Document ID: EPA-R08-OAR-2019-0140-0039
Agency: epa
Document Type: Proposed Rule
Title: Air Plan Approvals: Colorado; Addressing Remanded Portions of the Previously Approved Infrastructure Requirements for the 2015 Ozone National Ambient Air Quality Standards
Posted Date: 2022-05-06T04:00Z

[Federal Register Volume 87, Number 88 (Friday, May 6, 2022)]
[Proposed Rules]
[Pages 27050-27059]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2022-09449]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R08-OAR-2019-0140; EPA-HQ-OAR-2021-0663; FRL-9782-01-R8]

Air Plan Approval; Colorado; Addressing Remanded Portions of the 
Previously Approved Infrastructure Requirements for the 2015 Ozone 
National Ambient Air Quality Standards

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: On January 5, 2021, the United States Court of Appeals for the 
Tenth Circuit granted the Environmental Protection Agency's (EPA) 
motion for a voluntary remand without vacatur of two parts of EPA's 
2020 final rule approving Colorado's infrastructure state 
implementation plan (SIP) submission for the 2015 8-hour ozone national 
ambient air quality standards (NAAQS) (2020 final rule). In this 
document, EPA proposes to address those two remanded parts of the 2020 
final rule: EPA's conclusion that Colorado's infrastructure SIP 
submission met the State's good neighbor obligation under Clean Air Act 
(CAA) section 110(a)(2)(D)(i)(I); and EPA's conclusion that Colorado's 
infrastructure SIP submission provided ``necessary assurances'' of the 
State's authority to regulate agricultural sources under CAA section 
110(a)(2)(E)(i). EPA is proposing to approve Colorado's infrastructure 
SIP submission pursuant to CAA section 110.

DATES: Written comments must be received on or before June 6, 2022.

ADDRESSES: You may send comments, identified as Docket No. EPA-R08-OAR-
2019-0140, using the Federal eRulemaking Portal at https://www.regulations.gov, following the online instructions for submitting 
comments. Include Docket ID No. EPA-R08-OAR-2019-0140 in the subject 
line of the message.
    Instructions: All submissions received must include Docket ID No. 
EPA-R08-OAR-2019-0140. 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. Out 
of an abundance of caution for members of the public and our staff, EPA 
Docket Center and Reading Room are open to the public by appointment 
only to reduce the risk of transmitting COVID-19. Our Docket Center 
staff also continues to provide remote customer service via email, 
phone, and webform. For further information on EPA Docket Center 
services and the current status, please visit us online at https://www.epa.gov/dockets. Please email or call a person listed in the FOR 
FURTHER INFORMATION CONTACT section if you need to make alternative 
arrangements for access to the docket.

FOR FURTHER INFORMATION CONTACT: Amrita Singh, Air and Radiation 
Division, EPA, Region 8, Mailcode 8ARD-IO, 1595 Wynkoop Street, Denver, 
Colorado, 80202-1129, telephone number: (303) 312-6103, email address: 
[email protected]; or

[[Page 27051]]

Ellen Schmitt, telephone number: (303) 312-6728, email address: 
[email protected].

SUPPLEMENTARY INFORMATION:  Public participation: Submit your comments, 
identified by Docket No. EPA-R08-OAR-2019-0140, at https://www.regulations.gov. Once submitted, comments cannot be edited or 
removed from the docket. EPA may publish any comment received to its 
public docket. Do not submit to 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. 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 proposed action, EPA-R08-OAR-
2019-0140 and EPA-HQ-OAR-2021-0663. Docket No. EPA-R08-OAR-2019-0140 
contains information specific to Colorado, 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 proposed action. All comments regarding 
information in either of these dockets must be made in Docket No. EPA-
R08-OAR-2019-0140. For additional submission methods, please email or 
call a person listed in the FOR FURTHER INFORMATION CONTACT. For the 
full EPA 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 to Docket No. EPA-HQ-OAR-2021-0663 is available 
electronically at https://www.regulations.gov. While all documents in 
that docket are listed in the index, some information may not be 
publicly available due to docket file size restrictions or content 
(e.g., CBI).
    Throughout this document wherever ``we,'' ``us,'' or ``our'' is 
used, we mean EPA.

Table of Contents

I. Background
    A. Statutory and Regulatory Background
    1. Ozone Transport
    a. Background on EPA's Ozone Transport Modeling Information
    2. Necessary Assurances of State Authority
    B. EPA's 2020 Action and the 2021 Voluntary Remand
II. EPA's Evaluation and Proposed Approval of Colorado's 
Infrastructure SIP Submission Under CAA Section 110(a)(2)(D)(i)(I) 
and (E)(i)
    A. Good Neighbor Provision
    1. EPA's Approach to Evaluating Interstate Transport SIP 
Submissions for the 2015 8-Hour Ozone NAAQS
    2. Selection of Analytic Year
    3. Step 1 of the 4-Step Interstate Transport Framework
    4. Step 2 of the 4-Step Interstate Transport Framework
    5. Step 3 of the 4-Step Interstate Transport Framework
    6. Step 4 of the 4-Step Interstate Transport Framework
    7. EPA's Evaluation of Colorado's CAA Section 110(a)(D)(i)(I) 
Submission
    8. Emissions Assumptions Used in Modeling
    B. Colorado's Authority To Regulate Agricultural Emissions
    1. EPA's Prior Approval
    2. EPA's Revised Analysis on Remand Under CAA Section 
110(a)(2)(E)(i)
III. Proposed Action
IV. Statutory and Executive Order Reviews

