9798 Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R06–OAR–2021–0801, EPA–HQ– 

OAR–2021–0663; FRL–9338–01–R6] 

Air Plan Disapproval; Arkansas, 

Louisiana, Oklahoma, and Texas; 

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 Federal Clean 

Air Act (CAA or the Act), the 

Environmental Protection Agency (EPA 

or Agency) is proposing to disapprove 

State Implementation Plan (SIP) 

submittals from Arkansas, Louisiana, 

Oklahoma and Texas regarding 

interstate transport for the 2015 8-hour 

ozone national ambient air quality 

standard (NAAQS). This provision 

requires that each state’s SIP contain 

adequate provisions to prohibit 

emissions from within the state from 

significantly contributing to 

nonattainment or interfering with 

maintenance of the NAAQS in other 

states. The ‘‘good neighbor’’ or 

‘‘interstate transport’’ 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: Written comments must be 

received on or before April 25, 2022. 

ADDRESSES: You may send comments, 

identified as Docket No. EPA–R06– 

OAR–2021–0801, 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 fuerst.sherry@

epa.gov. Include Docket ID No. EPA– 

R06–OAR–2021–0801 in the subject line 

of the message. 

Instructions: All comments submitted 

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. Out of an abundance of 

caution for members of the public and 

our staff, the EPA Docket Center and 

Reading Room are open to the public by 

appointment only to reduce the risk of 

transmitting COVID–19. Our Docket 

Center staff also continues to provide 

remote customer service via email, 

phone, and webform. For further 

information on the EPA Docket Center 

services and the current status, please 

visit us online at https://www.epa.gov/ 

dockets. 

FOR FURTHER INFORMATION CONTACT: 

Sherry Fuerst, EPA Region 6 Office, AR– 

SI, 214–665–6454, fuerst.sherry@

epa.gov. We encourage the public to 

submit comments via https://

www.regulations.gov, as there will be a 

delay in processing mail and no courier 

or hand deliveries will be accepted. 

Please call or email the contact above if 

you need alternative access to material 

indexed but not provided in the docket. 

SUPPLEMENTARY INFORMATION: Public 

Participation: Submit your comments, 

identified by Docket ID No. EPA–R06– 

OAR–2021–0801, 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 

the EPA’s docket at https://

www.regulations.gov any information 

you consider to be Confidential 

Business Information (CBI) or other 

information whose disclosure is 

restricted by statute. Multimedia 

submissions (audio, video, etc.) must be 

accompanied by a written comment. 

The written comment is considered the 

official comment and should include 

discussion of all points you wish to 

make. The EPA will generally not 

consider comments or comment 

contents located outside of the primary 

submission (i.e., on the web, cloud, or 

other file sharing system). 

There are two dockets supporting this 

action, EPA–R06–OAR–2021–0801 and 

EPA–HQ–OAR–2021–0663. Docket No. 

EPA–R06–OAR–2021–0801 contains 

information specific to Arkansas, 

Louisiana, Oklahoma, and Texas, 

including the notice of proposed 

rulemaking, submittals from the states, 

and the EPA Region 6 2015 8-Hour 

Ozone Transport SIP Proposal Technical 

Support Document (EPA Region 6 TSD). 

Docket No. EPA–HQ–OAR–2021–0663 

contains additional modeling files, 

emissions inventory files, technical 

support documents, and other relevant 

supporting documentation regarding 

interstate transport of emissions for the 

2015 8-hour ozone NAAQS which are 

being used to support this action, 

including Preparation of Emissions 

Inventories for the 2016v2 North 

American Emissions Modeling Platform, 

and Air Quality Modeling TSD for 2015 

ozone NAAQS Transport SIP Proposed 

Actions. All comments regarding 

information in either of these dockets 

are to be made in Docket No. EPA–R06– 

OAR–2021–0801. For additional 

submission methods, please contact 

Sherry Fuerst, 214–665–6454, 

fuerst.sherry@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-epadockets. Due to public health concerns 

related to COVID–19, the EPA Docket 

Center and Reading Room are open to 

the public by appointment only. Our 

Docket Center staff also continues to 

provide remote customer service via 

email, phone, and webform. For further 

information and updates on EPA Docket 

Center services, please visit us online at 

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

The EPA continues to carefully and 

continuously monitor information from 

the Centers for Disease Control and 

Prevention (CDC), local area health 

departments, and our Federal partners 

so that we can respond rapidly as 

conditions change regarding COVID–19. 

The index to the dockets for this 

action, Docket No. EPA–R06–OAR– 

2021–0801 and EPA–HQ–OAR–2021– 

0663, are available electronically at 

https://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 4-Step 

Interstate Transport Regulatory Process 

C. Background on the EPA’s Ozone 

Transport Modeling Information 

D. The EPA’s Approach to Evaluating 

Interstate Transport SIPs for the 2015 

Ozone NAAQS 

1. Selection of Analytic Year 

2. Step 1 of the 4-Step Interstate Transport 

Framework 

3. Step 2 of the 4-Step Interstate Transport 

Framework 


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1 ‘‘National Ambient Air Quality Standards for 

Ozone’’, Final Rule, 80 FR 65292 (October 26, 

2015). Although the level of the standard is 

specified in the units of ppm, ozone concentrations 

are also described in parts per billion (ppb). For 

example, 0.070 ppm is equivalent to 70 ppb. 

2SIP revisions that are intended to meet the 

applicable requirements of section 110(a)(1) and (2) 

of the CAA are often referred to as infrastructure 

SIPs and the applicable elements under section 

110(a)(2) are referred to as infrastructure 

requirements. 

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

11 (D.C. Cir. 2008). 

4See ‘‘Federal Implementation Plans: Interstate 

Transport of Fine Particulate Matter and Ozone and 

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

8, 2011). 

5See ‘‘Cross-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 (D.C. Cir. 2019). The 

Revised CSAPR Update for the 2008 Ozone 

NAAQS, 86 FR 23054 (April 30, 2021), responded 

to the remand of the CSAPR Update in Wisconsin 

and the vacatur of a separate rule, the ‘‘CSAPR 

Close-Out,’’ 83 FR 65878 (December 21, 2018), in 

New York v. EPA, 781 F. App’x. 4 (D.C. Cir. 2019). 

