in response to comments. For the final rule, the point source oil and
gas emissions for 2016 were based on the 2016v2 point inventory except
that most 2014 NEI-based emissions were replaced with 2017 NEI
emissions. Additionally, in response to comments, state-provided
emissions equivalent to those in the 2016v1 platform were used for
Colorado, and some New Mexico emissions were replaced with data
backcast from 2020 to 2016. To develop inventories for 2023 and 2026
for the final rule, the year 2016 oil and gas point source inventories
were first projected to 2021 values based on actual historical
production data, then those 2021 emissions were projected to 2023 and
2026 using regional projection factors based on AEO 2022 projections.
This was an update from the proposal approach that used actual data
only through the year 2019, because 2021 data were not yet available.
NOX and VOC reductions resulting from co-benefits of NSPS
for Stationary Reciprocating Internal Combustion Engines (RICE) are
reflected, along with Natural Gas Turbine and Process Heater NSPS
NOX controls and Oil and Gas NSPS VOC controls. In some
cases, year 2019 point source inventory data were used instead of the
projected future year emissions except for the Western Regional Air
Partnership (WRAP) states of Colorado, New Mexico, Montana, Wyoming,
Utah, North Dakota, and South Dakota. The WRAP future year inventory
\148\ was used in these WRAP states in all future years except in New
Mexico where the WRAP base year emissions were projected using the EIA
historical and AEO forecasted production data. Estimated impacts from
the New Mexico Administrative code 20.2.50 \149\ were also included.
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    \148\ http://www.wrapair2.org/pdf/WRAP_OGWG_2028_OTB_RevFinalReport_05March2020.pdf.
    \149\ https://www.srca.nm.gov/parts/title20/20.002.0050.html.
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4. Development of Emissions 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. The EPA

developed the onroad mobile source emissions for states other than
California using the EPA's Motor Vehicle Emissions Simulator (MOVES).
MOVES3 was released in November 2020 and has been followed by some
minor releases that improved the usage of the model but that do not
have substantive impacts on the emissions estimates. For the proposal,
MOVES3 was run using inputs provided by state and local agencies
through the 2017 NEI where available, in combination with nationally
available data sets to develop a complete inventory. Onroad emissions
were developed based on emissions factors output from MOVES3 runs 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 through the 2016v1
process, and the remaining activity data were derived from those used
to develop the 2017 NEI. The onroad emissions were computed within
SMOKE by multiplying emissions factors developed using MOVES with the
appropriate activity data. Prior to computing the final rule emissions,
updates to some onroad inputs were made in response to comments and to
implement corrections. Onroad mobile source emissions for California
were consistent with the updated emissions data provided by the state
for the final rule.
    The 2023 and 2026 onroad emissions reflect projected changes to
fuel properties and usage, along with the impact of the rules included
in MOVES3 for each of those years. MOVES emissions factors for the
years 2023 and 2026 were used. A comprehensive list of control programs
included for onroad mobile sources is available in the 2016v3 Emissions
Modeling TSD. Year 2023 and 2026 activity data for onroad mobile
sources were provided by some state and local agencies, and otherwise
were projected to 2023 and 2026 by first projecting the 2016 activity
to year 2019 based on county level vehicle miles traveled (VMT) from
the Federal Highway Administration. Because VMT for onroad mobile
sources were substantially impacted by the pandemic and took about two
years to rebound to pre-pandemic levels, in the 2016v3 platform no
growth in VMT was implemented from 2019 to. The estimated 2021 VMT were
then grown from 2021 to 2023 and 2026 using AEO 2022-based factors.
Recent updates to inspection and maintenance programs in North Carolina
and Tennessee were reflected in the MOVES inputs for the final rule
modeling. The 2023 and 2026 onroad mobile emissions were computed
within SMOKE by multiplying the respective emissions factors developed
using MOVES with the year-specific activity data. Prior to computing
the final rule emissions for 2023, the EPA made updates to some onroad
inputs in response to comments and to implement corrections.
5. Development of Emissions Inventories for Commercial Marine Vessels
    The commercial marine vessel (CMV) emissions in the 2016 base case
emissions inventory for this rule were based on those in the 2017 NEI.
Factors were 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 category 3 (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 2026 were incorporated into the
projected emissions for CMV sources. The CMV emissions were split into
emissions inventories from the larger C3 engines, and those from the
smaller category 1 and 2 (C1C2) engines. CMV emissions in California
are based on emissions provided by the state. The CMV emissions are
consistent with the emissions for the 2016v1 platform updated CMV
emissions released by February 2020 although they include projected
emissions for the years of 2023 and 2026 instead of 2023 and 2028. In
addition, in response to comments, the EPA implemented an improved
process for spatial allocating CMV emissions along state and county
boundaries.
6. Development of Emissions Inventories for Other Nonroad Mobile
Sources
    The EPA developed nonroad mobile source emissions inventories
(other than CMV, locomotive, and aircraft emissions) for 2016, 2023,
and 2026 from monthly, county, and process level emissions output from
MOVES3. 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. The nonroad emissions for the final rule were
unchanged from those at the proposal. The nonroad mobile emissions
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 2016v3
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 MOVES3. Year 2016 locomotive emissions were developed through the
2016v1 collaborative process and the year 2016 emissions are mostly
consistent with those in the 2017 NEI. More information on the
development of the Class I, Class II and III, and commuter rail line
haul locomotive emissions is available in the 2016v3 Emissions Modeling
TSD. The projected locomotive emissions for 2023 and 2026 were
developed by applying factors to the 2016 emissions using activity data
based on AEO freight rail energy use growth rate projections along with
emissions rates adjusted to account for recent historical trends. The
emission factors used for NOX, PM10 and VOC for line haul
locomotives in the analytic years were derived from trend lines based
on historic line-haul emission factors from the period of 2007 through
2017 and extrapolated to 2023 and 2026.
7. Development of Emissions Inventories for Nonpoint Sources
    For stationary nonpoint sources, some emissions in the 2016 base
case emissions inventory come directly from the 2017 NEI, others were
adjusted from the 2017 NEI to represent 2016 levels, and the remaining
emissions including those from oil and gas, fertilizer, and solvents
were computed specifically to represent 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. The emissions sources derived from the 2017
NEI include agricultural livestock, fugitive dust, residential wood
combustion, waste disposal (including composting), bulk gasoline
terminals, and miscellaneous non-industrial sources such as cremation,
hospitals, lamp breakage, and automotive repair shops. A recent method
to compute solvent VOC emissions was used.\150\
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    \150\ https://doi.org/10.5194/acp-21-5079-2021.
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    Where comments were provided about projected control measures or

changes in nonpoint source emissions, those inputs were first reviewed
by the EPA. Those found to be based on reasonable data for affected
emissions sources were incorporated into the projected inventories for
2023 and 2026 to the extent possible. Where possible, projection
factors based on the AEO used data from AEO 2022, the most recent AEO
at the time available at the time the inventories were developed.
Federal regulations that impact the nonpoint sources were reflected in
the inventories. Adjustments for state fuel sulfur content rules for
fuel oil in the Northeast were included along with solvent controls
applicable within the ozone transport region. Details are available in
the 2016v3 Emissions Modeling TSD.
    Nonpoint oil and gas emissions inventories for many states were
developed based on outputs from the 2017 NEI version of the EPA Oil and
Gas Tool using activity data for year 2016. Production-related
emissions data from the 2017 NEI were used for Oklahoma, 2016v1
emissions were used for Colorado and for Texas production-related
sources to response to comments. Data for production-related nonpoint
oil and gas emissions in the states of Colorado, Montana, New Mexico,
North Dakota, South Dakota, Utah, and Wyoming were obtained from the
WRAP baseline inventory.\151\ A California Air Resources Board-provided
inventory was used for 2016 oil and gas emissions in California.
Nonpoint oil and gas inventories for 2023 and 2026 were developed by
first projecting the 2016 oil and gas inventories to 2021 values based
on actual production data. Next, those 2021 emissions were projected to
2023 and 2026 using regional projection factors by product type based
on AEO 2022 projections. A 2017-2019 average inventory was used for oil
and natural gas exploration emissions in 2023 and 2026 except for
California and in the WRAP states in which data from the WRAP future
year inventory \152\ were used. NOX and VOC reductions that
are co-benefits to the NSPS for RICE are reflected, along with Natural
Gas Turbines and Process Heaters NSPS NOX controls and NSPS
Oil and Gas VOC controls. The WRAP future year inventory was used for
oil and natural gas production sources in 2023 and 2026 except in New
Mexico where the WRAP Base year emissions were projected using the EIA
historical and AEO forecasted production data. Estimated impacts from
the New Mexico Administrative Code 20.2.50 were included.
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    \151\ http://www.wrapair2.org/pdf/WRAP_OGWG_Report_Baseline_17Sep2019.pdf.
    \152\ http://www.wrapair2.org/pdf/WRAP_OGWG_2028_OTB_RevFinalReport_05March2020.pdf.
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D. Air Quality Modeling 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 2015
ozone NAAQS in the 2023 and 2026 analytic years. Where the EPA's
analysis shows that an area or site does not fall under the definition
of a nonattainment or maintenance receptor in these analytic years,
that site is excluded from further analysis under this rule.
    In the proposed rule, the EPA applied the same approach used in the
CSAPR Update and the Revised CSAPR Update to identify nonattainment and
maintenance receptors for the 2008 ozone NAAQS.\153\ See 86 FR 23078-
79. The 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.\154\ 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.
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    \153\ See 86 FR 23078-79.
    \154\ 531 F.3d at 910-911 (holding that the EPA must give
``independent significance'' to each prong of CAA section
110(a)(2)(D)(i)(I)).
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    In the CSAPR Update and the Revised CSAPR Update, the 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 the
EPA defined nonattainment receptors as those areas that both currently
monitor nonattainment and that the EPA projects will be in
nonattainment in the future compliance year.\155\
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    \155\ 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 the EPA has the most
confidence in our projections of nonattainment for those monitoring
sites that also measure nonattainment for the most recent period of
available ambient data. The EPA separately identified maintenance
receptors as those monitoring sites that would have difficulty
maintaining the relevant NAAQS in a scenario that accounts for
historical variability in air quality at that site. The variability in
air quality was determined by evaluating the ``maximum'' future design
value at each monitoring site based on a projection of the maximum
measured design value over the relevant period. The 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). The EPA also recognizes
that previously experienced meteorological conditions (e.g., dominant
wind direction, temperatures, and 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.\156\ 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.
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    \156\ The EPA's air quality modeling guidance identifies the use
of the highest of the relevant base period design values as a means
to evaluate future year attainment under meteorological conditions
that are especially conducive to ozone formation. See 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.
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    Therefore, applying this methodology in this rule, the EPA assessed
the magnitude of the projected maximum design values for 2023 and 2026
at each monitoring site in relation to the 2015 ozone NAAQS and, where
such a value exceeds the NAAQS, the EPA determined that receptor to be
a ``maintenance'' receptor for purposes of defining interference with
maintenance, consistent with the method used in CSAPR and upheld by the
D.C. Circuit in EME Homer City II.\157\ That is,

