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

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

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'

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

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.

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

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

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

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,

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.
---------------------------------------------------------------------------

    \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.
---------------------------------------------------------------------------

    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.
---------------------------------------------------------------------------

    \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).
---------------------------------------------------------------------------

    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

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.

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-

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

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

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

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

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.

      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