approach, and that considerable time and resources may be needed to
fully identify and properly characterize all SO2 sources
(including those emitting less than 100 tons of SO2 per
year) that may potentially cause or contribute to a violation of the
new SO2 NAAQS, we also had to assess how and when to best
use modeling as the primary method in implementation.

    The approach that EPA expects to take, which is described in
sections V and VI of the preamble, is consistent with the language of
the Clean Air Act and would accommodate the time needed for an accurate
assessment of ambient air quality levels for the 1-hour SO2
standard. Section 107(d)(1) requires areas to be designated
``attainment'' if they meet the standard, ``nonattainment'' if they do
not meet the standard or contribute to a nearby violation, or
``unclassifiable'' if they cannot be designated on the basis of
available information. EPA's expected approach would enable us to make
the appropriate designation decision required by the CAA, based on the
record of information that will be before EPA regarding each area.
Areas would be designated ``nonattainment'' if either available
monitoring data or modeling shows that a violation exists, or
``attainment'' if both available monitoring data and modeling indicate
the area is attaining. All other areas would be designated
``unclassifiable,'' as required by section 107(d)(1)(A).
    We currently anticipate that our projected post-designation
implementation approach would look to robust CAA section 110(a)(1)
SIPs, which have sometimes been previously referred to as
``maintenance'' or ``infrastructure'' SIPs but for the new
SO2 NAAQS would serve as substantive ``attainment'' SIPs.
Our current thinking is that, to be approved by EPA, such plans would
need to provide for attainment and maintenance of the new 1-hour
SO2 NAAQS as expeditiously as practicable, which we expect
to be no later than five years after initial designation (or
approximately August 2017) in all areas of the State, including any
area initially designated ``nonattainment,'' and also including any
area designated ``unclassifiable'' that has SO2 sources with
the potential to cause or contribute to a violation of the NAAQS. The
CAA establishes deadlines for States to submit these plans to EPA.\23\
State plans that address areas designated as ``nonattainment'' (i.e.,
``nonattainment area SIPs'') are due within 18 months from the
effective date of the designation, under CAA section 192. EPA
anticipates that this deadline would be February 2014. State plans
addressing all other areas (i.e., ``maintenance SIPs'') are due within
3 years following the promulgation of the new NAAQS, or June 2013,
under CAA section 110(a)(1).
---------------------------------------------------------------------------

    \23\ The schedule for State plans addressing areas designated
``nonattainment'' is governed by CAA section 191. The schedule for
State plans for all other areas, including areas designated
``unclassifiable'' and ``attainment,'' is governed by CAA section
110(a)(1).
---------------------------------------------------------------------------

    Section 110(a)(1), unlike section 192, does not specify a maximum
deadline by which States are required to show they have met the
requirements to implement, maintain, and enforce a NAAQS. EPA believes,
however, that August 2017 is the latest date by which areas should show
they have achieved attainment and maintenance of the standard because
this deadline is the same as would be required for areas designated
nonattainment in June 2012. It is therefore presumptively reasonable as
it is identical to the period Congress provided for nonattainment areas
to reach attainment. Moreover, EPA notes that the maintenance SIPs will
be due in June 2013, rather than in February 2014, giving States and
sources at least as much time between SIP development and submission
and the date by which attainment should be achieved as they would have
had the area been designated nonattainment in 2012. These section
110(a)(1) SIPs would be able to rely on modeling reflecting any
SO2 reductions that we expect to result before the
attainment date from compliance with the rules EPA expects to
promulgate before 2013, (including technology-based standards under CAA
section 112(d) for certain source categories emitting large amounts of
SO2 such as Electric Generating Units and industrial
boilers, and revised rules establishing further limits on
SO2 emitted by sources in upwind States which contribute
significantly to downstream States' inability to attain or maintain the
PM2.5 NAAS (the so-called Clean Air Interstate Replacement rule)).
Thus, we intend that a State's section 110(a)(1) SIP may account for
projected emissions reductions, including any from national and
regional rules that are promulgated before these SIP submissions,
provided that those reductions occur under a schedule that ensures
attainment as expeditiously as practicable. We expect that date to be
no later than 5 years from the date of initial designation or August
2017.
    Under this anticipated approach, attainment SIPs for nonattainment
areas would have to include enforceable emissions limitations,
timetables for compliance, and appropriate testing/reporting to assure
compliance, and demonstrate attainment through air quality modeling for
all sources contributing to monitored and modeled violations, or that
have the potential to cause or contribute to a violation of the NAAQS.
The SIPs under section 110(a)(1) would need to demonstrate through
refined air quality modeling that any source or group of sources that
have the potential to cause or contribute to a violation of the NAAQS
are, or will be, sufficiently controlled to ensure timely attainment
and maintenance of the NAAQS. We would expect this to include any
individual sources with the potential to emit 100 or more tons per year
of SO2, and other sources that may also cause or contribute
to violations of the new SO2 NAAQS. We expect to develop
guidance for the States' use on how best to identify and assess the
impact of sources that may have this potential. As mentioned
previously, we intend to provide an opportunity for notice and comment
on this guidance before finalizing it.
    EPA again notes that it anticipates several forthcoming national
and regional rules, such as the pending Industrial Boilers MACT
standard under CAA section 112(d), that are likely to require
significant reductions in SO2 emissions over the next
several years. A limited qualitative assessment based on the results of
preliminary modeling of some sample facilities indicates that well
controlled sources should meet the new SO2 NAAQS (see Brode
2010b). Exceptions could include unique sources with specific
characteristics that contribute to higher ambient impacts (short stack
heights, complex terrain, etc.). These national and regional rules are
expected to lead to SO2 reductions that will help achieve
compliance with the new SO2 NAAQS prior to 2017. If, upon
EPA review of submitted SIPs that rely upon those reductions or other
local controls, it appears that States will nevertheless fail to attain
the NAAQS as expeditiously as practicable (and no later than August
2017), the Clean Air Act provides authorities for EPA to solve such
failure, including, as appropriate, disapproving submitted SIPs, re-
designating unclassifiable areas to nonattainment, issuing SIP calls,
and promulgating FIPs.
    For the reasons discussed above, EPA has determined that it is
appropriate and efficient to principally use modeling to assess
compliance for medium to larger sources, and to rely more on monitoring
for groups of smaller sources and sources not as conducive to modeling.
EPA's revised monitoring network requirements have been developed to be
consistent with this approach. However, EPA is still considering how
monitoring and modeling data would be used together in specific
situations to define attainment and nonattainment boundaries and under
what circumstances it may be appropriate to rely on monitoring data
alone to make attainment determinations. EPA intends

to address these issues as it develops implementation guidance.
    In light of the new approach that EPA intends to take with respect
to implementation of the SO2 NAAQS, EPA intends to solicit
public comment on guidance regarding modeling, and also solicit public
comment on additional implementation planning guidance, including the
content of the maintenance plans required under section 110(a)(1) of
the Clean Air Act. EPA also notes that State monitoring plans and the
SIP submissions that States will make will also be subject to public
notice and comment.

IV. Amendments to Ambient Monitoring and Reporting Requirements

    In this section of the preamble, we describe the proposal, the
public comments that we received on the proposed monitoring and
reporting requirements, and the final requirements for the
SO2 monitoring network. We are modifying our proposed
approach to the amount of monitoring to require following consideration
of public comments and a review of our historic practice in assessing
compliance with the SO2 NAAQS. As we explain above in
section III, we will use a hybrid approach that combines monitoring and
modeling, using each of these analytic tools where they are most
appropriate and effective. This approach and its requirements are
intended to support the revised SO2 NAAQS, described in
section II above. For a short-term 1-hour standard, dispersion modeling
of stationary sources will generally be more technically appropriate,
efficient, and effective because it takes into account fairly
infrequent combinations of meteorological and source operating
conditions that can contribute to peak ground-level concentrations of
SO2. Even an expansive monitoring network could fail to
identify all such locations. Consequently, we have revised the scope of
the monitoring network, reflecting a modified and expanded set of
objectives. This section also describes and explains the final
requirements for the new SO2 Federal Reference Method (FRM),
and the SO2 network design, monitoring objectives, data
reporting, and data quality objectives that support the revised primary
SO2 NAAQS.

A. Monitoring Methods

1. Requirements for SO2 Federal Reference Method (FRM)
    The proposal to promulgate an automated SO2 FRM was
based on a need to update the cumbersome existing manual wet-chemistry
(pararosaniline) method to a continuous-type automated method that can
readily provide 1-hour SO2 measurement capability. See 74 FR
at 64846-849. The following paragraphs provide background, rationale,
and the final changes to the automated SO2 Federal Reference
Method (FRM) and to the associated performance specifications for
automated SO2 analyzers.
a. Proposed Ultraviolet Fluorescence SO2 FRM and Its
Implementation
    FRMs, set forth in several appendices to 40 CFR Part 50, serve (1)
To provide a specified methodology for definitively measuring
concentrations of ambient air pollutants for comparison to the NAAQS in
Part 50, and (2) to provide a standard of comparison for determining
equivalency of alternative pollutant measurement methods that can be
used in lieu of the FRM for such monitoring.
    The FRM for measuring SO2 in the ambient air was
promulgated on April 30, 1971 in conjunction with the first primary
SO2 NAAQS (36 FR 8196). This SO2 FRM is specified
in Appendix A of Part 50 and identified as the pararosaniline manual
method. See generally 74 FR at 64846. In the interim, EPA has
designated many SO2 methods as equivalent methods (FEMs),
most of which are based on the ultraviolet fluorescence (UVF) measuring
technique. Id. In fact, virtually all SO2 monitoring data
are now obtained with FEMs that use the UVF technique.
    In light of this, EPA proposed to establish a new automated
SO2 FRM based on UVF--the same measurement technique
employed by FEM analyzers now in widespread use by most State and local
monitoring agencies and having the measurement capability needed to
implement the proposed 1-hour SO2 NAAQS. FRM analyzers using
this UVF technique can provide the needed detection limits, precision,
and accuracy and fulfill other purposes of an FRM, including use as an
appropriate standard of reference for testing and designation of new
FEM analyzers. At proposal, EPA specified the new method in
performance-based form, describing a generic reference measurement
principle and associated calibration procedure in a new Appendix A-1 to
40 CFR Part 50. Associated performance requirements applicable to
candidate automated SO2 analyzers (both FRMs and FEMs) were
proposed in 40 CFR Part 53.
    EPA also proposed retaining the existing manual pararosaniline FRM
for SO2. Although EPA recognized that the existing method is
cumbersome for one-hour measurements, it is capable of making
measurements of 1 hour or even 30 minute periods. 74 FR at 64846; see
also Part 50 Appendix A at 1.1 (``[t]he method is applicable to the
measurement of ambient SO2 concentrations using sampling
periods ranging from 30 minutes to 24 hours''). Supersession of the
existing manual FRM, as defined in Sec.  53.16, would require not only
withdrawal of that existing FRM but also the cancellation of the
designations of all existing SO2 FEMs. Loss of the use of
these FEM analyzers would leave State and local monitoring agencies
with no approved SO2 monitors until new FRM and FEM
analyzers could be designated under the new FRM. The resulting costs
and disruptions to monitoring agencies is unnecessary because the
current SO2 FEMs readily and accurately measure (and report)
one-hour ambient measurements. See 74 FR at 64847. Accordingly, EPA
concluded that supersession of the existing FRM was not warranted,
given the costs and disruptions which would occur to State monitoring
programs and the limited benefits from such an action given the
suitability of the in-use FEMs. Id. at 68646; see also section
53.16(b)(1) stating that in exercising its discretion as to whether to
proceed with supersession of an FRM, EPA will consider the benefits (in
terms of requirements and purposes of the Act) from specifying a new
reference method, potential economic consequences of such supersession
for State and local monitoring agencies, and disruption to State and
local air quality monitoring programs. Instead, EPA proposed to add the
new UVF FRM while retaining the existing FRM for some period of time to
support the continued approval of existing SO2 FEM
analyzers.
b. Public Comments on the Proposed FRM and Implementation
    EPA received comments from State and local groups (e.g., City of
Houston, Houston-Galveston Area Council, KY, NC, NY, PA, SC, SD, and
WI) and industry (e.g., AirQuality Research and Logistics (AQRL),
Consumers Energy, ExxonMobil, Montana Sulfur and Chemical Company, Inc.
(MSCC), and the Utility Air Regulatory Group (UARG)), all generally
supporting EPA's proposal to adopt the proposed automated UVF as an
FRM. For example, South Dakota supported adding the UVF SO2
method as an additional FRM and stated that this method is currently
being used in the network and will reduce the cost of implementing the
new monitoring