7 In addition to the CSAPR rulemakings, other 

regional rulemakings addressing ozone transport 

include the ‘‘NOX SIP Call,’’ 63 FR 57356 (October 

27, 1998), and the ‘‘Clean Air Interstate Rule’’ 

(CAIR), 70 FR 25162 (May 12, 2005). 

4. Step 3 of the 4-Step Interstate Transport 

Framework 

5. Step 4 of the 4-Step Interstate Transport 

Framework 

II. Arkansas SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS and the EPA 

Evaluation of the SIP Submission 

A. Summary of ADEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

B. EPA Evaluation of the ADEQ SIP 

Submission 

1. Evaluation of Information Provided by 

ADEQ Regarding Step 1 

2. Evaluation of Information Provided by 

ADEQ Regarding Step 2 

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

Modeling and Findings for Arkansas 

4. Evaluation of Information Provided by 

ADEQ Regarding Step 3 

5. Evaluation of Information Provided by 

ADEQ Regarding Step 4 

6. Conclusion 

III. Louisiana SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS and the EPA 

Evaluation of the SIP Submission 

A. Summary of LDEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

B. EPA Evaluation of the LDEQ SIP 

Submission 

1. Evaluation of Information Provided by 

LDEQ Regarding Steps 1 and 2 

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

Modeling and Findings for Louisiana 

3. Evaluation of Information Provided by 

LDEQ Regarding Step 3 

4. Evaluation of Information Provided by 

LDEQ Regarding Step 4 

5. Conclusion 

IV. Oklahoma SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS and the EPA 

Evaluation of the SIP Submission 

A. Summary of ODEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

B. EPA Evaluation of the ODEQ SIP 

Submission 

1. Evaluation of Information Provided by 

ODEQ Regarding Steps 1 and 2 

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

Modeling and Findings for Oklahoma 

3. Evaluation of Information Provided by 

ODEQ Regarding Step 3 

4. Evaluation of Information Provided by 

ODEQ Regarding Step 4 

5. Conclusion 

C. Impact on Areas of Indian Country 

V. Texas SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS and the EPA 

Evaluation of the SIP Submission 

A. Summary of TCEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

B. EPA Evaluation of the TCEQ SIP 

Submission 

1. Evaluation of Information Provided by 

TCEQ Regarding Step 1 

i. Evaluation of TCEQ’s Methodology for 

Identifying Maintenance Receptors 

ii. Evaluation of the TCEQ Modeling 

2. Evaluation of Information Provided by 

TCEQ Regarding Step 2 

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

Modeling and Findings for Texas 

4. Evaluation of Information Provided by 

TCEQ Regarding Step 3 

5. Evaluation of Information Provided by 

TCEQ Regarding Step 4 

6. Conclusion 

VI. Proposed Action 

VII. Statutory and Executive Order Reviews 

I. Background 

A. Description of Statutory Background 

On October 1, 2015, the EPA 

promulgated a revision to the 2015 8- 

hour ozone NAAQS (2015 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 requirements, often referred to 

as ‘‘prongs’’ within CAA section 

110(a)(2)(D)(i)(I). A SIP for a new or 

revised NAAQS must contain adequate 

provisions prohibiting any source or 

other type of emissions activity within 

the state from emitting air pollutants in 

amounts that will significantly 

contribute to nonattainment of the 

NAAQS in another state (prong 1) or 

interfere with maintenance of the 

NAAQS in another state (prong 2). The 

EPA and states must give independent 

significance to prong 1 and prong 2 

when evaluating downwind air quality 

problems under CAA section 

110(a)(2)(D)(i)(I).3

B. Description of the EPA’s 4-Step 

Interstate Transport Regulatory Process 

The EPA is using the 4-Step interstate 

transport framework (or 4-Step 

framework) described in detail below to 

evaluate states’ SIP submittals 

addressing the interstate transport 

provision for the 2015 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 CrossState Air Pollution Rule (CSAPR), 

which addressed interstate transport 

with respect to the 1997 ozone NAAQS 

as well as the 1997 and 2006 fine 

particulate matter standards,4 and the 

Cross-State Air Pollution Rule Update 

(CSAPR Update) 5 and the Revised 

CSAPR Update, both of which 

addressed the 2008 ozone NAAQS.6

Through the development and 

implementation of the CSAPR 

rulemakings and prior regional 

rulemakings pursuant to the interstate 

transport provision,7 the EPA, working 

in partnership with states, developed 

the following 4-Step framework to 

evaluate a state’s obligations to 

eliminate interstate transport emissions 

under the interstate transport provision 

for the ozone NAAQS: (1) Identify 

monitoring sites that are projected to 

have problems attaining and/or 

maintaining the NAAQS (i.e., 

nonattainment and/or maintenance 

receptors); (2) identify states that impact 

those air quality problems in other (i.e., 

downwind) states sufficiently such that 

the states are considered ‘‘linked’’ and 

therefore warrant further review and 

analysis; (3) identify the emissions 

reductions necessary (if any), applying a 

multifactor analysis, to eliminate each 

linked upwind state’s significant 

contribution to nonattainment or 

interference with maintenance of the 

NAAQS at the locations identified in 

Step 1; and (4) adopt permanent and 

enforceable measures needed to achieve 

those emissions reductions. 


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8A design value is a statistic that describes the 

air quality status of a given location relative to the 

level of the NAAQS. Design values are typically 

used to designate and classify nonattainment areas, 

as well as to assess progress towards meeting the 

NAAQS. See https://www.epa.gov/air-trends/airquality-design-values#report. 

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 at 1735. 

11See EPA memorandum, ‘‘Information on the 

Interstate Transport State Implementation Plan 

Submissions for the 2008 Ozone National Ambient 

Air Quality Standards under Clean Air Act section 

110(a)(2)(D)(i)(I)’’, October 27, 2017, (‘‘October 2017 

memorandum’’) available in Docket ID No. EPA– 

HQ–OAR–2021–0663 or at https://www.epa.gov/ 

interstate-air-pollution-transport/interstate-airpollution-transport-memos-and-notices. 

12See EPA memorandum, ‘‘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 or at https://www.epa.gov/ 

interstate-air-pollution-transport/interstate-airpollution-transport-memos-and-notices. 

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 interstate transport provision. Any such 

determination would be made through notice-andcomment rulemaking.’’ 

14See EPA memorandums, ‘‘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 or at https://www.epa.gov/airmarkets/memoand-supplemental-information-regarding-interstatetransport-sips-2015-ozone-naaqs. 

