Update. Application of step 2 for this rule is described in section V.
---------------------------------------------------------------------------

    \68\ For ozone the impacts would include those from (VOC) and
NOX, and from all sectors.
    \69\ The number of days used in calculating the average
contribution metric has historically been determined in a manner
that is generally consistent with EPA's recommendations for
projecting future year ozone design values. Our ozone attainment
demonstration modeling guidance at the time of CSAPR recommended
using all model-predicted days above the NAAQS to calculate future
year design values (https://www3.epa.gov/ttn/scram/guidance/guide/final-03-pm-rh-guidance.pdf). In 2014 EPA issued draft revised
guidance that changed the recommended number of days to the top-10
model predicted days (https://www3.epa.gov/ttn/scram/guidance/guide/Draft-O3-PM-RH-Modeling_Guidance-2014.pdf). For the CSAPR Update EPA
transitioned to calculating design values based on this draft
revised approach. The revised modeling guidance was finalized in
2019 and, in this regard, EPA is calculating both the ozone design
values and the contributions based on a top-10 day approach. U.S.
Environmental Protection Agency, 2018. Modeling Guidance for
Demonstrating Attainment of Air Quality Goals for Ozone,
PM2.5, and Regional Haze, Research Triangle Park, NC.
(https://www3.epa.gov/ttn/scram/guidance/guide/O3-PM-RH-Modeling_Guidance-2018.pdf).
---------------------------------------------------------------------------

    Step 3--For states that are linked in step 2 to downwind air
quality problems, the CSAPR and the CSAPR Update evaluated
NOX reductions that were available in upwind states by
applying a uniform control stringency (represented by a cost per ton of
NOX reduced) to entities in these states. EPA evaluated
multiple factors, including NOX reduction potential, cost,
and downwind air quality improvements available at several control
stringencies in the multi-factor test. This evaluation quantified the
magnitude of emissions that significantly contribute to nonattainment
or interfere with maintenance of a NAAQS downwind and apportioned
upwind responsibility among linked states, an approach upheld by the
U.S. Supreme Court in EPA v. EME Homer City.\70\ In this action, EPA
applies this same approach to identify NOX emission
reductions necessary to address significant contribution for the 2008
ozone NAAQS.
---------------------------------------------------------------------------

    \70\ EPA v. EME Homer City Generation, L.P., 572 U.S. 489
(2014).
---------------------------------------------------------------------------

    In EME Homer City, the Supreme Court held that ``EPA cannot require
a State to reduce its output of pollution by more than is necessary to
achieve attainment in every downwind State or at odds with the one-
percent threshold the Agency has set.'' 572 U.S. at 521. The Court
acknowledged that ``instances of `over-control' in particular downwind
locations may be incidental to reductions necessary to ensure
attainment elsewhere.'' Id. at 492.

    ``Because individual upwind States often `contribute
significantly' to nonattainment in multiple downwind locations, the
emissions reductions required to bring one linked downwind State
into attainment may well be large enough to push other linked
downwind States over the attainment line. As the Good Neighbor
Provision seeks attainment in every downwind State, however,
exceeding attainment in one State cannot rank as `over-control'
unless unnecessary to achieving attainment in any downwind State.
Only reductions unnecessary to downwind attainment anywhere fall
outside the Agency's statutory authority.''

Id. at 522 (footnotes excluded).
    The Court further explained that ``while EPA has a statutory duty
to avoid over-control, the Agency also has a statutory obligation to
avoid `under-control,' i.e., to maximize achievement of attainment
downwind.'' Id. at 523. Therefore, in the CSAPR Update, EPA evaluated
possible over-control by considering whether an upwind state is linked
solely to downwind air quality problems that can be resolved at a lower
cost threshold, or if upwind states would reduce their emissions at a
lower cost threshold to the extent that they would no longer meet or
exceed the 1 percent air quality contribution threshold. See 81 FR at
74551-52. This evaluation of cost, NOX reductions, and air
quality improvements, including consideration of potential over-
control, results in EPA's determination of upwind emissions that
significantly contribute to nonattainment or interfere with maintenance
of the NAAQS downwind and should therefore be eliminated. This allows
EPA to then determine an enforceable emissions limit (often embodied in
the form of an emissions budget) for the covered sources. Emissions
budgets are the remaining allowable emissions after the elimination of
emissions identified as significantly contributing to nonattainment or
interfering with maintenance of the standard downwind.
    In both the CSAPR and the CSAPR Update, EPA focused its step 3
analysis on EGUs. In the CSAPR Update, EPA did not quantify non-EGU
stationary source emission reductions to address interstate ozone
transport for the 2008 ozone NAAQS for two reasons. First, EPA
explained that there was greater uncertainty in EPA's assessment of
non-EGU NOX mitigation potential, and that more time would
be required for states and EPA to improve non-EGU point source data and
pollution control assumptions before it could develop emission
reduction obligations based on that data. See 81 FR 74542. Second, EPA
explained that it did not believe that significant, certain, and
meaningful non-EGU NOX reduction was in fact feasible for
the 2017 ozone season. Id. In Wisconsin, the D.C. Circuit found that
the practical obstacles EPA identified with respect to its evaluation
of non-EGUs did not rise to the level of an ``impossibility,'' 938 F.3d
at 318-20. The court also found that EPA must make a higher showing of
uncertainty regarding non-EGU point-source NOX mitigation
potential before declining to regulate such sources on such a basis,
id. Therefore, as discussed in more detail in section VI, in this final
action on remand from Wisconsin, EPA has included all major stationary
source sectors in the linked upwind states in its ``significant
contribution'' analysis at step 3 of the 4-step framework.
    Step 4--the CSAPR and the CSAPR Update established interstate
trading programs to implement the necessary emission reductions. Each
state subject to the program is assigned an emissions budget for the
covered sources. Emissions allowances are allocated to units covered by
the trading program, and the covered units then surrender allowances
after the close of each control period in an amount equal to their
ozone season EGU NOX emissions. Emissions allowances are
allocated to units covered by the respective trading program, and the
covered units then surrender allowances after the close of each control
period in an amount equal to their ozone season EGU NOX
emissions.
    All of EPA's trading programs established under the good neighbor
provision allow for interstate trading. However, in order to ensure
that each state achieves reductions proportional to the level of their
significant contribution, beginning with the CSAPR, EPA established
``assurance levels'' set as percentage of each state's budget (e.g.,
121 percent) above which emissions from sources in that state become
subject to a higher ``penalty'' surrender ratio. These assurance levels
are designed to allow for a certain level of year-to-year variability
within power sector emissions to account for fluctuations in demand and
EGU

operations. The levels are therefore set by determining a ``variability
limit,'' calculated based on an analysis of the historical level of
variability in EGU operations.
    Thus, both the CSAPR and the CSAPR Update set assurance levels
equal to the sum of each state's emissions budget plus its variability
limit. The CSAPR and the CSAPR Update included assurance provisions to
limit state emissions to levels below 121 percent of the state's ozone
season NOx emissions budget by requiring additional allowance
surrenders in the instance that emissions in the state exceed this
level. This limit on the degree to which a state's emissions can exceed
its budget is responsive to previous court decisions (see discussion in
section VII.C.2 of this preamble) and was not part of the CSAPR Update
aspects remanded to EPA in Wisconsin. EPA is applying the same
variability limits and assurance provisions in this rule.\71\
Implementation using a trading program is further described in section
VII.
---------------------------------------------------------------------------

    \71\ Historical heat input and NOX emissions in
states covered by the CSAPR programs may be found in the
``Historical CSAPR Update Emissions and Heat Input 2000 to
2019.xlsx'' file.
---------------------------------------------------------------------------

    EPA received several comments related to its overall approach in
this rulemaking. These comments related to the following topics: (1)
Whether this rule remains only a partial remedy in terms of both the
amount of emission reductions achieved and the timing of
implementation; (2) whether any additional EGU emission reductions
relative to the CSAPR Update are permissible in light of the CSAPR
Update record and the scope of the D.C. Circuit's decision in
Wisconsin; and (3) EPA's use of cost to define significant
contribution. Other comments on EPA's overall approach in this action
are addressed in the RTC document.
    Comment: Numerous commenters asserted that despite EPA purporting
to fully address the covered states' good neighbor obligations, the
rule remains only a partial solution, and allows upwind states'
significant contribution to nonattainment and interference with
maintenance of the 2008 ozone NAAQS to continue past the next
attainment date. One commenter asserts that this rule will ``hinder''
attainment of the 2015 ozone NAAQS in downwind states. Many commenters
claim that the rule is insufficient to ensure downwind attainment of
the NAAQS. The commenters question EPA's application of the 4-step
framework and disagree with the Agency's conclusions drawn from that
analysis, particularly with respect to the EPA's determinations at step
3 and the emissions controls adopted at step 4. Some commenters also
challenge the legal basis for the selection of the 2021 analytic year,
as opposed to 2020, and whether EPA has met the requirement to obtain
reductions ``as expeditiously as practicable'' or otherwise complied
with the holdings in Wisconsin and New York to eliminate significant
contribution on par with the relevant downwind attainment deadlines.
See, e.g. Wisconsin, 938 F.3d at 315. Some argue that EPA depends on
claims of technical infeasibility or scientific uncertainty and flawed
cost effectiveness considerations in not requiring more emission
reductions on a shorter timeframe. Others believe the implementation
timeframe of this rule to be a phased plan in direct conflict with
Wisconsin and New York. One commenter concludes there is a ``mismatch''
between EPA's 4-step framework's multi-factor test at step 3 and the
implementation timeframes in this rule. They also argue that EPA should
consider the cost of RACT in downwind states when analyzing the
maximized cost effectiveness of controls in upwind states. Several
commenters also brought attention to the length of time between when
2008 ozone NAAQS good neighbor SIPs were initially due and the proposed
rule in October 2020.
    Response: This rule is a full remedy for the good neighbor
provision for the covered upwind states for the 2008 ozone NAAQS based
on EPA's analysis. The good neighbor provision does not obligate upwind
states to fully resolve a downwind nonattainment or maintenance
problem. CAA section 110(a)(2)(D)(i)(I) only requires that upwind
states prohibit those emissions that ``contribute significantly to
nonattainment'' or ``interfere with maintenance of the NAAQS.'' As
such, the objective of the good neighbor provision is the elimination
of upwind significant contribution or interference with maintenance. It
does not require that the upwind states bear the full burden of
bringing downwind states into attainment. Ultimate achievement of the
NAAQS downwind is accomplished through the larger framework of the CAA,
including under sections 110, 181, 182 and other provisions to attain
the NAAQS. Thus, in this action, EPA must determine what amount of
upwind contribution is significant (or interferes with maintenance) and
require elimination of that significant contribution while avoiding
overcontrol or undercontrol. EPA v. EME Homer City Generation, L.P.,
572 U.S. 489, 521-23 (2014).
    Further, it is not correct to say that good neighbor obligations
can only be found to be fully addressed when there is no longer any
remaining air quality problem at the downwind receptors. Indeed, the
Supreme Court recognized in EME Homer City, 572 U.S. at 521-22, that
under the framework EPA has adopted, EPA could not require a state to
further reduce its emissions once it is at or below the 1 percent
contribution threshold at all receptors. The aim of the good neighbor
provision is to eliminate significant contribution to nonattainment and
interference with maintenance, not to achieve final attainment at the
downwind receptor. Further, in upholding EPA's approach to defining and
allocating upwind responsibility in the CSAPR, the Court in EME Homer
City recognized the discretion EPA has in defining what constitutes
``significant'' contribution, and did not hold that obligations on
upwind states must be imposed to ``maximize'' downwind attainment
without consideration of any other factors. Accord Wisconsin, 938 F.3d
at 320 (recognizing EPA's discretion to interpret ``significant
contribution'').
    The comments do not establish a basis for asserting that EPA's
approach to defining significant contribution or interference with
maintenance is unlawful or unreasonable. They do not explain what is
meant by ``excessive amounts of ozone pollution,'' ``excessive upwind
contributions,'' ``sufficient emission reductions,'' or ``sufficient
upwind reductions.'' These comments do not inform how EPA should define
significant contribution nor do they recognize that EPA has discretion
to define significant contribution. The D.C. Circuit first upheld the
validity of using cost as part of the method for determining
``significance'' in Michigan v. EPA, 213 F.3d 663, 675-79 (D.C. Cir.
2000). The Supreme Court upheld that same approach in EPA v. EME Homer
City Generation, L.P., 572 U.S. 489, 512-20 (2014) (``Eliminating those
amounts that can cost[hyphen]effectively be reduced is an efficient and
equitable solution to the allocation problem the Good Neighbor
Provision requires the Agency to address.''). EPA applied this approach
again in the CSAPR Update, its first action to address good neighbor
obligations under the 2008 ozone NAAQS. And while that action only
provided a partial remedy, no party in Wisconsin challenged as a
general matter EPA's ability to use cost-effectiveness in determining
and allocating upwind responsibility. Wisconsin and New York recognized
EPA's discretion to define significant