requirements for this rule. The UARG stated that the proposal to
specify a different FRM to judge compliance is entirely reasonable, and
UARG generally supported the proposed specifications for a new FRM but
maintained that the current FRM could not be used along with a new FRM.
ExxonMobil stated that it supports ``* * * EPA allowing monitoring
agencies to choose mobile monitoring that meets monitoring quality
requirements.'' AQRL stated that ``EPA is correct in choosing to
designate [promulgate] a new (automated) FRM for measurement of
SO2.''
    EPA did not receive any public comments opposing the proposed
automated UVF SO2 FRM but did receive a few technical
comments on specific provisions of the method. EPA proposed use of an
inlet line particle filter as a requirement for new UVF SO2
FRM analyzers, believing that use of a particle filter is advantageous
to prevent interference, malfunction, or damage to the analyzer from
particles in the sampled air. The State of Missouri questioned this
requirement, noting that such a filter can sometimes cause problems and
that filter requirements for other FRM and FEM analyzers have been
analyzer-specific depending on the manufacturer's stipulation. EPA
believes, however, that for new SO2 FRM analyzers, the
benefits and uniformity provided by a mandatory filter requirement
outweigh possible disadvantages of such a filter.
    Missouri also suggested that the language of proposed Sections
4.1.1 and 4.1.2 regarding calibration system flow rate requirements
were somewhat confusing, and that the high (50-100 ppm) concentration
requirement for the calibration standard specified in Section <IP_ADDRESS>
is sometimes a problem. In response to these comments, the language of
Sections 4.1.1 and 4.1.2 has been clarified, and the concentration of
the standard specified in Section <IP_ADDRESS> has been reduced to 10 ppm.
    EPA received a number of comments from States (e.g., NC, NYSDEC,
PA, SC, and SD) that supported the EPA proposed plan of temporary
retention of the existing wet-chemistry pararosaniline FRM and for FEMs
approved based on that method. For example, Pennsylvania stated
``[t]his methodology should enable State and local agencies to continue
using their existing monitoring equipment and [thereby] avoid large
capital fund outlays for samplers and ultimately avoid any delays in
collecting data that would be comparable to the proposed new primary
sulfur dioxide NAAQS.'' North Carolina requested ``* * * that the EPA
maintain the current reference method for at least an additional 10
years.'' Wisconsin and the Center for Biological Diversity (CBD)
suggested expeditiously phasing out the existing manual SO2
FRM.
    In contrast, however, EPA also received comments from industry that
opposed the retention of the existing pararosaniline FRM while
promulgating a new automated UVF FRM. In particular, UARG stated ``* *
* having two FRMs specified for a given NAAQS--is not viable,''
pointing out that there is only one FRM for each NAAQS under the
present standards, a result UARG appears to believe is legally
mandated.
    EPA disagrees with this comment. First, there is nothing in the Act
that mandates a single FRM for each NAAQS. Section 109 of the Act, in
fact, does not address this issue at all. Second, as noted previously,
there are sound policy reasons for not withdrawing the existing FRM at
this time. Therefore, EPA sees no legal or other obstacle in adding a
new automated UVF FRM while retaining the existing manual FRM.
    UARG further maintained that EPA provided no support for its
statement that the existing FEMs, which constitute the bulk of the
existing SO2 monitoring network, are adequate for the
current and proposed new SO2 NAAQS. UARG also stated that
``although the FEMs may be adequate for many other purposes, they may
only be used to judge compliance with the 1-hour NAAQS if they are
shown to qualify as FRMs or FEMs under the new FRM definition.''
    EPA disagrees with this comment also. In answer to UARG's second
point, it is not necessary that these existing FEMs be re-designated as
FRMs pursuant to the new automated FRM to continue their approved use.
There is no legal impediment to such continued use, since they are (and
will continue to be) FEMs approved based on an FRM that adequately
measures one-hour ambient SO2 concentrations. Nor is there
any technical impediment to the continued use of these FEMs, given that
they are automated continuous monitoring methods capable of measuring
SO2 concentrations ranging from a few minutes to a 1-hour
period. The existing FEMs in the network use the same UVF technology as
the proposed (and now final) automated FRM and have been reporting 1-
hour monitoring data for decades. These FRMs have been tested against
the test and performance requirements of Part 53, which are designed
specifically to test such continuous methods. Further, the proposed
SO2 method performance specifications for the standard
measurement range were derived from data submitted in FEM applications
for analyzers that were subsequently designated as FEMs. Therefore,
these FEMs are technically and legally sound to judge compliance with
the one-hour NAAQS.
    EPA has clarified the regulatory text so that the rules state
unambiguously that both SO2 FRMs apply to the new one-hour
standard (as well as to the 24-hour and annual standards so long as
they are retained), as do all presently-designated FEMs.
c. Conclusions on Ultraviolet Fluorescence SO2 FRM and
Implementation
    We are finalizing the proposed new automated SO2 FRM,
which is based on UVF technology, with the following minor technical
changes: The language of Sections 4.1.1 and 4.1.2 has been clarified,
and the minimum concentration of the calibration standard specified in
Section <IP_ADDRESS> has been reduced to 10 ppm. The new FRM is codified as
Appendix A-1 to 40 CFR Part 50 and titled ``Reference Measurement
Principle and Calibration Procedure for the Measurement of Sulfur
Dioxide in the Atmosphere (Ultraviolet Fluorescence Method).'' EPA is
retaining the previously existing manual pararosaniline SO2
FRM for the time being and re-codifying it as Appendix A-2 to 40 CFR
Part 50. However, EPA plans to rescind this manual FRM at a future time
when new SO2 FRM analyzers have adequately permeated State
monitoring networks.
2. Requirements for Automated SO2 Methods
a. Performance Specifications for Automated Methods
    In association with the proposal to adopt a new automated FRM, EPA
proposed to update the performance-based designation requirements for
FEM SO2 analyzers currently specified in 40 CFR Part 53. As
noted in the proposal preamble (74 at 64846), these requirements were
established in the 1970's, based primarily on the wet-chemical
measurement technology available at that time. Those initial
requirements have become significantly outdated and need to be modified
to match current technology, particularly because they would apply to
new SO2 FRM analyzers under the proposed new FRM. The better
instrumental performance available with the proposed new UVF FRM
technique allows the performance requirements in Part 53 to be made
more stringent for

both FRM and FEM SO2 analyzers. Updating these performance
requirements is needed to ensure that, going forward, all new
SO2 monitors will have improved performance.
    EPA solicited comments on the proposed new performance requirements
for automated SO2 methods that were included in Table B-1
(Performance Specifications for Automated Methods) of Part 53. We
proposed revised performance specifications for noise, lower detectable
limit, interference equivalent, zero drift, span drift, lag time, rise
time, fall time, and precision. EPA proposed to reduce the allowable
noise limit from 5 to 1 ppb, the lower detectable limit from 10 to 2
ppb, the interference equivalent limits from 20 ppb to
5 ppb for each interferent, and from 60 ppb to 20 ppb for
the total of all interferents, the zero drift limit from 20
to 4 ppb, the lag time limit from 20 to 2 minutes, both
rise and fall time limits from 15 to 2 minutes, and the precision
limits from 15 ppb to 2 percent of the upper range limit. EPA further
proposed to eliminate the requirements for span drift at 20% of the
upper range limit. In addition, to address the need for more sensitive,
lower measurement ranges for SO2 analyzers, EPA proposed a
separate set of performance requirements that would apply specifically
to narrower measurement ranges, i.e. ranges extending from zero to
concentrations less than 0.5 ppm. Other minor changes were proposed in
the wording of a few sections of Part 53 Subparts A and B, including
provision for alternate data recording devices in Sec.  53.21 to
supplement the older language relating specifically to strip chart
recorders.
b. Public Comments
    EPA received a number of comments from industry (AQRL and UARG) and
from the multi-State organization NESCAUM regarding the proposed
interferent limit requirements listed in Table B-1. UARG submitted
comments supportive of all the proposed requirements for the new UVF
SO2 FRM, except for the proposed total interferent limits of
20 ppb. UARG acknowledged that EPA proposed to reduce the total
interferent level substantially from 60 ppb to 20 ppb, but maintained
that the proposed level of 20 ppb is still too high because it amounts
to 20%-40% of the levels being considered for the NAAQS (50-150 ppb).
AQRL recommended limiting ``* * * each interferent to no more than
3 ppb and total interference to no more than 12 ppb.''
NESCAUM recommended tightening the nitric oxide (NO) interference limit
from 100:1 to 300:1 (i.e., one third of the proposed value of 5 ppb). NESCAUM states that the proposed interferent value of
5 ppb results in substantial NO interference at sites with
low SO2 levels in urban areas.
    EPA revisited the issue of the interferent equivalent limit for
SO2 analyzers in context of the above comments and
reconsidered what is reasonably feasible with current technology. We
reviewed the current instrument specifications and test data submitted
for numerous SO2 FEM applications. We also took into account
that the test concentrations of most of these interferents are
substantially higher than the concentrations normally observed in
ambient air. EPA considered lowering the testing concentrations of
these interferents, which would have correspondingly lowered the
interferent equivalent for each analyte. However, EPA took a more
conservative approach and retained the existing test concentrations for
H2S, NO2, NO, O3, m-xylene, and water
vapor. Based on this review, we found that it is not feasible to
further lower the limit requirement for these interferents below 5 ppb. However, in response to the NESCAUM comment, EPA
determined that the interferent equivalent limit requirement for NO
interference could be reduced to 3 ppb (166:1) for the new,
lower measurement range to reduce possible NO interference at sites
with low SO2 levels in urban area.
    In regard to the total limit for all interferent equivalents for
SO2 analyzers, EPA notes that many of the interferents for
which testing is required (specified in Table B-3 of Part 53) would
likely react with each other and would thus not co-exist in ambient air
at the specified test concentrations. Therefore, EPA determined that
the limit requirement for total interference equivalent can be
eliminated, and Table B-1 now reflects this change.
    EPA received comment from AQRL on the existing span drift
requirement for SO2 analyzers specified in Table B-1. AQRL
recommended lowering the span drift requirement at 80% URL to 2.5%,
stating that ``ambient air monitors in the 21st century should be able
to hold span drift to no more than 2.5% under the
conditions specified in EPA testing * * *.'' Based on information from
FEM testing laboratories and manufacturers' data (EPA, 2009c), EPA
largely agrees with this comment and concludes that the span drift
requirement at 80% can be lowered to 3%. Table B-1 has been
changed to include this revised limit.
    EPA received comment from the State of Wisconsin suggesting that
the proposed revised provisions of section 53.21 (Test conditions) be
further changed to more specifically recognize use of digital recorders
for obtaining test results rather than maintaining the tie to analog
strip chart recorder technology. EPA acknowledges that industry has
moved away from strip chart recording technology to digital data
recording. However, the proposed language of Sec.  53.21 calls for a
graphic representation of analyzer responses to test concentrations to
facilitate visual examination of test results and allows any
``alternative measurement data recording device'' as long as it can
provide such a graphic representation. Describing the analog strip
chart recorder in this section provides an appropriate model to help
define the type of graphic representation needed for the Part 53 tests.
EPA believes that the proposed language of Sec.  53.21 is adequately
broad to permit digital or other types of data recording devices.
c. Conclusions for Performance Specifications for SO2
Automated Methods
    Based on typical performance capabilities of current UVF analyzers
and manufacturers' actual testing data, we are keeping the limit for
each interference equivalent for SO2 analyzers at 5 ppb. However, we are lowering the interference equivalent
requirement for NO to 3 ppb for the lower measurement
range. A footnote denoting this specific requirement is being added to
Table B-1. We are eliminating the total interference equivalent
requirement for SO2 analyzers, and Table B-1 is being
revised to incorporate this change.
    The 24-hour span drift at 80% of the upper range limit for
SO2 analyzers is being lowered to 3% in Table B-
1 to be in line with current technology. Also, unrelated to
SO2, a typographical error for the noise requirement for CO
analyzers is being corrected to 0.5 ppm in Table B-1.
    Finally, information on generation and verification of test
concentrations for naphthalene was inadvertently omitted from Table B-
2, Test Atmospheres, even though it was added as a required interferent
test in our proposal. Therefore, we are adding that information for
naphthalene. Also in Table B-2, we are correcting the verification
information for nitric oxide.