I. Background

A. Statutory and Regulatory Background

    On October 1, 2015, 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 three years 
after promulgation of a new or revised standard, SIP submissions 
meeting the applicable requirements of CAA section 110(a)(2).\2\
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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 submissions 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.
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1. Ozone Transport
    One of the applicable requirements of section 110(a)(2) 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 
submission 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). 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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    \3\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir. 
2008).
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    EPA is using the 4-step interstate transport framework (or 4-step 
framework) to evaluate state SIP submissions addressing the interstate 
transport provision for the 2015 8-hour ozone NAAQS. 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 
CSAPR Update,\5\ and the Revised CSAPR Update, both of which addressed 
the 2008 ozone NAAQS.\6\
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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 United States Court of Appeals for the D.C. 
Circuit 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).
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    Through the development and implementation of the CSAPR rulemakings 
and prior regional rulemakings pursuant to the interstate transport 
provision,\7\ EPA, working in partnership with states, developed the 
following 4-step interstate transport framework to evaluate a state's 
obligations to eliminate interstate transport emissions under the 
interstate

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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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    \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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a. Background on EPA's Ozone Transport Modeling Information
    In general, 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, 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 (VOC) in individual upwind states.
    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, 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, 
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 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 
interstate transport framework.\12\ EPA subsequently issued two more 
memoranda in August and October 2018, providing additional information 
to states developing interstate transport SIP submissions for the 2015 
ozone NAAQS concerning, respectively, potential contribution thresholds 
that may be appropriate to apply in Step 2 of the 4-step framework, and 
considerations for identifying downwind areas that may have problems 
maintaining the standard 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 1735.
    \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, available in Docket No. EPA-
HQ-OAR-2021-0663 or at https://www.epa.gov/node/194139/.
    \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 or at https://www.epa.gov/airmarkets/memo-and-supplemental-information-regarding-interstate-transport-sips-2015-ozone-naaqs.
    \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.'' 
March 2018 memorandum at 2.
    \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 or at https://www.epa.gov/airmarkets/memo-and-supplemental-information-regarding-interstate-transport-sips-2015-ozone-naaqs.
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    Since the release of the modeling data shared in the March 2018 
memorandum, 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 EPA, the MJOs, and states to develop a new, more recent 
emissions platform for use by EPA and states in regulatory modeling as 
an improvement over the dated 2011-based platform that EPA had used to 
project ozone design values and contribution data provided in the 2017 
and 2018 memoranda. 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, EPA 
released and accepted public comment on 2023 modeling that used the 
2016v1 emissions platform.\15\ Although the Revised CSAPR 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 Revised CSAPR Update for the 2008 Ozone NAAQS, 85 FR 
68964, 68981 (October 30, 2020).
    \16\ See the Air Quality Modeling Technical Support Document for 
the Final Revised Cross-State Air Pollution Rule Update, included in 
the Headquarters Docket No. EPA-HQ-OAR-2021-0663.
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    Following the final Revised CSAPR Update, EPA made further updates 
to the 2016 emissions platform to include mobile emissions from 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 the updated emissions platform, 2016v2, is described in an emissions 
modeling technical support document (TSD).\18\ EPA performed air

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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\
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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. Dated: February 2022. (2016v2 TSD). Included 
under Docket No. EPA-HQ-OAR-2021-0663.
    \19\ Ramboll Environment and Health, January 2021, www.camx.com.
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    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 and 
generally references it within this action as 2016v2 modeling for 2023. 
By using the updated modeling results, 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 EPA's 
2016v2 modeling. EPA is accepting public comment on this updated 2023 
modeling, which uses a 2016v2 emissions platform as the modeling 
pertains to this proposed action. Comments on EPA's air quality 
modeling as used in this proposed action should be submitted in the 
Regional docket for this action, Docket No. EPA-R08-OAR-2019-0140. EPA 
is not accepting comments in Docket No. EPA-HQ-OAR-2021-0663.
2. Necessary Assurances of State Authority
    CAA section 110(a)(2)(E)(i) requires that a state provide 
``necessary assurances'' that it will have, among other things, 
adequate authority under state law to carry out its SIP to meet CAA 
requirements with respect to the relevant NAAQS.\20\ Specifically, a 
state's infrastructure SIP submission should show that the state has 
the legal authority to carry out the provisions identified in the 
state's infrastructure SIP submission and is not prohibited by federal 
or state law from carrying out the SIP submission.
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    \20\ 42 U.S.C. 7410(a)(2)(E)(i).
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B. EPA's 2020 Action and the 2021 Voluntary Remand

