[Federal Register Volume 87, Number 100 (Tuesday, May 24, 2022)]
[Proposed Rules]
[Pages 31495-31510]
[FR Doc No: 2022-11153]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R08-OAR-2022-0268; EPA-HQ-OAR-2021-0663; FRL-9805-01-R8]

Air Plan Disapproval; Wyoming; Interstate Transport of Air
Pollution for the 2015 8-Hour Ozone National Ambient Air Quality
Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: Pursuant to the Clean Air Act (CAA or the Act), the
Environmental Protection Agency (EPA) is proposing to disapprove the
State Implementation Plan (SIP) submittal from Wyoming regarding
interstate transport for the 2015 8-hour ozone national ambient air
quality standards (NAAQS). The ``good neighbor'' or ``interstate
transport'' provision requires that each state's SIP contain adequate
provisions to prohibit emissions from within the state from
significantly contributing to nonattainment or interfering with
maintenance of the NAAQS in other states. This requirement is part of
the broader set of ``infrastructure'' requirements, which are designed
to ensure that the structural components of each state's air quality
management program are adequate to meet the state's responsibilities
under the CAA. This disapproval, if finalized, will establish a 2-year
deadline for the EPA to promulgate a Federal Implementation Plan (FIP)
to address the relevant interstate transport requirements, unless the
EPA approves a subsequent SIP submittal that meets these requirements.
Disapproval does not start a mandatory CAA sanctions clock.

DATES: Comments: Written comments must be received on or before July
25, 2022.

ADDRESSES: You may send comments, identified as Docket No. EPA-R08-OAR-
2022-0268, to the Federal eRulemaking Portal at https://www.regulations.gov following the online instructions for submitting
comments.
    Instructions: All submissions received must include Docket No. EPA-
R08-OAR-2022-0268. Comments received may be posted without change to
https://www.regulations.gov, including any personal information
provided. For detailed instructions on sending comments and additional
information on the rulemaking process, see the ``Public participation''
heading of the SUPPLEMENTARY INFORMATION section of this document. The
EPA Docket Office can be contacted at<PHONE_NUMBER>, and is located at
EPA Docket Center Reading Room, WJC West Building, Room 3334, 1301
Constitution Avenue NW, Washington, DC 20004. For further information
on EPA Docket Center services and the current hours of operation at the
EPA Docket Center, please visit us online at https://www.epa.gov/dockets.

FOR FURTHER INFORMATION CONTACT: Ellen Schmitt, Air and Radiation
Division, EPA, Region 8, Mailcode 8ARD-IO, 1595 Wynkoop Street, Denver,
Colorado, 80202-1129, telephone number:<PHONE_NUMBER>, email address:
[email protected].

SUPPLEMENTARY INFORMATION:
    Public participation: Submit your comments, identified by Docket
No. EPA-R08-OAR-2022-0268, at https://www.regulations.gov. Once
submitted, comments cannot be edited or removed from the docket. The
EPA may publish any comment received to its public docket. Do not
submit to the EPA's docket at https://www.regulations.gov any
information you consider to be Confidential Business Information (CBI)
or other information whose disclosure is restricted by statute.
Multimedia submissions (audio, video, etc.) must be accompanied by a
written comment. The written comment is considered the official comment
and should include discussion of all points you wish to make. The EPA
will generally not consider comments or comment contents located
outside of the primary submission (i.e., on the web, cloud, or other
file sharing system).
    There are two dockets supporting this action, EPA-R08-OAR-2022-0268
and EPA-HQ-OAR-2021-0663. Docket No. EPA-R08-OAR-2022-0268 contains
information specific to Wyoming, including the notice of proposed
rulemaking. Docket No. EPA-HQ-OAR-2021-0663 contains additional
modeling files, emissions inventory files, technical support documents,
and other relevant supporting documentation regarding interstate
transport of emissions for the 2015 8-hour ozone NAAQS which are being
used to support this action. All comments regarding information in
either of these dockets are to be made in Docket No. EPA-R08-OAR-2022-
0268. For additional submission methods, please contact Ellen Schmitt,
telephone number:<PHONE_NUMBER>, email address: [email protected].
For the EPA's full public comment policy, information about CBI or
multimedia submissions, and general guidance on making effective
comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets.
    The index for Docket No. EPA-HQ-OAR-2021-0663, is available
electronically at www.regulations.gov. While all documents in the
docket are listed in the index, some information may not be publicly
available due to docket file size restrictions or content (e.g., CBI).
    The EPA continues to carefully and continuously monitor information
from the Centers for Disease Control and Prevention (CDC), local area
health departments, and our federal partners so that we can respond
rapidly as conditions change regarding COVID-19.

    Throughout this document, ``we,'' ``us,'' and ``our'' means the
EPA.

Table of Contents

I. Background
    A. Description of Statutory Background
    B. Description of the EPA's 4-Step Interstate Transport
Regulatory Process
    C. Background on the EPA's Ozone Transport Modeling Information
    D. The EPA's Approach to Evaluating Interstate Transport SIPs
for the 2015 8-Hour Ozone NAAQS
II. Wyoming SIP Submission Addressing Interstate Transport of Air
Pollution for the 2015 8-Hour Ozone NAAQS
III. The EPA's Evaluation
    A. Evaluation of Information Provided by Wyoming Regarding Step
1 and Step 2
    B. Evaluation of Information Provided Regarding Step 3
    C. Evaluation of Information Provided Regarding Step 4
    D. Conclusion
IV. Proposed Action
V. Statutory and Executive Order Reviews

I. Background

A. Description of Statutory Background

    On October 1, 2015, the EPA promulgated a revision to the ozone
NAAQS (2015 8-hour ozone NAAQS), lowering the level of both the primary
and secondary standards to 0.070 parts per million (ppm).\1\ Section
110(a)(1) of the CAA requires states to submit, within 3 years after
promulgation of a new or revised standard, SIP submissions meeting the
applicable requirements of section 110(a)(2).\2\ One of these
applicable requirements is found in CAA section 110(a)(2)(D)(i)(I),
otherwise known as the ``interstate transport'' or ``good neighbor''
provision, which generally requires SIPs to contain adequate provisions
to prohibit in-state emissions activities from having certain adverse
air quality effects on other states due to interstate transport of
pollution. There are two so-called ``prongs'' within CAA section
110(a)(2)(D)(i)(I). A SIP for a new or revised NAAQS must contain
adequate provisions prohibiting any source or other type of emissions
activity within the state from emitting air pollutants in amounts that
will significantly contribute to nonattainment of the NAAQS in another
state (prong 1) or interfere with maintenance of the NAAQS in another
state (prong 2). The EPA and states must give independent significance
to prong 1 and prong 2 when evaluating downwind air quality problems
under CAA section 110(a)(2)(D)(i)(I).\3\
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    \1\ National Ambient Air Quality Standards for Ozone, Final
Rule, 80 FR 65292 (October 26, 2015). Although the level of the
standard is specified in the units of ppm, ozone concentrations are
also described in parts per billion (ppb). For example, 0.070 ppm is
equivalent to 70 ppb.
    \2\ SIP revisions that are intended to meet the applicable
requirements of section 110(a)(1) and (2) of the CAA are often
referred to as infrastructure SIPs and the applicable elements under
section 110(a)(2) are referred to as infrastructure requirements.
    \3\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir.
2008).
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B. Description of the EPA's 4-Step Interstate Transport Regulatory
Process

    The EPA is using the 4-Step Interstate Transport Framework (or 4-
Step Framework) to evaluate Wyoming's SIP submittal addressing the
interstate transport provision for the 2015 8-hour ozone NAAQS. The EPA
has addressed the interstate transport requirements of CAA section
110(a)(2)(D)(i)(I) with respect to prior ozone NAAQS in several
regional regulatory actions, including the Cross-State Air Pollution
Rule (CSAPR), which addressed interstate transport with respect to the
1997 ozone NAAQS as well as the 1997 and 2006 fine particulate matter
standards,\4\ and the Cross-State Air Pollution Rule Update (CSAPR
Update) \5\ and the Revised CSAPR Update, both of which addressed the
2008 ozone NAAQS.\6\ Through the development and implementation of the
CSAPR rulemakings and prior regional rulemakings pursuant to the
interstate transport provision,\7\ the EPA, working in partnership with
states, developed the following 4-Step Framework to evaluate a state's
obligations to eliminate interstate transport emissions under the
interstate transport provision for the ozone NAAQS: (1) Identify
monitoring sites that are projected to have problems attaining and/or
maintaining the NAAQS (i.e., nonattainment and/or maintenance
receptors); (2) identify states that impact those air quality problems
in other (i.e., downwind) states sufficiently such that the states are
considered ``linked'' and therefore warrant further review and
analysis; (3) identify the emissions reductions necessary (if any),
applying a multifactor analysis, to eliminate each linked upwind
state's significant contribution to nonattainment or interference with
maintenance of the NAAQS at the locations identified in Step 1; and (4)
adopt permanent and enforceable measures needed to achieve those
emissions reductions.
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    \4\ See Federal Implementation Plans: Interstate Transport of
Fine Particulate Matter and Ozone and Correction of SIP Approvals,
76 FR 48208 (August 8, 2011).
    \5\ Cross-State Air Pollution Rule Update for the 2008 Ozone
NAAQS, 81 FR 74504 (October 26, 2016).
    \6\ In 2019, the D.C. Circuit Court of Appeals remanded the
CSAPR Update to the extent it failed to require upwind states to
eliminate their significant contribution by the next applicable
attainment date by which downwind states must come into compliance
with the NAAQS, as established under CAA section 181(a). Wisconsin
v. EPA, 938 F.3d 303, 313 (D.C. Cir. 2019). The Revised CSAPR Update
for the 2008 Ozone NAAQS, 86 FR 23054 (April 30, 2021), responded to
the remand of the CSAPR Update in Wisconsin and the vacatur of a
separate rule, the ``CSAPR Close-Out,'' 83 FR 65878 (December 21,
2018), in New York v. EPA, 781 F. App'x. 4 (D.C. Cir. 2019).
    \7\ In addition to the CSAPR rulemakings, other regional
rulemakings addressing ozone transport include the ``NOX
SIP Call,'' 63 FR 57356 (October 27, 1998), and the ``Clean Air
Interstate Rule'' (CAIR), 70 FR 25162 (May 12, 2005).
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C. Background on the EPA's Ozone Transport Modeling Information

    In general, the EPA has performed nationwide air quality modeling
to project ozone design values which are used in combination with
measured data to identify nonattainment and maintenance receptors. To
quantify the contribution of emissions from specific upwind states on
2023 ozone design values for the identified downwind nonattainment and
maintenance receptors, the EPA performed nationwide, state-level ozone
source apportionment modeling for 2023. The source apportionment
modeling provided contributions to ozone at receptors from precursor
emissions of anthropogenic nitrogen oxides (NOX) and
volatile organic compounds (VOCs) in individual upwind states.
    The EPA has released several documents containing projected ozone
design values, contributions, and information relevant to evaluating
interstate transport with respect to the 2015 8-hour ozone NAAQS.
First, on January 6, 2017, the EPA published a notice of data
availability (NODA) in which we requested comment on preliminary
interstate ozone transport data including projected ozone design values
and interstate contributions for 2023 using a 2011 base year
platform.\8\ In the NODA, the EPA used the year 2023 as the analytic
year for this preliminary modeling because that year aligns with the
expected attainment year for Moderate ozone nonattainment areas for the
2015 8-hour ozone NAAQS.\9\ On October 27, 2017, we released a
memorandum (October 2017 memorandum) containing updated modeling data
for 2023, which incorporated changes made in response to comments on
the NODA, and noted that the modeling may be useful for states
developing SIPs to address interstate transport obligations for the

