Federal Register / Vol. 87, No. 100 / Tuesday, May 24, 2022 / Proposed Rules 31443 

posted when available at www.osha.gov/ 

stateplans. 

Notice of intention to appear, hearing 

testimony, and documentary evidence: 

You may submit your notice of 

intention to appear, hearing testimony, 

and documentary evidence, identified 

by docket number (OSHA–2021–0012), 

electronically at www.regulations.gov. 

Follow the instructions online for 

electronic submission of materials, 

including attachments. 

Instructions: All submissions must 

include the agency name and docket 

number for this rulemaking (Docket No. 

OSHA–2021–0012). All submissions, 

including any personal information, are 

placed in the public docket without 

change and may be available online at 

www.regulations.gov. Therefore, OSHA 

cautions you about submitting certain 

personal information, such as social 

security numbers and birthdates. For 

additional information on submitting 

notices of intention to appear, hearing 

testimony, or documentary evidence, 

see Section V, ‘‘Public Participation’’, 

and the DATES and ADDRESSES sections 

of the Notice preamble (87 FR 23783, 

23783–84, 23788–89; April 21, 2022). 

Docket: To read or download 

comments, notices of intention to 

appear, and materials submitted in 

response to this Federal Register notice, 

go to Docket No. OSHA–2021–0012 at 

www.regulations.gov. All comments and 

submissions are listed in the 

www.regulations.gov index; however, 

some information (e.g., copyrighted 

material) is not publicly available to 

read or download through that website. 

All comments and submissions are 

available for inspection and, where 

permissible, copying at the OSHA 

Docket Office, U.S. Department of 

Labor; telephone: (202) 693–2350 (TTY 

number: (877) 889–5627). 

Electronic copies of this Federal 

Register document are available at 

www.regulations.gov. This document, as 

well as news releases and other relevant 

information, is also available at OSHA’s 

website at www.osha.gov. 

FOR FURTHER INFORMATION CONTACT:

For press inquiries: Contact Frank 

Meilinger, OSHA Office of 

Communications, U.S. Department of 

Labor; telephone (202) 693–1999; email 

meilinger.francis2@dol.gov. 

For general and technical 

information: Contact Douglas J. 

Kalinowski, Director, OSHA Directorate 

of Cooperative and State Programs, U.S. 

Department of Labor; telephone (202) 

693–2200; email: kalinowski.doug@

dol.gov. 

SUPPLEMENTARY INFORMATION: OSHA 

published a Notice of Proposed 

Reconsideration and Revocation of Final 

Approval of the Arizona State Plan for 

Occupational Safety and Health (Notice) 

on April 21, 2022 (87 FR 23783). This 

Notice requested written comments and 

requests for a hearing by May 26, 2022, 

and notices of intention to appear at the 

public hearing by May 11, 2022; and the 

Notice tentatively scheduled an 

informal public hearing on this proposal 

to begin on August 16, 2022. OSHA is 

extending the deadline for submitting 

notices of intention to appear at the 

hearing by 55 days to July 5, 2022, and 

the deadline for submitting written 

comments and testimony and requests 

for a hearing by 40 days to July 5, 2022. 

OSHA is not changing the tentative 

hearing date; if an informal public 

hearing is held, the hearing will 

commence on August 16, 2022. 

Authority and Signature 

This document was prepared under 

the direction of Douglas L. Parker, 

Assistant Secretary of Labor for 

Occupational Safety and Health, U.S. 

Department of Labor, 200 Constitution 

Avenue NW, Washington, DC. OSHA is 

issuing this notice under the authority 

specified by Section 18 of the 

Occupational Safety and Health Act of 

1970 (29 U.S.C. 667), Secretary of 

Labor’s Order No. 8–2020 (85 FR 58393 

(Sept. 18, 2020)), and 29 CFR parts 

1902, 1952, 1953, 1954, and 1955. 

Signed at Washington, DC, on May 11, 

2022. 

Douglas L. Parker, 

Assistant Secretary of Labor for Occupational 

Safety and Health. 

[FR Doc. 2022–10714 Filed 5–23–22; 8:45 am] 

BILLING CODE 4510–26–P 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R09–OAR–2022–0394; EPA–HQ– 

OAR–2021–0663; FRL–9772–01–R9] 

Air Plan Disapproval; California; 

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 a State Implementation Plan 

(SIP) submittal from California 

addressing interstate transport for the 

2015 8-hour ozone national ambient air 

quality standards (NAAQS). The ‘‘good 

neighbor’’ or ‘‘interstate transport’’ 

provision of the Act 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 

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–R09– 

OAR–2022–0394, by any of the 

following methods: Federal 

eRulemaking Portal at https://

www.regulations.gov following the 

online instructions for submitting 

comments or via email to 

kelly.thomasp@epa.gov. Include Docket 

ID No. EPA–R09–OAR–2022–0394 in 

the subject line of the message. 

Instructions: All submissions received 

must include the Docket ID No. for this 

rulemaking. 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. 

FOR FURTHER INFORMATION CONTACT: Tom 

Kelly, EPA Region IX, 75 Hawthorne St., 

San Francisco, CA 94105. By phone: 

415–972–3856 or by email at 

kelly.thomasp@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Public Participation: Submit your 

comments, identified by Docket ID No. 

EPA–R09–OAR–2022–0394, at https://

www.regulations.gov (our preferred 

method), or the other methods 

identified in the ADDRESSES section. 

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 

EPA’s docket at https://

www.regulations.gov any information 


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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 (DC Cir. 2008). 

4See Federal Implementation Plans: Interstate 

Transport of Fine Particulate Matter and Ozone and 

Correction of SIP Approvals, 76 FR 48208 (Aug. 8, 

2011). 

5Cross-State Air Pollution Rule Update for the 

2008 Ozone NAAQS, 81 FR 74504 (Oct. 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 (DC 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 (DC 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). 

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–R09–OAR–2022–0394 and 

EPA–HQ–OAR–2021–0663. Docket No. 

EPA–R09–OAR–2022–0394 contains 

information specific to California, 

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 that are being used 

to support this action. All comments 

regarding information in either of these 

dockets are to be made in Docket No. 

EPA–R09–OAR–2022–0394. For 

additional submission methods, 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 Tom Kelly, (415) 972–3856, 

kelly.thomasp@epa.gov. For the full 

EPA public comment policy, 

information about CBI or multimedia 

submissions, and general guidance on 

making effective comments, please visit 

https://www.epa.gov/dockets/ 

commenting-epa-dockets. 

The index to the docket for this 

action, Docket No. EPA–R09–OAR– 

2022–0394, 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). 

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 Four 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. SIP Submission Addressing Interstate 

Transport of Air Pollution for the 2015 

8-Hour Ozone NAAQS 

A. Information Provided at Steps 1 and 2 

B. Information Provided at Step 3 

C. Information Provided at Step 4 

III. EPA Evaluation 

A. Evaluation of California Weight of 

Evidence Analysis 

B. Evaluation of Information Provided by 

California Regarding Step 1 and 2 

C. Results of the EPA’s Step 1 and Step 2 

Modeling and Findings for California 

D. Evaluation of Information Provided 

Regarding Step 3 

E. Evaluation of Information Provided 

Regarding Step 4 

F. Tribal Consultation 

G. 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). An interstate SIP 

submission for a new or revised NAAQS 

must contain adequate provisions 

prohibiting any source or other type of 

emissions activity within the state from 

emitting air pollutants in amounts that 

will significantly contribute to 

nonattainment of the NAAQS in another 

state (prong 1) or interfere with 

maintenance of the NAAQS in another 

state (prong 2). 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 Four Step 

Interstate Transport Regulatory Process 

The EPA is using the 4-step interstate 

transport framework (or 4-step 

framework) to evaluate the state’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 


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8More information on the source apportionment 

modeling can be found in the Air Quality Modeling 

Technical Support Document for the 2015 Ozone 

NAAQS Transport SIP Proposed Actions. 

9See 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). 

10 82 FR 1733, 1735. 

11See Information on the Interstate Transport 

State Implementation Plan Submissions for the 

2008 Ozone National Ambient Air Quality 

Standards under Clean Air Act Section 

110(a)(2)(D)(i)(I), October 27, 2017, available in 

docket ID No. EPA–HQ–OAR–2021–0663. 

12See 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 ID No. EPA– 

HQ–OAR–2021–0663. 

13The 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.’’ 

14See 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 (‘‘October 2018 

memorandum’’), available in docket ID No. EPA– 

HQ–OAR–2021–0663. 

15The results of this modeling, as well as the 

underlying modeling files, are included in docket 

ID No. EPA–HQ–OAR–2021–0663. 

16See 85 FR 68964, 68981. 

17See the Air Quality Modeling Technical 

Support Document for the Final Revised Cross-State 

Air Pollution Rule Update, included in the 

Headquarters docket ID No. EPA–HQ–OAR–2021– 

0663. 

18Additional details and documentation related 

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

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

19See Technical Support Document (TSD) 

Preparation of Emissions Inventories for the 2016v2 

North American Emissions Modeling Platform 

included in the Headquarters docket ID No. EPA– 

HQ–OAR–2021–0663. 

20Ramboll Environment and Health, January 

2021, www.camx.com. 

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 

The EPA has performed nationwide 

air quality modeling to project ozone 

design values that are used in 

combination with measured data to 

identify nonattainment and 

maintenance receptors. To quantify the 

contribution of emissions from specific 

upwind states to 2023 ozone design 

values for the identified downwind 

nonattainment and maintenance 

receptors, the EPA performed 

nationwide, state-level ozone source 

apportionment modeling. 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 states and other sources.8

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.9

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.10 On October 27, 2017, we 

released a memorandum using the ‘‘en’’ 

emissions inventory (‘‘October 2017 

memorandum’’) containing updated 

modeling data for 2023, which 

incorporated changes made in response 

to comments on the NODA, and noted 

that the modeling may be useful for 

states developing SIPs to address 

interstate transport obligations for the 

2008 ozone NAAQS.11 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.12 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 interstate transport 

framework.13 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 standard at 

Step 1 of the 4-step interstate transport 

framework.14

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.15 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.16 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.17

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 18 and 

updated emissions projections for 

electric generating units (EGUs) that 

reflect the emissions reductions from 

the Revised CSAPR Update, recent 

information on plant closures, and other 

sector trends. The construct of the 

updated emissions platform, 2016v2, is 

described in the emissions modeling 

technical support document (TSD) for 

this proposed rule.19 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 7.10.20 The EPA now 

proposes to primarily rely on modeling 

based on the updated and newly 


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21March 2018 memorandum, Attachment A. 

22 Id. at A–1. 

23 Id. 

24For 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). 

25We 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 

available 2016v2 emissions platform in 

evaluating these submissions with 

respect to Steps 1 and 2 of the 4-step 

interstate transport framework and 

generally 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.C. 

of this notice and the Air Quality 

Modeling TSD for 2015 Ozone NAAQS 

Transport SIP Proposed Actions, 

included in Docket ID No. EPA–HQ– 

OAR–2021–0663 for this proposal, 

contain additional detail on the EPA’s 

2016v2 modeling. In this notice, the 

EPA is accepting public comment on 

this updated 2023 modeling, which uses 

the 2016v2 emissions platform. 

Comments on the EPA’s air quality 

modeling should be submitted in the 

Regional docket for this action, docket 

ID No. EPA–R09–OAR–2022–0394. 

Comments are not being accepted in 

docket ID No. EPA–HQ–OAR–2021– 

0663. 

States may have chosen to rely on the 

results of 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.A. and III.B. we evaluate how 

California 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 submittals for 

the 2015 8-hour ozone NAAQS. These 

policy judgments reflect consistency 

with relevant case law and past agency 

practice as reflected in the CSAPR and 

related rulemakings. Nationwide 

consistency in approach is particularly 

important in the context of interstate 

ozone transport, which is a 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. 

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.21 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.’’ 22 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.’’ 23 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 submittals, 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).24 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 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, ‘‘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,25 which is now the 


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and 2 of the interstate transport 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. 

26See 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 Aug. 3, 2018). 