    On September 17, 2018, the State of Colorado submitted to EPA its 
infrastructure SIP submission for the 2015 ozone NAAQS. On July 29, 
2019, EPA proposed to approve Colorado's submission with respect to all 
relevant CAA elements.\21\ EPA proposed approval of the portion of 
Colorado's infrastructure SIP related to prongs 1 and 2 of CAA section 
110(a)(2)(D)(i)(I), primarily relying on the 2023 modeling (2011 base 
year platform) presented in the March 2018 memorandum.\22\ EPA's 
analysis of the 2023 modeling indicated that Colorado's largest impacts 
at any identified downwind receptor would be less than 1 percent (0.70 
ppb) of the 2015 ozone NAAQS.\23\ Thus, EPA proposed to find that 
Colorado's emissions would not significantly contribute to 
nonattainment or interfere with maintenance of the 2015 ozone NAAQS in 
any other state.\24\
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    \21\ Promulgation of State Implementation Plan Revisions; 
Infrastructure Requirements for the 2015 Ozone National Ambient Air 
Quality Standards; Colorado and North Dakota, 84 FR 36516 (July 29, 
2019). In the same rulemaking EPA also proposed to act on North 
Dakota's infrastructure SIP submission for the 2015 ozone NAAQS. 
EPA's final action with respect to North Dakota's SIP submission was 
neither challenged nor remanded and thus is not addressed in this 
action.
    \22\ 84 FR 36516, 36524-25.
    \23\ Id. n.24.
    \24\ 84 FR 36524-25.
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    On September 13, 2019, the United States Court of Appeals for 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).\25\ In our 2020 
final rule (published on April 10, 2020), EPA defended the use of the 
2023 analytical year on the basis of what was then, in the Agency's 
view, a position consistent with Wisconsin--specifically that the 
Wisconsin holding did not apply with respect to the attainment date for 
marginal areas.\26\ However, EPA also offered an alternative rationale. 
EPA used linear interpolation to estimate Colorado's maximum 
contribution to a potential receptor in 2021 and concluded that even if 
it were appropriate to use the 2021 marginal area date rather than the 
2023 moderate area date, Colorado's impacts would be similar to those 
projected for 2023 and thus it would not significantly contribute to 
nonattainment or interfere with maintenance in other states.\27\
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    \25\ 938 F.3d at 313.
    \26\ Approval and Promulgation of State Implementation Plan 
Revisions; Infrastructure Requirements for the 2015 Ozone National 
Ambient Air Quality Standards; Colorado and North Dakota, 85 FR 
20169, 20169-71 (April 10, 2020).
    \27\ Id. at 20169.
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    On May 19, 2020, the D.C. Circuit issued a ruling in Maryland v. 
EPA that cited the Wisconsin decision in holding that 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 EPA's denial of a petition under CAA section 
126(b).\28\ The court noted that ``section 126(b) incorporates the Good 
Neighbor Provision,'' and, therefore, ``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).
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    \28\ 958 F.3d 1185, 1203-04 (D.C. Cir. 2020).
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    On June 9, 2020, the Center for Biological Diversity (Center) filed 
a petition for review of the 2020 final rule in the United States Court 
of Appeals for the Tenth Circuit (Tenth Circuit).\29\ The Center 
challenged two sub-elements of the SIP approval: (1) EPA's conclusion 
that Colorado's infrastructure SIP submission satisfies the good 
neighbor provision, CAA section 110(a)(2)(D)(i)(I); and (2) EPA's 
conclusion that the State's infrastructure SIP submission satisfies 
Colorado's obligation to provide necessary assurances that the State 
has authority to regulate all agricultural sources of air pollution as 
may be required by the CAA section 110(a)(2)(E)(i).\30\
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    \29\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.).
    \30\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), Petitioner's Opening Brief at 10-11.
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    In challenging EPA's approval of the portion of Colorado's 
infrastructure SIP submission addressing CAA section 
110(a)(2)(D)(i)(I), the Center argued that EPA's analysis focused on 
the wrong analytical year, failed to adequately analyze all of the 
relevant potential out-of-state receptor locations, and should have 
accounted for air quality impacts from various proposed and final 
federal rules.\31\
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    \31\ Id.
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    With respect to the state authority issue, the Center argued that a 
provision of state law, Colo. Rev. Stat. Sec.  25-7-109(8)(a), bars 
Colorado from regulating agricultural sources other than those that are 
major sources. The Center argued that this means that Colorado's 
infrastructure SIP submission failed to provide ``necessary 
assurances'' of the State's authority to regulate all agricultural 
sources, as may be needed to comply with CAA requirements for SIPs, 
pursuant to CAA section 110(a)(2)(E)(i).\32\
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    \32\ Id.
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    On December 31, 2020, EPA filed a motion for a voluntary remand 
without vacatur of the two challenged parts of the 2020 final rule. EPA 
stated that it

[[Page 27054]]

intended to consider additional information, including the Maryland 
decision and new information developed after EPA issued the 2020 final 
rule that was not available in the administrative record for the 2020 
final rule.\33\ The Tenth Circuit granted EPA's motion on January 5, 
2021.\34\
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    \33\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), EPA's Motion for Voluntary Remand, Ex. 1, Declaration in 
Support of Motion for Voluntary Remand, at ] 8-10.
    \34\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), January 5, 2021 Order.
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    In this document, EPA proposes to address the two remanded portions 
of EPA's 2020 final rule by proposing to approve Colorado's 
infrastructure SIP submission for the 2015 ozone standards with respect 
to CAA section 110(a)(2)(D)(i)(I) and (E)(i). EPA seeks comment on its 
conclusions under CAA section 110(a)(2)(D)(i)(I) and (E)(i) in this 
proposed approval. We are not otherwise addressing or reopening for 
comment any of the other portions of our 2020 final rule. We will deem 
any comments on such portions beyond the scope of this action.

II. EPA's Evaluation and Proposed Approval of Colorado's Infrastructure 
SIP Submission Under CAA Section 110(a)(2)(D)(i)(I) and (E)(i)

A. Good Neighbor Provision

1. EPA's Approach to Evaluating Interstate Transport SIP Submissions 
for the 2015 8-Hour Ozone NAAQS
    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 the 
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.\35\
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    \35\ See EME Homer City Generation, LP v. EPA, 572 U.S. 489, 519 
(2014). As discussed later in this section, EPA recognizes that the 
nature of high ozone levels due to wintertime inversion conditions 
in the Uinta Basin in Utah raises unique analytical challenges in 
assessing whether there is transport from Colorado during those 
wintertime episodes. EPA has separately analyzed that unique 
situation and proposes to conclude that emissions from Colorado do 
not contribute to high ozone levels in Utah. That analysis, however, 
is separate from the generally applicable 4-step analytical 
framework for ozone transport described here.
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    The remainder of this section describes EPA's proposed framework 
with respect to analytic year, definition of nonattainment and 
maintenance receptors, selection of contribution threshold, and 
multifactor control strategy assessment.
2. Selection of Analytic Year
    In general, the states and EPA must implement the interstate 
transport provision in a manner ``consistent with the provisions of 
[title I of the CAA].'' \36\ 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).\37\ As 
discussed in Section I of this proposed rulemaking, recent case law 
makes clear that the states and the Agency are obligated, under the 
good neighbor provision, to assess downwind air quality as 
expeditiously as practicable and no later than the next applicable 
attainment date. This 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.\38\ 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.
---------------------------------------------------------------------------