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 (Oct. 30, 2020). 

17See the Air Quality Modeling Technical 

Support Document for the Final Revised Cross-State 

Air Pollution Rule Update, available in Docket ID 

No. EPA–HQ–OAR–2021–0063 for this action. 

18Additional details and documentation related 

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

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

19Ramboll Environment and Health, January 

2021, www.camx.com. 

C. Background on the EPA’s Ozone 

Transport Modeling Information 

In general, the EPA has performed 

nationwide air quality modeling to 

project ozone design values (DVs) 8

which are used in combination with 

measured data to identify 

nonattainment and maintenance 

receptors. To quantify the contribution 

of emissions from specific upwind 

states on 2023 ozone DVs for the 

identified downwind nonattainment 

and maintenance receptors, the EPA 

performed nationwide, state-level ozone 

source apportionment modeling for 

2023. The source apportionment 

modeling provided contributions to 

ozone at receptors from precursor 

emissions of anthropogenic nitrogen 

oxides (NOX) and volatile organic 

compounds (VOCs) in individual 

upwind states. 

The EPA has released several 

documents containing projected ozone 

design values, contributions, and 

information relevant to evaluating 

interstate transport with respect to the 

2015 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 DVs 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 

NAAQS.10 On October 27, 2017, we 

released a memorandum (October 2017 

memorandum) containing updated 

modeling data for 2023, which 

incorporated changes made in response 

to comments on the NODA, and noted 

that the modeling may be useful for 

states developing SIPs to address 

interstate transport obligations for the 

2008 ozone NAAQS.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 ozone NAAQS at 

Step 1 of the 4-Step framework.12 The 

March 2018 memorandum also included 

the then newly available contribution 

modeling data to assist states in 

evaluating their impact on potential 

downwind air quality problems for the 

2015 ozone NAAQS under Step 2 of the 

4-Step 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 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 DVs and contribution data 

provided in the 2017 and 2018 

memoranda. The EPA used the 2016v1 

emissions to project ozone DVs 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

See 85 FR 68964, 68981. Although the 

Revised CSAPR Update addressed 

transport for the 2008 ozone NAAQS, 

the projected DVs 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 Revised CSAPR Update 

final rule, 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 Technical Support 

Document (TSD) Preparation of 

Emissions Inventories for the 2016v2 

North American Emissions Modeling 

Platform, which is included in Docket 

ID No. EPA–HQ–OAR–2021–0663. The 

EPA performed air quality modeling of 

the 2016v2 emissions using the most 

recent publicly released version of the 

Comprehensive Air-quality Model with 

extensions (CAMx) photochemical 

modeling, version 7.10.19 The EPA now 

proposes to rely on the air quality 

modeling performed using CAMx, 

version 7.10, and the newly available 

2016v2 emissions platform in evaluating 

states’ submissions with respect to Steps 


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20See 63 FR 57356. The NOX SIP Call required 

22 eastern states and the District of Columbia to 

submit state implementation plans (SIPs) that set 

statewide ozone season NOx budgets which would 

reduce emissions of NOX. 

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 

Aug. 3, 2018). 

1 and 2 of the 4-Step 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. Sections II–V 

of this action 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 action, the 

EPA is inviting public comment on this 

updated 2023 modeling, which uses a 

2016v2 emissions platform. Per the 

instructions in the Supplementary 

Information section above, all public 

comments, including comments on the 

EPA’s air quality modeling should be 

submitted in the Regional docket for 

this action, Docket ID No. EPA–R06– 

OAR–2021–0801. Comments are not 

being accepted in Docket 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 Multi-Jurisdictional 

Organizations (MJOs) to evaluate 

downwind air quality problems and 

contributions as part of their 

submissions. In Sections II–V of this 

action, we evaluate how the states used 

air quality modeling information in 

their submissions. 

D. The EPA’s Approach to Evaluating 

Interstate Transport SIPs for the 2015 

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 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 1998 

NOX SIP Call 20 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 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 Attachment A 

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 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 analysis. 

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 interstate transport 

provision, to assess downwind air 

quality as expeditiously as practicable 

and no later than the next applicable 


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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 

and 2 of the 4-Step 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 896, 910– 

11 (D.C. Cir. 2008) (holding that the EPA must give 

‘‘independent significance’’ to each prong of CAA 

section 110(a)(2)(D)(i)(I)). 

28See 81 FR 74504 (October 26, 2016). This same 

concept, relying on both current monitoring data 

and modeling to define nonattainment receptor, 

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

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

531 F.3d at 913–14 (affirming as reasonable EPA’s 

approach to defining nonattainment in CAIR). 

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). 

attainment date,25 which is now the 

Moderate area attainment date under 

CAA section 181 for ozone 

nonattainment. The Moderate area 

attainment date for the 2015 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 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 ozone NAAQS. 

The EPA recognizes that the 

attainment date for nonattainment areas 

classified as Marginal for the 2015 

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 23074; see also 

Wisconsin, 938 F.3d at 322. 

Consequently, in this proposal the EPA 

proposes to 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 ozone NAAQS. 

2. Step 1 of the 4-Step Interstate 

Transport Framework 

In Step 1, the EPA identifies 

monitoring sites that are projected to 

have problems attaining and/or 

maintaining the NAAQS in the 2023 

analytic year. Where the EPA’s analysis 

shows that a site does not fall under the 

definition of a nonattainment or 

maintenance receptor, that site is 

excluded from further analysis under 

the EPA’s 4-Step framework. For sites 

that are identified as a nonattainment or 

maintenance receptor in 2023, we 

proceed to the next step of our 4-Step 

framework by identifying the upwind 

state’s contribution to those receptors. 