27See North Carolina v. EPA, 531 F.3d at 910– 

11 (holding that the EPA must give ‘‘independent 

significance’’ to each prong of CAA section 

110(a)(2)(D)(i)(I)). 28See 81 FR 74504 (October 26, 2016). This same 

concept, relying on both current monitoring data 

and modeling to define nonattainment receptors, 

was also applied in CAIR. See 70 FR 25162, 25241 

and 25249 (January 14, 2005); see also North 

Carolina, 531 F.3d at 913–14 (affirming as 

reasonable EPA’s approach to defining 

nonattainment in CAIR). 

29See 76 FR 48208 (August 8, 2011). CSAPR 

Update and Revised CSAPR Update also used this 

approach. See 81 FR 74504 (October 26, 2016) and 

86 FR 23054 (April 30, 2021). 

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, 2024.26 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 at 23054, 23074; see 

also Wisconsin, 938 F.3d at 322. 

Consequently, in this proposal the EPA 

will use the analytical year of 2023 to 

evaluate each state’s CAA section 

110(a)(2)(D)(i)(I) SIP submission with 

respect to the 2015 8-hour ozone 

NAAQS. 

2. Step 1 of the 4-Step Interstate 

Transport Framework 

In Step 1, the EPA identifies 

monitoring sites that are projected to 

have problems attaining and/or 

maintaining the NAAQS in the 2023 

analytic year. Where the EPA’s analysis 

shows that a site does not fall under the 

definition of a nonattainment or 

maintenance receptor, that site is 

excluded from further analysis under 

the EPA’s 4-step interstate transport 

framework. For sites that are identified 

as a nonattainment or maintenance 

receptor in 2023, we proceed to the next 

step of our 4-step interstate transport 

framework by identifying the upwind 

state’s contribution to those receptors. 

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.27

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).28

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).29 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), 


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30As examples of general approaches for how 

such an analysis could be conducted for their 

sources, states could look to the CSAPR Update, 81 

FR 74504, 74539–51; CSAPR, 76 FR 48208, 48246– 

63; CAIR, 70 FR 25162, 25195–229; or the NOX SIP 

Call, 63 FR 57356, 57399–405. See also Revised 

CSAPR Update, 86 FR 23054, 23086–23116. 

Consistently across these rulemakings, the EPA has 

developed emissions inventories, analyzed different 

levels of control stringency at different cost 

thresholds, and assessed resulting downwind air 

quality improvements. 

31Letter dated October 1, 2018, from Richard W. 

Corey, Executive Officer, CARB, to Michael Stoker, 

Regional Administrator, EPA Region IX. 

32 86 FR 16533 (March 30, 2021). 

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 

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 still the result of the 

collective impacts of contributions from 

many upwind states. Therefore, 

application of a consistent contribution 

threshold is necessary to identify those 

upwind states that should have 

responsibility for addressing their 

contribution to the downwind 

nonattainment and maintenance 

problems to which they collectively 

contribute. Continuing to use 1 percent 

of the NAAQS as the screening metric 

to evaluate collective contribution from 

many upwind states also allows 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 74504, 74518. See 

also 86 FR 23054, 23085 (reviewing and 

explaining rationale from CSAPR, 76 FR 

48,208, 48237–38, for selection of 1 

percent threshold). 

The EPA’s August 2018 memorandum 

recognized that in certain 

circumstances, a state may be able to 

establish that an alternative contribution 

threshold of 1 ppb is justifiable. Where 

a state relies on this alternative 

threshold, and where that state 

determined that it was not linked at 

Step 2 using the alternative threshold, 

the EPA will evaluate whether the state 

provided a technically sound 

assessment of the appropriateness of 

using this alternative threshold based on 

the facts and circumstances underlying 

its application in the 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. In 

general, the emissions-reducing effects 

of all existing emissions control 

requirements are already reflected in the 

air quality results of the modeling for 

Steps 1 and 2. If the state is shown to 

still be linked to one or more downwind 

receptor(s), states must provide a 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.30

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. SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 8-Hour Ozone NAAQS 

In California, the California Air 

Resources Board (CARB or ‘‘State’’) is 

the state agency responsible for the 

adoption and submission to the EPA of 

California state, county, and local SIPs 

and SIP revisions. CARB submitted its 

infrastructure SIP revision (‘‘2018 

Infrastructure SIP,’’ ‘‘California’s 2018 

Submittal,’’, or ‘‘2018 Submittal’’) for 

the 2015 ozone NAAQS on October 1, 

2018.31 In 2021, the EPA finalized 

action on most of the ‘‘infrastructure’’ 

requirements in that submittal but did 

not act on the interstate transport 

requirements of 110(a)(2)(D)(i)(I).32 We 

are proposing action on the interstate 

transport portions of California’s 2018 

Submittal in this action. 


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33California’s 2018 Submittal, p. A4–10. 

34 Id. at A4–11, Table 1. 

35 Id. at A4–10. 

36 Id. 

37 Id. at A4–13. 

38 Id. 

39 Id. at A4–13–A4–14. 

40 Id. at A4–17. 

41 Id. at A4–17–A4–18. 

42 Id. at A4–18. 

43 Id. 

44 Id. 

A. Information Provided at Steps 1 

and 2 

Enclosure 4 of California’s 2018 

Submittal contains the state’s ‘‘Good 

Neighbor State Implementation Plan.’’ 

For Steps 1 and 2 of its four-step 

analysis, California reviewed the results 

of the EPA’s modeling runs released 

with the January 2017 NODA, the 

October 2017 memorandum, and the 

March 2018 memorandum.33 CARB 

presented modeled design values for 

monitoring sites that the modeling 

released with the January 2017 NODA 

or the October 2017 memoranda 

projected to be nonattainment or 

maintenance receptors in 2023, using 

the EPA’s definition of those terms.34

CARB explained it focused its 

evaluation on receptor sites in Colorado 

and Arizona because those were the 

western states, other than California, 

where the EPA’s modeling identified 

nonattainment or maintenance sites in 

2023 using the data in the March 2018 

memorandum.35 Accordingly, CARB 

identified the following receptor sites 

and modeled design values, noting that 

the EPA’s modeling released with the 

October 2017 memorandum and March 

2018 memorandum yielded the same 

design values for 2023.36

TABLE 1—MODELED RECEPTOR DESIGN VALUES IN WESTERN STATES IN CALIFORNIA’S 2018 SUBMITTAL

Site County AQS No. 

January 2017 

modeling-average 2023 

DV 

(ppb) 

January 2017 

modeling-maximum 2023 

DV 

(ppb) 

October 2017 

modeling-average 2023 

DV 

(ppb) 

October 2017 

modeling-maximum 2023 

DV 

(ppb) 

Colorado 

Chatfield .............................. Douglas .............................. 08–035–0004 69.6 71.6 71.1 73.2 

Rocky Flats North ............... Jefferson ............................. 08–059–0006 70.5 72.9 71.3 73.7 

NREL ................................... Jefferson ............................. 08–059–0011 69.7 72.7 70.9 73.9 

Fort Collins West ................ Larimer ............................... 08–069–0011 68.6 70.4 71.2 73.0 

Highland Reservoir ............. Arapahoe ............................ 08–005–0002 68.0 70.0 69.3 71.3 

Weld Co. Tower .................. Weld ................................... 08–123–0009 67.2 68.3 70.2 71.4 

Arizona 

West Phoenix ...................... Maricopa ............................. 04–013–0019 67.9 70.0 69.3 71.4 

North Phoenix ..................... Maricopa ............................. 04–013–1004 68.7 69.8 69.8 71.0 

Source: California’s 2018 Submittal, p. A4–11, Table 1. 

To ‘‘assess the potential for transport 

impacts from California to Colorado 

receptors,’’ CARB identified geographic 

and meteorologic features of the Denver 

Metro/North Front Range nonattainment 

area.37 Geographic features identified by 

CARB included the Front Range, 

extending up to 8,000 feet in elevation 

on the western side of the metropolitan 

area, as well as mountains on the 

southern and southeastern end of the 

area. CARB noted that both form 

barriers to air flow.38 CARB also notes 

that to the east and north of the Denver 

area are gradually rising hills that 

generally are open to airflow with the 

Denver area. Meteorological conditions 

identified by CARB include sunlight, 

temperature and winds conducive to 

formation of ozone, and diurnal 

recirculation that allows emissions and 

ozone concentrations to build up over 

multi-day periods.39 CARB additionally 

notes that this terrain, combined with 

unique atmospheric conditions and high 

temperatures during the summer 

months, are highly conducive to the 

accumulation of local emissions and the 

formation of ozone in the DenverAurora-Lakewood Core Based Statistical 

Area, and, additionally, allows for 

ozone concentrations to remain higher 

for more hours, leading to higher 8-hour 

averages at monitoring sites. 

CARB notes additional local features 

that contribute to high ozone 

concentrations in the Denver 

nonattainment area, including upslope 

and downslope flow in the foothill 

regions on broad high-pressure days 

near several violating monitoring sites. 

CARB also claims that wildfires had an 

impact on ozone concentrations in the 

Denver area, specifically noting that on 

September 4, 2017, large wildfire events 

across five western states brought 

plumes of smoke southward along the 

Front Range, inevitably mixing with the 

surface, and as a result, elevated ozone 

concentrations were observed at the 

Rocky Flats North site, reaching 0.078 

ppm 8-hour average ozone 

concentrations. 

CARB conducted a trajectory analysis 

from California to Colorado using the 

National Oceanic and Atmospheric 

Administration (NOAA) Hybrid Single 

Particle Lagrangian Integrated Trajectory 

(HYSPLIT) model.40 CARB found that 

only two percent of nearly 500 

backward trajectories from Colorado 

receptor sites on high ozone days in 

June and July (initiated from 10, 1000, 

and 2000 meters above ground level 

with a duration of 96 hours) indicated 

air parcels may have traveled from a 

mixed layer within California, where 

pollutants become well dispersed, to a 

mixed layer at the Colorado receptor 

sites.41 CARB also found only one 

forward trajectory (starting at 10 meters 

above the ground with a duration of 96 

hours) from the mixed layer in 

California reached the mixed layer at a 

Colorado receptor site.42According to 

CARB, this suggests that the complexity 

of the physical environment between 

California and Colorado limits the 

reproducibility of modeled transport 

and that considerable multi-faceted 

analyses would be necessary to explore 

transport mechanisms through areas of 

complex terrain.43

CARB’s conclusion based on the 

HYSPLIT trajectories was that upperlevel air was almost always above the 

mixed layer over California, Colorado, 

or both.44 Without vertical mixing of the 

air between the mixed layer near the 

ground and the upper level, CARB 


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45 Id. 

46 Id. at A4–18–A4–19. 

47 Id. at A4–19. 

48 Id. at A4–23. 

49 Id. at A4–23. See also March 2018 

memorandum, Attachment A at A–2. 

50California’s 2018 Submittal at A4–23–A4–24 

(citing 83 FR 14807 (April 6, 2018) and 83 FR 31068 

(July 3, 2018)). 