    \36\ 42 U.S.C. 7410(a)(2)(D)(i).
    \37\ For attainment dates for the 2015 8-hour ozone NAAQS, refer 
to 42 U.S.C. 7511(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).
    \38\ See 42 U.S.C. 7511(a); 40 CFR 51.1303; 83 FR 25776.
---------------------------------------------------------------------------

    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, 
including Colorado, relied upon EPA modeling of the year 2023, and no 
state provided an alternative analysis using a 2021 analytic year (or 
the prior 2020 ozone season). EPA appreciates that among the arguments 
raised by the Center in challenging the 2020 final rule was the failure 
to analyze a year earlier than 2023. However, EPA must act on SIP 
submissions--even in this action on remand--using the information 
available at the time it takes such action. In this circumstance, EPA 
does not believe it would be appropriate to evaluate Colorado's 
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.\39\ It would not 
make sense to analyze air quality, contribution levels, or emissions 
control strategies for the 2021 attainment date, for purposes of 
interstate transport obligations, when no emissions reductions, if 
shown to be needed, could be implemented by that date anyway.\40\ 
Consequently, in this proposal EPA will use the analytical year of 2023 
to evaluate Colorado's CAA section 110(a)(2)(D)(i)(I) SIP submission 
with respect to the 2015 8-hour ozone NAAQS.
---------------------------------------------------------------------------

    \39\ See 86 FR 23074; see also Wisconsin, 938 F.3d at 322.
    \40\ Nor does EPA view 2022 as a reasonable analytic year for a 
similar reason: it would be impossible to finalize this action and 
implement any emissions reductions measures that could be shown to 
be needed by the 2022 ozone season. Thus, 2023 is the appropriate 
analytic year and also aligns with the next attainment date.
---------------------------------------------------------------------------

3. Step 1 of the 4-Step Interstate Transport Framework
    In Step 1, EPA identifies monitoring sites that are projected to 
have problems attaining and/or maintaining the NAAQS in the 2023 
analytic year. Where 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 EPA's 4-step interstate 
transport framework. For sites that are identified as a nonattainment 
or maintenance receptor in 2023, we proceed to the next step of our 4-
step interstate transport framework by identifying the upwind state's 
contribution to those receptors.
    EPA's approach to identifying ozone nonattainment and maintenance 
receptors in this action is consistent with the approach used in 
previous transport rulemakings. 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

[[Page 27055]]

D.C. Circuit's direction in North Carolina v. EPA.\41\
---------------------------------------------------------------------------

    \41\ See 531 F.3d at 910-11 (holding that EPA must give 
``independent significance'' to each prong of CAA section 
110(a)(2)(D)(i)(I)).
---------------------------------------------------------------------------

    For this proposal, 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 EPA 
defined nonattainment receptors as those areas that both currently 
measure nonattainment and that EPA projects will be in nonattainment in 
the future analytic year (i.e., 2023).\42\
---------------------------------------------------------------------------

    \42\ See 81 FR 74504. This same concept, relying on both current 
monitoring data and modeling to define nonattainment receptor, was 
also applied in CAIR. See 70 FR at 25241, 25249 (January 14, 2005); 
see also North Carolina, 531 F.3d at 913-14 (affirming as reasonable 
EPA's approach to defining nonattainment in CAIR).
---------------------------------------------------------------------------

    In addition, in this proposal, 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.\43\ 
Specifically, 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 
period. EPA interprets the projected maximum future design value to be 
a potential future air quality outcome consistent with the meteorology 
that yielded maximum measured concentrations in the ambient data set 
analyzed for that receptor (i.e., ozone conducive meteorology). EPA 
also recognizes that previously experienced meteorological conditions 
(e.g., dominant wind direction, temperatures, air mass patterns) 
promoting ozone formation that led to maximum concentrations in the 
measured data may reoccur in the future. The maximum design value gives 
a reasonable projection of future air quality at the receptor under a 
scenario in which such conditions do, in fact, reoccur. The projected 
maximum design value is used to identify upwind emissions that, under 
those circumstances, could interfere with the downwind area's ability 
to maintain the NAAQS.
---------------------------------------------------------------------------

    \43\ 795 F.3d 118, 136 (D.C. Cir. 2015); see 76 FR 48208 (August 
8, 2011). CSAPR Update and Revised CSAPR Update also used this 
approach. See 81 FR 74504 and 86 FR 23054.
---------------------------------------------------------------------------

    Recognizing that nonattainment receptors are also, by definition, 
maintenance receptors, 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, 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.
4. Step 2 of the 4-Step Interstate Transport Framework
    In Step 2, 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 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). 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 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. EPA 
continues to find 1 percent to be an appropriate threshold.
    For ozone, as EPA found in the Clean Air Interstate Rule (CAIR), 
CSAPR, and CSAPR Update, a portion of the nonattainment problems from 
anthropogenic sources in the United States 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. EPA's analysis shows that much of the ozone transport problem 
being analyzed in this proposed rule is still the result of the 
collective impacts of contributions from many 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 EPA (and 
states) to apply a consistent framework to evaluate interstate 
emissions transport under the interstate transport provision from one 
NAAQS to the next.\44\
---------------------------------------------------------------------------

    \44\ See 81 FR 74518. See also 86 FR 23085 (reviewing and 
explaining rationale from CSAPR, 76 FR 48237-38, for selection of 1 
percent threshold).
---------------------------------------------------------------------------