The EPA’s approach to identifying 

ozone nonattainment and maintenance 

receptors in this action is consistent 

with the approach used in previous 

transport rulemakings. The EPA’s 

approach gives independent 

consideration to both the ‘‘contribute 

significantly to nonattainment’’ and the 

‘‘interfere with maintenance’’ prongs of 

CAA section 110(a)(2)(D)(i)(I), 

consistent with the D.C. Circuit’s 

direction in North Carolina v. EPA.27

For the purpose of this proposal, the 

EPA identifies ‘‘nonattainment’’ 

receptors as those monitoring sites that 

are projected to have average DVs in 

2023 that exceed the NAAQS and that 

are also measuring nonattainment based 

on the most recent monitored DVs. 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 DV at each 

receptor based on a projection of the 

maximum measured DV over the 

relevant period. The EPA interprets the 

projected maximum future DV 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, 

air mass patterns) promoting ozone 

formation that led to maximum 

concentrations in the measured data 

may reoccur in the future. The 

maximum DV 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 DV 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 DVs above the level of 

the applicable NAAQS, but that are not 

currently measuring nonattainment 

based on the most recent official DVs. In 

addition, those monitoring sites with 

projected average DVs below the 

NAAQS, but with projected maximum 

DVs above the NAAQS are also 

identified as ‘‘maintenance only’’ 

receptors, even if they are currently 

measuring nonattainment based on the 

most recent official DVs. 

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 ozone NAAQS), the 

upwind state is not ‘‘linked’’ to a 

downwind air quality problem, and the 

EPA, therefore, concludes that the state 

does not significantly contribute to 


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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. 

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 of 

controls as part of a multifactor 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 continue 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 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. For ozone, as the 

EPA found in the Clean Air Interstate 

Rule (CAIR), CSAPR, and CSAPR 

Update, a portion of the nonattainment 

problem 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 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 at 74518. 

See also 86 FR at 23085 (reviewing and 

explaining rationale from CSAPR); 76 

FR at 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 

analysis 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 costeffectiveness 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 preparing a 

multifactor assessment that evaluates 

additional available control 

opportunities. 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 

In Step 4, states (or the EPA) develop 

permanent and federally enforceable 

control strategies to achieve the 

emissions reductions determined to be 

necessary in Step 3 to eliminate 

significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. For a state 

linked in Steps 1 and 2 to rely on an 

emissions control measure in 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 

section 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. Arkansas SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

and the EPA Evaluation of the SIP 

Submission 

A. Summary of ADEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

On October 10, 2019, the Arkansas 

Division of Environmental Quality 

(ADEQ) of the Arkansas Department of 

Energy and Environment made a SIP 

submission addressing interstate 

transport of air pollution for the 2015 

ozone NAAQS. The ADEQ SIP 

submission provided an analysis of 

Arkansas’s air emissions impact to 

downwind states using the EPA’s 4-Step 

framework and an analytic year of 2023 

and concluded that the State’s air 

emissions will not contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in other states. 

To identify downwind monitors 

projected to be in nonattainment and/or 

have maintenance issues in 2023 (Step 

1), ADEQ relied on the EPA’s interstate 

transport modeling results that are 

included as an attachment to the March 

2018 memorandum. The EPA modeling 

results included with the March 2018 

memorandum provide: (1) Projected 


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31 ‘‘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, available in Docket ID 

No. EPA–HQ–OAR–2021–0663 or at https://

www.epa.gov/airmarkets/memo-and-supplementalinformation-regarding-interstate-transport-sips2015-ozone-naaqs. 

32See EPA memorandum from Peter Tsirigotis, 

Director of the Office of Air Quality planning and 

Standards, April 17, 2018, ‘‘Guidance on Significant 

Impact Levels for Ozone and Fine Particles in the 

Prevention of Significant Deterioration Permitting 

Program’’ (‘‘SILs Guidance’’ or ‘‘April 2018 

memorandum’’), available at: https://www.epa.gov/ 

sites/default/files/2018-04/documents/sils_policy_

guidance_document_final_signed_4-17-18.pdf. 

33Table AR–1 lists all sites that the EPA projected 

to have a fy 2023 average DV or fy 2023 maximum 

DV greater than 70.9 ppb in our March 2018 

memorandum. As Arkansas stated in the SIP 

submission, the EPA considers sites matching these 

criteria to be projected nonattainment areas and 

projected maintenance areas, respectively. ADEQ 

ranked these sites by Arkansas’s potential 

contribution, which the EPA determined based on 

the daily eight-hour average contributions on the 

top ten concentration days in 2023. 

average DV and maximum DV for the 

future year 2023 (fy 2023) for ozone 

monitors projected to be potential 

nonattainment or maintenance receptors 

in the 48 contiguous States and (2) the 

expected contribution of State emissions 

to the projected ozone concentrations at 

each ozone monitor. 

At Step 2, ADEQ identified those 

states to which Arkansas contributes 

emissions and then applied a 1 ppb 

contribution threshold to determine 

projected nonattainment and/or 

maintenance receptors in other states 

that might be significantly impacted by 

emissions from Arkansas. ADEQ 

provided three rationales as a basis to 

support their decision to rely on a 1 ppb 

contribution threshold. First, ADEQ 

cited to the August 2018 

memorandum 31 that compares the 

collective contribution captured by 

three different contribution thresholds: 

1 Percent of the NAAQS, 1 ppb, and 2 

ppb. ADEQ summarized the August 

2018 memorandum and concluded that 

the 1 percent and 1 ppb contribution 

thresholds are generally comparable. 

Second, ADEQ referenced an April 2018 

memorandum 32 in which the EPA 

examined the use of a significant impact 

level (SIL) value of 1 ppb for 

determining whether a proposed 

prevention of significant deterioration 

(PSD) source causes or contributes to a 

violation of the corresponding 2015 

ozone NAAQS. Despite recognizing that 

a contribution threshold is not the same 

as a significance level, ADEQ claimed 

that a contribution threshold and 

significance level are sufficiently 

analogous to support the use of a 1 ppb 

contribution threshold. The final 

rationale ADEQ provided was based on 

the consistency with the reported 

precision of Federal reference monitors 

for ozone and the rounding 

requirements found in 40 CFR part 50, 

Appendix U, Interpretation of the 

Primary and Secondary National 

Ambient Air Quality Standards for 

Ozone. ADEQ noted that the 1 percent 

contribution threshold of 0.7 ppb is 

lower than the manufacturer’s reported 

precision of Federal reference monitors 

for ozone and that the requirements 

found in Appendix U truncates monitor 

values of 0.7 ppb to 0 ppb. 

As stated previously, ADEQ identified 

all potential nonattainment and 

maintenance receptors for fy 2023 

showing a contribution of emissions 

from Arkansas.33 These receptors are 

included in Table AR–1. 

