to meet CAA requirements must be included in the SIP). IEPA did not
attempt to revise Illinois' SIP to include these measures in order to
implement its good neighbor obligations. Further, the listing of
existing or on-the-way control measures, whether approved into the
State's SIP or not, does not substitute for a complete Step 3 analysis
under EPA's 4-step framework to define ``significant contribution.''
IEPA did not identify the control measures, provide an assessment of
the overall effects of these measures, note when the reductions would
be achieved, or explain what the overall resulting air quality effects
would be at identified out of state receptors. IEPA did not evaluate
additional, potential emissions control opportunities, or their costs
or impacts, or attempt to analyze whether, if applied more broadly
across linked states, the emissions reductions would constitute the
elimination of significant contribution on a regional scale. IEPA did
not offer an explanation as to whether any faster or more stringent
emissions reductions that may be available were prohibitively costly or
infeasible. Although EPA acknowledges states are not necessarily bound
to follow its own analytical framework at Step 3, IEPA did not attempt
to determine or justify an appropriate uniform cost-effectiveness
threshold for the more stringent 2015 ozone NAAQS. This would have been
similar to the approach to defining significant contribution that EPA
has applied in prior rulemakings such as CSAPR and or the CSAPR Update,
even if such an analysis is not technically mandatory.
    Finally, under the Wisconsin decision, states and EPA may not delay
implementation of measures necessary to address good neighbor
requirements beyond the next applicable attainment date without a
showing of impossibility or necessity. See 938 F.3d at 320. The IEPA's
submittal is insufficient to the extent the implementation timeframes
for identified control measures were left unidentified, unexplained, or
too uncertain to permit EPA to form a judgment as to whether the timing
requirements for good neighbor obligations have been met.
    IEPA also attempted to rely on a concept related to international
emissions identified in Attachment A to the March 2018 memorandum--a
concept that apparently had its origins outside EPA and was not
endorsed or recommend by EPA at the time or since. IEPA noted that
Illinois would be linked to only one receptor if international and
offshore emissions were simply subtracted from the receptor's maximum
design values. As explained in Section I.D above, the concepts
presented in Attachment A to the March 2018 memorandum were neither
guidance nor determined by EPA to be consistent with the CAA. EPA made
clear at the time that it would thoroughly review the technical and
legal justifications states put forward in relying on any concepts from
Attachment A to the March 2018 memorandum. In this case, what IEPA
proposes is clearly unacceptable.
    The state's reasoning related to international and offshore
emissions is inapplicable to the requirements of CAA section
110(a)(2)(D)(i)(I). The good neighbor provision requires states and EPA
to address interstate transport of air pollution that contributes to
downwind states' ability to attain and maintain NAAQS. Whether
emissions from other states or other countries also contribute to the
same downwind air quality issue is irrelevant in assessing whether a
downwind state has an air quality problem, or whether an upwind state
is significantly contributing to that problem. States are not obligated
under CAA section 110(a)(2)(D)(i)(I) to reduce emissions sufficient on
their own to resolve downwind receptors' nonattainment or maintenance
problems. Rather, states are obligated to eliminate their own
``significant contribution'' or ``interference'' with the ability of
other states to attain or maintain the NAAQS.
    Indeed, the D.C. Circuit in Wisconsin specifically rejected
petitioner arguments suggesting that upwind states should be excused
from good neighbor obligations on the basis that some other source of
emissions (whether international or another upwind state) could be
considered the ``but-for'' cause of downwind air quality problem. See
938 F.3d at 323-324. The court viewed petitioners' arguments as
essentially an argument ``that an upwind state `contributes
significantly' to downwind nonattainment only when its emissions are
the sole cause of downwind nonattainment.'' Id. at 324. The court
explained that ``an upwind state can `contribute' to downwind
nonattainment even if its emissions are not the but-for cause.'' Id. at
324-325. See also Catawba County v. EPA, 571 F.3d 20, 39 (D.C. Cir.
2009) (rejecting the argument ``that `significantly contribute'
unambiguously means `strictly cause' '' because there is ``no reason
why the statute precludes EPA from determining that [an] addition of
[pollutant] into the atmosphere is significant even though a nearby
county's nonattainment problem would still persist in its absence'');
Miss. Comm'n on Envtl. Quality v. EPA, 790 F.3d 138, 163 n.12 (D.C.
Cir. 2015) (observing that the argument that ``there likely would have
been no violation at all . . . if it were not for the emissions
resulting from [another source]'' is ``merely a rephrasing of the but-
for causation rule that we rejected in Catawba County''). Therefore, a
state is not excused from eliminating its significant contribution on
the basis that international emissions also contribute some amount of
pollution to the same receptors to which the state is linked. Illinois'
argument related to international and offshore emissions fails to
change the status of any receptor at Step 1, to eliminate Illinois'
linkages at Step 2, or to provide sufficient evidence that Illinois
does not contribute significantly to receptors at Step 3.
    We therefore propose that Illinois was required to analyze
emissions from the sources and other emissions activity from within the
state to determine whether its contributions were

significant, and we propose to disapprove its submission because
Illinois failed to do so.
5. Evaluation of Information Provided Regarding Step 4
    Step 4 of the 4-step interstate transport framework calls for
development of permanent and federally enforceable control strategies
to achieve the emissions reductions determined to be necessary at Step
3 to eliminate significant contribution to nonattainment or
interference with maintenance of the NAAQS. IEPA identified future
NOX RACT standards for the Chicago area and unnamed Federal
rules were sufficient to resolve Illinois' good neighbor obligations
for the 2015 ozone NAAQS. However, Illinois did not revise its SIP to
include these emission reductions in a revision to its SIP to ensure
the reductions were permanent and enforceable. As a result, EPA
proposes to disapprove Illinois' submittal on the separate,\66\
additional basis that the Illinois has not included permanent and
enforceable emissions reductions in its SIP as necessary to meet the
obligations of CAA section 110(a)(2)(d)(i)(I).
---------------------------------------------------------------------------

    \66\ Pointing to anticipated upcoming emission reductions is not
sufficient as a step 3 analysis, for the reasons discussed in
Section 4. In this section, we explain that to the extent such
anticipated reductions are not included in the SIP and rendered
permanent and enforceable, reliance on such anticipated reductions
is also insufficient at step 4.
---------------------------------------------------------------------------

6. Conclusion
    Based on EPA's evaluation of Illinois SIP submission, EPA is
proposing to find that the portion of Illinois' May 21, 2018 SIP
submission addressing CAA section 110(a)(2)(D)(i)(I) does not meet the
state's interstate transport obligations, because it fails to contain
the necessary provisions to eliminate emissions that will contribute
significantly to nonattainment or interfere with maintenance of the
2015 ozone NAAQS in any other state.

B. Indiana

1. Evaluation of Information Provided by Indiana Regarding Step 1
    IDEM relied on LADCO modeling released in 2018 to identify
nonattainment and maintenance receptors in 2023. As described
previously in this action, LADCO performed a modeling demonstration
like that of EPA's 2018 transport modeling efforts, except with use of
the ERTAC EGU Tool for EGU emissions. LADCO identified nonattainment
and maintenance receptors using EPA methodology identified in Section
I. IDEM elected to rely on LADCO's modeling results, which identified
similar receptors to EPA's modeling included in the March 2018 memo.
Since new modeling has been performed by EPA which includes updated
emission data using the 2016v2 platform, EPA proposes to primarily rely
on the most recent modeling to identify nonattainment and maintenance
receptors in 2023 (see Table 3 further in this action).
    Nonetheless, the LADCO modeling relied on by IDEM also identified a
number of nonattainment and maintenance receptor sites in 2023. See
Table 7 on page 30 of Attachment 1 to Indiana's submittal. Thus, even
under the LADCO modeling for 2023, IDEM acknowledges in its submittal
the existence of several nonattainment and maintenance receptors.
    IDEM essentially argues that two of the receptors to which Indiana
was linked would not actually be receptors in 2023. Based on updated
modeling of EPA's 2016v2 emissions platform, EPA agrees with IDEM that
that the Allegan, Michigan monitor is not expected to be a receptor in
2023, but not the receptor in Sheboygan, Wisconsin. Regardless, EPA
disagrees with the line of reasoning IDEM put forward to argue that
those two monitors would not be receptors to the extent such reasoning
could be applied to Indiana's linkages in EPA's modeling using the
2016v2 emissions platform. First, IDEM concluded that if ozone design
value trends continued as expected then those two receptors would reach
attainment before 2023. In addition, IDEM compared 2012-2017 monitoring
data with LADCO's and EPA's modeling and concluded that the Sheboygan,
Wisconsin and Allegan, Michigan receptor monitors were already below
the 2023 projections. Additionally, EPA's updated modeling, which
considers more recent design values and emissions, continues to find
that Indiana is linked to downwind nonattainment and maintenance
receptors, despite downward trends in emissions and design values
2. Evaluation of Information Provided by the State Regarding Step 2
    Although Indiana relied on alternative modeling to EPA's modeling,
Indiana acknowledged in its SIP submission that it is linked to one or
more downwind receptors above either a 1 percent of the NAAQS or 1 ppb
contribution threshold in 2023. Because the alternative modeling relied
on by IDEM also demonstrates that a linkage exists between the state
and one or more downwind receptors at Step 2, EPA need not conduct a
comparative assessment of the alternative modeling; the State concedes
that it is linked. IDEM's analysis corroborates the conclusion in EPA's
most recent modeling, described in the next section.
    IDEM additionally utilized a 1 ppb threshold at Step 2 to identify
whether it was linked to a projected downwind nonattainment or
maintenance receptor. As discussed in EPA's August 2018 memorandum,
with appropriate additional analysis it may be reasonable for states to
use a 1 ppb contribution threshold, as an alternative to a 1 percent
threshold, at Step 2 of the 4-Step interstate transport framework, for
the purposes of identifying linkages to downwind receptors. However,
IDEM's submission did not contain any additional analysis of
contributions at the receptors to which they were linked to support
their claim that a 1 ppb threshold was an appropriate Step 2 screening
threshold. Rather, IDEM claimed that a threshold of 1 percent was too
low because it is less than the ozone monitoring ``tolerance level'' of
1 ppb (i.e., precision) used for reporting measured ozone
concentrations. In its submittal IDEM failed to provide any basis for
asserting that the precision of an ozone monitor is applicable to the
precision of ozone contributions which are not a directly measured
quantity. Regardless, total upwind contributions are well above 1 ppb
at all receptors to which Indiana is linked based on modeling by LADCO
and by EPA. In addition, Indiana alone contributes above 1 ppb to
several downwind receptors. Because contributions are not directly
measured, modeling is used to apportion projected ozone design values
into contributions from individual states and other sources of ozone
precursors (e.g., fires and biogenic sources). The projected ozone
design values are calculated using the method recommended in EPA's
modeling guidance.\67\ As part of this method, projected design values
are reported with a precision of a tenth of a ppb. Consistent with our
modeling guidance, ozone contributions are evaluated with a precision
of a tenth of a ppb. For example, a contribution of 0.6999 . . . ppb is
reported as 0.69 ppb and evaluated as 0.6 ppb which is below the 1
percent threshold.
---------------------------------------------------------------------------

    \67\ Modeling Guidance for Demonstrating Air Quality Goals for
Ozone, PM2.5, and Regional Haze. November 2018. EPA 454-
R-18-009. https://www.epa.gov/scram/sip-modeling-guidance-documents.
---------------------------------------------------------------------------

    Indiana seemingly conflates the contribution threshold at Step 2
with a Step 3 determination of ``significance'' (which is reached only
after the application of a multi-factor analysis), regardless EPA does
not accept

Indiana's argument that a 1 percent of the NAAQS contribution threshold
at Step 2 is ``not appropriate'' for the 2015 ozone NAAQS on the basis
of unwarranted modeling reliability concerns.\68\ The explanation for
how the 1 percent contribution threshold was originally derived is
available in the 2011 CSAPR rulemaking. See 76 FR 48208, 48237-38.
Further, in the CSAPR Update, EPA re-analyzed the threshold for
purposes of the 2008 ozone NAAQS and determined it was appropriate to
continue to apply this threshold. EPA compared the 1 percent threshold
to a 0.5 percent of NAAQS threshold and a 5 percent of NAAQS threshold.
EPA found that the lower threshold did not capture appreciably more
upwind state contribution compared to the 1 percent threshold, while
the 5 percent threshold allowed too much upwind state contribution to
drop out from further analysis. See Final CSAPR Update Air Quality
Modeling TSD, at 27-30 (EPA-HQ-OAR-2015-0596-0144). If EPA were to
apply this analysis to the 2015 ozone NAAQS using the updated modeling
based on the 2016v2 emissions platform, a 5 percent of the NAAQS
contribution threshold would forgo nearly 30 percent of the total
upwind contribution, on average, for those receptors to which Indiana
is linked using a 1 percent threshold. As EPA noted in the August 2018
memorandum, the use of even a 2 ppb contribution threshold under the
modeling released with the March 2018 memorandum would only capture
about 55 percent of all upwind contributions, and therefore ``emission
reductions from states linked at that higher threshold may be
insufficient to address collective upwind state contribution to
downwind air quality problems.'' \69\
---------------------------------------------------------------------------