51 Id. at A4–23. 

52 Id. at A4–23–A4–24. 

53 Id. at A4–24. 

54 Id. 

55 Id. at A4–25 

56 Id. at A4–26. 

57 Id. A4–26. 

58 Id. 

59 Id. at A4–27. 

60 Id. at A4–23. 

61 Id. at A4–29. 

62 Id. At A4–29—A4–33 

63 Id. at A4–31–A4–32. 

64 Id. at A4–32–A4–33. 

65 Id. at A4–35. 

66 Id. 

67 Id. at A4–35. 

asserted, little to no impact from 

transport of emissions or pollutants 

would be expected at the surface.45

CARB concluded that transport from 

California emissions sources to 

Colorado is possible but extremely 

unlikely on high ozone days at the 

Colorado receptor sites identified by the 

EPA modeling.46 CARB also noted that 

this conclusion appeared consistent 

with Colorado’s weight of evidence 

(WOE) analysis for its Denver Metro/ 

North Front Range attainment plan SIP 

submittal for the 0.075 ppm 8-hour 

ozone standard.47

CARB also attempted to rely on one 

of the preliminary ‘‘potential 

flexibilities’’ in Attachment A to the 

EPA’s March 2018 memorandum to 

evaluate California’s impacts on 

receptors in Colorado.48 CARB cited the 

idea of ‘‘consider[ing] removal of certain 

data from modeling analysis for the 

purposes of projecting design values 

and calculating the contribution metric 

where data removal is based on model 

performance and technical analyses 

support the exclusion.’’ 49 In making use 

of this potential flexibility, CARB used 

exceptional event data from Colorado’s 

‘‘weight of evidence’’ (WOE) analysis 

included in its Denver Metro/North 

Front Range attainment SIP submittal 

for the 0.075 ppm 8-hour ozone 

standard.50 CARB asserted that, 

although Colorado did not submit 

formal demonstrations for these events 

under the Exceptional Event Rule 

because they did not affect design 

values in the area’s attainment year, the 

EPA concurred with Colorado’s 

assessment that the model Colorado 

used for its Denver Metro/North Front 

Range attainment SIP submittal was 

properly configured, met EPA 

performance requirements, and was 

appropriately used in its application.51

To recalculate projected design values 

excluding exceptional event data 

flagged by Colorado, CARB first 

calculated design values from prior 

years for the four monitors analyzed in 

Colorado’s Denver Metro/North Front 

Range attainment SIP submittal for the 

2008 8-hour ozone standard of 0.075 

ppm (Chatfield, Rocky Flats North, 

NREL, and Fort Collins West) as well as 

for two additional monitors: Highland 

Reservoir and Weld Co. Tower.52 CARB 

found that the average design values for 

the years 2009–2011, 2010–2012, and 

2011–2013 dropped by 1–2 ppb at all 

six sites when data flagged by Colorado 

was excluded.53 CARB found that 

maximum base year design values 

dropped by 2–3 ppb.54

CARB also attempted to replicate 

Colorado’s future design value 

calculations from Colorado’s WOE 

analysis to be consistent with 

Colorado’s approach by using the ‘‘el’’ 

version of the emissions inventory.55

CARB excluded Colorado’s flagged 

events to recalculate design values for 

future year modeling for 2023 based on 

EPA’s Good Neighbor SIP modeling 

released in the January 2017 NODA 

(using the ‘‘el’’ emissions inventory).56

CARB calculated that average design 

values at the six sites dropped by 1 to 

2 ppb, and maximum design values 

dropped by 2 ppb.57 CARB found this 

resulted in no nonattainment or 

maintenance receptors being projected 

in Colorado in 2023.58

CARB repeated the same process with 

EPA’s Good Neighbor SIP modeling 

released in the October 2017 

memorandum using the ‘‘en’’ emissions 

inventory.59 CARB found that excluding 

data flagged by Colorado reduced the 

2023 average design values by 0–2 ppb 

and the maximum design values by 1– 

2 ppb. CARB asserted that its approach 

was consistent with Attachment A to 

the March 2018 memorandum with 

respect to ‘‘collaboration among states 

linked to a common receptor and among 

linked upwind and downwind states in 

developing and applying a regionally 

consistent approach.’’ 60

CARB analyzed the projected design 

values in the October 2017 

memorandum and March 2018 

memorandum in order to understand 

why the 2023 design values in Colorado 

were higher in the latter modeling 

compared to the former.61 CARB noted 

that ‘‘receptors’’ in Colorado based on 

the January 2017 modeling were clean 

in 2023 after removing atypical events, 

but some of these sites became receptors 

in the March 2018 modeling even after 

atypical events were removed in the 

projection of 2023 design values. CARB 

performed an analysis of contributions 

and emissions to determine why four 

monitors in Denver changed from being 

clean (after removing atypical events) in 

the older January 2017 modeling to 

maintenance-only receptors (after 

removing atypical events) using the 

March 2018 modeling. CARB concluded 

that higher emissions, and therefore 

higher contributions, from Colorado 

sources were the primary reason why 

the four monitors changed from clean in 

the January 2017 modeling to 

maintenance-only in the March 2018 

modeling. CARB used this result to 

argue that emissions in Colorado, not 

California, are responsible for the 

projected maintenance problem at these 

four receptors.62 CARB also noted that 

differences in EPA’s methodology for 

calculating average contributions at 

individual monitors between the 

January 2017 NODA and the March 

2018 memorandum could have 

contributed to the receptor changes that 

CARB identified in its submittal. CARB 

further noted that the updated 

emissions inventory (‘‘en’’) would not 

have accounted for Colorado’s planned 

controls on Colorado’s nonpoint source 

emissions from oil and gas and therefore 

overstated those emissions.63

Ultimately, CARB decided it was 

more appropriate to rely on the ‘‘en’’ 

emissions inventory-based modeling 

released with the October 2017 

memorandum, stating that this would be 

the more conservative approach.64

To analyze receptors in, and 

California’s contributions to, Arizona, 

CARB first noted that the EPA’s 

modeling released with the October 

2017 memorandum identified the West 

Phoenix and North Phoenix monitoring 

sites in the Phoenix-Mesa 

nonattainment area as potential 

maintenance receptors, while the earlier 

version of the modeling released with 

the January 2017 NODA did not project 

any receptors in Arizona.65 CARB 

identified geographic features of the 

Phoenix-Mesa nonattainment area, 

including the Sierra Estrella Mountains 

to the southwest, the White Tank 

Mountains to the west, the Bradshaw 

Mountains to the north and northeast, 

the Superstition Mountains to the east, 

and the South Mountains to the south.66

CARB explained this ‘‘topographic 

bowl’’ significantly limits air flow 

during non-stormy periods.67 CARB also 

identified meteorological factors 

affecting the Phoenix area, such as 

upper-level high pressure systems over 


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68 Id. at A4–35–A4–36. 69 Id. at A4–26. 70 Id. at A4–39. 71 Id. 72 Id. at A4–40. 73 Id. A4–40–A4–41. 74 Id. at A4–42. 75 Id. at A4–43. 76 Id. 77 Id. at A4–44. 78 Id. 79 Id. 80 Id. 

81 Id. at A4–45. 

82 Id. at A4–46. 

83 Id. at A4–46–A4–56. 

84 Id. at A4–47–A4–48. 

the western U.S. that produce high 

temperatures, limit cloud formation, 

and generally lead to light winds.68

Additional meteorological factors 

identified by CARB included cooling in 

the evening that brings emissions and 

pollutants back to the metropolitan area 

at night, the summer monsoon, 

temperature inversions outside of 

monsoon season, atmospheric mixing 

heights of several thousand feet on hot 

afternoons, and ‘‘dry’’ thunderstorms 

that may ignite wildfires.69

CARB also conducted a simplified, 

short-ranged trajectory analysis.70 CARB 

viewed backward HYSPLIT model 

trajectories for the two receptor sites in 

Arizona to evaluate the potential for 

transport of ozone or ozone precursors 

from California. CARB concluded that 

air is typically from within the Phoenix 

area for trajectories at 100 and 500 

meters above ground level and that 

trajectories at 1000 meters are most 

frequently from the north-northeast, 

southeast, or southwest. CARB 

interpreted its analysis to suggest air 

from California was unlikely to be a 

significant factor contributing to higher 

ozone values in Phoenix.71

CARB then compared the two 

receptor sites’ 2023 projections from the 

two versions of transport modeling that 

EPA released in the January 2017 NODA 

(‘‘el’’ emissions inventory) and the 

October 2017 memorandum (‘‘en’’ 

emissions inventory). CARB noted that 

the design values at the two Arizona 

sites increased by 1–2 ppb from the 

earlier to the later version of the EPA’s 

modeling, and that based on the 

contribution data included in the March 

2018 memorandum, Arizona’s own 

contribution increased by 1.8–2.2 ppb.72

CARB concluded that the difference in 

Arizona’s design values appeared to be 

mainly driven from Arizona’s own 

contributions, and that the balance of 

difference between Arizona’s 

contributions and the changes to the 

design values came from emissions 

categorized as ‘‘Other’’, which included 

emissions from Canada and Mexico.73

Specifically, CARB noted that fire 

impacts increased at both sites and that 

international contributions increased at 

the West Phoenix site.74 At both sites, 

CARB explained that the home state’s 

contribution grew by more than the 

amount necessary to make the site a 

maintenance receptor in modeling 

released with the October 2017 

memorandum (using the ‘‘en’’ emissions 

inventory).75 CARB also noted that, 

while Arizona’s contribution increased 

from the modeling released with the 

January 2017 NODA (‘‘el’’ emissions 

inventory) to the modeling results 

released with the October 2017 

memorandum (‘‘en’’ emissions 

inventory), California’s contribution 

decreased at both monitoring sites 

between the same versions.76

CARB decided to give more weight to 

the modeling results released with the 

October 2017 memorandum (‘‘en’’ 

emissions inventory),77 which indicated 

that California contributes above 1 

percent of the NAAQs to two 

maintenance receptors in Arizona. 

However, CARB concluded that the 

differences between the modeling 

results based on the ‘‘el’’ and ‘‘en’’ 

emissions inventories resulting in 

increased modeled design values for the 

two maintenance receptors in Arizona 

were not due to increased contributions 

from California.78 Based on this 

information, CARB found it ‘‘reasonable 

to conclude that emissions from 

California do not significantly interfere 

with attainment/maintenance of the 

0.070 ppm 8-hour ozone NAAQS at the 

modeled ozone receptors in Arizona.’’ 79

Overall, California’s conclusion for 

Step 2 was that it is linked to downwind 

sites in Colorado and Arizona, based on 

the EPA modeling results released with 

the October 2017 memorandum.80

However, based on the additional 

analyses CARB provided at Step 2 (and 

further analysis of emissions control 

measures provided at Step 3 of their 

submittal), CARB concludes that 

California does not contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in other states. 

B. Information Provided at Step 3 

For Step 3, CARB reviewed and 

evaluated California’s emissions control 

measures.81 CARB noted that, based on 

the 2011 National Emissions Inventory 

(NEI) and based on 2023 projected 

emissions, a NOX control strategy would 

be most effective for reducing regional 

scale ozone transport.82 Table 2 below 

shows California emissions in 2011 and 

2023 by sector and percentage. 

TABLE 2—CALIFORNIA EMISSIONS IN 2021 AND 2023 BY SECTOR AND PERCENTAGE

Modeled emissions by sector 

NOX VOCs 

Mobile Stationary ≤Area Mobile Stationary Area 

Percent of 2011 NEI Emissions ............... 78.4 11.2 10.4 34.8 6.5 58.7 

Percent of 2023 Projected Emissions ..... 67.1 26.9 6.0 28.6 29.3 42.1 

Source: California’s 2018 Submittal, p. A4–46, Table 31. 

CARB noted that a NOX-focused 

approach is consistent with the EPA’s 

historical focus for transport control 

measures, then summarized its controls 

for mobile sources, stationary sources, 

and consumer products.83

CARB summarized its NOX controls 

for mobile sources, including the Smog 

Check program, low emission vehicle 

fleet standards and zero emission 

vehicle regulation, and California’s 

reformulated gasoline standard. CARB 

also described programs to reduce NOX

emissions from heavy-duty vehicles by 

nearly 70 percent by 2023 and from offroad equipment by 45 percent by 2031. 

In the South Coast Air Quality 

Management District, CARB stated that 

mobile source control programs are 

projected to reduce NOX emissions by 

153 tons per day (tpd) in 2023 and by 

184 tpd by 2031. CARB also noted that 

the federal government has primary 

regulatory authority over mobile sources 

such as ocean-going vessels, aircraft, 

and locomotives.84

CARB also described NOX controls for 

stationary sources, noting that 

California’s 35 air districts have primary 

authority over those sources. CARB 


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85 Id. at A4–48–A4–53. 86 Id. at A4–54–A4–55. 87 Id. at A4–54. 88 Id. at A4–54–A4–55. 89 Id. at A4–55. 90 Id. 91 Id. 92 Id. 

93 Id., p. A4–55 94 Id. at A4–56. 95 Id., p. A4–57 96 Id. 97 Id. at A4–58–A4–69. 98 Id. at A4–59. 99 Id. at A4–61–A4–67. 

100 Id. at A4–63. 

101 Id. at A4–67–A4–69. 

102 Id. at A4–68. 

103 Id. at A4–72–A4–73. 

104 Id. at A4–73–A4–74. 

provided examples of prohibitory rules 

for NOX and VOC already approved into 

the California SIP. These included rules 

controlling VOC emissions from 

Graphic Arts Operations in the Ventura 

County Air Pollution Control District 

(APCD), Placer County APCD, and San 

Diego County APCD. Other examples 

included rules controlling NOX

emissions from Natural Gas-Fired FanType Central Furnaces and Small Water 

Heaters in Santa Barbara APCD, from 

Gasoline Transfer and Dispensing in the 

South Coast Air Quality Management 

District, from Natural Gas-Fired Water 

Heaters, Small Boilers, and Process 

Heaters in Placer County APCD, and 

from Large Water Heaters and Small 

Boilers in Ventura County APCD. 