2008 ozone NAAQS.\10\ On March 27, 2018, we issued a memorandum (March
2018 memorandum) noting that the same 2023 modeling data released in
the October 2017 memorandum could also be useful for identifying
potential downwind air quality problems with respect to the 2015 8-hour
ozone NAAQS at Step 1 of the 4-Step Interstate Transport Framework.\11\
The March 2018 memorandum also included the then newly available
contribution modeling data for 2023 to assist states in evaluating
their impact on potential downwind air quality problems for the 2015 8-
hour ozone NAAQS under Step 2 of the 4-Step Framework.\12\ The EPA
subsequently issued two more memoranda in August and October 2018,
providing additional information to states developing interstate
transport SIP submissions for the 2015 8-hour ozone NAAQS concerning,
respectively, potential contribution thresholds that may be appropriate
to apply in Step 2 of the 4-Step Interstate Transport Framework, and
considerations for identifying downwind areas that may have problems
maintaining the NAAQS at Step 1 of the 4-Step Framework.\13\
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    \8\ See Notice of Availability of the Environmental Protection
Agency's Preliminary Interstate Ozone Transport Modeling Data for
the 2015 8-hour Ozone National Ambient Air Quality Standard (NAAQS),
82 FR 1733 (January 6, 2017).
    \9\ 82 FR 1733 at 1735 (January 6, 2017).
    \10\ See Information on the Interstate Transport State
Implementation Plan Submissions for the 2008 Ozone National Ambient
Air Quality Standards under Clean Air Act Section
110(a)(2)(D)(i)(I), October 27, 2017 (``October 2017 memorandum''),
available in Docket No. EPA-HQ-OAR-2021-0663.
    \11\ See Information on the Interstate Transport State
Implementation Plan Submissions for the 2015 Ozone National Ambient
Air Quality Standards under Clean Air Act Section
110(a)(2)(D)(i)(I), March 27, 2018 (``March 2018 memorandum''),
available in Docket No. EPA-HQ-OAR-2021-0663.
    \12\ The March 2018 memorandum, however, provided, ``While the
information in this memorandum and the associated air quality
analysis data could be used to inform the development of these SIPs,
the information is not a final determination regarding states'
obligations under the good neighbor provision. Any such
determination would be made through notice-and-comment rulemaking.''
    \13\ See Analysis of Contribution Thresholds 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, August 31, 2018 (``August 2018 memorandum''),
and 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, available in Docket No.
EPA-HQ-OAR-2021-0663.
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    Since the release of the modeling data shared in the March 2018
memorandum, the EPA performed updated modeling using a 2016-based
emissions modeling platform (i.e., 2016v1). This emissions platform was
developed under the EPA/Multi-Jurisdictional Organization (MJO)/state
collaborative project.\14\ This collaborative project was a multi-year
joint effort by the EPA, MJOs, and states to develop a new, more recent
emissions platform for use by the EPA and states in regulatory modeling
as an improvement over the dated 2011-based platform that the EPA had
used to project ozone design values and contribution data provided in
the 2017 and 2018 memoranda. The EPA used the 2016v1 emissions to
project ozone design values and contributions for 2023. On October 30,
2020, in the Notice of Proposed Rulemaking for the Revised CSAPR
Update, the EPA released and accepted public comment on 2023 modeling
that used the 2016v1 emissions platform.\15\ Although the Revised CSPAR
Update addressed transport for the 2008 ozone NAAQS, the projected
design values and contributions from the 2016v1 platform are also
useful for identifying downwind ozone problems and linkages with
respect to the 2015 ozone NAAQS.\16\
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    \14\ The results of this modeling, as well as the underlying
modeling files, are included in Docket No. EPA-HQ-OAR-2021-0663.
    \15\ See 85 FR 68964, 68981.
    \16\ See the Air Quality Modeling Technical Support Document for
the Final Revised Cross-State Air Pollution Rule Update, included in
the Docket No. EPA-HQ-OAR-2021-0663.
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    Following the final Revised CSAPR Update, the EPA made further
updates to the 2016 emissions platform to include mobile emissions from
the EPA's Motor Vehicle Emission Simulator MOVES3 model \17\ and
updated emissions projections for electric generating units (EGUs) that
reflect the emissions reductions from the Revised CSAPR Update, recent
information on plant closures, and other sector trends. The construct
of this updated emissions platform, 2016v2, is described in the
emissions modeling technical support document (TSD) supporting this
proposed rule.\18\ The EPA performed air quality modeling of the 2016v2
emissions using the most recent public release version of the
Comprehensive Air-quality Model with extensions (CAMx) photochemical
modeling, version 7.10.\19\ The EPA now proposes to primarily rely on
modeling based on the updated and newly available 2016v2 emissions
platform in evaluating these submissions with respect to Steps 1 and 2
of the 4-Step Framework. This modeling will generally be referenced
within this action as 2016v2 modeling for 2023. By using the updated
modeling results, the EPA is using the most current and technically
appropriate information for this proposed rulemaking. Section III of
this document and the Air Quality Modeling TSD for 2015 Ozone NAAQS
Transport SIP Proposed Actions, included in Docket No. EPA-HQ-OAR-2021-
0663 for this proposal, contain additional detail on the EPA's 2016v2
modeling. In this document, the EPA is accepting public comment on this
updated 2023 modeling, which uses a 2016v2 emissions platform. Comments
on the EPA's air quality modeling should be submitted in the Regional
docket for this action, Docket No. EPA-R08-OAR-2022-0268. Comments are
not being accepted in Docket No. EPA-HQ-OAR-2021-0663.
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    \17\ Additional details and documentation related to the MOVES3
model can be found at https://www.epa.gov/moves/latest-version-motor-vehicle-emission-simulator-moves.
    \18\ See Technical Support Document (TSD) Preparation of
Emissions Inventories for the 2016v2 North American Emissions
Modeling Platform included in Docket No. EPA-HQ-OAR-2021-0663.
    \19\ Ramboll Environment and Health, January 2021, www.camx.com.
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    States may have chosen to rely on the results of the EPA modeling
and/or alternative modeling performed by states or MJOs to evaluate
downwind air quality problems and contributions as part of their
submissions. In Section III we evaluate how the Wyoming Department of
Environmental Quality (WDEQ) used air quality modeling information in
their submission.

D. The EPA's Approach To Evaluating Interstate Transport SIPs for the
2015 8-Hour Ozone NAAQS

    The EPA proposes to apply a consistent set of policy judgments
across all states for purposes of evaluating interstate transport
obligations and the approvability of interstate transport SIP
submissions for the 2015 8-hour ozone NAAQS. These policy judgments
reflect consistency with relevant case law and past Agency practice as
reflected in CSAPR and related rulemakings. Nationwide consistency in
approach is particularly important in the context of interstate ozone
transport, which is a regional-scale pollution problem involving many
smaller contributors. Effective policy solutions to the problem of
interstate ozone transport going back to the NOX SIP Call
have necessitated the application of a uniform framework of policy
judgments in order to ensure an ``efficient and equitable'' approach.
See EME Homer City Generation, LP v. EPA, 572 U.S. 489, 519 (2014).
    In the March, August, and October 2018 memoranda, the EPA
recognized that states may be able to establish alternative approaches
to addressing their interstate transport obligations for the 2015 8-
hour ozone NAAQS that vary from a nationally uniform framework.

The EPA emphasized in these memoranda, however, that such alternative
approaches must be technically justified and appropriate in light of
the facts and circumstances of each particular state's submittal. In
general, the EPA continues to believe that deviation from a nationally
consistent approach to ozone transport must be substantially justified
and have a well-documented technical basis that is consistent with
relevant case law. Where states submitted SIPs that rely on any such
potential ``flexibilities'' as may have been identified or suggested in
the past, the EPA will evaluate whether the state adequately justified
the technical and legal basis for doing so.
    The EPA notes that certain concepts included in an attachment to
the March 2018 memorandum require unique consideration, and these ideas
do not constitute Agency guidance with respect to transport obligations
for the 2015 ozone NAAQS. Attachment A to the March 2018 memorandum
identified a ``Preliminary List of Potential Flexibilities'' that could
potentially inform SIP development,\20\ however, the EPA made clear in
that attachment that the list of ideas were not suggestions endorsed by
the Agency but rather ``comments provided in various forums'' on which
the EPA sought ``feedback from interested stakeholders.'' \21\ Further,
Attachment A stated, ``EPA is not at this time making any determination
that the ideas discussed below are consistent with the requirements of
the CAA, nor are we specifically recommending that states use these
approaches.'' \22\ Attachment A to the March 2018 memorandum,
therefore, does not constitute Agency guidance, but was intended to
generate further discussion around potential approaches to addressing
ozone transport among interested stakeholders. To the extent states
sought to develop or rely on these ideas in support of their SIP
submissions, the EPA will thoroughly review the technical and legal
justifications for doing so.
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    \20\ March 2018 memorandum, Attachment A.
    \21\ Id. at A-1.
    \22\ Id.
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    The remainder of this section describes the EPA's proposed
framework with respect to analytic year, definition of nonattainment
and maintenance receptors, selection of contribution threshold, and
multifactor control strategy assessment.
1. Selection of Analytic Year
    In general, the states and the EPA must implement the interstate
transport provision in a manner ``consistent with the provisions of
[title I of the CAA.]'' CAA section 110(a)(2)(D)(i). This requires,
among other things, that these obligations are addressed consistently
with the timeframes for downwind areas to meet their CAA obligations.
With respect to ozone NAAQS, under CAA section 181(a), this means
obligations must be addressed ``as expeditiously as practicable'' and
no later than the schedule of attainment dates provided in CAA section
181(a)(1).\23\ Several D.C. Circuit court decisions address the issue
of the relevant analytic year for the purposes of evaluating ozone
transport air-quality problems. On September 13, 2019, the D.C. Circuit
issued a decision in Wisconsin v. EPA, remanding the CSAPR Update to
the extent that it failed to require upwind states to eliminate their
significant contribution by the next applicable attainment date by
which downwind states must come into compliance with the NAAQS, as
established under CAA section 181(a). 938 F.3d 303 at 313.
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    \23\ For attainment dates for the 2015 8-hour ozone NAAQS, refer
to CAA section 181(a), 40 CFR 51.1303, and Additional Air Quality
Designations for the 2015 Ozone National Ambient Air Quality
Standards, 83 FR 25776 (June 4, 2018, effective August 3, 2018).
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    On May 19, 2020, the D.C. Circuit issued a decision in Maryland v.
EPA that cited the Wisconsin decision in holding that the EPA must
assess the impact of interstate transport on air quality at the next
downwind attainment date, including Marginal area attainment dates, in
evaluating the basis for the EPA's denial of a petition under CAA
section 126(b). Maryland v. EPA, 958 F.3d 1185, 1203-04 (D.C. Cir.
2020). The court noted that ``section 126(b) incorporates the Good
Neighbor Provision,'' and, therefore, the ``EPA must find a violation
[of section 126] if an upwind source will significantly contribute to
downwind nonattainment at the next downwind attainment deadline.
Therefore, the agency must evaluate downwind air quality at that
deadline, not at some later date.'' Id. at 1204 (emphasis added). The
EPA interprets the court's holding in Maryland as requiring the states
and the Agency, under the good neighbor provision, to assess downwind
air quality as expeditiously as practicable and no later than the next
applicable attainment date,\24\ which is now the Moderate area
attainment date under CAA section 181 for ozone nonattainment. The
Moderate area attainment date for the 2015 8-hour ozone NAAQS is August
3, 2024.\25\ The EPA believes that 2023 is now the appropriate year for
analysis of interstate transport obligations for the 2015 8-hour ozone
NAAQS, because the 2023 ozone season is the last relevant ozone season
during which achieved emissions reductions in linked upwind states
could assist downwind states with meeting the August 3, 2024 Moderate
area attainment date for the 2015 8-hour ozone NAAQS.
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    \24\ We note that the court in Maryland did not have occasion to
evaluate circumstances in which the EPA may determine that an upwind
linkage to a downwind air quality problem exists at Steps 1 and 2 of
the 4-Step Framework by a particular attainment date, but for
reasons of impossibility or profound uncertainty the Agency is
unable to mandate upwind pollution controls by that date. See
Wisconsin, 938 F.3d at 320. The D.C. Circuit noted in Wisconsin that
upon a sufficient showing, these circumstances may warrant
flexibility in effectuating the purpose of the interstate transport
provision.
    \25\ See CAA section 181(a); 40 CFR 51.1303; Additional Air
Quality Designations for the 2015 Ozone National Ambient Air Quality
Standards, 83 FR 25776 (June 4, 2018, effective August 3, 2018).
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    The EPA recognizes that the attainment date for nonattainment areas
classified as Marginal for the 2015 8-hour ozone NAAQS was August 3,
2021. Under the Maryland holding, any necessary emissions reductions to
satisfy interstate transport obligations should have been implemented
by no later than this date. At the time of the statutory deadline to
submit interstate transport SIPs (October 1, 2018), many states relied
upon the EPA modeling of the year 2023, and no state provided an
alternative analysis using a 2021 analytic year (or the prior 2020
ozone season). However, the EPA must act on SIP submittals using the
information available at the time it takes such action. In this
circumstance, the EPA does not believe it would be appropriate to
evaluate states' obligations under CAA section 110(a)(2)(D)(i)(I) as of
an attainment date that is wholly in the past, because the Agency
interprets the interstate transport provision as forward looking. See
86 FR 23054 at 23074; see also Wisconsin, 938 F.3d at 322.
Consequently, in this proposal the EPA will use the analytical year of
2023 to evaluate Wyoming's CAA section 110(a)(2)(D)(i)(I) SIP
submission with respect to the 2015 8-hour ozone NAAQS.
2. Step 1 of the 4-Step Interstate Transport Framework
    In Step 1, the EPA identifies monitoring sites that are projected
to have problems attaining and/or maintaining the NAAQS in the 2023
analytic year. Where the EPA's analysis shows that a site does not fall
under the definition of a nonattainment or maintenance receptor, that
site is excluded from further analysis under