    \68\ Indiana's SIP submission, Attachment 1 at 4.
    \69\ August 2018 memorandum at 4.
---------------------------------------------------------------------------

    With these figures in mind, IDEM's claims that the contribution
threshold should be substantially higher than 1 or even 2 ppb solely on
the basis of modeling uncertainty cannot be accepted. First, both
IDEM's and EPA's modeling techniques are sufficiently reliable and fit
for the purpose to measure upwind contribution levels down to at least
one percent of the NAAQS. EPA's recommended model attainment test is
based on application of the model in a relative sense rather than
relying upon absolute model predictions.\70\ All models have
limitations resulting from uncertainties in inputs and scientific
formulation. To minimize the effects of these uncertainties, the
modeling is anchored to base period measured data in EPA's guidance
approach for projecting design values. Notably, EPA also uses our
source apportionment modeling in a relative sense when calculating the
average contribution metric (used to identify linkages). In this method
the magnitude of the contribution metric is tied to the magnitude of
the projected average design value which is tied to the base period
average measured design value. EPA's guidance has not established a
bright-line criteria for judging whether or not statistical measures of
model performance constitute acceptable or unacceptable model
performance. So, contrary to what Indiana appears to be claiming with
regards to modeling biases, there are no EPA recommended measures of
allowable error. Although EPA does not typically focus on using
particular benchmarks as the sole criteria for model performance, EPA
notes that the model performance for the updated modeling based on the
2016v2 emissions platform is generally within the benchmarks
recommended by Emery, et al., (2017).\71\
---------------------------------------------------------------------------

    \70\ See Section 4.1' ``Overview of Modeled Attainment Test in
EPA Modeling Guidance for Demonstrating Air Quality Goals for Ozone,
PM2.5, and Regional Haze. November 2018. EPA 454-R-18-
009. https://www.epa.gov/scram/sip-modeling-guidance-documents.
    \71\ Christopher Emery, Zhen Liu, Armistead G. Russel, M. Talat
Odman, Greg Yarwood and Naresh Kumar (2017). Recommendations on
statistics and benchmarks to assess photochemical model performance,
Journal of the Air & Waste Management Association, 67:5,582-598,
DOI: 10.1080/10962247.2016.1265027.
---------------------------------------------------------------------------

    EPA has successfully applied a 1 percent of NAAQS threshold to
identify linked upwind states in three prior rulemakings. And the D.C.
Circuit has declined to establish bright line criteria for model
performance. In upholding EPA's approach to evaluating interstate
transport in CSAPR, the Supreme Court held that they would not
``invalidate EPA's predictions solely because there might be
discrepancies between those predictions and the real world. That
possibility is inherent in the enterprise of prediction.'' EME Homer
City Generation, L.P. v. EPA, 795 F.3d 118, 135 (2015). The court
continued to note that ``the fact that a `model does not fit every
application perfectly is no criticism; a model is meant to simplify
reality in order to make it tractable.' '' Id. at 135-36 (quoting
Chemical Manufacturers Association v. EPA, 28 F.3d 1259, 1264 (D.C.
Cir. 1994).
    EPA's August 2018 memorandum provided that whether use of a 1 ppb
threshold is appropriate must be based on an evaluation of state-
specific circumstances, and no such evaluation was included in the
submission. EPA's experience with the alternative Step 2 thresholds is
further discussed in Section I.D.3.i. As discussed there, EPA is
considering withdrawing the August 2018 memorandum.
3. Results of EPA's Step 1 and Step 2 Modeling and Findings for Indiana
    As described in Section I, EPA performed air quality modeling using
the 2016v2 emissions platform to project design values and
contributions for 2023. These data were examined to determine if
Indiana contributes at or above the threshold of 1 percent of the 2015
ozone NAAQS (0.70 ppb) to any downwind nonattainment or maintenance
receptor. As shown in Table 3, the data \72\ indicate that in 2023,
emissions from Indiana contribute greater than 1 percent of the
standard to nonattainment or maintenance-only receptors in Wisconsin,
Illinois, Connecticut, and Pennsylvania.\73\
---------------------------------------------------------------------------

    \72\ Design values and contributions at individual monitoring
sites nationwide are provide in the file:
2016v2_DVs_state_contributions.xlsx which is included in docket ID
No. EPA-HQ-OAR-2021-0663.
    \73\ These modeling results are consistent with the results of a
prior round of 2023 modeling using the 2016v1 emissions platform
which became available to the public in the fall of 2020 in the
Revised CSAPR Update, as noted in Section I. That modeling showed
that Indiana had a maximum contribution greater than 0.70 ppb to at
least one nonattainment or maintenance-only receptor in 2023. These
modeling results are included in the file ``Ozone Design Values And
Contributions Revised CSAPR Update.xlsx'' in docket ID No. EPA-HQ-
OAR-2021-0663.

                       Table 3--Indiana Linkage Results Based on EPA Updated 2023 Modeling
----------------------------------------------------------------------------------------------------------------
                                                                   2023  Average   2023  Maximum      Indiana
         Receptor ID               Location       Nonattainment/   design value    design value    contribution
                                                   maintenance         (ppb)           (ppb)           (ppb)
----------------------------------------------------------------------------------------------------------------
550590025....................  Kenosha, WI.....  Maintenance....            69.2            72.3            7.10
170310032....................  Cook, IL........  Maintenance....            69.8            72.4            7.03

550590019....................  Kenosha, WI.....  Nonattainment..            72.8            73.7            6.60
551010020....................  Racine, WI......  Nonattainment..            71.3            73.2            6.60
170317002....................  Cook, IL........  Maintenance....            70.1            73.0            6.33
170310076....................  Cook, IL........  Maintenance....            69.3            72.1            6.21
170310001....................  Cook, IL........  Maintenance....            69.6            73.4            5.44
170314201....................  Cook, IL........  Maintenance....            69.9            73.4            4.65
90099002.....................  New Haven, CT...  Nonattainment..            71.8            73.9            0.87
90019003.....................  Fairfield, CT...  Nonattainment..            76.1            76.4            0.76
90013007.....................  Fairfield, CT...  Nonattainment..            74.2            75.1            0.75
420170012....................  Bucks, PA.......  Maintenance....            70.7            72.2            0.73
----------------------------------------------------------------------------------------------------------------

    Therefore, based on EPA's evaluation of the information submitted
by IDEM, and based on EPA's most recent modeling results for 2023, EPA
proposes to find that Indiana is linked at Steps 1 and 2 and has an
obligation to assess potential emissions reductions from sources or
other emissions activity at Step 3 of the 4-Step framework. EPA
therefore will proceed to Step 3 of the 4-Step interstate transport
framework to assess the arguments the State presented as to why,
despite this linkage, the state should not be considered to
significantly contribute to nonattainment or interfere with maintenance
of the NAAQS in any other state.
4. Evaluation of Information Provided Regarding Step 3
    At Step 3 of the 4-Step interstate transport framework, a state's
emissions are further evaluated, in light of multiple factors,
including air quality and cost considerations, to determine what, if
any, emissions significantly contribute to nonattainment or interfere
with maintenance and, thus, must be eliminated under CAA section
110(a)(2)(D)(i)(I).
    To effectively evaluate which emissions in the state should be
deemed ``significant'' and therefore prohibited, states generally
should prepare an accounting of sources and other emissions activity
for relevant pollutants and assess potential, additional emissions
reduction opportunities and resulting downwind air quality
improvements. EPA has consistently applied this general approach (i.e.,
Step 3 of the 4-Step interstate transport framework) when identifying
emissions contributions that the Agency has determined to be
``significant'' (or interfere with maintenance) in each of its prior
Federal, regional ozone transport rulemakings, and this interpretation
of the statute has been upheld by the Supreme Court. See EME Homer
City, 572 U.S. 489, 519 (2014). While EPA has not directed states that
they must conduct a Step 3 analysis in precisely the manner EPA has
done in its prior regional transport rulemakings, state implementation
plans addressing the obligations in CAA section 110(a)(2)(D)(i)(I) must
prohibit ``any source or other type of emissions activity within the
state'' from emitting air pollutants which will contribute
significantly to downwind air quality problems. Thus, states must
complete something similar to EPA's analysis (or an alternative
approach to defining ``significance'' that comports with the statute's
objectives) to determine whether and to what degree emissions from a
state should be ``prohibited'' to eliminate emissions that will
``contribute significantly to nonattainment in, or interfere with
maintenance of'' the NAAQS in any other state. IDEM did not conduct
such an analysis in their SIP submission.
    IDEM first asserted that Indiana's rule amendments under CSAPR
meant that Indiana was already meeting the good neighbor requirements
for the 2015 ozone NAAQS. The submittal, however, did not contain a
demonstration at Step 3 that the State was adequately controlling its
emissions for purposes of the good neighbor provision, particularly
because the State conceded in its submission that it was potentially
significantly contributing to one or more receptors in 2023 at Steps 1
and 2. The SIP submittal pointed to the state's existing NOX
control measures, consent decree requirements, and future fuel switches
and retirements for large EGUs and non-EGUs for the years 2008 through
2017 to conclude Indiana is already meeting its good neighbor
obligations for the 2015 ozone NAAQS.
    However, the state's submittal does not include a sufficient
examination or a technical justification that could support the
conclusion that the state has no further good neighbor obligations for
the 2015 ozone NAAQS. In particular, the state did not conduct in its
submittal an analysis of potential additional emissions reductions
measures to further reduce its impact on the identified downwind
receptors. For example, although Indiana did include in its submission
a list of controls at individual emissions units at facilities in the
state, IDEM did not analyze additional potential NOX
emissions control technologies, their associated costs, estimated
emissions reductions, and downwind air quality improvements. Nor does
the submittal include an analysis of whether such potential, additional
control technologies or measures could reduce the impact of Indiana's
emissions on out of state receptors. Though there is not a prescribed
method for a Step 3 analysis, EPA has consistently applied Step 3 of
the good neighbor framework through a more rigorous evaluation of
potential additional control technologies or measures than what Indiana
provided in its submission. Identifying a range of various emissions
control measures that have been or may be enacted at the state level,
without analysis of the impact of those measures on the out of state
receptors, is not analytically sufficient. In general, the air quality
modeling that EPA has conducted (as well the modeling relied on by
Indiana in its submittal) already accounts for ``on-the-books''
emissions control measures. Both sets of modeling clearly establish
continued linkage from Indiana to downwind receptors in 2023 at Steps 1
and 2, despite those emissions control efforts.
    IDEM provided what they characterized as a weight of evidence
analysis consisting of monitoring data, emissions data, and
photochemical modeling to justify their conclusion that no additional
emission reductions would be necessary to satisfy Indiana's ozone
transport obligations. First, IDEM

presented evidence of downward trends of statewide ozone concentrations
and emissions, as well as a decrease in projected EGU emissions in 2023
relative to 2011. Despite these trends, however, the LADCO modeling
that Indiana depended on for its submittal still identified that
Indiana would contribute over 1 ppb to one or more receptors in 2023.
    As for downwind design value trends, EPA disagrees that IDEM's
reliance on trends data to conclude that the Harford, Maryland and
Richmond, New York monitors would reach attainment ``over time'' is
sufficient to support a conclusion that Indiana has no good neighbor
obligations. The states and EPA are to address interstate transport
obligations ``as expeditiously as practicable'' and no later than the
attainment schedule set in accordance with CAA section 181(a). See
North Carolina, 531 F.3d at 911-13; Wisconsin, 938 F.3d at 313-20;
Maryland, 958 F.3d at 1204; New York v. EPA, 964 F.3d 1214, 1226 (D.C.
Cir. 2020); New York v. EPA, 781 Fed. App'x 4, 6-7 (D.C. Cir. 2019).
IDEM asserted that EGUs are well controlled in Indiana and cited
several state and Federal regulations that EGUs may be subject to in
Indiana. In general, however, the listing of existing or on-the-way
control measures, whether approved into the state's SIP or not, does
not substitute for a complete Step 3 analysis under EPA's 4-Step
framework to define ``significant contribution.'' IDEM did not provide
an assessment of the overall effects of the identified control measures
or explain what the overall resulting air quality effects would be at
identified out of State receptors. IDEM did not perform an analysis of
all large NOX emitting EGU for factors that may affect the
facilities' emissions, including but not limited to allowance prices,
fuel prices, and enforceable limits. IDEM did not evaluate additional,
potential emissions control opportunities, or their costs or impacts,
or attempt to analyze whether, if applied more broadly across linked
states, the emissions reductions would constitute the elimination of
significant contribution on a regional scale. IDEM did not offer an
explanation as to whether any faster or more stringent emissions
reductions that may be available were prohibitively costly or
infeasible. Although EPA acknowledges states are not necessarily bound
to follow its own analytical framework at Step 3, IDEM did not attempt
to determine or justify an appropriate uniform cost-effectiveness
threshold. This would have been similar to the approach to defining
significant contribution that EPA has applied in prior rulemakings such
as CSAPR and or the CSAPR Update, even if such an analysis is not
technically mandatory. As discussed previously, both the LADCO modeling
relied on by the state and EPA's updated modeling indicates sources in
Indiana are linked to downwind air quality problems for the 2015 ozone
standard. However, Indiana's SIP submittal did not include an analysis
of potential NOX emissions control technologies, associated
costs, estimated emissions reductions, and downwind air quality in
order to determine whether the State had eliminated the State's
downwind contribution in amounts which will significantly contribute to
nonattainment or interfere with maintenance. Thus, EPA proposes to
disapprove Indiana's SIP submission on the separate, additional basis
that the SIP submittal did not assess additional emission control
opportunities.
    IDEM concluded it is not cost-effective to evaluate and implement
controls on non-EGUs in the state on the sole basis that the majority
of NOX emissions in the state come from EGUs. EPA cannot
accept the assertion as it is insufficiently supported. Cost-
effectiveness must be assessed in the context of the specific CAA
program; assessing cost-effectiveness in the context of ozone transport
should reflect a more comprehensive evaluation of the nature of the
interstate transport problem, the total emissions reductions available
at several cost thresholds, and the air quality impacts of the
reductions at downwind receptors. EPA notes that there are as many as
two dozen non-EGU facilities in Indiana with more than 300 tons per
year of NOX emissions each, but IDEM did not analyze control
opportunities at these sources at all in the SIP submission.
    IDEM also argued that additional emissions reductions from EGU and
non-EGU sources in Indiana ``are getting more difficult to mandate''
because of reduced effectiveness of controls to make significant
decreases in ozone values, operational concerns, and increased costs
for customers.\74\ Again, the SIP submission does not contain
sufficient evidence to support that conclusion. IDEM did not identify
controls that had reduced effectiveness or explain why they believed
they had reduced effectiveness. IDEM did not describe what any
operational concerns were for any controls, nor did IDEM provide any
information to support their claim that controls would increase costs
for consumers. While Indiana's existing control measures have
undoubtedly reduced the amount of transported ozone pollution to other
states and have contributed to the downward emissions trends and
improving air quality in the State as shown in the state's SIP
submittal, in the Revised CSAPR Update, EPA's analysis found that
despite Indiana's existing control programs, additional emissions
reductions were achievable from EGUs in the state, even under the level
of control stringency EPA determined appropriate to eliminate
significant contribution for the 2008 ozone NAAQS. In any case, EPA has
not established a benchmark cost-effectiveness threshold for good
neighbor obligations for the 2015 ozone NAAQS, and IDEM in its
submittal has not conducted an analysis to establish one for EPA to
evaluate.
---------------------------------------------------------------------------