Separately, CARB described California’s 

consumer product control program to 

regulate reactive organic gas emissions, 

a subset of VOCs.85

After summarizing these controls, 

CARB provided information about its 

electric generating units (EGUs) and 

non-EGU stationary sources.86 Noting 

that the EPA has historically targeted 

EGUs for reductions in ozone transport 

pollution, CARB stated that the only 

two EGUs in the state emitting NOX at 

rates higher than 0.061 lb/MMBtu are 

either ‘‘unlikely’’ to have further costeffective emission control opportunities 

or planned to retire by the end of 

2019.87 CARB also explained that the 

only EGU emitting more than 250 tpy 

NOX in 2011 ceased operation in 2014, 

and that two EGUs emitting over 100 

tpy NOX in the San Joaquin Valley 

APCD ceased operation in 2011.88

CARB also noted that in 2016, the 

EPA assessed further NOX reductions 

from EGUs and that the CSAPR Update 

resulted in a cost threshold of $1400 per 

ton.89 CARB stated that the EPA’s 

analysis showed ozone season EGU NOX

reductions in California would not 

occur until the $5000 per ton emissionscontrol scenario.90 CARB concluded 

that due to ‘‘strict and comprehensive 

emissions regulations on emissions, 

EGUs do not appreciably contribute to 

NOX such that the emissions could 

significantly contribute to ozone 

formation in another state.’’ 91

For non-EGUs, CARB noted that, 

although they emitted 6.7 times as much 

NOX as EGUs did in 2011 in California, 

they only represented 5.2 percent of the 

statewide NOX inventory.92 CARB 

concluded that, for the large non-EGU 

sources that are either subject to NOX

control measures that have not been 

submitted for approval into the 

California SIP, or fall outside the 

geographic jurisdiction of the applicable 

district rules, further emission controls 

would be unlikely to reduce any 

potential impact on downwind states’ 

air quality because such sources 

comprise no more than 0.8 percent of 

the total NOX emitted in California in 

2011.93 CARB also highlighted its 

consumer product control program, 

which regulates reactive organic gas.94

CARB’s Step 3 conclusion was that 

‘‘the State’s emission reduction control 

system leads the nation in stringency for 

most sectors of emission sources’’ and 

that ‘‘California’s emission reduction 

programs adequately prohibit the 

emission of air pollutants in amounts 

that will significantly contribute to 

nonattainment, or interfere with 

maintenance, of the 0.070 ppm 8-hour 

ozone standard in any downwind 

state.’’ 95

C. Information Provided at Step 4 

For its Step 4 analysis, CARB stated, 

‘‘Although linked to other western states 

with projected air quality problems in 

2023, California is not significantly 

contributing to nonattainment or 

maintenance problems in any other 

states. This is in large part due to the 

stringency of California’s air pollution 

control program. Therefore, no further 

reductions or measures are necessary for 

Good Neighbor SIP purposes.’’ 96

CARB then provided a weight of 

evidence (WOE) analysis. The WOE 

analysis purported to ‘‘describe the U.S. 

EPA’s contribution modeling when 

grouping upwind states’ contributions,’’ 

asserted that transport relationships 

among eastern and western states are 

different, and argued that the role of 

interstate transport in western states is 

a very small portion of projected design 

values and that the collective impact of 

all upwind states is also a small 

portion.97 CARB also asserted that 

previous rounds of EPA’s 

photochemical modeling identified 

smaller collective contribution for 

western states than eastern states.98

CARB’s WOE analysis further 

described differences it claims exist 

between transport in eastern and 

western states.99 Differences asserted by 

CARB included large populations in 

eastern states, the relatively small size 

of eastern states and consequent high 

population density, and numerous 

metropolitan areas in eastern states that 

cross state boundaries. CARB also noted 

the complex topography in western 

states, which presents a challenge to air 

quality modeling, as well as the relative 

distances between nonattainment areas 

and emissions sources in western states 

and the larger overall sizes of western 

states compared to eastern states.100

CARB continued its WOE analysis by 

reiterating that California has little 

impact on ozone levels outside its 

borders, specifically because of the 

distance between California’s eastern 

border and its emissions sources, as 

well as because of the Sierra Nevada 

mountains, which limit airflow from 

California to the east and which are 

sparsely populated.101 CARB’s WOE 

analysis concluded with a per-capita 

NOX emissions comparison across the 

states, in which California ranked nearly 

last (even though it ranked second 

highest in total NOX emissions (after 

Texas)), and stated, ‘‘U.S. EPA’s 

modeling of state contributions bears 

out the expectation that California’s 

impacts on other states would be very 

small.’’ 102

In summary, California’s 2018 

submittal concluded that, while 

California is linked to receptors in 

Arizona and Colorado with projected air 

quality problems in 2023, California is 

not significantly contributing to 

nonattainment or maintenance problems 

in any other states.103 Further, CARB 

asserted that its emissions reduction 

programs adequately prohibit the 

emission of air pollutants for transport 

purposes, and that California has 

already adopted and implemented 

permanent and enforceable measures of 

sufficient stringency to ensure that the 

state does not contribute significantly to 

ozone nonattainment or maintenance 

problems in downwind states.104

III. EPA Evaluation 

The EPA is proposing to find that 

California’s 2018 SIP Submittal 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 

based on the EPA’s evaluation of the SIP 

submission using the 4-step interstate 

transport framework, and the EPA is 


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105 Id. at A4–62. 

106 81 FR 74503, 74523. 

107 Id. 

108See 81 FR 31513 (May 19, 2016) (Arizona); 83 

FR 65093 (December 19, 2018) (California); 85 FR 

26361 (May 4, 2020) (New Mexico); 81 FR 71991 

(Oct. 19, 2016) (Utah prong 2); 82 FR 9155 

(February 3, 2017) (Utah prong 1); 84 FR 14270 

(April 10, 2019) (Wyoming). 

109 83 FR 5381 (February 7, 2018). See also 82 FR 

9155, 9157 (February 3, 2017). 

110See 84 FR 3389, 3391 (Feb. 12, 2019). See also 

81 FR 71991, 71994–95 (Oct. 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). 

111 Id. at A4–11 and Table 1. 112 Id. at A4–33. A4–34, A4–36. 113As explained in Section I, the October 2017 

memorandum provided projected ozone design 

values for 2023. The data released in the March 

2018 memorandum built off the information 

provided in the October 2017 memorandum by 

including contribution data to assist states in the 

development of their interstate transport SIPs for 

the 2015 ozone NAAQS. 114 Id. at A4–44. 115 Id. 

therefore proposing to disapprove 

California’s 2018 SIP Submittal. 

A. Evaluation of California Weight of 

Evidence Analysis 

As an initial matter, the EPA will 

address CARB’s ‘‘WOE analysis’’ that 

included the statement: ‘‘[b]y not 

promulgating a version of the CSAPR in 

the West, U.S. EPA could be viewed as 

tacitly acknowledging a disparity in the 

significance of interstate transport of 

ozone between western and eastern 

states.’’ 105 That is an incorrect 

interpretation. The EPA took comment 

on including western states in the 

CSAPR Update, but did not finalize due 

to the possibility that ‘‘there may be 

additional factors to consider in the 

EPA’s and state’s evaluations’’ such as 

unspecified ‘‘geographically specific 

factors[.]’’ 106 The EPA stated explicitly 

that even though no western state was 

included in the CSAPR Update, 

‘‘western states are not relieved of their 

statutory obligation to address interstate 

transport under the [CAA] section 

110(a)(2)(d)(i)(I)’’ and that the ‘‘EPA and 

western states, working together, are 

continuing to evaluate interstate 

transport obligations on a case-by-case 

basis.’’ 107

While the EPA has in limited 

circumstances found unique issues 

associated with addressing ozone 

transport in western states, the EPA has 

consistently applied the 4-step transport 

framework in western states and has 

identified ozone transport problems in 

the west that are similar to those in the 

east.108 For example, in a prior action 

addressing California’s interstate 

transport obligations for the 2008 ozone 

NAAQS, the EPA concluded that ‘‘the 

collective contribution of emissions 

from upwind states represent a 

considerable portion of the ozone 

concentrations at the maintenance 

receptors in the Denver area.’’ 109

Similarly, the EPA’s view in acting on 

Wyoming and Utah’s 2008 ozone 

NAAQS SIP submittals 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 percent), comparable to some 

eastern receptors . . . .’’ 110

The remaining discussion in CARB’s 

WOE analysis is of limited relevance to 

the question of assessing interstate 

ozone transport. CARB’s description of 

western geography, settlement patterns, 

early American expansionist policy, and 

historically low population density do 

not overcome the fundamental 

conclusion from each successive round 

of EPA modeling, discussed below: 

California contributes more than 1 

percent of the 2015 ozone NAAQS to 

multiple nonattainment and/or 

maintenance receptors in other states. 

As explained in further detail below, the 

EPA has examined the reliability of its 

nationwide modeling for characterizing 

ozone transport in the west and finds 

that the modeling is reliable. The 

remainder of CARB’s analysis at Steps 3 

and 4 is not approvable. CARB did not 

adequately evaluate additional 

emissions control opportunities to 

support its conclusion that emissions 

from sources in California do not 

significantly contribute to 

nonattainment or interfere with 

maintenance in other States. The EPA 

acknowledges that California may have 

one of, if not the, most stringent 

emissions control strategies in the 

country, but the state remains obligated 

to analyze additional control 

opportunities once a linkage has been 

established at Steps 1 and 2. Finally, 

while the EPA approved California’s 

transport SIP submittal for the 2008 

ozone NAAQS at Step 3 on the basis 

that the State’s emissions were overall 

relatively well-controlled, the EPA 

cannot reach the same conclusion here 

for the more stringent 2015 ozone 

NAAQS. In particular, the EPA finds 

persistent linkages between California 

and several out of state receptors under 

the more stringent NAAQS. Further, the 

EPA finds based on new modeling, see 

Section III.B. of this document, that the 

State failed to adequately assess 

emissions control opportunities at 

certain non-EGU facilities. 

B. Evaluation of Information Provided 

by California Regarding Step 1 and 2 

1. Different Versions of EPA Modeling 

and Regulatory Flexibility 

At Step 1 and 2 of the 4-step interstate 

transport framework, CARB assessed the 

EPA modeling released with the January 

2017 NODA and the October 2017 

memorandum, noting they yielded the 

same design values to identify 

nonattainment and maintenance 

receptors in Colorado and Arizona in 

2023.111 CARB used the EPA’s modeling 

released with the January 2017 NODA 

and the March 2018 memorandum to 

identify California’s contributions to 

receptors in Colorado and Arizona, and 

decided to give more weight to the 

contribution modeling released with the 

March 2018 memorandum (which was 

based on the ‘‘en’’ emissions 

inventory).112 113 With regard to the 

Arizona receptors, CARB compared 

different versions of the EPA’s modeling 

and noted that California’s contribution 

decreased from the earlier to later 

modeling versions, while Arizona’s own 

emissions increased. Ultimately, CARB 

chose to rely on the version of the EPA 

modeling that identified 2 receptors in 

Arizona, as well as contributions from 

California to those receptors above 1 

percent of the NAAQS.114 CARB 

acknowledged that California is linked 

to downwind air quality problems in 

Arizona above the 1 percent of the 

NAAQS threshold at Step 2.115

With regard to receptors in Colorado, 

CARB likewise compared different 

versions of the EPA’s modeling. 

Additionally, however, CARB attempted 

to rely on a potential ‘‘flexibility’’ 

identified in Attachment A to the March 

2018 memorandum to exclude 

exceptional events flagged by Colorado 

in its Denver Metro/North Front Range 

attainment SIP submittal for the 2008 

ozone NAAQS to revise base year design 

values. This analysis led CARB to 

conclude that there will be fewer 

receptors in Colorado in 2023 than 

under either the EPA’s modeled design 

values released with the January 2017 

NODA or the October 2017 

memorandum: Zero nonattainment or 

maintenance receptors using the January 

2017 NODA version of EPA’s modeling 

and the ‘‘el’’ emissions inventory, and 

zero nonattainment and four 

maintenance receptors using the 

October 2017 memorandum version of 

EPA’s modeling and the ‘‘en’’ emissions 

inventory. 

