. The SIP must show that the
area will attain the standard as expeditiously as practicable, and must
``provide for the implementation of all Reasonably Available Control
Measures (RACM) as expeditiously as practicable (including such
reductions in emissions from existing sources in the area as may be
obtained through the adoption, at a minimum, of Reasonably Available
Control Technology (RACT)).''
SIPs required under Part D of the CAA must also provide for
reasonable further progress (RFP). See section 172(c)(2) of the CAA.
The CAA defines RFP as ``such annual incremental reductions in
emissions of the relevant air pollution as are required by part D, or
may reasonably be required by the Administrator for the purpose of
ensuring attainment of the applicable NAAQS by the applicable
attainment date.'' See section 171 of the CAA. Historically, for some
pollutants, RFP has been met by showing annual incremental emission
reductions sufficient to maintain generally linear progress toward
attainment by the applicable attainment date.
All SO2 nonattainment area SIPs must include contingency
measures which must be implemented in the event that an area fails to
meet RFP or fails to attain the standards by its attainment date. See
section 172(c)(9) of the CAA. These contingency measures must be fully
adopted rules or control measures that take effect without further
action by the State or the Administrator. The EPA interprets this
requirement to mean that the contingency measures must be implemented
with only minimal further action by the State or the affected sources
with no additional rulemaking actions such as public hearings or
legislative review.
Emission inventories are also critical for the efforts of State,
local, and Federal agencies to attain and maintain the NAAQS that EPA
has established for criteria pollutants including SO2.
Section 191(a) in conjunction with section 172(c) requires that areas
designated as nonattainment for SO2 submit an emission
inventory to EPA no later than 18 months after designation as
nonattainment. In the case of SO2,
[[Page 35578]]
sections 191(a) and 172(c) also direct States to submit periodic
emission inventories for nonattainment areas. The periodic inventory
must include emissions of SO2 for point, nonpoint, mobile,
and area sources.
b. Current Approach
EPA did not receive any comments on this issue. Thus, EPA has no
changes to make to this discussion.
2. New Source Review and Prevention of Significant Deterioration
Requirements
a. Approach Described in the Proposal
We provided a discussion of the new source review and prevention of
significant deterioration programs in the preamble to the proposed
rule. The Prevention of Significant Deterioration (PSD) and
nonattainment New Source Review (NSR) programs contained in parts C and
D of Title I of the CAA govern preconstruction review of any new or
modified major stationary sources of air pollutants regulated under the
CAA as well as any precursors to the formation of that pollutant when
identified for regulation by the Administrator.\37\ The EPA rules
addressing these programs can be found at 40 CFR 51.165, 51.166, 52.21,
52.24, and Part 51, appendix S.
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\37\ The terms ``major'' and ``minor'' define the size of a
stationary source, for applicability purposes, in terms of an annual
emissions rate (tons per year, tpy) for a pollutant. Generally, a
minor source is any source that is not ``major.'' ``Major'' is
defined by the applicable regulations--PSD or nonattainment NSR.
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The PSD program applies when a major source located in an area that
is designated as attainment or unclassifiable for any criteria
pollutant is constructed or undergoes a major modification.\38\ The
nonattainment NSR program applies on a pollutant-specific basis when a
major source constructs or modifies in an area that is designated as
nonattainment for that pollutant. The minor NSR program addresses major
and minor sources that undergo construction or modification activities
that do not qualify as major, and it applies, as necessary to assure
attainment, regardless of the designation of the area in which a source
is located.
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\38\ In addition, the PSD program applies to non-criteria
pollutants subject to regulation under the Act, except those
pollutants regulated under section 112 and pollutants subject to
regulation only under section 211(o).
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The PSD requirements include but are not limited to the following:
Installation of Best Available Control Technology (BACT);
Air quality monitoring and modeling analyses to ensure
that a project's emissions will not cause or contribute to a violation
of any NAAQS or maximum allowable pollutant increase (PSD increment);
Notification of Federal Land Manager of nearby Class I
areas; and public comment on the permit.
To the extent necessary to address these PSD requirements for the
new 1-hour SO2 NAAQS, SIPs are due no later than 3 years
after the promulgation date. Generally, however, the owner or operator
of any major stationary source or major modification obtaining a final
PSD permit on or after the effective date of the new 1-hour
SO2 NAAQS will be required, as a prerequisite for the PSD
permit, to demonstrate that the emissions increases from the new or
modified source will not cause or contribute to a violation of that new
NAAQS. The EPA anticipates that individual sources will be able to
complete this demonstration under the PSD regulations based on current
guidance in EPA's Guideline on Air Quality Models, Appendix W of 40 CFR
Part 51.
The owner or operator of a new or modified source will still be
required to demonstrate compliance with the annual and 24-hour
SO2 increments, even when their counterpart NAAQS are
revoked. The annual and 24-hour increments are established in the CAA
and will need to remain in the PSD regulations because EPA does not
interpret the CAA to authorize EPA to remove them. It appears necessary
for Congress to amend the CAA to make appropriate changes to the
statutory SO2 increments. In 1990, the CAA was amended to
accommodate PM10 increments in lieu of the statutory TSP
increments.
In association with the requirement to demonstrate compliance with
the NAAQS and increments, the owner or operator of a new or modified
source must submit for review and approval a source impact analysis and
an air quality analysis. The source impact analysis, primarily a
modeling analysis, must demonstrate that allowable emissions increases
from the proposed source or modification, in conjunction with emissions
from other existing sources will not cause or contribute to either a
NAAQS or increment violation. The air quality analysis must assess the
ambient air quality in the area that the proposed source or
modification would affect.
For the air quality analysis, the owner or operator must submit in
its permit application air quality monitoring data that shall have been
gathered over a period of one year and is representative of air quality
in the area of the proposed project. If existing data representative of
the area of the proposed project is not available, new data may need to
be collected by the owner or operator of the source or modification.
Where data is already available, it might be necessary to evaluate the
location of the monitoring sites from which the SO2 data
were collected in comparison to any new siting requirements associated
with the 1-hour SO2 NAAQS. If existing sites are
inappropriate for providing the necessary representative data, then new
monitoring data will need to be collected by the owner or operator of
the proposed project.
Historically, EPA has allowed the use of several screening tools to
help facilitate the implementation of the new source review program by
reducing the permit applicant's burden, and streamlining the permitting
process for de minimis circumstances. These screening tools include a
significant emissions rate (SER), significant impact levels (SILs), and
a significant monitoring concentration (SMC). The SER, as defined in
tons per year for each regulated pollutant, is used to determine
whether any proposed source or modification will emit sufficient
amounts of a particular pollutant to require the review of that
pollutant under the NSR permit program. EPA will consider whether to
evaluate the existing SER for SO2 to see if it would change
substantially based on the NAAQS levels for the 1-hour averaging
period. Historically, for purposes of defining the SER, we have defined
a de minimis pollutant impact as one that results in a modeled ambient
impact of less than approximately 4% of the short-term NAAQS. The
current SER for SO2 (40 tpy) is based on the impact on the
24-hour SO2 NAAQS. See 45 FR 52676, 52707 (August 7, 1980).
We have typically used the most sensitive averaging period to calculate
the SER, and we may want to evaluate the new 1-hour period for
SO2 because it is likely to represent the most sensitive
averaging period for SO2.
The SIL, expressed as an ambient pollutant concentration (ug/m3),
is used to determine whether the impact of a particular pollutant is
significant enough to warrant a complete air quality impact analysis
for any applicable NAAQS and increments. EPA has promulgated
regulations under 40 CFR 51.165(b) which include SILs for
SO2 to determine whether a source's impact would be
considered to cause or contribute to a NAAQS violation for the 3-hour
(the secondary NAAQS), 24-hour or annual averaging periods. These SILs
were originally developed in 1978 to limit the application of air
quality dispersion models to a downwind
[[Page 35579]]
distance of no more than 50 kilometers or to ``insignificant levels.''
See 43 FR 26398, June 19, 1978. Through guidance, EPA has also allowed
the use of SILs to determine whether or not it is necessary for a
source to carry out a comprehensive source impact analysis and to
determine the extent of the impact area in which the analysis will be
carried out. The existing SILs for SO2 were not developed on
the basis of specific SO2 NAAQS levels, so there may be no
need to revise the existing SILs. Even upon revocation of the annual
and 24-hour NAAQS, the corresponding SIL should still be useful for
increment assessment. A SIL for the 1-hour averaging period does not
exist, and would need to be developed for use with modeling for 1-hour
SO2 NAAQS and any 1-hour increments.
Finally, the SMC, also measured as an ambient pollutant
concentration ([mu]g/m\3\), is used to determine whether it may be
appropriate to exempt a proposed project from the requirement to
collect ambient monitoring data for a particular pollutant as part of a
complete permit application. EPA first defined SMCs for regulated
pollutants under the PSD program in 1980. See 45 FR 52676, 52709-10
(August 7, 1980). The existing SMC for SO2, based on a 24-
hour averaging period, may need to be re-evaluated to consider the
effect of basing the SMC on the 1-hour averaging period, especially in
light of revocation of the NAAQS for the 24-hour averaging period.
Third, even if the 1-hour averaging period does not indicate the need
for a revised SMC for SO2, the fact that the original SMC
for SO2 is based on 1980 monitoring data (Lowest Detectable
Level, correction factor of ``5''), could be a basis for revising the
existing value. More up-to-date monitoring data and statistical
analyses of monitoring accuracy may yield a different--possibly lower--
correction factor today. The new 1-hour NAAQS will not necessarily
cause this result, but may provide a ``window of opportunity'' to re-
evaluate the SMC for SO2.
States which have areas designated as nonattainment for the
SO2 NAAQS are directed to submit, as a part of the SIP due
18 months after an area is designated as nonattainment, provisions
requiring permits for the construction and operation of new or modified
stationary sources anywhere in the nonattainment area. Prior to
adoption of the SIP revision addressing major source nonattainment NSR
for SO2 nonattainment areas, the requirements of 40 CFR part
51, appendix S will apply. Nonattainment NSR requirements include but
are not limited to:
Installation of Lowest Achievable Emissions Rate (LAER)
control technology;
Offsetting new emissions with creditable emissions
reductions;
A certification that all major sources owned and operated
in the State by the same owner are in compliance with all applicable
requirements under the CAA;
An alternatives and siting analysis demonstrating that the
benefits of a proposed source significantly outweigh the environmental
and social costs imposed as a result of its location, construction, or
modification; and
Public comment on the permit.
Minor NSR programs must meet the statutory requirements in section
110(a)(2)(C) of the CAA which requires ``* * * regulation of the
modification and construction of any stationary source * * * as
necessary to assure that the [NAAQS] are achieved.'' These programs
must be established in each State within 3 years of the promulgation of
a new or revised NAAQS.
b. Comments and Responses
Several commenters stated that in order to avoid confusion and lag
time as it relates to PSD/NSR and permitting activities, which must be
taken by States following the promulgation of the revised NAAQS, EPA
must provide guidance as soon as possible related to these issues.
Commenters also stated that EPA must develop guidance as soon as
possible to address the screening tools for PSD/NSR such as SILs, SERs,
SMCs, and the development of increments. Several commenters also stated
that guidance should be provided as it relates to the use of AERMOD to
address PSD issues.
The EPA acknowledges that a decision to promulgate a new short-term
SO2 NAAQS will have implications for the air permitting
process. The full extent of how a new short-term SO2 NAAQS
will affect the NSR process will need to be carefully evaluated. First,
major new and modified sources applying for NSR/PSD permits will
initially be required to demonstrate that their proposed emissions
increases of SO2 will not cause or contribute to a violation
of any NAAQS or PSD increments for SO2, including the new 1-
hour SO2 NAAQS. In addition, we believe that section 166(c)
of the CAA authorizes EPA to consider the need to promulgate a new 1-
hour increment. Historically, EPA has developed increments for each
applicable averaging period for which a NAAQS has been promulgated.
However, increments for a particular pollutant do not necessarily need
to match the averaging periods that have been established for NAAQS for
the same pollutant. Environmental Defense Fund, Inc. v. EPA, 898 F.2d
183, 189-190 (DC Cir. 1990) (``* * * the `goals and purposes' of the
PSD program, set forth in Sec. 160, are not identical to the criteria
on which the ambient standards are based.'') Thus, we would need to
evaluate the need for a new 1-hour SO2 increment in
association with the goals and purposes of the statutory PSD program
requirements.
We agree with the commenters that there may be a need for EPA to
provide additional screening tools or to revise existing screening
tools that are frequently used under the NSR/PSD program for reducing
the burden of completing SO2 ambient air impact analyses.
These screening tools include the SILs, as mentioned by the commenter,
but also include the SER for emissions of SO2 and the SMC
for SO2. The existing sceening tools apply to the averaging
periods used to define the existing NAAQS for SO2, including
the annual, 24-hour, and 3-hour averaging periods. EPA intends to
evaluate the need for possible changes or additions to each of these
useful screening tools for SO2 due to the revision of the
SO2 NAAQS to provide for a 1-hour standard. We believe it is
highly likely that in order to be most useful for implementing the new
1-hour averaging period for NSR purposes, new 1-hour screening values
will be appropriate.
Finally, in response to the comment concerning the need for
additional guidance as it relates to the use of AERMOD to address PSD
issues, EPA anticipates providing additional technical guidance on
modeling and analysis as a part of the SIP demonstration process. As
stated previously, EPA intends to solicit public comment on guidance
regarding modeling, and also solicit public comment on additional
implementation planning guidance. However, EPA believes that the air
quality models currently required for NSR/PSD permitting as provided in
the EPA's Guideline on Air Quality Models, Appendix W of CFR 40 Part 51
would be appropriate for demonstrating compliance with the revised
SO2 NAAQS under these programs. At this time, EPA is not
considering modifying the AERMOD dispersion model and its underlying
science for predicting SO2 concentrations to accommodate the
revised NAAQS for SO2.
[[Page 35580]]
c. Current Approach
In the preamble to the proposed regulation, EPA noted that ``PSD
permit requirements are effective on the promulgation date of a new or
revised standard.'' However, this statement did not reflect an
important distinction that needs to be clarified here. Under section
51.166(b)(49)(i) and 52.21(b)(50)(i) of EPA's regulations, a pollutant
that has not been regulated previously would become a ``regulated NSR
pollutant'' upon promulgation of a NAAQS. See, 75 FR 17004, 17018-19.
However, in the case of pollutants that are already ``regulated NSR
pollutants,'' at the time a new NAAQS is promulgated or an existing
NAAQS is revised, EPA interprets the CAA and EPA regulations to require
implementation of the new or revised standard in the Federal PSD
permitting process upon the effective date of any new or revised
standards. Section 165(a)(3) of the CAA and section 52.21(k) of EPA's
regulations require that a permit applicant demonstrate that it will
not cause or contribute to a violation of ``any'' NAAQS. See,
Memorandum from Stephen D. Page, Director of EPA Office of Air Quality
Planning and Standards, ``Applicability of the Federal Prevention of
Significant Deterioration Permit Requirements to New and Revised
National Ambient Air Quality Standards'' (April 1, 2010).
Amendments to the existing PSD requirements set forth in EPA
regulations concerning SILs, SERs and SMCs may involve notice and
comment rulemaking which could take at least one year to complete. For
PM2.5, EPA developed SERs under the initial NSR
implementation requirements for PM2.5. See 73 FR 28321, May
16, 2008. The SILs and SMC for PM2.5 are being developed
under a subsequent rulemaking simultaneously with the promulgation of
PM2.5 increments, pursuant to a CAA schedule that allows EPA
2 years from the promulgation of new and revised NAAQS to promulgate
increments. Under such an approach, SILs and SMC are not available
until the increments are promulgated. States and industry have
criticized that approach because it has left State permitting
authorities without an EPA-approved de minimis value that could be used
in determining the level of analysis that individual PSD sources must
undergo, and could result in more detailed analyses for sources that
will have only have de miminis impacts on the NAAQS.
To address this concern, we believe it is appropriate to proceed
with development of the PSD screening tools in advance of an increment
rulemaking to hasten their availability. In addition, we are assessing
the possibility of developing interim screening tools that can be used
by States prior to the completion of the SIP-development process if the
States establish an appropriate record for individual permitting
actions based on the supporting technical information provided by EPA.
It is our expectation, that if such interim screening tools are
appropriate, we would make the interim SIL and the supporting record
for EPA's assessment available before the effective date of the new 1-
hour SO2 NAAQS to facilitate more efficient PSD permit
reviews once the new standard becomes effective.
3. General Conformity
a. Approach Described in the Proposal
Section 176(c) of the CAA requires that all Federal actions conform
to an applicable implementation plan developed pursuant to section 110
and part D of the CAA. The EPA rules developed under section 176(c)
prescribe the criteria and procedures for demonstrating and assuring
conformity of Federal actions to a SIP. Each Federal agency must
determine that any actions covered by the general conformity rule
conform to the applicable SIP before the action is taken. The criteria
and procedures for conformity apply only in nonattainment areas and
those nonattainment areas redesignated to attainment since 1990
(``maintenance areas'') with respect to the criteria pollutants under
the CAA: \39\ carbon monoxide (CO), lead (Pb), nitrogen dioxide
(NO2), ozone (O3), particulate matter (PM2.5 and
PM10), and sulfur dioxide (SO2). The general
conformity rules apply one year following the effective date of
designations for any new or revised NAAQS.\40\
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\39\ Criteria pollutants are those pollutants for which EPA has
established a NAAQS under section 109 of the CAA.