TABLE AR–1—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS IDENTIFIED BY ARKANSAS BASED ON THE

EPA’S MARCH 2018 MEMORANDUM

Receptor 

(site ID, county, state) 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) 

Arkansas 

contribution 

(ppb) 

260050003, Allegan, MI ............................................................................................................... 69 71.7 1.64 

482011039, Harris, TX ................................................................................................................ 71.8 73.5 0.99 

480391004, Brazoria, TX ............................................................................................................. 74 74.9 0.90 

484392003, Tarrant, TX .............................................................................................................. 72.5 74.8 0.78 

481210034, Denton, TX .............................................................................................................. 69.7 72 0.58 

482011034, Harris, TX ................................................................................................................ 70.8 71.6 0.54 

551170006, Sheboygan, WI ........................................................................................................ 72.8 75.1 0.51 

550790085, Milwaukee, WI ......................................................................................................... 71.2 73 0.40 

482010024, Harris, TX ................................................................................................................ 70.4 72.8 0.29 

261630019, Wayne, MI ............................................................................................................... 69 71 0.27 

240251001, Harford, MD ............................................................................................................. 70.9 73.3 0.17 

90019003, Fairfield, CT ............................................................................................................... 73 75.9 0.13 

90013007, Fairfield, CT ............................................................................................................... 71 75 0.13 

361030002, Suffolk, NY ............................................................................................................... 74 75.5 0.12 

360810124, Queens, NY ............................................................................................................. 70.2 72 0.09 

90099002, New Haven, CT ......................................................................................................... 69.9 72.6 0.08 

90010017, Fairfield, CT ............................................................................................................... 68.9 71.2 0.07 

80590006, Jefferson, CO ............................................................................................................ 71.3 73.7 0.03 

80590011, Jefferson, CO ............................................................................................................ 70.9 73.9 0.02 

81230009, Weld, CO ................................................................................................................... 70.2 71.4 0.02 

80350004, Douglas, CO .............................................................................................................. 71.1 73.2 0.01 

Based on a 1 ppb contribution 

threshold, ADEQ identified only one fy 

2023 projected maintenance receptor, 

Allegan County, MI, and no fy 2023 

projected nonattainment receptors 

linked to Arkansas. ADEQ also cited 

other modeling performed by TCEQ and 

Midwest Ozone Group, which showed 

that when different modeling protocols 

were employed, future year DV 

projections and contributions could 

differ considerably. ADEQ therefore 

elected to consider other evidence 

regarding its linkage to air quality in 

Allegan County, MI. Specifically, ADEQ 

analyzed back trajectory information to 

infer that there is no consistent or 

persistent relationship between elevated 

ozone days in Allegan County, MI and 

air traveling through Arkansas. ADEQ 

assessed wind patterns on elevated 

ozone days—days with a maximum 

daily average 8-hour ozone (MDA8) 

greater than 70.9 ppb in Allegan County, 


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34HYbrid Single-Particle Lagrangian Integrated 

Trajectory (HYSPLIT) model is a complete system 

for computing both simple air parcel trajectories 

and complex dispersion and deposition 

simulations. The model is designed to support a 

wide range of simulations related to the 

atmospheric transport and dispersion of pollutants 

and hazardous materials to the Earth’s surface. 

35ADEQ analyzed ten years of HYSPLIT back 

trajectories to examine potential relationships 

between elevated ozone days at the Allegan County, 

MI monitor and emissions from Arkansas. In the 

SIP submission ADEQ stated their rationale for 

looking at an extended period of time is to gain a 

more complete picture of how Arkansas’s emissions 

might contribute to elevated ozone in Allegan 

County, MI, rather than relying entirely on the 

EPA’s modeling simulation, which is based on a 

single base year. 

36See the AirNow-Tech website at https://

www.airnowtech.org/. AirNow-Tech is a website for 

air quality data management analysis, and decision 

support used by the Federal, State, Tribal, and local 

air quality organizations. 

37 If the same maximum eight-hour value 

occurred multiple times a day, ADEQ evaluated all 

incidences of the value for that day. 

38EDAS is an intermittent data assimilation 

system that uses successive three-hour model 

forecasts to generate gridded meteorological fields 

that reflect observations covering the continental 

United States. EDAS is accessible at https://

ready.arl.noaa.gov/edas40.php. 

39Mixing heights (m), defined as the height above 

ground level of the layer adjacent to the ground 

over which an emitted or entrained inert nonbuoyant tracer will be mixed by turbulence. 

40Ambient air is the ‘‘portion of the atmosphere, 

external to buildings, to which the general public 

has access.’’ 40 CFR 50.1(e). 

41The number of days in a given year and the 

number of consecutive years is of particular 

relevance for the ozone NAAQS, which is 

calculated based the annual fourth-highest daily 

maximum eight-hour concentration averaged over 

three consecutive years. 

42MSA is defined as a geographic region with a 

high population density at its core and close 

economic ties throughout the area. 

43The eight linked states include Illinois, 42%; 

Indiana, 15%; Michigan, 7%; Missouri, 6%; Texas, 

5%; Wisconsin, 4%; Oklahoma, 3% and Arkansas, 

4%. The remaining contribution is labeled at 

‘‘Other’’. The linkages are based on the EPA’s 

modeling results that are attached to the March 

2018 memorandum. 

44The EPA’s Revised CSAPR Update, 86 FR 

23054 (April 20, 2021), states ‘‘. . . EPA adjusted 

its representative cost for optimizing existing SNCR 

control to $1,800 per ton in response to comments 

received on the proposed rule . . . EPA views 

$1,600 per ton for optimization of existing SCR 

control and installation of state-of-the are NOX

combustion controls and $1,800 per ton for 

optimization of existing SNCRs as comparable for 

policy purposes.’’ ADEQ’s screening analysis using 

the EPA tools (referencing the EPA’s Air Pollution 

Control Cost Estimation Spreadsheet for SCR) 

shows that cost-effectiveness values for ozoneseason operation of SCR and SNCR are: $12,605– 

$31,580/ton for SCR and $4,221–$45,581 for SNCR. 

ADEQ notes that any costs imposed to install 

controls at the examined EGUs would be passed on 

to Arkansas ratepayers. 