monitoring sites with a maximum design value that exceeds the NAAQS are
projected to have maintenance problems in the future analytic
years.\158\
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    \157\ See 795 F.3d at 136.
    \158\ The EPA issued a memorandum in October 2018, providing
additional information to states developing interstate transport SIP
submissions for the 2015 8-hour ozone NAAQS concerning
considerations for identifying downwind areas that may have problems
maintaining the standard at Step 1 of the 4-step interstate
transport framework. See Considerations for Identifying Maintenance
Receptors for Use in Clean Air Act Section 110(a)(2)(D)(i)(I)
Interstate Transport State Implementation Plan Submissions for the
2015 Ozone National Ambient Air Quality Standards, October 19, 2018
(``October 2018 memorandum''), available in Docket No. EPA-HQ-OAR-
2021-0668 or at https://www.epa.gov/airmarkets/memo-and-supplemental-information-regarding-interstate-transport-sips-2015-ozone-naaqs. EPA is not applying the suggested analytical approaches
in that memorandum in this rule, nor would those approaches be
appropriate in light of currently available data. Potential
alternative approaches would introduce unnecessary and substantial
additional analytical burdens that could frustrate timely and
efficient implementation of good neighbor obligations. In addition,
the information supplied in that memorandum is now outdated due to
several additional years of air quality monitoring data and updated
modeling results. EPA's current approach to defining ``maintenance''
receptors has been upheld and continues to provide an appropriate
approach to addressing the ``interference with maintenance'' prong
of the Good Neighbor provision. See EME Homer City, 795 F.3d 118,
136-37; Wisconsin, 938 F.3d at 325-26.
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    Recognizing that nonattainment receptors are also, by definition,
maintenance receptors, the EPA often uses the term ``maintenance-only''
to refer to receptors that are not also nonattainment receptors.
Consistent with the concepts for maintenance receptors, as described
previously, the EPA identifies ``maintenance-only'' receptors as those
monitoring sites that have projected average design values above the
level of the applicable NAAQS, but that are not currently measuring
nonattainment based on the most recent official design values. In
addition, those monitoring sites with projected average design values
below the NAAQS, but with projected maximum design values above the
NAAQS are also identified as ``maintenance only'' receptors, even if
they are currently measuring nonattainment based on the most recent
official design values.\159\
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    \159\ See https://www.epa.gov/air-trends/air-quality-design-values for design value reports. At the time of this action, the
most recent reports available are for the calendar year 2021.
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    Comment: The EPA received comments claiming that the projected
design values for 2023 were biased low compared to recent measured
data. Commenters noted that a number of monitoring sites that are
projected to be below the NAAQS in 2023 based on the EPA's modeling for
the proposed action are currently measuring nonattainment based on data
from 2020 and 2021. One commenter requested that the EPA determine
whether its past modeling tends to overestimate or underestimated
actual observed design values. If EPA finds that the agency's model
tends to underestimate future year design values, the commenter
requests that EPA re-run its ozone modeling, incorporating parameters
that account for this tendency.
    Response: In response to comments, the EPA compared the projected
2023 design values based on the proposal modeling to recent trends in
measured data. As a result of this analysis, the EPA agrees that
current data indicate that there are monitoring sites at risk of
continued nonattainment in 2023 even though the model projected average
and maximum design values at these sites are below the NAAQS (i.e.,
sites that are not modeling-based receptors). It would not be
reasonable to ignore recent measured ozone levels in many areas that
are clearly not fully consistent with certain concentrations in the
Step 1 analysis for 2023. Therefore, the EPA has also developed an
additional maintenance-only receptor category, which includes what we
refer to as ``violating monitor'' receptors, based on current ozone
concentrations measured by regulatory ambient air quality monitoring
sites.
    Specifically, the EPA has identified monitoring sites with measured
2021 and preliminary 2022 design values and 4th high maximum daily 8-
hour average (MDA8) ozone in both 2021 and 2022 (preliminary data) that
exceed the NAAQS, although projected to be in attainment in 2023, as
having the greatest risk of continuing to have a problem attaining the
standard in 2023. These criteria sufficiently consider measured air
quality data so as to avoid including monitoring sites that have
measured nonattainment data in recent years but could reasonably be
anticipated to not have a nonattainment or maintenance problem in 2023,
in line with our modeling results. Our methodology is intended only to
identify those sites that have sufficiently poor ozone levels that
there is clearly a reasonable expectation that an ozone nonattainment
or maintenance problem will persist in the 2023 ozone season. Moreover,
2023 is so near in time that recent measured ozone levels can be used
to reasonably project whether an air quality problem is likely to
persist. We view this approach to identifying additional receptors in
2023 as the best means of responding to the comments on this issue in
this action, while also identifying all transport receptors.
    For purposes of this action, we treat these violating monitors as
an additional type of maintenance-only receptor. Because our modeling
did not identify these sites as receptors, we do not believe it is
sufficiently certain that these sites will be in nonattainment such
that they should be considered nonattainment receptors. Rather, our
authority for treating these sites as receptors in 2023 flows from the
responsibility in CAA section 110(a)(2)(i)(I) to prohibit emissions
that interfere with maintenance of the NAAQS. See, e.g., North
Carolina, 531 F.3d at 910-11 (failing to give effect to the interfere
with maintenance clause ``provides no protection for downwind areas
that, despite EPA's predictions, still find themselves struggling to
meet NAAQS due to upwind interference . . . .'') (emphasis added).
Recognizing that no modeling can perfectly forecast the future, and ``a
degree of imprecision is inevitable in tackling the problem of
interstate air pollution,'' this approach in the Agency's judgement
best balances the need to avoid both ``under-control'' and
``overcontrol,'' EME Homer City, 572 U.S. at 523.
    We acknowledge that the traditional modeling plus monitoring
methodology we used at proposal and in prior ozone transport rules
would otherwise have identified such sites as being in attainment in
2023. Despite the implications of the current measured data suggesting
there will be a nonattainment problem at these sites in 2023, we cannot
definitively establish that such sites will be in nonattainment in 2023
in light of our modeling projections. In the face of this uncertainty,
we regard our ability to consider such sites as receptors for purposes
of good neighbor analysis under CAA section 110(a)(2)(D)(i)(I) to be a
function of the requirement to prohibit emissions that interfere with
maintenance of the NAAQS; even if an area may be technically in
attainment, we have reliable information indicating that there is an
identified risk that attainment will not in fact be achieved.

    However, because we did not identify this basis for receptor-
identification at proposal, in this final action we are only using this
receptor category on a confirmatory basis. That is, for states that we
find linked based on our traditional modeling-based methodology in
2023, we find in this final analysis that the linkage at Step 2 is
strengthened and confirmed if that state is also linked to one or more
``violating monitor'' receptors. If a state is only linked to a
violating-monitor receptor in this final analysis, we are deferring
taking final action on that state's SIP submittal. This is the case for
the State of Tennessee. Among the states that previously had their
transport SIPs fully approved for the 2015 ozone NAAQS, the EPA has
also identified a linkage to violating-monitor receptors for the State
of Kansas. The EPA intends to further review its air quality modeling
results and recent measured ozone levels, and we intend to address
these states' good neighbor obligations as expeditiously as practicable
in a future action.

E. Methodology for Projecting Future Year Ozone Design Values

    Consistent with the EPA's modeling guidance, the 2016 base year and
future year air quality modeling results were used in a relative sense
to project design values for 2023 and 2026. That is, the ratios of
future year model predictions to base year model predictions are used
to adjust ambient ozone design values \160\ 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, measured data for 2014-2018 (i.e., design values for 2016, 2017,
and 2018) were used to project average and maximum design values in
2023 and 2026.
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    \160\ The ozone design value at a particular monitoring site is
the 3-year average of the annual 4th highest daily maximum 8-hour
ozone concentration at that site.
---------------------------------------------------------------------------

    The ozone predictions from the 2016 and future year air quality
model simulations were used to project 2016-2018 average and maximum
ozone design values to 2023 and 2026 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 \161\ surrounding the location of the monitoring site to
calculate a Relative Response Factor (RRF) for that site.\162\ However,
the guidance also notes that an alternative array of grid cells may be
used in certain situations where local topographic or geographical
feature (e.g., a large water body or a significant elevation change)
may influence model response.
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    \161\ As noted in this section, each model grid cell is 12 x 12
km.
    \162\ The relative response factor represents the change in
ozone at a given site. To calculate the RRF, the EPA's modeling
guidance recommends selecting the 10 highest ozone days in an ozone
season at a given monitor in the base year, noting which of the grid
cells surrounding the monitor 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
grid cells, are then averaged. The fractional change between the
base year (2016 model run) average ozone concentration and the
future year (e.g., 2023 model run) average ozone concentration
represents the relative response factor.
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    The 2016-2018 base period average and maximum design values were
multiplied by the RRF to project each of these design values to each of
the three future years. In this manner, the projected design values are
grounded in monitored data, and not the absolute model-predicted future
year concentrations. Following the approach in the CSAPR Update and the
Revised CSAPR Update, the EPA also projected future year 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, the 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).\163\ 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 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 the EPA 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). The projected 2023 and 2026 design values using
both the ``3 x 3'' and ``no-water'' approaches are provided in the
docket for this final rule. For this final rule, the EPA is relying
upon design values based on the ``no water'' approach for identifying
nonattainment and maintenance receptors.\164\
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    \163\ https://www.mmm.ucar.edu/weather-research-and-forecasting-model.
    \164\ Using design values from the ``3 x 3'' approach, the
maintenance-only receptor at site 550590019 in Kenosha County, WI
would become a nonattainment receptor because the average design
value with the ``3 x 3'' approach is 72.0 ppb versus 70.8 ppb with
the ``no water'' approach. In addition, the maintenance-only
receptor at site 090099002 in New Haven County, CT would become a
nonattainment receptor using the ``3 x 3'' approach because the
average design value with the ``3 x 3'' approach is 71.2 ppb versus
70.5 ppb with the ``no water'' approach.
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    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.\165\ Therefore, projected design values that are
greater than or equal to 71 ppb are considered to be violating the 2015
ozone NAAQS. For those sites that are projected to be violating the
NAAQS based on the average design values in the future analytic years,
the Agency examined the measured design values for 2021, which are the
most recent official measured design values at the time of this final
rule. As noted earlier, the Agency is identifying 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
71 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 (i.e., ozone design values below the
level of the 2015 ozone NAAQS) and (2) those sites with projected
average design values below the level of the NAAQS, but with projected
maximum design values of 71 ppb or greater. In addition to the
maintenance-only receptors, 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 2023
and 2026 base case are used for assessing the contribution of emissions
in upwind states to downwind nonattainment and maintenance of the 2015
ozone NAAQS as part of this final rule.\166\
---------------------------------------------------------------------------

    \165\ 40 CFR part 50, appendix P--Interpretation of the Primary
and Secondary National Ambient Air Quality Standards for Ozone.
    \166\ In addition, there are 71 monitoring sites in California
with projected 2023 maximum design values above the NAAQS. With two
exceptions, as described in section IV.F of this document, the
Agency is not making a determination in this action that these
monitors are ozone transport receptors. The two exceptions are the
two monitoring sites that represent air quality impacts to lands of
the Morongo and Pechanga tribes. As explained in footnote 110 supra,
we treat these as transport receptors that are impacted by emissions
from California.
---------------------------------------------------------------------------

    Table IV.D-1 contains the 2016-centered \167\ base period average
and maximum 8-hour ozone design values, the 2023 base case average and
maximum design values and the measured 2021 design values for the sites
that are projected to be nonattainment receptors in 2023. Table IV.D-2
contains this same information for monitoring sites that are projected
to be maintenance-only receptors in 2023. 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 Air Quality Modeling Final Rule TSD.
---------------------------------------------------------------------------

    \167\ 2016-centered averaged design values represent the average
of the design values for 2016, 2017, and 2018. Similarly, the
maximum 2016-centered design value is the highest measured design
value from these three design value periods.