the EPA's 4-Step Framework. For sites that are identified as a
nonattainment or maintenance receptor in 2023, we proceed to the next
step of our 4-Step Framework by identifying the upwind state's
contribution to those receptors.
    The EPA's approach to identifying ozone nonattainment and
maintenance receptors in this action is consistent with the approach
used in previous transport rulemakings. The EPA's approach gives
independent consideration to both the ``contribute significantly to
nonattainment'' and the ``interfere with maintenance'' prongs of CAA
section 110(a)(2)(D)(i)(I), consistent with the D.C. Circuit's
direction in North Carolina v. EPA.\26\
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    \26\ See North Carolina v. EPA, 531 F.3d 896, 910-11 (D.C. Cir.
2008) (holding that the EPA must give ``independent significance''
to each prong of CAA section 110(a)(2)(D)(i)(I)).
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    For the purpose of this proposal, the EPA identifies 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
CSAPR Update, where the EPA defined nonattainment receptors as those
areas that both currently measure nonattainment and that the EPA
projects will be in nonattainment in the future analytic year (i.e.,
2023).\27\
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    \27\ See 81 FR 74504 (October 26, 2016). This same concept,
relying on both current monitoring data and modeling to define
nonattainment receptor, was also applied in CAIR. See 70 FR 25162 at
25241, 25249 (January 14, 2005); see also North Carolina, 531 F.3d
at 913-14 (affirming as reasonable the EPA's approach to defining
nonattainment in CAIR).
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    In addition, in this proposal, the EPA identifies a receptor to be
a ``maintenance'' receptor for purposes of defining interference with
maintenance, consistent with the method used in the CSAPR and upheld by
the D.C. Circuit in EME Homer City Generation, L.P. v. EPA, 795 F.3d
118, 136 (D.C. Cir. 2015).\28\ Specifically, the EPA identified
maintenance receptors as those receptors that would have difficulty
maintaining the relevant NAAQS in a scenario that takes into account
historical variability in air quality at that receptor. The variability
in air quality was determined by evaluating the ``maximum'' future
design value at each receptor based on a projection of the maximum
measured design value over the relevant base 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, vertical mixing, insolation, 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. 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.
---------------------------------------------------------------------------

    \28\ See 76 FR 48208 (August 8, 2011). The CSAPR Update and the
Revised CSAPR Update also used this approach. See 81 FR 74504
(October 26, 2016) and 86 FR 23054 (April 30, 2021).
---------------------------------------------------------------------------

    Recognizing that nonattainment receptors are also, by definition,
maintenance receptors, the EPA often uses the term ``maintenance-only''
to refer to those receptors that are not nonattainment receptors.
Consistent with the concepts for maintenance receptors, as described
above, 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.
3. Step 2 of the 4-Step Interstate Transport Framework
    In Step 2, the EPA quantifies the contribution of each upwind state
to each receptor in the 2023 analytic year. The contribution metric
used in Step 2 is defined as the average impact from each state to each
receptor on the days with the highest ozone concentrations at the
receptor based on the 2023 modeling. If a state's contribution value
does not equal or exceed the threshold of 1 percent of the NAAQS (i.e.,
0.70 ppb for the 2015 8-hour ozone NAAQS), the upwind state is not
linked to a downwind air quality problem, and the EPA, therefore,
concludes that the state does not significantly contribute to
nonattainment or interfere with maintenance of the NAAQS in the
downwind states. However, if a state's contribution equals or exceeds
the 1 percent threshold, the state's emissions are further evaluated in
Step 3, considering both air quality and cost as part of a multi-factor
analysis, to determine what, if any, emissions might be deemed
``significant'' and, thus, must be eliminated under CAA section
110(a)(2)(D)(i)(I). The EPA is proposing to rely in the first instance
on the 1 percent threshold for the purpose of evaluating a state's
contribution to nonattainment or maintenance of the 2015 8-hour ozone
NAAQS (i.e., 0.70 ppb) at downwind receptors. This is consistent with
the Step 2 approach that the EPA applied in CSAPR for the 1997 ozone
NAAQS, which has subsequently been applied in the CSAPR Update when
evaluating interstate transport obligations for the 2008 ozone NAAQS.
The EPA continues to find 1 percent to be an appropriate threshold. For
ozone, as the EPA found in the Clean Air Interstate Rule (CAIR), CSAPR,
and the CSAPR Update, a portion of the nonattainment problems from
anthropogenic sources 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, in some cases, substantially
larger contributions from a subset of particular upwind states. The
EPA's analysis shows that much of the ozone transport problem being
analyzed in this proposed rule is the result of the collective impacts
of contributions from multiple upwind states. Therefore, application of
a consistent contribution threshold is necessary to identify those
upwind states that should have responsibility for addressing their
contribution to the downwind nonattainment and maintenance problems to
which they collectively contribute. 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 81
FR at 74518. See also 86 FR at 23085 (reviewing and explaining
rationale from CSAPR, 76 FR at 48237-38, for selection of the 1 percent
threshold).
    The EPA's August 2018 memorandum recognized that in certain
circumstances, a state may be able to establish that an alternative
contribution threshold of 1 ppb is justifiable. Where a state relies on
this alternative threshold, and where that state determined that it was
not linked at Step 2 using the alternative threshold,

the EPA will evaluate whether the state provided a technically sound
assessment of the appropriateness of using this alternative threshold
based on the facts and circumstances underlying its application in that
particular SIP submission.
4. Step 3 of the 4-Step Interstate Transport Framework
    Consistent with the EPA's longstanding approach to eliminating
significant contribution or interference with maintenance, at Step 3,
states linked at Steps 1 and 2 are generally expected to prepare a
multifactor assessment of potential emissions controls. The EPA's
analysis at Step 3 in prior federal actions addressing interstate
transport requirements has primarily focused on an evaluation of cost-
effectiveness of potential emissions controls (on a marginal cost-per-
ton basis), the total emissions reductions that may be achieved by
requiring such controls (if applied across all linked upwind states),
and an evaluation of the air quality impacts such emissions reductions
would have on the downwind receptors to which a state is linked; other
factors may potentially be relevant if adequately supported. In
general, where the EPA's or alternative air quality and contribution
modeling establishes that a state is linked at Steps 1 and 2, it will
be insufficient at Step 3 for a state merely to point to its existing
rules requiring control measures as a basis for approval. Generally,
the emissions-reducing effects of all existing emissions control
requirements are already reflected in the air quality results of the
modeling for Steps 1 and 2. If the state is shown to still be linked to
one or more downwind receptor(s), states must provide a well-documented
evaluation determining whether their emissions constitute significant
contribution or interference with maintenance by evaluating additional
available control opportunities by preparing a multifactor assessment.
While the EPA has not prescribed a particular method for this
assessment, the EPA expects states at a minimum to present a sufficient
technical evaluation. This would typically include information on
emissions sources, applicable control technologies, emissions
reductions, costs, cost effectiveness, and downwind air quality impacts
of the estimated reductions, before concluding that no additional
emissions controls should be required.\29\
---------------------------------------------------------------------------

    \29\ As examples of general approaches for how such an analysis
could be conducted for their sources, states could look to the CSAPR
Update, 81 FR 74504, 74539-51; CSAPR, 76 FR 48208, 48246-63; CAIR,
70 FR 25162, 25195-229; or the NOX SIP Call, 63 FR 57356,
57399-405. See also the Revised CSAPR Update, 86 FR 23054, 23086-
23116. Consistently across these rulemakings, the EPA has developed
emissions inventories, analyzed different levels of control
stringency at different cost thresholds, and assessed resulting
downwind air quality improvements.
---------------------------------------------------------------------------

5. Step 4 of the 4-Step Interstate Transport Framework
    At Step 4, states (or the EPA) develop permanent and federally
enforceable control strategies to achieve the emissions reductions
determined to be necessary at Step 3 to eliminate significant
contribution to nonattainment or interference with maintenance of the
NAAQS. For a state linked at Steps 1 and 2 to rely on an emissions
control measure at Step 3 to address its interstate transport
obligations, that measure must be included in the state's SIP so that
it is permanent and federally enforceable. See CAA section 110(a)(2)(D)
(``Each such [SIP] shall . . . contain adequate provisions . . . .'').
See also CAA 110(a)(2)(A); Committee for a Better Arvin v. U.S. E.P.A.,
786 F.3d 1169, 1175-76 (9th Cir. 2015) (holding that measures relied on
by state to meet CAA requirements must be included in the SIP).