MI. ADEQ used the National Oceanic 

and Atmospheric Administration 

(NOAA) HYbrid Single Particle 

Lagrangian Integrated Trajectory 

(HYSPLIT) 34 model to evaluate wind 

back trajectories from over a 10-year 

period (2008–2017).35 Over the course 

of the 10-year period, ADEQ identified 

95 elevated ozone days (MDA8 > 70.9 

ppb) for the Allegan County, MI 

monitor.36 Next, ADEQ identified the 

maximum ozone value within these 

elevated ozone days.37 Using HYSPLIT, 

ADEQ ran 72-hour back trajectories 

using the hour of the maximum ozone 

value for each elevated day as the back 

trajectory start time. To consider the 

effects of vertical variations in wind 

flows on transport patterns, ADEQ used 

the following starting heights above 

ground level: 100m, 500m, 1000m, and 

1500m. ADEQ obtained 40 km grid 

meteorological data for the back 

trajectory analysis using Eta Data 

Assimilation System (EDAS) data.38 In 

total, ADEQ ran 152 back trajectories for 

each mixing height.39 ADEQ filtered the 

back trajectories to determine whether 

further analysis is warranted using two 

criteria. First, ADEQ filtered out back 

trajectories that had a starting hour 

mixing height below the back trajectory 

start height because ADEQ asserted 

these air parcels would not have 

reached ambient air 40 at the Allegan 

County, MI monitor site. Second, ADEQ 

filtered out any back trajectory that did 

not have a path through any portion of 

Arkansas. After ADEQ applied their 

filter criteria, 41 out of 608 back 

trajectories (6.74%) remained from 22 

out of the 95 elevated ozone days (23%) 

examined. Of the 10 years examined, 

ADEQ also found that air passing 

through Arkansas only reached Allegan 

County, MI on four or more days in one 

year: 2012.41 For 2012, HYSPLIT 

analyses indicated 14 Arkansas-Allegan 

County, MI linked back trajectories for 

7 days in total in 2012, whereas for 

2011, 2013, 2014, and 2016 the 

HYSPLIT analyses indicated three, two, 

zero and one days with ArkansasAllegan County, MI linked back 

trajectories, respectively. For the 10 

years ADEQ’s performed HYSPLITs, 

ADEQ’s HYSPLIT analysis indicated on 

average 2.2 days per year had 

trajectories with Arkansas-Allegan 

County, MI linked back trajectories. 

ADEQ also noted that these trajectories 

passed through other states and through 

Metropolitan Statistical Areas (MSAs) 42

both before and after traversing through 

Arkansas. Specifically, ADEQ stated 

that 37 trajectories passed through the 

Chicago-Naperville-Elgin, IL–IN–WI 

MSA prior to reaching Allegan County, 

MI. Based on these results, ADEQ 

concluded that other states and MSAs 

were more likely to have influenced 

ozone concentrations at the Allegan 

County, MI monitor on the days with 

back trajectories linked to Arkansas. 

In Step 3, ADEQ also considered air 

quality trends in Allegan County, MI, 

emission trends in other upwind states, 

relative contribution from other upwind 

states, and cost factors. ADEQ presented 

that ozone DVs in Allegan County, MI 

fluctuated over the 2008–2017 period 

with higher concentration occurring 

from 2012 through 2014 but declining 

since 2014. ADEQ also mentioned that 

despite the most recent 2017 DV for the 

Allegan County monitor continuing to 

show an exceedance of the 2015 ozone 

NAAQS, the EPA-projected 2023 ozone 

average DV at the Allegan County, MI 

monitor, based on data provided in the 

March 2018 memorandum, is 69.0 ppb, 

which would be in attainment of the 

2015 ozone NAAQS in 2023. 

Next, ADEQ included an evaluation of 

the relative contribution and the 

emission trends from the eight states 43

with contributions greater than 1 ppb to 

the Allegan County, MI receptor. The 

emission trends evaluation examined 

ozone precursors, nitrogen oxides (NOX) 

and volatile organic compounds (VOC), 

from 2011 to 2017 and the model 

projected fy 2023 emissions level. 

ADEQ noted that the two states with the 

highest contributions to Allegan County, 

MI—Illinois and Indiana—have both 

experienced year-over-year decreases in 

NOX emissions in excess of 20,000 tons 

of NOX reduced per year. Arkansas had 

also experienced decreases in NOX

emissions each evaluated year and 

emitted less NOX than any other of the 

potentially linked states. In addition, 

ADEQ referenced the EPA projections 

showing that most potentially linked 

states will continue to realize reductions 

in NOX, as well as VOCs, through 2023. 

ADEQ confirmed that based on this 

analysis, the overall general trends of 

NOX and VOC emissions are declining 

from Arkansas and the other linked 

states. The continuation of trends in the 

emissions reductions observed, 

particularly from Illinois and Indiana, 

are anticipated by ADEQ to result in air 

quality improvements in Allegan 

County, MI. 

In terms of cost analysis, ADEQ 

focused only on the cost of NOX

controls at electric generating units 

(EGUs) in the State because EGUs are 

the largest source of NOX emissions that 

ADEQ regulates. In its analysis, ADEQ 

found that the costs to install additional 

NOX controls (selective catalytic 

reduction, SCR and selective 

noncatalytic reduction, SNCR) at EGUs 

exceed the EPA’s cost thresholds used 

for the CSAPR and CSAPR Update 

rules.44 Based on ADEQ’s evaluation of 

the evidence, ADEQ concluded that no 

additional controls beyond pre-existing 


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45Per the instructions in the Supplementary 

Information section above, all public comments, 

including comments on the EPA’s air quality 

modeling should be submitted in the Regional 

docket for this action, Docket ID No. EPA–R06– 

OAR–2021–0801. Comments are not being accepted 

in Docket No. EPA–HQ–OAR–2021–0663. 

46We note that ADEQ identified additional 

modeling performed by TCEQ and Midwest Ozone 

Group, but simply concluded that different 

modeling can lead to differences in DV projections 

and ozone contributions of these two alternative 

modeling analyses, only TCEQ’s modeling using a 

2012 base year identified receptors in Texas that 

projected different DVs for the Texas receptors 

identified in the EPA’s 2011 base year. We discuss 

the EPA’s review of the TCEQ’s modeling elsewhere 

in this action and the Technical Support Document 

for this action ‘‘EPA Region 6 2015 8-Hour Ozone 

Transport SIP Proposal Technical Support 

Document’’ (EPA Region 6 2015 Ozone Transport 

SIP TSD.pdf) included in the Regional docket for 

this action (Docket ID No. EPA–R06–OAR–2021– 

0801), but we do conclude that TCEQ and recent 

monitoring data indicate that there are problematic 

receptors that are expected to be either 

nonattainment or maintenance receptors in 2023 

including the Texas receptors that the EPA 

identified in our March 2018 memorandum with 

Arkansas linkages. 

state and Federal regulations were 

warranted for Arkansas sources to 

satisfy interstate transport obligations 

for the 2015 ozone NAAQS. 