B. Network Design

    Ambient SO2 monitoring data are collected by State,
local, and Tribal monitoring agencies (``monitoring agencies'') in
accordance with the monitoring requirements contained in

40 CFR parts 50, 53, and 58. A monitoring network is generally designed
to measure, report, and provide related information on air quality data
as described in 40 CFR Part 58. To ensure that the data from the
network is accurate and reliable, the monitors in the network must meet
a number of requirements including the use of monitoring methods that
EPA has approved as Federal Reference Methods (FRMs) or Federal
Equivalent Methods (FEMs) (discussed in some detail above in section
IV.A), focusing on particular monitoring objectives, and following
specific siting criteria, data reporting, quality assurance and data
handling rules or procedures.
    With the revision to the SO2 NAAQS, which establishes a
new 1-hour averaging period intended to limit short-term exposures that
may occur anywhere in an area, EPA evaluated the existing network to
determine if it was adequate to support the revised SO2
NAAQS. A significant fact for ambient SO2 concentrations is
that stationary sources are the predominant emission sources of
SO2 and the peak, maximum SO2 concentrations that
may occur are most likely to occur nearer the parent stationary source,
as noted in the ISA (ISA, 2-1), section II.A.1 above, and in section
IV.B.1 below. According to the 2005 National Emissions Inventory, there
are 32,288 sources (facilities) emitting SO2, of which 1,928
are emitting 100 tons per year (tpy) or more. In the proposal (74 FR
64851), EPA had anticipated requiring 348 source-oriented monitors in
the network design based on a population and emissions metric and a
State's emissions contribution to the National Emissions Inventory
(NEI). In response to this proposal, EPA received numerous comments
arguing that the required number of monitors in the network would be
too small. Other commenters argued that expanding the monitoring to an
adequate scale would impose a large burden and expense on the States.
Some commenters referred to SO2 modeling in their
submissions as an addition or alternative to monitoring. Consequently,
as part of developing a balanced response to these comments, we
revisited how we had historically dealt with SO2 for various
purposes including designations and implementation through permitting
and emissions limitations. As explained in section III, this has been
realized through a combined monitoring and modeling approach. As set
out below, and in sections III, VI, and VII, our ultimate intention is
to utilize a combined monitoring and modeling approach, a hybrid
analytic approach, to assess compliance with the revised SO2
NAAQS.
    As a result of this contemplated hybrid analytic approach, the
minimum number of monitors required in the network through this
rulemaking is reduced to approximately 163 monitors from the
approximated 348 monitors that were proposed. This section of the
preamble includes a discussion of the proposal, the comments received,
and the details of and the rationale for the final changes to the
SO2 network design requirements.
1. Approach for Network Design
a. Proposed Approach for Network Design
    To fully support the proposed revision to the SO2 NAAQS,
EPA indicated the need to identify where short-term, peak ground-level
concentrations--i.e., concentrations from 5 minutes to one hour (or
potentially up to 24 hours)--may occur. Given that large stationary
sources are the predominant source of emissions, monitoring short-term,
peak ground-level concentrations would require monitors to be sited to
assess impacts of individual or groups of sources and therefore be
source-oriented in nature. As a result, under a monitoring-focused
approach, EPA proposed a two-pronged monitoring network of all source-
oriented monitors. However, due to the multiple variables that affect
ground level SO2 concentrations from individual or groups of
sources, including stack heights, emission velocities, stack diameters,
terrain, and meteorology, EPA could not specify a source specific
threshold, algorithm, or metric by which to require monitoring. The
design of the proposed network represented a primarily monitoring-
focused approach to assess compliance with the primary SO2
NAAQS.
    In preparation for the SO2 NAAQS proposal, EPA conducted
an analysis of the approximately 488 SO2 monitoring sites
operating during calendar year 2008 (Watkins and Thompson, 2009). This
analysis indicated that approximately ~ 35% of the monitoring network
was addressing locations of maximum (highest) concentrations, likely
linked to a specific source or group of sources. Meanwhile, just under
half (~ 46%) of the sites were reported to be for the assessment of
concentrations for general population exposure. These data allowed EPA
to conclude that the network \24\ was not properly focused to support
the revised NAAQS (under the assumption that source-oriented monitoring
data would be the primary tool for assessing compliance with the
NAAQS). As a result, EPA proposed a two-pronged monitoring network (74
FR 64850), based on the premise of a monitoring-focused approach, with
minimum requirements for: (1) Monitors in urban areas where there is a
higher coincidence of population and emissions, utilizing a Population
Weighted Emissions Index (PWEI), and (2) monitors in States based on
each State's contributions to the national SO2 emissions
inventory. In addition, all the monitors in the network would be sited
at locations of expected maximum hourly concentrations and therefore
likely be source-oriented. This two-pronged network would have resulted
in a minimum of approximately 348 monitors nationwide \25\ providing
data for comparison with the 1-hour standard and supporting its
implementation.
---------------------------------------------------------------------------

    \24\ Prior to this rulemaking there were no minimum monitoring
requirements, except for those required at the multi-pollutant
National Core (NCore) monitoring sites. The monitoring rule
promulgated in 2006 (71 FR 61236) removed minimum monitoring
requirements (except for those NCore stations). This change was
largely driven by the fact that there was no longer an
SO2 nonattainment problem under the then-existing
standards. However, this logic does not apply to the revised primary
SO2 NAAQS.
    \25\ Required monitor estimates were based on 2008 Census
estimates and the 2005 National Emissions Inventory.
---------------------------------------------------------------------------

    Under the first prong of the network design, EPA proposed that the
ambient SO2 monitoring network account for SO2
exposure by requiring monitors in locations where population and
emissions may lead to higher potential for population exposure to peak
hourly SO2 concentrations. In order to do this, EPA
developed a Population Weighted Emissions Index (PWEI) that uses
population and emissions inventory data at the CBSA \26\ level to
assign required monitoring for a given CBSA (with population and
emissions being obvious relevant factors in prioritizing numbers of
required monitors). The PWEI for a particular CBSA was proposed to be
calculated by multiplying the population (using the latest Census
Bureau estimates) of a CBSA by the total amount of SO2
emissions in that CBSA. The CBSA SO2 emission value would be
in tons per year, and calculated by aggregating the county level
emissions for each county in a CBSA. We would then divide the resulting
product of CBSA population and CBSA SO2 emissions by
1,000,000 to provide a PWEI value, the units of

which would be millions of people-tons per year.
---------------------------------------------------------------------------

    \26\ CBSAs are defined by the U.S. Census Bureau, and are
comprised of both Metropolitan Statistical Areas and Micropolitan
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    We proposed that the first prong of the SO2 network
design require monitors in CBSAs, according to the following criteria.
For any CBSA with a calculated PWEI value equal to or greater than
1,000,000, a minimum of three SO2 monitors would be required
within that CBSA. For any CBSA with a calculated PWEI value equal to or
greater than 10,000, but less than 1,000,000, a minimum of two
SO2 monitors would be required within that CBSA. For any
CBSA with a calculated PWEI value equal to or greater than 5,000, but
less than 10,000, a minimum of one SO2 monitor would be
required within that CBSA. EPA estimated that the proposed criteria
would have resulted in 231 required sites in 131 CBSAs.
    Under the second prong of the network design, EPA proposed to
require a monitor or monitors in each State, allocated by State-level
SO2 emissions. This prong of the network design was intended
to allow a portion of the overall required monitors to be placed where
needed, independent of the first prong of the network design, inside or
outside of CBSAs. EPA proposed to require monitors, using State
boundaries as the geographic unit for allocation purposes, in
proportion to a State's SO2 emissions, i.e., a State with
higher emissions would have been required to have a proportionally
higher number of monitors. The proposed percent contribution of
individual States would have been based on the most recent NEI, with
SO2 emissions being aggregated by State. The number of
required monitors per State would correspond to every one percent
(after rounding) of each State's contribution to the national
SO2 inventory. EPA also proposed that each State have at
least one monitor required as part of this second prong, even if a
particular State contributes less than 0.5% of the total anthropogenic
national emissions inventory. As a result, the proposed second prong
would have required approximately 117 monitoring sites based on State-
level SO2 emissions in the most recent NEI, which at the
time of the proposal, was the 2005 NEI.
    EPA also stated in the proposal that the multi-pollutant National
Core (NCore) monitoring sites would not have counted towards meeting
the proposed monitoring requirements. However, data from the NCore
would be compared to the NAAQS even though NAAQS comparisons are not
the sole objective of NCore monitors. The monitoring rule promulgated
in 2006 (71 FR 61236) and codified at 40 CFR Part 58 and its Appendices
established the NCore multi-pollutant network requirement to support
integrated air quality management data needs. In particular, NCore
sites are intended to provide long-term data for air quality trends
analysis, model evaluation, and, for urban sites, tracking metropolitan
air quality statistics. To do this, NCore sites are required to measure
various pollutants, including SO2, but they are not source
oriented monitoring sites, and therefore are not likely to be the
location of maximum expected concentration in an area. NCore sites are
intended to provide data representing concentrations at the broader
neighborhood and urban spatial scales. These reasons were the rationale
justifying why SO2 monitors at NCore stations would not have
been part of the minimum monitors required under the proposed network.
b. Alternative Network Design
    EPA also solicited comment on an alternative network design,
including alternative methods to determine the minimum number of
monitors per State (74 FR 64854). EPA requested comment on whether a
screening approach for assessing the likelihood of a NAAQS exceedance
could be developed and serve as a basis for determining the number and
location of required monitors. In particular, EPA requested comment on
whether it should utilize existing screening tools such as AERSCREEN or
SCREEN3, which use parameters such as effective stack height and
emissions levels to identify facilities with the potential to cause an
exceedance of the proposed standard. For that set of sources, EPA could
then require States to conduct more refined modeling (using the
American Meteorological Society (AMS)/EPA Regulatory Model (AERMOD)) to
determine locations where monitoring should be conducted. Any screening
or refined modeling would likely be carried out by States by using EPA
recommended models and techniques referenced by 40 CFR Part 51,
Appendix W, which provides guidance on air quality modeling. Such
screening or refined modeling uses facility emission tonnage, stack
heights, stack diameters, emission temperatures, emission velocities,
and accounts for local terrain and meteorology in determining where
expected maximum hourly concentrations may occur. In using this
approach, EPA would then require States to locate monitors at the point
of maximum concentration around sources identified as likely causing
NAAQS exceedances. EPA also noted that this alternative approach would
not distinctly use population as a factor for where monitors should be
placed.
c. Public Comments
    EPA received many comments on the proposed network design and the
alternative network design approaches. Based on comments that were
clear enough on the issue, EPA believes the commenters' positions on
the network design approach generally fell into one of three
categories: (1) Those who supported the two-prong approach, but
suggested some modification to it, (2) those who supported the
alternative network design, and (3) those who suggested other concepts
for the network instead of the two approaches EPA presented in the
proposal.
    The commenters who generally supported the two-prong network
design, but suggested some modification included some State and local
air agencies (e.g. NACAA and nine other State groups or agency
commenters) and industry groups (e.g. AQRL, ACC, and eight other
commenters). Of this group, some of the State and local air agencies
specifically commented on how EPA should modify one or both of the
prongs of the proposed network design. Some particular individual
suggestions will be addressed here and those comments not addressed
here will be addressed in the response to comment document. However,
one recurring suggestion from the State and local agency commenters in
this group was that the network design leads to some duplicative and/or
unneeded monitoring, and therefore they requested that EPA include a
provision to ``waive'' the monitoring network design requirements in
situations where minimum monitoring requirements appear duplicative or
unnecessary. In particular, NACAA stated that it ``* * * is concerned
that the two pronged approach in the proposed regulation will lead to
duplicative monitoring in some areas and require monitors in areas
where monitors are not needed. EPA recognizes the potential for
duplicative monitoring, but the proposal does not permit the removal of
duplicative monitors.'' This NACAA comment was echoed by some of the
other States who commented on the proposed approach (e.g. AK, FL, IL,
NC, SC, and WI). The industry commenters were also generally supportive
of the two-prong approach, with some making general suggestions to
modify the network design. For example, AQRL stated that the ``* * *
network design proposal seems to provide the flexibility for States and
the EPA regions to work together to arrive at the adequate monitoring
network.'' AQRL also