contribution. Wisconsin v. EPA, 938 F.3d 303, 319-20 (D.C. Cir. 2019)
(``EPA, though, possesses a measure of latitude in defining which
upwind contribution `amounts' count as `significant[ ]' and thus must
be abated.''); New York v. Envtl. Prot. Agency, 781 F. App'x 4, 7 (D.C.
Cir. 2019) (``[I]n determining what constitutes a significant
contribution to downwind nonattainment, the agency can consider the
amount of upwind states' contributions and the cost of abating
them.'').
    With respect to the timing of when such reductions must be
achieved, EPA agrees that ``as expeditiously as practicable'' is the
first-order statutory directive. See CAA section 181(a)(1); Wisconsin,
938 F.3d at 313. EPA's approach in this rule, after determining
significant contribution, implements all reductions that EPA identified
as possible by the 2021 attainment date, and requires additional
reductions of EGUs in later ozone seasons to the extent not possible by
that date to fully eliminate significant contribution. In this case,
implementing reductions any faster than the 2021 ozone season is
impossible because 2020 is in the past. Commenters are incorrect to
assert that EPA has unlawfully failed to require all necessary
reductions by the 2021 attainment date. EPA has required those
reductions that it has determined are possible by that date; EPA has
also made a determination that additional reductions that are only
possible after that date are nonetheless necessary to eliminate
significant contribution or interference with maintenance, as EPA has
interpreted those terms, and is requiring those later reductions as
expeditiously as practicable. Achieving necessary reductions past the
next attainment date when EPA finds it is impossible to do so
beforehand is consistent with the statute and prior caselaw. Wisconsin
and New York recognized these flexibilities available to EPA in
acknowledging that for reasons of necessity or impossibility, EPA may
deviate from the attainment schedule for downwind areas established in
the Act. Wisconsin v. EPA, 938 F.3d 303, 320 (D.C. Cir. 2019). New York
v. EPA, 781 F. App'x 4, 7 (D.C. Cir. 2019). Indeed, these commenters
are not asserting that EPA lacks authority to require reductions beyond
the attainment date, only that EPA should have required the reductions
by that date. But these comments fail to establish a technical or
evidentiary basis to overturn EPA's judgment that such additional
reductions are not in fact possible by the 2021 attainment date.
    EPA disagrees that Wisconsin held that it must address good
neighbor obligations by the full ozone season prior to the attainment
date (i.e., here, 2020). The decision recognized that the agency must
fully address good neighbor obligations (to the extent EPA determines
possible) by the attainment date itself. 938 F.3d at 315. EPA's
practice of addressing obligations by the full ozone season prior to
the attainment date, while not mandated by statute or caselaw,
continues to make good policy sense, because it assists downwind areas
with improved three-year design values \72\ used in determining whether
attainment has been achieved. However, in this instance, as one
commenter correctly notes, reductions in 2020 are not possible since
this rule was not proposed until after the 2020 ozone season. EPA
nonetheless can still meet the legal mandate to achieve those
reductions that are possible by the 2021 attainment date.
---------------------------------------------------------------------------

    \72\ The ozone design value at a particular monitoring site is
the 3-year average of the annual fourth highest daily maximum 8-hour
ozone concentration at that site.
---------------------------------------------------------------------------

    Further, EPA is not relying on ``scientific uncertainty'' as a
justification for not requiring reductions earlier. As explained
elsewhere in this record, EPA has determined the amount of time needed
for installation and operation of various control strategies. With
respect to the optimization of existing SNCR controls, EPA notes that
it is requiring that strategy as reflected in the final budgets by the
2021 attainment date, as explained in sections VI.B.1, C.1, and D.1.
    EPA defined significant contribution in this rule based on an
assessment of control alternatives under the 4-step good neighbor
framework's step 3 multi-factor test. EPA's determination of what
controls to require and when they can first be implemented are based on
EPA's technical evaluation and application of the third step multi-
factor analysis in the 4-step framework. The only ``mismatch'' that one
commenter identified at the third step is no mismatch at all; it is
simply the reality that some of the controls that EPA is requiring in
this rule cannot be installed before the 2021 ozone season, and some
controls that EPA assessed cannot be installed and operational before
air quality problems are projected to resolve under the 2008 ozone
NAAQS (i.e., by the 2025 ozone season). These comments have not
explained how EPA's evaluation of control options under that test was
arbitrary or capricious.
    The reasons for alleged past delays in implementing ozone transport
obligations is out of the scope of this action on remand. However, EPA
notes that the time it has taken to get reductions in place to address
interstate ozone transport is due to multiple factors, including past
judicial stays of major transport rules such as the NOX SIP
Call and the CSAPR. In addition, EPA had made a determination in the
CSAPR Close-out that it had fully addressed good neighbor obligations;
it was not until the D.C. Circuit ruled in Wisconsin that the basis for
this conclusion was revealed to be insufficient. The CSAPR Update has
and continues to achieve upwind reductions for the 2008 ozone NAAQS. As
explained elsewhere in the preamble to this action, EPA now finds it to
be a full remedy for nine upwind states.
    Comment: Several commenters said that the CSAPR Update was already
a complete remedy with regard to the EGU sector. One commenter
described EPA's response to the remand as ``unreasonable'' and its re-
application of the 4-step framework as ``erroneous.'' Other commenters
opined that EPA has no legal basis to require short-term EGU controls
under the Wisconsin remand. In their opinion, Wisconsin found that the
CSAPR Update fully eliminated significant contribution from EGUs, which
they supported by quoting portions of the decision. They asserted that
Wisconsin only authorized EPA to search for emission reductions from
non-EGUs and to narrowly reconsider the CSAPR Update in terms of the
statutory downwind attainment dates.
    Response: The commenters are incorrect that EPA lacks a legal basis
to re-assess and fully address good neighbor obligations for the
covered states under the Wisconsin remand. As an initial matter, the
CSAPR Update was, by EPA's own admissions, a partial rule. See 81 FR at
74521-22. The court's analysis upholding the portions of the rule in
Wisconsin cited by these commenters was against a backdrop that the
rule was only partial in nature. See, e.g., 938 F.3d at 327. Wisconsin
required EPA to provide a complete remedy by the next applicable
attainment date. This was confirmed in the New York decision vacating
the CSAPR Close-out. The D.C. Circuit found that rule violated the
holding in Wisconsin by failing to analyze the 2021 analytic year
without a sufficient showing of impossibility or necessity. To the
extent that EPA had attempted to fully address the relevant obligations
in the CSAPR Close-Out Rule, that action has been vacated. Therefore,
on remand, EPA not only needs to use a different analytic year to

inform its analysis under the 4-step framework, but it also needs to
apply that framework in order to determine what, if any, obligations
must be addressed, and what emission reductions must be required.
    EPA disagrees that Wisconsin prevents requiring additional
necessary controls on EGUs. As stated in the preamble to the CSAPR
Update, EPA did not view the CSAPR Update as necessarily fully
eliminating significant contribution from EGUs. See 81 FR 74522.
Wisconsin recognized that EPA anticipated ``further EGU reductions that
are achievable after 2017'' may be necessary to completely eliminate
significant contribution or interference with maintenance for the 2008
ozone NAAQS. Wisconsin v. EPA, 938 F.3d 303 (D.C. Cir. 2019) (quoting
81 FR 74522). In the present action, evaluation of a full remedy in
accordance with Wisconsin under the 4-step framework, and particularly
the step 3 multi-factor test, establishes that additional reductions
from EGUs should be required in 12 of the states currently subject to
the CSAPR Update. For nine other states, their continued obligations
under the CSAPR Update satisfy their good neighbor obligations for the
2008 ozone NAAQS. That same analysis shows that reductions from non-
EGUs are not justified under the same test.
    Comment: Some commenters argued that EPA's use of cost in defining
significant contribution has no statutory basis and is contrary to
NAAQS attainment planning caselaw and the Supreme Court's holding in
EPA v. EME Homer City Generation, L.P., 572 U.S. 489 (2014) (``EME
Homer City''), because it does not result in sufficient emission
reduction for attainment and maintenance of the NAAQS. The commenter
also said that even if EPA could use cost as a basis for defining
significant contribution for non-attainment, the Agency could not do so
for interference with maintenance. Another commenter described EPA's
proposed cost threshold of $1,600 per ton as ``arbitrary'' and
inconsistent with the CAA and EME Homer City, as this cost threshold is
insufficient to enable downwind states reach attainment or maintenance.
Further, commenters argued, EPA's use of cost-effectiveness as a metric
at step 3 fails to identify what the ultimate goal should be, as cost-
effectiveness can only be used to evaluate which way to best achieve a
goal. One commenter argued that EPA should require upwind reductions so
long as the downwind benefit of such reductions continues to outweigh
their cost.
    Response: The approach used here is materially the same approach
the Agency applied in the NOX SIP Call, the CSAPR, and in
the CSAPR Update. These comments essentially seek to relitigate EME
Homer City, as well as the D.C. Circuit's prior opinion in Michigan v.
EPA, 213 F.3d 663 (D.C. Cir. 2000). Contrary to the commenters'
interpretation, EME Homer City allowed the use of cost both to define
and to allocate upwind state responsibility. 572 U.S. 489, 518-520
(2014) (``The Agency, tasked with choosing which among equal
``amounts'' to eliminate, has chosen sensibly to reduce the amount
easier, i.e., less costly, to eradicate.''). Notably, in the CSAPR
rulemaking, EPA used cost as part of a multi-factor effectiveness
metric in the multi-factor test to determine the ``amount'' of upwind
contribution that is ``significant'' in a very similar manner as EPA
did in the CSAPR Update and now here in this action on remand. See 76
FR 48208, 48248-51 (Aug. 8, 2011). In the NOX SIP Call, EPA
took a similar approach. See 213 F.3d at 675 (``Although the dividing
line was a very low threshold of contribution, in the end EPA's rule
called for termination of only a subset of each state's contribution.
EPA decided that the 23 `significant contributors' need only reduce
their ozone by the amount achievable with `highly cost-effective
controls.' '') (emphasis added) (citing 63 FR at 57403).
    Commenters fail to identify why an alternative method for
determining ``contribution'' is compelled by the statute, or that EPA's
approach is unlawful, arbitrary, or capricious. Contrary to these
commenters' assertion, the good neighbor provision does not contemplate
that an upwind state's obligation can only ever be resolved once a
downwind receptor is fully in attainment. The Supreme Court recognized
in EME Homer City that the 1 percent contribution threshold used at
step 2 must necessarily be a stopping point in EPA's analysis because a
state that contributed less than that would not be assessed for
reductions at step 3 in the first place. 572 U.S. at 521. The Supreme
Court in EME Homer City recognized that the problem of defining
``significant contribution'' in the context of a regional pollutant
like ozone is inherently extremely complex. Id. at 514. The Court found
that using cost (and specifically, a uniform cost-effectiveness
threshold) to allocate the reduction obligation was both equitable and
efficient. Id. at 519.
    Further, the case law on barring use of cost considerations in the
attainment planning context cited by one commenter is inapplicable. EPA
has discretion to interpret significant contribution, as recognized by
Wisconsin and New York. Wisconsin v. EPA, 938 F.3d 303, 319-20 (D.C.
Cir. 2019) (``EPA, though, possesses a measure of latitude in defining
which upwind contribution `amounts' count as `significant[ ]' and thus
must be abated.''). New York v. Envtl. Prot. Agency, 781 F. App'x 4, 7
(D.C. Cir. 2019) (``[I]n determining what constitutes a significant
contribution to downwind nonattainment, the agency can consider the
amount of upwind states' contributions and the cost of abating
them.'').
    The comment that cost effectiveness does not provide an adequate
basis for EPA to select the correct level of stringency misapprehends
the full scope of the step 3 multi-factor analysis EPA applies in the
4-step framework. EPA's analysis at step 3 additionally considers the
total amount of reductions to be achieved by a control stringency as
well as the effect on air quality at downwind receptors. EPA also must
take into consideration the minimum amount of time needed for controls
to be installed and operational, because if an air quality problem is
no longer present by the time controls could be operational, then there
is no need for those controls to be required. See 572 U.S. at 521.
Thus, it is not just the relative cost effectiveness of a control
stringency but its ultimate effect on a downwind problem that informs
EPA's determination of ``significance.'' \73\
---------------------------------------------------------------------------

    \73\ To some degree, these commenters may be overstating the
relative importance of ``cost'' in EPA's step 3 analysis. EPA's
design of cost thresholds derives from the identification of
discrete types of NOX emission control strategies. EPA
then identifies a representative cost-effectiveness on a per ton
basis for that technology. In the step 3 analysis, it is not the
cost per ton value itself that is inherently meaningful, but rather
how that cost-effectivess value relates to other control
stringencies, how many emission reductions may be obtained, and how
air quality is ultimately impacted. Said differently, when EPA
determines not to require controls at a higher cost threshold, it is
not on grounds that they are simply ``too expensive for industry.''
Further, there are always inherent uncertainties in identifying a
precise cost per ton value for any particular control stringency,
but this in itself does not upset EPA's ability to render an overall
policy judgment based on the step 3 factors as to the level of
emission reductions required. As an example, EPA explains in Section
VI.D.1 why its cost thresholds for EGU control stringencies at
$1,600 per ton and $1,800 per ton in this action generate
essentially the same point on a cost curve for purposes of its step
3 analysis. In any case, EPA notes that the Agency's determination
not to require further EGU controls than EPA identified in this
action, and to a certain extent non-EGU controls, is based primarily
on timing, not a determination of relative cost-effectiveness.
Likewise, emission controls included in the emission budgets in this
rulemaking would likely still be included even if their
representative cost levels were somewhat higher, so long as they
still present a compelling result in the multi-factor test taking
timing and downwind air quality impacts into account.