Nonetheless, CARB’s analysis did not 

conclude that California was not linked 

below 1 percent of the NAAQS to the 


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116A4–33, A4–34. 

117 Id. at A4–44. 

118 In addition, the EPA’s most recent modeling 

identifies receptors in 2023 in Utah, Nevada, and 

on tribal lands. Yuma, Arizona is also identified as 

a receptor in EPA’s most recent modeling, while the 

Phoenix area no longer has receptors and now has 

a longer timeframe for attainment due to proposed 

changes in nonattainment classification. Projections 

for receptors that the EPA’s most recent modeling 

identifies are provided later in this notice in Table 

5. 

119Historic design values at individual 

monitoring sites nationwide are provided in the 

file: 2010–2020 Design Values.xlsx which is 

included in docket ID No. EPA–HQ–OAR–2021– 

0663. Design value reports can also be obtained on 

EPA’s website at https://www.epa.gov/air-trends/ 

air-quality-design-values. 

120 Design values obtained from https://

www.epa.gov/air-trends/air-quality-design-values, 

April 21, 2022. 

121CARB presented data for these two Maricopa 

County monitors in its submittal. Those monitors 

are no longer projected to be receptors in the EPA’s 

most recent modeling. However, Yuma, Arizona, 

along with monitors in Utah, Nevada, and on tribal 

land, as described in Table 5 of this notice, are still 

projected to be receptors. 

four maintenance sites it identified in 

Colorado. On the contrary, CARB 

acknowledged that it chose to rely on 

the EPA modeling released with the 

October 2017 memorandum (using the 

‘‘en’’ emissions inventory) and that 

California was linked to four 

maintenance receptors in Colorado 

using that version of the modeling, even 

after CARB removed flagged data.116

As explained in Section I.D. above, 

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. The 

EPA made clear at the time that it would 

thoroughly review the technical and 

legal justifications states put forward in 

relying on any concepts from 

Attachment A to the March 2018 

memorandum. In this case, what CARB 

proposes is potentially consistent with 

the EPA’s modeling guidance, insofar as 

the EPA has recognized that it may be 

appropriate to exclude certain flagged 

data associated with atypical events 

(e.g., wildfires) when calculating base 

period design values to project to a 

future year. However, CARB’s removal 

of atypical data did not change its 

conclusion that there are receptors in 

Colorado in 2023 and that California 

contributes above 1 percent of the 

NAAQS to one or more of them.117

2. Wildfires 

In response to California’s claim that 

recorded violations at projected 

receptors in both Colorado and Arizona 

are heavily influenced by wildfires 

experienced in western states, the EPA 

acknowledges that wildfires could 

influence downwind pollutant 

concentrations and that it is likely that 

wildfires would occur in 2023 and 

future years. However, there is no way 

to accurately forecast the timing, 

location, and extent of fires across a 

future three-year period that would be 

used to calculate ozone design values. 

In the EPA’s CSAPR Update Modeling 

provided in the March 2018 

memorandum and in the EPA’s 2016v2 

emissions platform based modeling, the 

EPA held the meteorological data and 

the fire and biogenic emissions constant 

at base year levels in the future year 

modeling, as those emissions are highlycorrelated with the meteorological 

conditions in the base year. 

CARB’sanalysis focused on changes in 

air quality projections at receptors after 

removing data associated with atypical 

events (e.g., wildfires) and questioned 

whether the number of receptors would 

be diminished or be nonexistent by 

2023 if those data were removed. 

However, we note that measured design 

values at the identified Colorado and 

Arizona receptors continue to have 

design values well in excess of the 2015 

ozone NAAQS, as shown in Tables 3 

and 4 below.118

TABLE 3—OZONE DESIGN VALUES FOR DENVER NONATTAINMENT AREA MONITORS 119

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 

TABLE 4—OZONE DESIGN VALUES FOR SELECTED ARIZONA MONITORS 120 121

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) b

04–013–0019 ...... Arizona ............... Maricopa ............ 73 74 74 73 74 

04–013–1004 ...... Arizona ............... Maricopa ............ 75 75 76 75 78 

40278011 ............ Arizona ............... Yuma .................. 74 72 71 71 68 

While elevated ozone levels in some 

instances may be associated with 

wildfires or other atypical events, 

presently neither Arizona or Colorado 

have sought, nor has the EPA concurred 

on, exceptional events demonstrations 

that would indicate official design 

values at these monitors should be 

appreciably lower than presently 

reported. 

3. Back Trajectory Analysis 

For both Colorado and Arizona, CARB 

analyzed HYSPLIT back trajectories, but 

these also did not affect CARB’s 

conclusions regarding California’s 

linkages to downwind monitors. 


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122 Id. at A4–18, A4–34. 

123 Id. at A4–39, A4–40 Figure 5. 

HYSPLIT back trajectory analyses use 

archived meteorological modeling that 

includes actual observed data (surface, 

upper air, airplane data, etc.) and 

modeled meteorological fields to 

estimate the most likely route of an air 

parcel transported to a receptor at a 

specified time. The method essentially 

follows a parcel of air backward in 

hourly steps for a specified length of 

time. HYSPLIT estimates the central 

path in both the vertical and horizontal 

planes. The HYSPLIT central path 

represents the centerline with the 

understanding that there are areas on 

each side horizontally and vertically 

that also contribute to the 

concentrations at the end point. The 

horizontal and vertical areas that 

potentially contribute to concentrations 

at the endpoint grow wider from the 

centerline the further back in time the 

trajectory goes. Therefore, a HYSPLIT 

centerline does not have to pass directly 

over emissions sources or emission 

source areas but merely relatively near 

emission source areas for those areas to 

contribute to concentrations at the 

trajectory endpoint. The EPA relies on 

back trajectory analysis as a corollary 

analysis along with observation-based 

meteorological wind fields at multiple 

heights to examine the general 

plausibility of the photochemical model 

‘‘linkages.’’ Because the back trajectory 

calculations do not account for any air 

pollution formation, dispersion, 

transformation, or removal processes as 

influenced by emissions, chemistry, 

deposition, etc., the trajectories cannot 

be used to develop quantitative 

contributions. Therefore, back 

trajectories cannot be used to 

quantitatively evaluate the magnitude of 

the existing photochemical 

contributions from upwind states to 

downwind receptors. In this regard, 

photochemical modeling simulations for 

ozone interstate transport assessments 

are relied upon by the EPA to simulate 

the formation and fate of oxidant 

precursors, primary and secondary 

particulate matter concentrations, and 

deposition over regional and urban 

spatial scales. Photochemical modeling 

is the most sophisticated tool available 

to estimate future ozone levels and 

contributions to those modeled future 

ozone levels. Consideration of the 

different processes that affect primary 

and secondary pollutants at the regional 

scale in different locations is 

fundamental to understanding and 

assessing the effects of emissions on air 

quality concentrations. 

CARB’s HYSPLIT back trajectory 

analysis showed that on high ozone 

days in Colorado at the receptors 

identified by the EPA in the March 2018 

memorandum ‘‘only one backward and 

forward trajectory pairing indicated that 

emissions in the California mixed layer 

should have reached the mixed layer at 

a Colorado receptor site.’’ CARB claims 

this suggests that ‘‘the complexity of the 

physical environment between 

California and Colorado limits the 

reproducibility of modeled transport 

and that considerable multi-faceted 

analyses would be necessary to explore 

transport mechanisms through areas of 

complex terrain.’’ 122 For Arizona, CARB 

concluded that ‘‘[o]nly a few trajectories 

extend from California’’ to the Phoenix 

area.123 CARB’s trajectory analysis 

confirmed that California is an upwind 

area for the receptors in Colorado and 

Arizona often enough to potentially 

contribute to nonattainment or interfere 

with maintenance. The analysis did not 

provide evidence that was contrary to 

the conclusions of the EPA’s 

photochemical modeling analyses (i.e., 

the EPA’s modeling results in the March 

2018 memorandum and EPA 2016v2 

model). 

Further, the EPA finds CARB’s back 

trajectory analysis to be deficient in 

proving that California does not 

contribute significantly to 

nonattainment or maintenance at the 

monitors in Colorado that the State was 

linked to in the EPA’s modeling results 

in the March 2018 memorandum. 

CARB’s back trajectory analysis for 

Colorado shows a linkage between 

California and the monitors when 

evaluating three altitudes: 10 meters, 

1,000 meters, and 2,000 meters, during 

June and July, when most exceedances 

occurred at these sites. By only 

evaluating these altitudes, CARB 

neglects to consider the wide range of 

heights that might show back 

trajectories leading back to sources in 

California, which could potentially 

further tie the state to more ozone 

exceedance events. In addition, by 

excluding trajectories with a centerline 

above the mixed layer the analysis fails 

to consider transported pollutants at 

elevations below the centerline which 

may be in the mixed layer and therefore 

impact ground level ozone 

concentrations. 

Similarly for its analysis of Arizona 

linkages, CARB’s back trajectory 

analysis shows a linkage between 

California and the monitors when 

evaluating three altitudes: 100 meters, 

500 meters, and 1,000 Meters on ozone 

exceedance days in 2015 and 2016. 

CARB’s back trajectories for Arizona use 

a relatively short 24-hour time period, 

which limits their reliability for 

evaluating long-distance transport of 

emissions. As evident from Figure 5 in 

CARB’s SIP submission, there were a 

number of exceedance days in Phoenix 

with 24-hour back trajectories that point 

westward toward California. These 

trajectories may have crossed portions 

of California if the trajectories were 

calculated for a longer time period, such 

as 48 hours. This would further 

strengthen the linkage to California that 

is already indicated by CARB’s analysis. 

In California’s 2018 submittal, CARB 

noted that projected 2023 DVs in Denver 

and Phoenix increased from the ‘‘el’’ 

modeling released in the January 2017 

NODA and the ‘‘en’’ modeling released 

in the October 2017 memorandum. For 

Denver, CARB noted that there were 

three 2023 receptors, all maintenanceonly, based on the ‘‘el’’ modeling, 

whereas with the ‘‘en’’ modeling 

projected three monitors to be 

nonattainment and three monitors to be 

maintenance-only. For Phoenix, CARB 

noted that there were no receptors 

projected for 2023 based on the ‘‘el’’ 

modeling, whereas with the ‘‘en’’ 

modeling, two monitors are projected to 

be maintenance-only. Based on an 

analysis of the change in ‘‘home state’’ 

emissions and contributions vs 

contributions and emissions from 

California in the ‘‘el’’ modeling vs the 

‘‘en’’ modeling, CARB argues that the 

nonattainment and maintenance 

receptors found in the ‘‘en’’ modeling in 

Denver and Phoenix are due to increases 

in emissions in the ‘‘home state’’ rather 

than contributions from California. 

With respect to the information 

California provided that is related to 

local emissions and the impact on air 

quality at the Denver nonattainment 

area receptors, this information is 

insufficient to approve California’s 2018 

SIP submittal. As an initial matter, we 

do not agree with CARB’s conclusions 

that the remaining nonattainment or 

maintenance problems in Arizona or in 

Colorado (after accounting for flagged 

data) should be ascribed solely to an 

increase in local emissions in the home 

state. While CARB asserts that its 

relative contribution to the problems 

has declined, CARB does not establish 

with any quantitative evidence that this 

contribution drops below 1 percent of 

the NAAQS. 

More fundamentally, regardless of 

whether local emissions are the largest 

contributor to a specific nonattainment 

or maintenance receptor, the good 

neighbor provision requires that upwind 

states prohibit emissions that contribute 

significantly to nonattainment or 

interfere with maintenance of the 

NAAQS in downwind states. The EPA 


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124See, e.g., A4–13–14. 

125Final 2016 Air Quality Management Plan, at 

7–23, South Coast Air Quality Management District, 

March 2017. 

126Historic design values at individual 

monitoring sites nationwide are provided in the 

file: 2010–2020 Design Values.xlsx that is included 

in docket ID No. EPA–HQ–OAR–2021–0663. Design 

value reports can also be obtained on EPA’s website 

at https://www.epa.gov/air-trends/air-qualitydesign-values. 

127See ‘‘CALIFORNIA Final Area Designations 

for the 2015 Ozone National Ambient Air Quality 

Standards Technical Support Document (TSD)’’ at 

pg 67, 178; available in docket ID No. EPA–HQ– 

OAR–2017–0548 (83 FR 25776, April 30, 2018). 