\40\ Transportation conformity is required under CAA section
176(c) (42 U.S.C. 7506(c) to ensure that Federally supported highway
and transit project activities are consistent with (``conform to'')
the purpose of the SIP. Transportation conformity applies to areas
that are designated nonattainment, and those areas redesignated to
attainment after 1990 (``maintenance areas'' with plans developed
under CAA section 175A) for transportation-related criteria
pollutants. Due to the relatively small amounts of sulfur in
gasoline and on-road diesel fuel, transportation conformity does not
apply to the SO2 NAAQS. 40 CFR 93.102(b)(1).
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The general conformity determination examines the impacts of direct
and indirect emissions related to Federal actions. The general
conformity rule provides several options to satisfy air quality
criteria, such as modeling or offsets, and requires the Federal action
to also meet any applicable SIP requirements and emissions milestones.
The general conformity rule also requires that notices of draft and
final general conformity determinations be provided directly to air
quality regulatory agencies and to the public by publication in a local
newspaper.
b. Current Approach
EPA did not receive any comments on this aspect of the discussion
in the proposal and expects to follow that approach.
E. Transition From the Existing SO2 NAAQS to a Revised
SO2 NAAQS
a. Proposal
In addition to proposing a short-term 1-hour SO2 NAAQS,
EPA proposed to revoke the annual and 24-hour standards (annual 0.03
ppm and 24-hour 0.14 ppm). Specifically, EPA proposed that the level
for the 1-hour standard for SO2 be a range between 50-100
ppb, and took comment on setting the level of the standard up to 150
ppb. We explained that if the Administrator sets the 1-hour standard at
100 ppb or lower, EPA proposed to revoke the 24-hour standard. If the
Administrator set the level of the 1-hour standard between a range of
100-150 ppb, then EPA proposed to retain the 24-hour standard.
We explained that if EPA revised the SO2 NAAQS and
revoked either the annual or 24-hour standard, EPA would need to
promulgate adequate anti-backsliding provisions. The CAA establishes
anti-backsliding requirements where EPA relaxes a NAAQS. Here, in EPA
replacing the annual and 24-hour standards with a short term 1-hour
standard, EPA must address the section 172(e) anti-backsliding
provision of the CAA and determine whether it applies on its face or by
analogy, and what provisions are appropriate to provide for transition
to the new standard. States will need to insure that the health
protection provided under the prior SO2 NAAQS continues to
be achieved as well as maintained as States begin to implement the new
NAAQS. This means that States are directed to continue implementing
attainment and maintenance SIPs associated with the prior
SO2 NAAQS until such time as they are subsumed by any new
planning and control requirements associated with the new NAAQS.
Whether or not section 172(e) directly applies to EPA's final
action on the SO2 NAAQS, EPA has previously looked to other
provisions of the CAA to determine how to address anti-
[[Page 35581]]
backsliding. The CAA contains a number of provisions that indicate
Congress's intent to not allow provisions from implementation plans to
be altered or removed if the plan revision would jeopardize the air
quality protection being provided by the existing plan when EPA revises
a NAAQS to make it more stringent. For example, section 110(l) provides
that EPA may not approve a SIP revision if it interferes with any
applicable requirement concerning attainment and RFP, or any other
applicable requirement under the CAA. In addition, section 193 of the
CAA prohibits the modification of a control, or a control requirement,
in effect or required to be adopted as of November 15, 1990 (i.e.,
prior to the promulgation of the Clean Air Act Amendments of 1990),
unless such a modification would ensure equivalent or greater emissions
reductions. Further, section 172(e) of the CAA specifies that if EPA
revises a NAAQS to make it less stringent than a previous NAAQS,
control obligations no less stringent than those that apply in
nonattainment area SIPs may not be relaxed, and adopting those controls
that have not yet been adopted as needed may not be avoided. The intent
of Congress, concerning the aforementioned sections of the CAA, was
confirmed in a recent DC Circuit Court opinion on the Phase I ozone
implementation rule. See South Coast Air Quality Management Dist. v.
EPA, 472 F.3d 882 (DC Cir. 2006).
To ensure that the anti-backsliding provisions and principles of
section 172(e) are met and applied upon EPA revocation of the annual
and 24-hour standards, EPA is providing that those SO2 NAAQS
will remain in effect for one year following the effective date of the
initial designations under section 107(d)(1) for the new SO2
NAAQS before the current NAAQS are revoked in most attainment areas.
However, any existing SIP provisions under CAA sections 110, 191 and
192 associated with the annual and 24-hour SO2 NAAQS will
remain in effect, including all currently implemented planning and
emissions control obligations, including both those in the State's SIP
and that have been promulgated by EPA in FIPs. This will ensure that
both the new nonattainment NSR requirements and the general conformity
requirements for a revised standard are in place so that there will be
no gap in the public health protections provided by these two programs.
It will also ensure that all nonattainment areas under the annual and/
or 24-hour NAAQS and all areas for which SIP calls have been issued
will continue to be protected by currently required control measures.
EPA is also providing that the annual and 24-hour NAAQS remain in
place for any current nonattainment area, or any area for which a State
has not fulfilled the requirements of a SIP call, until the affected
area submits, and EPA approves, a SIP with an attainment,
implementation, maintenance and enforcement SIP which fully addresses
the attainment and maintenance requirements of the new SO2
NAAQS. This, in combination with the CAA mechanisms provided in
sections 110(l), 193, and 172(e) will help to ensure that continued
progress is made toward timely attainment of the SO2 NAAQS.
Also, in light of the nature of the new SO2 NAAQS, the lack
of classifications (and mandatory controls associated with such
classifications pursuant to the CAA), and the small number of current
nonattainment areas, and areas subject to SIP calls, EPA believes that
retaining the current standard for a limited period of time until
attainment and maintenance SIPs are approved for the new standard in
current nonattainment areas and SIP call areas, and one year after
designations in other areas, will adequately serve the anti-backsliding
requirements and goals of the CAA.\41\
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\41\ The areas that are currently designated as nonattainment
for the pre-existing SO2 primary NAAQS are Hayden, AZ;
Armstrong, PA; Laurel, MT; Piti, GU; and Tanguisson, GU. The areas
that are designated nonattainment for both the primary and the
secondary standards are East Helena, MT, Salt Lake Co, MT, Toole Co,
UT, and Warren Co, NJ. (See http://www.epa.gov/oar/oaqps/greenbk/lnc.html). The Billings/Laurel, MT, area is the only area currently
subject to a SIP call.
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b. Comments and Responses
Several commenters stated that they support EPA's proposal stating
that the annual and 24-hour SO2 NAAQS EPA would remain in
effect for one year following the effective date of the initial
designations under section 107(d)(1) for the revised SO2
NAAQS before the current NAAQS are revoked in most attainment areas.
The commenters also support EPA's proposal that any existing SIP
provisions under CAA sections 110, 191 and 192 associated with the
annual and 24-hour SO2 NAAQS would remain in effect,
including all currently implemented planning and emissions control
obligations, including both those in the State's SIP and that have been
promulgated by EPA in FIPs. Several commenters also stated that they
support EPA's proposal that an area's nonattainment designation and the
subsequent CAA requirements under the current SO2 NAAQS will
remain in effect until the affected State submits, and EPA approves a
SIP which meets all of the relevant CAA requirements for the affected
nonattainment area. EPA appreciates the support of the commenters on
its strategy for addressing the anti-backsliding requirements related
to the current and revised SO2 standard, pursuant to section
172(e) of the CAA.
One commenter, however, stated that while they support EPA's
proposal to address the anti-backsliding provisions of section 172(e)
of the CAA, they believe that EPA's proposal is deficient in several
respects. The commenter stated that EPA's proposal to not terminate the
annual and 24-hour standards for SO2 in any nonattainment
area, or any area for which a State has not fulfilled the requirements
of a SIP call, until after the affected area submits and EPA approves a
SIP with an attainment demonstration which fully ``addresses'' the
attainment requirements of the revised SO2 NAAQS is flawed.
The commenter states that EPA's use of the term ``addresses'' is
impermissibly and arbitrarily ambiguous and that the agency needs to
clarify that ``fully addressing'' the attainment requirements of the
revised NAAQS actually means providing for timely attainment of the
NAAQS, and the submittal of a SIP that fully meets all of the
requirements of section 110 and part D of Title I of the CAA, including
sections 172, 173, and 191-193 of the CAA.
Another commenter stated that the 24-hour SO2 standard
should not be revoked in attainment areas until EPA approves section
110(a)(2) ``infrastructure'' SIPs under the new 1-hour standard for
such areas, in order to avoid delays in between attainment designation
and such SIP approvals resulting in leaving the public unprotected or
creating inter-state conflict that triggers section 126 petitions. This
commenter further stated that the annual SO2 standard should
not be revoked until EPA approves SIPs in attainment areas under the
future SO2 secondary standard, which may also be based on an
annual averaging time.
EPA agrees with the comment made by the commenter regarding the
need to approve SIPs in nonattainment areas (and in SIP call areas)
before revoking the 24-hour and annual NAAQS for such areas. EPA
clarifies that for those areas designated as nonattainment for the
current NAAQS, or areas which have not met the requirements of a SIP
call, that the State must submit a SIP that meets all of the applicable
CAA requirements as they relate to section 110 and part D of Title I of
the CAA, including sections 110(a), 172, 173, and 191-193 of the CAA.
In addition to the
[[Page 35582]]
submittal of the SIP related to these requirements, EPA must approve
the submittal for the area before the current standard can be revoked
for the affected area.
EPA disagrees with the comment. This rulemaking concerns only the
primary standards for SO2. 74 FR at 64812 n. 2. The annual
SO2 standard is a primary standard, not a secondary
standard. See 40 CFR section 50.4 (a). The exclusive secondary standard
for SO2 is the 3-hour standard codified in 40 CFR section
50.5. EPA is not determining the adequacy of this secondary standard in
this review or this rulemaking, as just noted. The commenter's request
to retain the annual primary standard until SIPs reflecting a new
secondary standard are approved is effectively a request to amend the
present secondary standard, and is therefore inappropriate given the
scope of this review. In any case, in the event that any substantive
responsive to this comment is required, air quality information
indicates that a 1-hour standard of 75 ppb is estimated to generally
keep annual SO2 concentrations well below the level of the
current annual standard. 74 FR at 64845. Thus, there would be no loss
of protection to public welfare due to revocation of the annual primary
standard.
EPA further disagrees with the commenter's request that we not
revoke the 24-hour standard in attainment areas before section
110(a)(2) ``infrastructure'' SIPs are approved under the new 1-hour
SO2 standard. An area that has shown it has attained the 24-
hour standard and that is not the subject of a SIP call, even after
revocation of the 24-hour standard, will still have in its SIP its
prior ``infrastucture'' SIP elements. There is no need to delay
revocation when that will not cause the area to become subject to a new
SIP under the new 1-hour NAAQS any faster than the statute already
requires (i.e., three years from the date of promulgation of the new
NAAQS). Furthermore, as we have explained in sections III, IV, V and VI
of this preamble, all areas are required by section 110(a)(1) of the
Clean Air Act to submit such SIPs by June 2013, and we expect that to
be approved they will all need to show attainment, implementation,
maintenance and enforcement of the new NAAQS as expeditiously as
practicable, which we believe is no later than August 2017. EPA
believes this anticipated approach would more than sufficiently address
the backsliding concerns raised by the commenter.
c. Final
EPA is making no changes to the proposed rule's discussion of the
transition strategy discussion for SO2 with the exception of
the clarifications noted above.
VII. Appendix T--Interpretation of the Primary NAAQS for Oxides of
Sulfur and Revisions to the Exceptional Events Rule
EPA proposed to add Appendix T, Interpretation of the Primary
National Ambient Air Quality Standards for Oxides of Sulfur, to 40 CFR
Part 50 in order to provide monitoring data handling procedures for the
proposed SO2 1-hour primary standard. The proposed section
50.17 which sets the averaging period, level, indicator, and form of
the NAAQS referred to this Appendix T. The proposed Appendix T detailed
the computations necessary for determining when the proposed 1-hour
primary SO2 NAAQS is met based on data from ambient
monitoring and also addressed monitoring data reporting, data
completeness considerations, and rounding conventions.
EPA proposed two versions of Appendix T. The first applied to a 1-
hour primary standard based on the annual 4th high value form, while
the second applied to a 1-hour primary standard based on the 99th
percentile daily value form. The final version of the Appendix reflects
our choice to adopt the 99th percentile daily form (see section II. E.3
above).
For the 1-hour primary standard, EPA proposed monitoring data
handling procedures, a cross-reference to the Exceptional Events Rule,
a grant of discretion for the Administrator to consider otherwise
incomplete monitoring data to be complete, and a provision addressing
the possibility of there being multiple SO2 monitors at one
site. EPA is finalizing these proposals, with one change from the
proposal with regard to the multiple monitor provision.
EPA is also making certain drafting changes to the proposed
regulatory text to clarify certain points and to assure that the
regulatory text conforms with EPA's intentions as stated in the
preamble. Specifically, EPA has slightly edited the text of the rule
from that proposed by adding the phrase ``at an ambient air monitoring
site'' to section 50.17 (b) and to section 1.1 of Appendix T to part
50, and also by adding a section 50.17 (c) stating that the level of
the standard is to be measured by an FRM found in Appendix A or A-1 to
Part 50, or by a properly designated FEM. Both of these provisions are
being added to conform the text of the new 1-hour standard to the
language of other NAAQS. See. e.g. the text of the 8-hour primary
standard for ozone in section 50.10 (a) and (b). The reference to ``at
an ambient monitoring site'' makes clear that the regulatory text
refers to situations where compliance with a NAAQS is measured by means
of monitoring. This text does not restrict or otherwise address
approaches which EPA or States may use to implement the new 1-hour
NAAQS, which may include, for example, use of modeling (see sections
III--VI above). See CAA sections 107 (d) (3) (A) (any ``air quality
data'' may be used for redesignations); 110 (a) (1) (which does not
address the issue of the types of data States may use in devising plans
for implementation, maintenance, and enforcement of a primary NAAQS);
192 (a) (which does not specify the types of data that may support a
demonstration that a non-attainment area has attained a NAAQS).
Similarly, EPA notes that Appendix T applies when ambient monitoring
data is gathered and utilized in support of the new 1-hour
SO2 NAAQS. As noted in sections III, IV, V, and VI above,
there are circumstances when EPA is considering use of modeling in the
SO2 NAAQS implementation effort, and other considerations
would apply if and to the extent modeling is utilized.
The EPA is also making SO2-specific changes to the
deadlines in 40 CFR 50.14, by which States must flag ambient air data
that they believe have been affected by exceptional events and submit
initial descriptions of those events, and to the deadlines by which
States must submit detailed justifications to support the exclusion of
those data from EPA monitoring-based determinations of attainment or
nonattainment with the NAAQS.
A. Interpretation of the Primary NAAQS for Oxides of Sulfur
The purpose of a monitoring data interpretation rule for the
SO2 NAAQS is to give effect to the form, level, averaging
time, and indicator specified in the regulatory text at 40 CFR 50.17,
anticipating and resolving in advance various future ambiguities that
could otherwise occur regarding use of ambient monitoring data. The new
Appendix T provides definitions and requirements that apply to the new
1-hour primary standard for SO2. The requirements concern
how ambient monitoring data are to be reported, what ambient monitoring
data are to be considered (including the issue of which of multiple
monitors' data sets will be used when more than one monitor has
operated at a site), and the
[[Page 35583]]
applicability of the Exceptional Events Rule to the primary
SO2 NAAQS.
1. Proposed Interpretation of the Standard Based on Data From Ambient
Monitoring
With regard to monitoring data completeness for the proposed 1-hour
primary standard, the proposed Appendix T followed past EPA practice
for other NAAQS pollutants by requiring that in general at least 75% of
the monitoring data that should have resulted from following the
planned monitoring schedule in a period must be available for the key
air quality statistic from that period to be considered valid. For the
1-hour primary SO2 NAAQS, the key air quality statistics are
the daily maximum 1-hour concentrations in three successive years. It
is important that sampling within a day encompass the period when
concentrations are likely to be highest and that all seasons of the
year are well represented. Hence, the 75% requirement was proposed to
be applied at the daily and quarterly levels.
Recognizing that there may be years with incomplete data, the
proposed Appendix T for the 99th percentile form provided that a design
value derived from incomplete monitoring data will nevertheless be
considered valid if the relevant one of two diagnostic substitution
tests validated such a design value as being either above the NAAQS
level or equal to or below the NAAQS level.