suggests that ``a State/local area should have the option to shutdown
or relocate any site mandated [by monitoring requirements] if measured
design values at the site are less than 75% of the selected standard
level.'' Multiple industry commenters (e.g. API, LEC, and RRI Energy)
expressed concern that the proposed network design had no monitoring
required specifically to measure background concentrations of
SO2. Dow Chemical suggested that EPA maintain some of the
existing monitors that characterize population exposure and other non-
source oriented sites for trends analysis.
    Those commenters who did not support the proposed network design,
and instead generally supported the concepts of the alternative network
design, include public health and environmental groups (e.g. ALA, CBD,
EDF, EJ, NRDC, and SC) and the States of Delaware and Iowa. In
particular, ALA, EDF, NRDC, and SC stated ``* * * the proposed 348
monitors are a grossly inadequate number to detect peak concentrations
from the nearly 2,000 major sources that emit more than 100 tons per
year of sulfur dioxide * * *'' and that ``it is most appropriate to use
screening tools to site all the monitors in the areas of highest
expected concentration * * *'' The Center for Biological Diversity,
with regard to the proposed network design, stated that ``* * * a
number of communities with very significant SO2 emissions
will not have any monitoring stations at all * * *'' Further, the State
of Iowa claimed that ``the proposed design of the SO2
ambient monitoring network provides insufficient assurances that the
public is protected from the health effects of SO2
exposure,'' and suggested that ``* * * the final rule contain
provisions that require monitors to be sited only at locations where
dispersion modeling indicates that the NAAQS is violated.''
    Commenters also suggested other concepts for the monitoring network
design in lieu of the approaches discussed in the proposal. NESCAUM,
NYSDEC, and PADEP, all suggested using an emissions-only approach to
trigger required monitoring instead of using the PWEI to require
monitors in an area. For example, NYSDEC suggests that the proposed
approach, using the PWEI, is ``* * * not more predictive than using
emissions data alone.'' NYSDEC went on to recommend that monitors be
required in CBSAs with aggregated emissions of 50,000 tons per year or
more and that ambient monitoring be considered for point sources with
20,000 tons per year. PADEP made several suggestions on network design,
including monitoring in any CBSA ``where there is a sulfur dioxide
source or combination of sources within 50 miles emitting a total of at
least 20,000 tons of SO2 per year * * *''
    Among all three groups of commenters discussed above, there was a
subset of commenters who specifically mentioned using modeling in some
form. Modeling was a component of the alternative network design, where
monitors would be required based on screening models and possibly
refined modeling of individual sources. EPA also expected that under
the proposed approach, many States would use modeling as a quantitative
analysis tool to site required monitors. Finally, source modeling is a
critical element for PSD and facility permitting. In their comments,
NESCAUM recommended that EPA allow modeling to be used in conjunction
with monitoring data to better determine nonattainment areas. North
Carolina advocated that EPA require SO2 sources, without
specifying a threshold size for sources, to perform modeling to
demonstrate that fence-line (ambient) air does not exceed the NAAQS due
to that particular source's emissions. North Carolina went on to
suggest that if a source's modeling showed an exceedance of the NAAQS,
the source could ``then be required to reduce emissions from the stack,
install continuous emissions monitoring (CEM) in the stack itself, or
require a fence-line monitor at the target facility.'' North Carolina
also stated, in the context of discussing its own PSD program, that
``the costs for modeling are small compared to the costs for
monitoring.'' Sierra Club stated that EPA should ``* * * employ modern
computer models to determine whether areas should be designated
nonattainment because they do not meet the NAAQS in areas where there
is no monitor.'' From these comments, EPA gathers that some public
commenters find modeling a useful tool and support the use of modeling
to ascertain ambient concentrations of SO2.
2. Modeling Ambient SO2 Concentrations
    EPA considered the various and sometimes competing concerns raised
by the commenters including duplicative monitoring, lack of adequate
number of monitors, insufficient flexibility, the monitoring burden,
and the modeling suggestions. EPA considered its historic practice and
the analytic tools available to arrive at a balanced approach that took
into account these concerns. In the past, EPA used a combination of
modeling and monitoring for SO2 during permitting,
designations, and re-designations in recognition of the fact that a
single monitoring site is generally not adequate to fully characterize
ambient concentrations, including the maximum ground level
concentrations, which exist around stationary SO2 sources.
With representative and appropriate meteorological and other input
data, refined dispersion models are able to characterize air quality
impacts from the modeled sources across the domain of interest on an
hourly basis with a high degree of spatial resolution, overcoming the
limitations of an approach based solely on monitoring. By simulating
plume dispersion on an hourly basis across a grid of receptor
locations, dispersion models are able to estimate the detailed spatial
gradients of ambient concentrations resulting from SO2
emission sources across a full range of meteorological and source
operating conditions. The 1-hour NAAQS is intended to provide
protection against short-term (5 minute to 24 hour) peak exposures,
whether they result from typical meteorological conditions or not.
Because ambient monitors are in fixed locations and a single monitor
can only represent impacts which occur at the location of the monitor,
a single monitor cannot identify all instances of peak ground-level
concentrations if, for example, different wind directions on various
days cause peak ground-level concentrations in different areas that do
not overlap. The uncertainty associated with this limitation is much
higher for an hourly standard than a long-term standard due to the
higher degree of spatial and temporal variability associated with peak
hourly impacts (discussed in ISA chapters 2.4 and 2.5). This limitation
of ambient monitoring may be true even if the source-oriented ambient
monitor was sited with the aid of modeling data, since the model is
less reliable at predicting the precise location of maximum impacts
than at predicting the distribution of impacts across the full modeling
domain, and no single monitor can be sited in a way to always measure
the peak ground-level SO2 concentrations that may be
occurring in the area around a source.
    EPA's Guideline on Air Quality Models, Appendix W to 40 CFR Part
51, provides recommendations on modeling techniques and guidance for
estimating pollutant concentrations in order to assess control
strategies and determine emission limits. These recommendations were
originally published in April 1978 and were incorporated by reference
in the PSD regulations, 40 CFR sections 51.166 and

52.21 in June 1978 (43 FR 26382). The purpose of Appendix is to promote
consistency in the use of modeling within the air quality management
process. Appendix W is periodically revised to ensure that new model
developments or expanded regulatory requirements are incorporated. The
most recent revision to Appendix W was published on November 9, 2005
(70 FR 68218), wherein EPA adopted AERMOD as the preferred dispersion
model for a wide range of regulatory applications in all types of
terrain. AERMOD is a steady-state plume dispersion model that employs
hourly sequential preprocessed meteorological data to simulate
transport and dispersion from multiple point, area, or volume sources
for averaging times from one hour to multiple years, based on an
advanced characterization of the atmospheric boundary layer. AERMOD
also accounts for building wake effects (i.e., downwash) on plume
dispersion. To support the promulgation of AERMOD as the preferred
model for near-field dispersion (50 km or less), EPA evaluated the
performance of the model across a total of 17 field study data bases
(Perry, et al., 2005; EPA, 2003), including several field studies based
on model-to-monitor comparisons of SO2 concentrations from
operating power plants.
    EPA anticipates that additional guidance for States may be needed
to clarify how to conduct dispersion modeling under Appendix W to
support the implementation of the new 1-hour SO2 NAAQS.
Although AERMOD is identified as the preferred model under Appendix W
for a wide range of applications and will be appropriate for most
modeling applications to support the new SO2 NAAQS, Appendix
W allows flexibility to consider the use of alternative models on a
case-by-case basis when an adequate demonstration can be made that the
alternative model performs better than, or is more appropriate than,
the preferred model for a particular application.
    In conclusion, EPA believes that a hybrid analytic approach that
uses a combination of modeling and monitoring information addresses the
varying and competing concerns expressed by the commenters. Modeling
large emission sources, along with smaller sources with the potential
to violate the NAAQS, deals effectively with the concern that the
monitoring network is not large enough to account for all sources that
could have high ambient SO2 concentrations. EPA believes
that more SO2 sources will ultimately be directly addressed
through modeling alone versus the number of sources which would have
been monitored under the proposed network design (which proposed a
minimum of 348 monitors). Because modeling provides a technically
appropriate and efficient method to identify locations of maximum
concentrations attributable to the major stationary SO2
sources, in the final network design (discussed below in section
IV.B.4), EPA is not requiring that monitors must be in locations of
expected maximum concentration, and thus, typically source-oriented.
Instead, monitors required under the final network design now can
address multiple monitoring objectives (discussed in IV.B.3 below),
with fewer number of monitors required overall than the number
estimated in the proposal. The flexibility that States now have, where
relatively fewer required monitors may be sited to meet multiple
objectives, effectively addresses concerns about duplicative monitoring
and the need for waivers, the need for measuring background
concentrations, and that emissions data rather than the PWEI could be
more predictive of high ambient SO2 concentrations as a
basis on which to require monitoring. The comments that suggested the
use of modeling, along with an examination of past practice, resulted
in the change to a hybrid approach where we use both modeling and
monitoring to assess ambient SO2 concentrations.
3. Monitoring Objectives
    Because EPA contemplates an ultimate approach that combines both
monitoring and modeling, the monitor objectives of the final network
design are now broadened to include assessment of source impacts,
highest concentration, population exposure, general background
concentrations, SO2 transport, and long-term trends. The
following paragraphs provide background, rationale, and details for the
final changes to monitoring objectives.
a. Proposed Monitoring Objectives
    EPA proposed that all minimally required monitoring sites in the
proposed two-prong network design be sited at locations of expected
maximum 1-hour concentrations, which would also likely discern 5-minute
peaks. EPA noted that in general, such locations would be close to
larger emitting sources (in tons per year) and/or areas of relatively
high emissions densities where multiple sources may be contributing to
peak ground-level concentrations. As a result, the proposed monitoring
network would have been comprised primarily of source-oriented
monitors. EPA also proposed that when selecting monitoring sites from
among a pool of candidate locations (which would be source-oriented
under the proposed network design), States prioritize these sites based
on where the maximum expected hourly concentrations would occur in
greater proximity to populations. EPA solicited general comments on the
role of population exposure in the site selection process.
b. Public Comments
    Commenters discussed a variety of issues on the subject of
monitoring objectives including the importance of considering
population exposure, the need for flexibility in monitor placement,
monitoring for background concentrations, monitoring for long term
trends analysis, and characterizing potential long-range transport of
SO2.
    EPA received many comments from States (e.g., NACAA, DE, IL, IN,
MO, SD, WI), the public health group ATS, and industry (e.g., AQRL,
Consumers Energy, Dominion, Dow, EPRI, ExxonMobil, Montana Sulfur and
Chemical, NPRA, Portland Cement, Rio Tinto, and UARG) suggesting that
required monitors account for, or be focused on, population exposure.
EPA also received many comments from States (e.g., NACAA, NESCAUM, FL,
IL, IN, IA, MI, OH, SC, and WI) and industry (e.g., API, Dow, and
TxOGA) asking for more flexibility in (source-oriented) monitor
placement with regard to both the target source and the physical
location of a monitor relative to that source. For example NACAA stated
that ``for source oriented monitors, placement at the point of 1-hour
maximum concentration must be realistic and flexible. EPA must allow
agencies to determine the most scientifically defensible location,
while taking into account potential exposures and access to locations
with adequate siting.'' Wisconsin stated that ``* * * monitor siting
should be balanced toward population-based monitors with a preference
toward maximum exposure.'' Wisconsin added that ``* * * placing
monitors at the maximum downwind location does not necessarily result
in effective protection of public health.''
    EPA received a number of comments on background monitoring \27\
from industry (API, LEC, and RRI Energy) and from the State of South
Carolina. API stated that ``because the monitors provide background
concentrations

needed to model impacts of new sources or sources undergoing major
modification in addition to providing data for judging compliance with
the NAAQS, it is important that some monitors be sited in a manner
suitable for assessing this background.'' API went on to state that ``*
* * EPA should encourage States to site an appropriate number of area-
wide monitors for use in establishing ambient background levels of
SO2.'' South Carolina states that ``to better support the
monitoring objectives, in particular those improving our understanding
and context for the source oriented monitoring data, the monitoring
requirements must include the ability for States to address the needs
for area and regional background concentration measurements.''
---------------------------------------------------------------------------

    \27\ Background monitoring can be considered to be
representative of ambient concentrations upwind of (and therefore
not typically influenced by) a geographic area such as an urban
area, or of an individual or group of emission sources.
---------------------------------------------------------------------------