  Table IV.D-1--Average and Maximum 2016-Centered and 2023 Base Case 8-Hour Ozone Design Values and 2021 Design Values (ppb) at Projected Nonattainment
                                                                        Receptors
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                2016        2016
               Monitor ID                          State                    County             Centered    Centered      2023        2023        2021
                                                                                average     maximum     Average     Maximum
--------------------------------------------------------------------------------------------------------------------------------------------------------
060650016...............................  CA                      Riverside.................        79.0        80.0        72.2        73.1          78
060651016...............................  CA                      Riverside.................        99.7       101.0        91.0        92.2          95
080350004...............................  CO                      Douglas...................        77.3          78        71.3        71.9          83
080590006...............................  CO                      Jefferson.................        77.3          78        72.8        73.5          81
080590011...............................  CO                      Jefferson.................        79.3          80        73.5        74.1          83
090010017...............................  CT                      Fairfield.................        79.3          80        71.6        72.2          79
090013007...............................  CT                      Fairfield.................        82.0          83        72.9        73.8          81
090019003...............................  CT                      Fairfield.................        82.7          83        73.3        73.6          80
481671034...............................  TX                      Galveston.................        75.7          77        71.5        72.8          72
482010024...............................  TX                      Harris....................        79.3          81        75.1        76.7          74
490110004...............................  UT                      Davis.....................        75.7          78        72.0        74.2          78
490353006...............................  UT                      Salt Lake.................        76.3          78        72.6        74.2          76
490353013...............................  UT                      Salt Lake.................        76.5          77        73.3        73.8          76
551170006...............................  WI                      Sheboygan.................        80.0          81        72.7        73.6          72
--------------------------------------------------------------------------------------------------------------------------------------------------------

Table IV.D-2--Average and Maximum 2016-Centered and 2023 Base Case 8-Hour Ozone Design Values and 2021 Design Values (ppb) at Projected Maintenance-Only
                                                                        Receptors
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                2016        2016
               Monitor ID                          State                    County             Centered    Centered      2023        2023        2021
                                                                                average     maximum     Average     Maximum
--------------------------------------------------------------------------------------------------------------------------------------------------------
040278011...............................  AZ                      Yuma......................        72.3          74        70.4        72.1          67
080690011...............................  CO                      Larimer...................        75.7          77        70.9        72.1          77
090099002...............................  CT                      New Haven.................        79.7          82        70.5        72.6          82
170310001...............................  IL                      Cook......................        73.0          77        68.2        71.9          71
170314201...............................  IL                      Cook......................        73.3          77        68.0        71.5          74
170317002...............................  IL                      Cook......................        74.0          77        68.5        71.3          73
350130021...............................  NM                      Dona Ana..................        72.7          74        70.8        72.1          80
350130022...............................  NM                      Dona Ana..................        71.3          74        69.7        72.4          75
350151005...............................  NM                      Eddy......................        69.7          74        69.7        74.1          77
350250008...............................  NM                      Lea.......................        67.7          70        69.8        72.2          66
480391004...............................  TX                      Brazoria..................        74.7          77        70.4        72.5          75
481210034...............................  TX                      Denton....................        78.0          80        69.8        71.6          74
481410037...............................  TX                      El Paso...................        71.3          73        69.8        71.4          75
482010055...............................  TX                      Harris....................        76.0          77        70.9        71.9          77
482011034...............................  TX                      Harris....................        73.7          75        70.1        71.3          71
482011035...............................  TX                      Harris....................        71.3          75        67.8        71.3          71
530330023...............................  WA                      King......................        73.3          77        67.6        71.0          64
550590019...............................  WI                      Kenosha...................        78.0          79        70.8        71.7          74
551010020...............................  WI                      Racine....................        76.0          78        69.7        71.5          73
--------------------------------------------------------------------------------------------------------------------------------------------------------

    In total, in the 2023 base case there are a total of 33 projected
modeling-based receptors nationwide including 14 nonattainment
receptors in 9 different counties and 19 maintenance-only receptors in
13 additional counties (Harris County, TX, has both nonattainment and
maintenance-only receptors).\168\ Of the 14 nonattainment receptors in
2023, 7 remain nonattainment receptors, 5 are projected to become
maintenance-only receptors and 2 are projected to be in attainment in
2026. Of the 19 maintenance-only receptors in 2023, 7 are projected to
remain maintenance-only receptors and 12 are projected to be in
attainment in 2026. The projected average and maximum design values in
2026 for all receptors are included in the Air Quality Modeling Final
Rule TSD.
---------------------------------------------------------------------------

    \168\ The EPA's modeling also projects that three monitoring
sites in the Uintah Basin (i.e., monitor 490472003 in Uintah County,
Utah, and monitors 490130002 and 490137011 in Duchesne County, Utah)
will have average design values above the NAAQS in 2023. However, as
noted in the proposed rule, the Uinta Basin nonattainment area was
designated as nonattainment for the 2015 ozone NAAQS not because of
an ongoing problem with summertime ozone (as is usually the case in
other parts of the country), but instead because it violates the
ozone NAAQS in winter. The main causes of the Uinta Basin's
wintertime ozone are sources located at low elevations within the
Basin, the Basin's unique topography, and the influence of the
wintertime meteorologic inversions that keep ozone and ozone
precursors near the Basin floor and restrict air flow in the Basin.
Because of the localized nature of the ozone problem at these sites
the EPA has not identified these three monitors as receptors in Step
1 of this final rule.
---------------------------------------------------------------------------

    Comment: EPA received comments saying that the projected design
values for 2023 were biased low compared to recent measured data.
Commenters noted that a number of monitoring sites that are projected
to be below the NAAQS in 2023 based on EPA's modeling for the proposed
rule are currently measuring nonattainment. Because 2023 is only a year
later than the most recent measured data some commenters said that EPA
should give greater weight to measured data when identifying downwind
receptors.
    Response: Based on an analysis of model projections for 2023 and
recent trends in measured data, the EPA agrees that current data
indicate that there are monitoring sites at risk of continued
nonattainment in 2023 even though the model projected average and
maximum design values at these sites are below the NAAQS (i.e., sites
that are not modeling-based receptors).\169\ Specifically, the EPA
believes that monitoring sites with measured design values and 4th high
maximum daily 8-hour average (MDA8) ozone based on 2021 and preliminary
2022 data have the greatest risk of continuing to have a problem
attaining the standard in 2023, even when the modeling projects these
sites will attain. These criteria are sufficiently conservative that we
avoid including monitoring sites that have measured nonattainment data
in recent years but could reasonably be anticipated to not have a
nonattainment or maintenance problem in 2023, in line with our modeling
results. Our methodology is intended only to identify those sites that
have sufficiently poor ozone levels that there is clearly a reasonable
expectation that an ozone nonattainment or maintenance problem will
persist in the 2023 ozone season. We do not apply this methodology for
the 2026 analytic year, because that year is sufficiently farther in
the future that we do not believe there would be a reasonable basis to
supplement our modeling analysis with this ``violating monitor''
methodology. By comparison, 2023 is so near in time that recent
measured ozone levels can be used reasonably to project whether an air
quality problem is likely to persist. We view this approach to
identifying additional receptors in 2023 as the best means of
responding to the comments on this issue in this action. The monitoring
sites that meet these criteria, along with the corresponding measured
and modeled data, are provided in Table IV.D-3.
---------------------------------------------------------------------------

    \169\ In addition, we note that comparing the projected 2023
maximum design values at modeling-based receptors listed in Table
IV.D-1 and Table IV.D-2 to the 2021 design values measured at these
sites indicates that the projected maximum values are lower than the
measured data at most receptors. These differences are particularly
evident at receptors in coastal Connecticut and in Denver. (See Air
Quality Modeling Final Rule TSD for details).
---------------------------------------------------------------------------

    For purposes of this action, we will treat these sites as an
additional type of maintenance-only receptor. Because our modeling did
not identify these sites as receptors, we do not believe it is
sufficiently certain that these sites will be in nonattainment that
they should be considered nonattainment receptors for purposes of this
final rule. Rather, our authority for treating these sites as receptors
in 2023 flows from the responsibility in CAA section 110(a)(2)(i)(I) to
prohibit emissions that interfere with maintenance of the NAAQS. See,
e.g., North Carolina, 531 F.3d at 910-11 (failing to give effect to the
interfere with maintenance clause ``provides no protection for downwind
areas that, despite EPA's predictions, still find themselves struggling
to meet NAAQS due to upwind interference . . . .'') (emphasis added).
Recognizing that no modeling can perfectly forecast the future, and ``a
degree of imprecision is inevitable in tackling the problem of
interstate air pollution,'' this approach in the Agency's judgement
best balances the need to avoid both ``under-control'' and
``overcontrol,'' EME Homer City, 572 U.S. at 523.
    In this action, we identify ``violating monitor'' maintenance-only
receptors for purposes of more firmly establishing that the states we
have otherwise identified as linked at Step 2 in our modeling-based
methodology can indeed be reasonably anticipated to be linked to air
quality problems in downwind states in 2023 for reasons that extend
beyond that methodology. In this sense, this approach is
``confirmatory'' and does not alter the geography of the final rule
compared to the application of the modeling-based receptor definitions
used at proposal. Rather, it strengthens the analytical basis for our
Step 2 findings by establishing that many upwind states covered in this
action are also projected to contribute above 1 percent of the NAAQS to
these types of receptors. For purposes of this final rule, we will not
finalize FIPs for any states that this analysis indicates contribute
greater than 1 percent of the NAAQS only to a ``violating monitor''
receptor. Our analysis suggests this would be the case for two states,
Kansas and Tennessee (see section IV.F of this document).\170\ We are
making no final decisions with respect to these states in this action
and intend to address these states in a subsequent action.
---------------------------------------------------------------------------

    \170\ We have not conducted an analysis in this action to
determine whether violating-monitor receptors may exist in
California.