    \74\ Indiana's SIP submission, Attachment 1 at 37.
---------------------------------------------------------------------------

    IDEM also identified several planned retirements or retrofits to
coal fired EGUs in Indiana that were not included in any modeling
available at the time of Indiana's submission and stated they would
reduce emissions several thousand tons beyond the modeling. Further,
EPA's assessment of future air quality conditions generally accounts
for on-the-books emission reductions and the most up-to-date forecast
of future emissions in the absence of the transport policy being
evaluated (i.e., base case conditions).\75\ As described in more detail
in Section I, EPA's latest projections of the baseline EGU emissions
uses the version 6--Summer 2021 Reference Case of the IPM. The IPM
version 6--Summer 2021 Reference Case uses the NEEDS v6 database as its
source for data on all existing and planned-committed units. Units are
removed from the NEEDS inventory only if a high degree of certainty
could be assigned to future implementation of the announced future
closure or retirement. Any retirements excluded from the NEEDS v6
inventory can be viewed in the NEEDS spreadsheet.\76\ EPA looked into
the upcoming retirements cited by IDEM and following the guidelines
regarding retirements for the IPM version--6 Summer 2021 Reference Case
certain units are not excluded from the NEEDS v6 inventory. There are
other retirements that were not included in the SIP submission that
were excluded

from the NEEDS v6 inventory for the 2023 projections. This includes
retirements at AES Petersburg, Merom, and RM Schahfer. In other words,
in general, any changes in the emissions inventory and on-the-books
controls relevant to emissions in 2023 have now been incorporated into
the EPA's modeling using the 2016v2 emissions platform, which projects
a continuing contribution from Indiana to out of state receptors above
a threshold of 1 percent of the NAAQS (at Steps 1 and 2) despite these
measures. Therefore, in light of continuing contribution to out of
state receptors from Indiana notwithstanding these identified
retirements, IDEM's SIP submission should have evaluated the
availability of additional air quality controls to improve downwind air
quality at nonattainment and maintenance receptors at Step 3
Furthermore, under the Wisconsin decision, states and EPA may not delay
implementation of measures necessary to address good neighbor
requirements beyond the next applicable attainment date without a
showing of impossibility or necessity. See 938 F.3d at 320. The IDEM's
submittal is insufficient to the extent the implementation timeframes
for several claimed expected shutdowns were left unidentified,
unexplained, or too uncertain to permit EPA to form a judgment as to
whether the timing requirements for good neighbor obligations have been
met.
---------------------------------------------------------------------------

    \75\ See 81 FR 74504 at 74517; 85 FR 68964 at 68979.
    \76\ The ``Capacity Dropped'' and the ``Retired Through 2023''
worksheets in NEEDS lists all units that are removed from the NEEDS
v6 inventory--NEEDS v6 Summer 2021 Reference Case. This data can be
found on EPA's website at: https://www.epa.gov/airmarkets/national-electric-energy-data-system-needs-v6.
---------------------------------------------------------------------------

    Additionally, IDEM explained in only the most general terms how the
unaccounted emissions reductions would influence downwind air quality
or Indiana's contributions to other state. IDEM also did not quantify
how the emissions reductions they estimated would impact air quality at
downwind receptors or Indiana's contributions. IDEM did not demonstrate
that the downwind improvements from these regulations and programs
would be sufficient to eliminate Indiana's linkages or prohibit the
State's emissions in amounts that will contribute significantly to
nonattainment or interfere with maintenance of the NAAQS in any other
state.
    IDEM also made several arguments related to potential flexibilities
identified in Attachment A to the March 2018 memorandum.\77\ As
explained previously in Section I, the concepts presented in Attachment
A to the March 2018 memorandum were neither guidance nor determined by
EPA to be consistent with the CAA. EPA will thoroughly review the
technical and legal justifications IDEM put forward in their attempt to
use a potential flexibility from Attachment A to the March 2018
memorandum.
---------------------------------------------------------------------------

    \77\ Based on the reference to the potential flexibilities in
Attachment A to the March 2018 memorandum on page 2 of Attachment 1
to Indiana's SIP submission, EPA assumes the reference to
``flexibilities'' on page 38 of Attachment 1 likewise references
Attachment A to the March 2018 memorandum.
---------------------------------------------------------------------------

    IDEM suggested that local emissions reductions from the
jurisdiction where downwind receptors are located should first be
implemented and accounted for before imposing obligations on upwind
states under the interstate transport provision. IDEM represented that
EPA had concluded that monitors in the Northeast ``are impacted from
more local emissions'' by citing a May 14, 2018 presentation. The
purpose of that presentation was to share a technical, exploratory
analysis of ozone trends. IDEM misrepresented the contents of the
presentation, which labeled the results as ``preliminary'' and
indicated that ``[f]urther exploration of the relative contribution
from various source sectors within the NE Corridor and in nearby upwind
states might also be informative.'' \78\ These preliminary results of
that analysis are generally consistent with EPA's updated modeling
using the 2016v2 emissions platform. Although EPA's modeling shows that
a large portion of the transport problem affecting the receptors in
Coastal Connecticut is indeed from sources within the Ozone Transport
Region (OTR), a substantial portion of the transport problem at these
receptors, on the order of 25 percent, is the result of transport from
states outside the OTR. However, the relevance of that presentation to
the evaluation of Indiana's good neighbor obligations is not clear. As
already discussed, the statute and the case law (particularly the
holdings in Wisconsin and Maryland) make clear that good neighbor
obligations are not merely supplementary to or deferable until after
local emission reductions are achieved. Further, based on EPA's
modeling released with the March 2018 memorandum, nearly all of the
receptors to which Indiana is linked are also heavily impacted by
distant upwind state emissions in addition to local sources and sources
in neighboring states. The Wisconsin decision's holding in regard to
international contribution (discussed in more detail later) is equally
applicable to an upwind state's claims that some other state's
emissions, or local emissions, are more to blame than its own
emissions. See 938 F.3d 303 at 323-25 (``an upwind state can
`contribute' to downwind nonattainment even if its emissions are not
the but-for cause'').
---------------------------------------------------------------------------

    \78\ Indiana's SIP submission, Appendix E at 4, 17.
---------------------------------------------------------------------------

    There is nothing in the CAA that supports Indiana's position on
local sources, and Indiana does not provide grounds on which to approve
its SIP submission. The D.C. Circuit has held on five different
occasions that the timing framework for addressing interstate transport
obligations must be consistent with the downwind areas' attainment
schedule. In particular, for the ozone NAAQS, the states and EPA are to
address interstate transport obligations ``as expeditiously as
practicable'' and no later than the attainment schedule set in
accordance with CAA section 181(a). See North Carolina, 531 F.3d at
911-13; Wisconsin, 938 F.3d at 313-20; Maryland, 958 F.3d at 1204; New
York v. EPA, 964 F.3d 1214, 1226 (D.C. Cir. 2020); New York v. EPA, 781
Fed. App'x 4, 6-7 (D.C. Cir. 2019). The court in Wisconsin explained
its reasoning in part by noting that downwind jurisdictions often may
need to heavily rely on emissions reductions from upwind states in
order to achieve attainment of the NAAQS, 938 F.3d at 316-17; such
states would face increased regulatory burdens including the risk of
bumping up to a higher nonattainment classification if attainment is
not reached by the relevant deadline, Maryland, 958 F.3d at 1204. The
statutory framework of the CAA and these cases establish clearly that
states and EPA must address interstate transport obligations in line
with the attainment schedule provided in the CAA in order to timely
assist downwind states in attaining and maintain the NAAQS, and this
schedule is ``central to the regulatory scheme.'' Wisconsin, 938 F.3d
at 316 (quoting Sierra Club v. EPA, 294 F.3d 155, 161 (D.C. Cir.
2002)).
    IDEM similarly suggested that international and offshore emissions
contributions should be part of the good neighbor calculus. IDEM's
reasoning related to international and offshore emissions is
inapplicable to the requirements of CAA section 110(a)(2)(D)(i)(I). The
good neighbor provision requires states and EPA to address interstate
transport of air pollution that contributes to downwind states' ability
to attain and maintain NAAQS. Whether emissions from other states or
other countries also contribute to the same downwind air quality issue
is irrelevant in assessing whether a downwind state has an air quality
problem, or whether an upwind state is significantly contributing to
that problem. States are not obligated under CAA section
110(a)(2)(D)(i)(I) to reduce

emissions sufficient on their own to resolve downwind receptors'
nonattainment or maintenance problems. Rather, states are obligated to
eliminate their own ``significant contribution'' or ``interference''
with the ability of other states to attain or maintain the NAAQS.
    Indeed, the D.C. Circuit in Wisconsin specifically rejected
petitioner arguments suggesting that upwind states should be excused
from good neighbor obligations on the basis that some other source of
emissions (whether international or another upwind state) could be
considered the ``but-for'' cause of downwind air quality problem. See
938 F.3d at 323-324. The court viewed petitioners' arguments as
essentially an argument ``that an upwind state `contributes
significantly' to downwind nonattainment only when its emissions are
the sole cause of downwind nonattainment.'' Id. at 324. The court
explained that ``an upwind state can `contribute' to downwind
nonattainment even if its emissions are not the but-for cause.'' Id. at
324-325. See also Catawba County v. EPA, 571 F.3d 20, 39 (D.C. Cir.
2009) (rejecting the argument ``that `significantly contribute'
unambiguously means `strictly cause''' because there is ``no reason why
the statute precludes EPA from determining that [an] addition of
[pollutant] into the atmosphere is significant even though a nearby
county's nonattainment problem would still persist in its absence'');
Miss. Comm'n on Envtl. Quality v. EPA, 790 F.3d 138, 163 n.12 (D.C.
Cir. 2015) (observing that the argument that ``there likely would have
been no violation at all . . . if it were not for the emissions
resulting from [another source]'' is ``merely a rephrasing of the but-
for causation rule that we rejected in Catawba County''). Therefore, a
state is not excused from eliminating its significant contribution on
the basis that international emissions also contribute some amount of
pollution to the same receptors to which the state is linked.
    IDEM also calculated Indiana's portion of contribution to the
Harford, Maryland receptor was 0.077 ppb, and determined that Indiana
would need to reduce its contribution by 0.0077 ppb (based on a
contribution threshold of 1 ppb) to bring the Maryland receptor into
attainment. IDEM argued that 0.0077 ppb is well within the error of the
model and would be ``difficult'' to translate into an emission
reduction requirement.\79\ We first note that this approach is a
deviation from EPA's traditional approach of apportioning upwind-state
responsibility at Step 3 using a uniform cost of control metric set at
a level that maximizes cost-effectiveness of emissions reductions in
relation to downwind state impacts across all linked states. Thus, this
is not how EPA has interpreted the statutory term ``significant'' in
the past, and EPA does not reach a conclusion whether this approach
would be approvable, had IDEM had imposed emissions reductions in line
with this logic.
---------------------------------------------------------------------------

    \79\ Indiana's SIP submission, Attachment 1 at 42.
---------------------------------------------------------------------------