The first proposed diagnostic data substitution test, relevant when
the design value derived from incomplete data was equal to or below the
NAAQS level, was intended to identify those cases with incomplete
monitoring data in which it nevertheless is very likely, if not
virtually certain, that the daily 1-hour design value would have been
observed to be less than or equal to the level of the NAAQS if
monitoring data had been minimally complete. This test involved the
substitution of a high historical concentration for any missing data.
The second proposed diagnostic data substitution test, relevant when
the design value derived from incomplete data was above the NAAQS
level, was intended to identify those cases with incomplete monitoring
data in which it nevertheless is very likely, if not virtually certain,
that the daily 1-hour design value would have been observed to be above
the level of the NAAQS if monitoring data had been minimally complete.
This test involved the substitution of a low historical concentration
for any missing data.
It should be noted that one possible outcome of applying the
relevant proposed substitution test is that a 3-year period with
incomplete monitoring data may nevertheless be determined to not have a
valid design value and thus to be unusable in making 1-hour primary
NAAQS compliance determinations based on monitoring for that 3-year
period.
Also, we proposed that the Administrator have general discretion to
use incomplete monitoring data based on case specific factors, either
at the request of a State or at her own initiative. Similar provisions
existed already for some other NAAQS.
The 99th percentile version of the proposed Appendix T provided a
table for determining which day's maximum 1-hour concentration will be
used as the 99th percentile concentration for the year. The proposed
table is similar to one used now for the 24-hour PM2.5 NAAQS
and the new 1-hour NO2 NAAQS, which are both based on a 98th
percentile form, but adjusted to reflect a 99th percentile form for the
1-hour primary SO2 standard. The proposed Appendix T also
provided instructions for rounding (not truncating) the average of
three annual 99th percentile hourly concentrations before comparison to
the level of the primary NAAQS.
2. Comments on Interpretation of the Standard
Several commenters expressed support for EPA's proposed 75%
completeness requirement for daily and quarterly monitoring data. A
comment was received that the substitution test should not be used to
make attainment or non-attainment designations. This commenter also
said that the same completeness requirement as used for nonattainment
should be used for attainment. Another commenter agreed that there
should be completeness criteria, but thought that monitoring data
should be substituted to make the set only 75% complete. We received
one comment that the computation of design values where multiple
monitors are present at a site should be averaged and not taken from a
designated primary monitor. We received no comment on the provision
which would afford the Administrator (or her delegee) discretion to use
incomplete monitoring data based on specified factors and accordingly
are adopting that provision as proposed.
3. Conclusions on Interpretation of the Standard
Consistent with the Administrator's decision to adopt a 99th
percentile form for the 1-hour NAAQS, the final version of Appendix T
is based on that form.
We agree with the three comments expressing the view that the
requirement for 75% monitoring data completeness per quarter should
apply with respect to the 1-hour standard. The final rule includes this
requirement.
We agree that nonattainment based on data from ambient monitoring
should not be declared without a very high confidence that actual air
quality did not meet the NAAQS, but we believe the proposed (and final)
substitution test provides this irrefutable proof. In the relevant
substitution test (Appendix T section 3.c.iii), the lowest daily
maximum concentration observed in the same calendar quarter within the
3-year period is the value used in the substitution. Moreover, to guard
against the possibility that even this lowest observed value is
unrepresentative because only a small number of days that happened to
have had poor air quality have valid monitoring data, substitution is
permitted only if there are at least 200 days across the three matching
quarters of the three years under consideration for which 75 percent of
the hours in the day have reported concentrations. (If less than 200
days are available, the outcome is that no conclusion can be reached
based on data from monitoring as to whether the NAAQS is met, an
outcome which satisfies the concern expressed by the commenter.) While
it is conceivable that the actual daily maximum concentration on the
day(s) without sufficiently complete data could have been even lower
than the value selected as the substitute value, the value that is
selected for substitution will be quite low, and therefore it is
extremely unlikely to be a candidate for selection as the annual 99th
percentile daily maximum concentration. The actual effect of the data
substitution, if any, is to change which of the actually observed and
ranked daily maximum concentrations during the year is identified as
the 99th percentile; the direction of the change, if any, will always
be towards a lower design value. For example, if the substitution test
of section 3.c.iii is used because there is one quarter of 92 days is
missing 70 of its 92 daily maximum concentration values; causing there
to be only 295 days with valid daily values for the whole year, it
would be necessary to substitute 47 values to make that quarter 75
percent complete. This would result in 343 days of actual or
substituted monitoring data for the year. The increase from 292 days to
342 days would cause the annual 99th percentile value to shift from the
3rd highest value to the 4th highest. Since a low
[[Page 35584]]
concentration is being used for the substitution, it is impossible for
the 4th highest value to itself be a substituted value. If this shift
results in the 3-year design value remaining above the NAAQS, the
failure to meet the NAAQS is confirmed. If this shift results in the 3-
year design value changing to be equal to or below the NAAQS, under the
terms of the substitution test the outcome is that no conclusion could
be reached based on this ambient monitoring data as to whether the
NAAQS is met. Since either the same or a lower ranking actually
measured concentration will always be identified, it is impossible for
the outcome of the substitution test of section 3.c.iii to be that an
area truly meeting the NAAQS based on ambient monitoring data is
determined to not meet it based on ambient monitoring data.
The commenter who said that the same completeness requirement
should be used for nonattainment as for attainment appears to have been
referring to a particular feature of the proposed diagnostic
substitution test rather than to the basic completeness requirement of
75%, which in both the proposal and the final rule applies equally to
both attainment and nonattainment situations. This particular feature
is discussed in the next paragraph.
The commenter who said that it is appropriate to substitute data to
make the set only 75% complete appears to have taken note that in the
proposed substitution test relevant in the case of an incomplete design
value equal to or below the NAAQS (section 3.c.ii), data are
substituted until 100% completeness is reached for the affected
quarter, while in the test relevant in the case of an incomplete design
value above the NAAQS (section 3.c.iii) data are substituted only until
75% completeness is reached. EPA believes this distinction is
appropriate, and we have retained the 100% substitution limit in the
final rule. In the case of an incomplete design value that is equal to
or below the NAAQS, the concern is that the actual concentrations on
the days without a valid daily maximum 1-hour concentration may have
been quite high such that the concentration on one of those days would
have been selected as the annual 99th percentile value. To be selected
as the annual 99th percentile value, a daily maximum must be ranked no
lower than the 4th highest daily value for the year. If substitution
stopped when 75% of the days in a quarter had an actual or substituted
value, there could be a situation in which only one, two, or three
historical high values would need to be substituted to reach the 75%
limit. It would therefore be possible for one of the actually measured
concentrations (for the same or another quarter) to be identified as
the annual 99th percentile value even if the substitution value is
higher than any value actually measured, defeating the very purpose of
the diagnostic test for an incomplete design value below the NAAQS,
which is to essentially rule out the possibility of not meeting the
NAAQS (when making monitoring-based determinations). The simplest way
to ensure that at least four values are substituted (when there are at
least four missing daily values) is to require substitution up to the
100% limit.
With regard to situations with multiple monitors operating at one
site, we note that there are few cases of this situation for
SO2 monitoring. Of over 500 SO2 monitoring sites
in operation any time during 2007-2009, for example, only seven
stations reported 1-hour data to the Air Quality System under two or
more distinct Pollutant Occurrence Codes (POC). In the same period,
collocated monitors reported data to AQS under distinct POCs for only
one of over 400 nitrogen dioxide sites, for only two of almost 400
carbon monoxide sites, and for only eight of almost 1300 ozone sites.
Even so, we believe is it important to have a well defined monitor data
handling procedure for such situations. Also, there is a practical
advantage in implementation if the same or similar procedure is used
across NAAQS pollutants especially for these four gaseous pollutants
that are measured on a 1-hour basis. A procedure that is simple to
implement also has advantages in implementation. Finally, the procedure
should not introduce any upward or downward bias in the determination
of the design value for the monitoring site.\42\
---------------------------------------------------------------------------
\42\ Selecting the maximum or minimum observed concentration for
an hour, the maximum or minimum annual 99th percentile, or the
maximum or minimum three-year design value would introduce such a
bias. Averaging multiple 1-hour measurements when available,
designating one monitor as primary and using a second monitor's
measurement only when the primary monitor fails to give a valid
measurement, or simply choosing to use the data record from only one
of the monitors (on some basis that is independent of the
concentration values obtained) would not introduce such a bias.
---------------------------------------------------------------------------
The proposed procedure for multiple SO2 monitors was the
same as EPA recently proposed and finalized for the new 1-hour NAAQS
for nitrogen dioxide, where there were no adverse comments received on
the proposal (75 FR 6474, February 9, 2010). It is also the same as
recently proposed in the reconsideration of the 8-hour ozone NAAQS (75
FR 2938, January 19, 2010). In the proposed procedure, in general, data
from two monitors would never be mixed within a year but data from
different monitors in different years could be used to calculate the 3-
year design value. As noted above, one commenter on the SO2
proposal suggested that instead of designating a primary monitor when
there are two monitors at a site, the measurements for an hour from
multiple monitors should be averaged instead. EPA has also received at
least one comment disagreeing with the recent proposal regarding
multiple ozone monitors. The comment in the ozone rulemaking favored
hour-by-hour substitution of data from a secondary monitor when the
designated primary monitor has not given a value measurement, as
opposed to the proposed restriction against mixing data within a year.
These comments have caused us to rethink the direction set in the final
NO2 rule and in the proposals for SO2 and ozone.
We now believe that substitution of monitoring data hour-by-hour is an
acceptable and in some ways superior approach to the other possible
approaches, while averaging hour-by-hour would be unduly complex. Also,
averaging hour-by-hour might not be transparent depending on whether
the averaging is done at the monitoring agency before submission to EPA
or by EPA as part of calculating a design value. However, in light of
the rarity of collocated monitors, it would be an unwarranted demand on
limited EPA resources to develop and maintain software for hour-by-hour
data substitution. Also, an hour-by-hour data substitution approach
depends on the advance designation of a primary monitor, which itself
could introduce confusion and would require software changes to EPA's
data system. Therefore, EPA believes that the most practical, and still
a technically valid approach, is to allow monitoring agencies the
option of hour-by-hour substitution between secondary and primary
monitors before submission of data to EPA, and for EPA to select for
use in calculating design values the one monitoring data record which
has the highest degree of completeness for a given year. The final rule
is based on this approach. EPA will also consider this approach when
finalizing the ozone NAAQS reconsideration rule, and when proposing
data interpretation provisions for a planned rulemaking to review the
carbon monoxide NAAQS. The already finalized procedures for nitrogen
dioxide data interpretation will be
[[Page 35585]]
implemented as promulgated, but will affect only an extremely small
number of collocated SO2 monitoring situations.
Finally, as proposed, the final version of Appendix T has a cross
reference to the Exceptional Events Rule (40 CFR 50.14) with regard to
the exclusion of monitoring data affected by exceptional events. In
addition, the specific steps for including such data in completeness
calculations while excluding such data from actual design value
calculations is clarified in Appendix T.
B. Exceptional Events Information Submission Schedule
The Exceptional Events Rule at 40 CFR 50.14 contains generic
deadlines for a State to submit to EPA specified information about
exceptional events and associated air pollutant concentration data. A
State must initially notify EPA that data have been affected by an
event by July 1 of the calendar year following the year in which the
event occurred; this is done by flagging the data in AQS and providing
an initial event description. The State must also, after notice and
opportunity for public comment, submit a demonstration to justify any
claim within 3 years after the quarter in which the data were
collected. However, if a regulatory decision based on the data (for
example, a designation action) is anticipated, the schedule to flag
data in AQS and submit complete documentation to EPA for review is
shortened, and all information must be submitted to EPA no later than
one year before the decision is to be made.
These generic deadlines are suitable for the period after initial
designations have been made under a NAAQS, when the decision that may
depend on data exclusion is a redesignation from attainment to
nonattainment or from nonattainment to attainment. However, these
deadlines present problems with respect to initial designations under a
newly revised NAAQS. One problem is that some of the deadlines,
especially the deadlines for flagging some relevant data, may have
already passed by the time the revised NAAQS is promulgated. Until the
level and form of the NAAQS have been promulgated a State does not know
whether the criteria for excluding data (which are tied to the level
and form of the NAAQS) were met on a given day. Another problem is that
it may not be feasible for information on some exceptional events that
may affect final designations to be collected and submitted to EPA at
least one year in advance of the final designation decision. This could
have the unintended consequence of EPA designating an area
nonattainment because of uncontrollable natural or other qualified
exceptional events.
The Exceptional Events Rule at section 50.14(c)(2)(v) indicates
``when EPA sets a NAAQS for a new pollutant, or revises the NAAQS for
an existing pollutant, it may revise or set a new schedule for flagging
data for initial designation of areas for those NAAQS.''
For the specific case of SO2, the signature date for the
revised SO2 NAAQS is June 2, 2010. State/Tribal area
designations recommendations will be due by June 2, 2011, and EPA will
make initial area designations under the revised NAAQS by June 1, 2012
(since June 2, 2012 would be on a Saturday) and will be informed by air
quality data from the years 2008-2010 or 2009-2011 if there is
sufficient data for these data years and by any refined modeling that
is conducted. (See Sections III, V and VI above for more detailed
discussions of the designation schedule and what data EPA expects to
use.) Because final designations would be made by June 1, 2012, all
events to be considered during the designations process would have to
be flagged and fully documented by States one year prior to
designations, by June 1, 2011. A State would not be able to flag and
submit documentation regarding events that occurred between June to
December 2011 by one year before designations are made in June 2012.
EPA is adopting revisions to 40 CFR 50.14 only to change submission
dates for information supporting claimed exceptional events affecting
SO2 data. The rule text at the end of this notice shows the
changes that will apply to the new 1-hour SO2 NAAQS. For air
quality data collected in 2008, we are extending the generic July 1,
2009 deadline for flagging data (and providing a brief initial
description of the event) to October 1, 2010. EPA believes this
extension will provide adequate time for States to review the impact of
exceptional events from 2008 on the revised standard and notify EPA by
flagging the relevant data in AQS. EPA is not changing the
foreshortened deadline of June 1, 2011 for submitting documentation to
justify an SO2-related exceptional event from 2008. We
believe the generic deadline provides adequate time for States to
develop and submit proper documentation.
For data collected in 2009, EPA is extending the generic deadline
of July 1, 2010 for flagging data and providing initial event
descriptions to October 1, 2010. EPA is retaining the deadline of June
1, 2011 for States to submit documentation to justify an
SO2-related exceptional event from 2009. For data collected
in 2010, EPA is promulgating a deadline of June 1, 2011 for flagging
data and providing initial event descriptions and for submitting
documentation to justify exclusion of the flagged data. EPA believes
that this deadline provides States with adequate time to review and
identify potential exceptional events that occur in calendar year 2010,
even for those events that might occur late in the year. EPA believes
these deadlines will be feasible because experience suggests that
exceptional events affecting SO2 data are few in number and
easily assessed, so no State is likely to have a large workload.
If a State intends 2011 data to be considered in SO2
designations, 2011 data must be flagged and detailed event
documentation submitted 60 days after the end of the calendar quarter
in which the event occurred or by March 31, 2012, whichever date occurs
first. Again, EPA believes these deadlines will be feasible because
experience suggest that exceptional events affecting SO2
data are few in number and easily assessed, so no State is likely to
have a large workload.
Table 1 summarizes the designation deadlines discussed in this
section and provides designation schedule information from recent,
pending or prior NAAQS revisions for other pollutants. EPA is revising
the final SO2 exceptional event flagging and documentation
submission deadlines accordingly to provide States with reasonably
adequate opportunity to review, identify, and document exceptional
events that may affect an area designation under a revised NAAQS.
[[Page 35586]]
Table 1--Schedule for Exceptional Event Flagging and Documentation Submission for Data To Be Used in
Designations Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
Air quality
NAAQS pollutant/standard/(level)/ data collected Event flagging & initial Detailed documentation
promulgation date for calendar description deadline submission deadline
year
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m\3\) 2004-2006 October 1, 2007 \a\......... April 15, 2008 \a\.
Promulgated October 17, 2006.
Ozone/8-Hr Standard (0.075 ppm) 2005-2007 June 18, 2009 \a\........... June 18, 2009 \a\.
Promulgated March 12, 2008. 2008 June 18, 2009 \a\........... June 18, 2009 \a\.
2009 60 Days after the end of the 60 Days after the end of the
calendar quarter in which calendar quarter in which
the event occurred or the event occurred or
February 5, 2010, whichever February 5, 2010, whichever
date occurs first \b\. date occurs first \b\.
NO2/1-Hour Standard (80-100 PPB, 2008 July 1, 2010 \a\............ January 22, 2011 \a\.
final level TBD). 2009 July 1, 2010 \a\............ January 22, 2011 \a\.
2010 April 1, 2011 \a\........... July 1, 2011 \a\.
SO2/1-Hour Standard (50-100 PPB, 2008 October 1, 2010 \b\......... June 1, 2011 \b\.
final level TBD). 2009 October 1, 2010 \b\......... June 1, 2011 \b\.