Based on the determinations made by 

ADEQ at Steps 1 through 3, ADEQ did 

not include any new control measures 

in the SIP submission to reduce ozone 

precursor emissions as part of a Step 4 

analysis. 

B. EPA Evaluation of the ADEQ SIP 

Submission 

The EPA is proposing to find that 

ADEQ’s October 10, 2019, SIP 

submission does not meet the State’s 

obligations with respect to prohibiting 

emissions that contribute significantly 

to nonattainment or interfere with 

maintenance of the 2015 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 therefore 

proposing to disapprove ADEQ’s SIP 

submission. 

1. Evaluation of Information Provided 

by ADEQ Regarding Step 1 

At Step 1 of the 4-Step framework, 

ADEQ relied on the EPA modeling 

released in the March 2018 

memorandum to identify nonattainment 

and maintenance receptors in 2023. As 

described in Section I of this action, the 

EPA has recently performed updated 

modeling using the 2016v2 platform to 

evaluate interstate transport of ozone for 

a fy 2023.45 The EPA proposes to 

primarily rely on the EPA’s modeling 

using the 2016v2 platform (EPA 2016v2 

modeling), to identify projected 

nonattainment and maintenance 

receptors in fy 2023. Updating the base 

period from 2011 (base period used in 

data included in the March 2018 

memorandum) to a more recent year 

(2016) allows for better projections of 

which monitors will have problems 

attaining and/or maintaining the 2015 

ozone NAAQS and factors in more 

recent base year DVs. The EPA notes 

that with a switch from 2011 base 

period meteorology to 2016 base period 

meteorology, it is normal and expected 

that the potential downwind 

nonattainment or maintenance receptors 

would change due to the different 

weather patterns that occurred in the 

different base periods, which impacts 

both the transport of pollutants from 

upwind states and what receptors have 

higher monitored values within 

nonattainment/maintenance regions.46

Modeling using both the 2011 and 2016 

based years consistently project that 

certain areas will have problems 

attaining and/or maintaining the 2015 

ozone NAAQS including receptors in 

Texas. 

2. Evaluation of Information Provided 

by ADEQ Regarding Step 2 

As noted earlier, ADEQ utilized a 1 

ppb threshold at Step 2 to identify 

whether the State was ‘‘linked’’ to a 

projected downwind nonattainment or 

maintenance receptor. ADEQ identified 

linkages for Arkansas to one 2023 

projected maintenance receptor, Allegan 

County, MI, and no 2023 projected 

nonattainment receptors. 

As discussed in the EPA’s August 

2018 memorandum, with appropriate 

additional analysis it may be reasonable 

for states to use a 1 ppb contribution 

threshold as an alternative to a 1 percent 

threshold, at Step 2 of the 4-Step 

interstate transport framework, for the 

purposes of identifying linkages to 

downwind receptors. However, the 

EPA’s August 2018 memorandum 

provided that whether or not a 1 ppb 

threshold is appropriate must be based 

on an evaluation of state-specific 

circumstances, and no such evaluation 

was included in the state’s submittal. 

Instead, ADEQ cited to the EPA’s SILs 

Guidance as a basis to support the use 

of a 1 ppb threshold; however, ADEQ 

did not explain the relevance of the SILs 

Guidance to ADEQ’s statutory obligation 

under the interstate transport provision. 

The SILs Guidance relates to a different 

provision of the Clean Air Act regarding 

implementation of the prevention of 

significant deterioration (PSD) 

permitting program, i.e., a program that 

applies in areas that have been 

designated attainment of the NAAQS. 

The SILs Guidance is not applicable to 

the interstate transport provision, which 

requires states to eliminate significant 

contribution or interference with 

maintenance of the NAAQS at known 

and ongoing air quality problem areas in 

other states. The EPA does not, in this 

action, agree that the State has justified 

its application of the 1 ppb threshold. In 

any case, both the EPA’s most recent 

modeling, EPA 2016v2 modeling, and 

the modeling relied on by ADEQ in its 

SIP submittal, indicate that the State is 

projected to contribute greater than both 

the 1 percent and alternative 1 ppb 

thresholds. While the EPA does not, in 

this action, propose to approve of the 

State’s application of the 1 ppb 

threshold, because the State has 

linkages greater than 1 ppb to projected 

downwind nonattainment or 

maintenance receptors, (as shown in 

Table AR–2) the State’s use of this 

alternative threshold at Step 2 of the 4- 

Step interstate framework would not 

alter our review and proposed 

disapproval of this SIP submittal. 

Additionally, the EPA here shares 

further evaluation of its experience 

since the issuance of the August 2018 

memorandum regarding use of 

alternative thresholds at Step 2. This 

experience leads the Agency to now 

believe it may not be appropriate to 

continue to attempt to recognize 

alternative contribution thresholds at 

Step 2. The August 2018 memorandum 

stated that ‘‘it may be reasonable and 

appropriate’’ for states to rely on an 

alternative threshold of 1 ppb threshold 

at Step 2. (The memorandum also 

indicated that any higher alternative 

threshold, such as 2 ppb, would likely 

not be appropriate.) However, the EPA 

also provided that ‘‘air agencies should 

consider whether the recommendations 

in this guidance are appropriate for each 

situation.’’ Following receipt and review 

of 49 interstate transport SIP submittals 

for the 2015 ozone NAAQS, the EPA’s 

experience has been that nearly every 

state that attempted to rely on a 1 ppb 

threshold did not provide sufficient 

information and analysis to support a 

determination that an alternative 

threshold was reasonable or appropriate 

for that state. 

For instance, in nearly all submittals, 

the states did not provide the EPA with 

analysis specific to their state or the 

receptors to which its emissions are 

potentially linked. In one case, the 

proposed approval of Iowa’s SIP 

submittal, the EPA expended its own 

resources to attempt to supplement the 

information submitted by the state, in 

order to more thoroughly evaluate the 

state-specific circumstances that could 


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47 ‘‘Air Plan Approval; Iowa; Infrastructure State 

Implementation Plan Requirements for the 2015 

Ozone National Ambient Air Quality Standard’’, 85 

FR 12232 (March 2, 2020). The agency received 

adverse comments on this proposed approval and 

has not taken final action with respect to this 

proposal. 