    We do not need to reach that point in the analysis, however,
because, having selected that approach to defining its obligations,
IDEM proceeded to ignore the result. IDEM's submission identified
Indiana's proportional contribution as 0.077 ppb to the Harford,
Maryland receptor. Having acknowledged Indiana was responsible for
eliminating up to 0.0077 ppb of contribution, IDEM claimed that because
that amount was within the ``error of the model'' that it would be
``difficult'' to require that amount of reductions from Indiana
sources.
    This argument does not rise to the level of acceptable proof. EPA
has routinely been capable of successfully implementing good neighbor
obligations through the CSAPR framework, and achieving significant
downwind air quality improvements through upwind-state reductions, at
levels of ``significant contribution'' comparable or even less than
those found in Indiana's submission, irrespective of alleged modeling
errors. See Wisconsin, 938 F.3d at 322-23 (rejecting Wisconsin's
argument that it should not face good neighbor obligations on the basis
that its emission reductions would only improve a downwind receptor by
two ten-thousandths of a part per billion).
    After measuring Indiana's significant contribution, IDEM suggested
that modeling uncertainty was too great to either require emissions
reductions. But IDEM had measured the state's significant contribution
and was therefore identifying the measurable amount of significant
contribution the state was legally responsible for eliminating. See
Michigan v. EPA, 213 F.3d 663, 683-84 (D.C. Cir. 2000) (significant
contribution must be ``measurable''). Further, scientific uncertainty
may only be invoked to avoid comporting with the requirements of the
CAA when ``the scientific uncertainty is so profound that it precludes
. . . reasoned judgment'' Massachusetts v. EPA, 127 S.Ct. 1438 (2007).
See Wisconsin, 938 F.3d at 318-19 (``Scientific uncertainty, however,
does not excuse EPA's failure to align the deadline for eliminating
upwind States' significant contributions with the deadline for downwind
attainment of the NAAQS.''). See also EME Homer City, 795 F.3d 118,
135-36 (``We will not invalidate EPA's predictions solely because there
might be discrepancies between those predictions and the real world.
That possibility is inherent in the enterprise of prediction.'').
IDEM's arguments related to modeling uncertainty do not establish a
level of uncertainty so high as to preclude reasoned judgement.
    IDEM provided an analysis of back trajectories from the Harford and
Richmond receptors to support its contention that Indiana does not
contribute significantly to nonattainment or maintenance at those
monitors, and that the receptors are more impacted by local emissions
anyway. IDEM also relied on an EPA presentation from 2018 to support
this conclusion.
    As already discussed, the statute and the case law (particularly
the holdings in Wisconsin and Maryland) make clear that good neighbor
obligations are not merely supplementary to or deferable until after
local emission reductions are achieved. Further, all of the receptors
to which Indiana is linked are heavily impacted by upwind state
emissions in addition to local sources and conditions. The Wisconsin
decision's holding in regard to international contribution (discussed
previously) is equally applicable to an upwind state's claims that some
other state's emissions, or local emissions, are more to blame than its
own emissions. See 938 F.3d 303 at 323-25 (``an upwind state can
`contribute' to downwind nonattainment even if its emissions are not
the but-for cause'').
    Further, EPA finds Indiana's back trajectory analysis to be
deficient in proving that Indiana does not contribute significantly to
nonattainment or maintenance at the Harford and Richmond monitors that
the State was linked to in the LADCO modeling. Indiana's back
trajectory analysis shows a linkage between Indiana and the monitors
when evaluating two altitudes, 10 meters and 750 meters, on several of
the exceedance days at these monitoring sites. By only evaluating two
altitudes, Indiana neglects to consider the wide range of heights that
might show back trajectories leading back to Indiana, potentially
further tying the state to more exceedance events. Furthermore, 10
meters is too low of an altitude to measure long range transport and it
would have been appropriate for Indiana to analyze several higher
altitudes to bolster its back trajectory analysis.

    Back trajectories alone are not sufficient to disconnect upwind
States from downwind receptors. Relying solely on back trajectories for
establishing linkages neglects the myriad of factors, most importantly
photochemical reactions, that are important for determining the
magnitude of ozone and precursor transport from upwind states to
downwind receptors. In this regard, EPA and LADCO modeling which
accounts for 3 dimensional meteorological conditions, regional
emissions, and photochemical reactions is the most complete, and
technically sound method to establish linkages between upwind states
and downwind nonattainment and maintenance receptors.
    The information and claims presented by IDEM did not provide
sufficient evidence to support alternative conclusions that EPA is
proposing to make in this action: Namely, that several receptors exist,
Indiana contributes to those receptors above a 1 percent of the NAAQS
contribution threshold, and that Indiana continues to have good
neighbor obligations that need to be addressed for the 2015 ozone
NAAQS. We therefore propose that Indiana was required to analyze
emissions from the sources and other emissions activity from within the
state to determine whether its contributions were significant, and we
propose to disapprove its submission because Indiana failed to do so.
5. Evaluation of Information Provided Regarding Step 4
    Step 4 of the 4-Step interstate transport framework calls for
development of permanent and federally enforceable control strategies
to achieve the emissions reductions determined to be necessary at Step
3 to eliminate significant contribution to nonattainment or
interference with maintenance of the NAAQS. IDEM identified the State's
existing NOX control measures, consent decree requirements,
and future fuel switches and retirements for large EGUs and non-EGUs
for the years 2008 through 2017 \80\ States may not rely on non-SIP
measures to meet SIP requirements. See CAA section 110(a)(2)(D) (``Each
such [SIP] shall . . . contain adequate provisions . . . .''). See also
CAA section 110(a)(2)(A); Committee for a Better Arvin v. U.S. E.P.A.,
786 F.3d 1169, 1175-76 (9th Cir. 2015) (holding that measures relied on
by state to meet CAA requirements must be included in the SIP).
However, the state did not revise its SIP to include these emission
reductions in a revision to its SIP to ensure the reductions were
permanent and enforceable. As a result, EPA proposes to disapprove
Indiana's submittal on the separate, additional basis that Indiana has
not included permanent and enforceable emissions reductions in its SIP
as necessary to meet the obligations of CAA section
110(a)(2)(d)(i)(I).6.
---------------------------------------------------------------------------

    \80\ Pointing to anticipated upcoming emission reductions, even
if they were not included in the analysis at Steps 1 and 2, is not
sufficient as a Step 3 analysis, for the reasons discussed in
Section II.B.4. In this section, we explain that to the extent such
anticipated reductions are not included in the SIP and rendered
permanent and enforceable, reliance on such anticipated reductions
is also insufficient at Step 4.
---------------------------------------------------------------------------

6. Conclusion
    Based on EPA's evaluation of Indiana's SIP submission, EPA is
proposing to find that the portion of Indiana's November 12, 2018 SIP
submission addressing CAA section 110(a)(2)(D)(i)(I) does not meet the
state's interstate transport obligations, because it fails to contain
the necessary provisions to eliminate emissions that will contribute
significantly to nonattainment or interfere with maintenance of the
2015 ozone NAAQS in any other state.

C. Michigan

1. Evaluation of Information Provided by Michigan Regarding Step 1
    At Step 1 of the 4-step interstate transport framework, Michigan
relied primarily on the LADCO modeling released in 2018 to identify
nonattainment and maintenance receptors in 2023. As described
previously in this action, LADCO performed a modeling demonstration
like that of EPA modeling released with the March 2018 memorandum,
except with use of the ERTAC EGU Tool to replace specific EGU
information. LADCO identified nonattainment and maintenance receptors
using EPA methodology. EGLE elected to rely on LADCO's ``water only''
modeling results, but also presented results from EPA's modeling
released with the March 2018 memorandum. EGLE noted that in general,
design values in the LADCO modeling were lower. However, since new
modeling has been performed by EPA which includes updated emission data
using the 2016v2 platform, EPA proposes to primarily rely on the most
recent modeling to identify nonattainment and maintenance receptors in
2023. Nonetheless, the alternative modeling relied on by EGLE also
identified a number of nonattainment and maintenance receptor sites in
2023. See Table 2 on page 14 of EGLE's submittal. Thus, even under its
alternative modeling of 2023, EGLE acknowledges in its submittal the
existence of several nonattainment and maintenance receptors.
2. Evaluation of Information Provided by the State Regarding Step 2
    Although Michigan relied on alternative modeling to EPA's modeling,
EGLE acknowledged in their SIP submission that Michigan is linked above
either a 1 percent of the NAAQS or 1 ppb or threshold to one or more
downwind receptors in 2023 (1.85 ppb to Sheboygan, Wisconsin (Site ID:
36-081-0124), 1.22 ppb to Queens, New York (Site ID: 36-085-0067), and
1.03 ppb to Richmond, New York (Site ID: 55-117-0006)). Because the
alternative modeling relied on by the state also demonstrates that a
linkage exists between the state and downwind receptors at Step 2, EPA
need not conduct a comparative assessment of the alternative modeling;
the state concedes that it is linked. EGLE's analysis corroborates the
conclusion in EPA's most recent modeling, described in the next
section.
    EGLE, relying on a concept from outside parties listed in
Attachment A to the March 2018 memorandum, attempted to justify the use
of a 1 ppb threshold at Step 2 to identify whether the state was
``linked'' to a projected downwind nonattainment or maintenance
receptor. In part, EGLE attempted to justify the use of a 1 ppb
contribution threshold based on the 2018 PSD SIL guidance document.
EGLE also referenced EPA's August 2018 memorandum, which said that with
appropriate additional analysis it may be reasonable for states to use
a 1 ppb contribution threshold, as an alternative to a one percent
threshold, at Step 2 of the 4-Step interstate transport framework for
the purposes of identifying linkages to downwind receptors. As
explained in Section I above, the concepts presented in Attachment A to
the March 2018 memorandum were neither guidance nor determined by EPA
to be consistent with the CAA. Further, EGLE did not explain the
relevance of the SILs Guidance to which it referred. This guidance
relates to a different provision of the Clean Air Act regarding
implementation of the prevention of significant deterioration (PSD)
permitting program, i.e., a program that applies in areas that have
been designated attainment of the NAAQS, and it is not applicable to
the good neighbor provision, which requires

states to eliminate significant contribution or interference with
maintenance of the NAAQS at known and ongoing air quality problem areas
in other states. Further, it is not correct to conflate the use of the
term ``significance'' as used in the SIL guidance, with the term
``contribution,'' which is the appliable statutory term that EPA
applies at Step 2 of the 4-step interstate transport framework.
(``Significance'' within the 4-step framework is evaluated at Step 3
through a multifactor analysis, for those states that are determined to
``contribute'' to downwind receptors at Steps 1 and 2. See Section
I.D.4 above.) Given the fundamentally different statutory objectives
and context, EPA disagrees with EGLE's contention that the SIL guidance
is applicable in the good neighbor context.
    EGLE's attempt to show ``inflection points'' through collectively
presenting contribution data at each linked receptor and its claim that
1 ppb reflects the most meaningful inflection point are likewise not
compelling. The presented data show a range of upwind contribution
levels captured by different contribution thresholds depending on which
receptor is analyzed. Certain receptors show a substantial downward
trend in captured total upwind contribution well before a threshold of
1 ppb. Therefore, EPA does not find this evidence supportive of a 1 ppb
threshold.
    EPA does not accept Michigan's position that a 1 percent of the
NAAQS contribution threshold at Step 2 ``may not be appropriate'' for
the 2015 ozone NAAQS due to modeling biases and errors.\81\ The
explanation for how the 1 percent contribution threshold was originally
derived is available in the 2011 CSAPR rulemaking. See 76 FR 48208,
48237-38. Further, in the CSAPR Update, EPA re-analyzed the threshold
for purposes of the 2008 ozone NAAQS and determined it was appropriate
to continue to apply this threshold. EPA compared the 1 percent
threshold to a 0.5 percent of NAAQS threshold and a 5 percent of NAAQS
threshold. EPA found that the lower threshold did not capture
appreciably more upwind state contribution compared to the 1 percent
threshold, while the 5 percent threshold allowed too much upwind state
contribution to drop out from further analysis. See Final CSAPR Update
Air Quality Modeling TSD, at 27-30 (EPA-HQ-OAR-2015-0596-0144). If EPA
were to apply this analysis to the 2015 ozone NAAQS using the updated
modeling based on the 2016v2 emissions platform, a 5 percent of the
NAAQS contribution threshold (i.e., 3.5 ppb) only captures
approximately 50 percent of the total upwind contribution. Compared to
a 1 percent threshold, a 5 percent threshold would, on average, forgo
27 percent) of the total upwind contribution. As EPA noted in the
August 2018 memorandum, the use of a 2 ppb contribution threshold under
the modeling released with the March 2018 memorandum would only capture
about 55 percent of all upwind contributions, and therefore ``emission
reductions from states linked at that higher threshold may be
insufficient to address collective upwind state contribution to
downwind air quality problems.''\31\
---------------------------------------------------------------------------

    \81\ Michigan's SIP submission at 16.
---------------------------------------------------------------------------