---------------------------------------------------------------------------

    The uniform control stringency selected in this rule for EGUs
compares favorably with prior transport rulemakings in terms of cost-
effectiveness, overall cost, total reductions, and downwind benefits.
By contrast, when EPA analyzed the best available current data on non-
EGUs for potential control, EPA's analysis showed that at a comparable
cost level ($2,000/ton--on a weighted average basis, rather than the
90th percentile value used as a representative marginal cost used for
EGU SCR optimization, far fewer NOX emission reductions were
available and their corresponding effect on downwind receptors was much
smaller, on the order of a few hundredths of a ppb.
    Regarding the comment that EPA has failed to give independent
effect to the requirement to prohibit emissions that interfere with
maintenance of the NAAQS in other states (i.e., prong 2): EPA gives
effect to prong 2 through identifying receptors that may have trouble
attaining the NAAQS under varying air quality and meteorological
conditions. EME Homer City upheld EPA's approach to using cost to
determine ``amounts'' with respect to both prong 1 and 2, and this is
settled law. EPA v. EME Homer City Generation, 572 U.S. at 518-520.
EPA's use of the term ``significant contribution'' in its analysis at
the third step of the 4-step framework is applied for both prongs 1 and
2. This approach to giving effect to the ``interfere with maintenance''
prong has been upheld twice by the D.C. Circuit. See EME Homer City,
795 F.3d at 136; Wisconsin, 938 F.3d at 325-27. In effect, EPA's
determination of what level of upwind contribution constitutes
``interference'' with a maintenance receptor is the same determination
as what constitutes ``significant contribution'' for a nonattainment
receptor. Nonetheless, this continues to give independent effect to
prong 2 because EPA applies a broader definition for identifying
maintenance receptors, which accounts for the possibility of problems
maintaining the NAAQS under realistic potential future conditions.
While EPA and others may occasionally use the language of
``significance'' as a shorthand for determinations at the third step
under both prongs 1 and 2, this does not detract from the fact that EPA
gives prong 2 independent effect under the 4-step framework.
    EPA has explained elsewhere in the record for this action why the
selected control stringency selected in this rule is appropriate in
light of EPA's application of the step 3 multi-factor test of the 4-
step framework. To the extent commenters argue that EPA should have
selected a higher cost threshold or required more reductions based on
the technical data, those issues are addressed elsewhere in the record.

V. Analyzing Downwind Air Quality and Upwind-State Contributions

    In this section, EPA describes the air quality modeling and
analyses performed to identify nonattainment and/or maintenance
receptors and evaluate interstate contributions to these receptors from
individual upwind states for the 2021 analytic year. Although the air
quality modeling was performed using an air quality modeling platform
that covers the contiguous 48 states, the analysis to identify
receptors and evaluate contributions focuses on the 21 upwind states
that are the subject of this rule with respect to the 2008 ozone NAAQS.
In this action, EPA is not addressing the good neighbor obligations of
any other state, nor is it addressing the obligations of any state,
including the 21 covered by this action, with respect to the 2015 ozone
NAAQS.
    The year 2021 was selected as the appropriate future analytic year
for this rule because it coincides with the July 20, 2021, Serious area
attainment date under the 2008 ozone NAAQS. In the CSAPR Update, EPA
had aligned its analysis and implementation of emission reductions with
the 2017 ozone season (ozone seasons run each year from May 1-September
30) in order to assist downwind states with timely attainment of the
2008 ozone NAAQS by the Moderate area attainment date of July 20, 2018.
See 81 FR 74516. In order to demonstrate attainment by this deadline,
states were required to rely on design values calculated using ozone
season data from 2015 through 2017, since the July 20, 2018, deadline
did not afford enough time for measured data of the full 2018 ozone
season. Similarly, for the Serious area attainment date in 2021, states
will rely on design values calculated using ozone season data from 2018
through 2020. However, it is not possible to impose emission reductions
on upwind states in the 2020 ozone season, which has already passed.
Reductions in the 2021 ozone season will nonetheless occur in time for
the 2021 attainment date and therefore assist downwind states in
achieving attainment by the July 20, 2021, attainment date, in
compliance with the Wisconsin holding. See Wisconsin, 938 F.3d at 309
(the CSAPR Update was unlawful to the extent it allowed upwind states
to ``continue their significant contributions to downwind air quality
problems beyond the statutory deadlines by which downwind States must
demonstrate their attainment of air quality standards'') (emphasis
added). Further, EPA continues to interpret the good neighbor provision
as forward-looking, based on Congress's use of the future-tense
``will'' in section 110(a)(2)(D)(i), an interpretation upheld in
Wisconsin, 938 F.3d at 322. It would be ``anomalous,'' id., for EPA to
impose good neighbor obligations in 2021 and future years based solely
on finding that ``significant contribution'' had existed at some time
in the past.
    EPA has also conducted additional analysis of remaining air quality
receptors and contribution in years beyond 2021, in order to ensure a
complete step 3 analysis. EPA has analyzed these later years to
determine whether any additional emission reductions that are
impossible to obtain by the 2021 attainment date may yet be necessary
in order to fully address significant contribution. This comports with
the D.C. Circuit's direction in Wisconsin that implementing good
neighbor obligations beyond the dates established for attainment may be
justified on a proper showing of impossibility and/or necessity. See
938 F.3d at 320. However, for purposes of EPA's initial analysis of air
quality at step 1 of the 4-step framework, in accordance with
Wisconsin, EPA has selected the 2021 ozone season, corresponding with
the 2021 Serious area attainment date.
    The remainder of this section includes information on: (1) The air
quality modeling platform used in support of this final rule with a
focus on the base year and future year base case emission inventories,
(2) the method for projecting design values in 2021, and (3) the
approach for calculating ozone contributions from upwind states.\74\
The Agency also provides the design values for nonattainment and
maintenance receptors and the predicted interstate contributions that
are at or above the 1 percent of the NAAQS screening threshold. The
2016 base period and 2021, 2023, and 2028 future design values and
contributions for all ozone monitoring sites are provided in the docket
for this rule. The Air Quality Modeling Technical Support Document

(AQM TSD) in the docket for this rule contains more detailed
information on the air quality modeling aspects of this rule.
---------------------------------------------------------------------------

    \74\ For the 2023 and 2028 modeling used in the step 3 analysis,
EPA followed the same method for projecting design values and
approach for calculating contributions as described for the 2021
analytic year.
---------------------------------------------------------------------------

A. Overview of Air Quality Modeling Platform

    EPA used the 2016-based modeling platform for the air quality
modeling for this final rule. This modeling platform includes 2016 base
year emissions from anthropogenic and natural sources and 2016
meteorology. The platform also includes anthropogenic emission
projections for 2023 and 2028. The emissions data contained in this
platform were developed by EPA, Multi-Jurisdictional Organizations
(MJOs), and state and local air agencies as part of the Emissions
Inventory Collaborative Process. This process resulted in a common-use
set of emissions data for a 2016 base year and 2023 and 2028 that can
be leveraged by EPA and states for regulatory air quality modeling.\75\
The air quality modeling was performed for a modeling region (i.e.,
modeling domain) that covers the contiguous 48 states using a
horizontal resolution of 12 x 12 km. EPA used the CAMx version 7beta6
for air quality modeling for both the proposed rule and this final
rule.\76\ Additional information on the 2016-based air quality modeling
platform can be found in the AQM TSD.
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    \75\ http://views.cira.colostate.edu/wiki/wiki/9169.
    \76\ EPA did not receive any comments on the use of CAMx version
7beta6 for the air quality modeling for this rule.
---------------------------------------------------------------------------

B. Emission Inventories

    EPA developed emission inventories for the proposed rule, including
emission estimates for EGUs, non-EGU point sources, stationary nonpoint
sources, onroad mobile sources, nonroad mobile sources, wildfires,
prescribed fires, and biogenic emissions that are not the result of
human activities. EPA's air quality modeling relies on this
comprehensive set of emission inventories because emissions from
multiple source categories are needed to model ambient air quality and
to facilitate comparison of model outputs with ambient measurements. To
prepare the emission inventories for air quality modeling, EPA
processed the emission inventories using the Sparse Matrix Operator
Kernel Emissions (SMOKE) Modeling System version 4.7 to produce the
gridded, hourly, speciated, model-ready emissions for input to the air
quality model. Additional information on the development of the
emission inventories and on data sets used during the emissions
modeling process are provided in the Technical Support Document (TSD)
``Preparation of Emissions Inventories for the 2016v1 North American
Emissions Modeling Platform,'' hereafter known as the ``Emissions
Modeling TSD.'' This TSD is available in the docket for this rule and
at https://www.epa.gov/air-emissions-modeling/2016v1-platform.
1. Foundation Emission Inventory Data Sets
    Emissions data were developed that represented the year 2016 to
support air quality modeling of a base year from which future air
quality could be forecasted. As noted above, EPA used the Inventory
Collaborative 2016 version 1 (2016v1) Emissions Modeling Platform,
released in October 2019, as the primary basis for the inventories
supporting the air quality modeling. This platform was developed
through a national collaborative effort between EPA and state and local
agencies along with MJOs. The original starting point for the U.S.
portions of the 2016 inventory was the 2014 National Emissions
Inventory (NEI), version 2 (2014NEIv2), although all of the inventory
sectors were updated to better represent the year 2016 through the
incorporation of 2016-specific state and local data along with
nationally applied adjustment methods. The future base case inventories
developed for 2023 and 2028 represent projected changes in activity
data and predicted emission reductions from on-the-books actions,
planned emission control installations, and promulgated federal
measures that affect anthropogenic emissions.\77\
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    \77\ Biogenic emissions and emissions from wildfires and
prescribed fires were held constant between 2016 and the future
years because (1) these emissions are tied to the 2016
meteorological conditions and (2) the focus of this rule is on the
contribution from anthropogenic emissions to projected ozone
nonattainment and maintenance.
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2. Development of Emission Inventories for EGUs
    Annual NOX and SO2 emissions for EGUs in the
2016 base year inventory are based primarily on data from continuous
emission monitoring systems (CEMS) and other monitoring systems allowed
for use by qualifying units under 40 CFR part 75, with other EGU
pollutants estimated using emission factors and annual heat input data
reported to EPA. For EGUs not reporting under part 75, EPA used the
most recent data submitted to the NEI by the states. Emissions data for
sources that did not have data provided for the year 2016 were pulled
forward from data submitted for 2014. The Air Emissions Reporting Rule,
(80 FR 8787 February 19, 2015), requires that Type A point sources
large enough to meet or exceed specific thresholds for emissions be
reported to EPA every year, while the smaller Type B point sources must
only be reported to EPA every three years. For more information on how
the 2016 EGU emissions data were developed and prepared for air quality
modeling, see the Emissions Modeling TSD.
    EPA projected future 2023 and 2028 baseline EGU emissions using the
version 6--January 2020 reference case of the Integrated Planning Model
(IPM).\78\ \79\ IPM, developed by ICF Consulting, is a state-of-the-
art, peer-reviewed, multi-regional, dynamic, deterministic linear
programming model of the contiguous U.S. electric power sector. It
provides forecasts of least cost capacity expansion, electricity
dispatch, and emission control strategies while meeting energy demand
and environmental, transmission, dispatch, and reliability constraints.
EPA has used IPM for over two decades to better understand power sector
behavior under future business-as-usual conditions and to evaluate the
economic and emission impacts of prospective environmental policies.
The model is designed to reflect electricity markets as accurately as
possible. EPA uses the best available information from utilities,
industry experts, gas and coal market experts, financial institutions,
and government statistics as the basis for the detailed power sector
modeling in IPM. The model documentation provides additional
information on the assumptions discussed here as well as all other
model assumptions and inputs.\80\
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    \78\ https://www.epa.gov/powersectormodeling.
    \79\ The 2016v1 platform released in October 2019 used the May
2019 reference case. The January 2020 IPM reference case is a later
version than what was originally released with 2016v1.
    \80\ 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:
www.epa.gov/airmarkets/powersectormodeling.
---------------------------------------------------------------------------