5. Step 3 of the 4-Step Interstate Transport Framework
    Consistent with 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. EPA's analysis 
at Step 3 in prior federal actions addressing interstate transport 
requirements has focused primarily 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 EPA's or alternative air quality and contribution 
modeling establishes that a state is linked at Steps 1 and 2, it will

[[Page 27056]]

be insufficient at Step 3 for a state merely to point to its existing 
rules requiring control measures as a basis for approval. In general, 
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 EPA has not prescribed a particular method for this assessment, 
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.\45\
---------------------------------------------------------------------------

    \45\ 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 Revised CSAPR Update, 86 FR 23054, 23086-23116. 
Consistently across these rulemakings, EPA has developed emissions 
inventories, analyzed different levels of control stringency at 
different cost thresholds, and assessed resulting downwind air 
quality improvements.
---------------------------------------------------------------------------

6. Step 4 of the 4-Step Interstate Transport Framework
    At Step 4, states (or 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.\46\
---------------------------------------------------------------------------

    \46\ See 42 U.S.C. 7410(a)(2)(D) (``Each such [SIP] shall . . . 
contain adequate provisions . . . .''); see also 42 U.S.C. 
7410(a)(2)(A); Committee for a Better Arvin v. EPA, 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).
---------------------------------------------------------------------------

7. EPA's Evaluation of Colorado's CAA Section 110(a)(2)(D)(i)(I) 
Submission
    As mentioned above, the State of Colorado submitted a SIP 
submission to EPA on September 17, 2018, to meet the good neighbor 
requirements for the 2015 ozone NAAQS. In its prong 1 and prong 2 
analysis, Colorado's SIP submission relies on analysis of the year 2023 
(using a 2011 base year platform), among other things, to conclude that 
the State does not significantly contribute to nonattainment or 
interfere with maintenance of the 2015 ozone NAAQS in any other 
state.\47\ As explained in Section I of this proposed rulemaking, EPA 
has conducted an updated analysis for the 2023 analytical year (using a 
2016 base year platform) and proposes to rely primarily on this updated 
modeling to evaluate Colorado's transport SIP submission.
---------------------------------------------------------------------------

    \47\ Letter from Dr. Larry Wolk, Executive Director, Colorado 
Department of Health & Environment, to Douglas Benevento, Regional 
Administrator, EPA Region 8, Attachment 9, Adopted SIP at 4-5 
(August 16, 2018) (Colorado SIP Submission).
---------------------------------------------------------------------------

    As described in Section I, EPA performed air quality modeling to 
project design values and contributions for 2023 using the 2016v2 
emissions platform. EPA examined these data to determine if emissions 
in Colorado contribute 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 in this most recent round of modeling. The data 
\48\ indicate that the highest contribution in 2023 from Colorado to a 
downwind nonattainment or maintenance receptor is 0.06 ppb and 0.20 
ppb, respectively.\49\ Specifically, EPA's analysis indicates that 
Colorado will have a 0.06 ppb impact at the projected nonattainment 
receptor in Kenosha County, Wisconsin (Site ID 550590019), which has a 
2023 projected average design value of 72.8 ppb and a 2023 projected 
maximum design value of 73.7 ppb. EPA's analysis further indicates that 
Colorado will have a 0.20 ppb impact at a projected maintenance 
receptor in Denton County, Texas (Site ID 481210034), which has a 
projected 2023 average design value of 70.4 ppb and a 2023 projected 
maximum design value of 72.2 ppb. The data also indicate that the only 
contribution in 2023 from Colorado to any downwind monitor above the 1 
percent threshold is to a monitor in San Juan, New Mexico (0.99 ppb). 
This monitor's 2023 average and maximum design values are projected to 
be below the 2015 ozone NAAQS and the monitor is therefore not 
projected to be a nonattainment and/or maintenance receptor for the 
2015 ozone NAAQS. Accordingly, EPA proposes to conclude that the most 
recent data support EPA's conclusion that Colorado does not contribute 
to nonattainment or interfere with maintenance of the 2015 ozone NAAQS 
in any other state.
---------------------------------------------------------------------------

    \48\ 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.
    \49\ Both 0.06 ppb and 0.20 ppb are below the 1 percent 
threshold of the 2015 ozone NAAQS (.70 ppb).
---------------------------------------------------------------------------

    In its comments on the 2020 rule and in its brief in the Tenth 
Circuit litigation, the Center identified several downwind areas that 
it argued may have been in nonattainment in 2020 but that EPA had 
screened out by using the incorrect analytic year of 2023. These 
included: Tarrant and Denton County, Texas; the Northern and Southern 
Wasatch Fronts in Utah; and monitors in New Mexico.\50\ In response to 
this argument, EPA is providing in Table 1 the projected 2023 design 
values (DV) and associated contributions from Colorado for all monitors 
located in these areas for which EPA's modeling provides valid 
contribution data.\51\
---------------------------------------------------------------------------

    \50\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), Petitioner's Opening Brief at 29-31.
    \51\ As described in the Air Quality Modeling Technical Support 
Document 2015 Ozone NAAQS Transport SIP Proposed Actions, EPA's 
method for calculating an average contribution metric for use in 
Step 2 of the 4-step transport framework is based on the average of 
daily contributions on the top 10 ozone concentrations days as 
modeled in 2023. However, in order to avoid including contributions 
on days with low ozone concentrations, EPA requires at least 5 days 
with model-predicted maximum daily average 8-hour ozone 
concentrations greater than or equal to 60 ppb. In EPA's method, 
contribution metric values are not calculated for monitors with 
fewer than 5 days that meet the 60 ppb threshold. As a result of 
applying this criterion, there were three monitoring sites in the 
areas identified by the Center, excluding the Uinta Basin, that are 
projected to have problems attaining and/or maintaining the NAAQS in 
2023 for which EPA did not calculate contribution metric values. 
These monitors include two sites in Dona Ana County, New Mexico, and 
one site in Toole County, Utah. Although EPA does not have 
contribution data for these specific monitors, the data at near-by 
monitors indicate that the contributions from Colorado to Dona Ana 
and Toole Counties are expected to be well below the 1 percent 
threshold. Specifically, the contribution from Colorado to a 
monitoring site in El Paso, Texas, which is in the Dona Ana-El Paso 
interstate nonattainment area, is 0.04 ppb and, as indicated in 
Table 1, the contributions from Colorado to monitoring sites in Salt 
Lake County, which is closer to Colorado than Toole County, are 0.03 
ppb.