    A number of commenters, including States (e.g., Missouri, NESCAUM,
Ohio, and South Carolina), citizens (Valley Watch at the Atlanta public
hearing), the CBD, and Dow, commented on SO2 transport and
related cross-boundary monitoring. Dow stated that ``SO2
distribution has long been known as an interstate issue with the vast
majority of SO2 sources being power plants and other fossil
fuel combustion facilities. These facilities are more likely to impact
distant areas than local areas and the resultant ground-level
concentrations are often minimal.'' Ohio stated that, under the
proposed approach, ``* * * it is likely that OH, WV, KY, and IN will
find sources along the Ohio River which could result in monitors being
located across the river from each other.'' In such situations, Ohio
asserts that ``States are capable of working with our neighbors to
determine which State would be in the best position to site and operate
a monitor.''
c. Conclusions on Monitoring Objectives
    A hybrid analytical approach, as noted above in section III and
IV.B.1 would ultimately make the most appropriate use of available
tools such as modeling and monitoring. Thus, unlike under the proposal,
the monitoring network will not have to be focused solely at locations
of expected maximum concentration relative to an SO2 source
given the anticipated adoption of a hybrid analytical approach. The
final network design is intended to be flexible to meet multiple
monitoring objectives, most of which were identified in the public
comments. Ambient monitoring networks are generally designed to meet
three primary monitoring objectives, as listed in 40 CFR Part 58
Appendix D, Section 1, including: (1) Providing air pollution data to
the general public in a timely manner, (2) support compliance with
ambient air quality standards and emissions strategy development, and
(3) support air pollution research studies (which includes health
studies and research). In order to support these air quality management
objectives, monitoring networks can have a variety of monitoring sites
that can be sited, as necessary, to characterize (a) emission sources
(i.e., source-oriented monitoring), (b) the highest concentration in an
area, (c) population exposure, (d) general background concentrations,
(e) regional transport, and (f) welfare-based impact.
    In light of the approach described in section III and further in
IV.B.1 above, EPA is finalizing an SO2 network design, with
broadened objectives, which EPA believes will address the concerns
noted in the public comments above, particularly those regarding siting
flexibility, population exposure, cross-boundary impacts, and the need
for the network to address multiple monitoring objectives. The final
network design requires that any SO2 monitors required in a
particular CBSA as determined based on PWEI values, discussed below in
section IV.B.4, shall satisfy the minimum monitoring requirements if
they are sited at locations where they can meet any one or more of the
following objectives (see Part 58 Appendix D section 4.4.2 as added by
today's final rule):
    (1) Source-Oriented Monitoring: This is accomplished with a monitor
sited to determine the impact of significant sources or source
categories on air quality. In some situations, such monitoring sites
may also be classified as high concentration sites (discussed below).
Examples of source-oriented monitors include those sited to capture or
assess peak ground-level concentrations from one or more major
SO2 sources, or those sited in an area with multiple smaller
sources with overlapping plumes.
    (2) Highest Concentration: This is assessed by a monitor sited to
measure the highest concentrations expected to occur in the area
covered by the network. Such a location may, or may not, also be
considered a source-oriented location (discussed above). Depending on
the case, this location is representative of the highest concentration
occurring across a relatively homogeneous area with spatial scales
typically ranging from tens of meters up to four kilometers.\28\
---------------------------------------------------------------------------

    \28\ Spatial scales are defined in 40 CFR Part 58 Appendix D,
section 1. Each scale is a description of the physical dimensions of
an air parcel nearest a monitoring site throughout which pollutant
concentrations are reasonably similar.
---------------------------------------------------------------------------

    (3) Population Exposure: This is assessed by a monitor sited to
measure typical concentrations in areas of (relatively) high population
density. Some examples are a monitor placed in an area of elevated or
high SO2 concentrations that also has a high population
density, an area that might be included in public health studies, or in
areas with vulnerable and susceptible populations.
    (4) General Background: This is assessed by placing a monitor in an
area to determine general background concentrations. Such locations
might be considered to be representative of ambient concentrations
upwind of (and therefore not typically influenced by) a geographic area
such as an urban area, or of an individual or group of emission
sources. EPA notes that although a required monitor is allowed to be
sited to assess background concentrations, the required monitor is not
allowed to be sited outside of the parent CBSA (whose PWEI value
triggered required monitoring, discussed in section IV.B.4 and IV.B.5).
If a State believes that there is a need to conduct background
monitoring outside of CBSAs with required monitoring, EPA notes that
States always have the prerogative to conduct monitoring above the
minimum requirements in any location the State believes is appropriate.
    (5) Regional Transport: This is assessed by placing a monitor in a
location to determine the extent of regional pollutant transport. Such
locations could be either upwind or downwind of urban areas,
characterizing the entry or exit of the pollutant in a region,
respectively. EPA notes that although a required monitor is allowed to
be sited to assess regional transport, the required monitor is not
allowed to be sited outside of the parent CBSA (whose PWEI value
triggered required monitoring, discussed in section IV.B.4 and IV.B.5).
If a State believes that there is a need to conduct background
monitoring outside of CBSAs with required monitoring, EPA notes that
States always have the prerogative to conduct monitoring above the
minimum requirements in any location the State believes is appropriate.
    In regard to the public comments expressing concerns on the issue
of cross-boundary transport, i.e., a source on one side of a political
boundary contributes to peak ground-level concentrations on the other
side of that boundary, EPA will allow a required monitor to be placed
outside of the parent CBSA (whose PWEI value triggered monitoring,
discussed in section IV.B.4 and IV.B.5) under one

particular condition. A source-oriented monitor may be sited outside of
the parent CBSA, whose PWEI value triggered required monitoring, if
that monitor is characterizing the location of expected maximum
concentration of a source inside that parent CBSA. If a State chooses
to exercise this flexibility in source-oriented monitor siting, the
State must provide clear rationale for their choice in their annual
monitoring plan, which is subject to EPA regional approval. If the
source-oriented monitor is to be placed in another State, such as the
example provided by the State of Ohio in the public comments above, the
two States are responsible for collaboration on the location and
operation of that monitoring site.
    Further, due to the broadened objectives of the final network
design, EPA also is finalizing the provision that an NCore
SO2 monitor within a CBSA (where a CBSAs PWEI value
triggered required monitoring) can be counted towards meeting the
minimum monitoring requirements in this rulemaking (discussed in
section IV.B.4) because they can meet some of the expanded objectives
of the network. NCore sites are intended to provide long-term data for
air quality trends analysis, model evaluation, and, for urban sites,
tracking metropolitan air quality statistics, and therefore are
appropriate to allow to count towards minimum monitoring requirements
under the revised monitoring scheme.
    Finally, EPA strongly encourages State and local air agencies to
consider using required monitoring, as appropriate, to characterize
those sources which are not as conducive to dispersion modeling and to
assess population exposure. Sources that are not conducive to
dispersion modeling include (1) sources classified as non-point sources
(a.k.a. ``area-sources'') such as shipping ports, (2) a source situated
in an area of complex terrain and/or situated in a complex
meteorological regime, and (3) locations that have multiple, relatively
small sources with overlapping plumes.
4. Final Monitoring Network Design
    The use of a hybrid analytic approach (discussed above in section
III and IV.B.1) makes it unnecessary for the final monitoring network
design to be distinctly focused on monitoring locations of expected
maximum concentration (and thus be primarily source-oriented), as
discussed in section IV.B.3 above. Instead, with the dual use of
modeling and monitoring for designations, the final monitoring network
is designed to provide flexibility for required monitors to address the
multiple monitoring objectives just discussed in the preceding section.
This flexibility in monitoring objectives is in response, in part, to
the many public comments received from States (e.g., NACAA and six
other States), industry (API, EPRI, UARG, and eight other groups), and
from the American Thoracic Society (ATS), urging EPA to ensure that
some or all of the required monitors be sited and suited to
characterize population exposure and, from many of these same
commenters, to allow flexibility in implementing the siting
requirements for the monitors. Under a hybrid approach, and the
different monitoring objectives resulting thereof, the final monitoring
network design also does not need to be a two-prong approach like the
one proposed. Therefore, EPA is adopting a modified version of the
first prong of the proposed network design, which will use PWEI values
to require monitors in certain CBSAs where there is increased
coincidence of population and SO2 emissions. There is no
second prong in the final network design by which monitors are required
based on a State's individual contribution to the national
anthropogenic SO2 inventory, as was proposed.
    The final monitoring network design requires monitoring in CBSAs
based on calculated PWEI values, where a PWEI shall be calculated (as
discussed in section IV.B.5 below) for each CBSA. For any CBSA with a
calculated PWEI value equal to or greater than 1,000,000, a minimum of
three SO2 monitors are required within that CBSA. This
requirement remains the same as proposed. For any CBSA with a
calculated PWEI value equal to or greater than 100,000, but less than
1,000,000, a minimum of two SO2 monitors are required within
that CBSA. For any CBSA with a calculated PWEI value equal to or
greater than 5,000, but less than 100,000, a minimum of one
SO2 monitor is required within that CBSA. EPA has adjusted
the thresholds for requiring one or two monitors in a CBSA and the
rationale for this adjustment is explained more fully below in section
IV.B.5. As just explained in section III.B.3, these monitors shall be
sited to meet one or more of a number of monitoring site objectives,
including the assessment of source impacts, highest concentrations,
population exposure, general background, and regional transport. EPA
believes that the monitors required within these PWEI breakpoints
provide a reasonable minimum number of monitors in a CBSA, where there
is a relatively increased coincidence of population and SO2
emissions and therefore increased potential for exposures, because we
are directly accounting for both population and emissions that exist in
individual CBSAs.\29\ EPA estimates that these minimum monitoring
criteria (based on 2008 population and 2005 NEI data) require 163
monitors within 131 CBSAs. EPA also intends for SO2 monitors
at NCore stations to satisfy these minimum monitoring requirements.
Based on analysis of proposed and approved NCore sites (as of April
2010), all of which are scheduled to be operational no later than
January 1, 2011, EPA estimates that 52 of the total 80 SO2
monitors at NCore stations are within the 131 CBSAs that have required
monitors based on their PWEI values. As a result, EPA estimates that
between these minimum monitoring requirements and the NCore network,
there will be at least 191 SO2 monitors operating across the
country.
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    \29\ The rationale for finalizing the use of the PWEI and the
number of monitors required through its application are discussed in
section III.B.4.
---------------------------------------------------------------------------

5. Population Weighted Emissions Index
    In the proposal, EPA had introduced a metric based on population
and emissions as a basis for locating monitors in the network. EPA
anticipated that this metric would characterize the potential for
exposure based on the proximity of source emissions to populations. The
following paragraphs provide background, rationale, and details for the
final changes of the calculation and use of the Population Weighted
Emissions Index in determining minimum monitoring requirements.
a. Proposed Use of the Population Weighted Emissions Index
    In the proposed network design approach, which utilized a two-prong
network design, EPA created the Population Weighted Emissions Index
(PWEI) in an attempt to focus monitoring resource where there was a
higher proximity of population and SO2 emissions. In effect,
areas with higher PWEI values have higher potential for population
exposure to short-term SO2 emissions. EPA proposed that the
PWEI be calculated using population and emissions inventory data at the
Core Based Statistical Area (CBSA) \30\ level to assign required
monitoring for a given CBSA, with population and emissions being the
relevant factors. To calculate the PWEI for a particular CBSA, using

the latest Census Bureau estimates, the population of a CBSA must be
multiplied by the total amount of SO2 emissions in that
CBSA. The CBSA emission value is in tons per year (using the latest
available National Emissions Inventory [NEI] data), and is calculated
by aggregating the county level emissions for each county in a CBSA. We
then divide the resulting product of CBSA population and CBSA
SO2 emissions by 1,000,000 to provide a PWEI value in more
manageable units of millions of people-tons per year.
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    \30\ CBSAs are defined by the U.S. Census Bureau, and are
comprised of both Metropolitan Statistical Areas and Micropolitan
Statistical Areas (http://www.census.gov).
---------------------------------------------------------------------------