    The IPM version 6--January 2020 reference base case accounts for
updated federal and state environmental regulations, committed EGU
retirements and new builds, and technology cost and performance
assumptions as of late 2019. This projected base case accounts for the
effects of the finalized Mercury and Air Toxics Standards rule, the
CSAPR and the CSAPR Update, New Source Review settlements, and other
on-the-books federal and state rules through 2019 \81\ impacting
SO2, NOX,

directly emitted particulate matter, and CO2, and final
actions EPA has taken to implement the Regional Haze Rule.
---------------------------------------------------------------------------

    \81\ For any specific version of IPM there is a cutoff date
after which it is no longer possible to incorporate updates into the
input databases. For version 6--January reference case, that cutoff
date was November 2019.
---------------------------------------------------------------------------

    Additional 2021 EGU emissions baseline levels were developed
through engineering analytics as an alternative approach that did not
involve IPM. EPA developed this inventory for use in step 3 of this
final rule, where it determines emission reduction potential and
corresponding emission budgets. IPM includes optimization and perfect
foresight in solving for least cost dispatch. Given that this final
rule will likely become effective either immediately prior to or
slightly after the start of the 2021 ozone season, EPA adopted a
similar approach to the CSAPR Update where it relied on IPM in a
relative way in step 3 to avoid overstating optimization and dispatch
decisions that were not possible in the short time frame. EPA does this
by using the difference in emission rate observed between IPM runs with
and without the cost threshold applied, rather than using absolute
values. In both the CSAPR Update and in this rule at step 3, EPA
complemented that projected IPM EGU outlook with historical (e.g.,
engineering analytics) perspective based on historical data that only
factors in known changes to the fleet. This 2021 engineering analytics
data set is described in more detail in the Ozone Transport Policy
Analysis Final Rule TSD.
3. Development of Emission Inventories for Non-EGU Point Sources
    The non-EGU point source emissions in the 2016 base case inventory
match those in the 2016v1 platform. Some non-EGU point source emissions
were based on data submitted for 2016, others were projected from 2014
to 2016, and the emissions for remaining small sources were kept at
2014 levels. Prior to air quality modeling, the emission inventories
were processed into a format that is appropriate for the air quality
model to use. Projection factors and percent reductions in this final
rule reflect comments received as a result of the Inventory
Collaborative development process, along with emission reductions due
to national and local rules, control programs, plant closures, consent
decrees, and settlements. Reductions from several Maximum Achievable
Control Technology and National Emission Standards for Hazardous Air
Pollutants (NESHAP) standards are included. Projection approaches for
corn ethanol and biodiesel plants, refineries and upstream impacts
represent requirements pursuant to the Energy Independence and Security
Act of 2007 (EISA). Details on the development and processing of the
non-EGU emissions inventories for 2016, 2023, and 2028 are available in
the Emissions Modeling TSD.
    For aircraft emissions at airports, the emissions used were based
on adjustments to emissions in the 2017 NEI (see https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-data
for data and a TSD). EPA developed and applied factors to adjust the
2017 emissions to 2016, 2023, and 2028 based on activity growth
projected by the Federal Aviation Administration Terminal Area Forecast
\82\ system, published in 2018.
---------------------------------------------------------------------------

    \82\ https://www.faa.gov/data_research/aviation/taf/.
---------------------------------------------------------------------------

    Emissions at rail yards were represented as non-EGU point sources.
The 2016 rail yard emissions are largely consistent with the 2017 NEI
rail yard emissions. The 2016, 2023, and 2028 rail yard emissions were
developed through the Inventory Collaborative process. The rail yard
emissions were interpolated from the 2016 and 2023 emissions. Class I
rail yard emissions were projected using the Energy Information
Administration's 2019 Annual Energy Outlook (AEO) freight rail energy
use growth rate projections for 2016, 2023, and 2028 with the fleet mix
assumed to be constant throughout the period.
    Point source oil and gas emissions for 2016 were based on the
2016v1 point inventory, while nonpoint oil and gas emissions were
primarily based on a run of EPA Oil and Gas Tool for the year 2016. The
2016 oil and gas inventories were projected to 2023 and 2028 using
regional projection factors by product type based on AEO 2019
projections. NOX and VOC reductions that are co-benefits to
the NESHAP and New Source Performance Standards (NSPS) for Stationary
Reciprocating Internal Combustion Engines (RICE) are reflected for
select source categories. In addition, Natural Gas Turbines and Process
Heaters NSPS NOX controls and NSPS Oil and Gas VOC controls
are reflected for select source categories. Additional information on
the development and modeling of the oil and gas emission inventories
can be found in the Emissions Modeling TSD.
4. Development of Emission Inventories for Onroad Mobile Sources
    Onroad mobile sources include exhaust, evaporative, and brake and
tire wear emissions from vehicles that drive on roads, parked vehicles,
and vehicle refueling. Emissions from vehicles using regular gasoline,
high ethanol gasoline, diesel fuel, and electric vehicles were
represented, along with buses that used compressed natural gas. EPA
developed the onroad mobile source emissions for states other than
California using EPA's Motor Vehicle Emissions Simulator (MOVES) 2014b.
MOVES2014b was used with inputs provided by state and local agencies,
where available, in combination with nationally available data sets.
Onroad emissions for the platform were developed based on emissions
factors output from MOVES2014b run for the year 2016, coupled with
activity data (e.g., vehicle miles traveled and vehicle populations)
representing the year 2016. The 2016 activity data were provided by
some state and local agencies, and the remaining activity data were
derived from the 2014NEIv2. The onroad emissions were computed within
SMOKE by multiplying emission factors developed using MOVES with the
appropriate activity data. Onroad mobile source emissions for
California were consistent with the emissions provided by the state.
    The future-year emissions for onroad mobile sources represent all
national control programs known at the time of modeling except for the
Greenhouse Gas Emissions and Fuel Efficiency Standards for Medium- and
Heavy-Duty Engines and Vehicles (HDGHG)--Phase 2 \83\ and the Safer
Affordable Fuel-Efficient (SAFE) Vehicles Rule.\84\ Finalized rules
incorporated into the onroad mobile source emissions include: Tier 3
Standards (March 2014), the Light-Duty Greenhouse Gas Rule (March
2013), Heavy (and Medium)-Duty Greenhouse Gas Rule (August 2011), the
Renewable Fuel Standard (February 2010), the Light Duty Greenhouse Gas
Rule (April 2010), the Corporate-Average Fuel Economy standards for
2008-2011 (April 2010), the 2007 Onroad Heavy-Duty Rule (February
2009), and the Final Mobile Source Air Toxics Rule (MSAT2) (February
2007). Estimates of the impacts of rules that were in effect in 2016
are included in the 2016 base year

emissions at a level that corresponds to the extent to which each rule
had penetrated into the fleet and fuel supply by the year 2016. Local
control programs such as the California LEV III program are included in
the onroad mobile source emissions. The future year onroad emissions
reflect projected changes to fuel properties and usage. MOVES was run
for the years 2023 and 2028 to generate the emissions factors relevant
to those years. Future year activity data for onroad mobile sources
were provided by some state and local agencies, and otherwise were
projected to 2023 and 2028 using AEO 2019-based factors. The future
year emissions were computed within SMOKE by multiplying the future
year emission factors developed using MOVES with the year-specific
activity data. Additional information on the approach for generating
the onroad mobile source emissions is available in the Emissions
Modeling TSD.
---------------------------------------------------------------------------

    \83\ The effect of the HDGHG Phase 2 rule on criteria pollutants
is estimated in Table 5-48 of the Regulatory Impact Analysis,
available from https://nepis.epa.gov/Exe/ZyPDF.cgi/P100P7NS.PDF?Dockey=P100P7NS.PDF.
    \84\ Information on the SAFE vehicles rule is available from
https://www.epa.gov/regulations-emissions-vehicles-and-engines/safer-affordable-fuel-efficient-safe-vehicles-final-rule.
Preliminary analysis by the Office of Transportation and Air Quality
of the impact of this rule on criteria pollutants show impacts of
less than 1 percent for VOC and no impact for NOX.
---------------------------------------------------------------------------

5. Development of Emission Inventories for Commercial Marine Vessels
    The commercial marine vessel (CMV) emissions in the 2016 base case
emission inventory for this rule were based on those in the 2017 NEI.
Factors were then applied to adjust the 2017 NEI emissions backward to
represent emissions for the year 2016. The CMV emissions reflect
reductions associated with the Emissions Control Area proposal to the
International Maritime Organization control strategy (EPA-420-F-10-041,
August 2010); reductions of NOX, VOC, and CO emissions for
new C3 engines that went into effect in 2011; and fuel sulfur limits
that went into effect prior to 2016. The cumulative impacts of these
rules through 2023 and 2028 were incorporated into the projected
emissions for CMV sources. The CMV emissions were split into emissions
inventories from the larger category 3 (C3) engines, and those from the
smaller category 1 and 2 (C1C2) engines. Some minor adjustments to the
CMV emissions were implemented following the October 2019 2016v1
release. These updated CMV inventories were released publicly by
February, 2020.\85\
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    \85\ See 2016emissions, 2023 emissions, and 2028 emissions under
ftp://newftp.epa.gov/air/emismod/2016/v1/.
---------------------------------------------------------------------------

6. Development of Emission Inventories for Other Nonroad Mobile Sources
    Nonroad mobile source emission inventories (other than CMV,
locomotive, and aircraft emissions) were developed from monthly,
county, and process level emissions output from MOVES2014b. MOVES2014b
included important updates to nonroad engine population growth rates.
Types of nonroad equipment include recreational vehicles, pleasure
craft, and construction, agricultural, mining, and lawn and garden
equipment. State-submitted emissions data for nonroad sources were used
for California.
    EPA also ran MOVES2014b for 2023 and 2028 to prepare nonroad mobile
emissions inventories for future years. The nonroad mobile emission
control programs include reductions to locomotives, diesel engines, and
recreational marine engines, along with standards for fuel sulfur
content and evaporative emissions. A comprehensive list of control
programs included for mobile sources is available in the Emissions
Modeling TSD.
    Line haul locomotives are also considered a type of nonroad mobile
source but the emissions inventories for locomotives were not developed
using MOVES2014b. Year 2016 locomotive emissions were developed through
the Inventory Collaborative and are mostly consistent with those in the
2017 NEI. The projected locomotive emissions for 2023 and 2028 were
developed by applying factors to the base year emissions using activity
data based on 2018 AEO freight rail energy use growth rate projections
and emission rates adjusted to account for recent historical trends.
7. Development of Emission Inventories for Nonpoint Sources
    The emissions for stationary nonpoint sources in our 2016 base case
emission inventory are largely consistent with those in the 2014NEIv2,
although some were adjusted to more closely reflect year 2016 using
factors based on changes to human population from 2014 to 2016.
Stationary nonpoint sources include evaporative sources, consumer
products, fuel combustion that is not captured by point sources,
agricultural livestock, agricultural fertilizer, residential wood
combustion, fugitive dust, and oil and gas sources. For more
information on the nonpoint sources in the 2016 base case inventory,
see the Emissions Modeling TSD and the 2014NEIv2 TSD.
    Where states provided the Inventory Collaborative information about
projected control measures or changes in nonpoint source emissions,
those inputs were incorporated into the projected inventories for 2023
and 2028. Adjustments for state fuel sulfur content rules for fuel oil
in the Northeast were included. Projected emissions for portable fuel
containers reflect the impact of projection factors required by the
final MSAT2 rule and the EISA, including updates to cellulosic ethanol
plants, ethanol transport working losses, and ethanol distribution
vapor losses.
    For 2016, nonpoint oil and gas emissions inventories were developed
based on a run of EPA Oil and Gas Tool for 2016. To develop the future
year inventories, regional projection factors for nonpoint oil and gas
sources were developed by product type based on AEO 2019 projections to
2023 and 2028. Estimates of criteria air pollutant (CAP) co-benefit
reductions resulting from the NESHAP for RICE and NSPS rules and Oil
and Gas NSPS VOC controls for select source categories were included.
Additional details on the application of these rules and projections
for nonpoint sources are available in the Emissions Modeling TSD. EPA
received comments on the emissions inventories used in the proposed
rule. These comments and EPA's responses are provided below and in the
RTC.
    Comment: EPA received comments that contend that the Agency did not
include emission reductions from all ``on the books'' control programs
in certain states. These commenters say that monitoring sites that were
identified as nonattainment and/or maintenance receptors might not be
receptors if the Agency had accounted for the impacts of all control
programs.\86\
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    \86\ For emissions sectors other than EGUs, EPA received only a
limited set of comments on the base year and projected emissions
inventories. Comments on emission inventories are addressed
elsewhere in this document and in the RTC.
---------------------------------------------------------------------------