[[Page 27057]]

                                                   Table 1--Colorado Contributions and Select Monitors
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                                                             Colorado
          Monitor (AQS site ID)                        State                        County             Projected 2023    Projected 2023    contribution
                                                                                                         average DV        maximum DV          (ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
350010029................................  New Mexico..................  Bernalillo.................              62.0              62.7            0.27
350450018................................  New Mexico..................  San Juan...................              64.7              66.6            1.00
350610008................................  New Mexico..................  Valencia...................              62.2              63.9            0.30
481210034................................  Texas.......................  Denton.....................              70.4              72.2            0.20
481211032................................  Texas.......................  Denton.....................              67.2              69.0            0.22
484393009................................  Texas.......................  Tarrant....................              68.0              68.7            0.17
481410029................................  Texas.......................  El Paso....................              62.3              64.6            0.04
490030003................................  Utah........................  Box Elder..................              65.2              66.5            0.02
490110004................................  Utah........................  Davis......................              72.9              75.1            0.03
490353006................................  Utah........................  Salt Lake..................              73.6              75.3            0.03
490353013................................  Utah........................  Salt Lake..................              74.4              74.9            0.03
490570002................................  Utah........................  Weber......................              70.6              72.5            0.02
490571003................................  Utah........................  Weber......................              70.5              71.5            0.02
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Table 1 shows that there are six monitors predicted to be violating 
the 2015 ozone NAAQS in 2023, one in Texas and five in Utah.\52\ 
However, Colorado's projected contribution to each of these monitors is 
below the 1 percent threshold. Thus, no further analysis is required to 
address Colorado's good neighbor obligations for the areas relevant to 
the listed monitors at Step 3.
---------------------------------------------------------------------------

    \52\ See monitors 481210034, 490110004, 490353006, 490353013, 
490570002, and 490571003.
---------------------------------------------------------------------------

    The Center also claimed that it could not find any documents in the 
record which address Colorado's contribution to nonattainment in the 
Uinta Basin.\53\ EPA projected the design values for several of the 
monitoring sites in Duchesne County and Uintah County, Utah, but the 
Agency's modeling represents summertime ozone conditions and is not 
designed to capture the conditions that result in the high wintertime 
ozone concentrations in the Uinta Basin nonattainment area.
---------------------------------------------------------------------------

    \53\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), Petitioner's Opening Brief at 30.
---------------------------------------------------------------------------

    In order to characterize potential transport from Colorado to the 
Uinta Basin nonattainment area in the absence of reliable modeling to 
inform wintertime ozone levels and contributions, EPA conducted a 
separate analysis for the Uinta Basin, which is provided in a Uinta 
Basin TSD accompanying this action and included in Docket EPA-R08-OAR-
2019-0140.\54\ To summarize EPA's TSD findings, the ozone levels in the 
Uinta Basin nonattainment area are caused by a combination of 
meteorological inversion conditions, the unique topography of the Uinta 
Basin, and significant emissions of ozone precursors from sources 
within Utah. Generally, EPA concludes that ozone-precursor emissions do 
not transport into the Uinta Basin from outside the area during 
wintertime inversion episodes that produce high ozone conditions. 
Further, with respect to the portion of Colorado located within the 
regional Uinta Basin, available data shows that, because of low wind 
speed during wintertime inversion conditions and the unique 
topographical features within the regional Uinta Basin, emissions from 
the relevant area of Colorado are unlikely to transport to the Utah 
portion of the Uinta Basin.\55\
---------------------------------------------------------------------------

    \54\ EPA, Technical Support Document, Ozone Transport Analysis: 
Colorado and the Uinta Basin Nonattainment Area, April 2022 (Uinta 
Basin TSD).
    \55\ Id.
---------------------------------------------------------------------------

    EPA reaches these conclusions recognizing the unique challenges 
associated with characterizing wintertime ozone concentrations and 
contributions in the Uinta Basin. As such, for this portion of the 
analysis, EPA is supplementing the consistently applied 4-step 
interstate transport framework used to characterize ozone transport at 
a broader, regional scale and during the summertime ozone season. Based 
on the information and analysis presented in the Uinta Basin TSD, EPA 
proposes to find that it is reasonable to conclude that Colorado does 
not significantly contribute to nonattainment or interfere with 
maintenance of the 2015 ozone NAAQS in the Utah portion of the Uinta 
Basin.\56\
---------------------------------------------------------------------------

    \56\ Id.
---------------------------------------------------------------------------