Also available on EPA’s website at https://

www.epa.gov/sites/default/files/2018-05/ 

documents/ca_tsd_combined_final_0.pdf. 

128Langford, A., Senff, C., Alvarez, R., Banta, R., 

Hardesty, R.: Long-range transport of ozone from 

the Los Angeles Basin: A case study, J. Geophys. 

Res., 37, L06807, doi:10.1029/2010GL042507. 

129Li, J., Georgescu, M., Hyde, P., Mahalov, A., 

and Moustaoui, M.: Regional-scale transport of air 

pollutants: Impacts of Southern California 

emissions on Phoenix ground-level ozone 

concentrations, Atmos. Chem. Phys., 15, 9345– 

9360, https://doi.org/10.5194/acp-15-9345-2015, 

2015. 

evaluates a state’s obligations to 

eliminate interstate transport emissions 

under the interstate transport provision 

according to the EPA’s 4-step process, 

and the EPA’s updated modeling at 

Steps 1 and 2 has identified a linkage 

between emission from California 

sources and downwind nonattainment 

and maintenance receptors. 

Further, the EPA disagrees with the 

implication that local emissions 

reductions from the jurisdiction where 

the downwind receptor is located must 

first be implemented and accounted for 

before imposing obligations on upwind 

states under the interstate transport 

provision. There is nothing in the CAA 

that supports that position, and it does 

not provide grounds on which to 

approve California’s 2018 SIP submittal. 

The D.C. Circuit has held on five 

different occasions that the timing 

framework for addressing interstate 

transport obligations must be consistent 

with the downwind areas’ attainment 

schedule. In particular, for the ozone 

NAAQS, the states and the EPA are to 

address interstate transport obligations 

‘‘as expeditiously as practicable’’ and no 

later than the attainment schedule set in 

accordance with CAA section 181(a). 

See North Carolina, 531 F.3d at 911–13; 

Wisconsin, 938 F.3d at 313–20; 

Maryland, 958 F.3d at 1204; New York 

v. EPA, 964 F.3d 1214, 1226 (D.C. Cir. 

2020); New York v. EPA, 781 Fed. App’x 

4, 6–7 (D.C. Cir. 2019). The court in 

Wisconsin explained its reasoning in 

part by noting that downwind 

jurisdictions often may need to heavily 

rely on emissions reductions from 

upwind states in order to achieve 

attainment of the NAAQS, 938 F.3d at 

316–17; such states would face 

increased regulatory burdens including 

the risk of bumping up to a higher 

nonattainment classification if 

attainment is not reached by the 

relevant deadline. Maryland, 958 F.3d at 

1204. The statutory framework of the 

CAA and these cases establish clearly 

that states and the EPA must address 

interstate transport obligations in line 

with the attainment schedule provided 

in the Act in order to timely assist 

downwind states in attaining and 

maintain the NAAQS, and this 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)). 

Additionally, the 2018 SIP submittal 

does not assess whether California’s 

own emissions contributed to 

nonattainment or interfered with 

maintenance at the linked receptors, or 

substantiate that emissions from 

California’s sources were not interacting 

with these monitors. Consequently, the 

application of local emission reduction 

measures does not absolve upwind 

states and sources from the 

responsibility of addressing their 

significant contribution. Moreover, 

California still has an obligation under 

the Act to address its downwind 

contribution to ozone nonattainment or 

interference with maintenance 

regardless of the emission reduction 

potential for local control measures. 

Furthermore, given that the EPA’s 

updated modeling indicates that 

California is linked to nonattainment 

and maintenance receptors at Step 2, the 

EPA disagrees with CARB’s claims 

regarding the application of local 

emission reduction measures with 

respect to its downwind linkages in the 

most recent modeling. 

CARB presents a number of 

arguments that the unique topography 

and/or meteorology in the western 

region and, in particular, in and 

surrounding the Denver and Phoenix 

nonattainment areas support a 

conclusion that California does not 

significantly contribute or interfere with 

maintenance in those areas. For 

example, CARB argues that the 

mountainous topography in California 

traps ozone-precursors in-state, and that 

the Rocky Mountains in Colorado and 

the mountains around Phoenix, Arizona 

also form barriers to the transport of 

ozone pollution.124 First, we note that 

despite these potential considerations, 

CARB itself acknowledges in its 2018 

submittal that California is linked to at 

least some receptors in Colorado and 

Arizona at Step 2 based on the modeling 

analysis on which it primarily relies. 

Second, even if CARB intended these 

arguments to support an alternative 

argument that it is not linked to those 

receptors, the EPA finds that these 

entirely qualitative discussions are 

insufficient to overcome the robust, 

quantitative basis to find linkages exist 

based on the modeling. 

We agree with CARB that the terrain 

in the western U.S. is complex. A 

complex topography can have a number 

of impacts on the transport of air and air 

pollutants, such as enhance vertical 

mixing of air, serve as a barrier to 

transported air pollution, enhance 

accumulation of local emissions in 

basins and valleys, and influence air 

flows up, down, and across valleys. 

While topography can have a significant 

effect on pollutant (e.g., ozone) 

formation and transport, it does not 

prevent transport within the State and 

beyond. Mountain passes through 

surrounding ranges can serve as 

‘‘transport corridors’’ for ozone. For 

example, in Southern California, areas 

where upwind pollution is funneled 

through valley topography experience 

some of the highest measured ozone 

concentrations, despite lower local 

emissions. 

In Southern California there are 

several examples of transport corridors 

that funnel ozone and ozone precursors. 

The Riverside County (Coachella) 2015 

8-hour ozone NAAQS design value for 

2020 was 88 ppb. The area is affected 

by transported emissions from the South 

Coast Air Basin through the San 

Gorgonio Pass.125 Similarly, the Kern 

County (Eastern Kern), CA 2015 8-hour 

ozone NAAQS design value for 2020 

was 86 ppb and is primarily influenced 

by emissions transported from the San 

Joaquin Valley through the Tehachapi 

Pass.126 127 Ozone and its precursors can 

be transported into the southern Mojave 

Desert Air Basin from the greater Los 

Angeles Air Basin through the Cajon 

Pass. Ozone can also be transported 

eastward to the Salton Sea Air Basin 

through the San Gorgonio Pass and from 

the San Diego Air Basin through other 

mountain passes continuing into 

Arizona. In addition to transport within 

the mixed layer, orographic lifting of 

ozone from the surface to the free 

troposphere by the so called ‘‘mountain 

chimney effect’’ is a potential additional 

pathway for venting of pollutants into 

the free troposphere and making them 

available for long-range transport to 

downwind states (Langford et. al., 2010 

and Li et. al., 2015).128 129 While 

Southern California offers evidence of 

funneling of pollution through 

mountain passes and upwelling of 

pollution into troposphere, we have no 

reason to conclude these effects could 


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130See Appendix A—Model Performance 

Evaluation for 2016v2 Base Year CAMx Simulation, 

of Air Quality Modeling TSD for 2015 Ozone 

NAAQS Transport SIP Proposed Actions, at A–10. 

131 Design values and contributions at individual 

monitoring sites nationwide are provide in the file: 

2016v2_DVs_state_contributions.xlsx which is 

included in docket ID No. EPA–HQ–OAR–2021– 

0663. 

132These 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. That 

modeling showed that California had a maximum 

contribution 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 EPA–HQ– 

OAR–2021–0663. 

133We note that, consistent with the EPA’s prior 

good neighbor actions in California, the regulatory 

ozone monitor located on the Morongo Band of 

Mission Indians (‘‘Morongo’’) reservation is a 

projected downwind receptor in 2023. See 

monitoring site 060651016 in Table 3. We also note 

that the Temecula, California regulatory ozone 

monitor is a projected downwind receptor in 2023 

and in past regulatory actions has been deemed 

representative of air quality on the Pechanga Band 

of Luisen ̃ o Indians (‘‘Pechanga’’) reservation. See, 

e.g., Approval of Tribal Implementation Plan and 

Designation of Air Quality Planning Area; Pechanga 

Band of Luisen ̃ o Mission Indians, 80 FR 18120, at 

18121–18123 (April 3, 2015); see also monitoring 

site 060650016 in Table 3. The presence of 

receptors on, or representative of, the Morongo and 

Pechanga reservations does not trigger obligations 

for the Morongo and Pechanga Tribes. Nevertheless, 

these receptors are relevant to the EPA’s assessment 

of any linked upwind states’ good neighbor 

obligations. See, e.g., Approval and Promulgation of 

Air Quality State Implementation Plans; California; 

Interstate Transport Requirements for Ozone, Fine 

Particulate Matter, and Sulfur Dioxide, 83 FR 65093 

(December 19, 2018). Under 40 CFR 49.4(a), tribes 

are not subject to the specific plan submittal and 

implementation deadlines for NAAQS-related 

requirements, including deadlines for submittal of 

plans addressing transport impacts. 

not occur in the Sierra Nevada 

Mountains and the Denver Metro/North 

Front Range. 

The EPA has previously explained 

that its nationwide photochemical grid 

modeling is reliable for applications in 

the western region of the U.S. In 

disapproving Utah’s 2008 ozone 

transport SIP submittal for prong 2, the 

EPA rejected comments that its CAMx 

modeling (the same modeling software 

used here) did not account for unique 

western geographical considerations. 

See 81 FR 71991, 71992–93 (Oct. 19, 

2016). In particular, the EPA noted that 

the modeling accounted for differences 

in emissions (including wildfires), 

meteorology, and topography’’ across all 

regions of the U.S. Id. at 71993. The 

EPA found that neither the commenters, 

nor the state in its SIP submittal, had 

adduced any additional factors that 

would be relevant for projecting ozone 

concentrations in the west that were not 

already factored into and documented 

in both the modeling itself and in the 

EPA’s technical support documents 

explaining that modeling. Id. The same 

holds true here. As explained in 

Appendix A of the Air Quality 

Modeling TSD included in docket ID 

No. EPA–HQ–OAR–2021–0663, the EPA 

has found that its updated 2016v2 

emissions platform-based modeling 

performs equally as well in eastern and 

western regions in terms of replicating 

the relative magnitude of concentrations 

and day-to-day variability that are 

characteristic of observed 8-hour daily 

maximum ozone concentrations in each 

region. It is also important to note that 

the model accurately captures 

substantial geographical difference in 

the temporal nature of ozone 

concentrations at the receptors in the 

west, including in the Denver 

nonattainment area, compared to 

receptors in the East.130 The EPA 

continues to find its modeling reliable 

for characterizing ozone concentrations 

and contribution values in the western 

region of the United States. As such, 

CARB’s qualitative discussions of 

western geography fail to present 

evidence that calls into question the 

results of the EPA’s photochemical grid 

modeling. 

C. Results of the EPA’s Step 1 and Step 

2 Modeling and Findings for California 

As described in section I, 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 California contributes at or 

above the threshold of 1 percent of the 

2015 8-hour ozone NAAQS (0.70 ppb) to 

any downwind nonattainment or 

maintenance receptor. As shown in 

Table 3, the data 131 indicate that in 

2023, emissions from California 

contribute greater than 1 percent of the 

standard (i.e., 0.70 ppb) to 

nonattainment or maintenance-only 

receptors in Arizona, Colorado, Nevada, 

and Utah.132 Emissions from California 

also contribute greater than 1 percent of 

the standard to nonattainment receptors 

on, or representative of, the Morongo 

and Pechanga reservations.133

Therefore, based on the EPA’s 

evaluation of the information submitted 

by California, and based on the EPA’s 

most recent modeling results for 2023, 

the EPA proposes to find that California 

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. 