     Table IV.D-3--Average and Maximum 2023 Base Case 8-Hour Ozone, and 2021 and Preliminary 2022 Design Values (ppb) and 4th High Concentrations at
                                                                   Violating Monitors
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                2023        2023                           2021 4th   2022 P 4th
              Monitor ID                      State                  County            Average     Maximum     2021    2022 P *      high        high
--------------------------------------------------------------------------------------------------------------------------------------------------------
40070010.............................  AZ                   Gila...................        67.9        69.5       77          76          75          74

40130019.............................  AZ                   Maricopa...............        69.8        70.0       75          77          78          76
40131003.............................  AZ                   Maricopa...............        70.1        70.7       80          80          83          78
40131004.............................  AZ                   Maricopa...............        70.2        70.8       80          81          81          77
40131010.............................  AZ                   Maricopa...............        68.3        69.2       79          80          80          78
40132001.............................  AZ                   Maricopa...............        63.8        64.1       74          78          79          81
40132005.............................  AZ                   Maricopa...............        69.6        70.5       78          79          79          77
40133002.............................  AZ                   Maricopa...............        65.8        65.8       75          75          81          72
40134004.............................  AZ                   Maricopa...............        65.7        66.6       73          73          73          71
40134005.............................  AZ                   Maricopa...............        62.3        62.3       73          75          79          73
40134008.............................  AZ                   Maricopa...............        65.6        66.5       74          74          74          71
40134010.............................  AZ                   Maricopa...............        63.8        66.9       74          76          77          75
40137020.............................  AZ                   Maricopa...............        67.0        67.0       76          77          77          75
40137021.............................  AZ                   Maricopa...............        69.8        70.1       77          77          78          75
40137022.............................  AZ                   Maricopa...............        68.2        69.1       76          78          76          79
40137024.............................  AZ                   Maricopa...............        67.0        67.9       74          76          74          77
40139702.............................  AZ                   Maricopa...............        66.9        68.1       75          77          72          77
40139704.............................  AZ                   Maricopa...............        65.3        66.2       74          77          76          76
40139997.............................  AZ                   Maricopa...............        70.5        70.5       76          79          82          76
40218001.............................  AZ                   Pinal..................        67.8        69.0       75          76          73          77
80013001.............................  CO                   Adams..................        63.0        63.0       72          77          79          75
80050002.............................  CO                   Arapahoe...............        68.0        68.0       80          80          84          73
80310002.............................  CO                   Denver.................        63.6        64.8       72          74          77          71
80310026.............................  CO                   Denver.................        64.5        64.8       75          77          83          72
90079007.............................  CT                   Middlesex..............        68.7        69.0       74          73          78          73
90110124.............................  CT                   New London.............        65.5        67.0       73          72          75          71
170310032............................  IL                   Cook...................        67.3        69.8       75          75          77          72
170311601............................  IL                   Cook...................        63.8        64.5       72          73          72          71
181270024............................  IN                   Porter.................        63.4        64.6       72          73          72          73
260050003............................  MI                   Allegan................        66.2        67.4       75          75          78          73
261210039............................  MI                   Muskegon...............        67.5        68.4       74          79          75          82
320030043............................  NV                   Clark..................        68.4        69.4       73          75          74          74
350011012............................  NM                   Bernalillo.............        63.8        66.0       72          73          76          74
350130008............................  NM                   Dona Ana...............        65.6        66.3       72          76          79          78
361030002............................  NY                   Suffolk................        66.2        68.0       73          74          79          74
390850003............................  OH                   Lake...................        64.3        64.6       72          74          72          76
480290052............................  TX                   Bexar..................        67.1        67.8       73          74          78          72
480850005............................  TX                   Collin.................        65.4        66.0       75          74          81          73
481130075............................  TX                   Dallas.................        65.3        66.5       71          71          73          72
481211032............................  TX                   Denton.................        65.9        67.7       76          77          85          77
482010051............................  TX                   Harris.................        65.3        66.3       74          73          83          72
482010416............................  TX                   Harris.................        68.8        70.4       73          73          78          71
484390075............................  TX                   Tarrant................        63.8        64.7       75          76          76          77
484391002............................  TX                   Tarrant................        64.1        65.7       72          77          76          80
484392003............................  TX                   Tarrant................        65.2        65.9       72          72          74          72
484393009............................  TX                   Tarrant................        67.5        68.1       74          75          75          75
490571003............................  UT                   Weber..................        69.3        70.3       71          74          77          71
550590025............................  WI                   Kenosha................        67.6        70.7       72          73          72          71
550890008............................  WI                   Ozaukee................        65.2        65.8       71          72          72          72
--------------------------------------------------------------------------------------------------------------------------------------------------------
* 2022 preliminary design values are based on 2022 measured MDA8 concentrations provided by state air agencies to the EPA's Air Quality System (AQS), as
  of January 3, 2023.

F. Pollutant Transport From Upwind States

1. Air Quality Modeling To Quantify Upwind State Contributions
    This section documents the procedures the EPA used to quantify the
impact of emissions from specific upwind states on ozone design values
in 2023 and 2026 for the identified downwind nonattainment and
maintenance receptors. The 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 benefit 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.
    The EPA performed nationwide, state-level ozone source
apportionment modeling using the CAMx Ozone Source Apportionment
Technology/Anthropogenic Precursor Culpability Analysis (OSAT/APCA)
technique \171\ to quantify the contribution of 2023 and 2026 base case
NOX and VOC emissions from all sources in each state to the

corresponding projected ozone design values in 2023 and 2026 at air
quality monitoring sites. The CAMx OSAT/APCA model run was performed
for the period May 1 through September 30 using the projected future
base case emissions and 2016 meteorology for this time period. 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).
---------------------------------------------------------------------------

    \171\ As part of this technique, ozone formed from reactions
between biogenic VOC and NOX with anthropogenic
NOX and VOC are assigned to the anthropogenic emissions.
---------------------------------------------------------------------------

    In the state-by-state source apportionment model runs, the 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 air quality modeling domain;
     Tribes--the emissions from those tribal lands for which
the Agency has point source inventory data in the 2016v3 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 (the
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.
    For the Step 2 analysis, the EPA calculated a contribution metric
that considers the average contribution on the 10 highest ozone
concentration days (i.e., top 10 days) in 2023. This average
contribution metric is intended to provide a reasonable representation
of the contribution from individual states to projected future year
design values, 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 analytic steps for calculating the contribution metric for the
2023 analytic year 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 average the concentrations for
the top 10 modeled ozone concentration days in 2023;
    (3) Divide the average contribution by the corresponding average
concentration to obtain a Relative Contribution Factor (RCF) for each
monitoring site;
    (4) Multiply the 2023 average design values by the 2023 RCF at each
site to produce the average contribution metric values in 2023.\172\
---------------------------------------------------------------------------

    \172\ Note that a contribution metric value was not calculated
for any receptor at which there were fewer than 5 days with model-
predicted MDA8 ozone concentrations greater than or equal to 60 ppb
in 2023. The monitoring site in Seattle, King County, Washington
(530330023), was the only receptor which did not meet this
criterion.
---------------------------------------------------------------------------

    This same approach was applied to calculate contribution metric
values at individual monitoring sites for 2026.\173\
---------------------------------------------------------------------------

    \173\ To provide consistency in the contributions for 2023 and
2026, the contribution metric values for 2026 are based on the 2026
daily contributions for the same days that were used to calculate
the contribution metric values for 2023.
---------------------------------------------------------------------------

    The resulting contributions from each tag to each monitoring site
in the U.S. for 2023 and 2026 can be found in the docket for this final
rule. Additional details on the source apportionment modeling and the
procedures for calculating contributions can be found in the Air
Quality Modeling Final Rule TSD. The EPA's response to comments on the
method for calculating the contribution metric can be found in the RTC
document for this final rule.
    The largest contribution from each state that is the subject of
this rule to modeled 8-hour ozone nonattainment and maintenance
receptors in downwind states in 2023 and 2026 are provided in Table
IV.F-1 and Table IV.F-2, respectively. The largest contribution from
each state to a ``violating monitor'' maintenance-only receptor is
provided in Table IV.F-3.

       Table IV.F-1--Largest Contribution to Downwind 8-Hour Ozone
             Nonattainment and Maintenance Receptors in 2023
                                  [ppb]
------------------------------------------------------------------------
                                        Largest             Largest
                                    contribution to     contribution to
          Upwind state                 downwind            downwind
                                     nonattainment     maintenance-only
                                       receptors           receptors
------------------------------------------------------------------------
Alabama.........................                0.75                0.65
Arizona.........................                0.54                1.69
Arkansas........................                0.94                1.21
California......................               35.27                6.31
Colorado........................                0.14                0.18
Connecticut.....................                0.01                0.01
Delaware........................                0.44                0.56
District of Columbia............                0.03                0.04
Florida.........................                0.50                0.54
Georgia.........................                0.18                0.17
Idaho...........................                0.42                0.41
Illinois........................               13.89               19.09

Indiana.........................                8.90               10.03
Iowa............................                0.67                0.90
Kansas..........................                0.46                0.52
Kentucky........................                0.84                0.79
Louisiana.......................                9.51                5.62
Maine...........................                0.02                0.01
Maryland........................                1.13                1.28
Massachusetts...................                0.33                0.15
Michigan........................                1.59                1.56
Minnesota.......................                0.36                0.85
Mississippi.....................                1.32                0.91
Missouri........................                1.87                1.39
Montana.........................                0.08                0.10
Nebraska........................                0.20                0.36
Nevada..........................                1.11                1.13
New Hampshire...................                0.10                0.02
New Jersey......................                8.38                5.79
New Mexico......................                0.36                1.59
New York........................               16.10               11.29
North Carolina..................                0.45                0.66
North Dakota....................                0.18                0.45
Ohio............................                2.05                1.98
Oklahoma........................                0.79                1.01
Oregon *........................                0.46                0.31
Pennsylvania....................                6.00                4.36
Rhode Island....................                0.04                0.01
South Carolina..................                0.16                0.18
South Dakota....................                0.05                0.08
Tennessee.......................                0.60                0.68
Texas...........................                1.03                4.74
Utah............................                1.29                0.98
Vermont.........................                0.02                0.01
Virginia........................                1.16                1.76
Washington......................                0.16                0.09
West Virginia...................                1.37                1.49
Wisconsin.......................                0.21                2.86
Wyoming.........................                0.68                0.67
------------------------------------------------------------------------