    In summary, based on the analyses provided in this document and in 
the Uinta Basin TSD, EPA proposes to conclude that emissions from 
sources in Colorado will not contribute to nonattainment or interfere 
with maintenance of the 2015 ozone NAAQS in any other state. 
Accordingly, EPA proposes to approve Colorado's infrastructure SIP 
submission for the 2015 ozone NAAQS under CAA section 
110(a)(2)(D)(i)(I).
8. Emissions Assumptions Used in Modeling
    The Center argued that in the context of evaluating Colorado's good 
neighbor SIP submission, EPA should have accounted for air quality 
impacts from various proposed and final federal rules.\57\ EPA's normal 
practice is to include in its modeling only changes in emissions from 
final regulatory actions because, until such rules are finalized, any 
potential changes in NOX or VOC emissions are speculative. 
EPA's updated 2023 modeling using the 2016v2 platform reflects an 
updated assessment of the emissions inventory nationwide based on 
changes in federal and state rules and other relevant changes in the 
emissions inventory known at the time this latest modeling was 
conducted. All assumptions that formed the basis of the updated 2023 
modeling (2016v2) are available in the emissions modeling TSD.\58\ EPA 
encourages commenters to review this information, which supports the 
updated basis for this proposed action. This information supersedes the 
older modeling of 2023 that had been used in the 2020 final rule (2011 
base year platform).
---------------------------------------------------------------------------

    \57\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), Petitioner's Opening Brief at 33-37.
    \58\ See generally 2016v2 TSD; see also, e.g., 2016v2 TSD 
Section 4, 157-213.
---------------------------------------------------------------------------

B. Colorado's Authority To Regulate Agricultural Emissions

1. EPA's Prior Approval
    CAA section 110(a)(2)(E)(i) requires that a state must provide 
``necessary assurances'' that it has, among other things, adequate 
authority under state law to carry out the provisions of its SIP with 
respect to the relevant NAAQS. In the context of an infrastructure SIP 
submission, EPA expects states to

[[Page 27058]]

provide such necessary assurances for the new or revised NAAQS at 
issue.
    In its September 17, 2018 infrastructure SIP submission, Colorado 
stated that ``[t]here are no state or federal provisions prohibiting 
the implementation of any provision of the Colorado SIP.'' 
Specifically, Colorado cited to its ``general authority to adopt the 
rules and regulations necessary to implement the SIP'' as ``set out in 
the Colorado Air Pollution Prevention and Control Act Section 25-7-105 
of the Colorado Revised Statutes (C.R.S);'' general authority to 
administer and enforce the program in C.R.S. 25-7-111; additional 
authority to regulate air pollution and implement provisions in the SIP 
in the Colorado Air Pollution Prevention and Control Act, Article 7 of 
title 25; and authority delegated under C.R.S. 42-4-301 through 42-4-
414 (concerning motor vehicle emissions) and 42-4-414, C.R.S. 
(concerning emissions from diesel-powered vehicles).\59\
---------------------------------------------------------------------------

    \59\ Colorado SIP Submission, Attachment 9, Adopted SIP at 6.
---------------------------------------------------------------------------

    The Center commented on EPA's proposed approval of the State's 
infrastructure SIP submission, stating that C.R.S. 25-7-109(8)(a) 
prohibits Colorado from regulating agricultural sources of air 
pollution unless they are major sources. EPA evaluated the Center's 
concern with respect to Colorado's authority. In response, EPA 
explained that the provision cited by the Center does not bar the State 
from carrying out its existing SIP, and that in fact, the provision 
requires regulation of agricultural sources if they are major 
stationary sources, or if regulation is required by Part C, Part D, or 
title V of the CAA. In other words, EPA interpreted the provision to 
mean that if it is necessary to regulate agricultural sources beyond 
those that are major sources in order to attain and maintain the NAAQS, 
then the State has authority to do so. EPA noted that whether Colorado 
will need additional emission limitations and other control measures 
for areas designated nonattainment for the 2015 ozone NAAQS will be 
evaluated by the State and EPA as part of the State's attainment plan 
under CAA title I part D through a separate process. Thus, EPA found 
that Colorado does not lack authority to implement the SIP and 
concluded instead that Colorado's infrastructure SIP satisfied CAA 
section 110(a)(2)(E)(i).\60\
---------------------------------------------------------------------------

    \60\ 85 FR 20171.
---------------------------------------------------------------------------

2. EPA's Revised Analysis on Remand Under CAA Section 110(a)(2)(E)(i)

    In its brief filed in the Tenth Circuit litigation, the Center 
renewed its argument challenging EPA's approval of Colorado's 
infrastructure SIP submission as meeting CAA section 110(a)(2)(E)(i) 
for the 2015 ozone NAAQS. The Center argued that EPA erred in approving 
Colorado's infrastructure SIP submittal under CAA section 
110(a)(2)(E)(i) because C.R.S. 25-7-109(8)(A) bars Colorado from 
regulating agricultural sources other than those that are major 
sources. In particular, the Center argued that agricultural emissions 
are largely not from major stationary sources, but rather from fugitive 
emissions due to pesticide application, gases emitted from soil after 
fertilizer application, minor stationary sources, and mobile sources. 
The Center argued that Colorado state law thus is inadequate to provide 
authority to control these sources of pollution.\61\
---------------------------------------------------------------------------

    \61\ Center for Biological Diversity v. EPA, No. 20-9560 (Tenth 
Cir.), Petitioner's Opening Brief at 38-44.
---------------------------------------------------------------------------

    As explained in the 2020 final rule, EPA disagreed with the 
Center's interpretation of the C.R.S. 25-7-109(8)(A) and instead 
concluded that Colorado is not prohibited under state law from 
regulating emissions from agricultural sources (however small) \62\ as 
necessary to implement the 2015 ozone NAAQS.\63\ In relevant part, the 
agricultural provision states that ``the [State] shall regulate 
emissions from [agriculture, horticultural, or floricultural 
production, including pesticide application] . . . if they are `major 
stationary sources', . . . or are required by Part C (prevention of 
significant deterioration), Part D (nonattainment), or Title V (minimum 
elements of a permit program), . . . .'' \64\ Thus, as stated in the 
2020 final rule, the statute plainly requires regulation of emissions 
from agricultural sources, including from nonpoint sources, soils and 
pesticides, mobile sources, and minor sources, if required under the 
CAA, including as necessary under Part D for attainment of the NAAQS.
---------------------------------------------------------------------------