    Response: The emissions inventories used for the step 1 and step 2
air quality modeling of 2023 and 2028 were developed through a
collaborative process through which input from state and local agencies
and multijurisdictional organizations was solicited and accepted. For
point sources, the 2016 inventories were derived from state and local
submissions to the 2016 NEI as required by the Air Emissions Reporting
Rule see 80 FR 8787 (February 19, 2015). Any rules promulgated by 2016
that would have impacted emissions in the year 2016 would be included
in those inventories. EPA then accounted for known changes in those
inventories that would occur by 2023 and 2028 using EPA projection
methods along with stakeholder-developed information. The Midatlantic
Regional Air Management Association (MARAMA) worked with their member
states and Ozone Transport Commission (OTC) states to develop
projection and control factors for the years 2023 and 2028. These
factors were provided to EPA in May

2019 and reflect rules impacting nonpoint sources that were promulgated
prior to 2019. Through the Inventory Collaborative process, the
inventories used for modeling included the ``on the books'' control
programs that were identified by EPA and the state and
multijurisdictional organization (MJO) partners such as MARAMA that
provided inputs to the collaborative inventories. Rules related to
emissions for sources other than EGUs promulgated in 2019 or later
following the completion of the inventories for those sources are not
included in the modeling for this rule.
    The commenter has listed multiple pages of various state-level
NOX and VOC control programs and regulations, promulgated
over multiple decades. The commenter did not provide quantitative
information or data to support their claim that EPA failed to include
the control programs cited by the commenter in the emissions
inventories used to support the proposed rule, what the effect would be
had they been included or characterized differently, and whether the
effect would have changed any of the regulatory outcomes in EPA's
analysis. This comment is further addressed in the RTC.
    Comment: EPA received comment suggesting changes to its EGU
emissions inventory used in its step 1 and step 2 evaluations based on
more recent data.
    Response: EPA is not changing the emissions inventory derived from
its IPM modeling that incorporated the latest data at the time of
execution in January of 2020 used at step 1 and step 2 of the 4-step
framework. However, both in the proposed rule and at final, EPA
reaffirmed its step 1 and step 2 findings using an updated/alternative
EGU emissions inventory from the engineering analytics tool used in
step 3 and discussed in the Ozone Transport Policy Analysis Final Rule
TSD. This tool reflects known changes (e.g., retirements and new
builds) applied to historical data to estimate future year EGU
emissions. It represents alternative EGU emissions inventory
perspective as it does not factor in model-projected changes. Moreover,
it incorporates the latest available data and commenter input regarding
any fleet changes. EPA, in the proposed and final rule, uses this
alternative inventory in conjunction with its air quality assessment
tool (AQAT) to estimate air quality impacts and upwind state
contributions. Both in the proposed rule and final, this alternative
emissions inventory and subsequent AQAT sensitivity analysis led to the
same step 1 and step 2 findings as the IPM-based EGU emissions
inventory and related CAMx modeling results. That is, EPA has examined
a range of EGU inventories using different future year projections and
incorporating the latest available data and commenter input. Across
this range of EGU emission inventory estimates, EPA reaches the same
conclusion for step 1 and step 2 downwind receptors and upwind
linkages. Therefore, EPA's EGU emission inventories and corresponding
step 1 and step 2 analytic findings have been robustly examined, tested
across a range of assumptions, and are robust to a variety of
assumptions, including the unit updates suggested by the commenter. For
a complete unit-by-unit inventory of all EGUs included in the future
year baseline for the engineering analytic tool, see the Ozone
Transport Policy Analysis Final Rule TSD; Appendix A. The data in this
Appendix reflect future unit level operating status taking into account
retirement and new build announcements from both commenter input and
the latest EIA Form 860 monthly (October 2020) available.

C. Air Quality Modeling and Analyses To Identify Nonattainment and
Maintenance Receptors

    In this section the Agency describes the air quality modeling and
analyses performed in Step 1 to identify locations where the Agency
expects there to be nonattainment or maintenance receptors for the 2008
8-hour ozone NAAQS in the 2021 analytic future year. Where EPA's
analysis shows that an area or site does not fall under the definition
of a nonattainment or maintenance receptor in 2021, that site is
excluded from further analysis under EPA's good neighbor framework.
    In this final rule, EPA is not reopening the approach used in the
CSAPR Update to identify nonattainment and maintenance receptors.
Wisconsin upheld EPA's approach to identifying nonattainment and
maintenance receptors against specific challenges. See 938 F.3d at 325-
27.\87\ As this action is taken in response to the Wisconsin remand and
to complete the good neighbor obligations that were partially addressed
in the CSAPR Update, it is entirely appropriate to continue to apply
the same approach to identifying receptors to fully address the
outstanding obligations as EPA took in partially addressing them.
Indeed, to do otherwise would be anomalous and could lead to
inconsistent treatment of states under the 4-step framework for
purposes of the 2008 ozone NAAQS. However, as an aid to understanding
EPA's approach to identifying receptors, a summary of this approach
follows.
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    \87\ The Court's holding rested in part on the partial nature of
the CSAPR Update, id. at 327, and rejected the remainder of the
challenge to EPA's treatment of maintenance receptors because
petitioners in the case failed to establish actual over-control.
Here, EPA has also conducted a rigorous overcontrol analysis showing
that this action does not result in overcontrol. See Ozone Policy
Analysis Final Rule TSD for details.
---------------------------------------------------------------------------

    EPA's approach gives independent effect to both the ``contribute
significantly to nonattainment'' and the ``interfere with maintenance''
prongs of section 110(a)(2)(D)(i)(I), consistent with the D.C.
Circuit's direction in North Carolina.\88\ Further, in its decision on
the remand of the CSAPR from the Supreme Court in the EME Homer City
case, the D.C. Circuit confirmed that EPA's approach to identifying
maintenance receptors in the CSAPR comported with the court's prior
instruction to give independent meaning to the ``interfere with
maintenance'' prong in the good neighbor provision. EME Homer City II,
795 F.3d at 136.
---------------------------------------------------------------------------

    \88\ 531 F.3d at 910-911 (holding that EPA must give
``independent significance'' to each prong of CAA section
110(a)(2)(D)(i)(I)).
---------------------------------------------------------------------------

    In the CSAPR Update, EPA identified nonattainment receptors as
those monitoring sites that are projected to have average design values
that exceed the NAAQS and that are also measuring nonattainment based
on the most recent monitored design values. This approach is consistent
with prior transport rulemakings, such as the NOX SIP Call
and CAIR, where EPA defined nonattainment receptors as those areas that
both currently monitor nonattainment and that EPA projects will be in
nonattainment in the future compliance year.\89\
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    \89\ See 63 FR 57375, 57377 (October 27, 1998); 70 FR
25241(January 14, 2005). See also North Carolina, 531 F.3d at 913-
914 (affirming as reasonable EPA's approach to defining
nonattainment in CAIR).
---------------------------------------------------------------------------

    The Agency explained in the NOX SIP Call and CAIR and
then reaffirmed in the CSAPR Update that EPA has the most confidence in
our projections of nonattainment for those counties that also measure
nonattainment for the most recent period of available ambient data. EPA
separately identified maintenance receptors as those receptors that
would have difficulty maintaining the relevant NAAQS in a scenario that
takes into account historical variability in air quality at that
receptor. The variability in air quality was determined by evaluating
the ``maximum'' future design value at each receptor based on a
projection of the maximum measured design value

over the relevant period. EPA interprets the projected maximum future
design value to be a potential future air quality outcome consistent
with the meteorology that yielded maximum measured concentrations in
the ambient data set analyzed for that receptor (i.e., ozone conducive
meteorology). EPA also recognizes that previously experienced
meteorological conditions (e.g., dominant wind direction, temperatures,
air mass patterns) promoting ozone formation that led to maximum
concentrations in the measured data may reoccur in the future. The
maximum design value gives a reasonable projection of future air
quality at the receptor under a scenario in which such conditions do,
in fact, reoccur. The projected maximum design value is used to
identify upwind emissions that, under those circumstances, could
interfere with the downwind area's ability to maintain the NAAQS.
    Therefore, applying this methodology in this final rule, EPA
assessed the magnitude of the maximum projected design value for 2021
at each receptor in relation to the 2008 ozone NAAQS and, where such a
value exceeds the NAAQS, EPA determined that receptor to be a
``maintenance'' receptor for purposes of defining interference with
maintenance, consistent with the method used in the CSAPR and upheld by
the DC Circuit in EME Homer City II.\90\ That is, monitoring sites with
a maximum design value that exceeds the NAAQS are projected to have a
maintenance problem in 2021.
---------------------------------------------------------------------------

    \90\ See 795 F.3d at 136.
---------------------------------------------------------------------------

    Recognizing that nonattainment receptors are also, by definition,
maintenance receptors, EPA often uses the term ``maintenance-only'' to
refer to receptors that are not also nonattainment receptors.
Consistent with the methodology described above, monitoring sites with
a projected maximum design value that exceeds the NAAQS, but with a
projected average design value that is below the NAAQS, are identified
as maintenance-only receptors. In addition, those sites that are
currently measuring ozone concentrations below the level of the
applicable NAAQS, but are projected to be nonattainment based on the
average design value and that, by definition, are projected to have a
maximum design value above the standard are also identified as
maintenance-only receptors.
    As described above in section V.B., EPA is using the 2016 and 2023
base case emissions developed under the EPA/MJO/state collaborative
project as the primary source for base year and 2023 future year
emissions data for this final rule. Because this platform does not
include emissions for 2021, EPA developed an interpolation technique
based on modeling for 2023 and measured ozone data to determine ozone
concentrations for 2021. To estimate average and maximum design values
for 2021, EPA first performed air quality modeling for 2016 and 2023 to
obtain design values in 2023. The 2023 design values were then coupled
with the corresponding 2016 measured design values to estimate design
values in 2021 using the interpolation technique described below.
    Consistent with EPA's modeling guidance, the 2016 and 2023 air
quality modeling results were used in a ``relative'' sense to project
design values for 2023. That is, the ratios of future year model
predictions to base year model predictions are used to adjust ambient
ozone design values up or down depending on the relative (percent)
change in model predictions for each location. The modeling guidance
recommends using measured ozone concentrations for the 5-year period
centered on the base year as the air quality data starting point for
future year projections. This average design value is used to dampen
the effects of inter-annual variability in meteorology on ozone
concentrations and to provide a reasonable projection of future air
quality at the receptor under ``average'' conditions. In addition, the
Agency calculated maximum design values from within the 5-year base
period to represent conditions when meteorology is more favorable than
average for ozone formation. Because the base year for the air quality
modeling used in this final rule is 2016, the base period 2014-2018
ambient ozone design value data was used in order to project average
and maximum design values in 2023.
    The ozone predictions from the 2016 and 2023 air quality model
simulations were used to project 2014-2018 average and maximum ozone
design values to 2023 using an approach similar to the approach in
EPA's guidance for attainment demonstration modeling. This guidance
recommends using model predictions from the ``3 x 3'' array of grid
cells \91\ surrounding the location of the monitoring site to calculate
a Relative Response Factor (RRF) for that site.\92\ The 2014-2018
average and maximum design values were multiplied by the RRF to project
each of these design values to 2023. In this manner, the projected
design values are grounded in monitored data, and not the absolute
model-predicted 2023 concentrations. In light of comments on the Notice
of Data Availability (82 FR 1733; January 6, 2017) and other analyses,
EPA also projected 2023 design values based on a modified version of
the ``3 x 3'' approach for those monitoring sites located in coastal
areas. In this alternative approach, EPA eliminated from the RRF
calculations the modeling data in those grid cells that are dominated
by water (i.e., more than 50 percent of the area in the grid cell is
water) and that do not contain a monitoring site (i.e., if a grid cell
is more than 50 percent water but contains an air quality monitor, that
cell would remain in the calculation). The choice of more than 50
percent of the grid cell area as water as the criteria for identifying
overwater grid cells is based on the treatment of land use in the
Weather Research and Forecasting model (WRF).\93\ Specifically, in the
WRF meteorological model those grid cells that are greater than 50
percent overwater are treated as being 100 percent overwater. In such
cases the meteorological conditions in the entire grid cell reflect the
vertical mixing and winds over water, even if part of the grid cell
also happens to be over land with land-based emissions, as can often be
the case for coastal areas. Overlaying land-based emissions with
overwater meteorology may be representative of conditions at coastal
monitors during times of on-shore flow associated with synoptic
conditions and/or sea-breeze or lake-breeze wind flows. But there may
be other times, particularly with off-shore wind flow when vertical
mixing of land-based emissions may be too limited due to the presence
of overwater meteorology. Thus, for our modeling EPA calculated 2023
projected average and maximum design values at individual monitoring
sites based on both the ``3 x 3'' approach as well as the alternative
approach that eliminates overwater cells in the RRF calculation

for near-coastal areas (i.e., ``no water'' approach).
---------------------------------------------------------------------------