TABLE 5—CALIFORNIA LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location Nonattainment/ 

maintenance 

2023 Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

California 

contribution 

(ppb) 

40278011 .......... Yuma (AZ) .............................................................. Maintenance-only .... 70.5 72.2 5.09 

80350004 .......... Denver/Chatfield (CO) ............................................ Nonattainment ......... 71.7 72.3 0.91 

80590006 .......... Rocky Flats (CO) .................................................... Nonattainment ......... 72.6 73.3 1.03 

80590011 .......... Denver/NREL (CO) ................................................ Nonattainment ......... 73.8 74.4 1.17 

320030075 ........ Las Vegas/Northwest (NV) ..................................... Maintenance-only .... 70.0 71.0 7.44 

490110004 ........ SLC/Bountiful (UT) ................................................. Nonattainment ......... 72.9 75.1 2.25 

490353006 ........ SLC/Hawthorne (UT) .............................................. Nonattainment ......... 73.6 75.3 2.46 

490353013 ........ SLC/Herriman (UT) ................................................ Nonattainment ......... 74.4 74.9 1.42 

490570002 ........ SLC/Ogden (UT) .................................................... Maintenance-only .... 70.6 72.5 2.24 

490571003 ........ SLC/Harrisonville (UT) ........................................... Maintenance-only .... 70.5 71.5 2.16 

060651016 ........ Morongo Band of Mission Indians ......................... Nonattainment ......... 89.8 90.9 34.24 

060650016 ........ Pechanga Band of Mission Indians (represented 

by Temecula (CA)).

Nonattainment ......... 72.0 72.9 26.32 


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134See discussion further in this Section 

discussing why EPA finds its analysis in the 

approval of California’s 2008 ozone NAAQS 

transport SIP to be appropriate or sufficient for 

purposes of this action. 

135 Id. at A4–57. 

136The EPA notes that it has proposed a costeffectiveness threshold of $11,000 per ton for EGUs 

in determining good neighbor obligations for the 

2015 ozone NAAQS after assessing the full range of 

NOX mitigation strategies that could be applied to 

fossil-fuel fired EGUs. ‘‘Federal Implementation 

Based on the EPA’s evaluation of the 

information provided in California’s 

2018 submittal and based on the results 

of the EPA’s 2016v2 emissions platform 

modeling, the EPA will proceed to 

evaluate these additional arguments at 

Step 3 of the 4-step interstate transport 

framework. 

D. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-step interstate 

transport 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 

interstate transport framework) when 

identifying emissions contributions that 

the Agency has determined to be 

‘‘significant’’ (or interferes 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. California did not conduct 

such an analysis in its 2018 SIP 

Submittal. 

As previously indicated in section 

II.B. California’s 2018 SIP Submittal 

provided an overview of NOX emissions 

by sector for 2011 NEI emissions and 

2023 projections. CARB also provided a 

summary of regulations controlling NOX

and VOCs at the state and district level 

for various sectors, many of which had 

been approved into California’s SIP. 

CARB asserted in the 2018 Submittal 

that, despite its contributions, California 

had met its good neighbor obligations 

through the implementation and 

enforcement of stringent NOX and VOC 

control measures that go beyond the 

EPA’s presumptive cost threshold in the 

CSAPR Update for highly cost-effective 

emissions reductions, and through the 

ongoing adoption and revision of 

additional control measures to further 

ensure the reduction of ozone in both 

California and downwind areas. 

CARB, however, did not provide an 

adequate demonstration at Step 3 that 

California was adequately controlling its 

emissions for the purposes of the good 

neighbor provision for the 2015 ozone 

NAAQS, particularly because CARB 

acknowledged in its 2018 SIP Submittal 

that its emissions were linked to 

Arizona and Colorado receptors at Steps 

1 and 2. In general, the air quality 

modeling that the EPA has conducted as 

well as the modeling relied on by CARB 

in its submittal already account for ‘‘onthe-books’’ emissions control measures. 

Both sets of modeling analyzed by 

CARB (confirmed by the EPA’s most 

recent modeling) clearly establish 

continued linkages from California to 

downwind receptors in 2023 at Steps 1 

and 2. In general, the listing of existing 

or on-the-way control measures, 

whether approved into the state’s SIP or 

not, does not substitute for a complete 

Step 3 analysis under the EPA’s 4-Step 

framework to define ‘‘significant 

contribution.’’ 134 CARB’s submittal 

does not include an assessment of the 

overall effects of the identified control 

measures it identifies or explain what 

the overall resulting air quality effects 

would be at identified out-of-state 

receptors. 

Further, CARB did not conduct in its 

submittal any analysis of potential 

additional emissions-reduction 

measures to further reduce its impact on 

the identified downwind receptors. For 

example, CARB did not include in the 

2018 SIP Submittal an accounting of 

facilities in the State along with an 

analysis of potential NOX emissions 

control technologies, their associated 

costs, estimated emissions reductions, 

and downwind air quality 

improvements. Nor does the submittal 

include an analysis of whether such 

potential additional control 

technologies or measures could reduce 

the impact of California’s emissions on 

out of state receptors. Though there is 

not a prescribed method for a Step 3 

analysis, the EPA has consistently 

applied Step 3 of the good neighbor 

framework through a more rigorous 

evaluation of potential additional 

control technologies or measures than 

what was provided in the SIP 

submission. Identifying a range of 

various emissions controls measures 

that have been or may be enacted at the 

state or local level, without analysis of 

the impact of those measures on the out 

of state receptors, is not analytically 

sufficient. 

CARB did not offer an explanation as 

to whether any more stringent emissions 

reductions that may be available were 

prohibitively costly or infeasible. CARB 

did note that the EPA’s 2016 costeffectiveness analysis of EGU emission 

reductions in the CSAPR Update for the 

2008 ozone NAAQS found that NOX

emission reductions at California EGUs 

would be achieved at a significantly 

higher cost threshold than the cost 

threshold finalized for the states 

ultimately included in the CSAPR 

Update. CARB further stated that what 

the ‘‘EPA found true with respect to the 

0.075 ppm 8-hour ozone standard is 

equally valid concerning the 0.070 ppm 

8-hour ozone standard. California’s 

emission reduction programs adequately 

prohibit the emission of air pollutants in 

amounts that will significantly 

contribute to nonattainment, or interfere 

with maintenance, of the 0.070 ppm 8- 

hour ozone standard in any downwind 

state.’’ 135

However, this is incorrect. There is no 

reason to suppose that the EPA or states 

should conclude that the same degree of 

emissions-control stringency that was 

deemed approvable to address good 

neighbor obligations to meet a less 

stringent NAAQS should apply to a 

more stringent NAAQS. While the EPA 

has not finalized a benchmark costeffectiveness threshold for good 

neighbor obligations for the more 

stringent 2015 ozone NAAQS, it was not 

the EPA’s obligation to do so prior to 

states developing their SIP 

submissions.136 CARB, in its 2018 SIP 


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Plan Addressing Regional Ozone Transport for the 

2015 Ozone National Ambient Air Quality 

Standard’’ 87 FR 20036, 20091–93 (April 6, 2022). 

While this does not represent a final promulgated 

cost-effectiveness benchmark for EGUs that 

California is expected to have applied, the EPA’s 

Step 3 analysis in the proposed FIP indicates the 

relative paucity of analysis in California’s SIP 

submittal regarding emissions control opportunities 

at Step 3. Nonetheless, the EPA has not proposed 

to apply the EGU control strategy in its proposed 

FIP action to California. See id. at 20088. The EPA 

continues to find in this proposal that California’s 

EGUs are sufficiently controlled for ozone-precursor 

emissions for purposes of good neighbor obligations 

under the 2015 ozone NAAQS. 

137The EPA reached this proposed conclusion for 

EGUs in California in the context of a recent 

proposed federal implementation plan and 

proposes the same conclusions for these sources in 

this action. See ‘‘Federal Implementation Plan 

Addressing Regional Ozone Transport for the 2015 

Ozone National Ambient Air Quality Standard,’’ 87 

FR 20036, 20088 (April 6, 2022). 

138The ‘‘Capacity Dropped’’ and the ‘‘Retired 

Through 2023’’ worksheets in NEEDS list all units 

that are removed from the NEEDS v6 inventory— 

NEEDS v6 Summer 2021 Reference Case. This data 

can be found on the EPA’s website at: https://

www.epa.gov/airmarkets/national-electric-energydata-system-needs-v6. 139 Id. at A4–55. 

Submittal, has not conducted an 

analysis to establish one for the EPA to 

evaluate, and this is grounds for 

disapproval. 

More fundamentally, relying on the 

CSAPR Update’s (or any other CAA 

program’s) determination of costeffectiveness without further Step 3 

analysis is insufficient. Costeffectiveness must be assessed in the 

context of the specific CAA program; 

assessing cost-effectiveness in the 

context of interstate ozone transport 

should reflect a more comprehensive 

evaluation of the nature of the interstate 

transport problem, the total emissions 

reductions available at several cost 

thresholds, and the air quality impacts 

of the reductions at downwind 

receptors. While the EPA has not 

finalized a benchmark cost-effectiveness 

value for the 2015 ozone NAAQS 

interstate transport obligations, because 

the 2015 ozone NAAQS is a more 

stringent and more protective air quality 

standard, it is reasonable to expect 

control measures or strategies to address 

interstate transport of ozone to reflect 

higher marginal control costs. As such, 

the marginal cost threshold of $1,400 

per ton for the CSAPR Update (which 

addresses the 2008 ozone NAAQS and 

is in 2011$) is not an appropriate cost 

threshold and cannot be approved as a 

benchmark to use for interstate transport 

SIP submissions for the 2015 ozone 

NAAQS. 

Although the EPA acknowledges 

states are not necessarily bound to 

follow the EPA’s own analytical 

framework at Step 3, CARB did not 

attempt to determine or justify an 

appropriate uniform cost-effectiveness 

threshold. This would have been similar 

to the approach to defining significant 

contribution that the EPA has applied in 

prior rulemakings such as CSAPR and 

the CSAPR Update, even if conducting 

precisely this type of analysis is not 

technically mandatory. For example, 

CARB did not conduct its own updated 

EGU analysis of all large NOX emitting 

EGUs. Nonetheless, the EPA finds based 

on its own analysis that additional 

emissions reductions are not required 

from EGUs to address California’s good 

neighbor obligations for the 2015 ozone 

NAAQS.137

As stated in the SIP submittal, the 

Greenleaf One unit emits at higher rates 

with a low utilization, resulting in only 

2 tons of NOX in the 2021 ozone season. 

Therefore, the EPA agrees it is unlikely 

that any significant cost-effective 

emission reduction opportunities exist 

at this facility. In addition, California 

has highlighted the retirements of the 

Redondo Beach units and the ACE 

Cogeneration facility. The EPA has 

confirmed the retirements of these and 

other units in California in the IPM 

version 6—Summer 2021 Reference 

Case database.138 The EPA IPM version 

6—Summer 2021 Reference Case uses 

the National Electric Energy Data 

System (NEEDS) v6 database as its 

source for data on all existing and 

planned-committed units. Units are 

removed from the NEEDS inventory 

only if a high degree of certainty could 

be assigned to future implementation of 

the announced future closure or 

retirement. The available retirementrelated information was reviewed for 

each unit, and the following rules are 

applied to remove: 

(i) Units that are listed as retired in 

the December 2020 EIA Form 860M; 

(ii) Units that have a planned 

retirement year prior to June 30, 2023 in 

the December 2020 EIA Form 860M; 

(iii) Units that have been cleared by 

a regional transmission operator (RTO) 

or independent system operator (ISO) to 

retire before 2023, or whose RTO/ISO 

clearance to retire is contingent on 

actions that can be completed before 

2023; 

(iv) Units that have committed 

specifically to retire before 2023 under 

federal or state enforcement actions or 

regulatory requirements; 

(v) And finally, units for which a 

retirement announcement can be 

corroborated by other available 

information. Units required to retire 

pursuant to enforcement actions or state 

rules on July 1, 2023 or later are 

retained in NEEDS v6. 

The majority of the EGUs in California 

have emissions controls and are 

sufficiently regulated, resulting in the 

lowest fossil fuel emission rate and 

highest share of renewable generation 

among the 26 states examined at the 

EPA’s Step 3 analysis for the proposed 

Federal Implementation Plan 

Addressing Regional Ozone Transport 

for the 2015 Ozone National Ambient 

Air Quality Standard. 87 FR 20036, 

20088. The EPA evaluated the EGU 

sources within the state of California 

and found there were no covered coal 

steam sources greater than 100 MW that 

would have emissions reduction 

potential according to the EPA’s 

assumed EGU SCR retrofit mitigation 

technologies. The NOX emission level 

for California was unchanged at 1,216 

tons of NOX across the various emission 

control scenarios. The EPA’s Step 3 

analysis, including analysis of the 

emissions reduction factors from EGU 

sources in the state, therefore resulted in 

no additional emission reductions 

required to eliminate significant 

contribution from any EGU sources in 

California. 