2010 June 1, 2011 \b\............ June 1, 2011 \b\.
2011 60 Days after the end of the 60 Days after the end of the
calendar quarter in which calendar quarter in which
the event occurred or March the event occurred or March
31, 2012, whichever date 31, 2012, whichever date
occurs first \b\. occurs first \b\.
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, and are shown in this table for
informational purposes--the Agency is not opening these dates for comment under this rulemaking.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final
initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most
notably, for data used by EPA for redesignations to attainment.
Note further that EPA is reprinting portions of this Table in
section 5014 but, with respect to the pollutants other than
SO2, is doing so only for readers' convenience and is not
reopening or otherwise reconsidering any aspect of the rules related to
these other pollutants.
VIII. Communication of Public Health Information
Information on the public health implications of ambient
concentrations of criteria pollutants is currently made available
primarily through EPA's Air Quality Index (AQI) program. The current
AQI has been in use since its inception in 1999 (64 FR 42530). It
provides accurate, timely, and easily understandable information about
daily levels of pollution (40 CFR 58.50). The AQI establishes a
nationally uniform system of indexing pollution levels for nitrogen
dioxide, carbon monoxide, ozone, particulate matter and SO2.
The AQI converts pollutant concentrations in a community's air to a
number on a scale from 0 to 500. Reported AQI values enable the public
to know whether air pollution levels in a particular location are
characterized as good (0-50), moderate (51-100), unhealthy for
sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-
300), or hazardous (300-500). The AQI index value of 100 typically
corresponds to the level of the short-term primary NAAQS for each
pollutant. An AQI value greater than 100 means that a pollutant is in
one of the unhealthy categories (i.e., unhealthy for sensitive groups,
unhealthy, very unhealthy, or hazardous) on a given day; an AQI value
at or below 100 means that a pollutant concentration is in one of the
satisfactory categories (i.e., moderate or good). Decisions about the
pollutant concentrations at which to set the various AQI breakpoints,
that delineate the various AQI categories, draw directly from the
underlying health information that supports the review of the primary
NAAQS.
The Agency recognizes the importance of revising the AQI in a
timely manner to be consistent with any revisions to the primary NAAQS.
Therefore, EPA proposed to finalize conforming changes to the AQI in
connection with the Agency's final decision on the SO2
NAAQS. Conforming changes that were proposed include setting the 100
level of the AQI at the same level as the revised primary
SO2 standard if a short-term primary standard was
promulgated, and revising the other AQI breakpoints at the lower end of
the AQI scale (i.e., AQI values of 50 and 150). EPA did not propose to
change breakpoints at the higher end of the AQI scale (from 200 to
500), which would apply to State contingency plans or the Significant
Harm Level (40 CFR 51.16), because the information from this review
does not inform decisions about breakpoints at those higher levels.
With regard to an AQI value of 50, the breakpoint between the good
and moderate categories, historically this value is set at the level of
the annual NAAQS, if there is one, or one-half the level of the short-
term NAAQS in the absence of an annual NAAQS (63 FR 67823, Dec. 12,
1998). Taking into consideration this practice, EPA proposed to set the
AQI value of 50 to be between 25 and 50 ppb SO2, 1-hour
average; stating that concentrations toward the lower end of this range
would be appropriate if the standard was set at the lower end of the
range of proposed standard levels, while concentrations toward the
higher end of this range would be more appropriate if the standard was
set at the higher end of the range of proposed standard levels. EPA
solicited comments on this range for an AQI value of 50 and the
appropriate basis for selecting an AQI value of 50.
With regard to an AQI value of 150, the breakpoint between the
unhealthy for sensitive groups and unhealthy categories, historically
values between the short-term standard and an AQI value of 500 are set
at levels that are approximately equidistant between the AQI values of
100 and 500 unless there is health evidence that suggests a specific
level would be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI
value of 150, EPA proposed to set the breakpoint within the range from
175 to 200 ppb SO2, 1-hour average, since it represents the
midpoint between the proposed range for the short-term
[[Page 35587]]
standard and the level of an AQI value of 200 (300 ppb SO2,
1-hour average).
EPA received few comments on the proposed breakpoints. Consistent
with the level of the short-term primary SO2 standard
promulgated in this rule, EPA is setting the AQI value of 100, the
breakpoint between the moderate and unhealthy for sensitive groups
category, at 75 ppb, 1-hour average. EPA is setting the AQI value of
50, the breakpoint between the good and moderate categories, at 35 ppb
SO2, 1-hour average, which is approximately one-half the
level of the new short-term standard, since the annual SO2
standard is being revoked. EPA is setting the AQI value of 150, the
breakpoint between the unhealthy for sensitive groups and unhealthy
categories, at 185 ppb SO2, 1-hour average, which represents
the approximate midpoint between the level of the new short-term
standard (75 ppb SO2, 1-hour average) and the level of an
AQI value of 200 (300 ppb SO2, 1-hour average).
EPA received comments from several State environmental
organizations and organizations of State and local air agencies about
forecasting and reporting the AQI for SO2. These commenters
expressed the view that forecasting hourly SO2
concentrations would be difficult. One commenter requested that EPA
delay the forecasting requirement for one year and other agencies
requested that EPA provide assistance in developing a forecast model.
Another commenter expressed the view that it is impractical to
incorporate SO2 into its forecasting and public health
notification program because SO2 does not behave like a
regional pollutant, and that exceedances may occur with little or no
warning and for two hours or less. This commenter requested EPA
consider the resources necessary for public communications at the State
and local levels, particularly in areas where other air quality
exceedances are relatively rare.
EPA recommends and encourages air quality forecasting but it is not
required (64 FR 42548; August 4, 1999). We agree that there will be new
challenges associated with creating and communicating an SO2
forecast, and will work with State and local agencies that want to
develop an SO2 forecasting program on issues including, but
not limited to, forecasting air quality for short time periods. We plan
to work with State and local air agencies to figure out the best way to
present this information to the public using the AQI.
With respect to the comment that it is impractical to incorporate
SO2 into a forecasting and public health notification
program because SO2 does not behave like a regional
pollutant, this final rule departs from the proposed rule in that it
allows for a combined monitoring and modeling approach. Because of
this, the monitoring network is not required to be wholly source-
oriented in nature. States have flexibility to allow required
monitoring sites to serve multiple monitoring objectives including
characterizing source impacts, highest concentrations, population
exposure, background, and regional transport. Further, EPA expects that
much of the existing network will be retained by States to satisfy the
minimum monitoring requirements. This means that it is unlikely that
AQI reporting and forecasting will be heavily driven by source-oriented
monitors. Rather, many of the existing monitors (a majority of which
are community-wide monitors) will remain in place, which prevents the
need for new geographic regions to be delineated. With respect to
concerns expressed about the resources required to report the AQI in
areas were exceedances of the standard are very rare, Appendix G to
Part 58 specifies that if the index value for a particular pollutant
remains below 50 for a season or year, then a State or local agency may
exclude the pollutant from the calculation of the AQI.
IX. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
Under section 3(f)(1) of Executive Order 12866 (58 FR 51735,
October 4, 1993), this action is an ``economically significant
regulatory action'' because it is likely to have an annual effect on
the economy of $100 million or more. Accordingly, EPA submitted this
action to the Office of Management and Budget (OMB) for review under EO
12866 and any changes made in response to OMB recommendations have been
documented in the docket for this action. In addition, EPA prepared a
Regulatory Impact Analysis (RIA) of the potential costs and benefits
associated with this action. However, the CAA and judicial decisions
make clear that the economic and technical feasibility of attaining the
national ambient standards cannot be considered in setting or revising
NAAQS, although such factors may be considered in the development of
State implementation plans to implement the standards. Accordingly,
although an RIA has been prepared, the results of the RIA have not been
considered by EPA in developing this final rule.
When estimating the SO2- and PM2.5-related
human health benefits and compliance costs in Table 2 below, EPA
applied methods and assumptions consistent with the state-of-the-
science for human health impact assessment, economics and air quality
analysis. EPA applied its best professional judgment in performing this
analysis and believes that these estimates provide a reasonable
indication of the expected benefits and costs to the nation of the
selected SO2 standard and alternatives considered by the
Agency. The Regulatory Impacts Analysis (RIA) available in the docket
describes in detail the empirical basis for EPA's assumptions and
characterizes the various sources of uncertainties affecting the
estimates below.
EPA's 2009 Integrated Science Assessment for Particulate Matter
concluded, based on the scientific literature, that a no-threshold log-
linear model most adequately portrays the PM-mortality concentration-
response relationship. Nonetheless, consistent with historical practice
and our commitment to characterizing the uncertainty in our benefits
estimates, EPA has included a sensitivity analysis with an assumed
threshold in the PM-mortality health impact function in the RIA. EPA
has included a sensitivity analysis in the RIA to help inform our
understanding of the health benefits which can be achieved at lower air
quality concentration levels. While the primary estimate and the
sensitivity analysis are not directly comparable, due to differences in
population data and use of different analysis years, as well as the
difference in the assumption of a threshold in the sensitivity
analysis, comparison of the two results provide a rough sense of the
proportion of the health benefits that occur at lower PM2.5
air quality levels. Using a threshold of 10 [mu]g/m\3\ is an arbitrary
choice (EPA could have assumed 6, 8, or 12 [mu]g/m\3\ for the
sensitivity analysis). Assuming a threshold of 10 [mu]g/m\3\, the
sensitivity analysis shows that roughly one-third of the benefits occur
at air quality levels below that threshold. Because the primary
estimates reflect EPA's current methods and data, EPA notes that
caution should be exercised when comparing the results of the primary
and sensitivity analyses. EPA appreciates the value of sensitivity
analyses in highlighting the uncertainty in the benefits estimates and
will continue to work to refine these analyses, particularly in those
instances in which air quality modeling data are available.
Table 2 shows the results of the cost and benefits analysis for
each standard alternative. As indicated above, implementation of the
SO2 control
[[Page 35588]]
measures identified from AirControlNET and other sources does not
result in attainment with the all target NAAQS levels in several areas.
In these areas, additional unspecified emission reductions might be
necessary to reach some alternative standard levels. The first part of
the table, labeled Partial attainment (identified controls), shows only
those benefits and costs from control measures we were able to
identify. The second part of the table, labeled Unidentified Controls,
shows only additional benefits and costs resulting from unidentified
controls. The third part of the table, labeled Full attainment, shows
total benefits and costs resulting from both identified and
unidentified controls. It is important to emphasize that we were able
to identify control measures for a significant portion of attainment
for many of those counties that would not fully attain the target NAAQS
level with identified controls. Note also that in addition to
separating full and partial attainment, the table also separates the
portion of benefits associated with reduced SO2 exposure
(i.e., SO2 benefits) from the additional benefits associated
with reducing SO2 emissions, which are precursors to
PM2.5 formation--(i.e., the PM2.5 co-benefits).
For instance, for the selected standard of 75 ppb, $2.2 million in
benefits are associated with reduced SO2 exposure while $15
billion to $37 billion are associated with reduced PM2.5
exposure.
Table 2--Monetized Benefits and Costs To Attain Alternate Standard Levels in 2020
[Millions of 2006$] \a\
--------------------------------------------------------------------------------------------------------------------------------------------------------
Number of
counties fully Discount rate Monetized SO2 Monetized PM2.5 co- Costs Net benefits
controlled (percent) benefits benefits,c,d
--------------------------------------------------------------------------------------------------------------------------------------------------------
Partial Attainment (identified controls)
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb........................... 40 3 \b\ $30,000 to $74,000.. $2,600 $27,000 to $71,000.
.............. 7 .............. $28,000 to $67,000.. .............. $25,000 to $64,000.
75 ppb........................... 20 3 \b\ $14,000 to $35,000.. $960 $13,000 to $34,000.
.............. 7 .............. $13,000 to $31,000.. .............. $12,000 to $30,000.
100 ppb.......................... 6 3 \b\ $6,900 to $17,000... $470 $6,400 to $17,000.
.............. 7 .............. $6,200 to $15,000... .............. $5,700 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Unidentified Controls
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb........................... 16 3 \b\ $4,000 to $9,000.... $1,800 $2,200 to $7,200.
.............. 7 .............. $3,000 to $8,000.... .............. $1,200 to $6,200.
75 ppb........................... 4 3 \b\ $1,000 to $3,000.... $500 $500 to $1,500.
.............. 7 .............. $1,000 to $3,000.... .............. $500 to $2,500.
100 ppb.......................... 3 3 \b\ $500 to $1,000...... $260 $240 to $740.
.............. 7 .............. $500 to $1,000...... .............. $240 to $740.
--------------------------------------------------------------------------------------------------------------------------------------------------------
Full Attainment
--------------------------------------------------------------------------------------------------------------------------------------------------------
50 ppb........................... 56 3 $8.50 $34,000 to $83,000.. $4,400 $30,000 to $79,000.
.............. 7 .............. $31,000 to $75,000.. .............. $27,000 to $71,000.
75 ppb........................... 24 3 $2.20 $15,000 to $37,000.. $1,500 $14,000 to $36,000
.............. 7 .............. $14,000 to $34,000.. .............. $13,000 to $33,000.
100 ppb.......................... 9 3 $0.60 $7,400 to $18,000... $730 $6,700 to $17,000.
.............. 7 .............. $6,700 to $16,000... .............. $6,000 to $15,000.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Estimates have been rounded to two significant figures and therefore summation may not match table estimates.
\b\ The approach used to simulate air quality changes for SO2 did not provide the data needed to distinguish partial attainment benefits from full
attainment benefits from reduced SO2 exposure. Therefore, a portion of the SO2 benefits is attributable to the known controls and a portion of the SO2
benefits are attributable to the unidentified controls. Because all SO2-related benefits are short-term effects, the results are identical for all
discount rates.
\c\ Benefits are shown as a range from Pope et al. (2002) to Laden et al. (2006). Monetized benefits do not include unquantified benefits, such as other
health effects, reduced sulfur deposition, or improvements in visibility.
\d\ These models assume that all fine particles, regardless of their chemical composition, are equally potent in causing premature mortality because
there is no clear scientific evidence that would support the development of differential effects estimates by particle type. Reductions in SO2
emissions from multiple sectors to meet the SO2 NAAQS would primarily reduce the sulfate fraction of PM2.5. Because this rule targets a specific
particle precursor (i.e., SO2), this introduces some uncertainty into the results of the analysis.
B. Paperwork Reduction Act
The information collection requirements in this final rule have
been submitted for approval to the Office of Management and Budget
(OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The
Information Collection Request (ICR) document prepared by EPA for these
revisions to part 58 has been assigned EPA ICR number 2370.02. The
information collected under 40 CFR part 53 (e.g., test results,
monitoring records, instruction manual, and other associated
information) is needed to determine whether a candidate method intended
for use in determining attainment of the NAAQS in 40 CFR part 50 will
meet the design, performance, and/or comparability requirements for
designation as a Federal reference method (FRM) or Federal equivalent
method (FEM). We do not expect the number of FRM or FEM determinations
to increase over the number that is currently used to estimate burden
associated with SO2 FRM/FEM determinations provided in the
current ICR for 40 CFR part 53 (EPA ICR numbers 2370.01). As such, no
change in the burden estimate for 40 CFR part 53 has been made as part
of this rulemaking.
The information collected and reported under 40 CFR part 58 is
needed to determine compliance with the NAAQS, to characterize air
quality and
[[Page 35589]]
associated health impacts, to develop emissions control strategies, and
to measure progress for the air pollution program. The amendments would
revise the technical requirements for SO2 monitoring sites,
require the siting and operation of additional SO2 ambient
air monitors, and the reporting of the collected ambient SO2
monitoring data to EPA's Air Quality System (AQS). The ICR is estimated
to involve 102 respondents for a total approximate cost of $15,203,762
(total capital, and labor and non-labor operation and maintenance) and
a total burden of 207,662 hours. The labor costs associated with these
hours is $11,130,409. Included in the $15,203,762 total are other costs
of other non-labor operations and maintenance of $1,104,377 and
equipment and contract costs of $2,968,975. In addition to the costs at
the State and local air quality management agencies, there is a burden
to EPA for a total of 14,749 hours and $1,060,621. Burden is defined at
5 CFR 1320.3(b). State, local, and Tribal entities are eligible for
State assistance grants provided by the Federal government under the
CAA which can be used for monitors and related activities. An agency
may not conduct or sponsor, and a person is not required to respond to,
a collection of information unless it displays a currently valid OMB
control number. The OMB control numbers for EPA's regulations in 40 CFR
are listed in 40 CFR part 9.
C. Regulatory Flexibility Act
The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
For purposes of assessing the impacts of this rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
After considering the economic impacts of this final rule on small
entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This final
rule will not impose any requirements on small entities. Rather, this
rule establishes national standards for allowable concentrations of
SO2 in ambient air as required by section 109 of the CAA.