II. Wyoming SIP Submission Addressing Interstate Transport of Air
Pollution for the 2015 8-Hour Ozone NAAQS

    On January 3, 2019, the WDEQ submitted a SIP revision addressing
the CAA section 110(a)(2)(D)(i)(I) interstate transport requirements
for the 2015 8-hour ozone NAAQS.\30\ The SIP submission provided WDEQ's
analysis of the State's impact to downwind states and concluded that
emissions from Wyoming will not significantly contribute to
nonattainment or interfere with maintenance of the 2015 ozone NAAQS in
other states in 2023.\31\ The WDEQ SIP submission cited the EPA's 4-
Step Framework approach, but also included a ``weight-of-evidence''
analysis.\32\ Throughout the submission, the WDEQ also incorporated
certain outside parties' ideas for ``flexibilities'' in assessing good
neighbor obligations that had been listed in Attachment A to the March
2018 memorandum.\33\ In their analysis, the WDEQ used the modeling
released with the March 2018 memorandum to identify nonattainment and
maintenance receptors in the Denver Metro/North Front Range
nonattainment area in 2023 (Step 1).\34\ The WDEQ also relied on the
EPA's modeling from the March 2018 memorandum to identify contributions
to projected nonattainment and/or maintenance receptors and emissions
from sources in the State in 2023 (Step 2).\35\ The WDEQ identified
five nonattainment and maintenance receptors to which the State was
projected to contribute equal to and greater than 0.70 ppb (1 percent
of the 2015 ozone NAAQS).\36\ Table 1 provides information on the five
nonattainment and maintenance receptors identified by the WDEQ in the
State's SIP submittal.
---------------------------------------------------------------------------

    \30\ In its SIP submission, the WDEQ references its Air Quality
Division (AQD). In this action, we simply reference the WDEQ.
Wyoming State Implementation Plan, Interstate Transport, To Satisfy
the Requirements of Clean Air Act 110(a)(2)(i)(I) for the 8-Hour
Ozone NAAQS Promulgated in October 2015, December 2018. Located in
the docket for this rulemaking at regulations.gov, Docket No. EPA-
R08-OAR-2022-0268.
    \31\ Wyoming State Implementation Plan, Attachment B at 10.
    \32\ See generally id. at 3-10.
    \33\ Id. at 3, 8.
    \34\ Id. at 3. The EPA notes that the modeling released with the
October 2017 and March 2018 memoranda both used 2011 base year
inventory data.
    \35\ Id. at 6.
    \36\ Id.

  Table 1--2023 Average and Maximum Design Values at Downwind Receptors With Wyoming Contributions Equal to and
                                             Greater Than 0.70 ppb a
----------------------------------------------------------------------------------------------------------------
                                                                                Wyoming
                                                                  Average design  Maximum design      modeled
            Site ID                    County           State       value (ppb)     value (ppb)    contribution
                                                                                (ppb)
----------------------------------------------------------------------------------------------------------------
80050002.......................  Arapahoe..........  CO                     69.3            71.3            1.04
80350004.......................  Douglas...........  CO                     71.1            73.2            1.00
80590006.......................  Jefferson.........  CO                     71.3            73.7            0.81

80590011.......................  Jefferson.........  CO                     70.9            73.9            1.03
80690011.......................  Larimer...........  CO                     71.2            73.0            0.88
----------------------------------------------------------------------------------------------------------------
\a\ Data according to March 2018 memorandum modeling.

    While the WDEQ presented all of the monitors that the modeling
projected Wyoming would contribute equal to and greater than the 1
percent threshold, the WDEQ indicated that they supported and applied
the use of different thresholds, such as the 1 ppb limit that was
established as part of the Significant Impact Level (SIL) for ozone,
used in the Prevention of Significant Deterioration (PSD) permitting
program. The WDEQ referenced the EPA's August 31, 2018 memorandum,
which the State interpreted as the EPA approving 1 ppb as an
alternative to the 1 percent of the NAAQS screening threshold at Step
2.\37\ The WDEQ noted that by using a 1 ppb threshold, the State is
only linked to three \38\ of the five receptors listed in Table 1.
---------------------------------------------------------------------------

    \37\ Id. at 6.
    \38\ Site ID 80050002 (Arapahoe), Site ID 80350004 (Douglas),
and Site ID 80590011 (Jefferson). Id.
---------------------------------------------------------------------------

    Although the WDEQ's use of a 1 ppb threshold eliminated only two of
the five receptor linkages, they relied on a weight-of-evidence
approach at Step 2 of the 4-Step Framework to assert that Wyoming does
not contribute to nonattainment or interfere with maintenance in
another state (i.e., at none of the five receptors to which Wyoming's
sources contribute greater than 1 percent of the NAAQS in the EPA's
2011-based modeling).\39\ The WDEQ stated that the weight-of-evidence
approach to evaluating transport in western states is appropriate,
since the EPA recognized in the CSAPR Update that it was not
appropriate to extend CSAPR to western states without first considering
important regional differences such as topography, prevalence of
wildfires, altitude, and other factors.\40\ The WDEQ referenced the
EPA's past actions on California's and Arizona's 2008 ozone interstate
transport SIPs as examples of the EPA relying on a weight-of-evidence
approach to support approval of the SIP for western states with
contributions greater than 1 percent of the NAAQS to a downwind
receptor.\41\
---------------------------------------------------------------------------

    \39\ Id. at 3, 8-9.
    \40\ Id. at 2.
    \41\ Id. at 2-3, 6.
---------------------------------------------------------------------------

    In their weight-of-evidence argument, the WDEQ considered the EPA's
approval of Arizona's interstate transport SIP for the 2008 ozone
NAAQS, in which the total contributions from all states that
contributed to the same receptor(s) were factored into the EPA's
analysis.\42\ The WDEQ stated that for Arizona's 2008 ozone transport
approval action, the EPA ``concluded that upwind state contribution to
the receptors Arizona was linked to were negligible, `particularly when
compared to the relatively large contributions from upwind states in
the East.' '' \43\
---------------------------------------------------------------------------

    \42\ See ``Partial Approval and Partial Disapproval of Air
Quality State Implementation Plans; Arizona; Infrastructure
Requirements to Address Interstate Transport for the 2008 Ozone
NAAQS,'' 81 FR 31513 (May 19, 2016).
    \43\ Wyoming State Implementation Plan, Attachment B at 6
(citing 81 FR 15203).
---------------------------------------------------------------------------

    Additionally, the WDEQ asserted that the EPA's modeling results in
the March 2018 memorandum illustrates a disparity between upwind
contributions from states in the East versus the West.\44\ The WDEQ
stated that the modeling showed that upwind contributions for one site
in Connecticut (Site ID 90019003) was 44.24 ppb, 12 times as much as
the in-state contributions of 3.71 ppb.\45\ The WDEQ compared this to
the relative contribution levels at the Colorado receptors previously
noted in Table 1. The WDEQ indicated that the highest collective
contributions from upwind states to these receptors was 7.06 ppb to
site 80590006 (one of the Jefferson County receptors) and that the in-
state (Colorado) contribution to the same receptor is 25.52 ppb. Table
2 of this document provides the WDEQ's summary of in-state and upwind
state contributions using the EPA's 2023 (2011 platform) modeling. The
WDEQ stated in their SIP submission that the total contributions from
upwind states to downwind receptors is much higher for eastern states
than for western states and therefore the 1 percent threshold needs to
be reevaluated for application in western states.\46\
---------------------------------------------------------------------------

    \44\ Id. at 6.
    \45\ Id.
    \46\ Id. at 7.
---------------------------------------------------------------------------

    The WDEQ concluded that the total collective contribution from
upwind states to the Colorado receptors (including the Arapahoe,
Douglas, and one Jefferson County receptor (Site ID 80590011)) is
``negligible.'' \47\ For the other Jefferson County receptor (Site ID
80580006) and the Larimer County receptor (the two receptors to which
the WDEQ identified that Wyoming contributes above 1 percent of the
NAAQs but below 1 ppb), the WDEQ did not include this argument
regarding negligible collective contribution, but reiterated that their
sources' contributions to this receptor are below 1 ppb, and therefore
do not contribute significantly.\48\ Citing a potential flexibility
from Attachment A to the March 2018 memorandum, the WDEQ also pointed
to international and non-anthropogenic emissions contributions as
additional support for concluding that it is unnecessary to reach a
Step 3 analysis for Wyoming, since contributions from these categories
make up over 50 percent of the total maximum design values at each of
the five receptors under evaluation.\49\
---------------------------------------------------------------------------

    \47\ Id. at 7-8.
    \48\ Id. at 8.
    \49\ Id. at 8-9.

                                              Table 2--In-State vs. Collective Upwind State Contributions a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Total
                                                                                Average design  Maximum design     In-state      contribution
                  Site                             County                   State           value (ppb)     value (ppb)    contribution     from upwind
                                                                                (ppb)       states (ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
80050002...............................  Arapahoe..................  CO                             69.3            71.3           22.94            5.98
80350004...............................  Douglas...................  CO                             71.1            73.2           24.71            5.94
80590006...............................  Jefferson.................  CO                             71.3            73.7           25.52            7.06
80590011...............................  Jefferson.................  CO                             70.9            73.9           24.72            6.98
80690011...............................  Larimer...................  CO                             71.2            73.0           21.74            6.33
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Data according to March 2018 memorandum modeling.

    The WDEQ's SIP submission also pointed to data that indicates that
the ``counties currently classified as nonattainment for the 2015
standards are projected to be in attainment by the year 2025.'' \50\
The WDEQ acknowledged that 2025 is not an applicable attainment year
for the 2015 ozone NAAQS, but wishes to include this projection as part
of its weight-of-evidence analysis and to help demonstrate that a
downward trend in ozone exists.\51\ Also included in Wyoming's SIP
submission is the State's projection for VOC and NOX
emissions reductions, including an expected reduction of over 32,000
tons per year (tpy) of NOX between 2011 and 2023.\52\ The
WDEQ notes that at the time of its SIP submission, 21,252 tpy of those
NOX reductions had yet to occur, but pointed to the Regional
Haze Rule and other agreements between the EPA and Wyoming
operators.\53\ The WDEQ also noted that additional emissions reductions
would be achieved through the Tier 3 Vehicle Emissions and Fuel
Standards national rulemaking.\54\ The State concluded that based on
the anticipated reductions, requiring additional reductions would not
be necessary.\55\
---------------------------------------------------------------------------

    \50\ Id. at 9.
    \51\ Id.
    \52\ Id.
    \53\ Id.
    \54\ Id.
    \55\ Id.
---------------------------------------------------------------------------

    Based on the results of its weight-of-evidence analysis at Step 2,
the WDEQ's 2019 SIP submission concluded that emissions from the State
are not linked to a downwind projected nonattainment or maintenance
receptor and therefore do not contribute to nonattainment or interfere
with the maintenance of the 2015 ozone NAAQS in any downwind state.\56\
---------------------------------------------------------------------------

    \56\ Id at 9-10.
---------------------------------------------------------------------------

III. The EPA's Evaluation

    The EPA is proposing to find that Wyoming's January 3, 2019, SIP
submission does not meet the State's obligations with respect to
prohibiting emissions that contribute significantly to nonattainment or
interfere with maintenance of the 2015 8-hour ozone NAAQS in any other
state. The Agency's decision to propose disapproval of Wyoming's SIP
submission is based on our evaluation of the SIP using the 4-Step
Framework.