    \62\ Emissions from agricultural sources make up a very small 
portion of NOX and VOC emissions statement in the Denver 
Metro/Northern Front Range nonattainment area. See 2017 NEI 
NOX VOC table, which is included in the docket for this 
action.
    \63\ 85 FR 20171.
    \64\ C.R.S. 25-7-109(8)(a).
---------------------------------------------------------------------------

    On remand, EPA verified that it properly interpreted Colorado law 
with respect to the State's authority to regulate agricultural sources, 
and, in particular, that Colorado law does not limit that regulatory 
authority to major sources. Indeed, Colorado has confirmed that it 
agrees with EPA's interpretation of C.R.S. 25-7-109(8)(A). In a letter 
submitted to EPA on July 29, 2021, Colorado acknowledged that C.R.S. 
25-7-109(8)(A) includes a ``limited restriction'' on the State's 
authority to regulate emissions from agricultural production activities 
but explains that there are ``important carve-outs'' to that limited 
restriction. Colorado confirmed that the State has explicit authority 
to regulate major stationary sources. Colorado further explained that 
the sources that qualify as ``major stationary sources'' depends on the 
classification of the nonattainment area at issue--the higher the 
classification the lower the emissions threshold to qualify as a major 
stationary source. Additionally, Colorado confirmed in the letter that 
the State has ``authority to regulate emissions from agricultural 
production, regardless of the size of the source, to the extent that 
such regulations are required by Part C (prevention of significant 
deterioration), Part D (nonattainment), or Title V (minimum elements of 
a permit program) of the federal [CAA].'' Moreover, Colorado confirmed 
that the State has explicit authority to regulate emissions from 
agricultural production to the extent that such regulation is required 
by CAA section 111 (new source performance standards) and explained 
that such regulation is conducted through the State's minor source, 
Prevention of Significant Deterioration, New Source Review, and Title V 
permitting programs. Finally, Colorado explained that the State has 
authority to promulgate, administer, and enforce emissions regulations 
that impact emissions from agricultural production, including mobile 
sources.\65\
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    \65\ Letter to Deb Thomas, Regional Administrator (Acting) and 
Deputy Regional Administrator, U.S. Environmental Protection Agency, 
Region 8, from Garrison Kaufman, Director, Air Pollution Control 
Division, July 29, 2021.
---------------------------------------------------------------------------

    Based on the above analysis and Colorado's July 29, 2021 letter, 
EPA has now verified its interpretation of the State's authority to 
regulate agricultural sources, as necessary to meet CAA requirements. 
Colorado has thus provided necessary assurances of the State's 
authority to regulate agricultural sources as required in 42 U.S.C. 
7410(a)(2)(E)(i). Accordingly, EPA is again proposing to approve 
Colorado's infrastructure SIP submission for the 2015 ozone NAAQS with 
respect to the requirements of CAA section 110(a)(2)(E)(i).

[[Page 27059]]

III. Proposed Action

    In this action, EPA proposes to conclude that Colorado's 
infrastructure SIP satisfies the interstate transport provision of the 
CAA, section 110(a)(2)(D)(i)(I), for the 2015 ozone NAAQS, and that the 
State has provided the necessary assurances of the State's authority to 
regulate all agricultural sources as may be required by the CAA under 
section 110(a)(2)(E)(i).

IV. Statutory and Executive Order Reviews

    Under the CAA, the Administrator is required to approve a SIP 
submission that complies with the provisions of the Act and applicable 
Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in 
reviewing SIP submissions, EPA's role is to approve state choices, 
provided that they meet the criteria of the CAA. Accordingly, this 
action merely proposes to approve state law as meeting Federal 
requirements and does not impose additional requirements beyond those 
imposed by state law. For that reason, this action:
     Is not a ``significant regulatory action'' subject to 
review by the Office of Management and Budget under Executive Orders 
12866 (58 FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21, 
2011);
     Does not impose an information collection burden under the 
provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
     Is certified as not having a significant economic impact 
on a substantial number of small entities under the Regulatory 
Flexibility Act (5 U.S.C. 601 et seq.);
     Does not contain any unfunded mandate or significantly or 
uniquely affect small governments, as described in the Unfunded 
Mandates Reform Act of 1995 (Pub. L. 104-4);
     Does not have Federalism implications as specified in 
Executive Order 13132 (64 FR 43255, August 10, 1999);
     Is not an economically significant regulatory action based 
on health or safety risks subject to Executive Order 13045 (62 FR 
19885, April 23, 1997);
     Is not a significant regulatory action subject to 
Executive Order 13211 (66 FR 28355, May 22, 2001);
     Is not subject to requirements of section 12(d) of the 
National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 
note) because application of those requirements would be inconsistent 
with the CAA; and
     Does not provide EPA with the discretionary authority to 
address, as appropriate, disproportionate human health or environmental 
effects, using practicable and legally permissible methods, under 
Executive Order 12898 (59 FR 7629, February 16, 1994).
    In addition, the SIP is not approved to apply on any Indian 
reservation land or in any other area where EPA or an Indian tribe has 
demonstrated that a tribe has jurisdiction. In those areas of Indian 
country, the proposed rule does not have tribal implications and will 
not impose substantial direct costs on tribal governments or preempt 
tribal law as specified by Executive Order 13175 (65 FR 67249, November 
9, 2000).

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by 
reference, Intergovernmental relations, Nitrogen dioxide, Ozone, 
Reporting and recordkeeping requirements, Volatile organic compounds.

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

    Dated: April 26, 2022.
K.C. Becker,
Regional Administrator, Region 8.
[FR Doc. 2022-09449 Filed 5-5-22; 8:45 am]
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