American Trucking Ass'ns v. EPA, 175 F.3d 1027, 1044-45 (DC Cir. 1999)
(NAAQS do not have significant impacts upon small entities because
NAAQS themselves impose no regulations upon small entities). Similarly,
the amendments to 40 CFR Part 58 address the requirements for States to
collect information and report compliance with the NAAQS and will not
impose any requirements on small entities.
D. Unfunded Mandates Reform Act
This action is not subject to the requirements of sections 202 and
205 of the UMRA. EPA has determined that this final rule does not
contain a Federal mandate that may result in expenditures of $100
million or more for State, local, and Tribal governments, in the
aggregate, or the private sector in any one year. The revisions to the
SO2 NAAQS impose no enforceable duty on any State, local or
Tribal governments or the private sector. The expected costs associated
with the monitoring requirements are described in EPA's ICR document,
but those costs are not expected to exceed $100 million in the
aggregate for any year. Furthermore, as indicated previously, in
setting a NAAQS, EPA cannot consider the economic or technological
feasibility of attaining ambient air quality standards. Because the CAA
prohibits EPA from considering the types of estimates and assessments
described in section 202 when setting the NAAQS, the UMRA does not
require EPA to prepare a written statement under section 202 for the
revisions to the SO2 NAAQS.
With regard to implementation guidance, the CAA imposes the
obligation for States to submit SIPs to implement the SO2
NAAQS. In this final rule, EPA is merely providing an interpretation of
those requirements. However, even if this rule did establish an
independent obligation for States to submit SIPs, it is questionable
whether an obligation to submit a SIP revision would constitute a
Federal mandate in any case. The obligation for a State to submit a SIP
that arises out of section 110 and section 191 of the CAA is not
legally enforceable by a court of law, and at most is a condition for
continued receipt of highway funds. Therefore, it is possible to view
an action requiring such a submittal as not creating any enforceable
duty within the meaning of U.S.C. 658 for purposes of the UMRA. Even if
it did, the duty could be viewed as falling within the exception for a
condition of Federal assistance under U.S.C. 658.
EPA has determined that this final rule contains no regulatory
requirements that might significantly or uniquely affect small
governments because it imposes no enforceable duty on any small
governments. Therefore, this rule is not subject to the requirements of
section 203 of the UMRA.
E. Executive Order 13132: Federalism
This final rule does not have federalism implications. It will not
have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding
the establishment and implementation of air quality improvement
programs as codified in the CAA. Under section 109 of the CAA, EPA is
mandated to establish NAAQS; however, CAA section 116 preserves the
rights of States to establish more stringent requirements if deemed
necessary by a State. Furthermore, this rule does not impact CAA
section 107 which establishes that the States have primary
responsibility for implementation of the NAAQS. Finally, as noted in
section E (above) on UMRA, this rule does not impose significant costs
on State, local, or Tribal governments or the private sector. Thus,
Executive Order 13132 does not apply to this rule.
F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments
Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by Tribal officials in the development of regulatory
policies that have Tribal implications.'' This final rule does not have
Tribal implications, as specified in Executive Order 13175. It does not
have a substantial direct effect on one or more Indian Tribes, on the
relationship between the Federal government and Indian Tribes, or on
the distribution of power and responsibilities between the
[[Page 35590]]
Federal government and Tribes. The rule does not alter the relationship
between the Federal government and Tribes as established in the CAA and
the TAR. Under section 109 of the CAA, EPA is mandated to establish
NAAQS; however, this rule does not infringe existing Tribal authorities
to regulate air quality under their own programs or under programs
submitted to EPA for approval. Furthermore, this rule does not affect
the flexibility afforded to Tribes in seeking to implement CAA programs
consistent with the TAR, nor does it impose any new obligation on
Tribes to adopt or implement any NAAQS. Finally, as noted in section E
(above) on UMRA, this rule does not impose significant costs on Tribal
governments. Thus, Executive Order 13175 does not apply to this rule.
G. Executive Order 13045: Protection of Children From Environmental
Health and Safety Risks
This action is subject to Executive Order (62 FR 19885, April 23,
1997) because it is an economically significant regulatory action as
defined by Executive Order 12866, and we believe that the environmental
health risk addressed by this action has a disproportionate effect on
children. The final rule will establish uniform national ambient air
quality standards for SO2; these standards are designed to
protect public health with an adequate margin of safety, as required by
CAA section 109. The protection offered by these standards may be
especially important for asthmatics, including asthmatic children,
because respiratory effects in asthmatics are among the most sensitive
health endpoints for SO2 exposure. Because asthmatic
children are considered a sensitive population, we have evaluated the
potential health effects of exposure to SO2 pollution among
asthmatic children. These effects and the size of the population
affected are discussed in chapters 3 and 4 of the ISA; chapters 3, 4,
7, 8, 9 of the REA, and sections II.A through II.E of this preamble.
H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use
This rule is not a ``significant energy action'' as defined in
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR
28355; May 22, 2001) because it is not likely to have a significant
adverse effect on the supply, distribution, or use of energy. The
purpose of this rule is to establish revised NAAQS for SO2.
The rule does not prescribe specific control strategies by which these
ambient standards will be met. Such strategies will be developed by
States on a case-by-case basis, and EPA cannot predict whether the
control options selected by States will include regulations on energy
suppliers, distributors, or users. Thus, EPA concludes that this rule
is not likely to have any adverse energy effects.
I. National Technology Transfer and Advancement Act
Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law 104-113, section 12(d) (15 U.S.C. 27)
directs EPA to use voluntary consensus standards in its regulatory
activities unless to do so would be inconsistent with applicable law or
otherwise impractical. Voluntary consensus standards are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) that are developed or adopted by
voluntary consensus standards bodies. The NTTAA directs EPA to provide
Congress, through OMB, explanations when the Agency decides not to use
available and applicable voluntary consensus standards.
This final rulemaking involves technical standards with regard to
ambient monitoring of SO2. The use of this voluntary
consensus standard would be impractical because the analysis method
does not provide for the method detection limits necessary to
adequately characterize ambient SO2 concentrations for the
purpose of determining compliance with the final revisions to the
SO2 NAAQS.
J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations
Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes
Federal executive policy on environmental justice. Its main provision
directs Federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
EPA has determined that this final rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health effects on
any population, including any minority or low-income population. The
final rule will establish uniform national standards for SO2
in ambient air.
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dose-response study of sulfur dioxide effects in normal, atopic, and
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Samet JM. (2009). Letter to EPA Administrator Lisa P. Jackson: Clean
Air Scientific Advisory Committee's (CASAC) Review of EPA's Risk and
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Primary National Ambient Air Quality Standards: Second Draft. EPA-
CASAC-09-007, May 18, 2009. Sulfur Dioxide Review Docket. Docket ID
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Thompson R and Stewart MJ. (2009). Air Quality Statistics for Cities
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List of Subjects
40 CFR Part 50
Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 53
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental
[[Page 35592]]
relations, Reporting and recordkeeping requirements.
40 CFR Part 58
Environmental protection, Administrative practice and procedure,
Air pollution control, Intergovernmental relations, Reporting and
recordkeeping requirements.
Dated: June 2, 2010.
Lisa P. Jackson,
Administrator.
0
For the reasons stated in the preamble, title 40, chapter I of the Code
of Federal Regulations is amended as follows:
PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS
0
1. The authority citation for part 50 continues to read as follows:
Authority: 42 U.S.C. 7401, et seq.
0
2. Section 50.4 is amended by adding paragraph (e) to read as follows:
Sec. 50.4 National primary ambient air quality standards for sulfur
oxides (sulfur dioxide).
* * * * *
(e) The standards set forth in this section will remain applicable
to all areas notwithstanding the promulgation of SO2
national ambient air quality standards (NAAQS) in Sec. 50.17. The
SO2 NAAQS set forth in this section will no longer apply to
an area one year after the effective date of the designation of that
area, pursuant to section 107 of the Clean Air Act, for the
SO2 NAAQS set forth in Sec. 50. 17; except that for areas
designated nonattainment for the SO2 NAAQS set forth in this
section as of the effective date of Sec. 50. 17, and areas not meeting
the requirements of a SIP call with respect to requirements for the
SO2 NAAQS set forth in this section, the SO2
NAAQS set forth in this section will apply until that area submits,
pursuant to section 191 of the Clean Air Act, and EPA approves, an
implementation plan providing for attainment of the SO2
NAAQS set forth in Sec. 50.17.
0
3. Section 50.14 is amended by revising paragraph (c)(2)(vi) to read as
follows:
Sec. 50.14 Treatment of air quality monitoring data influenced by
exceptional events.
* * * * *
(c) * * *
(2) * * *
(vi) When EPA sets a NAAQS for a new pollutant or revises the NAAQS
for an existing pollutant, it may revise or set a new schedule for
flagging exceptional event data, providing initial data descriptions
and providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS. Table 1 provides the schedule
for submission of flags with initial descriptions in AQS and detailed
documentation. These schedules shall apply for those data which will or
may influence the initial designation of areas for those NAAQS. EPA
anticipates revising Table 1 as necessary to accommodate revised data
submission schedules for new or revised NAAQS.
Table 1--Schedule of Exceptional Event Flagging and Documentation Submission for Data To Be Used in Designations
Decisions for New or Revised NAAQS
----------------------------------------------------------------------------------------------------------------
Air quality
NAAQS Pollutant/standard/(level)/ data collected Event flagging & initial Detailed documentation
promulgation date for calendar description deadline submission deadline
year
----------------------------------------------------------------------------------------------------------------
PM2.5/24-Hr Standard (35 [mu]g/m3) 2004-2006 October 1, 2007 \a\....... April 15, 2008. \a\
Promulgated October 17, 2006.
----------------------------------------------------------------------------------------------------------------
Ozone/8-Hr Standard (0.075 ppm) 2005-2007 June 18, 2009 \a\......... June 18, 2009 \a\
Promulgated March 12, 2008. 2008 June 18, 2009 \a\......... June 18, 2009 \1\
2009 60 days after the end of 60 days after the end of
the calendar quarter in the calendar quarter in
which the event occurred which the event occurred
or February 5, 2010, or February 5, 2010,
whichever date occurs whichever date occurs
first \b\. first.\b\
----------------------------------------------------------------------------------------------------------------
NO2/1-Hour Standard (80-100 PPB, final 2008 July 1, 2010 \a\.......... January 22, 2011. \a\
level TBD). 2009 July 1, 2010 \a\.......... January 22, 2011. \a\
2010 April 1, 2011 \a\......... July 1, 2010. \a\
----------------------------------------------------------------------------------------------------------------
SO 2/1-Hour Standard (50-100 PPB, final 2008 October 1, 2010 \b\....... June 1, 2011. \b\
level TBD). 2009 October 1, 2010 \b\....... June 1, 2011. \b\
2010 June 1, 2011. \b\......... June 1, 2011. \b\
2011 60 days after the end of 60 days after the end of
the calendar quarter in the calendar quarter in
which the event occurred which the event occurred
or March 31, 2012, or March 31, 2012,
whichever date occurs whichever date occurs
first \b\. first. \b\
----------------------------------------------------------------------------------------------------------------
\a\ These dates are unchanged from those published in the original rulemaking, or are being proposed elsewhere
and are shown in this table for informational purposes--the Agency is not opening these dates for comment
under this rulemaking.
\b\ Indicates change from general schedule in 40 CFR 50.14.
Note: EPA notes that the table of revised deadlines only applies
to data EPA will use to establish the final initial designations for
new or revised NAAQS. The general schedule applies for all other
purposes, most notably, for data used by EPA for redesignations to
attainment.
* * * * *
0
4. A new 50.17 is added to read as follows:
Sec. 50.17 National primary ambient air quality standards for sulfur
oxides (sulfur dioxide).
(a) The level of the national primary 1-hour annual ambient air
quality standard for oxides of sulfur is 75 parts per billion (ppb,
which is 1 part in 1,000,000,000), measured in the ambient air as
sulfur dioxide (SO2).
(b) The 1-hour primary standard is met at an ambient air quality
monitoring site when the three-year average of the annual (99th
percentile) of the daily maximum 1-hour average concentrations is less
than or equal to 75 ppb, as determined in accordance with Appendix T of
this part.
(c) The level of the standard shall be measured by a reference
method based on Appendix A or A-1 of this part, or by a Federal
Equivalent Method (FEM)
[[Page 35593]]
designated in accordance with part 53 of this chapter.
0
5. Add Appendix A-1 to Part 50 to read as follows:
Appendix A-1 to Part 50--Reference Measurement Principle and
Calibration Procedure for the Measurement of Sulfur Dioxide in the
Atmosphere (Ultraviolet Fluorescence Method)
1.0 Applicability
1.1 This ultraviolet fluorescence (UVF) method provides a
measurement of the concentration of sulfur dioxide (SO2)
in ambient air for determining compliance with the national primary
and secondary ambient air quality standards for sulfur oxides
(sulfur dioxide) as specified in Sec. 50.4, Sec. 50.5, and Sec.
50.17 of this chapter. The method is applicable to the measurement
of ambient SO2 concentrations using continuous (real-
time) sampling. Additional quality assurance procedures and guidance
are provided in part 58, Appendix A, of this chapter and in
Reference 3.
2.0 Principle
2.1 This reference method is based on automated measurement of
the intensity of the characteristic fluorescence released by
SO2 in an ambient air sample contained in a measurement
cell of an analyzer when the air sample is irradiated by ultraviolet
(UV) light passed through the cell. The fluorescent light released
by the SO2 is also in the ultraviolet region, but at
longer wavelengths than the excitation light. Typically, optimum
instrumental measurement of SO2 concentrations is
obtained with an excitation wavelength in a band between
approximately 190 to 230 nm, and measurement of the SO2
fluorescence in a broad band around 320 nm, but these wavelengths
are not necessarily constraints of this reference method. Generally,
the measurement system (analyzer) also requires means to reduce the
effects of aromatic hydrocarbon species, and possibly other
compounds, in the air sample to control measurement interferences
from these compounds, which may be present in the ambient air.
References 1 and 2 describe UVF method.
2.2 The measurement system is calibrated by referencing the
instrumental fluorescence measurements to SO2 standard
concentrations traceable to a National Institute of Standards and
Technology (NIST) primary standard for SO2 (see
Calibration Procedure below).
2.3 An analyzer implementing this measurement principle is shown
schematically in Figure 1. Designs should include a measurement
cell, a UV light source of appropriate wavelength, a UV detector
system with appropriate wave length sensitivity, a pump and flow
control system for sampling the ambient air and moving it into the
measurement cell, sample air conditioning components as necessary to
minimize measurement interferences, suitable control and measurement
processing capability, and other apparatus as may be necessary. The
analyzer must be designed to provide accurate, repeatable, and
continuous measurements of SO2 concentrations in ambient
air, with measurement performance as specified in Subpart B of Part
53 of this chapter.
2.4 Sampling considerations: The use of a particle filter on the
sample inlet line of a UVF SO2 analyzer is required to
prevent interference, malfunction, or damage due to particles in the
sampled air.
3.0 Interferences
3.1 The effects of the principal potential interferences may
need to be mitigated to meet the interference equivalent
requirements of part 53 of this chapter. Aromatic hydrocarbons such
as xylene and naphthalene can fluoresce and act as strong positive
interferences. These gases can be removed by using a permeation type
scrubber (hydrocarbon ``kicker''). Nitrogen oxide (NO) in high
concentrations can also fluoresce and cause positive interference.
Optical filtering can be employed to improve the rejection of
interference from high NO. Ozone can absorb UV light given off by
the SO2 molecule and cause a measurement offset. This
effect can be reduced by minimizing the measurement path length
between the area where SO2 fluorescence occurs and the
photomultiplier tube detector (e.g. <5 cm). A hydrocarbon scrubber,
optical filter and appropriate distancing of the measurement path
length may be required method components to reduce interference.
4.0 Calibration Procedure
Atmospheres containing accurately known concentrations of sulfur
dioxide are prepared using a compressed gas transfer standard
diluted with accurately metered clean air flow rates.
4.1 Apparatus: Figure 2 shows a typical generic system suitable
for diluting a SO2 gas cylinder concentration standard
with clean air through a mixing chamber to produce the desired
calibration concentration standards. A valve may be used to
conveniently divert the SO2 from the sampling manifold to
provide clean zero air at the output manifold for zero adjustment.
The system may be made up using common laboratory components, or it
may be a commercially manufactured system. In either case, the
principle components are as follows:
4.1.1 SO2 standard gas flow control and measurement
devices (or a combined device) capable of regulating and maintaining
the standard gas flow rate constant to within 2 percent
and measuring the gas flow rate accurate to within 2,
properly calibrated to a NIST-traceable standard.
4.1.2 Dilution air flow control and measurement devices (or a
combined device) capable of regulating and maintaining the air flow
rate constant to within 2 percent and measuring the air
flow rate accurate to within 2, properly calibrated to a
NIST-traceable standard.
4.1.3 Mixing chamber, of an inert material such as glass and of
proper design to provide thorough mixing of pollutant gas and
diluent air streams.