       Table IV.F-2--Largest Contribution to Downwind 8-Hour Ozone
             Nonattainment and Maintenance Receptors in 2026
                                  [ppb]
------------------------------------------------------------------------
                                        Largest             Largest
                                    contribution to     contribution to
          Upwind state                 downwind            downwind
                                     nonattainment     maintenance-only
                                       receptors           receptors
------------------------------------------------------------------------
Alabama.........................                0.20                0.69
Arizona.........................                0.44                1.34
Arkansas........................                0.53                1.16
California......................               34.03                6.16
Colorado........................                0.04                0.17
Connecticut.....................                0.00                0.01
Delaware........................                0.43                0.41
District of Columbia............                0.03                0.02
Florida.........................                0.46                0.17
Georgia.........................                0.13                0.16
Idaho...........................                0.27                0.36
Illinois........................                0.63               13.57
Indiana.........................                1.06                8.53
Iowa............................                0.14                0.62
Kansas..........................                0.14                0.42
Kentucky........................                0.79                0.76
Louisiana.......................                4.57                9.37

Maine...........................                0.00                0.01
Maryland........................                1.06                0.92
Massachusetts...................                0.06                0.31
Michigan........................                1.39                1.47
Minnesota.......................                0.15                0.32
Mississippi.....................                0.29                1.15
Missouri........................                0.29                1.68
Montana.........................                0.06                0.07
Nebraska........................                0.09                0.19
Nevada..........................                0.67                0.90
New Hampshire...................                0.01                0.09
New Jersey......................                8.10                7.04
New Mexico......................                0.35                0.46
New York........................               12.65               12.34
North Carolina..................                0.40                0.42
North Dakota....................                0.09                0.17
Ohio............................                1.95                1.93
Oklahoma........................                0.19                0.74
Oregon *........................                0.26                0.41
Pennsylvania....................                5.47                4.94
Rhode Island....................                0.00                0.03
South Carolina..................                0.14                0.15
South Dakota....................                0.03                0.04
Tennessee.......................                0.24                0.54
Texas...........................                0.48                4.34
Utah............................                1.05                0.81
Vermont.........................                0.01                0.02
Virginia........................                1.09                1.10
Washington......................                0.10                0.14
West Virginia...................                1.36                1.34
Wisconsin.......................                0.17                0.18
Wyoming.........................                0.40                0.59
------------------------------------------------------------------------

 Table IV.F-3--Largest Contribution to Downwind 8-Hour Ozone ``Violating
                  Monitor'' Maintenance-Only Receptors
                                  [ppb]
------------------------------------------------------------------------
                                                    Largest contribution
                                                         to downwind
                   Upwind state                       violating monitor
                                                      maintenance-only
                                                          receptors
------------------------------------------------------------------------
Alabama...........................................                  0.79
Arizona...........................................                  1.62
Arkansas..........................................                  1.16
California........................................                  6.97
Colorado..........................................                  0.39
Connecticut.......................................                  0.17
Delaware..........................................                  0.42
District of Columbia..............................                  0.03
Florida...........................................                  0.50
Georgia...........................................                  0.31
Idaho.............................................                  0.46
Illinois..........................................                 16.53
Indiana...........................................                  9.39
Iowa..............................................                  1.13
Kansas............................................                  0.82
Kentucky..........................................                  1.57
Louisiana.........................................                  5.06
Maine.............................................                  0.02
Maryland..........................................                  1.14
Massachusetts.....................................                  0.39
Michigan..........................................                  3.47

Minnesota.........................................                  0.64
Mississippi.......................................                  1.02
Missouri..........................................                  2.95
Montana...........................................                  0.12
Nebraska..........................................                  0.43
Nevada............................................                  1.11
New Hampshire.....................................                  0.10
New Jersey........................................                  8.00
New Mexico........................................                  0.34
New York..........................................                 12.08
North Carolina....................................                  0.65
North Dakota......................................                  0.35
Ohio..............................................                  2.25
Oklahoma..........................................                  1.57
Oregon *..........................................                  0.36
Pennsylvania......................................                  5.20
Rhode Island......................................                  0.08
South Carolina....................................                  0.23
South Dakota......................................                  0.12
Tennessee.........................................                  0.86
Texas.............................................                  3.83
Utah..............................................                  1.46
Vermont...........................................                  0.03
Virginia..........................................                  1.39
Washington........................................                  0.11
West Virginia.....................................                  1.79
Wisconsin.........................................                  5.10
Wyoming...........................................                  0.42
------------------------------------------------------------------------
* Does not include California monitoring sites.

2. Application of Contribution Screening Threshold
    In Step 2 of the interstate transport framework, the 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 state to each downwind
nonattainment or maintenance receptor that were used for the Step 2
evaluation can be found in the Air Quality Modeling Final Rule TSD.
    The EPA applies an air quality screening threshold of 1 percent of
the NAAQS, which has been used since the CSAPR rulemaking, including in
the CSAPR Update, the Revised CSAPR Update, and numerous actions
evaluating states' transport SIP submittals. The explanation for how
this value was originally derived is available in the CSAPR rulemaking
from 2011. See 76 FR 48208, 48237-38. As originally explained there,
the application of a relatively low threshold is intended to capture a
relatively large percentage of the contribution from upwind states to
downwind receptors in light of the regional-scale, collective
contribution problem associated with both ozone and PM2.5
NAAQS. Id. The Agency also explained that the use of a higher threshold
in transport rules prior to CSAPR was based on single-day maximum
contribution, whereas in CSAPR (and continuing in subsequent rules
including this one), the Agency uses a more robust, average
contribution metric over multiple days. Thus, it was not the case that
1 percent of NAAQS was substantially more stringent than that prior
approach. Id. at 48238. In the 2016 CSAPR Update, the EPA reviewed the
1 percent threshold (as coupled with multi-day averaging) and
determined it was appropriate to continue to apply this threshold. The
EPA compared the 1 percent threshold to a 0.5 percent of NAAQS
threshold and a 5 percent of NAAQS threshold. The 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.\174\ The EPA continues to observe that nonattainment
and maintenance receptors identified at Step 1 are impacted
collectively by emissions from numerous upwind contributors. Therefore,
application of a low, uniform screening threshold allows the EPA to
identify upwind states that share a responsibility under the interstate
transport provision to eliminate their significant contribution.
---------------------------------------------------------------------------

    \174\ See Final CSAPR Update Air Quality Modeling TSD, at 27-30
(EPA-HQ-OAR-2015-0596-0144). See also 86 FR 23054, 23085.
---------------------------------------------------------------------------

    As we explained at proposal, the EPA recognizes that in 2018 it
issued a memorandum indicating the potential for states to use a higher
threshold at Step 2 in the development of their good neighbor SIP
submissions where it could be technically justified. The August 2018
memorandum stated that ``it may be reasonable and appropriate'' for
states to rely on an alternative 1 ppb threshold at Step 2.\175\ (The
memorandum also indicated that any

higher alternative threshold, such as 2 ppb, would likely not be
appropriate.) The EPA nonetheless proposed to fulfill its role under
CAA section 110(c) in promulgating FIPs to directly implement good
neighbor requirements, and in this role, proposed retaining use of the
1 percent threshold for all states. We noted that in several documents
proposing transport SIP disapprovals, see, e.g., 87 FR 9498 and 87 FR
9510 (Feb. 22, 2022), we explained that our experience since the
issuance of the August 2018 memorandum regarding use of alternative
thresholds led the Agency to believe it may not be appropriate to
continue to attempt to recognize alternative contribution thresholds at
Step 2, either in the context of SIPs or FIPs.
---------------------------------------------------------------------------

    \175\ August 2018 memo at 4.
---------------------------------------------------------------------------

    We went on to explain that the EPA's experience since 2018 is that
allowing for alternative Step 2 thresholds may be impractical or
otherwise inadvisable for a number of additional policy reasons. For a
regional air pollutant such as ozone, consistency in requirements and
expectations across all states is essential. Using multiple different
thresholds at Step 2 with respect to the 2015 ozone NAAQS raises
substantial policy consistency and practical implementation
concerns.\176\ The application of different thresholds at Step 2 has
the potential to result in inconsistent determination of good neighbor
obligations. From the perspective of ensuring effective regional
implementation of good neighbor obligations, the more important
analysis is the evaluation of the emissions reductions needed, if any,
to address a state's significant contribution after consideration of a
multifactor analysis at Step 3, including a detailed evaluation that
considers air quality factors and cost. We explained that while
alternative thresholds for purposes of Step 2 may be ``similar'' in
terms of capturing the relative amount of upwind contribution (as
described in the August 2018 memorandum), nonetheless, use of
alternative thresholds would allow certain states to avoid further
evaluation of potential emissions controls while other states must
proceed to a Step 3 analysis. This could create significant equity and
consistency problems among states.
---------------------------------------------------------------------------

    \176\ We note that Congress has placed on the EPA a general
obligation to ensure the requirements of the CAA are implemented
consistently across states and regions. See CAA section 301(a)(2).
Where the management and regulation of interstate pollution levels
spanning many states is at stake, consistency in application of CAA
requirements is paramount.
---------------------------------------------------------------------------