    With the change in the approach discussed in section III and
section IV.B.1 above, and considering the final monitoring network
design discussed in IV.B.4 above, the use of the PWEI from that which
was proposed also changes. The following paragraphs discuss some of the
public comments received on the general use and calculation of the
PWEI; other comments that focused on the detailed application of the
PWEI as proposed will be addressed in the response to comments document
since our approach in applying the PWEI has changed.
b. Public Comments
    EPA received a number of comments from State and local groups
(e.g., NACAA and eight others) and industry (e.g., AQRL, ACC, and eight
others) who generally agreed with the two-pronged network design
concept which had the PWEI as a component. More specifically, some
State commenters (e.g. NACAA, AK, FL, IL, NC, SC, and WI) expressed
concern that the PWEI (along with the second prong of the proposed
network design) created monitoring requirements that were
``duplicative'' and also called for monitors in areas where they were
not needed. Even amongst some of the commenters who generally agreed
with the PWEI concept, some provided examples of where the PWEI
appeared to be duplicative in its proposed application. One example was
provided by the State of Florida, ``in the case of Homosassa Springs,
the [proposed network design] requires two monitors [in that CBSA as a
result of the proposed use of the PWEI]. The driving source is the
Crystal River Power Plant, with emissions in 2008 of over 85,000 tons
per year of SO2. The next largest source in the CBSA has
emissions of roughly two tons per year.'' EPA believes that Florida is
asserting that the one large source disproportionately drove the PWEI
too high for that particular CBSA and only one monitor was actually
needed. EPA notes that these particular comments on duplicative
monitoring were made under the premise that all proposed required
monitors would be sited in locations of expected maximum concentration,
and therefore would be source-oriented in nature. As a result, these
commenters believed it was necessary that a waiver provision be
included if they could show that the required number of monitors was
too many, as in Florida's example.
    As discussed in section IV.B.4 above, a hybrid approach results in
a final network design with a reduced number of required monitors from
the number proposed, a different application of the PWEI, and provides
flexibility in meeting additional monitoring objectives for the
required monitors, making the need for a waiver from the minimally
required monitors unnecessary. If a CBSA is required to have multiple
monitors now, those monitors are not specifically required to be
located near sources where maximum concentrations of SO2 are
expected to occur. Instead, they can be sited at different locations to
fulfill a variety of objectives, although, as noted in secion IV.B.3
above, EPA is strongly encouraging States to consider monitoring near
sources not conducive to dispersion modeling and for characterization
of population exposures.
    EPA received comments from Michigan, South Carolina, and CBD
requesting clarification on the logic behind the proposed PWEI
thresholds, or breakpoints, by which three, two, one, or no monitors
would be required in a given CBSA. In addition, some States (e.g., MI,
MO, SC, and WI) and industry (e.g., LCA, LMOGA, and LPPA) suggested
specific adjustments to the proposed application of the PWEI. For
example, Michigan suggested that the required monitor breakpoint values
be adjusted to the ``natural breakpoints in the overall distribution''.
South Carolina suggested EPA identify a way to normalize the PWEI
stating the PWEI would be more appropriate ``* * * if it used a value
that better addressed difference in area, population distribution, land
use, number, types of sources, etc.''
    In the proposed network design, EPA selected the PWEI values, or
breakpoints, to require one or more monitors based on the overall
distribution of PWEI values across all CBSAs. Based on U.S. Census
Bureau data (http://www.census.gov), there are approximately 939 CBSAs
in the country. EPA proposed and now requires that a PWEI value be
calculated for each of these CBSAs to determine if monitoring is
required in that CBSA. Based on 2008 census estimates and the 2005 NEI,
the average CBSA PWEI value is 21,900 while the median value is only
121. This indicates that a relatively small number of CBSAs with high
PWEI values are driving the very upper end of the PWEI distribution.
The proposed breakpoint where one monitor was required in a CBSA was a
PWEI value of 5,000. EPA estimated that 131 out of 939 CBSAs (~14%)
have a PWEI value of 5,000 or more. Further, these 131 CBSAs occupy
~98% of the sum of PWEI values across all 939 CBSAs, where high PWEI
values indicate increased coincidence in population and SO2
emissions. Within this group of CBSAs with PWEI values of 5,000 or
more, EPA considered the relative amounts of population, emissions, and
general frequency of occurrence of relatively larger SO2
sources (such as those that emit 100 tons per year or more) in
selecting the breakpoints to require two and three monitors in a CBSA
for the proposed network design. These considerations were made in an
effort to apply a nationally applicable process by which to require a
minimum number of monitors for an area, which all were to be sited in
locations of expected maximum concentration, and therefore likely
source-oriented monitors. In regard to the comments suggesting
modification to the calculation or to normalize the PWEI, EPA believes
that the proposed calculation, under a hybrid analytical approach, is
still most appropriate. Under a hybrid analytical approach, States have
the flexibility to move monitoring resources where needed within CBSAs
that have a high coincidence of population and emissions instead of
only being able to site monitors to characterize sources. States have
the option to consider additional factors such as those listed in South
Carolina's comments above in further identifying where required
monitoring may be most appropriate in their areas with required
monitoring.
    Several States (e.g. NESCAUM, NYSDEC, and PADEP) suggested
abandoning the PWEI concept altogether and instead using some form of
emissions-only approach to require monitors. For example, NESCAUM, who
generally supported a ``hot-spot'' monitoring approach, suggested that
the PWEI be abandoned and EPA instead ``* * * adopt an emissions-only
approach, resulting in fewer CBSA monitors. We [NESCAUM] suggest a
threshold of 50,000 tpy CBSA SO2 emissions to trigger the
first CBSA monitor and a second CBSA monitor required when emissions
exceed 200,000 tpy.'' NESCAUM states that the proposed use of the PWEI
``* * * can

result in multiple monitors in large cities that have relatively small
CBSA SO2 emissions, or no monitor in a CBSA with large
emissions.'' NYSDEC suggests that the proposed approach, using the
PWEI, is ``* * * not more predictive than using emissions data alone.''
NYSDEC went on to suggest that monitors be required in CBSAs with
aggregated emissions of 50,000 tons per year or more and that ambient
monitoring be considered for point sources with 20,000 tons per year.
PADEP made several suggestions on network design, with one that
suggested monitoring in any CBSA ``where there is a sulfur dioxide
source or combination of sources within 50 miles emitting a total of at
least 20,000 tons of SO2 per year * * *''
    EPA reviewed emissions and 2005 NEI data and compared the
suggestions provided by NESCAUM and NYSDEC to the requirement of the
final network design. Under NESCAUM's suggested design, EPA estimates
there would be 75 required monitors in 65 CBSAs. Of these 65 CBSAs, 6
CBSAs that are not covered by the final network design would be
included; however, 72 CBSAs that will have monitors under the final
network design would otherwise not have monitors under NESCAUM's
design. EPA believes that the exclusion of those 72 CBSAs would lead to
too sparse a network to adequately meet the monitoring objectives of
the network. Under NYSDEC's suggested network design, EPA estimates
that there would be a minimum of 65 monitors in the same 65 CBSAs of
the NESCAUM suggested design. Further, if States ensured that monitors
were placed near all sources emitting 20,000 tons per year (as NYSDEC
suggested should be ``considered'' for monitoring), there could be an
additional 69 monitors.\31\ EPA believes that the final network design
as discussed above in section IV.B.4, with the increased flexibility
for monitors to meet multiple monitoring objectives (discussed in
IV.B.3 above) including, among others, characterization of source
impacts or population exposure, is better served using PWEI values to
require monitors because it explicitly accounts for population to
require and distribute monitors as compared to an emissions-only
approach. If there is reason for concern that other CBSAs or areas not
included in the final network design, such as the six CBSAs that were
included in the NESCAUM and NYSDEC suggested network designs noted
above, warrant monitoring resources, States or the EPA Regional
Administrator may take action to require monitoring in such areas. The
authority of an EPA Regional Administrator to require additional
monitoring above the minimum requirements is discussed in section
IV.B.6 below.
---------------------------------------------------------------------------

    \31\ In simulating NYSDEC's suggested network design, EPA
assumed that no CBSA would have more than one monitor. According to
the 2005 NEI, there are 162 sources emitting 20,000 tpy or more a
year. 93 of those sources are estimated to be inside CBSAs that have
emissions of 50,000 tpy, leaving approximately 62 sources that would
need a monitor to satisfy NYSDEC's suggested network design.
---------------------------------------------------------------------------

    EPA received a number of comments from States (e.g., IA, NESCAUM,
NC, NYSDEC, SC, and WI) and industry (e.g., CE, Dominion, EEI, LCA,
LMOGA, LPPA, and UARG) raising concern over the way the PWEI is
calculated. Specifically, many commenters in this group indicated that
they believed that the 2005 NEI would be used in an exclusive or
permanent fashion to calculate the PWEI, and that updated NEI data
would not be used. For example, NESCAUM states that ``EPA should not
require States to rely solely on EPA's inventories [for calculating the
PWEI], such as the National Emissions Inventory (NEI), as they do not
always have the updated information that is necessary for such
regulatory decisions.'' Wisconsin ``* * * believes that States should
be allowed to use their own annual point source inventories instead of
EPA's National Emissions Inventory (NEI) for evaluating emission
sources. Wisconsin's point inventory is updated annually and has a
reporting threshold of five tons per year for SO2, making it
more sensitive to changes in facility operations than the NEI, which is
updated triennially.'' UARG stated that their ``primary concern with
this network design is its reliance on old emissions data. For electric
utilities which report their SO2 emissions to EPA annually,
the use of more recent data would be appropriate.''
    EPA does not intend for relatively old emissions data to be used in
calculating the PWEI values for individual CBSAs. As was detailed in
the proposed regulatory text for 40 CFR Part 58 Appendix D (74 FR
64880), EPA stated that ``The PWEI shall be calculated by multiplying
the population of each CBSA, using the most current census data, by the
total amount of SO2 in tons per year emitted within the CBSA
area, using an aggregate of the most recent county level emissions data
available in the National Emissions Inventory for each county in each
CBSA.'' Although commenters suggested that there may be other resources
from which emissions data may be obtained, particularly at the
individual State level, the NEI is comprised of emissions data which is
collected by EPA from the States themselves. The Air Emissions
Reporting Requirements (40 CFR Part 51), by which EPA sets out how
States are to report their emission inventories, was recently revised
in December of 2008. That rulemaking was intended to provide enhanced
options to States for emissions data collection and exchange and unify
reporting dates for various categories of inventories. EPA notes that
the NEI is updated in full every three years and the 2008 NEI is
scheduled to be available by January 2011. States will have submitted
their data by May 31, 2010, before this rule is promulgated and
published, and EPA will provide comment on these submittals during the
summer of 2010. States will have an opportunity to revise their 2008
data submissions in the fall of 2010. In the triennial update, both
point and nonpoint data are required to be submitted by States and are
included in the inventory. Further, States are required to submit
emissions data annually for all sources emitting 2,500 tons per year or
more of SO2 as well as for sources emitting other pollutants
in excess of thresholds set for those pollutants. In all point source
submittals to the NEI, States are also allowed to submit emissions data
for sources of any emissions level, but are not required to do so.
Starting with the 2009 NEI, the annual and triennial State NEI
submittals will be due one year after the end of the emissions year.
States have an additional opportunity to revise their submittals based
on EPA comment in the spring of the following year, with EPA publishing
the inventory no later than 6 months after the inventory submittal
dates (18 months after the end of the emissions year). This approach
and schedule is accelerated over past NEI schedules and has been
designed as part of the development of the new Emission Inventory
System (EIS). Rather than representing old emissions data, the NEI
available through EIS represents a timely and appropriate source of
emissions data.
    EPA believes that the process by which the NEI will be updated
(through use of the EIS) will be adjusted in a manner that will allow
for more frequent insertion of State supplied emissions data, allowing
for a more up-to-date inventory. EPA takes this opportunity to
encourage States to supply all of their available emissions information
to the NEI as soon as practicable. Therefore, EPA believes that the NEI
is an appropriate and nationally representative source of emissions
data by which PWEI calculations may be

made. PWEI calculations for all CBSAs will use the same year of data at
any given time, and States, local agencies, and Tribes will have
uniform opportunity for revising their emissions data for this purpose.
EPA again encourages States to view the NEI submittals as their
opportunity to submit their best available SO2 and other
inventory data with the knowledge that it will be used for the purpose
of PWEI values.
c. Conclusions on the Use of the Population Weighted Emissions Index
    In the final network design, EPA has determined that it is
appropriate to use PWEI values as the mechanism by which to require
monitors in certain CBSAs, similar to its use in the first prong of the
proposed two-prong network design. EPA believes that using the PWEI
metric to inform where monitoring is required is more appropriate for
the SO2 network design than utilizing a population-only or
emissions-only type of approach, because it takes into account not just
one factor, i.e., only population or only emissions, but instead takes
into account the exposure from SO2 emissions to groups of
people who are in greater proximity to such emissions.
    In the final rule, EPA is retaining the requirement to calculate
the PWEI by multiplying the population of each CBSA, using the most
current census data/estimates from the U.S. Census bureau, by the total
amount of SO2 in tons per year emitted within the CBSA area,
using an aggregate of county level emissions data available in the most
recent published version of the National Emissions Inventory for each
county in each CBSA. The resulting product shall be divided by one
million, providing a PWEI value, the units of which are million
persons-tons per year. For any CBSA with a calculated PWEI value equal
to or greater than 1,000,000, a minimum of three SO2
monitors are required within that CBSA. For any CBSA with a calculated
PWEI value equal to or greater than 100,000, but less than 1,000,000, a
minimum of two SO2 monitors are required within that CBSA.
For any CBSA with a calculated PWEI value equal to or greater than
5,000, but less than 100,000, a minimum of one SO2 monitor
is required within that CBSA. EPA believes that the monitors required
within these breakpoints provide a reasonable minimum number of
monitors in a CBSA that considers the combination of population and
emissions that exist in a CBSA. These criteria (based on 2008
population and 2005 NEI data) are estimated to require 163 monitors
within 131 CBSAs.
    EPA has changed the PWEI breakpoint in the final rule at which two
monitors are required in a CBSA to 100,000 from the breakpoint of
10,000 in the proposed network design based on multiple considerations.
First, EPA changed the breakpoint because of a hybrid analytic approach
and attendant changes in monitoring objectives (see section IV.B.3),
with the result being that the monitoring network is no longer intended
to be comprised primarily of source-oriented monitors that are sited at
locations of expected maximum concentration. This change in objective
of the network design allows fewer monitors to provide the necessary
amount of ambient monitoring data EPA to meet the multiple monitoring
objectives. Second, the breakpoint of 100,000 occurs near a ``natural''
breakpoint in the PWEI distribution, a consideration that Michigan
suggested, where the estimated 28 CBSAs with PWEI values of 100,000 or
more occupy ~87% of the sum of PWEI values across all 939 CBSAs.
Finally, EPA considered commenters' assertion that the first prong of
the proposed network design created duplicative monitoring in certain
CBSAs. This duplicative monitoring is especially recognized in some
CBSAs with relatively small populations and somewhat large emissions
which are dominated by a single source (such as the Homosassa Springs,
FL example discussed above). Raising the second breakpoint helps to
alleviate some of the duplicative monitoring that many of the State
commenters noted.
    EPA therefore is keeping the first and third breakpoints, which
require one monitor in a CBSA having a PWEI value of 5,000 and three
monitors in a CBSA having a PWEI value of 1,000,000. EPA believes
maintaining these breakpoints along with the revised 100,000 PWEI
breakpoint, will (1) ensure that highly populated areas will be
monitored for ambient SO2 concentrations even if the
emissions in that area are moderate, which is appropriate given the
fact that the greater population creates increased potential for
exposure to those moderate emissions, and (2) that those areas with
higher emissions or emission densities, with moderate or modest
populations will be monitored because those increased emissions are
likely to have a significant impact on nearby populations.
6. Regional Administrator Authority
    The following paragraphs provide background, rationale, and details
for the final changes to Regional Administrator authority to use
discretion in requiring additional SO2 monitors beyond the
minimum network requirements.
a. Proposed Regional Administrator Authority
    EPA proposed that the Regional Administrators will have discretion
to require monitoring above the minimum requirements, as necessary, to
address situations where the minimum monitoring requirements are not
sufficient to meet monitoring objectives. EPA recognized that the
minimum required monitors in the proposed two-pronged network design
were based on indicators that may not have always provided spatial
coverage for all the areas that have SO2 sources. Although
the network design and the objectives of the network design have
changed from those that were proposed because of our contemplated use
of a hybrid analytical approach, EPA believes it is still important for
Regional Administrators to have the discretion, and authority, to
require monitoring above the minimum requirements. Providing the RAs
with this discretion will allow them to fill any identified gaps in
meeting the monitoring objectives of the network.
b. Public Comments
    Some commenters (e.g., LCA, LMOGA, LPPA, and South Carolina)
expressed concerns with the proposed provision authorizing the Regional
Administrator to require additional monitoring above the minimum
requirements. The LCA, LMOGA, and LPPA stated that ``the EPA's proposal
to allow the Regional Administrator discretion to require a State to
add additional monitors is flawed in that it provides unfettered
discretion. Criteria should be added * * * that limit such discretion
and require the Regional Administrator to consider certain objective
factors when determining whether to require any additional ambient
SO2 monitors to the network.'' South Carolina stated that
``the Regional Administrators should not have the discretion to require
monitoring above the requirements described in [the proposal for] Part
58 and its Appendices. State monitoring organizations must be given
discretion to decide the appropriate use of resources to meet uniform
monitoring requirements. Additional monitoring requirements should not
be imposed without concurrence of the monitoring organization and
additional funding that completely supports the additional costs.''