4.1.4 Sampling manifold, constructed of glass,
polytetrafluoroethylene (PTFE TeflonTM), or other
suitably inert material and of sufficient diameter to insure a
minimum pressure drop at the analyzer connection, with a vent
designed to insure a minimum over-pressure (relative to ambient air
pressure) at the analyzer connection and to prevent ambient air from
entering the manifold.
4.1.5 Standard gas pressure regulator, of clean stainless steel
with a stainless steel diaphragm, suitable for use with a high
pressure SO2 gas cylinder.
4.1.6 Reagents
4.1.6.1 SO2 gas concentration transfer standard
having a certified SO2 concentration of not less than 10
ppm, in N2, traceable to a NIST Standard Reference
Material (SRM).
4.1.6.2 Clean zero air, free of contaminants that could cause a
detectable response or a change in sensitivity of the analyzer.
Since ultraviolet fluorescence analyzers may be sensitive to
aromatic hydrocarbons and O2-to-N2 ratios, it
is important that the clean zero air contains less than 0.1 ppm
aromatic hydrocarbons and O2 and N2
percentages approximately the same as in ambient air. A procedure
for generating zero air is given in reference 1.
4.2 Procedure
4.2.1 Obtain a suitable calibration apparatus, such as the one
shown schematically in Figure 1, and verify that all materials in
contact with the pollutant are of glass, TeflonTM, or
other suitably inert material and completely clean.
4.2.2 Purge the SO2 standard gas lines and pressure
regulator to remove any residual air.
4.2.3 Ensure that there are no leaks in the system and that the
flow measuring devices are properly and accurately calibrated under
the conditions of use against a reliable volume or flow rate
standard such as a soap-bubble meter or a wet-test meter traceable
to a NIST standard. All volumetric flow rates should be corrected to
the same reference temperature and pressure by using the formula
below:
[GRAPHIC] [TIFF OMITTED] TR22JN10.000
Where:
Fc = corrected flow rate (L/min at 25 [deg]C and 760 mm Hg),
Fm = measured flow rate, (at temperature, Tm and
pressure, Pm),
Pm = measured pressure in mm Hg, (absolute), and
Tm = measured temperature in degrees Celsius.
4.2.4 Allow the SO2 analyzer under calibration to
sample zero air until a stable response is obtained, then make the
proper zero adjustment.
4.2.5 Adjust the airflow to provide an SO2
concentration of approximately 80 percent of the upper measurement
range limit of the SO2 instrument and verify that the
total air flow of the calibration system exceeds the demand of all
analyzers sampling from the output manifold (with the excess
vented).
4.2.6 Calculate the actual SO2 calibration
concentration standard as:
[[Page 35594]]
[GRAPHIC] [TIFF OMITTED] TR22JN10.001
Where:
C = the concentration of the SO2 gas standard
Fp = the flow rate of SO2 gas standard
Ft = the total air flow rate of pollutant and diluent gases
4.2.7 When the analyzer response has stabilized, adjust the
SO2 span control to obtain the desired response
equivalent to the calculated standard concentration. If substantial
adjustment of the span control is needed, it may be necessary to re-
check the zero and span adjustments by repeating steps 4.2.4 through
4.2.7 until no further adjustments are needed.
4.2.8 Adjust the flow rate(s) to provide several other
SO2 calibration concentrations over the analyzer's
measurement range. At least five different concentrations evenly
spaced throughout the analyzer's range are suggested.
4.2.9 Plot the analyzer response (vertical or Y-axis) versus
SO2 concentration (horizontal or X-axis). Compute the
linear regression slope and intercept and plot the regression line
to verify that no point deviates from this line by more than 2
percent of the maximum concentration tested.
Note: Additional information on calibration and pollutant
standards is provided in Section 12 of Reference 3.
5.0 Frequency of Calibration
The frequency of calibration, as well as the number of points
necessary to establish the calibration curve and the frequency of
other performance checking will vary by analyzer; however, the
minimum frequency, acceptance criteria, and subsequent actions are
specified in Reference 3, Appendix D: Measurement Quality Objectives
and Validation Template for SO2 (page 9 of 30). The
user's quality control program should provide guidelines for initial
establishment of these variables and for subsequent alteration as
operational experience is accumulated. Manufacturers of analyzers
should include in their instruction/operation manuals information
and guidance as to these variables and on other matters of
operation, calibration, routine maintenance, and quality control.
6.0 References for SO2 Method
1. H. Okabe, P. L. Splitstone, and J. J. Ball, ``Ambient and Source
SO2 Detector Based on a Fluorescence Method'', Journal of
the Air Control Pollution Association, vol. 23, p. 514-516 (1973).
2. F. P. Schwarz, H. Okabe, and J. K. Whittaker, ``Fluorescence
Detection of Sulfur Dioxide in Air at the Parts per Billion Level,''
Analytical Chemistry, vol. 46, pp. 1024-1028 (1974).
3. QA Handbook for Air Pollution Measurement Systems--Volume II.
Ambient Air Quality Monitoring Programs. U.S. EPA. EPA-454/B-08-003
(2008).
BILLING CODE 6560-50-P
[GRAPHIC] [TIFF OMITTED] TR22JN10.002
[[Page 35595]]
[GRAPHIC] [TIFF OMITTED] TR22JN10.003
BILLING CODE 6560-50-C
0
6. Appendix A to Part 50 is redesignated as Appendix A-2 to Part 50.
0
7. Appendix T to Part 50 is added to read as follows:
Appendix T to Part 50--Interpretation of the Primary National Ambient
Air Quality Standards for Oxides of Sulfur (Sulfur Dioxide)
1. General
(a) This appendix explains the data handling conventions and
computations necessary for determining when the primary national
ambient air quality standards for Oxides of Sulfur as measured by
Sulfur Dioxide (``SO2 NAAQS'') specified in Sec. 50.17
are met at an ambient air quality monitoring site. Sulfur Dioxide
(SO2) is measured in the ambient air by a Federal
reference method (FRM) based on appendix A or A-1 to this part or by
a Federal equivalent method (FEM) designated in accordance with part
53 of this chapter. Data handling and computation procedures to be
used in making comparisons between reported SO2
concentrations and the levels of the SO2 NAAQS are
specified in the following sections.
(b) Decisions to exclude, retain, or make adjustments to the
data affected by exceptional events, including natural events, are
made according to the requirements and process deadlines specified
in Sec. Sec. 50.1, 50.14 and 51.930 of this chapter.
(c) The terms used in this appendix are defined as follows:
Daily maximum 1-hour values for SO2 refers to the
maximum 1-hour SO2 concentration values measured from
midnight to midnight (local standard time) that are used in NAAQS
computations.
Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
specified in section 5 of this appendix. The design value for the
primary 1-hour NAAQS is the 3-year average of annual 99th percentile
daily maximum 1-hour values for a monitoring site (referred to as
the ``1-hour primary standard design value'').
99th percentile daily maximum 1-hour value is the value below
which nominally 99 percent of all daily maximum 1-hour concentration
values fall, using the ranking and selection method specified in
section 5 of this appendix.
Pollutant Occurrence Code (POC) refers to a numerical code (1,
2, 3, etc.) used to distinguish the data from two or more monitors
for the same parameter at a single monitoring site.
Quarter refers to a calendar quarter.
Year refers to a calendar year.
2. Requirements for Data Used for Comparisons With the SO2
NAAQS and Data Reporting Considerations
(a) All valid FRM/FEM SO2 hourly data required to be
submitted to EPA's Air Quality System (AQS), or otherwise available
to EPA, meeting the requirements of part 58 of this chapter
including appendices A, C, and E shall be used in design value
calculations. Multi-hour average concentration values collected by
wet chemistry methods shall not be used.
(b) Data from two or more monitors from the same year at the
same site reported to EPA under distinct Pollutant Occurrence Codes
shall not be combined in an attempt to meet data completeness
requirements. The Administrator will combine annual 99th percentile
daily maximum concentration values from different monitors in
different years, selected as described here, for the purpose of
developing a valid 1-hour primary standard design value. If more
than one of the monitors meets the completeness requirement for all
four quarters of a year, the steps specified in section 5(a) of this
appendix shall be applied to the data from the monitor with the
highest average of the four quarterly completeness values to derive
a valid annual 99th percentile daily maximum concentration. If no
monitor is complete for all four quarters in a year, the steps
specified in section 3(c) and 5(a) of this appendix shall be applied
to the data from the monitor with the highest average of the four
quarterly completeness values in an attempt to derive a valid annual
99th percentile daily maximum concentration. This paragraph does not
prohibit a monitoring agency from making a local designation of one
physical monitor as the primary monitor for a Pollutant Occurrence
Code and substituting the 1-hour data from a second physical monitor
whenever a valid concentration value is not obtained from the
primary monitor; if a monitoring agency substitutes data in this
manner, each substituted value must be accompanied by an
[[Page 35596]]
AQS qualifier code indicating that substitution with a value from a
second physical monitor has taken place.
(c) Hourly SO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
3. Comparisons With the 1-Hour Primary SO2 NAAQS
(a) The 1-hour primary SO2 NAAQS is met at an ambient
air quality monitoring site when the valid 1-hour primary standard
design value is less than or equal to 75 parts per billion (ppb).
(b) An SO2 1-hour primary standard design value is
valid if it encompasses three consecutive calendar years of complete
data. A year meets data completeness requirements when all 4
quarters are complete. A quarter is complete when at least 75
percent of the sampling days for each quarter have complete data. A
sampling day has complete data if 75 percent of the hourly
concentration values, including State-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator, are reported.
(c) In the case of one, two, or three years that do not meet the
completeness requirements of section 3(b) of this appendix and thus
would normally not be useable for the calculation of a valid 3-year
1-hour primary standard design value, the 3-year 1-hour primary
standard design value shall nevertheless be considered valid if one
of the following conditions is true.
(i) At least 75 percent of the days in each quarter of each of
three consecutive years have at least one reported hourly value, and
the design value calculated according to the procedures specified in
section 5 is above the level of the primary 1-hour standard.
(ii) (A) A 1-hour primary standard design value that is equal to
or below the level of the NAAQS can be validated if the substitution
test in section 3(c)(ii)(B) results in a ``test design value'' that
is below the level of the NAAQS. The test substitutes actual
``high'' reported daily maximum 1-hour values from the same site at
about the same time of the year (specifically, in the same calendar
quarter) for unknown values that were not successfully measured.
Note that the test is merely diagnostic in nature, intended to
confirm that there is a very high likelihood that the original
design value (the one with less than 75 percent data capture of
hours by day and of days by quarter) reflects the true under-NAAQS-
level status for that 3-year period; the result of this data
substitution test (the ``test design value'', as defined in section
3(c)(ii)(B)) is not considered the actual design value. For this
test, substitution is permitted only if there are at least 200 days
across the three matching quarters of the three years under
consideration (which is about 75 percent of all possible daily
values in those three quarters) for which 75 percent of the hours in
the day, including State-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator, have
reported concentrations. However, maximum 1-hour values from days
with less than 75 percent of the hours reported shall also be
considered in identifying the high value to be used for
substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture but at least 50 percent data capture, including State-
flagged data affected by exceptional events which have been approved
for exclusion by the Administrator; if any quarter has less than 50
percent data capture then this substitution test cannot be used.
Identify for each quarter (e.g., January-March) the highest reported
daily maximum 1-hour value for that quarter, excluding State-flagged
data affected by exceptional events which have been approved for
exclusion by the Administrator, looking across those three months of
all three years under consideration. All daily maximum 1-hour values
from all days in the quarter period shall be considered when
identifying this highest value, including days with less than 75
percent data capture. If after substituting the highest reported
daily maximum 1-hour value for a quarter for as much of the missing
daily data in the matching deficient quarter(s) as is needed to make
them 100 percent complete, the procedure in section 5 yields a
recalculated 3-year 1-hour standard ``test design value'' less than
or equal to the level of the standard, then the 1-hour primary
standard design value is deemed to have passed the diagnostic test
and is valid, and the level of the standard is deemed to have been
met in that 3-year period. As noted in section 3(c)(i), in such a
case, the 3-year design value based on the data actually reported,
not the ``test design value'', shall be used as the valid design
value.
(iii) (A) A 1-hour primary standard design value that is above
the level of the NAAQS can be validated if the substitution test in
section 3(c)(iii)(B) results in a ``test design value'' that is
above the level of the NAAQS. The test substitutes actual ``low''
reported daily maximum 1-hour values from the same site at about the
same time of the year (specifically, in the same three months of the
calendar) for unknown hourly values that were not successfully
measured. Note that the test is merely diagnostic in nature,
intended to confirm that there is a very high likelihood that the
original design value (the one with less than 75 percent data
capture of hours by day and of days by quarter) reflects the true
above-NAAQS-level status for that 3-year period; the result of this
data substitution test (the ``test design value'', as defined in
section 3(c)(iii)(B)) is not considered the actual design value. For
this test, substitution is permitted only if there are a minimum
number of available daily data points from which to identify the low
quarter-specific daily maximum 1-hour values, specifically if there
are at least 200 days across the three matching quarters of the
three years under consideration (which is about 75 percent of all
possible daily values in those three quarters) for which 75 percent
of the hours in the day have reported concentrations. Only days with
at least 75 percent of the hours reported shall be considered in
identifying the low value to be used for substitution.
(B) The substitution test is as follows: Data substitution will
be performed in all quarter periods that have less than 75 percent
data capture. Identify for each quarter (e.g., January-March) the
lowest reported daily maximum 1-hour value for that quarter, looking
across those three months of all three years under consideration.
All daily maximum 1-hour values from all days with at least 75
percent capture in the quarter period shall be considered when
identifying this lowest value. If after substituting the lowest
reported daily maximum 1-hour value for a quarter for as much of the
missing daily data in the matching deficient quarter(s) as is needed
to make them 75 percent complete, the procedure in section 5 yields
a recalculated 3-year 1-hour standard ``test design value'' above
the level of the standard, then the 1-hour primary standard design
value is deemed to have passed the diagnostic test and is valid, and
the level of the standard is deemed to have been exceeded in that 3-
year period. As noted in section 3(c)(i), in such a case, the 3-year
design value based on the data actually reported, not the ``test
design value'', shall be used as the valid design value.
(d) A 1-hour primary standard design value based on data that do
not meet the completeness criteria stated in 3(b) and also do not
satisfy section 3(c), may also be considered valid with the approval
of, or at the initiative of, the Administrator, who may consider
factors such as monitoring site closures/moves, monitoring
diligence, the consistency and levels of the valid concentration
measurements that are available, and nearby concentrations in
determining whether to use such data.
(e) The procedures for calculating the 1-hour primary standard
design values are given in section 5 of this appendix.
4. Rounding Conventions for the 1-Hour Primary SO2 NAAQS
(a) Hourly SO2 measurement data shall be reported to
AQS in units of parts per billion (ppb), to at most one place after
the decimal, with additional digits to the right being truncated
with no further rounding.
(b) Daily maximum 1-hour values and therefore the annual 99th
percentile of those daily values are not rounded.
(c) The 1-hour primary standard design value is calculated
pursuant to section 5 and then rounded to the nearest whole number
or 1 ppb (decimals 0.5 and greater are rounded up to the nearest
whole number, and any decimal lower than 0.5 is rounded down to the
nearest whole number).
5. Calculation Procedures for the 1-Hour Primary SO2 NAAQS
(a) Procedure for identifying annual 99th percentile values.
When the data for a particular ambient air quality monitoring site
and year meet the data completeness requirements in section 3(b), or
if one of the conditions of section 3(c) is met, or if the
Administrator exercises the discretionary authority in section 3(d),
identification of annual 99th percentile value is accomplished as
follows.
(i) The annual 99th percentile value for a year is the higher of
the two values resulting from the following two procedures.
[[Page 35597]]
(1) Procedure 1. For the year, determine the number of days with
at least 75 percent of the hourly values reported.
(A) For the year, determine the number of days with at least 75
percent of the hourly values reported including State-flagged data
affected by exceptional events which have been approved for
exclusion by the Administrator.
(B) For the year, from only the days with at least 75 percent of
the hourly values reported, select from each day the maximum hourly
value excluding State-flagged data affected by exceptional events
which have been approved for exclusion by the Administrator.
(C) Sort all these daily maximum hourly values from a particular
site and year by descending value. (For example: (x[1], x[2], x[3],
* * *, x[n]). In this case, x[1] is the largest number and x[n] is
the smallest value.) The 99th percentile is determined from this
sorted series of daily values which is ordered from the highest to
the lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days with
valid data for year y (cny). The corresponding ``n''
value in the right column identifies the rank of the annual 99th
percentile value in the descending sorted list of daily site values
for year y. Thus, P0.99, y = the nth largest value.
(2) Procedure 2. For the year, determine the number of days with
at least one hourly value reported.
(A) For the year, determine the number of days with at least one
hourly value reported including State-flagged data affected by
exceptional events which have been approved for exclusion by the
Administrator.
(B) For the year, from all the days with at least one hourly
value reported, select from each day the maximum hourly value
excluding State-flagged data affected by exceptional events which
have been approved for exclusion by the Administrator.