    The EPA further proposed that, in promulgating FIPs to address
these obligations on a nationwide scale, national ozone transport
policy would not be well-served by applying a single, less stringent
threshold at Step 2. The EPA recognized in the August 2018 memo that
there was some similarity in the amount of total upwind contribution
captured (on a nationwide basis) between 1 percent and 1 ppb. However,
the EPA noted at proposal that while this may be true in some sense,
that is hardly a compelling basis to move to a 1 ppb threshold. Indeed,
the 1 ppb threshold has the disadvantage of losing a certain amount of
total upwind contribution for further evaluation at Step 3. Considering
the core statutory objective of ensuring elimination of all significant
contribution to nonattainment or interference of the NAAQS in downwind
states and the broad, regional nature of the collective contribution
problem with respect to ozone, EPA could not identify a compelling
policy imperative to move to a 1 ppb threshold.
    In the proposal, we also found consistency with past interstate
transport actions such as CSAPR, and the CSAPR Update and Revised CSAPR
Update rulemakings (which used a Step 2 threshold of 1 percent of the
NAAQS for two less protective ozone NAAQS) to be an important
consideration. Continuing to use a 1 percent of NAAQS approach ensures
that as the NAAQS are revised and made more stringent, an appropriate
increase in stringency at Step 2 occurs, so as to ensure an
appropriately larger amount of total upwind-state contribution is
captured for purposes of fully addressing interstate transport for the
more protective NAAQS.
    The Agency also questioned whether it would be a good use of
limited resources to attempt to further justify the use of alternative
thresholds for certain states at Step 2 for purposes of the 2015 ozone
NAAQS. Therefore, while EPA articulated the possibility of an
alternative threshold in the August 2018 memorandum, the EPA concluded
in the proposal that our experience and further evaluation since the
issuance of that memo has revealed substantial programmatic and policy
difficulties in attempting to implement this approach, and therefore we
proposed to apply the 1 percent of NAAQS threshold.
    Comment: Many commenters disagreed with our proposal to continue
using a 1 percent of NAAQS threshold. They argued that the EPA was
reversing course from its policy as articulated in the August 2018
memorandum and that the EPA was now bound to use a 1 ppb threshold
rather than 1 percent of NAAQS, even in promulgating a FIP rather than
evaluating SIPs. Commenters further argued that a 1 ppb threshold would
be more consistent with the EPA's ``significant impact level'' (SIL)
guidance related to implementing prevention of significant
deterioration (PSD) permitting requirements. They argued that the 1
percent threshold was below precision limits of regulatory ozone
monitors, and they argued it was within the ``margin of error'' of the
EPA's modeling.
    Response: The EPA is finalizing its proposed approach of
consistently using a 1 percent of the NAAQS threshold at Step 2 in this
action to determine which states contribute to identified nonattainment
and maintenance receptors. This approach ensures both national
consistency across all states and consistency and continuity with our
prior interstate transport actions for other NAAQS. We do not agree
that this approach is inconsistent with or a reversal in policy from
the August 2018 memorandum, which only suggested that states in the
development of their SIPs ``may'' be able to establish that 1 ppb could
be an appropriate alternative threshold. The EPA has been consistent in
that memorandum, and since that time, that final determinations on
alternative thresholds would be made through rulemaking action, as the
EPA is taking here.
    The August 2018 memorandum made clear that the Agency had
substantial doubts that any threshold greater than 1 ppb (such as 2
ppb) would be acceptable, and the Agency is affirming that a threshold
higher than 1 ppb would not be justified under any circumstance for
purposes of this action. No commenter credibly provided a basis for
using a threshold even higher than 1 ppb, and so this issue is
primarily limited to the difference between a 0.7 ppb threshold (the 1
percent of the NAAQS threshold discussed previously in this section)
and a 1.0 ppb threshold. Therefore, before proceeding in responding to
these comments, we note that this issue is only relevant to a small
number of states whose contributions to any receptor are above 1
percent of the NAAQS but lower than 1 ppb. Under the 2016v3 modeling of
2023 being used in this final rule, the states in this rule with
contributions that fall between 0.70 ppb and 1 ppb are Alabama,
Kentucky, and Minnesota. Similarly, the EPA applies the 1 percent
threshold in its 2026 modeling projections to determine if any states
will not be linked to an ozone receptor by that year, and therefore
should not be subject to the more stringent requirements that take
effect in 2026. The states in this rule in that year with contribution
between 0.70 ppb and 1 ppb are

Kentucky, Nevada, and Oklahoma. For all other states covered in this
action, at least one linkage exists in 2023 (and, as relevant, in 2026)
that is greater than 1 ppb, and therefore the question of whether the
EPA must recognize a 1 ppb threshold would not have a dispositive
effect on the regulatory determination being made at Step 2.
    The 1 percent of the NAAQS threshold is consistent with the Step 2
approach that the EPA applied in CSAPR for the 1997 ozone NAAQS and has
subsequently been applied in the CSAPR Update and Revised CSAPR Update
when evaluating determining interstate transport obligations for the
2008 ozone NAAQS. The EPA continues to find 1 percent of the ozone
NAAQS to be an appropriate threshold. For ozone, as the EPA found in
CAIR, CSAPR, and the CSAPR Update, a portion of the nonattainment and
maintenance problems in the U.S. results from the combined impact of
relatively small contributions from many upwind states, along with
contributions from in-state sources and other sources. The EPA's
analysis shows that the ozone transport problem being analyzed in this
rule is still the result of the collective impacts of emissions from
multiple upwind contributors. Therefore, application of a consistent
contribution threshold is necessary to identify those upwind states
that should have responsibility for addressing their contribution (to
the extent found ``significant'' at Step 3) to the downwind
nonattainment and maintenance problems to which they collectively
contribute. Where a great number of geographically dispersed emissions
sources contribute to a downwind air quality problem, which is the case
for ozone, EPA believes that, in the context of CAA section
110(a)(2)(D)(i)(I), a state-level threshold of 1 percent of the NAAQS
is a reasonably small enough value to identify only the greater-than-de
minimis contributors yet is not so large that it unfairly focuses
attention for further action only on the largest single or few upwind
contributors. Continuing to use 1 percent of the NAAQS as the screening
metric to evaluate collective contribution from many upwind states also
allows the EPA (and states) to apply a consistent framework to evaluate
interstate emissions transport under the interstate transport provision
from one NAAQS to the next. See 86 FR 23054, 23085; 81 FR 74504, 74518;
76 FR 48208, 48237-38.
    Further, the EPA notes that the role of the Step 2 threshold is
limited and just one step in the larger 4-Step Framework. It serves to
screen in states for further evaluation of emissions control
opportunities applying a multifactor analysis at Step 3. Thus, as the
Supreme Court has recognized, the contribution threshold essentially
functions to exclude states with ``de miminis'' impacts. EME Homer
City, 572 U.S. 489, 500.
    Comments related to the August 2018 memorandum argued that the EPA
legally committed itself to approving SIP submissions from states with
contributions below 1 ppb and so now the EPA must apply that threshold
in this FIP action. (Comments regarding this issue as related to the
EPA's action on SIPs is addressed in that rulemaking and is beyond the
scope of this action.) This is not what the memorandum said. The
memorandum merely provided an analysis regarding ``the degree to which
certain air quality threshold amounts capture the collective amount of
upwind contribution from upwind states.'' \177\ It interpreted ``that
information to make recommendations about what thresholds may be
appropriate for use in'' SIP submissions (emphasis added).\178\
Specifically, the August 2018 memorandum said, ``Because the amount of
upwind collective contribution capture with the 1 percent and the 1 ppb
thresholds is generally comparable, overall, we believe it may be
reasonable and appropriate for states to use a 1 ppb contribution
threshold, as an alternative to a 1 percent threshold, at Step 2 of the
4-step framework in developing their SIP revisions addressing the good
neighbor provision for the 2015 ozone NAAQS'' (emphasis added).\179\
Thus, the text of the August 2018 memorandum in no way committed that
the EPA would be using a 1 ppb threshold going forward either in its
evaluation of SIPs or in promulgating a FIP. The August 2018 memorandum
indicated that ``[f]ollowing these recommendations does not ensure that
EPA will approve a SIP revision in all instances where the
recommendations are followed, as the guidance may not apply to the
facts and circumstances underlying a particular SIP. Final decisions by
the EPA to approve a particular SIP revision will only be made based on
the requirements of the statute and will only be made following an air
agency's final submission of the SIP revision to the EPA, and after
appropriate notice and opportunity for public review and comment.''
\180\ Further, the August 2018 memorandum said that ``EPA and air
agencies should consider whether the recommendations in this guidance
are appropriate for each situation.'' \181\ The memorandum said nothing
regarding what threshold the EPA would apply if promulgating a FIP.
---------------------------------------------------------------------------

    \177\ August 2018 memorandum, at 1.
    \178\ Id.
    \179\ Id. at 4.
    \180\ Id. at 1.
    \181\ Id.
---------------------------------------------------------------------------

    As explained in the SIP disapproval action and again here, the EPA
finds it would not be sound policy to apply an alternative contribution
threshold or thresholds to one or more states within the 4-step
interstate transport framework for the 2015 ozone NAAQS. However, the
EPA disagrees with commenters' claims that the agency has reversed
course on applying the August 2018 memorandum, because the memorandum
never adopted a view that the use of 1 ppb or other alternative
thresholds would in fact be acceptable. Although the EPA said at
proposal that the EPA may rescind the guidance in the future, we took
comment on the subject and also stated, ``EPA is not at this time
rescinding the August 2018 memorandum.'' \182\ The EPA is not formally
rescinding the August 2018 memorandum in this action or at this time.
However, it is not required that agencies must ``rescind'' a memorandum
or guidance the moment it becomes outdated or called into question. The
August 2018 memorandum was not issued through notice-and-comment
rulemaking and is not binding on the Agency or other parties. While the
willingness of the Agency as expressed in that memorandum to entertain
the possibility of an alternative threshold of 1 ppb may be considered
a kind of policy position, agencies may change their non-binding
policies without going through notice and comment rulemaking. Catawba
County v. EPA, 571 F.3d 20, 34 (D.C. Cir. 2009). In this case, we went
through notice and comment rulemaking on this topic in the SIP-
disapproval action (88 FR 9336) and here, even though the August 2018
memorandum was issued without such opportunity for public input. We
further address the basis for the consistent use of a 1 percent of
NAAQS threshold and summarize our conclusions under the FCC v. Fox
factors below.
---------------------------------------------------------------------------

    \182\ 87 FR 9545, 9551 (Feb. 22, 2022) (Alabama, Mississippi,
Tennessee); 87 FR 9498, 9510 (Feb. 22, 2022) (Kentucky); 87 FR 9838,
9844 (Feb. 22, 2022) (Illinois, Indiana, Michigan, Minnesota, Ohio,
Wisconsin); 87 FR 9798, 9807, 9813, 9820 (Feb. 22, 2022) (Arkansas,
Louisiana, Oklahoma, Texas); 87 FR 9533, 9542 (Feb. 22, 2022)
(Missouri); 87 FR 31470, 31479 (May 24, 2022) (Utah); 87 FR 31495,
31504 (May 24, 2022) (Wyoming); 87 FR 31485, 31490 (May 24, 2022)
(Nevada).
---------------------------------------------------------------------------

    We continue to believe, as set forth in our proposed action, that
national ozone transport policy is not well served by

allowing for less protective thresholds than 1 percent of the NAAQS at
Step 2. Furthermore, the EPA disagrees with commenters who suggest that
national consistency is an inappropriate consideration in the context
of interstate ozone transport. The Good Neighbor provision, CAA section
110(a)(2)(D)(i)(I), requires to a unique degree of concern for
consistency, parity, and equity across state lines.\183\ For a regional
air pollutant such as ozone, consistency in requirements and
expectations across all states is essential. Based on the EPA's review
of good neighbor SIP submissions to-date and after further
consideration of the policy implications of attempting to recognize an
alternative Step 2 threshold for certain states, the Agency concludes
that the attempted use of different thresholds at Step 2 with respect
to the 2015 8-hour ozone NAAQS raises substantial policy consistency
and practical implementation concerns. The availability of different
thresholds at Step 2 has the potential to result in inconsistent
application of good neighbor obligations based solely on the strength
of a state's SIP submission at Step 2 of the 4-step interstate
transport framework. The steps of the analysis that lead up to
evaluating emissions reductions opportunities to address states'
significant contribution at Step 3 should be applied on a consistent
basis. Where alternative thresholds for purposes of Step 2 may be
``similar'' in terms of capturing the relative amount of upwind
contribution (as described in the August 2018 memorandum), nonetheless,
use of an alternative threshold would allow certain states to avoid
further evaluation of potential emissions controls while other states
must proceed to a Step 3 analysis. This can create significant equity
and consistency problems among states and could lead to ineffective or
inefficient approaches to eliminating significant contribution.
---------------------------------------------------------------------------