    \91\ As noted above, each model grid cell is 12 x 12 km.
    \92\ The RRF represents the change in ozone based on emission
changes at a given site. In order to calculate the RRF, EPA's
modeling guidance recommends selecting the 10 highest ozone days in
an ozone season at any given monitor in the base year, noting which
of the grid cells in the 3x3 array experienced the highest ozone
concentrations in the base year, and averaging those ten highest
concentrations. The model is then run using the projected year
emissions, in this case 2023, with all other model variables held
constant. Ozone concentrations from the same ten days, in the same
ten grid cells, are then averaged. The fractional change between the
base year (2011 model run) averaged ozone concentrations and the
future year (2023 model run) averaged ozone concentrations
represents the relative response factor.
    \93\ https://www.mmm.ucar.edu/weather-research-and-forecasting-model.
---------------------------------------------------------------------------

    The 2023 average and maximum design values for both the ``3 x 3''
and ``no water'' approaches were then paired with the corresponding
base period measured design values at each ozone monitoring site.
Design values for 2021 for both approaches were calculated by linearly
interpolating between the 2016 base period and 2023 projected
values.\94\ The steps in the interpolation process for estimating 2021
average and maximum design values are as follows:
---------------------------------------------------------------------------

    \94\ EPA examined the 2019 design values as a way to support the
set of monitoring sites that were identified as receptors based on
the 2021 interpolated design values. The outcome of this analysis
was that each of the five receptors in 2021 had 2019 measured design
values that exceeded the 2008 NAAQS. In addition, there are four
other monitoring sites in the eastern U.S. that are not projected to
be receptors in 2021, but that have 2019 design values that exceeded
the NAAQS. Because the measured design values at these sites are
only 1 or 2 ppb above the NAAQS, it is reasonable to assume that
these four sites will be clean by 2021--which is consistent with the
projections for these monitoring sites. Thus, the analysis of 2019
measured data and 2021 projections provides confidence in the
approach for identifying nonattainment/maintenance receptors in
2021.
---------------------------------------------------------------------------

    (1) Calculate the ppb change in design values between the 2016 base
period and 2023;
    (2) Divide the ppb change by 7 to calculate the ppb change per year
over the 7-year period between 2016 and 2023;
    (3) Multiply the ppb per year value by 5 to calculate the ppb
change in design values over the 5-year period between 2016 and 2021;
    (4) Subtract the ppb change between 2016 to 2021 from the 2016
design values to produce the design values for 2021.
    The projected 2021 and 2023 design values using both the ``3 x 3''
and ``no-water'' approaches are provided in the AQM TSD.\95\ For this
final rule, EPA is relying upon design values based on the ``no water''
approach for identifying nonattainment and maintenance receptors.
---------------------------------------------------------------------------

    \95\ Based on the 2021 design values, there are 129 monitoring
sites that have different design values based on the ``3 x 3''
approach vs the ``no-water'' approach. For these 129 monitoring
sites, the average difference is 0.41 ppb and the median difference
is 0.28 ppb. The average and median percent differences between the
``3 x 3'' and ``no-water'' design values at these 129 monitoring
sites are 0.65 percent and 0.52 percent, respectively. Thus, there
is not much difference in the design values between these two
approaches.
---------------------------------------------------------------------------

    Consistent with the truncation and rounding procedures for the 8-
hour ozone NAAQS, the projected design values are truncated to integers
in units of ppb.\96\ Therefore, projected design values that are
greater than or equal to 76 ppb are considered to be violating the 2008
ozone NAAQS. For those sites that are projected to be violating the
NAAQS based on the average design values in 2021, the Agency examined
the design values for 2019, which are the most recent certified
measured ozone design values at the time of this action. As noted
above, the Agency identified nonattainment receptors in this rulemaking
as those sites that are violating the NAAQS based on current measured
air quality and also have projected average design values of 76 ppb or
greater. Maintenance-only receptors include both (1) those sites with
projected average design values above the NAAQS that are currently
measuring clean data and (2) those sites with projected average design
values below the level of the NAAQS, but with projected maximum design
values of 76 ppb or greater. In addition to the maintenance-only
receptors, the 2021 ozone nonattainment receptors are also maintenance
receptors because the maximum design values for each of these sites is
always greater than or equal to the average design value. The
monitoring sites that the Agency projects to be nonattainment and
maintenance receptors for the ozone NAAQS in the 2021 base case are
used for assessing the contribution of emissions in upwind states to
downwind nonattainment and maintenance of ozone NAAQS as part of this
action.
---------------------------------------------------------------------------

    \96\ 40 CFR part 50, Appendix P to Part 50--Interpretation of
the Primary and Secondary National Ambient Air Quality Standards for
Ozone.
---------------------------------------------------------------------------

    Table V.C-1 contains the 2014-2018 base period average and maximum
8-hour ozone design values, the 2021 base case average and maximum
design values,\97\ and the 2019 design values for the two sites that
are projected to be nonattainment receptors in 2021 and the two sites
that are projected to be maintenance-only receptors in 2021.\98\ The
design values for all monitoring sites in the U.S. are provided in the
docket for this rule. Additional details on the approach for projecting
average and maximum design values are provided in the AQM TSD.
---------------------------------------------------------------------------

    \97\ The design values for 2021 in this table are based on the
``no water'' approach.
    \98\ Using design values from the ``3 x 3'' approach does not
change the total number of receptors in 2021. However, with the ``3
x 3'' approach the maintenance-only receptor in New Haven County, CT
has a projected maximum design value of 75.5 ppb and would,
therefore, not be a receptor using this approach. In contrast,
monitoring site 090010017 in Fairfield County, CT has projected
average and maximum design value of 75.7 and 76.3 ppb, respectively,
with the ``3 x 3'' approach and would, therefore, be a maintenance-
only receptor with this approach.

     Table V.C-1--Average and Maximum 2014-2018 and 2021 Base Case 8-Hour Ozone Design Values and 2019 Preliminary Design Values (ppb) at Projected
                                                        Nonattainment and Maintenance-Only Sites
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                          Average design  Maximum design
             Monitor ID                  State              Site            value 2014-     value 2014-   Average design  Maximum design    2019 Design
                                                                               2018            2018         value 2021      value 2021         value
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                 Nonattainment Receptors
--------------------------------------------------------------------------------------------------------------------------------------------------------
090013007..........................  CT            Stratford............            82.0              83            76.5            77.4              82
090019003..........................  CT            Westport.............            82.7              83            78.5            78.8              82
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                               Maintenance-Only Receptors
--------------------------------------------------------------------------------------------------------------------------------------------------------
090099002..........................  CT            Madison..............            79.7              82            73.9            76.1              82
482010024..........................  TX            Houston..............            79.3              81            75.5            77.1              81
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Comment: Some commenters said that EPA's interpolation method for
determining design values in 2021 is flawed because (1) the method
incorrectly assumes that ozone precursor emissions in all source
sectors in all states change at an equal rate between 2016 and 2023,
(2) linearly interpolated EGU emissions for 2021 overstate EPA's IPM-
predicted EGU emissions for 2021, and (3) the method does not account
for the non-linear

response of ozone to emissions changes. These commenters say that EPA
should have developed a 2021 specific emissions inventory or at a
minimum developed an interpolated 2021 emission inventory and then
rerun the photochemical model to account for the reactivity of ozone
formation from the distribution of ozone precursor emissions. The
commenters contend that failing to take this step, EPA has introduced
significant uncertainty into the air quality projections of the
proposed rule and potentially subjected multiple upwind states to
unnecessary additional control requirements.
    Response: As an initial matter, there is no legal obligation for
EPA to directly model the selected analytic year, here 2021, in order
to make regulatory determinations within the 4-step good neighbor
framework. Given the limited amount of time EPA had to complete this
rulemaking in order to meet the court-ordered March 15 deadline, EPA
reasonably chose to use existing air quality modeling and contribution
information to derive an appropriately reliable projection of air
quality conditions and contributions in 2021. The Supreme Court
recognized in EME Homer City that it is not possible to perfectly
account for all factors that will affect downwind air quality problems
in a future year. Regulators, the Court noted, ``must account for the
vagaries of the wind'' and in assigning upwind responsibility face a
``thorny causation problem.'' 572 U.S. 489, 497, 514. EPA's ultimate
task is not to achieve a perfect understanding of atmospheric
conditions in some future year, but ``to quantify the amount of upwind
gases . . . that must be reduced to enable downwind states to keep
their levels of ozone . . . in check. Id. 497. See also EME Homer City
Generation, L.P. v. EPA, 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. . . . [A] model is meant to
simplify reality in order to make it tractable.'').
    EPA continues to view the interpolation analysis presented at
proposal as sufficiently reliable for purposes of the regulatory
determinations made in this rulemaking. Commenters assert that it is
possible EPA may have found certain upwind state linkages not to exist
had EPA taken a different approach to developing its projections for
2021. But no commenter has established an actual instance of
overcontrol, which the courts have held must be clearly established
through as-applied challenges. See Wisconsin, 938 F.3d at 325 (``
`[T]he Supreme Court has made clear . . . that the way to contest
instances of over-control is not through generalized claims that EPA's
methodology would lead to over-control, but rather through a
``particularized, as-applied challenge.'' ' '') (quoting EME Homer
City, 795 F.3d at 137).
    Nonetheless, in consideration of these comments, EPA has performed
additional analysis, which confirms the regulatory determinations EPA
proposed and is now finalizing. EPA was able to construct an emissions
inventory for 2021, using available data and the same approach as EPA
used to develop projection inventories for 2023 and 2028. Details on
the construct of the 2021 emissions are provided in the Emissions
Modeling TSD. There was, however, insufficient time to perform air
quality modeling using this newly constructed 2021 inventory. Instead
EPA used the Air Quality Assessment Tool (AQAT) to perform a
sensitivity analysis to determine whether there would be any change in
the outcome of this rule if the projection of 2021 air quality were
based on projected 2021 emissions rather than EPA's interpolation
method, as described above. In brief, AQAT uses the results of existing
base year and future year air quality modeling as part of an
interpolation technique to estimate ozone design values and
contributions for analytic years that are not modeled as well as to
analyze the air quality impacts of control scenarios in step 3 of the
4-step transport framework. AQAT is calibrated using model simulations
to account for the non-linearity response of ozone to emissions
changes. As noted by the commenter, EPA's interpolation approach
inherently assumes that the relative change in emissions between 2016
and 2023 is the same across all states. Because this application of
AQAT considered 2021 state level emissions on a state-by-state basis,
the analysis accounted for any state-to-state differences in the change
in emissions between 2016 and 2023. As part of this sensitivity
analysis EPA coupled the 2021 emissions and 2023 model-predicted ozone
design values and contributions to estimate design values and
contributions in 2021. EPA also used the 2021 emissions in AQAT to
create a more-refined interpolated 2022 emission inventory. EPA then
used the AQAT to examine the effects of this refined 2022 emission
inventory on ozone design values and contributions. The results
indicate that any changes in the nonattainment or maintenance status of
individual receptors using 2021 and 2022 projected emissions would not
affect which upwind states significantly contribute to nonattainment
and/or interfere with maintenance of the 2008 NAAQS in another
state.\99\ Details on AQAT and this sensitivity analysis can be found
in the Ozone Policy Analysis Final Rule TSD.
---------------------------------------------------------------------------