A. Evaluation of Information Provided by Wyoming Regarding Step 1 and
Step 2

    At Step 1 and Step 2 of the 4-Step Framework, Wyoming relied on the
EPA modeling released in the March 2018 memorandum to identify
nonattainment and maintenance receptors and upwind state linkages to
those nonattainment and maintenance receptors in 2023. In this
proposal, the EPA relies on the Agency's most recently available
modeling (2016v2) to identify upwind contributions and linkages to
downwind air quality problems in 2023. The earlier modeling relied on
by the WDEQ identified a number of nonattainment and maintenance
receptor sites in 2023 as did the more recent 2016v2 2023 modeling.
Thus, the EPA agrees with the WDEQ that for Step 1 under the 4-Step
Interstate Transport Framework, a number of nonattainment and
maintenance receptors for the 2015 ozone NAAQS were projected for 2023.
    As noted in Section II, at Step 2, the WDEQ completed a weight-of-
evidence analysis to conclude that it was not linked to any projected
downwind receptors. The WDEQ stated that a weight-of-evidence approach
at Step 2 was the most appropriate method for evaluating interstate
transport obligations for western states, and that this approach was
consistent with the EPA's past practice of relying on a weight-of-
evidence approach in evaluating interstate transport in the West under
the 2008 ozone NAAQS. The EPA has not prescribed to states any specific
methodology for developing SIP submissions, although the EPA has
historically relied on the 4-Step Framework to complete its evaluation
of state SIP submissions for ozone transport. Under the previous 2008
ozone NAAQS, the EPA's action on western states' interstate transport
SIP submissions has been informed using EPA modeling results and has
considered additional factors as appropriate. In the CSAPR Update, the
EPA stated, ``The EPA and western states, working together, are
continuing to evaluate interstate transport obligations on a case-by-
case basis. The EPA will fulfill its backstop role with respect to
issuing FIPs for western states if and when that becomes necessary.''
\57\ The EPA did note that there ``may be'' geographic factors to
consider when acting on western states but did not attempt to elucidate
what these were or how they may be relevant to interstate transport
policy.\58\
---------------------------------------------------------------------------

    \57\ See 81 FR 74504, at 74523.
    \58\ See id. The EPA also noted that the western states on which
it was not acting in the CSAPR Update were not thereby relieved of
their statutory obligations to address interstate transport and that
the analyses developed for the CSAPR Update, including air quality
modeling and emissions control potential ``can be useful for western
states in developing SIPs.'' Id.
---------------------------------------------------------------------------

    The EPA did not provide, as the WDEQ SIP submission suggests,
specific regional differences or a set criteria that must be considered
prior to acting on western state SIPs. Further, as discussed in more
detail later in this section, the EPA's proposed action on this SIP
submission is entirely consistent with the reasoning it previously
applied in acting on SIP submissions for western states including
Arizona, Utah, and Wyoming under the 2008 ozone NAAQS. Nonetheless, we
will evaluate the evidence and arguments supplied by the WDEQ to
determine whether the State's conclusion, that no further controls are
necessary for Wyoming to meet its obligations under CAA
110(a)(2)(D)(i)(I), is adequately supported.
    The first argument the WDEQ relied on to support its conclusion was
an alternative threshold at Step 2 to determine a linkage. The WDEQ
noted

their support of an alternative threshold to 1 percent of the
NAAQS.\59\ As noted in Section II of the preamble, the WDEQ referred to
the 1 ppb limit of the ozone SIL used in the PSD permitting program and
relied on the August 2018 memorandum to conclude using a 1 ppb
alternative contribution threshold at Step 2 is ``appropriate'' for
Wyoming.\60\ The WDEQ further argued that there is a ``need to
reevaluate the application of CSAPR and the associated 1 percent
threshold in the West.'' \61\ As an initial matter, the EPA does not
agree with the WDEQ's assessment that 1 percent is not an appropriate
threshold for western states. The explanation for how the 1 percent
contribution threshold was originally derived is available in the 2011
CSAPR rulemaking. See 76 FR 48208, 48237-38. Further, in the CSAPR
Update, the EPA re-analyzed the threshold for purposes of the 2008
ozone NAAQS and determined it was appropriate to continue to apply this
threshold. See 86 FR 23054, 23085 (summarizing CSAPR and the CSAPR
Update basis for use of 1 percent of the NAAQS threshold).
---------------------------------------------------------------------------

    \59\ Wyoming State Implementation Plan, Attachment B at 6.
    \60\ Id. at 6.
    \61\ Id. at 7.
---------------------------------------------------------------------------

    Further, the EPA has explained in prior actions on western states'
ozone interstate transport SIPs that a 1 percent of the NAAQS threshold
may be appropriate in the West just as much as in the East. In acting
on Wyoming's interstate transport SIP submissions for the 2008 ozone
NAAQS, the EPA consistently applied the 1 percent threshold, and
rejected use of a higher threshold. The EPA explained that a 1 percent
threshold was appropriate to apply for a Colorado receptor ``because
the air quality problem in that area resulted in part from the
relatively small individual contribution of upwind states that
collectively contribute a larger portion of the ozone contributions
(9.7%), comparable to some eastern receptors . . . .'' See 84 FR 3389,
3391 (February 12, 2019).\62\
---------------------------------------------------------------------------

    \62\ While the EPA ultimately approved Wyoming's transport SIP
submission as proposed in the 2019 action, the approval was on the
basis of a unique air quality demonstration developed by Colorado
itself to establish that there would be no air quality problem in
Colorado with respect to the 2008 ozone NAAQS once air quality
monitoring data influenced by ``atypical events'' were removed
(assuming 2023 was the correct analytical year). See 84 FR 3392-94;
84 FR 14270 (April 10, 2019) (final action; no comments received).
No such basis for approval of Wyoming's transport SIP has been
developed or submitted with respect to Colorado's ongoing air
quality problems under the 2015 ozone NAAQS. Further, as presented
in Table 5 in this document, at least four Colorado monitoring sites
continue to have design values as of 2020 that are in excess of the
2008 ozone NAAQS, let alone the 2015 ozone NAAQS.
---------------------------------------------------------------------------

    In the EPA's action on Utah's SIP submission as to prong 2 for the
2008 ozone NAAQS, the EPA further addressed the basis for applying a 1
percent threshold at least as to Colorado receptors, and rejected
comments advocating for a higher threshold. 81 FR 71991, 71994-95
(October 19, 2016). As in its Wyoming actions, the EPA explained the
basis for the 1 percent threshold as derived in CSAPR and the CSAPR
Update rulemakings, and then explained that the same reasoning would
hold true with respect to the Colorado receptors to which Utah was
linked. Id. The EPA noted that Utah's state agency's advocacy for a
higher contribution threshold of 2 percent of the NAAQS was not
technically supported and ``appears to only be justified by the
conclusion that Utah would not have been linked to Denver receptors at
this level.'' Id. at 71995.
    When the EPA took action on Arizona's 2008 ozone NAAQS transport
SIP submittal, it again found the 1 percent threshold appropriate to
apply as to that western state. 81 FR 15200, 15202-03 (March 22, 2016).
We stated that we disagreed with Arizona's contention that it is
unclear what screening threshold is significant for southwestern states
when addressing interstate transport contributions. We explained that
we believe contribution from an individual state equal to or above 1
percent of the NAAQS could be considered significant where the
collective contribution of emissions from one or more upwind states is
responsible for a considerable portion of the downwind air quality
problem regardless of where the receptor is geographically located. See
id. 15202.
    As discussed in further detail later in the section, the EPA found,
based on an analysis of the California monitoring sites at issue in
that action, that Arizona was not contributing to downwind
nonattainment or maintenance problems. But this conclusion was not
reached on the basis of an alternative threshold at Step 2, which, as
explained previously, the EPA did not find justified to assume for an
entire region such as the southwest.\63\
---------------------------------------------------------------------------

    \63\ The EPA received no comment on the Arizona 2008 ozone NAAQS
interstate transport proposal and therefore finalized its approval
without further analysis. 81 FR 31513 (May 19, 2016).
---------------------------------------------------------------------------

    The WDEQ also seeks to rely on the EPA's August 2018 memorandum as
a basis for using a 1 ppb threshold. However, that memorandum provided
that whether use of a 1 ppb threshold is appropriate must be based on
an evaluation of state-specific circumstances, and no such evaluation
was included in the WDEQ's submission. The August 2018 memorandum did
not establish a rule that the application of a 1 ppb threshold to
determine a linkage would always be approvable, as the WDEQ appears to
assume in its SIP submission. Rather, the EPA suggested that where the
percentage of upwind state emissions is comparable to the amount
captured at 1 percent, it may be reasonable for states to use a 1 ppb
contribution threshold, as an alternative to a 1 percent threshold, at
Step 2 of the 4-Step Framework, for the purposes of identifying
linkages to downwind receptors. This indicates that a more
determinative conclusion of appropriateness would require further
state-and-receptor-specific analysis.\64\ However, the WDEQ's SIP
submission does not evaluate whether the level of upwind state
contribution captured at the 1 ppb threshold is sufficiently comparable
to the amount captured at 1 percent at each linked receptor. The WDEQ
does not include any further technical analysis to sufficiently justify
use of an alternative 1 ppb threshold at the linked receptors.
---------------------------------------------------------------------------

    \64\ The EPA provided comments on November 1, 2018, regarding
the use of a 1 ppb threshold on the WDEQ's draft SIP submittal
during the State's comment period. In these comments, the EPA
indicated that the August 2018 memorandum suggested that, depending
on the particular facts and circumstances, it may be reasonable and
appropriate to use a 1 ppb threshold, and if the WDEQ wished to use
a 1 ppb threshold in its SIP development, the EPA recommended
Wyoming review the August 2018 memorandum and revise their arguments
accordingly. The EPA's comments were included in the WDEQ's SIP
submission and are also included in Docket No. EPA-R08-OAR-2022-
0268.
---------------------------------------------------------------------------

    The WDEQ also referred to the EPA's use of the ozone SIL in the PSD
permitting program as additional justification for use of a 1 ppb
threshold. The EPA's SIL guidance relates to a different provision of
the CAA regarding implementation of the PSD permitting program, i.e., a
program that applies in areas that have been designated attainment or
unclassifiable for the NAAQS, and it is not applicable to the good
neighbor provision, which requires states to eliminate significant
contribution or interference with maintenance of the NAAQS at known and
ongoing air quality problem areas in other states.
    The analytical gaps identified previously, as well as the EPA's
consistent policy of applying a 1 percent of the NAAQS threshold even
in the case of western states like Wyoming (particularly with respect
to the Colorado receptors), indicate that the use of a 1 ppb threshold
for the State is not approvable.