    \183\ EPA notes that Congress has placed on EPA a general
obligation to ensure the requirements of the CAA are implemented
consistently across states and regions. See CAA section 301(a)(2).
Where the management and regulation of interstate pollution levels
spanning many states is at stake, consistency in application of CAA
requirements is paramount.
---------------------------------------------------------------------------

    One commenter suggested the EPA could address this potentially
inequitable outcome by simply adopting a 1 ppb contribution threshold
for all states. However, the August 2018 memorandum did not conclude
that 1 ppb would be appropriate for all states and the EPA does not
view that conclusion to be supported at present. The EPA recognized in
the August 2018 memorandum that there was some similarity in the amount
of total upwind contribution captured (on a nationwide basis) between 1
percent and 1 ppb. However, while this may be true in some sense, that
is hardly a compelling basis to move to a 1 ppb threshold for every
state. Indeed, the 1 ppb threshold has the disadvantage of losing a
certain amount of total upwind contribution for further evaluation at
Step 3 (e.g., roughly 7 percent of total upwind state contribution was
lost according to the modeling underlying the August 2018 memorandum;
in the EPA's 2016v2 modeling, the amount lost is 5 percent; in the
EPA's 2016v3 modeling used for final, the amount lost is also 5
percent). Further, this logic has no end point. A similar observation
could be made with respect to any incremental change. For example,
should the EPA next recognize a 1.2 ppb threshold because that would
only cause some small additional loss in capture of upwind state
contribution as compared to 1 ppb? If the only basis for moving to a 1
ppb threshold is that it captures a ``similar'' (but actually smaller)
amount of upwind contribution, then there is no basis for moving to
that threshold at all. Considering the core statutory objective of
ensuring elimination of all significant contribution to nonattainment
or interference with maintenance of the NAAQS in other states and the
broad, regional nature of the collective contribution problem with
respect to ozone, we continue to find no compelling policy reason to
adopt a new threshold for all states of 1 ppb.
    Nor have commenters explained why use of a 1 ppb threshold would be
appropriate under the more protective 2015 ozone NAAQS when a 1 percent
of the NAAQS contribution threshold has been used for less protective
ozone NAAQS. To illustrate, a state contributing greater than 0.75 ppb
but less than 1 ppb to a receptor under the 2008 ozone NAAQS was
``linked'' at Step 2,\184\ but if a 1 ppb threshold were used for the
2015 ozone NAAQS then that same state would not be ``linked'' to a
receptor at Step 2 under a NAAQS that is set to be more protective of
human health and the environment. Consistency with past interstate
transport actions such as CSAPR, and the CSAPR Update and Revised CSAPR
Update rulemakings (which all used the 1 percent of the NAAQS for less
protective ozone NAAQS), is an important consideration. We affirm our
view in CSAPR that continuing to use a 1 percent of NAAQS approach
ensures that if the NAAQS are revised and made more stringent, an
appropriate increase in stringency at Step 2 occurs, so as to ensure an
appropriately larger amount of total upwind-state contribution is
captured for purposes of fully addressing interstate transport. See 76
FR 48208, 48237-38.
---------------------------------------------------------------------------

    \184\ See 86 FR 23054, 23058 (April 30, 2021).
---------------------------------------------------------------------------

    We note further that application of a 1 percent of NAAQS threshold
has been the EPA's consistent approach in each of our notice-and-
comment rulemakings beginning with CSAPR and continuing with the CSAPR
Update, the Revised CSAPR Update, and numerous actions on ozone
transport SIP submissions. In each case, the 1 percent of the NAAQS
threshold was subject to rigorous vetting through public comment and
the Agency's response to those comments, including through the use of
analytical evaluations of alternative thresholds. See, e.g., 81 FR
74518-19. By contrast, the August 2018 memorandum was not issued
through notice-and-comment rulemaking procedures, and the EPA was
careful to caveat its utility and ultimate reliability for that reason.
    The EPA disagrees with claims that the EPA is applying the August
2018 memorandum inconsistently based on the EPA's actions with regard
to Arizona, Iowa, and Oregon. The EPA withdrew a previously proposed
approval of Iowa's SIP submission that was premised on a 1 ppb
contribution threshold, and re-proposed and finalized approval of that
SIP based on a different rationale using a 1 percent of the NAAQS
contribution threshold. 87 FR 9477 (Feb. 22, 2022); 87 FR 22463 (April
15, 2022). The EPA also disagrees with any claim that Oregon and
Arizona were ``allowed'' to use a 1 ppb or higher threshold. The EPA
approved Oregon's SIP submission for the 2015 ozone NAAQS on May 17,
2019, and both Oregon and the EPA relied on a 1 percent of the NAAQS
contribution threshold. 84 FR 7854, 7856 (March 5, 2019) (proposal); 84
FR 22376 (May 17, 2019) (final). In the proposal for this action, the
EPA explained it was not proposing to conduct an error correction for
Oregon even though updated modeling indicated Oregon contributed above
1 percent of the NAAQS to monitors in California.
    The EPA is deferring finalizing a finding at this time for Oregon
(see section IV.G of this document for additional information). In
2016, the EPA approved Arizona's SIP for the earlier 2008 ozone NAAQS
based on a similar rationale with regard to certain monitors in
California. 81 FR 15200 (March 22, 2016) (proposal); 81 FR 31513 (May
19, 2016) (final rule). We are deferring finalizing a finding at this
time that such a rationale is appropriate

with respect to the more protective 2015 ozone NAAQS. While Arizona and
Oregon's interstate transport obligations for the 2015 ozone NAAQS
remain pending (along with several other states), there is no
inconsistency in the treatment of these states or any other state at
Step 2.
    Some commenters claim the EPA must use a 1 ppb threshold based on
the identification of 1 ppb as a significance threshold in one step of
the PSD permitting process. The EPA's SIL guidances, however, relate to
a different provision of the Clean Air Act regarding implementation of
the prevention of significant deterioration (PSD) permitting program.
This program applies in areas that have been designated attainment of
the NAAQS and is intended to ensure that such areas remain in
attainment even if emissions were to increase as a result of new
sources or major modifications to existing sources located in those
areas. This purpose is different than the purpose of the good neighbor
provision, which is to assist downwind areas (in some cases hundreds or
thousands of miles away) in resolving ongoing nonattainment of the
NAAQS or difficulty maintaining the NAAQS through eliminating the
emissions from other states that are significantly contributing to
those problems. In addition, as discussed in preceding paragraphs, the
purpose of the Step 2 threshold within the EPA's interstate transport
framework for ozone is to broadly sweep in all states contributing to
identified receptors above a de minimis level in recognition of the
collective-contribution problem associated with regional-scale ozone
transport. The threshold used in the context of PSD SIL serves a
different purpose, and so it does not follow that they should be made
equivalent. Further, commenters incorrectly associate the EPA's Step 2
contribution threshold with the identification of ``significant''
emissions (which does not occur until Step 3), and so it is not the
case that the EPA is interpreting the same term differently.
    The EPA has previously explained this distinction between the good
neighbor framework and PSD SILs. See 70 FR 25162, 25190-25191 (May 12,
2005); 76 FR 48208, 48237 (Aug. 8, 2011). Importantly, the implication
of the PSD SIL threshold is not that single-source contribution below
this level indicates the absence of a contribution or that no emissions
control requirements are warranted. Rather, the PSD SIL threshold
addresses whether further, more comprehensive, multi-source review or
analysis of air quality impacts are required of the source to support a
demonstration that it meets the criteria for a permit. A source with
estimated impacts below the PSD SIL may use this to demonstrate that it
will not cause or contribute (as those terms are used within the PSD
program) to a violation of an ambient air quality standard, but is
still subject to meeting applicable control requirements, including
best available control technology, designed to moderate the source's
impact on air quality.
    Moreover, other aspects of the technical methodology in the SILs
guidance compared to the good neighbor framework make a direct
comparison between these two values misleading. For instance, in PSD
permit modeling using a single year of meteorology the maximum single-
day 8-hour contribution is evaluated with respect to the SIL. The
purpose of the contribution threshold at Step 2 of the 4-step good
neighbor framework is to determine whether the average contribution
from a collection of sources in a state is small enough not to warrant
any additional control for the purpose of mitigating interstate
transport, even if that control were highly cost effective. Using a 1
percent of the NAAQS threshold is more appropriate for evaluating
multi-day average contributions from upwind states than a 1 ppb
threshold applied for a single day, since that lower value of 1 percent
of the NAAQS will capture variations in contribution. If EPA were to
use a single day reflecting the maximum amount of contribution from an
upwind state to determine whether a linkage exists at Step 2,
commenters' arguments for use of the PSD SIL might have more force.
This would in effect be a return to the pre-CSAPR contribution
calculation methodology of using a single day, see 76 FR 48238.
However, that would likely cause more states to become linked, not
less. And in any case, consistent with the method in our modeling
guidance for projecting future attainment/nonattainment and as the EPA
concluded in 2011 in CSAPR, the present good neighbor methodology of
using multiple days provides a more robust approach to establishing
that a linkage exists at the state level than relying on a single day
of data.
    A commenter also claimed the 1 percent of NAAQS threshold is
inconsistent with the standards of precision for Federal reference
monitors for ozone and the rounding requirements found in 40 CFR part
50, appendix U, Interpretation of the Primary and Secondary National
Ambient Air Quality Standards for Ozone. Commenter claimed that the 1
percent contribution threshold of 0.7 ppb is lower than the
manufacturer's reported precision of these reference monitors and that
the requirements found in Appendix U truncates monitor values of 0.7
ppb to 0 ppb. However, the commenter is mistaken in applying criteria
related to the precision of monitoring technology to the modeling
methodology by which we project contributions when quantifying and
evaluating interstate transport at Step 2. Indeed, contributions by
source or state cannot be derived from the total ambient concentration
of ozone at a monitor at all but must be apportioned through modeling.
Under our longstanding methodology for doing so, the contribution
values identified from upwind states are based on a robust assessment
of the average impact of each upwind state's ozone-precursor emissions
over a range of scenarios, as explained in the 2016v3 modeling's Air
Quality Modeling Final Rule TSD, in the docket for this rule, Docket ID
No. EPA-HQ-OAR-2021-0668. This analysis is in no way connected with or
dependent on monitoring instruments' precision of measurement. See EME
Homer City, 795 F.3d 118, 135-36 (``[A] model is meant to simplify
reality in order to make it tractable.' '') (quoting Chemical
Manufacturers Association v. EPA, 28 F.3d 1259, 1264 (D.C. Cir. 1994).
    To the extent that commenters argue that the EPA consider a less
stringent threshold as a result of modeling uncertainty, the EPA
disagrees with this notion. The EPA has successfully applied a 1
percent of NAAQS threshold to identify linked upwind states using
modeling in three prior FIP rulemakings and numerous state-specific
actions on good neighbor obligations. This continues to be a reasonable
approach, and indeed courts have repeatedly declined to establish
bright line criteria for model performance. In upholding the EPA's
approach to evaluating interstate transport in CSAPR, the D.C. Circuit
held that it would not ``invalidate EPA's predictions solely because
there might be discrepancies between those predictions and the real
world. That possibility is inherent in the enterprise of prediction.''
EME Homer City Generation, L.P. v. EPA, 795 F.3d 118, 135 (2015).
``[T]he fact that a `model does not fit every application perfectly is
no criticism; a model is meant to simplify reality in order to make it
tractable.' '' Id. at 135-36 (quoting Chemical Manufacturers
Association v. EPA, 28 F.3d 1259, 1264 (D.C. Cir. 1994). See also
Sierra Club v. EPA, 939 F.3d 649, 686-87 (5th Cir. 2019) (upholding
EPA's modeling in the