(C) Sort all these daily maximum values from a particular site
and year by descending value. (For example: (x[1], x[2], x[3], * *
*, x[n]). In this case, x[1] is the largest number and x[n] is the
smallest value.) The 99th percentile is determined from this sorted
series of daily values which is ordered from the highest to the
lowest number. Using the left column of Table 1, determine the
appropriate range (i.e., row) for the annual number of days with
valid data for year y (cny). The corresponding ``n''
value in the right column identifies the rank of the annual 99th
percentile value in the descending sorted list of daily site values
for year y. Thus, P0.99,y = the nth largest value.
(b) The 1-hour primary standard design value for an ambient air
quality monitoring site is mean of the three annual 99th percentile
values, rounded according to the conventions in section 4.
Table 1
------------------------------------------------------------------------
P0.99,y is the nth
Annual number of days with valid data for year maximum value of the
``y'' (cny) year, where n is the
listed number
------------------------------------------------------------------------
1-100............................................. 1
101-200........................................... 2
201-300........................................... 3
301-366........................................... 4
------------------------------------------------------------------------
PART 53-AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS
0
8. The authority citation for part 53 continues to read as follows:
Authority: Sec. 301(a) of the Clean Air Act (42 U.S.C. sec.
1857g(a)), as amended by sec. 15(c)(2) of Pub. L. 91-604, 84 Stat.
1713, unless otherwise noted.
Subpart A--[Amended]
0
9. Section 53.2 is amended by revising paragraphs (a)(1) and (b) to
read as follows:
Sec. 53.2 General requirements for a reference method determination.
* * * * *
(a) Manual methods--(1) Sulfur dioxide (SO2) and Lead.
For measuring SO2 and lead, appendixes A-2 and G of part 50
of this chapter specify unique manual FRM for measuring those
pollutants. Except as provided in Sec. 53.16, other manual methods for
lead will not be considered for a reference method determination under
this part.
* * * * *
(b) Automated methods. An automated FRM for measuring
SO2, CO, O3, or NO2 must utilize the
measurement principle and calibration procedure specified in the
appropriate appendix to part 50 of this chapter (appendix A-1 only for
SO2 methods) and must have been shown in accordance with
this part to meet the requirements specified in this subpart A and
subpart B of this part.
0
10. Section 53.8 is amended by revising paragraph (c) to read as
follows:
Sec. 53.8 Designation of reference and equivalent methods.
* * * * *
(c) The Administrator will maintain a current list of methods
designated as FRM or FEM in accordance with this part and will send a
copy of the list to any person or group upon request. A copy of the
list will be available via the Internet and may be available from other
sources.
0
11. Table A-1 to Subpart A is revised to read as follows:
Table A-1 to Subpart A of Part 53--Summary of Applicable Requirements for Reference and Equivalent Methods for Air Monitoring of Criteria Pollutants
--------------------------------------------------------------------------------------------------------------------------------------------------------
Applicable subparts of part 53
Pollutant Reference or Manual or automated Applicable part 50 appendix -----------------------------------------------------------
equivalent A B C D E F
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO2............... Reference........... Manual.............. A-2
Automated........... A-1 [check] [check]
Equivalent.......... Manual.............. A-1 [check] [check]
Automated........... A-1 [check] [check] [check]
CO................ Reference........... Automated........... C [check] [check]
Equivalent.......... Manual.............. C [check] [check]
Automated........... C [check] [check] [check]
O3................ Reference........... Automated........... D [check] [check]
Equivalent.......... Manual.............. D [check] [check]
Automated........... D [check] [check] [check]
NO2............... Reference........... Automated........... F [check] [check]
Equivalent.......... Manual.............. F [check] [check]
Automated........... F [check] [check] [check]
Pb................ Reference........... Manual.............. G
Equivalent.......... Manual.............. G [check] [check]
Automated........... G [check] [check]
PM10-Pb........... Reference........... Manual.............. Q
Equivalent.......... Manual.............. Q [check] [check]
[[Page 35598]]
Automated........... Q [check] [check]
PM10.............. Reference........... Manual.............. J [check] [check]
Equivalent.......... Manual.............. J [check] [check] [check]
Automated........... J [check] [check] [check]
PM2.5............. Reference........... Manual.............. L [check] [check]
Equivalent Class I.. Manual.............. L [check] [check] [check]
Equivalent Class II. Manual.............. L\1\ [check] [check]\ [check] [check]\
2\ 1 2\
Equivalent Class III Automated........... L\1\ [check] [check] [check] [check]\
1\
PM10 2.5.......... Reference........... Manual.............. L, O [check] [check]
Equivalent Class I.. Manual.............. L, O [check] [check] [check]
Equivalent Class II. Manual.............. L, O [check] [check]\ [check] [check]\
2\ 1 2\
Equivalent Class III Automated........... L\1\, O\1\ [check] [check] [check] [check]\
1\
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator.
2. Alternative Class III requirements may be substituted.
Subpart B--[Amended]
0
12. Section 53.20 is amended by revising paragraph (b) and Table B-1 in
paragraph (c) to read as follows:
Sec. 53.20 General provisions.
* * * * *
(b) For a candidate method having more than one selectable
measurement range, one range must be that specified in table B-1
(standard range for SO2), and a test analyzer representative
of the method must pass the tests required by this subpart while
operated in that range. The tests may be repeated for one or more
broader ranges (i.e., ones extending to higher concentrations) than the
range specified in table B-1, provided that the range does not extend
to concentrations more than four times the upper range limit specified
in table B-1. For broader ranges, only the tests for range
(calibration), noise at 80% of the upper range limit, and lag, rise and
fall time are required to be repeated. The tests may be repeated for
one or more narrower ranges (ones extending to lower concentrations)
than that specified in table B-1. For SO2 methods, table B-1
specifies special performance requirements for narrower (lower) ranges.
For methods other than SO2, only the tests for range
(calibration), noise at 0% of the measurement range, and lower
detectable limit are required to be repeated. If the tests are
conducted or passed only for the specified range (standard range for
SO2), any FRM or FEM method determination with respect to
the method will be limited to that range. If the tests are passed for
both the specified range and one or more broader ranges, any such
determination will include the additional range(s) as well as the
specified range, provided that the tests required by subpart C of this
part (if applicable) are met for the broader range(s). If the tests are
passed for both the specified range and one or more narrower ranges,
any FRM or FEM method determination for the method will include the
narrower range(s) as well as the specified range. Appropriate test data
shall be submitted for each range sought to be included in a FRM or FEM
method determination under this paragraph (b).
(c) * * *
Table B-1--Performance Specifications for Automated Methods
--------------------------------------------------------------------------------------------------------------------------------------------------------
SO 2
---------------------------- Definitions and test
Performance parameter Units \1\ Lower O 3 CO NO 2 procedures
Std. range \3\ range 2 3
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Range........................... ppm................... 0-0.5 <0.5 0-0.5 0-50 0-0.5 Sec. 53.23(a).
2. Noise........................... ppm................... 0.001 0.0005 0.005 0.5 0.005 Sec. 53.23(b).
3. Lower detectable limit.......... ppm................... 0.002 0.001 0.010 1.0 0.010 Sec. 53.23(c).
4. Interference equivalent
Each interferent............... ppm................... 0.005 minus>0.00 minus>0.02 minus>1.0 minus>0.02
5
Total, all interferents........ ppm................... -- -- 0.06 1.5 0.04 Sec. 53.23(d).
5. Zero drift, 12 and 24 hour...... ppm................... 0.004 minus>0.00 minus>0.02 minus>1.0 minus>0.02
2
6. Span drift, 24 hour
20% of upper range limit....... Percent............... -- -- 20.0 minus>10.0 minus>20.0
80% of upper range limit....... Percent............... 3.0 minus>3.0 minus>5.0 minus>2.5 minus>5.0
7. Lag time........................ Minutes............... 2 2 20 10 20 Sec. 53.23(e).
8. Rise time....................... Minutes............... 2 2 15 5 15 Sec. 53.23(e).
9. Fall time....................... Minutes............... 2 2 15 5 15 Sec. 53.23(e).
10. Precision
20% of upper range limit....... ppm................... -- -- 0.010 0.5 0.020 Sec. 53.23(e).
Percent............... 2 2 .......... ........... .......... Sec. 53.23(e).
80% of upper range limit....... ppm................... -- -- 0.010 0.5 0.030 Sec. 53.23(e).
Percent............... 2 2 -- -- -- Sec. 53.23(e).
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. To convert from parts per million (ppm) to [mu]g/m\3\ at 25 [deg]C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the gas.
Percent means percent of the upper range limit.
2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range shows that
the lower range specification is met for each of these test parameters.
[[Page 35599]]
3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and conduct
suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision are
within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct
calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM
designation.
4. For nitric oxide interference for the SO2 UVF method, interference equivalent is 0.003 ppm for the lower range.
* * * * *
0
13. Section 53.21 is amended by revising paragraph (a) to read as
follows:
Sec. 53.21 Test conditions.
(a) Set-up and start-up of the test analyzer shall be in strict
accordance with the operating instructions specified in the manual
referred to in Sec. 53.4(b)(3). Allow adequate warm-up or
stabilization time as indicated in the operating instructions before
beginning the tests. The test procedures assume that the test analyzer
has an analog measurement signal output that is connected to a suitable
strip chart recorder of the servo, null-balance type. This recorder
shall have a chart width of a least 25 centimeters, chart speeds up to
10 cm per hour, a response time of 1 second or less, a deadband of not
more than 0.25 percent of full scale, and capability either of reading
measurements at least 5 percent below zero or of offsetting the zero by
at least 5 percent. If the test analyzer does not have an analog signal
output, or if other types of measurement data output are used, an
alternative measurement data recording device (or devices) may be used
for the tests, provided it is reasonably suited to the nature and
purposes of the tests and an analog representation of the analyzer
measurements for each test can be plotted or otherwise generated that
is reasonably similar to the analog measurement recordings that would
be produced by a conventional chart recorder.
* * * * *
0
14. Section 53.22(d) is amended by revising Table B-2 to read as
follows:
Sec. 53.22 Generation of test atmospheres.
* * * * *
(d) * * *
Table B-2--Test Atmospheres
------------------------------------------------------------------------
Test gas Generation Verification
------------------------------------------------------------------------
Ammonia....................... Permeation device. Indophenol method,
Similar to system reference 3.
described in
references 1 and 2.
Carbon dioxide................ Cylinder of zero Use NIST-certified
air or nitrogen standards whenever
containing CO2 as possible. If NIST
required to obtain standards are not
the concentration available, obtain
specified in Table 2 standards from
B-3. independent
sources which
agree within 2
percent, or obtain
one standard and
submit it to an
independent
laboratory for
analysis, which
must agree within
2 percent of the
supplier's nominal
analysis.
Carbon monoxide............... Cylinder of zero Use a FRM CO
air or nitrogen analyzer as
containing CO as described in
required to obtain reference 8.
the concentration
specified in Table
B-3.
Ethane........................ Cylinder of zero Gas chromatography,
air or nitrogen ASTM D2820,
containing ethane reference 10. Use
as required to NIST-traceable
obtain the gaseous methane or
concentration propane standards
specified in Table for calibration.
B-3.
Ethylene...................... Cylinder of pre- Do.
purified nitrogen
containing
ethylene as
required to obtain
the concentration
specified in Table
B-3.
Hydrogen chloride............. Cylinder\1\ of pre- Collect samples in
purified nitrogen bubbler containing
containing distilled water
approximately 100 and analyze by the
ppm of gaseous mercuric
HCL. Dilute with thiocyante method,
zero air to ASTM (D612), p.
concentration 29, reference 4.
specified in Table
B-3.
Hydrogen sulfide.............. Permeation device Tentative method of
system described analysis for H2S
in references 1 content of the
and 2. atmosphere, p.
426, reference 5.
Methane....................... Cylinder of zero Gas chromatography
air containing ASTM D2820,
methane as reference 10. Use
required to obtain NIST-traceable
the concentration methane standards
specified in Table for calibration.
B-3.
Naphthalene................... 1. Permeation Use NIST-certified
device as standards whenever
described in possible. If NIST
references 1 and 2. standards are not
2. Cylinder of pre- available, obtain
purified nitrogen 2 standards from
containing 100 ppm independent
naphthalene. sources which
Dilute with zero agree within 2
air to percent, or obtain
concentration one standard and
specified in Table submit it to an
B-3.. independent
laboratory for
analysis, which
must agree within
2 percent of the
supplier's nominal
analysis.
Nitric oxide.................. Cylinder\1\ of pre- Use NIST-certified
purified nitrogen standards whenever
containing possible. If NIST
approximately 100 standards are not
ppm NO. Dilute available, obtain
with zero air to 2 standards from
required independent
concentration. sources which
agree within 2
percent, or obtain
one standard and
submit it to an
independent
laboratory for
analysis, which
must agree within
2 percent of the
supplier's nominal
analysis.
Nitrogen dioxide.............. 1. Gas phase 1. Use an FRM NO2
titration as analyzer
described in calibrated with a
reference 6. gravimetrically
2. Permeation calibrated
device, similar to permeation device.
system described 2. Use an FRM NO2
in reference 6.. analyzer
calibrated by gas-
phase titration as
described in
reference 6.
Ozone......................... Calibrated ozone Use an FEM ozone
generator as analyzer
described in calibrated as
reference 9. described in
reference 9.
Sulfur dioxide................ 1. Permeation Use an SO2 FRM or
device as FEM analyzer as
described in described in
references 1 and 2. reference 7.
2. Dynamic dilution
of a cylinder
containing
approximately 100
ppm SO2 as
described in
Reference 7..
[[Page 35600]]
Water......................... Pass zero air Measure relative
through distilled humidity by means
water at a fixed of a dew-point
known temperature indicator,
between 20[deg] calibrated
and 30[deg] C such electrolytic or
that the air piezo electric
stream becomes hygrometer, or wet/
saturated. Dilute dry bulb
with zero air to thermometer.
concentration
specified in Table
B-3.
Xylene........................ Cylinder of pre- Use NIST-certified
purified nitrogen standards whenever
containing 100 ppm possible. If NIST
xylene. Dilute standards are not
with zero air to available, obtain
concentration 2 standards from
specified in Table independent
B-3. sources which
agree within 2
percent, or obtain
one standard and
submit it to an
independent
laboratory for
analysis, which
must agree within
2 percent of the
supplier's nominal
analysis.
Zero air...................... 1. Ambient air
purified by
appropriate
scrubbers or other
devices such that
it is free of
contaminants
likely to cause a
detectable
response on the
analyzer.
2. Cylinder of ...................
compressed zero
air certified by
the supplier or an
independent
laboratory to be
free of
contaminants
likely to cause a
detectable
response on the
analyzer.
------------------------------------------------------------------------
\1\ Use stainless steel pressure regulator dedicated to the pollutant
measured.
Reference 1. O'Keefe, A. E., and Ortaman, G. C. ``Primary Standards for
Trace Gas Analysis,'' Anal. Chem. 38, 760 (1966).
Reference 2. Scaringelli, F. P., A. E. Rosenberg, E., and Bell, J. P.,
``Primary Standards for Trace Gas Analysis.'' Anal. Chem. 42, 871
(1970).
Reference 3. ``Tentative Method of Analysis for Ammonia in the
Atmosphere (Indophenol Method)'', Health Lab Sciences, vol. 10, No. 2,
115-118, April 1973.
Reference 4. 1973 Annual Book of ASTM Standards, American Society for
Testing and Materials, 1916 Race St., Philadelphia, PA.
Reference 5. Methods for Air Sampling and Analysis, Intersociety
Committee, 1972, American Public Health Association, 1015.
Reference 6. 40 CFR 50 Appendix F, ``Measurement Principle and
Calibration Principle for the Measurement of Nitrogen Dioxide in the
Atmosphere (Gas Phase Chemiluminescence).''
Reference 7. 40 CFR 50 Appendix A-1, ``Measurement Principle and
Calibration Procedure for the Measurement of Sulfur Dioxide in the
Atmosphere (Ultraviolet FIuorscence).''
Reference 8. 40 CFR 50 Appendix C, ``Measurement Principle and
Calibration Procedure for the Measurement of Carbon Monoxide in the
Atmosphere'' (Non-Dispersive Infrared Photometry)''.
Reference 9. 40 CFR 50 Appendix D, ``Measurement Principle and
Calibration Procedure for the Measurement of Ozone in the
Atmosphere''.
Reference 10. ``Standard Test Method for C, through C5 Hydrocarbons in
the Atmosphere by Gas Chromatography'', D 2820, 1987 Annual Book of
Aston Standards, vol 11.03, American Society for Testing and
Materials, 1916 Race St., Philadelphia, PA 19103.
0
15. Section 53.23(d) is amended by revising Table B-3 to read as
follows:
Sec. 53.23 Test procedures.