    With these figures in mind, EGLE's claim based on unwarranted
concerns over modeling uncertainty cannot be accepted. Both LADCO's and
EPA's modeling techniques are sufficiently reliable and fit for the
purpose to measure upwind contribution levels down to at least one 1
percent of the NAAQS. EPA's recommended model attainment test is based
on application of the model in a relative sense rather than relying
upon absolute model predictions.\82\ All models have limitations
resulting from uncertainties in inputs and scientific formulation. To
minimize the effects of these uncertainties, the modeling is anchored
to base period measured data in EPA's guidance approach for projecting
design values. Notably, EPA also uses our source apportionment modeling
in a relative sense when calculating the average contribution metric
(used to identify linkages). In this method the magnitude of the
contribution metric is tied to the magnitude of the projected average
design value which is tied to the base period average measured design
value. EPA's guidance has not established a bright-line criteria for
judging whether or not statistical measures of model performance
constitute acceptable or unacceptable model performance. So, contrary
to what Michigan appears to be claiming with regards to modeling
biases, there are no EPA recommended measures of allowable error.
Although EPA does not typically focus on using particular benchmarks as
the sole criteria for model performance, EPA notes that the model
performance for the updated modeling based on the 2016v2 emissions
platform is generally within the benchmarks recommended by Emery.\83\
---------------------------------------------------------------------------

    \82\ See Section 4.1' ``Overview of Modeled Attainment Test in
EPA Modeling Guidance for Demonstrating Air Quality Goals for Ozone,
PM2.5, and Regional Haze. November 2018. EPA 454-R-18-
009. https://www.epa.gov/scram/sip-modeling-guidance-documents.
    \83\ Christopher Emery, Zhen Liu, Armistead G. Russel, M. Talat
Odman, Greg Yarwood and Naresh Kumar (2017). Recommendations on
statistics and benchmarks to assess photochemical model performance,
Journal of the Air & Waste Management Association, 67:5,582-598,
DOI: 10.1080/10962247.2016.1265027.
---------------------------------------------------------------------------

    EPA has successfully applied a 1 percent of the NAAQS threshold to
identify linked upwind states in three prior rulemakings. And the D.C.
Circuit has also declined to establish bright line criteria for model
performance. In upholding EPA's approach to evaluating interstate
transport in CSAPR, the D.C. Circuit held that they would not
``invalidate EPA's predictions solely because there might be
discrepancies between those predictions and the real world. That
possibility is inherent in the enterprise of prediction.'' EME Homer
City Generation, L.P. v. EPA, 795 F.3d 118, 135 (2015). The court
continued to note that ``the fact that a `model does not fit every
application perfectly is no criticism; a model is meant to simplify
reality in order to make it tractable.' '' Id. at 135-36 (quoting
Chemical Manufacturers Association v. EPA, 28 F.3d 1259, 1264 (D.C.
Cir. 1994).
    EPA's August 2018 memorandum provided that whether use of a 1 ppb
threshold is appropriate must be based on an evaluation of state-
specific circumstances, and no such evaluation was included in the
submission. EPA's experience with the alternative Step 2 thresholds is
further discussed in Section I.D.3.i. As discussed there, EPA is
considering withdrawing the August 2018 memorandum.
    Based on EPA's updated modeling (as well as the LADCO's 2018
modeling (with water) the state elected to rely on in its SIP
submission), the state is projected to contribute greater than both the
1 percent and alternative 1 ppb thresholds. While EPA does not, in this
action, approve of the state's application of the 1 ppb threshold,
based on its linkages greater than 1 ppb to projected downwind
nonattainment or maintenance receptors, the state's use of this
alternative threshold at Step 2 of the 4-Step interstate framework is
inconsequential to our action on this SIP submission.
3. Results of EPA's Step 1 and Step 2 Modeling and Findings for
Michigan
    As described in Section I, EPA performed air quality modeling using
the 2016v2 emissions platform to project design values and
contributions for 2023. These data were examined to determine if
Michigan contributes at or above the threshold of 1 percent of the

2015 ozone NAAQS (0.70 ppb) to any downwind nonattainment or
maintenance receptor. As shown in Table 4, the data \84\ indicate that
in 2023, emissions from Michigan contribute greater than one percent of
the standard to nonattainment or maintenance-only receptors in
Illinois, Connecticut, Wisconsin, and Pennsylvania.\85\
---------------------------------------------------------------------------

    \84\ Design values and contributions at individual monitoring
sites nationwide are provide in the file
``2016v2_DVs_state_contributions.xlsx'' which is included in docket
ID No. EPA-HQ-OAR-2021-0663.
    \85\ These modeling results are consistent with the results of a
prior round of 2023 modeling using the 2016v1 emissions platform
which became available to the public in the fall of 2020 in the
Revised CSAPR Update, as noted in Section I. That modeling showed
that Illinois had a maximum contribution greater than 0.70 ppb to at
least one nonattainment or maintenance-only receptor in 2023. These
modeling results are included in the file ``Ozone Design Values &
Contributions Revised CSAPR Update.xlsx'' in docket ID No. EPA-HQ-
OAR-2021-0663.

                      Table 4--Michigan Linkage Results Based on EPA Updated 2023 Modeling
----------------------------------------------------------------------------------------------------------------
                                                                   2023  Average   2023  Maximum     Michigan
         Receptor ID               Location       Nonattainment/   design value    design value    contribution
                                (county, state)    maintenance         (ppb)           (ppb)           (ppb)
----------------------------------------------------------------------------------------------------------------
170314201....................  Cook, IL........  Maintenance....            69.9            73.4            1.67
170310076....................  Cook, IL........  Maintenance....            69.3            72.1            1.54
90099002.....................  New Haven, CT...  Nonattainment..            71.8            73.9            1.27
170317002....................  Cook, IL........  Maintenance....            70.1            73.0            1.26
170310032....................  Cook, IL........  Maintenance....            69.8            72.4            1.21
550590025....................  Kenosha, WI.....  Maintenance....            69.2            72.3            1.17
550590019....................  Kenosha, WI.....  Nonattainment..            72.8            73.7            1.07
90010017.....................  Fairfield, CT...  Nonattainment..            73.0            73.7            1.07
551010020....................  Racine, CT......  Nonattainment..            71.3            73.2            1.02
90013007.....................  Fairfield, CT...  Nonattainment..            74.2            75.1            0.94
170310001....................  Cook, IL........  Maintenance....            69.6            73.4            0.93
90019003.....................  Fairfield, CT...  Nonattainment..            76.1            76.4            0.92
420170012....................  Bucks, PA.......  Nonattainment..            70.7            72.2            0.75
----------------------------------------------------------------------------------------------------------------

    Therefore, based on EPA's evaluation of the information submitted
by EGLE, and based on EPA's most recent modeling results for 2023, EPA
proposes to find that Michigan is linked at Steps 1 and 2 and has an
obligation to assess potential emissions reductions from sources or
other emissions activity at Step 3 of the 4-step framework. EPA
therefore will proceed to Step 3 of the 4-step interstate transport
framework to assess the arguments the state presented as to why,
despite this linkage, the state should not be considered to
significantly contribute to nonattainment or interfere with maintenance
of the NAAQS in any other state.
4. Evaluation of Information Provided Regarding Step 3
    At Step 3 of the 4-step interstate transport framework, a state's
emissions are further evaluated, in light of multiple factors,
including air quality and cost considerations, to determine what, if
any, emissions significantly contribute to nonattainment or interfere
with maintenance and, thus, must be eliminated under CAA section
110(a)(2)(D)(i)(I).
    To effectively evaluate which emissions in the state should be
deemed ``significant'' and therefore prohibited, states generally
should prepare an accounting of sources and other emissions activity
for relevant pollutants and assess potential, additional emissions
reduction opportunities and resulting downwind air quality
improvements. EPA has consistently applied this general approach (i.e.,
Step 3 of the 4-step interstate transport framework) when identifying
emissions contributions that the Agency has determined to be
``significant'' (or interfere with maintenance) in each of its prior
Federal, regional ozone transport rulemakings, and this interpretation
of the statute has been upheld by the Supreme Court. See EME Homer
City, 572 U.S. 489, 519 (2014). While EPA has not directed states that
they must conduct a Step 3 analysis in precisely the manner EPA has
done in its prior regional transport rulemakings, state implementation
plans addressing the obligations in CAA section 110(a)(2)(D)(i)(I) must
prohibit ``any source or other type of emissions activity within the
State'' from emitting air pollutants which will contribute
significantly to downwind air quality problems. Thus, states must
complete something similar to EPA's analysis (or an alternative
approach to defining ``significance'' that comports with the statute's
objectives) to determine whether and to what degree emissions from a
state should be ``prohibited'' to eliminate emissions that will
``contribute significantly to nonattainment in, or interfere with
maintenance of'' the NAAQS in any other state.
    EGLE did not conduct a sufficient step 3 analysis in Michigan's SIP
submission. As explained previously, at Step 3 EGLE instead applied a
weight of evidence analysis to argue that the state needed no
additional emission reductions despite concluding Michigan was linked
to three receptors at Step 2. The evidence presented in EGLE's
submittal consisted primarily of support for the argument that upwind
states should have a lower responsibility to other states when the
upwind state is only linked to maintenance receptors. EGLE's analysis
focused on the Sheboygan, Wisconsin maintenance receptor (Site ID: 36-
081-0124), as EGLE concluded it was the receptor to which Michigan was
projected to contribute the most in 2023 at 1.85 ppb. EGLE also relied
on several ideas in Attachment A to the March 2018 memorandum to
further discount the importance of its own emissions. As noted in
Section I, the ideas listed in Attachment A to the March 2018
memorandum were not agency guidance nor had EPA determined them to be
consistent with the requirements of the CAA. EPA will thoroughly review
the technical and legal justifications ELGE made put forward in their
attempt to use them as flexibilities.
    In its submittal, EGLE cited a concept in Attachment A to the March
2018 memorandum to ``[c]onsider whether the remedy for upwind states
linked to maintenance receptors could be less

stringent than those linked to nonattainment receptors'' and argued
that because the CAA incudes different SIP development requirements for
nonattainment and maintenance areas, that likewise nonattainment and
maintenance areas should be treated differently in good neighbor SIPs.
EGLE posited that because the CAA does not require emission reductions
from maintenance areas, then upwind states can potentially make a
sufficient showing they have no obligation to reduce emissions to
monitors in other states projected to be maintaining the NAAQS. EGLE
specifically noted that (1) the projected exceedance at the Sheboygan,
Wisconsin receptor is very small, (2) the majority of the projected
contribution to the Sheboygan, Wisconsin receptor is from federally
regulated sources or sources Michigan cannot otherwise regulate, (3)
Michigan's projected contribution to all three linked receptors is
small compared to the projected contribution from other states and
sources, (4) there are large projected contributions to the Sheboygan,
Wisconsin receptor from international emissions, (5) Michigan's
contributions to projected exceedance at the three maintenance
receptors are small relative to other sources that also contribute more
than 1 ppb to those receptors, (6) the modeling variability is greater
than Michigan's contributions to the amount of the projected exceedance
at each linked receptor and (7) there is a downward emissions trend in
Michigan.
    As a general matter, EPA disagrees with EGLE's premise that if no
emission reductions are needed for the receptor to which Michigan
contributes the most, that automatically no emission reductions are
needed for the other receptors to which Michigan is linked. EGLE
unreasonably failed to analyze receptor-specific circumstances present
at other receptors to which it was linked, and this is particularly the
case because EGLE chose to rely so heavily on receptor-specific
information to support their conclusions with respect to the Sheboygan
receptor. Further, while the set of receptors to which Michigan is
linked has changed in the most recent modeling (and now includes
nonattainment receptors), EPA disagrees with Michigan's arguments to
the extent such reasoning could be applied to Michigan's linkages
identified in EPA's 2016v2 emissions platform modeling.
    EGLE argued that because the Sheboygan, Wisconsin receptor, had a
small projected exceedance over the NAAQS, requiring additional
emission reductions in Michigan would be ``premature'' and
``burdensome.'' \86\ EGLE's premise goes beyond the concept in
Attachment A to the 2018 memorandum that emission-reduction obligations
as to maintenance receptors may be different; rather, EGLE argues that
not only should Michigan have lower obligations with respect to
maintenance receptors, but no obligations at all. Under the D.C.
Circuit's decision in North Carolina, states and EPA are required to
give independent significance to the ``interference with maintenance''
prong of section 110(a)(2)(D)(i)(I). 531 F.3d at 910. Since CSAPR,
EPA's nationally consistent policy framework for addressing interstate
ozone transport has given meaning to this prong through a separate
definition of maintenance receptors at step 1 of the 4-step interstate
transport framework. For states linked only to those receptors, EPA has
found it appropriate to apply an emissions control solution that is
uniform with the strategy applied for states that are linked to
nonattainment receptors. See 76 FR at 48271. EPA's approach to
addressing interference with maintenance under prong 2 for ozone NAAQS
has been upheld twice, including on remand from the Supreme Court
decision EGLE cited. See EME Homer City Generation, L.P., 795 F.3d at
136; Wisconsin, 938 F.3d at 325-27. See also 86 FR at 23074.
Particularly given this context, Michigan's SIP does not provide
sufficient evidence to support less stringent or even no standards of
emissions reductions relative to what would result from EPA's
historical approach of addressing emissions activities from upwind
states that are linked to maintenance-only receptors.
---------------------------------------------------------------------------

    \86\ See Michigan SIP submission p. 20.
---------------------------------------------------------------------------