    \99\ Because EPA directly modeled 2023 and 2028, EPA relied
solely on that modeling, and associated inventories, for its
analysis of 2023 and later years.
---------------------------------------------------------------------------

    Comment: Other commenters claim that there is a disconnect between
EPA's projected 2021 design values and current ozone monitoring data.
These commenters said that EPA should give priority to monitored data
over modeled data when evaluating which areas need transport
obligations resolved. Specifically, one commenter performed an analysis
to estimate 2021 design values by first estimating a fourth high
maximum daily average 8-hour (MDA8) ozone concentration in 2021 based
on the four-year average of the measured fourth high values during the
period 2017 through 2020 and second, calculating the 2021 design value
as the average of the measured fourth high value in 2019, the
preliminary fourth high value in 2020 and the estimated fourth high
value in 2021.\100\ Another commenter performed a statistical linear
regression analysis of the fourth highest measured values for each of
three time periods: 2012 through 2020, 2014 through 2020, and 2016
through 2020 to estimate fourth highest values in 2021 that would
result in nonattainment in 2021 at individual monitoring sites. This
commenter said that an assessment of actual ambient monitor data, such
as the analysis performed by this commenter, should be given as much
weight, if not more, in identifying receptors in 2021 as the modeling-
based analysis performed by EPA. Both commenters said that the results
of their analyses support EPA's finding that the four monitoring sites
identified in Table V.C-1, above will be receptors in 2021. However,
both commenters claim that the Madison, Connecticut monitoring site
090099002 will be a nonattainment receptor, whereas EPA projects this
site to be a maintenance-only receptor in 2021. Also, both commenters
claim that there will be an additional 2021 nonattainment receptor at
the Greenwich, Connecticut monitoring site 090010017. One commenter
noted that identifying the Madison monitoring site as nonattainment
instead of maintenance-only and the Greenwich

monitoring site as a receptor will not alter the outcome of EPA's
determination of which upwind states are linked to downwind receptors
at step 2 of the 4-step transport framework.
---------------------------------------------------------------------------

    \100\ Ozone design values and fourth high maximum daily 8-hour
ozone concentrations for 2020 are preliminary and have not yet been
cerified by EPA.
---------------------------------------------------------------------------

    In addition to the 2021 receptors in Connecticut, one commenter
said that there will be two additional monitoring sites in the eastern
U.S. that each have a chance of being a nonattainment or maintenance
receptor in 2021. These monitoring sites are Houston-Deer Parksite
492011039 and Dallas-Grapevine site 48439007). The other commenter said
that their analysis shows that there will be up to four additional
nonattainment receptors in 2021 in the eastern U.S. outside of
Connecticut. These monitoring sites include the Chicago-Northbrook,
Illinois monitoring site 170314201, the Michigan City, Indiana
monitoring site 180910005, the El Paso, Texas monitoring site
481410037, and the Dallas-Eagle Mountain Rock monitoring site
484390075.
    Response: EPA agrees with these commenters that the four monitoring
sites identified by EPA as receptors in Table V.C-1 will be receptors
in 2021. EPA also agrees that there would be no change in the upwind
states covered by this rule if the Madison, Connecticut maintenance-
only receptor is a nonattainment receptor rather than maintenance-only
receptor. As described above, a maintenance-only receptor is a
monitoring site that is at risk of being in nonattainment under
meteorological conditions that are more conducive than average for
ozone formation. Also, upwind states that are linked to maintenance-
only receptors are evaluated by EPA using the same approach as those
upwind states linked to nonattainment receptors in EPA's analysis of
significant contribution in step 3 of the 4-step transport framework.
Regarding the Greenwich, Connecticut monitoring site, EPA's
contribution data, as provided in the docket for this rule, shows that
there would be no additional upwind states covered by this rule if this
monitoring site was included as a receptor in 2021. That is, all the
upwind states that are linked to this monitoring site, using a 1
percent of the NAAQS threshold, are also linked to one or more of the
other 2021 nonattainment and/or maintenance receptors in Connecticut
that are identified in Table V.C-1.
    EPA disagrees with the commenters that the six additional
monitoring sites (i.e., Chicago/Northbrook, Dallas/Eagle Mountain Rock,
Dallas/Grapevine, El Paso, Houston/Deer Park, and Michigan City) will
be nonattainment or maintenance receptors in 2021. First, as explained
in the Air Quality TSD, these sites are not identified in the
methodology EPA uses to identify nonattainment and maintenance
receptors. These conclusions are bolstered by EPA's review of measured
design values for the period 2012 through 2019 at each of these six
monitoring sites (see Table V.C-2). These data show that each of these
sites, except for the site in Michigan City, is not measuring
nonattainment based on their 2019 design value, which are the most
recent official design values based on state-certified data. Moreover,
the monitoring site in El Paso has not measured a violation during this
entire eight-year time period; the Houston/Deer Park site has not
measured a violation in the most recent 6 years; the Dallas/Eagle
Mountain Lake site has not measured a violation in the most recent 4
years; the Chicago/Northbrook site has measured only 1 violation in the
most recent 6 years; and the Dallas/Grapevine site has measured only
one violation in the most recent 4 years. At the Michigan City site,
there are no official measured design values in 2016, 2017, and 2018
because there was no valid fourth high MDA8 ozone concentration in
2016. As a result, the data at this site did not meet the criteria in
EPA's modeling guidance for calculating valid future year design
values. As such, EPA has not calculated projected design values nor any
contributions for this site.

                               Table V.C-2--Ozone Design Values at Monitoring Sites Identified as Receptors by Commenters
--------------------------------------------------------------------------------------------------------------------------------------------------------
         Site ID               State              County                 Site name         2012    2013    2014    2015    2016    2017    2018    2019
--------------------------------------------------------------------------------------------------------------------------------------------------------
170314201................  IL             Cook..................  Chicago/Northbrook....      78      77      74      68      71      72      77      74
180910005................  IN             LaPorte...............  Michigan City.........      83      83      79      68  ......  ......  ......      76
481410037................  TX             El Paso...............  El Paso...............      72      72      72      71      70      71      73      75
482011039................  TX             Harris................  Houston/Deer Park.....      84      79      72      69      67      68      71      75
484390075................  TX             Tarrant...............  Dallas/Eagle Mountain       82      81      79      76      72      71      70      73
                                                                   Lake.
484393009................  TX             Tarrant...............  Dallas/Grapevine......      86      86      80      78      75      75      76      75
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Comment: In the proposed rule EPA requested comment on applying the
``3 x 3'' approach and the ``no water cell'' approach, described above,
to identify modeled-grid cells for use in projecting ozone design
values to a future year. One commenter said that both the ``3 x 3'' and
``no water cell'' approaches are acceptable, a second commenter
supported the use of the ``no water cell'' approach, while a third
commenter suggested that EPA modify the ``no water cell'' approach to
exclude from the calculation of projected design values any data from
the grid cell containing the monitoring site, if the monitor grid cell
is also dominated by water.
    Response: EPA has considered these comments and will continue to
rely upon the ``no water cell'' approach used for the proposed rule to
calculate projected design values at monitoring sites in coastal areas.
The alternative suggested by one commenter to exclude model data from
the grid cell containing the monitoring site, if that grid cells is
classified as a ``water'' grid cell, ignores the modeling data for the
location of the monitoring state which is contrary to EPA's air quality
modeling guidance. This guidance recommends that the calculation of
ozone relative response factors, which are used in projecting future
year design values, include the modeled data in grid cells immediately
surrounding the monitoring site along with the grid cell in which the
monitor is located. For coastal monitoring sites, the grid cell in
which the monitor is located is more likely to be representative of the
monitor locations, than adjacent over-water grid cells. In this regard,
the approach suggested by the commenter is too restrictive in that
modeling data in the grid cell containing the monitoring site would
never be used in projecting design values for that monitor.

D. Pollutant Transport From Upwind States

1. Air Quality Modeling To Quantify Upwind State Contributions
    This section documents the procedures EPA used to quantify the
impact of emissions from specific upwind states on 2021 8-hour design
values for the identified downwind nonattainment and maintenance

receptors. EPA used CAMx photochemical source apportionment modeling to
quantify the impact of emissions in specific upwind states on downwind
nonattainment and maintenance receptors for 8-hour ozone. CAMx employs
enhanced source apportionment techniques that track the formation and
transport of ozone from specific emissions sources and calculates the
contribution of sources and precursors to ozone for individual receptor
locations. The strength of the photochemical model source apportionment
technique is that all modeled ozone at a given receptor location in the
modeling domain is tracked back to specific sources of emissions and
boundary conditions to fully characterize culpable sources.
    EPA performed nationwide, state-level ozone source apportionment
modeling using the CAMx Ozone Source Apportionment Technology/
Anthropogenic Precursor Culpability Analysis (OSAT/APCA) technique
\101\ to quantify the contribution of 2023 base case NOX and
VOC emissions from all sources in each state to projected 2023 ozone
design values at air quality monitoring sites. The CAMx OSAT/APCA model
run was performed for the period May 1 through September 30 using the
projected 2023 base case emissions and 2016 meteorology for this time
period. As described below, in the source apportionment modeling the
Agency tracked (i.e., tagged) the amount of ozone formed from
anthropogenic emissions in each state individually as well as the
contributions from other sources (e.g., natural emissions).
---------------------------------------------------------------------------

    \101\ As part of this technique, ozone formed from reactions
between biogenic VOC and anthropogenic NOX or biogenic
NOX and anthropogenic VOC are assigned to the
anthropogenic emissions. This approach is designed to fully capture
as part of the anthropogenic contribution the total amount of ozone
formed from photochemical reactions that involve emissions from all
anthropogenic sources. In this manner, ozone is assigned to the
controllable (i.e., anthropogenic) precursors that react with non-
controllable (i.e., biogenic) precursors.
---------------------------------------------------------------------------

    To determine upwind contributions in 2021 the Agency applied the
contributions from the 2023 modeling in a relative manner to the 2021
ozone design values. The analytic steps in the process are as follows:
    (1) Calculate the 8-hour average contribution from each source tag
to each monitoring site for the time period of the 8-hour daily maximum
modeled concentrations in 2023;
    (2) Average the contributions and concentrations for each of the
top 10 modeled ozone concentration days in 2023 \102\ and then divide
the average contribution by the corresponding concentration to obtain a
Relative Contribution Factor (RCF) for each monitoring site; and
---------------------------------------------------------------------------

    \102\ The number of days used in calculating the average
contribution metric has historically been determined in a manner
that is generally consistent with EPA's recommendations for
projecting future year ozone design values. Our ozone attainment
demonstration modeling guidance at the time of the CSAPR recommended
using all model-predicted days above the NAAQS to calculate future
year design values (https://www3.epa.gov/ttn/scram/guidance/guide/final-03-pm-rh-guidance.pdf). In 2014 EPA issued draft revised
guidance that changed the recommended number of days to the top-10
model predicted days (https://www3.epa.gov/ttn/scram/guidance/guide/Draft-O3-PM-RH-Modeling_Guidance-2014.pdf). For the CSAPR Update EPA
transitioned to calculating design values based on this draft
revised approach. The revised modeling guidance was finalized in
2019 and, in this regard, EPA is calculating both the ozone design
values and the contributions based on a top-10 day approach (https://www3.epa.gov/ttn/scram/guidance/guide/O3-PM-RH-Modeling_Guidance-2018.pdf).
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    (3) Multiply the 2021 design values by the 2023 RCF at each site to
produce the average contribution metric values in 2021.\103\ The
resulting 2021 contributions from each tag to each monitoring site in
the U.S. along with additional details on the source apportionment
modeling and the procedures for calculating contributions can be found
in the AQM TSD.
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    \103\ The method for calculating the average contribution metric
values in 2021 was also applied to 2023 and 2028 based on the
projected design values and contribution modeling for each of those
years, respectively.
---------------------------------------------------------------------------

    In the source apportionment model run, EPA tracked the ozone formed
from each of the following tags:
     States--anthropogenic NOX and VOC emissions
from each state tracked individually (emissions from all anthropogenic
sectors in a given state were combined);
     Biogenics--biogenic NOX and VOC emissions
domain-wide (i.e., not by state);
     Boundary Concentrations--concentrations transported into
the modeling domain;
     Tribes--the emissions from those tribal lands for which
the Agency has point source inventory data in the 2016v1 emissions
modeling platform (EPA did not model the contributions from individual
tribes);
     Canada and Mexico--anthropogenic emissions from sources in
the portions of Canada and Mexico included in the modeling domain (EPA
did not model the contributions from Canada and Mexico separately);
     Fires--combined emissions from wild and prescribed fires
domain-wide (i.e., not by state); and
     Offshore--combined emissions from offshore marine vessels
and offshore drilling platforms.
    The contribution modeling provided contributions to ozone from
anthropogenic NOX and VOC emissions in each state,
individually. The contributions to ozone from chemical reactions
between biogenic NOX and VOC emissions were modeled and
assigned to the ``biogenic'' category. The contributions from wildfire
and prescribed fire NOX and VOC emissions were modeled and
assigned to the ``fires'' category. That is, the contributions from the
``biogenic'' and ``fires'' categories are not assigned to individual
states nor are they included in the state contributions.
    The average contribution metric is intended to provide a reasonable
representation of the contribution from individual states to the
projected 2021 design value, based on modeled transport patterns and
other meteorological conditions generally associated with modeled high
ozone concentrations at the receptor. An average contribution metric
constructed in this manner is beneficial since the magnitude of the
contributions is directly related to the magnitude of the design value
at each site.
    The largest contribution from each state that is the subject of
this rule to 8-hour ozone nonattainment and maintenance receptors in
downwind states in 2021 is provided in Table V.D-1.