* * * * *
(d) * * *
Table B-3--Interferent Test Concentration,\1\ Parts per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hydro- Hydro- Carbon
Pollu- tant Analyzer type chloric Ammo- gen Sulfur Nitrogen Nitric Carbon Ethy- Ozone M- Water vapor mon- Meth- Ethane Naphthalene
acid nia sulfide dioxide dioxide oxide dioxide lene xylene oxide ane
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2................... Ultraviolet fluorescence ....... ....... \5\ 0.1 \4\ 0.5 0.5 ....... ....... 0.5 0.2 20,000 ....... ....... ....... \6\ 0.05
0.14
SO2................... Flame photometric....... ....... ....... 0.01 \4\ ........ ....... 750 ....... ....... ....... \3\ 20,000 50 ....... ....... ...........
0.14
SO2................... Gas chromatography...... ....... ....... 0.1 \4\ ........ ....... 750 ....... ....... ....... \3\ 20,000 50 ....... ....... ...........
0.14
SO2................... Spectrophotometric-wet 0.2 0.1 0.1 \4\ 0.5 ....... 750 ....... 0.5 ....... ........... ....... ....... ....... ...........
chemical 0.14
(pararosanaline).
SO2................... Electrochemical......... 0.2 0.1 0.1 \4\ 0.5 0.5 ....... 0.2 0.5 ....... \3\ 20,000 ....... ....... ....... ...........
0.14
SO2................... Conductivity............ 0.2 0.1 ....... \4\ 0.5 ....... 750 ....... ....... ....... ........... ....... ....... ....... ...........
0.14
SO2................... Spectrophotometric-gas ....... ....... ....... \4\ 0.5 ....... ....... ....... 0.5 0.2 ........... ....... ....... ....... ...........
phase, including DOAS. 0.14
O3.................... Chemiluminescent........ ....... ....... \3\ 0.1 ....... ........ ....... 750 ....... \4\ ....... \3\ 20,000 ....... ....... ....... ...........
0.08
O3.................... Electrochemical......... ....... \3\ 0.1 ....... 0.5 0.5 ....... ....... ....... \4\ ....... ........... ....... ....... ....... ...........
0.08
O3.................... Spectrophotometric-wet ....... \3\ 0.1 ....... 0.5 0.5 \3\ 0.5 ....... ....... \4\ ....... ........... ....... ....... ....... ...........
chemical (potassium 0.08
iodide).
O3.................... Spectrophotometric-gas ....... ....... ....... 0.5 0.5 0.5 ....... ....... \4\ 0.02 20,000 ....... ....... ....... ...........
phase, including 0.08
ultraviolet absorption
and DOAS.
CO.................... Infrared................ ....... ....... ....... ....... ........ ....... 750 ....... ....... ....... 20,000 \4\ 10 ....... ....... ...........
CO.................... Gas chromatography with ....... ....... ....... ....... ........ ....... ....... ....... ....... ....... 20,000 \4\ 10 ....... 0.5 ...........
flame ionization
detector.
CO.................... Electrochemical......... ....... ....... ....... ....... ........ 0.5 ....... 0.2 ....... ....... 20,000 \4\ 10 ....... ....... ...........
CO.................... Catalytic combustion- ....... 0.1 ....... ....... ........ ....... 750 0.2 ....... ....... 20,000 \4\ 10 5.0 0.5 ...........
thermal detection.
CO.................... IR fluorescence......... ....... ....... ....... ....... ........ ....... 750 ....... ....... ....... 20,000 \4\ 10 ....... 0.5 ...........
CO.................... Mercury replacement-UV ....... ....... ....... ....... ........ ....... ....... 0.2 ....... ....... ........... \4\ 10 ....... 0.5 ...........
photometric.
NO2................... Chemiluminescent........ ....... \3\ 0.1 ....... 0.5 \4\ 0.1 0.5 ....... ....... ....... ....... 20,000 ....... ....... ....... ...........
NO2................... Spectrophotometric-wet ....... ....... ....... 0.5 \4\ 0.1 0.5 750 ....... 0.5 ....... ........... ....... ....... ....... ...........
chemical (azo-dye
reaction).
NO2................... Electrochemical......... 0.2 \3\ 0.1 ....... 0.5 \4\ 0.1 0.5 750 ....... 0.5 ....... 20,000 50 ....... ....... ...........
NO2................... Spectrophotometric-gas ....... \3\ 0.1 ....... 0.5 \4\ 0.1 0.5 ....... ....... 0.5 ....... 20,000 50 ....... ....... ...........
phase.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. Concentrations of interferent listed must be prepared and controlled to 10 percent of the stated value.
2. Analyzer types not listed will be considered by the Administrator as special cases.
3. Do not mix with the pollutant.
[[Page 35601]]
4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test.
6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate
response for interference
* * * * *
Subpart C [Amended]
0
16. Section 53.32 is amended by revising paragraph (e)(2) to read as
follows:
Sec. 53.32 Test procedures for methods for SO2, CO,
O3, and NO2.
* * * * *
(e) * * *
(2) For a candidate method having more than one selectable range,
one range must be that specified in table B-1 of subpart B of this
part, and a test analyzer representative of the method must pass the
tests required by this subpart while operated on that range. The tests
may be repeated for one or more broader ranges (i.e., ones extending to
higher concentrations) than the one specified in table B-1 of subpart B
of this part, provided that such a range does not extend to
concentrations more than four times the upper range limit specified in
table B-1 of subpart B of this part and that the test analyzer has
passed the tests required by subpart B of this part (if applicable) for
the broader range. If the tests required by this subpart are conducted
or passed only for the range specified in table B-1 of subpart B of
this part, any equivalent method determination with respect to the
method will be limited to that range. If the tests are passed for both
the specified range and a broader range (or ranges), any such
determination will include the broader range(s) as well as the
specified range. Appropriate test data shall be submitted for each
range sought to be included in such a determination.
* * * * *
0
17. Table C-1 to Subpart C is revised to read as follows:
Table C-1 to Subpart C of Part 53--Test Concentration Ranges, Number of Measurements Required, and Maximum Discrepancy Specifications
--------------------------------------------------------------------------------------------------------------------------------------------------------
Simultaneous measurements required Maximum
---------------------------------------------------- discrepancy
Pollutant Concentration range, parts per 1-hour 24-hour specification,
million (ppm) ---------------------------------------------------- parts per
First set Second set First set Second set million
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ozone.......................................... Low 0.06 to 0.10................. 5 6 ........... ........... 0.02
Med. 0.15 to 0.25................ 5 6 ........... ........... 0.03
High 0.35 to 0.46................ 4 6 ........... ........... 0.04
������������������������������������������������
Total......................... 14 18 ........... ........... ................
������������������������������������������������
Carbon monoxide................................ Low 7 to 11...................... 5 6 ........... ........... 1.5
Med. 20 to 30.................... 5 6 ........... ........... 2.0
High 25 to 45.................... 4 6 ........... ........... 3.0
������������������������������������������������
Total......................... 14 18 ........... ........... ................
������������������������������������������������
Sulfur dioxide................................. Low 0.02 to 0.05................. 5 6 3 3 0.02
Med. 0.10 to 0.15................ 5 6 2 3 0.03
High 0.30 to 0.50................ 4 6 2 2 0.04
������������������������������������������������
Total......................... 14 18 7 8 ................
������������������������������������������������
Nitrogen dioxide............................... Low 0.02 to 0.08................. ........... ........... 3 3 0.02
Med. 0.10 to 0.20................ ........... ........... 2 2 0.02
High 0.25........................ ........... ........... 2 2 0.03
������������������������������������������������
Total......................... ........... ........... 7 8 ................
--------------------------------------------------------------------------------------------------------------------------------------------------------
PART 58--AMBIENT AIR QUALITY SURVEILLANCE
0
The authority citation for part 58 continues to read as follows:
Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619.
Subpart B [AMENDED]
0
19. Section 58.10, is amended by adding paragraph (a)(6) to read as
follows:
Sec. 58.10 Annual monitoring network plan and periodic network
assessment.
* * * * *
(a) * * *
(6) A plan for establishing SO2 monitoring sites in
accordance with the requirements of appendix D to this part shall be
submitted to the EPA Regional Administrator by July 1, 2011 as part of
the annual network plan required in paragraph (a) (1). The plan shall
provide for all required SO2 monitoring sites to be
operational by January 1, 2013.
* * * * *
0
20. Section 58.12 is amended by adding paragraph (g) to read as
follows:
Sec. 58.12 Operating Schedules
* * * * *
(g) For continuous SO2 analyzers, the maximum 5-minute
block average concentration of the twelve 5-minute blocks in each hour
must be collected except as noted in Sec. 58.12 (a).
* * * * *
0
21. Section 58.13 is amended by adding paragraph (d) to read as
follows:
Sec. 58.13 Monitoring network completion.
* * * * *
(d) The network of SO2 monitors must be physically
established no later than January 1, 2013, and at that time, must be
operating under all of the requirements of this part, including the
[[Page 35602]]
requirements of appendices A, C, D, and E to this part.
0
22. Section 58.16 is amended by adding paragraph (g) to read as
follows:
Sec. 58.16 Data submittal and archiving requirements.
* * * * *
(g) Any State or, where applicable, local agency operating a
continuous SO2 analyzer shall report the maximum 5-minute
SO2 block average of the twelve 5-minute block averages in
each hour, in addition to the hourly SO2 average.
0
23. Appendix A to Part 58 is amended as by adding paragraph 2.3.1.6 to
read as follows:
Appendix A to Part 58--Quality Assurance Requirements for SLAMS, SPMs
and PSD Air Monitoring
* * * * *
2.3.1.6 Measurement Uncertainty for SO2. The goal for
acceptable measurement uncertainty for precision is defined as an
upper 90 percent confidence limit for the coefficient of variation
(CV) of 10 percent and for bias as an upper 95 percent confidence
limit for the absolute bias of 10 percent.
* * * * *
0
24. Appendix D to Part 58 is amended as by revising paragraph 4.4 to
read as follows:
Appendix D to Part 58--Network Design Criteria for Ambient Air Quality
Monitoring
* * * * *
4.4 Sulfur Dioxide (SO2) Design Criteria.
4.4.1 General Requirements. (a) State and, where appropriate,
local agencies must operate a minimum number of required
SO2 monitoring sites as described below.
4.4.2 Requirement for Monitoring by the Population Weighted
Emissions Index. (a) The population weighted emissions index (PWEI)
shall be calculated by States for each core based statistical area
(CBSA) they contain or share with another State or States for use in
the implementation of or adjustment to the SO2 monitoring
network. The PWEI shall be calculated by multiplying the population
of each CBSA, using the most current census data or estimates, and
the total amount of SO2 in tons per year emitted within
the CBSA area, using an aggregate of the most recent county level
emissions data available in the National Emissions Inventory for
each county in each CBSA. The resulting product shall be divided by
one million, providing a PWEI value, the units of which are million
persons-tons per year. For any CBSA with a calculated PWEI value
equal to or greater than 1,000,000, a minimum of three
SO2 monitors are required within that CBSA. For any CBSA
with a calculated PWEI value equal to or greater than 100,000, but
less than 1,000,000, a minimum of two SO2 monitors are
required within that CBSA. For any CBSA with a calculated PWEI value
equal to or greater than 5,000, but less than 100,000, a minimum of
one SO2 monitor is required within that CBSA.
(1) The SO2 monitoring site(s) required as a result
of the calculated PWEI in each CBSA shall satisfy minimum monitoring
requirements if the monitor is sited within the boundaries of the
parent CBSA and is one of the following site types (as defined in
section 1.1.1 of this appendix): population exposure, highest
concentration, source impacts, general background, or regional
transport. SO2 monitors at NCore stations may satisfy
minimum monitoring requirements if that monitor is located within a
CBSA with minimally required monitors under this part. Any monitor
that is sited outside of a CBSA with minimum monitoring requirements
to assess the highest concentration resulting from the impact of
significant sources or source categories existing within that CBSA
shall be allowed to count towards minimum monitoring requirements
for that CBSA.
4.4.3 Regional Administrator Required Monitoring. (a) The
Regional Administrator may require additional SO2
monitoring stations above the minimum number of monitors required in
4.4.2 of this part, where the minimum monitoring requirements are
not sufficient to meet monitoring objectives. The Regional
Administrator may require, at his/her discretion, additional
monitors in situations where an area has the potential to have
concentrations that may violate or contribute to the violation of
the NAAQS, in areas impacted by sources which are not conducive to
modeling, or in locations with susceptible and vulnerable
populations, which are not monitored under the minimum monitoring
provisions described above. The Regional Administrator and the
responsible State or local air monitoring agency shall work together
to design and/or maintain the most appropriate SO2
network to provide sufficient data to meet monitoring objectives.
4.4.4 SO2 Monitoring Spatial Scales. (a) The
appropriate spatial scales for SO2 SLAMS monitors are the
microscale, middle, neighborhood, and urban scales. Monitors sited
at the microscale, middle, and neighborhood scales are suitable for
determining maximum hourly concentrations for SO2.
Monitors sited at urban scales are useful for identifying
SO2 transport, trends, and, if sited upwind of local
sources, background concentrations.
(1) Microscale--This scale would typify areas in close proximity
to SO2 point and area sources. Emissions from stationary
point and area sources, and non-road sources may, under certain
plume conditions, result in high ground level concentrations at the
microscale. The microscale typically represents an area impacted by
the plume with dimensions extending up to approximately 100 meters.
(2) Middle scale--This scale generally represents air quality
levels in areas up to several city blocks in size with dimensions on
the order of approximately 100 meters to 500 meters. The middle
scale may include locations of expected maximum short-term
concentrations due to proximity to major SO2 point, area,
and/or non-road sources.
(3) Neighborhood scale--The neighborhood scale would
characterize air quality conditions throughout some relatively
uniform land use areas with dimensions in the 0.5 to 4.0 kilometer
range. Emissions from stationary point and area sources may, under
certain plume conditions, result in high SO2
concentrations at the neighborhood scale. Where a neighborhood site
is located away from immediate SO2 sources, the site may
be useful in representing typical air quality values for a larger
residential area, and therefore suitable for population exposure and
trends analyses.
(4) Urban scale--Measurements in this scale would be used to
estimate concentrations over large portions of an urban area with
dimensions from 4 to 50 kilometers. Such measurements would be
useful for assessing trends in area-wide air quality, and hence, the
effectiveness of large scale air pollution control strategies. Urban
scale sites may also support other monitoring objectives of the
SO2 monitoring network such as identifying trends, and
when monitors are sited upwind of local sources, background
concentrations.
4.4.5 NCore Monitoring. (a) SO2 measurements are
included within the NCore multipollutant site requirements as
described in paragraph (3)(b) of this appendix. NCore-based
SO2 measurements are primarily used to characterize
SO2 trends and assist in understanding SO2
transport across representative areas in urban or rural locations
and are also used for comparison with the SO2 NAAQS.
SO2 monitors at NCore sites that exist in CBSAs with
minimum monitoring requirements per section 4.4.2 above shall be
allowed to count towards those minimum monitoring requirements.
* * * * *
0
25. Appendix G to Part 58 is amended as by revising Table 2 to read as
follows:
Appendix G to Part 58--Uniform Air Quality Index (AQI) and Daily
Reporting
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[[Page 35603]]
Table 2--Breakpoints for the AQI
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These breakpoints Equal these AQI's
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PM2.5 PM10
O3 (ppm) 8-hour O3 (ppm) ([mu]g/m ([mu]g/ CO (ppm) SO2 (ppm) 1-hour NO2 (ppm) 1-hour AQI Category
1-hour \1\ \3\) m \3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0.000-0.059................ .......... 0.0-15.4 0-54 0.0-4.4 0-0.035 0-0.053 0-50 Good.
0.060-0.075................ .......... 15.5-40.4 55-154 4.5-9.4 0.036-0.075 0.054-0.100 51-100 Moderate.
0.076-0.095................ 0.125-0.16 40.5-65.4 155-254 9.5-12.4 0.076-0.185 0.101-0.360 101-150 Unhealthy for Sensitive Groups.
4
0.096-0.115................ 0.165-0.20 \3\ 65.5- 255-354 12.5-15. \4\ 0.186-0.304 0.361-0.64 151-200 Unhealthy.
4 150.4 4
0.116-0.374................ 0.205-0.40 \3\ 150.5- 355-424 15.5-30. \4\ 0.305-0.604 0.65-1.24 201-300 Very Unhealthy.
4 250.4 4
(\2\)...................... 0.405-0.50 \3\ 250.5- 425-504 30.5-40. \4\ 0.605-0.804 1.25-1.64 301-400 .................................
4 350.4 4
(\2\)...................... 0.505-0.60 \3\ 350.5- 505-604 40.5-50. \4\ 0.805-1.004 1.65-2.04 401-500 Hazardous.
4 500.4 4
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\1\ Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour
ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be
calculated, and the maximum of the two values reported.
\2\ 8-hour O3 values do not define higher AQI values (>=301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations.
\3\ If a different SHL for PM2.5 is promulgated, these numbers will change accordingly.
\4\ 1-hr SO2 values do not define higher AQI values (>=200). AQI values of 200 or greater are calculated with 24-hour SO2 concentrations.
* * * * *
[FR Doc. 2010-13947 Filed 6-21-10; 8:45 am]
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