    The EPA here shares further evaluation of its experience since the
issuance of the August 2018 memorandum regarding use of alternative
thresholds at Step 2. This experience leads the Agency to now believe
it may not be appropriate to continue to attempt to recognize
alternative contribution thresholds at Step 2. The August 2018
memorandum stated that ``it may be reasonable and appropriate'' for
states to rely on an alternative threshold of 1 ppb threshold at Step
2.\65\ (The memorandum also indicated that any higher alternative
threshold, such as 2 ppb, would likely not be appropriate.) However,
the EPA also provided that ``air agencies should consider whether the
recommendations in this guidance are appropriate for each situation.''
Following receipt and review of 49 good neighbor SIP submittals for the
2015 8-hour ozone NAAQS, the EPA's experience has been that nearly
every state that attempted to rely on a 1 ppb threshold did not provide
sufficient information and analysis to support a determination that an
alternative threshold was reasonable or appropriate for that state. For
instance, in nearly all submittals, the states did not provide the EPA
with analysis specific to their state or the receptors to which its
emissions are potentially linked. In one case, the proposed approval of
Iowa's SIP submittal, the EPA expended its own resources to attempt to
supplement the information submitted by the State, in order to more
thoroughly evaluate the state-specific circumstances that could support
approval.\66\ It was at the EPA's sole discretion to perform this
analysis in support of the State's submittal, and the Agency is not
obligated to conduct supplemental analysis to fill the gaps whenever it
believes a state's analysis is insufficient. The Agency no longer
intends to undertake supplemental analysis of SIP submittals with
respect to alternative thresholds at Step 2 for purposes of the 2015
ozone NAAQS. Furthermore, 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. Based on its review of
submittals to-date and after further consideration of the policy
implications of attempting to recognize an alternative Step 2 threshold
for certain states, the Agency now believes the attempted use of
different thresholds at Step 2 with respect to the 2015 ozone NAAQS
raises substantial policy consistency and practical implementation
concerns.\67\ 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 submittal at
Step 2 of the 4-Step Framework. 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. 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 emission controls while other states must
proceed to a Step 3 analysis. This can create significant equity and
consistency problems among states. Further, it is not clear that
national ozone transport policy is best served by allowing for less
stringent thresholds at Step 2. 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, the EPA notes 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 (e.g.,
roughly seven percent of total upwind state contribution was lost
according to the modeling underlying the August 2018 memorandum; \68\
in the EPA's updated modeling, the amount lost is five percent).
Considering the core statutory objective of ensuring elimination of all
significant contribution to nonattainment or interference of the NAAQS
in other states and the broad, regional nature of the collective
contribution problem with respect to ozone, there does not appear to be
a compelling policy imperative in allowing some states to use a 1 ppb
threshold while others rely on a 1 percent of NAAQS threshold.
---------------------------------------------------------------------------

    \65\ August 2018 memorandum at 4.
    \66\ See ``Air Plan Approval; Iowa; Infrastructure State
Implementation Plan Requirements for the 2015 Ozone National Ambient
Air Quality Standard,'' 85 FR 12232 (March 2, 2020). The Agency
received adverse comment on this proposed approval and has
subsequently formally withdrawn the proposed approval. 87 FR 9477
(February 22, 2022).
    \67\ 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.
    \68\ See August 2018 memorandum at 4.
---------------------------------------------------------------------------

    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 stringent ozone
NAAQS), is also important. 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. Accord
76 FR 48237-38. Therefore, notwithstanding the August 2018 memorandum's
recognition of the potential viability of alternative Step 2
thresholds, and in particular, a potentially applicable 1 ppb
threshold, the EPA's experience since the issuance of that memorandum
has revealed substantial programmatic and policy difficulties in
attempting to implement this approach. Nonetheless, the EPA is not at
this time rescinding the August 2018 memorandum. The basis for
disapproval of Wyoming's SIP submission with respect to the Step 2
analysis is, in the Agency's view, warranted even under the terms of
the August 2018 memorandum. The EPA invites comment on this broader
discussion of issues associated with alternative thresholds at Step 2.
Depending on comment and further evaluation of this issue, the EPA may
determine to rescind the 2018 memorandum in the future.
    As described in Section I of this preamble, the EPA performed air
quality modeling using the 2016v2 emissions platform to project design
values and contributions for 2023. These data were examined to
determine if Wyoming contributes at or above the threshold of 1 percent
of the 2015 8-hour ozone NAAQS (0.70 ppb) to any downwind nonattainment
or maintenance receptor.

As shown in Table 3, the data \69\ indicate that in 2023, emissions
from Wyoming contribute greater than 1 percent of the NAAQS to a
nonattainment receptor in Douglas County, Colorado (Site ID
80350004).\70\
---------------------------------------------------------------------------

    \69\ Design values and contributions at individual monitoring
sites nationwide are provided in the file:
2016v2_DVs_state_contributions.xlsx which is included in Docket No.
EPA-HQ-OAR-2021-0663.
    \70\ These modeling results are consistent with the results of a
prior round of 2023 modeling using the 2016v1 emissions platform
which became available to the public in the fall of 2020 in the
Revised CSAPR Update, as noted in Section I of the document. That
modeling showed that Wyoming had a maximum contribution equal to or
greater than 0.70 ppb to at least one nonattainment or maintenance-
only receptor in 2023. These modeling results are included in the
file ``Ozone Design Values and Contributions Revised CSAPR
Update.xlsx'' in Docket No. EPA-HQ-OAR-2021-0663.

                                       Table 3--Wyoming Linkage Results Based on the EPA's Updated 2023 Modeling a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                2023 Average    2023  Maximum       Wyoming
               Receptor ID                           Location              Nonattainment/maintenance     design value     design value     contribution
                                                                                (ppb)            (ppb)            (ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
80350004.................................  Douglas County, Colorado....  Nonattainment...............            71.7             72.3             0.81
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ According to data from 2016v2 platform modeling.

    Another argument the WDEQ made as part of its Step 2 weight-of-
evidence evaluation was a determination that emissions from sources in
Wyoming are ``negligible'' when compared to in-state, non-U.S., and
nonanthropogenic contributions to three receptors to which it
contributed greater than 1 percent of the NAAQS (using the EPA's March
2018 memorandum modeling results). Regarding the WDEQ's argument that
contributions from Wyoming are negligible when considering total
collective contributions from all upwind states to the same receptors,
the EPA disagrees.
    The WDEQ makes reference to the EPA's approval of Arizona's 2008
ozone NAAQS transport SIP as a basis for the claim that its emissions
are negligible.\71\ In that action the EPA made an assessment of the
nature of certain monitoring sites in California. The EPA noted that a
``factor [. . .] relevant to determining the nature of a projected
receptor's interstate transport problem is the magnitude of ozone
attributable to transport from all upwind states collectively
contributing to the air quality problem.'' 81 FR at 15203. The EPA
observed that only one upwind state (Arizona) was linked above 1
percent of the 2008 ozone NAAQS to the two relevant monitoring sites in
California, and the cumulative ozone contribution from all upwind
states (including those linked and unlinked) to those sites was 2.5
percent and 4.4 percent of the total ozone concentration, respectively.
The EPA determined the size of those cumulative upwind contributions
was ``negligible, particularly when compared to the relatively large
contributions from upwind states in the East or in certain other areas
of the West.'' Id. (emphasis added). In the Arizona action, the EPA
concluded the two California sites to which Arizona was linked should
not be treated as receptors for the purposes of determining good
neighbor obligations for the 2008 ozone NAAQS. Id.
---------------------------------------------------------------------------

    \71\ See 81 FR 15200 (March 22, 2016) (proposal); 81 FR 31513
(May 19, 2016) (final rule; no comments received).
---------------------------------------------------------------------------

    As an initial matter, we note that this analysis is properly
considered at Step 1 of the 4-Step Framework rather than at Step 2, as
it is a determination of whether an interstate-pollution transport
problem should be considered to exist at all, before reaching a
determination as to which states contribute to that problem. As the EPA
explained in its Arizona action, it considered the 1 percent of the
NAAQS threshold appropriate to apply at Step 2. Id. at 15202. See also
id. at 15203 (``EPA believes the emissions that result in transported
ozone from upwind states have limited impacts on the projected air
quality problems in El Centro, California and Los Angeles, California,
and therefore should not be treated as receptors for purposes of
determining the interstate transport obligations of upwind states.'').
However, because Wyoming has presented this argument as a part of its
weight-of-evidence analysis at Step 2, we present this analysis in turn
here, as related to the WDEQ's Step 2 arguments.
    Turning to the substance of the WDEQ's argument that the EPA's
Arizona action supports an approval here: The conclusions the EPA
reached regarding El Centro and Los Angeles California cannot be
reached with respect to the three receptors in Colorado examined by the
WDEQ,\72\ and the EPA has consistently taken this same position across
several prior actions addressing Wyoming's and Utah's interstate
transport obligations, where we have concluded that the receptors in
Colorado are ``substantially'' influenced by upwind-state emissions.
See 82 FR 9155, 9157 (February 3, 2017). When acting on Wyoming's and
Utah's 2008 ozone NAAQS interstate transport SIP submissions, the EPA's
view was that ``the air quality problem in [the Denver nonattainment
area of Colorado] resulted in part from the relatively small individual
contribution of upwind states that collectively contribute a larger
portion of the ozone contributions (9.7%), comparable to some eastern
receptors . . . .'' See 84 FR 3389, 3391 (February 12, 2019). See also
81 FR 71991, 71994-95 (October 19, 2016); 81 FR 28807, 28810 (May 10,
2016) (Colorado receptors are impacted by interstate transport where
total upwind state contribution is 11 percent of the total ozone
concentration, and five states were projected to be linked).
---------------------------------------------------------------------------

    \72\ Site ID 80050002 (Arapahoe), Site ID 80350004 (Douglas),
and Site ID 80590011 (Jefferson).
---------------------------------------------------------------------------

    Indeed, the EPA has specifically addressed this precise comparison
between the circumstances of Arizona's approval and the nature of the
receptors in Colorado. In approving Utah's interstate transport SIP as
to prong 1 for the 2008 ozone NAAQS, the EPA found its analysis as to
Arizona's impact on California sites did not apply to Utah's impact on
Colorado's sites (which the EPA found remained to be at least
maintenance receptors as to the 2008 ozone NAAQS). See 82 FR 9155, 9157
(February 3, 2017) (``The EPA's assessment concluded that emissions
reductions from Arizona are not necessary to address interstate
transport because the total collective upwind state ozone contribution
to these receptors is relatively low compared to the air quality
problems typically addressed by the good neighbor provision. As
discussed previously, the EPA similarly evaluated collective
contribution to the Douglas County, Colorado monitor and finds the
collective contribution of

transported pollution to be substantial.'') (emphasis added).\73\
---------------------------------------------------------------------------

    \73\ As noted in that action, because Utah was found to still be
linked to Colorado's maintenance receptors under the 2008 ozone
NAAQS, the EPA's disapproval of the SIP as to prong 2 remained in
place, and accordingly, there is an outstanding obligation to
resolve Utah's transport obligations with respect to the 2008 ozone
NAAQS. See id. at 9156.
---------------------------------------------------------------------------

    The modeling data on which the WDEQ relied in its SIP submission
continue to bear out these conclusions (see Appendix B, pages 6-8).
That modeling showed contributions from more than one upwind state
above 1 percent of the NAAQS at all Colorado receptors and showed total
upwind contribution to be between 8 and 10 percent of the total ozone
concentrations at those receptors.
    The EPA acknowledges that in its most recent modeling of 2023
(using the 2016v2 platform), the degree of the interstate transport
problem to Colorado is now projected to lessen somewhat compared to
previous projections of 2023. However, these projected improvements are
still not sufficient to draw a conclusion that Colorado is not impacted
to a considerable degree by out of state emissions. The EPA's recent
air quality modeling continues to show that multiple upwind states
collectively contribute to projected downwind nonattainment or
maintenance receptors in Colorado--specifically, California, Utah, and
Wyoming all contribute above 1 percent of the NAAQS to at least one of
Colorado's receptors in 2023. (In contrast, at the time EPA approved
Arizona's 2008 ozone NAAQS good neighbor SIP, Arizona was the only
state linked above 1 percent at the relevant California monitoring
sites.) Further, our most recent modeling shows that the total upwind
state contribution to ozone concentrations (from linked and unlinked
states) at identified downwind air quality problems in Colorado is
approximately 6 to 7 percent, as shown in Table 4. That remains higher
than the 2 to 4 percent range of total upwind contribution the EPA
found to be negligible with respect to the California sites analyzed in
the Arizona action. Therefore, the EPA continues to find that the
collective contribution of emissions from upwind states represents a
significant portion of the ozone concentrations at projected
nonattainment and maintenance receptors in Colorado.