c. Conclusions on Regional Administrator Authority
    The authority of Regional Administrators to require additional
monitoring above the minimum required is not unique to the
SO2 NAAQS. For example, Regional Administrators have the
authority to use their discretion to require additional NO2
or Pb monitors (40 CFR Part 58 Appendix D section 4.3.4 and 4.5,
respectively) and to work with State and local air agencies in
designing and/or maintaining an appropriate ozone monitoring network
(40 CFR Part 58 Appendix D section 4.1). EPA believes that the
nationally applicable final network design, although somewhat dictated
by local factors (population and emissions), may not account for all
locations where monitors should be sited, including where potentially
high concentrations of SO2 may be occurring. Examples
include locations that have the potential to violate or contribute to
violations of the NAAQS, areas that might have high concentrations of
SO2 that are not characterized by modeling or have sources
that are not conducive to modeling, and locations with susceptible and
vulnerable populations. As a result, EPA believes it is important for
Regional Administrators to have the authority to address possible gaps
in the minimally required monitoring network, especially near sources
or areas that are not conducive to modeling by granting them authority
to require monitoring above the minimum requirements. However, in
response to public comments, EPA notes that Regional Administrators
would use this authority in collaboration with State agencies to design
and/or maintain the most appropriate SO2 monitoring network
to meet the needs of a given area. For all the situations where the
Regional Administrators may require additional monitoring, it is
expected that the Regional Administrators will work on a case-by-case
basis with State or local air agencies. Further, any monitor required
through the Regional Administrator and selected by the State agency, or
any new monitor proposed by the State itself, is not done so with
unfettered discretion, since any such action would be included in the
Annual Monitoring Network Plan per Sec.  58.10, which must be made
available for public inspection or comment, and approval by the EPA
Regional Administrator.
    Therefore, EPA is finalizing the proposal that Regional
Administrators may use their authority to require monitoring above the
minimum requirements, as necessary, in any area, to address situations
where the minimally required monitoring network is not sufficient to
meet monitoring objectives. In all cases in which a Regional
Administrator may consider the need for additional monitoring, it is
expected that the Regional Administrators will work with the State or
local air agencies to evaluate evidence or needs to determine if a
particular area may warrant additional monitoring.
7. Monitoring Network Implementation
    The following paragraphs provide background, rationale, and details
for the final approach for the monitoring network implementation.
a. Proposed Monitoring Network Implementation
    EPA proposed that State and, where appropriate, local air
monitoring agencies submit a plan for deploying SO2 monitors
in accordance with the proposed requirements discussed above by July 1,
2011. EPA also proposed that the SO2 network be physically
established no later than January 1, 2013. EPA also proposed that the
number of sites required to operate as a result of the Population
Weighted Emissions Index (PWEI) values calculated for each CBSA be
reviewed and revised for each CBSA through the 5-year network
assessment cycle required in Sec.  58.10.
b. Public Comments
    EPA received comments from the ALA, EDF, NRDC, and SC that
supported ``* * * a more accelerated deployment of new monitoring than
the 2013 target date proposed by EPA. The sooner monitors are in place,
the sooner the public will experience the health benefits of the new
standard.'' However, EPA received comment from States (e.g., IA, MI,
NC, SC and WI), industry (e.g., LCA, LMOGA, and LPPA) and public health
and environmental groups (e.g., ALA, EDF, NRDC, and SC) expressing
concern with the proposed deployment schedule of the proposed
SO2 network in that it was too fast or needed to be phased
in. The States of Iowa, South Carolina, and Wisconsin suggested that
EPA allow the proposed network to deploy on a phased schedule. For
example, South Carolina recommended a ``phased implementation with
largest source/highest probability population exposure areas designated
for implementation in 2013 (some proportion of the highest PWEI
monitors) and establishment of the remaining PWEI and the State level
emissions triggered monitoring required the following year.''
Meanwhile, the States of Michigan and North Carolina, along with the
industry commenters LCA, LMOGA, and LPPA, suggested EPA reconsider
implementation dates in light of the multiple rulemakings that impose
mandates on States that have and will be occurring in the future. For
example, North Carolina stated that ``EPA must keep in mind that it is
simultaneously revising numerous ambient standards and associated
monitoring requirements. EPA seems to view each of these proposals as
independent actions; but the State and local agencies must consider the
cumulative impact of EPA's various regulatory actions on their ability
to comply.'' North Carolina goes on to say that ``EPA must allow States
the flexibility to prioritize among the new requirements to get
community based monitors in place first and to establish the others as
funding and personnel resources allow.''
    EPA believes that with the use of a hybrid analytical approach, the
concerns raised by States and industry commenters suggesting a phased
or delayed implementation are addressed because the final network
minimum design requirements result in fewer monitors being required
than in the proposed network design. EPA's analysis of the existing
network had indicated that a substantial number of monitors were not
sited at locations of maximum concentrations. These monitors would have
had to be re-located to count towards minimum monitoring requirements
under the proposed monitoring-focused approach. Under a combined
modeling and monitoring approach, the required monitors can be used to
satisfy multiple monitoring objectives and therefore, many of the
monitors in the existing network will satisfy the requirements in the
final network design, eliminating any need for a phased or delayed
network implementation. In regard to the suggestion by public health
and environmental groups to speed up implementation, EPA notes that
under a hybrid analytical approach much of the existing network will
fulfill minimum monitoring requirements, and an accelerated schedule is
not necessary; the network implementation date provides a balance
between ensuring the minimally required network is fully in place in a
reasonable amount of time and providing States adequate time to fulfill
all the requirements in this rulemaking.\32\
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    \32\ Moreover, as explained in section IV.A, the existing FEM
monitors in operation may continue to be used to monitor compliance
with the NAAQS.
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    EPA received comment on the frequency by which the minimally

required network will be reviewed and possibly adjusted based on
updated population and emissions inventories. The State commenters
listed above, and some others including NACAA, indicated that they
believed that the proposal for reviewing the SO2 network
every five years was intended to be a separate review from the required
5-year network assessments required in Sec.  58.10(d). NACAA stated
``EPA proposes that the SO2 monitoring network be evaluated
every five years. This is an unnecessary duplication of effort in light
of the current requirements for the annual network plan and five year
network review.'' NACAA went on to say that ``the current requirements
[in Sec.  58.10] should be regarded as the primary source of monitoring
network information for all NAAQS pollutant monitoring, regardless of
the pollutant.''
    EPA concurs with NACAA's statements that the existing requirements
for network assessment are an appropriate primary source of monitoring
network information. In the proposal, EPA did not intend for a required
5-year review of the SO2 network to be an additional effort
on top of the existing required network assessments but instead to be
included as part of the 5-year assessment in Sec.  58.10(d). EPA notes
that CBSA populations and emissions inventories change over time,
suggesting a need for periodic review of the monitoring network. At the
same time, EPA recognizes the advantages of a stable monitoring
network. However, after considering comments, EPA is not finalizing the
proposed language for 40 CFR Part 58 Appendix D, section 4.4.3(2) which
simply referenced back to Sec.  58.10. This proposed text it is not
needed and appears to simply cause confusion. EPA asserts that the
existing requirements in Sec.  58.10 provide a sufficient and
appropriate mechanism for network updates and assessment.
c. Conclusions on Monitoring Network Implementation
    Based on the public comments, and due to the contemplated use of a
hybrid analytical approach, EPA is finalizing, as was proposed, that
State and, where appropriate, local air monitoring agencies submit a
plan for deploying SO2 monitors in accordance with the
proposed requirements presented below by July 1, 2011. Minimally
required SO2 monitors shall be physically established no
later than January 1, 2013.

C. Data Reporting

    The following paragraphs provide background, rationale, and details
for monitor data reporting requirements.
a. Proposed Data Reporting
    Controlled human exposure studies indicate that exposures to peaks
of SO2 on the order of 5 to 10 minutes result in moderate or
greater decrements in lung function and/or respiratory symptoms in
exercising asthmatics (section II.B.1 above, ISA section 5.2, REA
section 7.2.3, and REA section <IP_ADDRESS>). As a result, the 1-hour
standard is intended to protect against short term exposures, including
exposures on the order of 5 minutes up to 24 hours, as is discussed in
section II.F.2 above. Therefore, in support of the revised NAAQS and
its intent, EPA proposed that State and local agencies shall report to
AQS the maximum 5-minute block average of the twelve 5-minute block
averages of SO2 for each hour. This 5-minute block reporting
requirement is in addition to the existing requirement to report the 1-
hour average. In addition, EPA solicited comment on the advantages and
disadvantages (including associated resource burdens) of alternatively
requiring State and local agencies to report all twelve 5-minute
SO2 values for each hour or the maximum 5-minute
concentration in an hour based on a moving 5-minute averaging period
rather than time block averaging.
    EPA also proposed Data Quality Objectives (DQOs) for the
SO2 network. DQOs generally specify the tolerable levels for
potential decision error used as a basis for establishing the quality
and quantity of data needed to support the objectives of the monitors.
EPA proposed the goal for acceptable measurement uncertainty for
SO2 methods to be defined as an upper 90 percent confidence
limit for the coefficient of variation (CV) of 15 percent for precision
and as an upper 95 percent confidence limit for the absolute bias of 15
percent for bias.
b. Public Comments
    EPA received many comments on the reporting of 5-minute data
values. The comments generally fell into one of the following
categories: \33\ (1) Those State, public health, and environmental
groups who supported the proposed requirement to report the maximum 5-
minute block average of the twelve 5-minute block averages of
SO2 for each hour (e.g., Missouri, NESCAUM, North Carolina,
ALA, EJ, EDF, NRDC, and SC), (2) those State, public health, and
environmental groups who supported the reporting of all twelve 5-minute
averages of each hour (e.g., Kentucky, NYSDEC, AQRL, ALA, ATS, CBD, EJ,
EDF, NRDC, and SC), (3) those State, public health, and environmental
groups who supported reporting the maximum 5-minute concentration in an
hour based on a moving 5-minute average (e.g., South Dakota, ALA, CBD,
EJ, EDF, NRDC, and SC), and (4) those State and industry groups who did
not support the reporting of any 5-minute data (e.g., Iowa, South
Carolina, LEC, and RRI Energy).
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    \33\ Note that some commenters supported more than one form of
reported 5-minute data.
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    Public health and environmental groups (e.g. ALA, CBD, EJ, EDF,
NRDC, and SC) supported an approach where 5-minute data must be
reported. However, these commenters were flexible in their position and
supported multiple forms or types of 5-minute data reporting. The ALA,
EJ, EDF, NRDC, and SC stated that ``we support the proposed requirement
for State and local monitoring agencies to report both hourly average
and maximum 5-minute averages out of the twelve 5-minute block averages
of SO2 for each hour.'' They also expressed a preference for
alternative 5-minute data reporting stating that they ``strongly prefer
that States be required to report the peak 5-minute concentrations of
SO2 based on a rolling average.'' Similarly, CBD stated that
``* * * EPA should require that State and local agencies report all 12
five-minute SO2 values for each hour in addition to 1-hour
averages. Where possible, EPA also should require reporting of rolling
five-minute averages rather than block data * * *''
    Missouri generally supported the proposed requirement to report the
maximum 5-minute average in the hour, saying ``it is not a problem to
report both the hourly average and the maximum 5-minute block
average.'' Nevertheless, Missouri went on to note constraints, stating
that ``* * * [their] data logger and associated software do not have
the capability to report all twelve 5-minute SO2 values for
each hour'' and that they ``* * * could not do this without software
being developed for this purpose and it could be time intensive to
validate this data.''
    Kentucky did not support the proposal to report the maximum 5-
minute data block in the hour because of the limitations in their data
acquisition systems. They explained that ``the data acquisition system
used by the [State] does not have the capability to automatically
report the maximum 5-minute block of data from an hour concentration.
[State] personnel would have to manually determine that