Table V.D-1--Largest Contribution to Downwind 8-Hour Ozone Nonattainment
                    and Maintenance Receptors in 2021
------------------------------------------------------------------------
                                Largest downwind      Largest downwind
                                 contribution to       contribution to
        Upwind state              nonattainment       maintenance-only
                               receptors for ozone   receptors for ozone
                                      (ppb)                 (ppb)
------------------------------------------------------------------------
Alabama.....................                  0.11                  0.27

Arkansas....................                  0.18                  0.15
Illinois....................                  0.81                  0.80
Indiana.....................                  1.26                  1.08
Iowa........................                  0.17                  0.22
Kansas......................                  0.13                  0.11
Kentucky....................                  0.87                  0.79
Louisiana...................                  0.27                  4.68
Maryland....................                  1.21                  1.56
Michigan....................                  1.71                  1.62
Mississippi.................                  0.10                  0.37
Missouri....................                  0.36                  0.33
New Jersey..................                  8.62                  5.71
New York....................                 14.44                 12.54
Ohio........................                  2.55                  2.35
Oklahoma....................                  0.20                  0.14
Pennsylvania................                  6.86                  5.64
Texas.......................                  0.59                  0.36
Virginia....................                  1.30                  1.69
West Virginia...............                  1.49                  1.55
Wisconsin...................                  0.23                  0.23
------------------------------------------------------------------------

    Comment: One commenter said that the future year average
contribution metric should be calculated using the modeled
contributions on the same days that were used to calculate the RRFs for
projecting future ozone design values.
    Response: EPA believes that its approach, as described above, for
calculating the future year average contribution metric provides a more
technically reliable estimate of contributions than the method
suggested by the commenter. In calculating the average contribution
metric, EPA uses modeled contributions on the 10 days in the future
year with the highest model-predicted concentrations.\104\ In part
because the formation of ozone from precursor emissions can be highly
nonlinear and dependent on meteorological conditions, the response of
ozone to emission reductions can vary from day to day. In this regard,
the days with the highest model-predicted ozone concentrations in the
2016 base year that are used for projecting ozone design values may not
be among the highest ozone days in the future analytic year. In this
situation, the calculation of the contribution metric could exclude
days with higher concentrations in the future year in favor of lower
future-concentration days that happened to correspond to the highest
days in 2016. The problems with basing the calculation of future year
average contributions on the days that were used to project design
values are illustrated in Table V.D-2. Table V.D-2 includes the data
for all the days that were either used to project design values and/or
to calculate the average contribution values from each upwind state to
a particular receptor. The data in the ``2016 Modeled'' column are the
2016 base year MDA8 ozone concentrations and the data in the ``2023
Modeled'' column are the MDA8 ozone concentrations in 2023. The data in
the table are ranked based on the magnitude of the 2016 MDA8
concentrations.\105\ Comparing the 2023 MDA8 ozone concentrations to
the corresponding 2016 values shows that the days with the highest MDA8
ozone concentrations in 2016 are not the same days as the highest MDA8
ozone concentrations in 2023. Of importance, the top 10 days based on
2016 model predictions includes five days with 2023 MDA8 ozone
concentrations below 60 ppb. In calculating the average contribution
metric EPA excludes from the calculation all days with future year
modeled MDA8 concentrations below 60 ppb. Thus, using EPA's approach
the average contribution metric in this example would be calculated
based on daily contribution data for the top 6 MDA8 concentration days
in 2023, because the remaining top 10 future year days are below 60 ppb
(i.e., 05/06, 05/13, 06/08, 09/12, and 09/28). Moreover, even though
the concentration on the sixth-highest day in 2023 is 60 ppb, the
contribution data on this day would be excluded from the calculations
because this day is not among the top 10 days used to project design
values.
---------------------------------------------------------------------------

    \104\ If there are fewer than 5 days with model-predicted future
year ozone concentrations greater than or equal to 60 ppb, then an
average contribution metric is not calculated because. Using the 60
ppb criteria aligns with the criteria for projecting future year
design values, as recommended in EPA's air quality modeling
guidance.
    \105\ Top 10 days that have modeled MDA8 ozone predictions less
than 60 ppb are not included in the RRF calculation.

     Table V.D-2--MDA8 Ozone Concentrations in 2016 Used To Project Design Values and the 2023 Modeled MDA8
                                      Concentrations on the Same Days (ppb)
----------------------------------------------------------------------------------------------------------------
                      Date                           2016 Rank     2016 Modeled      2023 Rank     2023 Modeled
----------------------------------------------------------------------------------------------------------------
07/01...........................................               1            79.4               3            69.1
06/27...........................................               2            79.4               1            74.5
05/12...........................................               3            76.4               2            69.7

06/08...........................................               4            71.9               7            59.5
09/12...........................................               5            69.4              13            51.8
09/28...........................................               6            68.5              10            56.3
08/09...........................................               7            68.5               5            61.0
05/13...........................................               8            67.8               9            57.1
09/19...........................................               9            67.5               4            61.3
05/06...........................................              10            67.1               8            58.1
08/08...........................................              11            65.8              12            54.4
07/21...........................................              12            65.2              11            55.9
06/30...........................................              13            64.8              14            50.0
05/10...........................................              14            63.4               6            60.0
----------------------------------------------------------------------------------------------------------------

    It is obviously impossible for EPA, or anyone, to predict which
exact days in a future year will have high ozone levels, nor does it
make sense to analyze contribution on modeled days of low ozone
concentration. EPA's methodology is reasonable in projecting where
ozone problems are likely to recur in a future year and analyzing who
is contributing to those problems under the conditions for high ozone
formation in those locations.
    Comment: One commenter said that EPA should base the calculation of
the future year contribution metric on days with measured exceedances
of the NAAQS. Specifically, the comment asked EPA to examine the 2016
measured concentrations at receptors in Connecticut to ensure that the
contribution from Illinois to these receptors was calculated on days
when the monitor measured exceedances.
    Response: EPA continues to believe that the future year
contribution metric should be based on the highest ozone concentration
days in the future year. However, as a sensitivity analysis EPA
recalculated the average contribution from Illinois to the three
receptors in Connecticut using the daily contributions on days with
measured exceedances of the NAAQS, after applying the 60 ppb screening
criteria to eliminate from the calculations those days with future year
model-predicted MDA8 ozone concentrations below 60 ppb. The results of
this sensitivity analysis, as provided in Table V.D-3, show that
Illinois would contribute above the 1 percent of the NAAQS screening
threshold to each of the three Connecticut receptors using the approach
suggested by the commenter.

     Table V.D-3--Contributions From Illinois (ppb) to Receptors in
                               Connecticut
------------------------------------------------------------------------
                                                           Contribution
                                           Contribution      based on
                Receptor                  based on EPA's     measured
                                              method        exceedance
                                                               days
------------------------------------------------------------------------
Stratford...............................            0.69            0.98
Westport................................            0.81            0.76
Madison.................................            0.80            1.03
------------------------------------------------------------------------

2. Application of Screening Threshold
    EPA evaluated the magnitude of the contributions from each upwind
state to downwind nonattainment and maintenance receptors. In step 2 of
the good neighbor framework, EPA uses an air quality screening
threshold to identify upwind states that contribute to downwind ozone
concentrations in amounts sufficient to ``link'' them to these to
downwind nonattainment and maintenance receptors. The contributions
from each of the CSAPR Update states to each downwind nonattainment
and/or maintenance receptor that were used for the step 2 evaluation
can be found in the AQM TSD.
    As discussed above in section IV, EPA is not reopening the air
quality screening threshold of 1 percent of the NAAQS used in the CSAPR
Update. Therefore, as in the CSAPR Update, EPA uses an 8-hour ozone
value for this air quality threshold of 0.75 ppb as the quantification
of 1 percent of the 2008 ozone NAAQS.
    Comment: Several commenters said that EPA's 1 percent of the NAAQS
threshold is too low and that, instead, a threshold of 1 ppb or 2 ppb
should be used as the contribution screening threshold in step 2.
    Response: As noted above, the Agency is not reopening the use of
the 1 percent threshold in this action to address the remand of the
CSAPR Update. This action is taken in response to the Wisconsin remand
and to complete the good neighbor obligations that were partially
addressed in the CSAPR Update. It is entirely appropriate to continue
to apply the same screening threshold to identifying receptors to fully
address the outstanding obligations as EPA took in initially addressing
them. Indeed, to do otherwise would be anomalous and pose a risk of
inconsistent requirements for different states. While the Agency is not
reopening the application of the 1 percent threshold in this action on
remand, explanation for how this value was originally derived is
available in the CSAPR rulemaking in 2011. 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.\106\ EPA therefore determined the 1
percent threshold was appropriate for purposes of good neighbor
obligations under the 2008 ozone NAAQS. This determination was not
challenged in the Wisconsin case. Thus, EPA is applying the 1 percent
threshold at step 2, consistent with its initial analysis of
obligations in the CSAPR Update and without reopening its prior
determination on this issue in that rule.
---------------------------------------------------------------------------

    \106\ See Final CSAPR Update Air Quality Modeling TSD, at 27-30
(EPA-HQ-OAR-2015-0596-0144).
---------------------------------------------------------------------------

a. States That Contribute Below the Screening Threshold
    Of the 21 states that are the subject of this final rule, EPA has
determined that the contributions from each of the

following states to nonattainment and/or maintenance-only receptors in
the 2021 analytic year are below the threshold: Alabama, Arkansas,
Iowa, Kansas, Mississippi, Missouri, Oklahoma, Texas, and Wisconsin.
Because these states are considered not to contribute to projected
downwind air quality problems, EPA is determining that the CSAPR Update
FIPs for these states (or, in the case of Alabama and Missouri, the SIP
revisions later approved to replace the states' CSAPR Update FIPs) are
a complete remedy to address their significant contribution under the
good neighbor provision for the 2008 ozone NAAQS. These states remain
subject to the ozone season NOX emission budgets established
in the CSAPR Update, and EPA is not reopening the determinations in the
CSAPR Update regarding these states.\107\
---------------------------------------------------------------------------

    \107\ EPA notes that the updated modeling establishing that
these states no longer contribute as of 2021 assumes in its baseline
the continued implementation of the CSAPR Update budgets in these
states.
---------------------------------------------------------------------------

b. States That Contribute at or Above the Screening Threshold
    In this final rule, states with remanded emission budgets under the
CSAPR Update that contribute to a specific receptor in an amount at or
above the screening threshold in 2021 are considered linked to that
receptor. The ozone contributions and emissions (and available emission
reductions) for these states are analyzed further at step 3, as
described in section VI, to determine whether and to what extent
emission reductions might be required from each state.
    Based on the maximum downwind contributions in Table V.D-1, the
step 2 analysis identifies that the following 11 states contribute at
or above the 0.75 ppb threshold to downwind nonattainment receptors: