Federal Register / Vol. 87, No. 100 / Tuesday, May 24, 2022 / Proposed Rules 31495 

61A 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. 

62The 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. 

contributions at steps 1 and 2 of that 

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.61 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).62

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. 

Martha Guzman Aceves, 

Regional Administrator, Region IX. 

[FR Doc. 2022–11151 Filed 5–23–22; 8:45 am] 

BILLING CODE 6560–50–P 

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. 

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 (202) 566–1744, 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: (303) 312–6728, 

email address: schmitt.ellen@epa.gov. 

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: (303) 312–6728, 

email address: schmitt.ellen@epa.gov. 

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-epadockets. 

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. 


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1National 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. 

2SIP 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. 

3See North Carolina v. EPA, 531 F.3d 896, 909– 

11 (D.C. Cir. 2008). 

4See Federal Implementation Plans: Interstate 

Transport of Fine Particulate Matter and Ozone and 

Correction of SIP Approvals, 76 FR 48208 (August 

8, 2011). 

5Cross-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). 

8See 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). 

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 socalled ‘‘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

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. 

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 


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10See 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. 

11See 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. 

12The 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-andcomment rulemaking.’’ 

13See 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. 

14The results of this modeling, as well as the 

underlying modeling files, are included in Docket 

No. EPA–HQ–OAR–2021–0663. 15See 85 FR 68964, 68981. 16See 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. 17Additional details and documentation related 

to the MOVES3 model can be found at https://

www.epa.gov/moves/latest-version-motor-vehicleemission-simulator-moves. 

18See 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. 

19Ramboll Environment and Health, January 

2021, www.camx.com. 

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

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/MultiJurisdictional 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

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. 

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 regionalscale 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. 


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20March 2018 memorandum, Attachment A. 

21 Id. at A–1. 

22 Id. 

23For 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). 

24We 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. 

25See 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). 

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. 

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. 

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. 

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 


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26See 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)). 27See 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). 

28See 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). 

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

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

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. 

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, 


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29As 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. 

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. 

31Wyoming State Implementation Plan, 

Attachment B at 10. 

32See 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. 

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 welldocumented 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

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. 

TABLE 1—2023 AVERAGE AND MAXIMUM DESIGN VALUES AT DOWNWIND RECEPTORS WITH WYOMING CONTRIBUTIONS

EQUAL TO AND GREATER THAN 0.70 PPB a

Site ID County State 

Average 

design value 

(ppb) 

Maximum 

design value 

(ppb) 

Wyoming 

modeled 

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 


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37 Id. at 6. 

38Site ID 80050002 (Arapahoe), Site ID 80350004 

(Douglas), and Site ID 80590011 (Jefferson). Id. 

39 Id. at 3, 8–9. 

40 Id. at 2. 

41 Id. at 2–3, 6. 

42See ‘‘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). 

43Wyoming State Implementation Plan, 

Attachment B at 6 (citing 81 FR 15203). 

44 Id. at 6. 

45 Id. 

46 Id. at 7. 

47 Id. at 7–8. 

48 Id. at 8. 

49 Id. at 8–9. 

TABLE 1—2023 AVERAGE AND MAXIMUM DESIGN VALUES AT DOWNWIND RECEPTORS WITH WYOMING CONTRIBUTIONS

EQUAL TO AND GREATER THAN 0.70 PPB a—Continued 

Site ID County State 

Average 

design value 

(ppb) 

Maximum 

design value 

(ppb) 

Wyoming 

modeled 

contribution 

(ppb) 

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. 

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-ofevidence 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

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

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

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 nonanthropogenic 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


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50 Id. at 9. 51 Id. 52 Id. 53 Id. 54 Id. 55 Id. 56 Id at 9–10. 

57See 81 FR 74504, at 74523. 

58See 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. 