value and then manually enter that data into AQS.'' Kentucky goes on to
suggest that ``the only feasible option for the [State] to submit 5-
minute data to AQS would be to submit all twelve 5-minute blocks of
data for each hour to AQS.''
    South Dakota stated that its ``* * * preference would be to report
the maximum 5-minute average for each hour calculated using a 5-minute
rolling average.'' South Dakota goes on to state that ``* * * while
doubling the work required to validate data and load the data into AQS,
the additional data should help determine if the selected standard
concentration level has achieved the necessary reduction in high
concentration 5-minute levels and provide the necessary data for
further study of health impacts * * *''
    South Carolina stated that it ``* * * does not support mandatory
reporting of 5-minute averages in addition to the 1-hour average
required for comparison to the standard. The validation and reporting
of 5-minute averages imposes a significant additional burden on the
reporting organization and its Quality System.'' Iowa, who also did not
support any form of 5-minute data reporting stated that ``the five-
minute data is not used to determine compliance with the NAAQS, and
represents ancillary data,'' and that ``validating and uploading the
five-minute data will take at least as much staff time as generating
the hourly data used for compliance.'' As a result, Iowa states that
``if EPA determines that five-minute data is needed, we recommend that
EPA require the maximum five-minute average in each hour, rather than
all twelve five-minute averages, in order to reduce the burden
associated with generation of the ancillary data set.''
    With regard to the proposed DQOs, EPA received comments from some
States (e.g., Kentucky, North Carolina, NYSDEC, and South Carolina)
providing general support for the goals for acceptable measurement
uncertainty for precision and bias. North Carolina stated that the ``*
* * precision and bias measurement uncertainty criteria should emulate
those that have been established for other recent NAAQS and NCore
pollutants.'' NYSDEC stated that ``the proposal does not seem
unreasonable, however these statistics are now expressed in terms of
confidence limits: Precision--90% confidence of a CV of 15% and Bias--
95% confidence of a CV of 15%.'' NYSDEC raises concern that ``* * * the
results are now dependent on the number of audits performed. This is
highly variable because some agencies run automatic audits every night,
[while] others use the old standard of once every 2 weeks.''
    In regard to comments on the proposed DQOs, EPA notes that the
precision and bias estimation technique on which NYSDEC comments were
focused were proposed and adopted in the monitoring rule promulgated on
October 6, 2006 and EPA did not intend to reopen those requirements for
comment. Moreover, SO2 precision and bias estimates have
been performed in this manner for the past four years and there have
been no adverse effects on data quality at the minimum required level
of performance checks every two weeks. The statistics for the precision
and bias estimates and the DQO goals are based on the accumulation of
the one-point precision checks aggregated at the frequencies required
in CFR which is every two weeks. Any organization performing more
frequent checks (such as every night) would accumulate more data for
the precision and bias estimates, have higher confidence in the data,
and would have less potential for outliers or higher than normal values
effecting the precision and bias estimate. In addition, monitoring
organizations running precision checks every 24 hours would be more
able to control data quality to meet the DQO goals than organizations
running the check every two weeks.
c. Conclusions on Data Reporting
    EPA received a fairly diverse set of comments on the
appropriateness of reporting 5-minute data and in what particular
format it may be provided in. EPA has considered the comments by the
States regarding validation of potentially 13 data values per hour
(instead of 1 or 2) and some States' lack of data acquisition capacity
or processing capability to report any particular type of 5-minute
value. EPA believes that in light of these comments, adopting a
requirement for continuous SO2 analyzers to report all
twelve 5-minute values or a rolling 5-minute value does not appear to
provide enough added value for the potential increased burden on
States, such as increased staff time dedicated to data processing and
QA, or in improving or adjusting data acquisition capabilities.
However, EPA also believes that obtaining some form of 5-minute data is
appropriate because such data have been critical to this NAAQS review,
and are anticipated to be of high value to inform future health studies
and, subsequently, future SO2 NAAQS reviews.\34\ Indeed, as
noted earlier, it was EPA's failure to adequately explain the absence
of protection from elevated short-term (5- to 10-minute exposure)
SO2 concentrations for heavily breathing asthmatics that
occasioned the remand of the 1996 SO2 primary NAAQS
(American Lung Association, 134 F.3d at 392). This belief is supported
further by the expectation that a significant portion of the monitors
operating to satisfy the final monitoring network design will likely be
sited for population exposures, which have traditionally provided
ambient data that is often utilized by epidemiologic health studies.
Therefore, EPA is finalizing the requirement that State and local air
agencies operating continuous SO2 analyzers shall report the
maximum 5-minute block average out of the twelve 5-minute block
averages in each hour, for each hour of the day, and that State and
local air agencies operating any type of SO2 analyzer shall
report the integrated 1-hour average value, as was proposed. EPA
encourages States capable of reporting all twelve 5-minute data blocks
in an hour to report such data to AQS. AQS is currently set-up to take
the 5-minute maximum value in an hour under parameter code 42406 and
can take all twelve 5-minute values under parameter code 42401 (with a
duration code of H). EPA notes that if a State were to choose to submit
all twelve 5-minute blocks in the hour, by default, they would be
submitting the maximum 5-minute data block within that hour, although
they have not singled out that particular value. Since the 5-minute
data is not directly being used for comparison to the NAAQS, EPA
believes that any State electing to submit all twelve 5-minute values
is still satisfying the intent of having the maximum 5-minute value
reported. Therefore, if a State chooses to submit all twelve 5-minute
values in an hour, they will be considered to be satisfying the data
reporting requirement of submitting the maximum 5-minute value in an
hour, and they do not have to separately report the maximum 5-minute
value from within that set of data values to AQS under parameter code
42406.
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    \34\ The REA assessed exposure and risks associated with 5-
minute SO2 concentrations above 5-minute health effect
benchmark levels derived from controlled human exposure studies. In
the analyses, the REA noted that very few State and local agencies
report ambient 5-minute SO2 data (REA, section <IP_ADDRESS>)
and that the lack of 5-minute data necessitated the use of
statistically estimated 5-minute SO2 data in order to
expand the geographic scope of the exposure and risk analyses (REA,
section 7.2.3).
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    EPA proposed new regulation text for 40 CFR Part 58 Appendix C,
which would have added section 2.1.2 that would have required any
SO2 FRM or

FEM used for making NAAQS decisions to be capable of providing both 1-
hour and 5-minute averaged concentration data. EPA is not finalizing
this proposed language, as the manual wet-chemistry pararosaniline
reference method cannot provide 5-minute data. Therefore, the proposed
language is inappropriate. However, both the UVF FEM and the new UVF
FRM continuous methods are capable of providing 5-minute averaged data.
As a result, the language in 58.12(g) and 58.16(g) requiring 5-minute
SO2 data has been adjusted to appropriately specify that
only those States operating continuous FRM or FEMs are required to
report the maximum 5-minute data value for each hour.
    With regard to acceptable measurement uncertainties, EPA reviewed
summary data for each Primary Quality Assurance Organization (PQAO) in
the 2008 Data Quality Indicator Report on SO2 data within
the 2008 Criteria Pollutant Quality Indicator Summary Report for AQS
Data (http://www.epa.gov/ttn/amtic/qareport.html). Of the 100 PQAOs in
the report, none of those organizations had summary CV or bias values
exceeding 10 percent. Thus, EPA believes that the SO2
network can and does easily attain measurement uncertainty criteria
more stringent than the finalized goal values and the monitoring
required under the final network design should be able to maintain this
level of performance. Therefore, in consideration of comments and
existing quality assurance data, EPA is changing the final goals from
those which were proposed for acceptable measurement uncertainty for
SO2 methods to be defined for precision as an upper 90
percent confidence limit for the coefficient of variation (CV) of 10
percent and for bias as an upper 95 percent confidence limit for the
absolute bias of 10 percent.

V. Initial Designation of Areas for the 1-Hour SO2 NAAQS

    This section of the preamble further addresses the process under
which EPA intends to identify whether areas of the country attain or do
not attain or are ``unclassifiable'' regarding the new 1-hour
SO2 NAAQS. After EPA establishes a new NAAQS, the CAA
directs States and EPA to take this first step, known as the ``initial
area designations,'' in ensuring that the NAAQS is ultimately attained.
    We are revising our discussion of an expected approach toward
issuing initial area designations in response to comments we received
on the proposed rule's treatment of monitoring and modeling (both
generally and in the specific context of designations), and to make the
expected process more consistent with our historical approach to
implementing the SO2 NAAQS. A revised anticipated approach
for issuing designations logically follows from our revised hybrid
approach to monitoring and modeling as discussed above in sections III
and IV. It would also affect a revised expected implementation approach
that we later discuss in section VI. 1. Designations.

a. Clean Air Act Requirements

    The CAA requires EPA and the States to take steps to ensure that
the new NAAQS are met following promulgation. The first step is for EPA
to identify whether areas of the country meet, do not meet, or cannot
yet be classified as either meeting or not meeting the new NAAQS.
Section 107(d)(1)(A) provides that, ``By such date as the Administrator
may reasonably require, but not later than 1 year after promulgation of
a new or revised NAAQS for any pollutant under section 109, the
Governor of each State shall * * * submit to the Administrator a list
of all areas (or portions thereof) in the State'' that should be
designated as nonattainment, attainment, or unclassifiable for the new
NAAQS. 42 U.S.C. 7407(d)(1)(A)(i)-(iii). Section 107(d)(1)(B)(i)
further provides, ``Upon promulgation or revision of a NAAQS, the
Administrator shall promulgate the designations of all areas (or
portions thereof) * * * as expeditiously as practicable, but in no case
later than 2 years from the date of promulgation. Such period may be
extended for up to one year in the event the Administrator has
insufficient information to promulgate the designations within 2
years.'' 42 U.S.C. 7407(d)(1)(B)(i).
    Under CAA section 107(d)(1)(B)(ii), no later than 120 days prior to
promulgating designations, EPA is required to notify States of any
intended modifications to their boundaries as EPA may deem necessary,
and States will have an opportunity to comment on EPA's tentative
decision. Whether or not a State provides a recommendation, the EPA
must promulgate the designation that it deems appropriate. 42 U.S.C.
7407(d)(1)(B)(ii).
    Accordingly, since the new 1-hour SO2 NAAQS is being
promulgated today, Governors should submit their initial SO2
designation recommendations to EPA no later than June 2, 2011. If the
Administrator intends to modify any State's boundary recommendation,
the EPA will notify the Governor no later than 120 days prior to
designations or, February 2012. States that believe the Administrator's
modification is inappropriate will have an opportunity to demonstrate
why they believe their recommendation is more appropriate before
designations are finalized in June 2012.
    For initial designations that will be finalized in June 2012,
States should use monitoring data from the existing SO2
network for the years 2008-2010, as well as any refined SO2
dispersion modeling (see Appendix W to 40 CFR Part 51) for sources that
may have the potential to cause or contribute to a NAAQS violation,
provided that it is recent and available. EPA will then issue
designations based on the record of information for that area. Under
our anticipated approach, an area that has monitoring data or refined
modeling results showing a violation of the NAAQS would be designated
as ``nonattainment.'' An area that has both monitoring data and