    Further, EPA believes it would be inconsistent with the CAA for EPA
to identify receptors that are at risk of NAAQS violations given
certain conditions due to transported upwind emissions and then not
prohibit the emissions that place the receptor at risk. The Supreme
Court held that it was a permissible interpretation of the statute to
apportion responsibility for states linked to nonattainment receptors
considering ``both the magnitude of upwind states' contributions and
the cost associated with eliminating them.'' EME Homer City, 572 U.S.
at 518-19. It is equally reasonable and permissible to use these
factors to apportion responsibility among upwind states linked to
maintenance receptors because the goal in both instances is to prohibit
the ``amounts'' of pollution that will either significantly contribute
to nonattainment or interfere with maintenance of the NAAQS downwind.
See Id. 515 n.18 (finding EPA's uniform-cost approach reasonable as to
both prongs of the good neighbor provision). EPA's updated modeling
indicates that Michigan will remain linked to downwind nonattainment
and maintenance receptors for the 2015 ozone standard at least through
2023. Consequently, EPA believes EGLE's assertion that upwind states
linked to maintenance-only receptors should be held to less stringent
standards of emissions reductions (as compared to states linked to a
nonattainment receptor) is inappropriate, whether applied to its
downwind linkages in either the modeling EGLE relied on or in EPA's
more recent modeling.
    EGLE also claimed that Attachment A to the March 2018 memorandum
suggested states linked only to maintenance receptors should consider
whether emissions reduction factors should be influenced by high
international contributions and high contributions from other states
and sources. As a concept presented by outside parties, Attachment A to
the March 2018 memorandum listed an idea that states may consider
whether air quality, cost, or emission reduction factors should be
weighted differently in areas where international contributions are
relatively high. EPA did not at the time endorse this concept, nor does
it do so now. However, EGLE did not present an approach or explain how
international contributions to the linked receptors should influence
the weighting of air quality, cost, or emission reductions at Step 3.
Rather, EGLE suggested that if a receptor is near an international
border, then international contribution could simply be removed from
that monitor's projected design value. This is neither appropriate nor
acceptable under the good neighbor provision or any other provision of
the Clean Air Act. Michigan's approach effectively takes the position
that no air quality problem should be deemed to exist at a downwind
receptor location under the false assumption that the international
portion of emissions affecting that area simply do not exist. EPA
categorically rejects this approach as an entirely unacceptable form of
air quality planning.
    EGLE further cited contributions from other states and sources to
the linkages it identified to conclude it would be ``unreasonable'' for
linked states with relatively low contributions to reduce their
contributions.\87\ The Step 2 threshold (whether at 1 percent or 1

ppb) is intended to reflect the ``collective contribution'' nature of
the interstate ozone transport problem and the complexity of the
various linkages among states. Cf. EME Homer City, 572 U.S. at 515-16.
The threshold functions as a screening step toward a more detailed
analysis of emission-reduction opportunities across all of the states
that contribute to some extent (i.e., above the threshold) to a
downwind air quality problem. To simply conclude that nothing need be
done regarding emissions that exceed the step 2 threshold because those
emissions can be characterized as ``small'' compared to others'
emissions (by the upwind state's lights at least) is an attempt to
simply move the ``contribution'' threshold at Step 2 and is clearly
insufficient at Step 3.
---------------------------------------------------------------------------

    \87\ See Michigan SIP submission p. 32.
---------------------------------------------------------------------------

    Whether emissions from other states or other countries also
contribute to the same downwind air quality issue is irrelevant in
assessing whether a downwind state has an air quality problem, or
whether an upwind state is significantly contributing to that problem.
States are not obligated under CAA section 110(a)(2)(D)(i)(I) to reduce
emissions sufficient on their own to resolve downwind receptors'
nonattainment or maintenance problems. Rather, states are obligated to
eliminate their own ``significant contribution'' or ``interference''
with the ability of other states to attain or maintain the NAAQS.
    Further, the court in Wisconsin explained that downwind
jurisdictions often may need to heavily rely on emissions reductions
from upwind states in order to achieve attainment of the NAAQS, 938
F.3d at 316-17; such states would face increased regulatory burdens
including the risk of bumping up to a higher nonattainment
classification if attainment is not reached by the relevant deadline,
Maryland, 958 F.3d at 1204. Indeed, the D.C. Circuit in Wisconsin
specifically rejected petitioner arguments suggesting that upwind
states should be excused from good neighbor obligations on the basis
that some other source of emissions (whether international or another
upwind state) could be considered the ``but-for'' cause of downwind air
quality problem. 938 F.3d at 323-324. The court viewed petitioners'
arguments as essentially an argument ``that an upwind state
`contributes significantly' to downwind nonattainment only when its
emissions are the sole cause of downwind nonattainment.'' 938 F.3d at
324. The court explained that ``an upwind state can `contribute' to
downwind nonattainment even if its emissions are not the but-for
cause.'' Id. at 324-325. See also Catawba County v. EPA, 571 F.3d 20,
39 (D.C. Cir. 2009) (rejecting the argument ``that `significantly
contribute' unambiguously means `strictly cause' '' because there is
``no reason why the statute precludes EPA from determining that [an]
addition of [pollutant] into the atmosphere is significant even though
a nearby county's nonattainment problem would still persist in its
absence''); Miss. Comm'n on Envtl. Quality v. EPA, 790 F.3d 138, 163
n.12 (D.C. Cir. 2015) (observing that the argument that ``there likely
would have been no violation at all . . . if it were not for the
emissions resulting from [another source]'' is ``merely a rephrasing of
the but-for causation rule that we rejected in Catawba County.'').
Therefore, a state is not excused from eliminating its significant
contribution on the basis that emissions from other sources also
contribute some amount of pollution to the same receptors to which the
state is linked. Thus, the state's arguments related to contributions
from other sources, including removing international emissions from
projected design values at the Sheboygan, Wisconsin monitor, are
insufficient at Step 3 of the analysis.
    EGLE's submission included an apportionment analysis to quantify
individual states' relative responsibility of the projected exceedances
at the three linked receptors. EGLE cited Attachment A to the March
2018 memorandum as well as EME Homer City Generation to suggest
Michigan could be found to be only responsible for eliminating its
share of the projected exceedances relative to other states that also
contribute more than 1 ppb to the same receptors. We first note that
this approach is a deviation from EPA's traditional approach of
apportioning upwind-state responsibility at Step 3 using a uniform cost
of control metric set at a level that maximizes cost-effectiveness of
emissions reductions in relation to downwind state impacts across all
linked states. Thus, this is not how EPA has interpreted the statutory
term ``significant'' in the past, and EPA does not reach a conclusion
whether this approach would be approvable, had EGLE had imposed
emissions reductions in line with this logic. We do not need to reach
that point in the analysis, however, because, having selected that
approach to defining its obligations, EGLE proceeded to ignore the
result.
    EGLE's submission identified Michigan's proportional contribution
as less than 0.12 ppb to the three linked receptors and .05 ppb to the
Sheboygan, Wisconsin receptor. Having acknowledged Michigan was
responsible for eliminating up to 0.12 ppb of contribution to the
downwind receptors, EGLE claimed that modeling ``noise'' made it
``difficult'' to require that amount of reductions from Michigan
sources. EGLE further opined that the downwind jurisdiction's share of
responsibilities likely made Michigan's contributions even lower and
the projected exceedances were so small that those three receptors were
likely to not have difficulty attaining the NAAQS anyway. EPA has
routinely been capable of successfully implementing good neighbor
obligations through the CSAPR framework, and achieving significant
downwind air quality improvements through upwind-state reductions, at
levels of ``significant contribution'' comparable or even less than
those found in Michigan's submittal, irrespective of alleged ``modeling
noise.'' See Wisconsin, 938 F.3d at 322-23 (rejecting Wisconsin's
argument that it should not face good neighbor obligations on the basis
that its emission reductions would only improve a downwind receptor by
two ten-thousandths of a part per billion).
    After measuring Michigan's significant contribution, EGLE suggested
that modeling uncertainty was too great to either require emissions
reductions or demonstrate that EGLE had any linkages to maintenance
receptors at all. But EGLE had measured the state's significant
contribution and was therefore identifying the measurable amount of
significant contribution the state was legally responsible for
eliminating. See Michigan v. EPA, 213 F.3d 663, 683-84 (D.C. Cir. 2000)
(significant contribution must be ``measurable''). Further, scientific
uncertainty may only be invoked to avoid comporting with the
requirements of the CAA when ``the scientific uncertainty is so
profound that it precludes . . . reasoned judgment'' Massachusetts v.
EPA, 127 S.Ct. 1438 (2007). See Wisconsin, 938 F.3d at 318-19
(``Scientific uncertainty, however, does not excuse EPA's failure to
align the deadline for eliminating upwind States' significant
contributions with the deadline for downwind attainment of the
NAAQS.''). See also EME Homer City, 795 F.3d 118, 135-36 (``We will not
invalidate EPA's predictions solely because there might be
discrepancies between those predictions and the real world. That
possibility is inherent in the enterprise of prediction.''). EGLE's
arguments related to modeling uncertainty or ``noise'' do not establish
a level of uncertainty so high as to

preclude reasoned judgement. EGLE argued that the three maintenance
receptors at issue could maintain the NAAQS without further emissions
reductions from any linked upwind state. In support, EGLE's submission
provided a list of on-the-way and on-the-books emission reductions
measures to argue that Michigan's good neighbor obligations were
already satisfied. EGLE provided references to certain facility
retirements in Michigan, Federal mobile source rules, Federal rules
reducing NOX and VOCs such as MATS and the Oil and Natural
Gas Industry Standards, the NOX SIP Call, and CSAPR Update.
    EPA's assessment of future air quality conditions generally already
accounts for on-the-books emission reductions and the most up-to-date
forecast of future emissions in the absence of the transport policy
being evaluated (i.e., base case conditions).\88\ As described in more
detail in Section I, EPA's latest projections of the baseline EGU
emissions uses the version 6--Summer 2021 Reference Case of the
IPM.\89\ The IPM version 6--Summer 2021 Reference Case uses the NEEDS
v6 database as its source for data on all existing and planned-
committed units. Units are removed from the NEEDS inventory only if a
high degree of certainty could be assigned to future implementation of
the announced future closure or retirement. Any retirements excluded
from the NEEDS v6 inventory can be viewed in the NEEDS spreadsheet.\90\
The inventory for these projections takes account of the retirement of
the Marquette Board of Light & Power Shiras Steam Plant, Lansing Board
of Water and Light, Eckert Station, Units 1 and 3-6; DTE, River Rouge,
Unit 3; We Energies, Presque Isle Power Plant, Units 5-9; DTE St.
Clair, Units 1-4 and 6-7; DTE Trenton Channel, Unit 9; Wyandotte, Unit
5; Consumers Energy Karn, Units 1-2.
---------------------------------------------------------------------------

    \88\ See 81 FR 74504 at 74517; 85 FR 68964 at 68979.
    \89\ Detailed information and documentation of EPA's Base Case,
including all the underlying assumptions, data sources, and
architecture parameters can be found on EPA's website at: https://www.epa.gov/airmarkets/epas-power-sector-modeling-platform-v6-using-ipm-summer-2021-reference-case.
    \90\ We note that for one of the units EGLE listed as projected
to retire, Wyandotte--Unit 5, this facility was still included in
the NEEDS as operating. Additionally, the unit IDs listed by EGLE in
the SIP submittal may be different from those listed in EPA's NEEDS
v6 inventory--NEEDS v6 Summer 2021 Reference Case, however we have
verified that these emissions decreases have been accounted for in
our most recent modeling.
---------------------------------------------------------------------------

    Additionally, EPA's modeling using the 2016v2 emissions platform
accounts for the onroad and nonroad rules that Michigan identified,
such as the Tier 3 Motor Vehicle Emission and Fuel Standards, to the
extent still on the books and projected to have ozone-precursor
emissions consequences.\91\
---------------------------------------------------------------------------

    \91\ See Technical Support Document (TSD) Preparation of
Emissions Inventories for the 2016v2 North American Emissions
Modeling Platform included in the Headquarters docket ID No. EPA-HQ-
OAR-2021-0663.
---------------------------------------------------------------------------

    In other words, changes in the emissions inventory and on-the-books
controls relevant to emissions in 2023 that EGLE claims EPA missed in
its prior modeling have now been incorporated into EPA's most recent
modeling of 2023 using the 2016v2 emissions platform. This modeling
projects a continuing contribution from Michigan to thirteen out-of-
state receptors above a threshold of 1 percent of the NAAQS (at Steps 1
and 2) despite these measures--nine of which have contribution from
Michigan above 1 ppb and seven of which are nonattainment receptors
(see Table 4).\92\ Therefore, in light of continuing contribution to
out of state receptors from Michigan notwithstanding these identified
on-the-books control measures, EGLE's SIP submission should have
evaluated the availability of additional air quality controls to
improve downwind air quality at nonattainment and maintenance receptors
at Step 3.
---------------------------------------------------------------------------

    \92\ Notably, in focusing its Step 3 analysis only on a single
receptor, EGLE gave no weight to the scope of its contribution to
downwind air quality problems. Linkages to thirteen receptor sites
in EPA's most recent modeling indicate that Michigan's emissions
have widespread effects in other states--effects that the State's
SIP submittal would do nothing to address.
---------------------------------------------------------------------------