TABLE 2—IN-STATE VS. COLLECTIVE UPWIND STATE CONTRIBUTIONS a

Site County State 

Average 

design value 

(ppb) 

Maximum 

design value 

(ppb) 

In-state 

contribution 

(ppb) 

Total 

contribution 

from upwind 

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-ofevidence 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

Based on the results of its weight-ofevidence 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

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

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 


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59Wyoming State Implementation Plan, 

Attachment B at 6. 

60 Id. at 6. 

61 Id. at 7. 

62While 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. 

63The 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). 

64The 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. 

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). 

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

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

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. 

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. 


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65August 2018 memorandum at 4. 

66See ‘‘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). 

67We 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. 68See August 2018 memorandum at 4. 

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. 

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. 


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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. 

70These 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. 

71See 81 FR 15200 (March 22, 2016) (proposal); 

81 FR 31513 (May 19, 2016) (final rule; no 

comments received). 

72Site ID 80050002 (Arapahoe), Site ID 80350004 

(Douglas), and Site ID 80590011 (Jefferson). 

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

TABLE 3—WYOMING LINKAGE RESULTS BASED ON THE EPA’S UPDATED 2023 MODELING a 

Receptor ID Location Nonattainment/maintenance 

2023 Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Wyoming 

contribution 

(ppb) 

80350004 ............................... Douglas County, Colorado ... Nonattainment ...................... 71.7 72.3 0.81 

aAccording 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. 

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 upwindstate 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). 

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 


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73As 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. 

74Wyoming SIP submission, Attachment B at 3, 

8. 

transported pollution to be 

substantial.’’) (emphasis added).73

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 

Site ID State County 2023 Avg 

(ppb) 

2023 Max 

(ppb) 

Contribution of 

all upwind 

states 

combined 

(ppb) 

Percent 

contribution of 

all upwind 

states 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 

aBased 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 nonanthropogenic 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. 

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 


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75Wyoming SIP submission, Attachment B at 9. 

76 Id. 

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 attainmentplanning 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 nonanthropogenic 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 nonanthropogenic 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. 

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. 


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77 Id. at 10. 

TABLE 5. OZONE DESIGN VALUES FOR DENVER NONATTAINMENT AREA MONITORS a

AQS Site ID State County 

2014–2016 

Design value 

(ppb) 

2015–2017 

Design value 

(ppb) 

2016–2018 

Design value 

(ppb) 

2017–2019 

Design value 

(ppb) 

2018–2020 

Design value 

(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 

aAccording 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. 

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 


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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. 

79A finding of nationwide scope or effect is also 

appropriate for actions that cover states in multiple 

Continued 

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

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 


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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. 

80The 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. 

1The State of California refers to reactive organic 

gases (ROG) rather than VOC in some of its ozonerelated SIP submissions. As a practical matter, ROG 

and VOC refer to the same set of chemical 

constituents, and for the sake of simplicity, we refer 

to this set of gases as VOC in this proposed rule. 

2See ‘‘Fact Sheet—2008 Final Revisions to the 

National Ambient Air Quality Standards for Ozone’’ 

dated March 2008. 

3The ozone NAAQS promulgated in 1979 was 

0.12 parts per million (ppm) averaged over a 1-hour 

period. See 44 FR 8202 (February 8, 1979). The 

ozone NAAQS promulgated in 1997 was 0.08 ppm 

averaged over an 8-hour period. See 62 FR 38856 

(July 18, 1997). 

4See 73 FR 16436 (March 27, 2008). 

5 Information on the 2015 ozone NAAQS is 

available at 80 FR 65292 (October 26, 2015). 

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

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 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R09–OAR–2018–0535; FRL–9690–01– 

R9] 

Withdrawal and Partial Approval/Partial 

Disapproval of Clean Air Plans; San 

Joaquin Valley, California; 

Contingency Measures for 2008 Ozone 

Standards 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Proposed rule. 