face of complaints regarding an alleged ``margin of error,'' noting
challengers face a ``considerable burden'' in overcoming a
``presumption of regularity'' afforded ``the EPA's choice of analytical
methodology'') (citing BCCA Appeal Grp. v. EPA, 355 F.3d 817, 832 (5th
Cir. 2003)).
    The Agency will continue to use the CAMx model to evaluate
contributions from upwind states to downwind areas. The agency has used
CAMx routinely in previous notice and comment transport rulemakings to
evaluate contributions relative to the 1 percent threshold for both
ozone and PM2.5. In fact, in the original CSAPR, the EPA
found that ``[t]here was wide support from commenters for the use of
CAMx as an appropriate, state-of-the science air quality tool for use
in the [Cross-State Air Pollution] Rule. There were no comments that
suggested that the EPA should use an alternative model for quantifying
interstate transport.'' 76 FR 48229 (August 8, 2011). In this action,
the EPA has taken a number of steps based on comments and new
information to ensure to the greatest extent the accuracy and
reliability of its modeling projections at Step 1 and 2, as discussed
elsewhere in this section.
    The EPA disagrees with commenters that case law reviewing changes
in agency positions such as FCC v. Fox TV Stations, Inc., 556 U.S. 502,
515 (2009), is applicable with respect to this issue. As explained
above, under the terms of the August 2018 memorandum, the Agency did
not conclude that the use of an alternative contribution threshold was
justified for any states. But even if it were found that the Agency's
position had changed between this rulemaking action and the August 2018
memorandum, the FCC v. Fox factors are met. We have explained above
that there are good reasons for continuing to use a 1 percent of NAAQS
threshold. We also are aware that we are not using a 1 ppb threshold
despite acknowledging the potential for doing so in the August 2018
memorandum. We do not believe that any party has a serious reliance
interest that would be sufficient to overcome the countervailing public
interest that is served through the EPA's determination to maintain
continuity with its longstanding, more protective 1 percent of NAAQS
threshold in this action. Cf. 88 FR 9373 (reviewing reliance in the
context of the SIP-disapproval action).
    The EPA therefore will continue its longstanding practice of
applying the 1 percent of NAAQS threshold in this action.
a. States That Contribute Below the Screening Threshold
    Based on the EPA's modeling and considering measured data at
violating monitors, the contributions from each of the following states
to nonattainment or maintenance-only receptors in the 2023 analytic
year are below the 1 percent of the NAAQS threshold: Colorado,
Connecticut, the District of Columbia, Delaware, Florida, Georgia,
Idaho, Maine, Massachusetts, Montana, Nebraska, New Hampshire, North
Carolina, North Dakota, Rhode Island, South Carolina, South Dakota,
Vermont, and Washington.\185\ The EPA has already approved these
states' 2015 ozone good neighbor SIP submittals. Because the
contributions from these states to projected downwind air quality
problems are below the screening threshold in the current modeling,
these states are not within the scope of this final rule. Additionally,
the EPA has made final determinations that two states outside the
modeling domain for the air quality modeling analyzed in this final
rulemaking--Hawaii \186\ and Alaska \187\--do not significantly
contribute to nonattainment or interfere with maintenance of the NAAQS
in any other state.
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    \185\ The status of monitoring sites in California to which
Oregon may be linked is under review. See section IV.G.
    \186\ The EPA approved Hawaii's 2015 ozone transport SIP on
December 27, 2021. See 86 FR 73129.
    \187\ The EPA approved Alaska's 2015 ozone transport SIP on
December 18, 2019. See 84 FR 69331.
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    With respect to Wyoming, our methodology when applied using the
2016v3 modeling suggests that whether the state is linked is uncertain
and warrants further analysis. The EPA intends to expeditiously review
its assessment with respect to Wyoming and take action addressing
Wyoming's good neighbor obligations for the 2015 ozone NAAQS through a
separate action.
b. States That Contribute at or Above the Screening Threshold
    Based on the maximum downwind contributions in Table IV.F-1, the
Step 2 analysis identifies that the following 21 states contribute at
or above the 0.70 ppb threshold to downwind nonattainment receptors in
2023: Alabama, Arkansas, California, Illinois, Indiana, Kentucky,
Louisiana, Maryland, Michigan, Mississippi, Missouri, Nevada, New
Jersey, New York, Ohio, Oklahoma, Pennsylvania, Texas, Utah, Virginia,
and West Virginia. Based on the maximum downwind contributions in Table
IV.F-1, the following 23 states contribute at or above the 0.70 ppb
threshold to downwind modeling-based maintenance-only receptors in
2023: Arizona, Arkansas, California, Illinois, Indiana, Iowa, Kentucky,
Louisiana, Maryland, Michigan, Minnesota, Mississippi, Missouri,
Nevada, New Jersey, New Mexico, New York, Ohio, Oklahoma, Texas,
Virginia, West Virginia, and Wisconsin. Based on the maximum downwind
contribution in Table IV.F-3, the following additional states
contribute at or above the 0.70 ppb threshold to downwind violating
monitor maintenance-only receptors in 2023: Kansas and Tennessee.
(However, the EPA is not taking final action based on this analytical
result for these two states at this time.) The levels of contribution
between each of these linked upwind states and downwind nonattainment
receptors and maintenance-only receptors are provided in the Air
Quality Modeling Final Rule TSD.
    Among the linked states are several western states--California,
Nevada, and Utah. While the EPA has not previously included action on
linked western states in its prior CSAPR rulemakings, the EPA has
consistently applied the 4-step framework in evaluating good neighbor
obligations from these states. On a case-by-case basis, the EPA has
found in some instances with respect to the 2008 ozone NAAQS that a
unique consideration has warranted approval of a western state's good
neighbor SIP submittal that might otherwise be found to contribute
above 1 percent of the NAAQS without concluding that additional
emissions reductions are required at Step 3 of the framework.\188\ The
EPA has also explained in prior actions that its air quality modeling
is reliable for assessing downwind air quality problems and ozone
transport contributions from upwind states throughout the nationwide
modeling domain.\189\ The EPA is deferring finalizing a finding at this
time for Oregon (see section IV.G of this document for additional
information).
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    \188\ See interstate transport approval actions under the 2008
ozone NAAQS for Arizona, California, and Wyoming at 81 FR 36179
(June 6, 2016), 83 FR 65093 (December 19, 2018), and 84 FR 14270
(April 10, 2019)), respectively.
    \189\ See 81 FR 71991 (October 19, 2016), 82 FR 9155 (February
3, 2017).
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    As explained in the following section, the EPA is not, in this
action, altering its prior approval of Oregon's good neighbor SIP
submission for the 2015 ozone NAAQS. For the remaining western states
included in this rule, the EPA's modeling supports a conclusion that
these states are linked above the

contribution threshold to identified ozone transport receptors in
downwind states, and therefore, consistent with the treatment of all
other states within the modeling domain, the EPA proposes to proceed to
evaluate these states for a determination of ``significant
contribution'' at Step 3.
    In conclusion, as described above, states with contributions that
equal or exceed 1 percent of the NAAQS to either nonattainment or
maintenance-only receptors are identified as ``linked'' at Step 2 of
the good neighbor framework and warrant further analysis for
significant contribution to nonattainment or interference with
maintenance under Step 3. The EPA finds that for purposes of this final
rule, the following 23 states are linked at Step 2 in 2023: Alabama,
Arkansas, California, Illinois, Indiana, Kentucky, Louisiana, Maryland,
Michigan, Minnesota, Mississippi, Missouri, Nevada, New Jersey, New
York, Ohio, Oklahoma, Pennsylvania, Texas, Utah, Virginia, West
Virginia, and Wisconsin. In addition, the EPA finds that the following
20 States are linked at Step 2 in 2026: Arkansas, California, Illinois,
Indiana, Kentucky, Louisiana, Maryland, Michigan, Mississippi,
Missouri, Nevada, New Jersey, New York, Ohio, Oklahoma, Pennsylvania,
Texas, Utah, Virginia, and West Virginia. We note that our updated
modeling for this final rule shows that two states, Minnesota and
Wisconsin, that we found linked in 2026 at proposal are no longer
projected to be linked in that year but are linked in 2023.\190\ As at
proposal, Alabama is only projected to be linked in 2023, not 2026.
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    \190\ Minnesota and Wisconsin were linked to maintenance-only
receptors in Cook County, IL in 2023. Minnesota and Wisconsin are
not linked in 2026 because the 2026 average and maximum design
values at the monitoring sites are projected to show attainment.
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    For six states, the EPA's analysis at this time indicates that a
linkage may exist in 2023 for which the EPA had not proposed FIP
requirements, or the updated analysis for this final rule suggests that
linkages we had previously found in the proposed action are now
uncertain and warrant further analysis. The EPA intends to
expeditiously address these states in a separate action or actions:
Arizona, Iowa, Kansas, New Mexico, Tennessee, and Wyoming.

G. Treatment of Certain Monitoring Sites in California and Implications
for Oregon's Good Neighbor Obligations for the 2015 Ozone NAAQS

    The EPA previously approved Oregon's September 25, 2018 transport
SIP submittal for the 2015 ozone NAAQS on May 17, 2019 (84 FR 22376),
because in an earlier round of modeling Oregon was not projected to
contribute above 1 percent of the NAAQS to any downwind receptors. In
the EPA's updated modeling used at proposal (2016v2) and again in the
final modeling (2016v3), Oregon is modeled to contribute above the 1
percent of NAAQS threshold to several monitoring sites in California
that would generally meet the EPA's definition of nonattainment or
maintenance ``receptors'' at Step 1.\191\ At proposal, the EPA
explained that our analysis of the nature of the air quality problem at
these monitoring sites led us to propose a determination that these
monitoring sites should not be treated as receptors for purposes of
determining interstate transport obligations of upwind states under CAA
section 110(a)(2)(D)(i)(I). We explained that we reached this
conclusion at Step 1 of our 4-step framework.