    Nor does EGLE's listing of existing control measures or overall
emission trends serve as an adequate substitute for a Step 3 analysis
of additional potential emission reductions. In general, the listing of
existing or on-the-way control measures, whether approved into the
State's SIP or not, does not substitute for a complete step 3 analysis
under EPA's 4-step framework to define ``significant contribution.''
ELGE did not provide an assessment of the overall effects of these
measures, when the emissions reductions would be achieved, and what the
overall resulting air quality effects would be at identified out of
state receptors. EGLE did not identify which portion of ongoing
emissions trends were not already accounted for in steps 1 and 2 of the
analysis (EPA addresses specific identified changes in emissions
inventory in the discussion above). EGLE did not evaluate additional,
potential emissions control opportunities, or their costs or impacts,
or attempt to analyze whether, if applied more broadly across linked
states, the emissions reductions would constitute the elimination of
significant contribution on a regional scale. The state did not offer
an explanation as to whether any faster or more stringent emissions
reductions that may be available were prohibitively costly or
infeasible. Although EPA acknowledges states are not necessarily bound
to follow its own analytical framework at step 3, we note that the
state did not attempt to determine or justify an appropriate uniform
cost-effectiveness threshold for the more stringent 2015 ozone NAAQS.
This would have been similar to the approach to defining significant
contribution that EPA has applied in prior rulemakings such as CSAPR
and or the CSAPR Update, even if such an analysis is not technically
mandatory.
    Further, the state's attempt to categorize certain sectors of
emissions as beyond its regulatory control is unpersuasive. Clearly the
state possesses regulatory authority over its EGU and non-EGU large
stationary sources as well as authority over other types of ``emissions
activity within the state,'' see CAA section 110(a)(2)(D)(i). And while
mobile sources are generally regulated at the Federal level under title
II of the Clean Air Act, the state also has the authority to undertake
any number of measures to reduce emissions from mobile sources through
means and techniques that are not preempted by title II. See, e.g., CAA
sections 182(b)(3), 182(b)(4), 182(c)(3), 182(c)(4), 182(c)(5),
182(d)(1), 182(e)(3), and 182(e)(4) (identifying programs to control
mobile source emissions that states are required to implement depending
on the degree of ozone nonattainment). Specifically with respect to
EGUs, EPA notes that no EGU NOX control program has yet been
established to implement good neighbor requirements for the 2015 ozone
NAAQS. Thus reliance on prior programs, such as the CSAPR Update or
Revised CSAPR Update, is misplaced, since those programs only addressed
good neighbor obligations under the less stringent 2008 ozone NAAQS.
    Finally, under the Wisconsin decision, states and EPA may not delay
implementation of measures necessary to address good neighbor
requirements beyond the next applicable attainment date without a
showing of impossibility or necessity. See 938 F.3d at 320. In those
cases where the measures identified by Michigan had implementation
timeframes beyond the next relevant attainment dates, the submission
did not offer a demonstration of impossibility of earlier
implementation of those control measures that would go into effect
after

2024. Similarly, the State's submittal is insufficient to the extent
the implementation timeframes for identified control measures were left
unidentified, unexplained, or too uncertain to permit EPA to form a
judgment as to whether the timing requirements for good neighbor
obligations have been met.
    For the reasons listed above, EPA proposes to find that Michigan
has not satisfied its obligations of the good neighbor SIP provisions
at Step 3 of the 4-step transport framework. We propose that Michigan
was required to analyze emissions more fully from the sources and other
emissions activity from within the state to determine whether its
contributions were significant, and we propose to disapprove its
submission because Michigan failed to do so.
5. Evaluation of Information Provided Regarding Step 4
    Step 4 of the 4-step interstate transport framework calls for
development of permanent and federally enforceable control strategies
to achieve the emissions reductions determined to be necessary at Step
3 to eliminate significant contribution to nonattainment or
interference with maintenance of the NAAQS. EGLE provided references to
on the books and on the way Federal mobile source rules, MATS and the
Oil and Natural Gas Industry Standards, the NOX SIP Call,
and CSAPR Update. As an initial matter, pointing to or listing existing
state or Federal control measures is not what is called for at Step 4.
Rather Step 4 requires the development of permanent and enforceable
measures to implement those measures determined to be required at Step
3. EGLE claimed that nothing was required of Michigan at Step 3 and
thus EGLE stated that it did not believe anything was required at Step
4. Therefore, we do not interpret the list of existing state or Federal
measures to be EGLE's attempt at implementation at Step 4.
    Because Michigan's SIP submission did not contain an evaluation of
additional emission control opportunities (or establish that no
additional controls are required), no information was provided at Step
4. As a result, EPA proposes to disapprove Michigan's submittal on the
separate, additional basis that the state has not developed permanent
and enforceable emissions reductions necessary to meet the obligations
of CAA section 110(a)(2)(d)(i)(I).
6. Conclusion
    Based on EPA's evaluation of EGLE's SIP submission, EPA is
proposing to find that the portion of Michigan's March 5, 2019 SIP
submission addressing CAA section 110(a)(2)(D)(i)(I) does not meet the
state's interstate transport obligations, because it fails to contain
the necessary provisions to eliminate emissions that will contribute
significantly to nonattainment or interfere with maintenance of the
2015 ozone NAAQS in any other state.

D. Minnesota

1. Evaluation of Information Provided by Minnesota Regarding Steps 1
and 2
    At Step 1 of the 4-step interstate transport framework, Minnesota
relied on both LADCO modeling and EPA modeling released in the March
2018 memorandum and to identify nonattainment and maintenance receptors
in 2023. As described previously, LADCO performed a modeling
demonstration like that of EPA's 2018 transport modeling, except with
use of the ERTAC EGU Tool to supplement state specific EGU information.
LADCO identified nonattainment and maintenance receptors using EPA
methodology. MPCA presented several nonattainment and maintenance
receptors identified by both LADCO modeling, showing ``no water'' and
``with water'' results and EPA modeling released with the March 2018
memorandum. Since new modeling has been performed by EPA with updated
emission data, EPA proposes to primarily rely on the most recent
modeling to identify nonattainment and maintenance receptors in 2023.
MPCA made several criticisms of EPA's method for projecting EGU
emissions in EPA's modeling released with the March 2018 memorandum.
Although EPA does not agree with those criticisms, we note that EPA is
relying on a different method for projecting emissions from EGUs in the
updated modeling using the 2016v2 emissions platform as explained in
more detail in Section I.
    Nonetheless, the alternative modeling relied on by MPCA also
identified a number of nonattainment and maintenance receptor sites in
2023. See Tables 2 and 3 on pages 8 and 9 of MPCA's submittal. Thus,
even under the alternative modeling of 2023, MPCA acknowledges in its
submittal the existence of several nonattainment and maintenance
receptors.
    At Step 2 of the 4-Step interstate transport framework, MPCA relied
on both LADCO modeling and EPA modeling released in the March 2018
memorandum to identify upwind state linkages to nonattainment and
maintenance receptors in 2023. Based on both modeling results, MPCA
concluded that Minnesota would contribute below 1 percent of the NAAQS
to receptors in 2023. However, in this proposal, EPA relies on the
Agency's most recently available modeling, which uses a more recent
base year and more up-to-date emissions inventories, to identify upwind
contributions and ``linkages'' to downwind air quality problems in 2023
using a threshold of 1 percent of the NAAQS. As shown in Table 5
(explained in the next section), the updated EPA modeling identifies
Minnesota's maximum contribution to a downwind nonattainment or
maintenance receptor is greater than 1 percent of the standard (i.e.,
0.70 ppb). Although the state did not rely on a 1 ppb contribution
threshold in its SIP submittal, EPA recognizes that the modeling the
MPCA used relied on the most recently available EPA modeling at the
time the state submitted its SIP submittal (EPA modeling released in
the March 2018 memorandum as well as the LADCO modeling). The 2018
modeling indicated the state was not projected to contribute above one
1 percent of the NAAQS to a projected downwind nonattainment or
maintenance receptor. Therefore, the state may not have considered
analyzing the reasonableness and appropriateness of a 1 ppb threshold
at Step 2 of the 4-step Step interstate transport framework per the
August 2018 memorandum. EPA's August 2018 memorandum provided that
whether use of a 1 ppb threshold is appropriate must be based on an
evaluation of state-specific circumstances, and no such evaluation was
included in the submission. EPA's experience with the alternative Step
2 thresholds is further discussed in Section I.D.3.i. As discussed
there, EPA is considering withdrawing the August 2018 memorandum.
2. Results of EPA's Step 1 and Step 2 Modeling and Findings for
Minnesota
    As described in Section I, EPA performed air quality modeling using
the 2016v2 emissions platform to project design values and
contributions for 2023. These data were examined to determine if
Minnesota contributes at or above the threshold of 1 percent of the
2015 ozone NAAQS (0.70 ppb) to any downwind nonattainment or
maintenance receptor. As shown in

Table 5, the data \93\ indicate that in 2023, emissions from Minnesota
contribute greater than 1 percent of the standards to two maintenance-
only receptors in Illinois. These modeling results are consistent with
the results of a prior round of 2023 modeling using the 2016v1
emissions platform that became available to the public in the fall of
2020 in the Revised CSAPR Update, as noted in Section I, which showed
that Minnesota had a maximum contribution of 0.86 ppb to a
nonattainment or maintenance receptor in 2023.\94\
---------------------------------------------------------------------------

    \93\ Design values and contributions at individual monitoring
sites nationwide are provide in the file:
2016v2_DVs_state_contributions.xlsx which is included in docket ID
No. EPA-HQ-OAR-2021-0663.
    \94\ These modeling results are consistent with the results of a
prior round of 2023 modeling using the 2016v1 emissions platform
which became available to the public in the fall of 2020 in the
Revised CSAPR Update, as noted in Section I. That modeling showed
that Minnesota had a maximum contribution greater than 0.70 ppb to
at least one nonattainment or maintenance-only receptor in 2023.
These modeling results are included in the file ``Ozone Design
Values And Contributions Revised CSAPR Update.xlsx'' in docket EPA-
HQ-OAR-2021-0663.

                      Table 5--Minnesota Linkage Results Based on EPA Updated 2023 Modeling
----------------------------------------------------------------------------------------------------------------
                                                                   2023  Average   2023  Maximum     Minnesota
         Receptor ID               Location       Nonattainment/   design value    design value    contribution
                                                   maintenance         (ppb)           (ppb)           (ppb)
----------------------------------------------------------------------------------------------------------------
170310001....................  Cook............  Maintenance....            69.6            73.4            0.97
170310076....................  Cook............  Maintenance....            69.3            72.1            0.79
----------------------------------------------------------------------------------------------------------------

    Based on EPA's evaluation of the information submitted by MPCA, and
based on EPA's most recent modeling results for 2023 using the 2016v2
emissions platform, EPA proposes to find that Minnesota is linked at
Steps 1 and 2 and has an obligation to assess potential emissions
reductions from sources or other emissions activity at Step 3 of the 4-
Step framework. Despite the linkage EPA determines exists at Step 2,
the state concluded in its submission based on other factors that it
should not be considered to significantly contribute to nonattainment
or interfere with maintenance of the NAAQS in other states. Therefore,
EPA will proceed to evaluate MPCA's additional analyses at Step 3 of
the 4-Step interstate transport framework.
4. Evaluation of Information Provided Regarding Step 3
    At Step 3 of the 4-Step interstate transport framework, a state's
emissions are further evaluated, in light of multiple factors,
including air quality and cost considerations, to determine what, if
any, emissions significantly contribute to nonattainment or interfere
with maintenance and, thus, must be eliminated under CAA section
110(a)(2)(D)(i)(I).
    To effectively evaluate which emissions in the state should be
deemed ``significant'' and therefore prohibited, states generally
should prepare an accounting of sources and other emissions activity
for relevant pollutants and assess potential, additional emissions
reduction opportunities and resulting downwind air quality
improvements. EPA has consistently applied this general approach (i.e.,
Step 3 of the 4-step interstate transport framework) when identifying
emissions contributions that the Agency has determined to be
``significant'' (or interfere with maintenance) in each of its prior
Federal, regional ozone transport rulemakings, and this interpretation
of the statute has been upheld by the Supreme Court. See EME Homer
City, 572 U.S. 489, 519 (2014). While EPA has not directed states that
they must conduct a Step 3 analysis in precisely the manner EPA has
done in its prior regional transport rulemakings, state implementation
plans addressing the obligations in CAA section 110(a)(2)(D)(i)(I) must
prohibit ``any source or other type of emissions activity within the
state'' from emitting air pollutants which will contribute
significantly to downwind air quality problems. Thus, states must
complete something similar to EPA's analysis (or an alternative
approach to defining ``significance'' that comports with the statute's
objectives) to determine whether and to what degree emissions from a
state should be ``prohibited'' to eliminate emissions that will
``contribute significantly to nonattainment in or interfere with
maintenance of'' the NAAQS in any other state. MPCA did not conduct
such an analysis in their SIP submission.
    Neither the LADCO modeling nor EPA modeling released with the March
2018 memorandum indicated that Minnesota would contribute over 1
percent of the NAAQS to any nonattainment or maintenance receptor in
2023. Therefore, MPCA stated they did not consider it necessary to
consider further emission reductions because Minnesota was not
projected to contribute to downwind air quality issues above the
contribution threshold. Despite this, Minnesota provided supporting
analysis to strengthen the conclusions of the modeling results. MPCA
presented evidence that ambient ozone concentrations in Minnesota had
been at or below the NAAQS from the late 1990s to 2017, and that
NOX and VOCs emissions had been steadily decreasing from
2002 through 2015. MPCA asserted that these trends would translate to
continued reductions in ozone being transported from the state to
nonattainment or maintenance receptors. Additionally, MPCA listed
several state and Federal regulatory programs that control or
incentivize NOX and VOC limits, including the CSAPR
NOX trading program.
    EPA does not dispute the evidence about ambient ozone
concentrations and NOX and VOC emissions trends or existence
of the NOX and VOC controls presented by Minnesota.\95\
However, as explained in Section I.C, the most recent EPA modeling
captures numerous updates to the 2016 emissions platform, including all
existing CSAPR trading programs, in the baseline,\96\ and that modeling
confirms that most these control programs were not sufficient to
eliminate Minnesota's linkage at Steps 1 and 2 under the 2015 ozone
NAAQS. The state therefore has good neighbor obligations under the 2015
8-hour NAAQS and is obligated at Step 3 to assess additional control
measures using a multifactor analysis.
---------------------------------------------------------------------------