SUMMARY: The Environmental Protection 

Agency (EPA) is proposing to withdraw 

the portion of the March 25, 2019 final 

action conditionally approving state 

implementation plan (SIP) submissions 

from the State of California under the 

Clean Air Act (CAA or ‘‘Act’’) to address 

contingency measure requirements for 

the 2008 ozone national ambient air 

quality standards (NAAQS or 

‘‘standards’’) in the San Joaquin Valley, 

California ozone nonattainment area. 

The SIP revisions include the portions 

of the ‘‘2016 Ozone Plan for the 2008 8- 

Hour Ozone Standard and the 2018 

Updates to the California State 

Implementation Plan’’ that address the 

contingency measure requirement for 

San Joaquin Valley. Simultaneously, the 

EPA is proposing a partial approval and 

partial disapproval of these SIP 

submissions. These proposed actions 

are in response to a decision issued by 

the U.S. Court of Appeals for the Ninth 

Circuit (Association of Irritated 

Residents v. EPA, Ninth Circuit, No. 19– 

71223, opinion filed August 26, 2021) 

remanding the EPA’s conditional 

approval of the contingency measure 

SIP submissions. 

DATES: Written comments must arrive 

on or before June 23, 2022. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–R09– 

OAR–2018–0535 at https://

www.regulations.gov. For comments 

submitted at Regulations.gov, follow the 

online instructions for submitting 

comments. Once submitted, comments 

cannot be edited or removed from 

Regulations.gov. The EPA may publish 

any comment received to its public 

docket. Do not submit electronically 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). For 

additional submission methods, please 

contact the person identified in the FOR

FURTHER INFORMATION CONTACT section. 

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. If you need 

assistance in a language other than 

English or if you are a person with 

disabilities who needs a reasonable 

accommodation at no cost to you, please 

contact the person identified in the FOR

FURTHER INFORMATION CONTACT section. 

FOR FURTHER INFORMATION CONTACT: 

Laura Lawrence, EPA Region IX, (415) 

972–3407, lawrence.laura@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Throughout this document, ‘‘we,’’ ‘‘us,’’ 

and ‘‘our’’ refer to the EPA. 

Table of Contents 

I. Background 

II. Proposed Action and Clean Air Act 

Consequences 

III. Request for Public Comment 

IV. Statutory and Executive Order Reviews 

I. Background 

Ground-level ozone pollution is 

formed from the reaction of volatile 

organic compounds (VOC) and oxides of 

nitrogen (NOX) in the presence of 

sunlight.1 These two pollutants, referred 

to as ozone precursors, are emitted by 

many types of sources, including on-and 

off-road motor vehicles and engines, 

power plants and industrial facilities, 

and smaller area sources such as lawn 

and garden equipment and paints. 

Scientific evidence indicates that 

adverse health effects occur following 

exposure to elevated levels of ozone, 

particularly in children and adults with 

lung disease. Breathing air containing 

ozone can reduce lung function and 

inflame airways, which can increase 

respiratory symptoms and aggravate 

asthma or other lung diseases.2

Under section 109 of the CAA, the 

EPA promulgates NAAQS for pervasive 

air pollutants, such as ozone. The EPA 

has previously promulgated NAAQS for 

ozone in 1979 and 1997.3 In 2008, the 

EPA revised and further strengthened 

the ozone NAAQS by setting the 

acceptable level of ozone in the ambient 

air at 0.075 parts per million (ppm) 

averaged over an 8-hour period.4

Although the EPA further tightened the 

8-hour ozone NAAQS to 0.070 ppm in 

2015, this action relates to the 

requirements for the 2008 ozone 

NAAQS.5

Following promulgation of a new or 

revised NAAQS, the EPA is required 

under CAA section 107(d) to designate 

areas throughout the country as 

attaining or not attaining the NAAQS. 

The EPA classifies ozone nonattainment 

areas under CAA section 181 according 

to the severity of the ozone pollution 

problem, with classifications ranging 

from ‘‘Marginal’’ to ‘‘Extreme.’’ State 

planning and emissions control 

requirements for ozone are determined, 

in part, by the nonattainment area’s 

classification. The EPA designated the 


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