The EPA proposes that California’s 

Step 3 analysis was likewise insufficient 

for non-EGU stationary sources. But 

whereas EPA is able to conclude, based 

on the foregoing analysis, that 

additional emissions reductions are not 

required from EGU sources in 

California, we can reach no such 

conclusion with respect to other 

industrial sources of emissions in the 

State. For non-EGUs, CARB did not 

complete an evaluation of cost effective 

control opportunities, and instead 

simply provided a cursory analysis that 

provided a few examples of regulations 

to conclude that ‘‘further emission 

controls would be unlikely to reduce 

any potential impact on downwind 

states’ air quality[.]’’ 139 CARB did not 

investigate additional potential 

emissions control opportunities, or their 

costs or impacts, or attempt to analyze 

whether, if applied more broadly across 

linked states, the emissions reductions 

would constitute the elimination of 

significant contribution on a regional 

scale. 

The EPA acknowledges that it has 

previously approved California’s 2008 

ozone NAAQS transport SIP at Step 3 

based on a relatively cursory review of 

California’s existing emissions control 

programs. See 83 FR 65093, 65094–95 

(Dec. 19, 2018). That approval pre-dates 

the D.C. Circuit’s decision in Wisconsin 

v. EPA, 938 F.3d 903 (D.C. Cir. 2019), 

in which the court found the EPA had 

not properly justified failing to analyze 

emissions reduction opportunities from 

industrial sources outside the power 


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140See, e.g., ‘‘Disapproval of Interstate Transport 

Requirements for the 2008 Ozone National Ambient 

Air Quality Standards; New York and New Jersey’’, 

86 FR 60602 (November 3, 2021). 141 87 FR 20036 (April 6, 2022). 

142See California’s 2018 Submittal at A4–33–A4– 

34, A4–44. 

sector. Id. at 918–20. At that time, the 

CSAPR Update had only addressed 

reductions from the power sector and 

applied a cost threshold of $1400 per 

ton. The EPA’s analysis of California 

focused on the fact that California’s EGU 

fleet was very well controlled and that 

all receptors for the 2008 ozone NAAQS 

were projected to be clean by 2023. 83 

FR 65093, 65095. The EPA engaged in 

an extremely limited review of other 

emissions control opportunities in 

California at non-EGU industrial 

sources, despite acknowledging that 

these sources emitted 6.7 times as much 

NOX as EGUs, and 19 large stationary 

sources each individually emitted over 

500 tons per year. Id. 

The EPA finds that good reasons exist 

for no longer considering such a cursory 

analysis of emissions reduction 

opportunities beyond the power sector 

to be adequate for purposes of CAA 

section 110(a)(2)(D)(i)(I). First, the EPA 

and the states are implementing the 

more stringent 2015 ozone NAAQS of 

70 ppb. Under that more stringent 

NAAQS, our analysis at Steps 1 and 2 

indicates a continuing linkage between 

California’s emissions and persistent air 

quality problems (at least through 2026) 

in other states in the EPA’s modeling. 

Further, while California may be 

relatively ‘‘well controlled’’ as a state 

overall on a per capita basis, the same 

could be said of other states throughout 

the country that continue to contribute 

above 1 percent of the NAAQS to at 

least one out of state receptor despite 

relatively stringent ozone-precursor 

emissions control programs. In the 

CSAPR Update and the Revised CSAPR 

Update, the EPA has found that states 

such as New York and New Jersey may 

nonetheless be found to have additional 

emissions control obligations in order to 

address their significant contribution 

under Section 110(a)(2)(D)(i)(I).140

Further, the relevance of a per capita 

emissions rate, which CARB cites as 

relatively low for California, is not 

readily apparent. California is a large 

and very populous state, and by CARB’s 

own admission, total NOX emissions 

from the State are second highest in the 

country, behind only Texas. Finally, the 

EPA recognizes the critical importance 

of consistency in application of good 

neighbor requirements across all states, 

especially with respect to regional-scale 

pollutants such as ozone. The EPA’s 

regional analysis in the proposed FIP 

(discussed below) indicates emissions 

control opportunities at non-EGUs in 

California at the same stringency as 

EPA’s Step 3 assessment of 22 other 

states. Therefore, for all of these reasons, 

the EPA does not view the degree of 

analysis at Step 3 that supported 

approval in the prior California 

transport action to be sufficient to 

justify approval in this case. 

The EPA notes that in the proposed 

FIP for California for the 2015 ozone 

NAAQS, we identified several potential 

cost-effective NOX controls for nonEGUs in California.141 The EPA’s nonEGU analysis in the proposed FIP 

focused on several industrial sectors 

and found impactful emissions 

reduction opportunities up to $7,500 

per ton, which the EPA proposed are 

needed to address 23 upwind state’s 

(including California’s) good neighbor 

obligations for the 2015 ozone NAAQS. 

See 87 FR 20036, 20089–90. In 

particular for California, the EPA found 

1,666 tons of ozone season NOX

emissions reduction available from a 

2019 baseline of 14,579 tons of ozone 

season emissions from the non-EGU 

sectors analyzed. Id. at 20090. The EPA 

proposed to require these reductions in 

part because, in conjunction with the 

other emission control strategies 

proposed in the FIP across the entire 

region of linked upwind states, the EPA 

found ozone levels would improve on 

average by 0.64 ppb across all impacted 

receptors, including those receptors 

affected by California’s emissions. Id. at 

20096. The EPA proposed to determine 

that these controls would eliminate 

significant contribution and interference 

with maintenance for the 2015 ozone 

NAAQS. 

The EPA acknowledges that California 

need not have conducted a Step 3 

analysis in precisely the manner as the 

proposed FIP, and we further 

acknowledge that our FIP for California 

and other states is only a proposal at 

this stage and is currently undergoing 

public comment. Nonetheless, the 

proposed FIP presents an example of 

how a potentially approvable Step 3 

analysis could have been conducted by 

CARB and highlights that cost-effective 

emissions reduction opportunities likely 

exist in California that could address 

interstate transport obligations, which 

CARB failed to analyze in the 2018 SIP 

Submittal. 

CARB also attempted to support its 

conclusion that California does not 

significantly contribute to 

nonattainment or maintenance in other 

states in part because it suggested that 

emissions originating from outside 

California, such as local emissions in 

Arizona and Colorado, as well as 

international emissions, and wildfires, 

were the primary driver of higher 

modeled design values at monitoring 

sites in those states using the EPA 

modeling released with the January 

2017 NODA and the October 2017 

memorandum.142

With respect to local, international, 

and non-anthropogenic emissions 

contributions, CARB’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 

downwind states’ ability to attain and 

maintain the 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) 


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143 Id. at A4–58 

144Letter dated February 14, 2022, from Elizabeth 

J. Adams, Director, Air and Radiation Division, EPA 

Region IX to Mark Macarro, Chairperson, Pechanga 

Band of Luisen ̃ o Indians of the Pechanga 

Reservation, Re: Invitation to Consult on 

California’s Interstate Transport State 

Implementation Plan for the 2015 Ozone National 

Ambient Air Quality Standards; and letter dated 

February 14, 2022, from Elizabeth J. Adams, 

Director, Air and Radiation Division, EPA Region IX 

to Charles Martin, Chairperson, Morongo Band of 

Mission Indians, Re: Invitation to Consult on 

California’s Interstate Transport State 

Implementation Plan for the 2015 Ozone National 

Ambient Air Quality Standards. 

145 86 FR 16533 (March 30, 2021). 

(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. 

In conclusion, at Step 3, we propose 

that California was required to analyze 

emissions from the sources and other 

emissions activity from within the state 

to determine whether its contributions 

to nonattainment were significant or 

interfered with maintenance of the 

NAAQS in downwind states, and we 

propose to disapprove the 2018 SIP 

Submittal on the separate, additional 

basis that it did not assess additional 

emission control opportunities. 

E. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-step interstate 

transport 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, California’s 2018 

SIP Submittal 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. Instead, CARB concluded that 

the state already has ‘‘measures of 

sufficient stringency to ensure that this 

State does not contribute significantly to 

downwind ozone problems, whether 

they be nonattainment or maintenance, 

in other states.’’ 143 As a result, the EPA 

proposes to disapprove California’s 

submittal 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) for the 2015 ozone 

NAAQS. 

F. Tribal Consultation 

On February 15, 2022, the EPA sent 

letters to the Morongo and Pechanga 

tribes inviting consultation on this 

proposed action.144 On March 2, 2022, 

the EPA held an informational meeting 

with the Morongo Tribe. The Morongo 

and Pechanga tribes did not request 

consultation on this Regional action. On 

April 7, 2022, the EPA opened a 30-day 

window for federally recognized tribes 

to request consultation on the national 

FIP proposal. 

G. Conclusion 

Based on the EPA’s evaluation of 

California’s SIP submission, the EPA is 

proposing to find that the portion of 

California’s October 1, 2018 SIP 

Submittal addressing CAA section 

110(a)(2)(D)(i)(I) does not meet the 

State’s interstate transport obligations, 

because it fails to contain the necessary 

provisions to eliminate emissions that 

will contribute significantly to 

nonattainment or interfere with 

maintenance of the 2015 8-hour ozone 

NAAQS in any other state. 

IV. Proposed Action 

We are proposing to disapprove 

Enclosure 4 of California’s 2018 SIP 

Submittal 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 California 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 

submittal that meets these requirements. 

Disapproval does not start a mandatory 

sanctions clock for California. The 

remaining elements of the State’s 

October 1, 2018 SIP Submittal are not 

addressed in this action and have been 

acted on in a separate rulemaking.145

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 (OMB) for review. 

B. Paperwork Reduction Act (PRA) 

This action does not impose an 

information collection burden under the 

PRA, because this proposed SIP 

disapproval, if finalized, will not inand-of itself create any new information 

collection burdens, but will simply 

disapprove certain State requirements 

for inclusion in the SIP. 

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 will not 

impose any requirements on small 

entities. This proposed SIP disapproval, 

if finalized, will not in-and-of itself 

create any new requirements but will 

simply disapprove certain State 

requirements for inclusion in the SIP. 

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. This action proposes to 

disapprove pre-existing requirements 

under State or local law, and imposes 

no new requirements. Accordingly, no 

additional costs to State, local, or tribal 

governments, or to the private sector, 

result from this action. 

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: Coordination 

With Indian Tribal Governments 

This action does not have tribal 

implications, as specified in Executive 

Order 13175, because the SIP revision 

that the EPA is proposing to disapprove 

would not apply on any Indian 

reservation land or in any other area 

where the EPA or an Indian tribe has 

demonstrated that a tribe has 

jurisdiction and will not impose 

substantial direct costs on tribal 

governments or preempt tribal law. 

Thus, Executive Order 13175 does not 

apply to this action. 


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146 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. 

147A 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. 

148The 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. 

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 this proposed SIP disapproval, 

if finalized, will not in-and-of itself 

create any new regulations, but will 

simply disapprove certain State 

requirements for inclusion in the SIP. 

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 (NTTAA) 

Section 12(d) of the NTTAA directs 

the EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. The EPA believes that this 

action is not subject to the requirements 

of section 12(d) of the NTTAA because 

application of those requirements would 

be inconsistent with the CAA. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Population 

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).146

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 

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.147 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).148

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 15, 2022. 

Martha Guzman Aceves, 

Regional Administrator, Region IX. 

[FR Doc. 2022–11150 Filed 5–23–22; 8:45 am] 

BILLING CODE 6560–50–P 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R05–OAR–2018–0689; FRL–9654–01– 

R5] 

Air Plan Approval; Minnesota; 

Approval of Infrastructure SIP 

Requirements for the 2015 Ozone 

NAAQS 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Proposed rule. 

SUMMARY: The Environmental Protection 

Agency (EPA) is proposing to approve 

elements of a State Implementation Plan 

(SIP) submission from Minnesota 

regarding the infrastructure 

requirements of section 110 of the Clean 

Air Act (CAA) for the 2015 ozone 

National Ambient Air Quality Standards 

(NAAQS). The infrastructure 

requirements 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. 

DATES: Comments must be received on 

or before June 23, 2022. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–R05– 

OAR–2018–0689 at https://

www.regulations.gov, or via email to 

arra.sarah@epa.gov. For comments 

submitted at Regulations.gov, follow the 

online instructions for submitting 

comments. Once submitted, comments 

cannot be edited or removed from 


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