    \95\ See Minnesota's SIP submittal Figures 1-3, pages 10-11.
    \96\ For a complete explanation of air quality modeling of the
2016v2 emissions platform modeling, please see ``AQ Modeling
TSD_2016v2 Platform.pdf'' included in docket ID No. EPA-HQ-OAR-2021-
0663.
---------------------------------------------------------------------------

    MPCA identified state permitting programs, rules, voluntary
programs, and the CSAPR NOX trading program,

among others, as NOX and VOC control measures which satisfy
Minnesota's good neighbor obligations under the 2015 ozone NAAQs. In
general, however, the listing of existing or on-the-way control
measures, whether approved into the state's SIP or not, does not
substitute for a complete Step 3 analysis under EPA's 4-step framework
to define ``significant contribution.'' Minnesota's submission does not
include an assessment of the overall effects of these measures, when
the reductions would be achieved, and what the overall resulting air
quality effects would be observed at identified out-of-state receptors.
Minnesota's submission does not include an evaluation of additional
potential emissions control opportunities, or their costs or impacts,
or attempt to analyze whether, if applied more broadly across linked
states, the emissions reductions would constitute the elimination of
significant contribution on a regional scale. The state's submission
did not contain an explanation as to whether any faster or more
stringent emissions reductions that may be available were prohibitively
costly or infeasible. Furthermore, states may not rely on non-SIP
measures to meet SIP requirements, and Minnesota has not revised its
SIP to contain the CSAPR NOX trading program or the non-SIP
approved rules MPCA identified. See CAA section 110(a)(2)(D) (``Each
such [SIP] shall . . . contain adequate provisions . . . .''). See also
CAA section 110(a)(2)(A); Committee for a Better Arvin v. U.S. E.P.A.,
786 F.3d 1169, 1175-76 (9th Cir. 2015) (holding that measures relied on
by state to meet CAA requirements must be included in the SIP).
    As mentioned above, EPA has newly available information that
indicates sources in Minnesota are linked to downwind air quality
problems for the 2015 ozone standard. Therefore, EPA proposes to
disapprove Minnesota's August 20, 2018 interstate transport SIP
submission on the separate, additional basis that the SIP submittal did
not assess additional emissions control opportunities.
5. Evaluation of Information Provided Regarding Step 4
    Step 4 of the 4-Step interstate transport frameworks calls for
development of permanent and federally enforceable control strategies
to achieve the emissions reductions determined to be necessary at Step
3 to eliminate significant contribution to nonattainment or
interference with maintenance of the NAAQS. MPCA identified state
permitting programs, rules, voluntary programs, and the CSAPR
NOX trading program, among others, as NOX and VOC
control measures which are not all part of Minnesota's SIP. Although
the state has since incorporated some of these control measures into
their SIP, Minnesota did not revise its SIP to include all these
emission reductions in a revision to its SIP to ensure the reductions
were permanent and enforceable and eliminate their significant
contribution to nonattainment or interference with maintenance of the
NAAQS. As a result, EPA proposes to disapprove Minnesota's submittal on
the separate, additional basis that the Minnesota has not developed
permanent and enforceable emissions reductions necessary to meet the
obligations of CAA section 110(a)(2)(d)(i)(I).
6. Conclusion
    Based on EPA's evaluation of Minnesota's SIP submission and after
consideration of updated EPA modeling using the 2016-based emissions
modeling platform, EPA is proposing to find that the portion of
Minnesota's October 1, 2018 SIP submission addressing CAA section
110(a)(2)(D)(i)(I) does not meet the state's interstate transport
obligations for 2015 ozone NAAQS, because it fails to contain the
necessary provisions to eliminate emissions that will contribute
significantly to nonattainment or interfere with maintenance of the
NAAQS in any other state.

E. Ohio

1. Evaluation of Information Provided by Ohio Regarding Steps 1
    At Step 1 of the 4-step interstate transport framework, OEPA relied
on LADCO modeling released in 2018 to identify nonattainment and
maintenance receptors in 2023. As described previously in this action,
LADCO performed modeling similar to EPA's modeling released in the
March 2018 memorandum, except with use of ERTAC for projecting future
year EGU emissions. LADCO identified nonattainment and maintenance
receptors using EPA methodology. OEPA elected to rely on LADCO's
``3x3'' modeling results, which identified similar receptors to EPA's
modeling included in the March 2018 memorandum.
    However, OEPA elected to use an alternative method developed by
TCEQ for identifying maintenance receptors at Step 1 of the 4-step
framework. Using the TCEQ method to identify maintenance receptors OEPA
claimed that four maintenance receptors based on EPA's approach would
not have difficulty maintaining the NAAQS in 2023. OEPA relied on the
potential flexibilities in Attachment A to the March 2018 in support of
its use of the TCEQ method. As explained in Section I.C above, the
concepts presented in Attachment A to the March 2018 memorandum were
neither guidance nor determined by EPA to be consistent with the CAA.
OEPA submitted Ohio's SIP submission before EPA released its October
2018 memorandum discussing maintenance receptors. Regardless, EPA will
examine the legal and technical merits of OEPA's arguments related to
the use of an alternative maintenance-only definition in light of the
October 2018 memorandum. OEPA has not adequately explained or justified
how TCEQ's method for identifying maintenance receptors reasonably
identifies areas that will have difficulty maintaining the NAAQS. That
is, EPA proposes to find that OEPA has provided no sound technical
basis for how TCEQ's methodology gives meaning to the CAA's instruction
that states submit good neighbor SIPs that prohibit their states'
emissions from interfering with the maintenance of the NAAQS in another
state.
    In North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir. 2008),
the D.C. Circuit rejected EPA's CAIR on the basis that EPA had not
adequately given meaning to the phrase ``interfere with maintenance''
in the good neighbor provision. Specifically, North Carolina argued
that it had counties that were projected to attain the NAAQS in the
future analytic year, but were at risk of falling back into
nonattainment due to interference from upwind sources, particularly
given year-to-year variability in ozone levels. The court agreed,
holding that EPA's rule did not adequately protect ``[a]reas that find
themselves barely meeting attainment.'' Id. at 910. Consequently, EPA
has developed a methodology, used in its 2011 CSAPR and its 2016 CSAPR
Update and Revised CSAPR Update, for identifying areas that may
struggle to maintain the NAAQS. See 76 FR at 48227-28. EPA's approach
to addressing maintenance receptors was upheld in the EME Homer City
litigation. See 795 F.3d 118, 136-37. It was also upheld in Wisconsin.
938 F.3d at 325-26. In Wisconsin, the court noted that four upwind
states were linked only to maintenance receptors and rejected the
argument that application of the same control level as EPA imposes for
those states linked to nonattainment receptors was unreasonable or
unlawful absent a

particularized showing of overcontrol. Id. at 327.
    In order to explain the differences between TCEQ's and EPA's
methodology for identifying maintenance receptors, it is helpful to
provide some additional context for how EPA projects future air
quality. EPA's air quality modeling guidance has long recommended
developing a base design value (i.e., the design value that will be
used as a starting point to model and analyze for purposes of
projecting future air quality concentrations) that is the average of
three design values spanning a five-year period, centered around one
year for which an emissions inventory will be submitted (e.g., if 2011
was the base emissions inventory year, a state would use monitored
values from 2009-2011, 2010-2012, 2011-2013 as the starting point for
projecting air quality concentrations in future years). The average of
these three design values is then multiplied by a relative response
factor to generate an average design value for the future year. If a
receptor's average future year design value is greater than or equal to
the level of the NAAQS, and the receptor has recent monitored data that
violates the NAAQS, that receptor is considered a ``nonattainment''
receptor at step 1. To identify maintenance receptors, EPA's
methodology looks to the highest design value of the three DVs used to
calculate the 5-year weighted average design value (e.g., in the 2011
example, if 2009-2011 had the highest design value of 2009-2011, 2010-
2012, and 2011-2013). EPA then applies the same relative response
factor to that highest design value to generate a projected future
maximum design value. Where a receptor's maximum design value exceeds
the level of the NAAQS, EPA has deemed those receptors to be
``maintenance'' receptors. This methodology was designed to address the
D.C. Circuit's holding that the CAA's ``interference with maintenance''
prong requires states and EPA to protect areas that may struggle with
maintaining the standard in the face of variable conditions.
    For its maintenance receptors, TCEQ elected not to use the highest
design value of the three DVs making up the base period average design
value. Instead, Texas (and by extension, Ohio), used the most recent
design value of the three DVs, regardless of whether the most recent
design value was highest or lowest. OEPA's proffered explanation for
using the most recent design value to identify maintenance receptors
was that the latest design value ``takes into consideration . . . any
emissions reductions that might have occurred.'' \97\ OEPA in its
submission did not explain why or how this methodology identifies those
areas that may be meeting the NAAQS or that may be projected to meet
the NAAQS but may nevertheless struggle to maintain the NAAQS, given
interannual variability in ozone conducive meteorology. In fact,
because the TCEQ's methodology adopted by OEPA uses the most recent
design value to capture more recent emissions reductions rather than
capture variable conditions, the methodology appears to be aimed at
limiting receptors which could be identified as maintenance receptors,
compared to EPA's methodology, which was designed to identify those
areas that might struggle to maintain the NAAQS in ozone conducive
conditions.
---------------------------------------------------------------------------

    \97\ TCEQ submission at 3-39 to 3-40.
---------------------------------------------------------------------------

    EPA disagrees that the use of latest three years for calculating a
DV properly accounts for the effects of meteorological variability for
the purpose of identifying projected maintenance receptors. Rather, the
use of a three-year average is intended to average out, not account
for, the effects of inter-annual variability in ozone conducive
meteorology. EPA reviewed the information provided by OEPA and proposes
to find that the information is insufficient to support the use of an
alternative approach. OEPA analysis of meteorological information did
not discuss or consider how other meteorological factors that are
typically associated with high ozone episodes such as humidity, solar
radiation, vertical mixing, and/or other meteorological indicators such
as cooling-degree days to confirm whether conditions affecting these
monitors may have been conducive to ozone formation during the 2009
through 2013 base period. In addition, the ozone trends data provided
in OEPA submittal indicate that several of the receptors in Coastal
Connecticut to which Ohio is linked by more than 1 ppb continue to
measure ozone design values close to or exceeding 80 ppb with no
overall downward trend in the most recent data in the submittal.\98\ In
any event, OEPA's use of an alternative approach to identifying
maintenance receptors does not result in a dispositive change in
receptor status for purposes of EPA's evaluation of OEPA's SIP
submittal at Step 1 because OEPA did not reach the conclusion that
there were no receptors in 2023 or claim at Step 2 that Ohio was not
linked to any receptor on the basis of the use of an alternative
definition of maintenance receptor.
---------------------------------------------------------------------------

    \98\ See ``2010 Thru 2020 Ozone Design Values.xlsx'' in docket
ID No. EPA-HQ-OAR-2021-0663.
---------------------------------------------------------------------------

    In conclusion, the modeling relied on by OEPA identified a number
of nonattainment and maintenance receptor sites in the Midwest and
Northeast in 2023. See Table 1 on page 8 of OEPA's submittal. Under
EPA's approach to defining nonattainment and maintenance receptors,
Ohio was shown to be linked to three ``nonattainment/maintenance''
receptors and six ``maintenance'' receptors. Under an alternative
approach to defining receptors (discussed below), OEPA concluded that
Ohio was shown to be linked to two ``nonattainment'' receptors, one
``nonattainment/maintenance'' receptor, and four ``maintenance''
receptors. Thus, based on using the LADCO's 2023 modeling and even
under an alternative approach to defining ``maintenance'' receptors,
OEPA acknowledges in its submittal the existence of several
nonattainment and maintenance receptors in the Midwest and Northeast.
EPA further evaluates Ohio's linkage to these receptors in the
following section.
2. Evaluation of Information Provided by the State Regarding Step 2
    Although OEPA relied on alternative modeling to EPA's modeling,
OEPA acknowledged in their SIP submission that Ohio is linked above
either a 1 percent of the NAAQS or a 1 ppb contribution threshold to