                                    Table 4--All Upwind State Contributions to Nonattainment Receptors in Colorado a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Percent
                                                                                Contribution    contribution
               Site ID                           State                    County          2023 Avg (ppb)  2023 Max (ppb)   of all upwind   of all upwind
                                                                                states          states
                                                                                combined (ppb)   combined \b\
--------------------------------------------------------------------------------------------------------------------------------------------------------
80350004.............................  Colorado................  Douglas................            71.7            72.3            5.17               7
80590006.............................  Colorado................  Jefferson..............            72.6            73.3            4.23               6
80590011.............................  Colorado................  Jefferson..............            73.8            74.4            4.34               6
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ Based on data from 2016v2 platform modeling.
\b\ Calculated using the projected 2023 average design values for the applicable receptors.

    As noted, the Agency has consistently found that the 1 percent of
the NAAQS threshold is appropriate for identifying interstate transport
linkages for states collectively contributing to downwind ozone
nonattainment or maintenance problems because that threshold captures a
high percentage of the total pollution transport affecting downwind
receptors. The EPA believes contribution from an individual state equal
to or above 1 percent of the NAAQS could be considered significant
where the collective contribution of emissions from one or more upwind
state is responsible for a considerable portion of the downwind air
quality problem regardless of where the receptor is geographically
located. In this case, three states contributing to those identified
receptors, including Wyoming, contribute emissions greater than or
equal to 1 percent of the 2015 ozone NAAQS. In addition, the total
upwind state contribution to ozone levels at the Colorado receptors is
on the order of 4 to 5 ppb, or 6 to 7 percent of total ozone
concentration, as shown in Table 4. Given these results, and the EPA's
consistent use of the 1 percent threshold, in ozone transport actions
across all areas of the country (including actions related to Wyoming
and Utah's interstate transport obligations with respect to these same
receptors), the EPA is proposing to determine that Wyoming contributes
to nonattainment and interferences with maintenance of the 2015 ozone
NAAQS for the Denver, Colorado area.
    The WDEQ, relying on potential ``flexibilities'' in Attachment A to
the March 2018 memorandum, also claims that receptors in the West are
predominantly impacted by local emissions and ``uncontrollable''
emissions such as those from non-U.S. sources or non-anthropogenic
sources, and so the State ``contends that it is unnecessary to consider
Step 3 in this analysis.'' \74\ As explained previously in the preamble
of this document, the concepts presented in Attachment A to the March
2018 memorandum were neither guidance nor determined by the EPA to be
consistent with the CAA. While in-state, non-U.S., and non-
anthropogenic sources emissions may be contributing to an area's
nonattainment or maintenance status, there is nothing in the CAA to
suggest that these emissions serve to absolve upwind states of their
obligations to control their own emissions.
---------------------------------------------------------------------------

    \74\ Wyoming SIP submission, Attachment B at 3, 8.
---------------------------------------------------------------------------

    With respect to local or in-state emissions, there is no statutory
basis to conclude that such emissions must be controlled first before a
contributing state's share can be controlled under CAA section
110(a)(2)(D)(i)(I). The D.C. Circuit has held on five different
occasions that the timing framework for addressing interstate transport
obligations must be consistent with the downwind areas' attainment
schedule. In particular, for the ozone NAAQS, the states and the EPA
are to address interstate transport obligations ``as expeditiously as
practicable'' and no later than the attainment schedule set in
accordance with CAA section 181(a). See North Carolina, 531 F.3d at
911-13; Wisconsin, 938 F.3d at 313-20; Maryland, 958 F.3d at 1204; New
York v. EPA, 964 F.3d 1214, 1226 (D.C. Cir. 2020); New York v. EPA, 781
Fed. App'x 4, 6-7 (D.C. Cir. 2019). The court in Wisconsin explained
its reasoning in part by noting that downwind jurisdictions often may
need to heavily rely on emissions reductions from upwind states in
order to achieve attainment of the NAAQS, 938 F.3d at

316-17; such states would face increased regulatory burdens including
the risk of bumping up to a higher nonattainment classification if
attainment is not reached by the relevant deadline, Maryland, 958 F.3d
at 1204. The statutory framework of the CAA and these cases establish
clearly that states and the EPA must address interstate transport
obligations in line with the attainment schedule provided in the CAA
(i.e., not after attainment-planning measures have been taken by the
downwind state) in order to timely assist downwind states in attaining
and maintain the NAAQS, and this attainment schedule is ``central to
the regulatory scheme.'' Wisconsin, 938 F.3d at 316 (quoting Sierra
Club v. EPA, 294 F.3d 155, 161 (D.C. Cir. 2002)).
    With respect to international and non-anthropogenic emissions
contributions, the WDEQ's reasoning is inapplicable to the requirements
of CAA section 110(a)(2)(D)(i)(I). The good neighbor provision requires
states and the EPA to address interstate transport of air pollution
that contributes to a downwind states' ability to attain and maintain
NAAQS. Whether emissions from other states or other countries also
contribute to the same downwind air quality issue is irrelevant in
assessing whether a downwind state has an air quality problem, or
whether an upwind state is significantly contributing to that problem.
States are not obligated under CAA section 110(a)(2)(D)(i)(I) to reduce
emissions sufficient on their own to resolve downwind receptors'
nonattainment or maintenance problems. Rather, states are obligated to
eliminate their own ``significant contribution'' or ``interference''
with the ability of other states to attain or maintain the NAAQS.
    Indeed, the D.C. Circuit in Wisconsin specifically rejected
petitioner arguments suggesting that upwind states should be excused
from good neighbor obligations on the basis that some other source of
emissions (whether international or another upwind state) could be
considered the ``but-for'' cause of downwind air quality problem. 938
F.3d 303 at 323-324. The court viewed petitioners' arguments as
essentially an argument ``that an upwind State `contributes
significantly' to downwind nonattainment only when its emissions are
the sole cause of downwind nonattainment.'' 938 F.3d 303 at 324. The
court explained that ``an upwind State can `contribute' to downwind
nonattainment even if its emissions are not the but-for cause.'' Id. at
324-325. See also Catawba County v. EPA, 571 F.3d 20, 39 (D.C. Cir.
2009) (rejecting the argument ``that `significantly contribute'
unambiguously means `strictly cause''' because there is ``no reason why
the statute precludes EPA from determining that [an] addition of
[pollutant] into the atmosphere is significant even though a nearby
county's nonattainment problem would still persist in its absence'');
Miss. Comm'n on Envtl. Quality v. EPA, 790 F.3d 138, 163 n.12 (D.C.
Cir. 2015) (observing that the argument that ``there likely would have
been no violation at all ... if it were not for the emissions resulting
from [another source]'' is ``merely a rephrasing of the but-for
causation rule that we rejected in Catawba County.''). Therefore, a
state is not excused from eliminating its significant contribution on
the basis that international emissions also contribute some amount of
pollution to the same receptors to which the state is linked.
    Further, the data supplied in Wyoming's SIP submission tends to be
self-refuting on this point. Table 3 in Appendix B to the State's SIP
submission indicates, according to the WDEQ, that ``more than 50
percent'' of the total ozone concentrations at the Colorado receptors
are from non-anthropogenic or non-U.S. emissions sources. Assuming
those numbers are correct, this means that nearly 50 percent of the
ozone levels at the Colorado receptors are the result of anthropogenic
emissions originating in the U.S. Those emissions are clearly within
the authority of states and the EPA to redress and reducing some
portion of those emissions can be assumed to improve air quality at the
Colorado receptors. While not all of those U.S. anthropogenic emissions
can be attributed to Wyoming, Wyoming's emissions are shown by the
modeling to contribute to Colorado's air quality problem at levels
sufficient to warrant evaluation of emissions control opportunities at
Step 3 of the EPA's longstanding analytical framework.
    The next analysis the WDEQ included in its SIP submission is
NOX and VOC emissions trends. The WDEQ points to a projected
downward trend of ozone levels at monitors within the Colorado
nonattainment area through 2025, as well as an observed reduction since
2011 in emissions of VOCs and NOX emissions in Wyoming
through a combination of regulatory and permitting actions.\75\ The
WDEQ also pointed to an estimate provided in the State's 2008 ozone
NAAQS infrastructure SIP, which projected a decrease in NOX
emissions between 2011 and 2023 of 32,985.5 tpy and a decrease of VOC
emissions between 2011 and 2023 of 905.6 tpy as a result of the Wyoming
Existing Source Rule.\76\ The EPA considers the measures the WDEQ
described to be beneficial in reducing VOC and NOX
emissions, and the EPA's most recent modeling has projected that there
will be a downward trend of ozone at the Denver nonattainment monitors.
However, the WDEQ has not provided any analysis to demonstrate that the
reductions in their State will be sufficient to eliminate its
contribution above 1 percent of the NAAQS to Colorado receptors, or
that those receptors will cease to exist by 2023. The WDEQ did not
quantify the total anticipated reductions in NOX and VOC
emissions from its permitting actions and existing regulatory
requirements nor did it evaluate the impact of those reductions in
downwind air quality at the Denver area receptors. In general, the air
quality modeling that the EPA has conducted already accounts for ``on-
the-books'' emissions control measures, including the expected
reductions those measures achieve through 2023. Both the 2016v1 and the
more current 2016v2 modeling clearly establish continued linkage from
Wyoming to downwind receptors in 2023 at Steps 1 and 2, despite those
emissions control efforts.
---------------------------------------------------------------------------

    \75\ Wyoming SIP submission, Attachment B at 9.
    \76\ Id.
---------------------------------------------------------------------------

    As explained previously in this document, the WDEQ's SIP submission
does not provide an adequate technical analysis demonstrating that the
SIP contains adequate provisions prohibiting emissions that will
significantly contribute to nonattainment or interfere with the 2015
ozone NAAQS in any other state. Moreover, Denver monitors continue to
violate the 2015 ozone NAAQS and in fact many Denver monitors showed an
increase in 2020 ozone design values when compared to the 2019 design
values. See Table 5 which includes the last five years of the 3-year
design values for the Denver nonattainment area monitors.

                                          Table 5. Ozone Design Values for Denver Nonattainment Area Monitors a
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                             2014-2016       2015-2017       2016-2018       2017-2019       2018-2020
           AQS Site ID                   State              County         Design value    Design value    Design value    Design value    Design value
                                                                               (ppb)           (ppb)           (ppb)           (ppb)           (ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
80013001........................  Colorado..........  Adams.............              67              67              67              65              69
80050002........................  Colorado..........  Arapahoe..........  ..............  ..............              73              74              77
80050006........................  Colorado..........  Arapahoe..........              67              67              69              69              71
80310002........................  Colorado..........  Denver............              66              68              69              68              70
80350004........................  Colorado..........  Douglas...........              77              77              78              78              81
80590005........................  Colorado..........  Jefferson.........              72              75              72              71              71
80590006........................  Colorado..........  Jefferson.........              77              77              78              76              79
80590011........................  Colorado..........  Jefferson.........              80              79              79              76              80
80690007........................  Colorado..........  Larimer...........              69              68              70              68              70
80690011........................  Colorado..........  Larimer...........              75              75              77              75              75
80691004........................  Colorado..........  Larimer...........              70              68              69              67              67
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ According to data from 2016v2 platform modeling.

    As shown in Table 5, the 3-year design values for majority of the
Denver monitors increased between 2019 and 2020, indicating the end of
any downward trend of ground-level ozone which the WDEQ may have seen
previously. Additionally, the EPA's most recent modeling continues to
indicate that emissions from Wyoming are projected to contribute to one
downwind nonattainment or maintenance receptors in the Denver, Colorado
area through 2023.
    In its January 2019 SIP submittal, the WDEQ acknowledges that
receptors in the Denver, Colorado nonattainment area could be impacted
by emissions from Wyoming, but despite the modeling results that
indicate that, the WDEQ concludes that Wyoming is not ``linked'' at
Step 2 and that emissions from the State do not significantly
contribute to nonattainment in the Denver area.\77\ Overall, the EPA
believes that Wyoming has not adequately addressed the modeled
contributions to projected downwind receptors identified by the EPA's
modeling. Therefore, based on the EPA's evaluation of the information
submitted by the WDEQ, and based on the EPA's most recent modeling
results for 2023, the EPA proposes to find that Wyoming is linked at
Steps 1 and 2 and has an obligation to assess potential emissions
reductions from sources or other emissions activity at Step 3 of the 4-
step framework.
---------------------------------------------------------------------------

    \77\ Id. at 10.
---------------------------------------------------------------------------

B. Evaluation of Information Provided Regarding Step 3

    At Step 3 of the 4-Step Framework, a state's emissions are further
evaluated, in light of multiple factors, including air quality and cost
considerations, to determine what, if any, emissions significantly
contribute to nonattainment or interfere with maintenance and, thus,
must be eliminated under CAA section 110(a)(2)(D)(i)(I).
    To effectively evaluate which emissions in the state should be
deemed ``significant'' and therefore prohibited, states generally
should prepare an accounting of sources and other emissions activity
for relevant pollutants and assess potential, additional emissions
reduction opportunities and resulting downwind air quality
improvements. The EPA has consistently applied this general approach
(i.e., Step 3 of the 4-Step Framework) when identifying emissions
contributions that the Agency has determined to be ``significant'' (or
interfere with maintenance) in each of its prior federal, regional
ozone transport rulemakings, and this interpretation of the statute has
been upheld by the Supreme Court. See EME Homer City, 572 U.S. 489, 519
(2014). While the EPA has not directed states that they must conduct a
Step 3 analysis in precisely the manner the EPA has done in its prior
regional transport rulemakings, state implementation plans addressing
the obligations in CAA section 110(a)(2)(D)(i)(I) must prohibit ``any
source or other type of emissions activity within the State'' from
emitting air pollutants which will contribute significantly to downwind
air quality problems. Thus, states must complete something similar to
the EPA's analysis (or an alternative approach to defining
``significance'' that comports with the statute's objectives) to
determine whether and to what degree emissions from a state should be
``prohibited'' to eliminate emissions that will ``contribute
significantly to nonattainment in, or interfere with maintenance of''
the NAAQS in any other state.
    Wyoming did not conduct such an analysis in its SIP submission, as
a result of their conclusions pursuant to Step 1 and Step 2. As
explained in connection with the evaluation of the WDEQ's Step 1 and
Step 2 analyses, the EPA disagrees with those conclusions and
accordingly the WDEQ should have proceeded to evaluate which emissions
in the State should be deemed ``significant'' and therefore prohibited.
We therefore propose that Wyoming was required to analyze emissions
from the sources and other emissions activity from within the State to
determine whether its contributions were significant, and we propose to
disapprove its SIP submission because Wyoming failed to do so.

C. Evaluation of Information Provided Regarding Step 4

    Step 4 of the 4-Step Framework calls for development of permanent
and federally enforceable control strategies to achieve the emissions
reductions determined to be necessary at Step 3 to eliminate
significant contribution to nonattainment or interference with
maintenance of the NAAQS. As mentioned previously, Wyoming's SIP
submission did not contain an evaluation of additional emission control
opportunities (or establish that no additional controls are required),
thus, no information was provided at Step 4. As a result, the EPA
proposes to disapprove Wyoming's January 3, 2019 SIP submission on the
separate, additional basis that the State has not developed permanent
and enforceable emissions reductions necessary to meet the obligations
of CAA section 110(a)(2)(D)(i)(I).

D. Conclusion

    Based on the EPA's evaluation of the WDEQ's SIP submission, the
Agency is proposing to find that the portion of the State's January 3,
2019 SIP submission addressing CAA section 110(a)(2)(D)(i)(I) does not
meet Wyoming's interstate transport obligations, because it fails to
contain the necessary provisions to eliminate emissions that will
contribute significantly to nonattainment or

interfere with maintenance of the 2015 8-hour ozone NAAQS in any other
state.

IV. Proposed Action

    We are proposing to disapprove the WDEQ's SIP submission pertaining
to interstate transport of air pollution which will significantly
contribute to nonattainment or interfere with maintenance of the 2015
8-hour ozone NAAQS in other states. Under CAA section 110(c)(1),
disapproval would establish a 2-year deadline for the EPA to promulgate
a FIP for Wyoming to address the CAA section 110(a)(2)(D)(i)(I)
interstate transport requirements pertaining to significant
contribution to nonattainment and interference with maintenance of the
2015 8-hour ozone NAAQS in other states, unless the EPA approves a SIP
that meets these requirements. Disapproval does not start a mandatory
CAA sanctions clock for Wyoming. The remaining elements of the State's
January 3, 2019 submission are not addressed in this action and have
been acted on in a separate rulemaking.

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review

    This action is not a significant regulatory action and was
therefore not submitted to the Office of Management and Budget for
review.

B. Paperwork Reduction Act (PRA)

    This proposed action does not impose an information collection
burden under the PRA because it does not contain any information
collection activities.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action merely proposes to disapprove a SIP submission as not meeting
the CAA.

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect
small governments. The action imposes no enforceable duty on any state,
local or tribal governments or the private sector.

E. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    This action does not have tribal implications as specified in
Executive Order 13175. This action does not apply on any Indian
reservation land, any other area where the EPA or an Indian tribe has
demonstrated that a tribe has jurisdiction, or non-reservation areas of
Indian country. Thus, Executive Order 13175 does not apply to this
action.

G. Executive Order 13045: Protection of Children From Environmental
Health Risks and Safety Risks

    The EPA interprets Executive Order 13045 as applying only to those
regulatory actions that concern environmental health or safety risks
that the EPA has reason to believe may disproportionately affect
children, per the definition of ``covered regulatory action'' in
section 2-202 of the Executive Order. This action is not subject to
Executive Order 13045 because it merely proposes to disapprove a SIP
submission as not meeting the CAA.

H. Executive Order 13211, Actions That Significantly Affect Energy
Supply, Distribution or Use

    This action is not subject to Executive Order 13211, because it is
not a significant regulatory action under Executive Order 12866.

I. National Technology Transfer and Advancement Act

    This rulemaking does not involve technical standards.

J. Executive Order 12898: Federal Actions to Address Environmental
Justice in Minority Populations and Low-Income Populations

    The EPA believes the human health or environmental risk addressed
by this action will not have potential disproportionately high and
adverse human health or environmental effects on minority, low-income
or indigenous populations. This action merely proposes to disapprove a
SIP submission as not meeting the CAA.

K. CAA Section 307(b)(1)

    Section 307(b)(1) of the CAA governs judicial review of final
actions by the EPA. This section provides, in part, that petitions for
review must be filed in the D.C. Circuit: (i) When the agency action
consists of ``nationally applicable regulations promulgated, or final
actions taken, by the Administrator,'' or (ii) when such action is
locally or regionally applicable, if ``such action is based on a
determination of nationwide scope or effect and if in taking such
action the Administrator finds and publishes that such action is based
on such a determination.'' For locally or regionally applicable final
actions, the CAA reserves to the EPA complete discretion whether to
invoke the exception in (ii).\78\
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    \78\ In deciding whether to invoke the exception by making and
publishing a finding that an action is based on a determination of
nationwide scope or effect, the Administrator takes into account a
number of policy considerations, including his judgment balancing
the benefit of obtaining the D.C. Circuit's authoritative
centralized review versus allowing development of the issue in other
contexts and the best use of agency resources.
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    If the EPA takes final action on this proposed rulemaking, the
Administrator intends to exercise the complete discretion afforded to
him under the CAA to make and publish a finding that the final action
(to the extent a court finds the action to be locally or regionally
applicable) is based on a determination of ``nationwide scope or
effect'' within the meaning of CAA section 307(b)(1). Through this
rulemaking action (in conjunction with a series of related actions on
other SIP submissions for the same CAA obligations), the EPA interprets
and applies section 110(a)(2)(D)(i)(I) of the CAA for the 2015 ozone
NAAQS based on a common core of nationwide policy judgments and
technical analysis concerning the interstate transport of pollutants
throughout the continental U.S. In particular, the EPA is applying here
(and in other proposed actions related to the same obligations) the
same, nationally consistent 4-Step Framework for assessing good
neighbor obligations for the 2015 ozone NAAQS. The EPA relies on a
single set of updated, 2016-base year photochemical grid modeling
results of the year 2023 as the primary basis for its assessment of air
quality conditions and contributions at Steps 1 and 2 of that 4-Step
Framework. Further, the EPA proposes to determine and apply a set of
nationally consistent policy judgments to apply the 4-Step Framework.
The EPA has selected a nationally uniform analytic year (2023) for this
analysis and is applying a nationally uniform approach to nonattainment
and maintenance receptors and a nationally uniform approach to
contribution threshold analysis.\79\ For these reasons, the

Administrator intends, if this proposed action is finalized, to
exercise the complete discretion afforded to him under the CAA to make
and publish a finding that this action is based on one or more
determinations of nationwide scope or effect for purposes of CAA
section 307(b)(1).\80\
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    \79\ A finding of nationwide scope or effect is also appropriate
for actions that cover states in multiple judicial circuits. In the
report on the 1977 Amendments that revised section 307(b)(1) of the
CAA, Congress noted that the Administrator's determination that the
``nationwide scope or effect'' exception applies would be
appropriate for any action that has a scope or effect beyond a
single judicial circuit. See H.R. Rep. No. 95-294 at 323, 324,
reprinted in 1977 U.S.C.C.A.N. 1402-03.
    \80\ The EPA may take a consolidated, single final action on all
of the proposed SIP disapproval actions with respect to obligations
under CAA section 110(a)(2)(D)(i)(I) for the 2015 ozone NAAQS.
Should the EPA take a single final action on all such disapprovals,
this action would be nationally applicable, and the EPA would also
anticipate, in the alternative, making and publishing a finding that
such final action is based on a determination of nationwide scope or
effect.
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List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by
reference, Ozone.

    Authority: 42 U.S.C. 7401 et seq.

    Dated: May 16, 2022.
KC Becker,
Regional Administrator, Region 8.
[FR Doc. 2022-11153 Filed 5-23-22; 8:45 am]
BILLING CODE 6560-50-P