48We note that Congress has placed on the EPA 

a general obligation to ensure the requirements of 

the CAA are implemented consistently across states 

and regions. See CAA section 301(a)(2). Where the 

management and regulation of interstate pollution 

levels spanning many states is at stake, consistency 

in application of CAA requirements is paramount. 49See August 2018 memorandum, at page 4. 

50 Design values and contributions at individual 

monitoring sites nationwide are provided in the 

file: 2016v2_DVs_state_contributions.xlsx which is 

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

0663. 

support approval.47 It was at the EPA’s 

sole discretion to perform this analysis 

in support of the state’s submittal, and 

the Agency is not obligated to conduct 

supplemental analysis to fill the gaps 

whenever it believes a state’s analysis is 

insufficient. The Agency no longer 

intends to undertake supplemental 

analysis of SIP submittals with respect 

to alternative thresholds at Step 2 for 

purposes of the 2015 ozone NAAQS. 

Furthermore, the EPA’s experience 

since 2018 is that allowing for 

alternative Step 2 thresholds may be 

impractical or otherwise inadvisable for 

a number of additional policy reasons. 

For a regional air pollutant such as 

ozone, consistency in requirements and 

expectations across all states is 

essential. Based on its review of 

submittals to-date and after further 

consideration of the policy implications 

of attempting to recognize an alternative 

Step 2 threshold for certain states, the 

Agency now believes the attempted use 

of different thresholds at Step 2 with 

respect to the 2015 ozone NAAQS raises 

substantial policy consistency and 

practical implementation concerns.48

The availability of different thresholds 

at Step 2 has the potential to result in 

inconsistent application of interstate 

transport obligations based solely on the 

strength of a state’s SIP submittal at Step 

2 of the 4-Step interstate transport 

framework. From the perspective of 

ensuring effective regional 

implementation of interstate transport 

obligations, the more important analysis 

is the evaluation of the emissions 

reductions needed, if any, to address a 

state’s significant contribution after 

consideration of a multifactor analysis 

at Step 3, including a detailed 

evaluation that considers air quality 

factors and cost. Where alternative 

thresholds for purposes of Step 2 may 

be ‘‘similar’’ in terms of capturing the 

relative amount of upwind contribution 

(as described in the August 2018 

memorandum), nonetheless, use of an 

alternative threshold would allow 

certain states to avoid further evaluation 

of potential emission controls while 

other states must proceed to a Step 3 

analysis. This can create significant 

equity and consistency problems among 

states. 

Further, it is not clear that national 

ozone transport policy is best served by 

allowing for less stringent thresholds at 

Step 2. The EPA recognized in the 

August 2018 memorandum that there 

was some similarity in the amount of 

total upwind contribution captured (on 

a nationwide basis) between 1 percent 

and 1 ppb. However, the EPA notes that 

while this may be true in some sense, 

that is hardly a compelling basis to 

move to a 1 ppb threshold. Indeed, the 

1 ppb threshold has the disadvantage of 

losing a certain amount of total upwind 

contribution for further evaluation at 

Step 3 (e.g., roughly seven percent of 

total upwind state contribution was lost 

according to the modeling underlying 

the August 2018 memorandum; 49 in 

EPA 2016v2 modeling, the amount lost 

is five percent). Considering the core 

statutory objective of ensuring 

elimination of all significant 

contribution to nonattainment or 

interference of the NAAQS in other 

states and the broad, regional nature of 

the collective contribution problem with 

respect to ozone, there does not appear 

to be a compelling policy imperative in 

allowing some states to use a 1 ppb 

threshold while others rely on a 1 

percent of NAAQS threshold. 

Consistency with past interstate 

transport actions such as CSAPR, and 

the CSAPR Update and Revised CSAPR 

Update rulemakings (which used a Step 

2 threshold of 1 percent of the NAAQS 

for two less stringent ozone NAAQS), is 

also important. Continuing to use a 1 

percent of NAAQS approach ensures 

that as the NAAQS are revised and 

made more stringent, an appropriate 

increase in stringency at Step 2 occurs, 

so as to ensure an appropriately larger 

amount of total upwind-state 

contribution is captured for purposes of 

fully addressing interstate transport. 

Accord 76 FR 48237–38. 

Therefore, notwithstanding the 

August 2018 memorandum’s 

recognition of the potential viability of 

alternative Step 2 thresholds, and in 

particular, a potentially applicable 1 

ppb threshold, the EPA’s experience 

since the issuance of that memorandum 

has revealed substantial programmatic 

and policy difficulties in attempting to 

implement this approach. Nonetheless, 

the EPA is not, at this time, rescinding 

the August 2018 memorandum. The 

basis for the EPA’s proposed 

disapproval of ADEQ’s SIP submission 

with respect to the Step 2 analysis is, in 

the Agency’s view, warranted even 

under the terms of the August 2018 

memorandum. The EPA invites 

comment on this broader discussion of 

issues associated with alternative 

thresholds at Step 2. (See 

Supplementary Information section 

above for details and docket to submit 

comments). Depending on public 

comments received in relation to this 

action and further evaluation of this 

issue, the EPA may determine to rescind 

the 2018 memorandum in the future. 

ADEQ included information in its SIP 

submission regarding back trajectories, 

emissions trends, and EGU cost controls 

to conclude that emissions from 

Arkansas should not be considered to 

contribute significantly to 

nonattainment or interfere with 

maintenance of the NAAQS in other 

states because there is not a persistent 

and consistent pattern of contribution 

from the State. While it is not entirely 

clear whether ADEQ was analyzing 

these factors under Step 2 or Step 3, the 

EPA is evaluating such arguments under 

Step 3, as we view these statements in 

the SIP submission to speak to whether 

or not a contribution is ‘‘significant’’ 

once a linkage is established. 

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

2 Modeling and Findings for Arkansas 

As described in Section I of this 

action, the EPA performed air quality 

modeling using the 2016v2 emissions 

platform to project DVs and 

contributions for 2023 (EPA 2016v2 

modeling). This data was examined to 

determine if Arkansas contributes at or 

above the threshold of 1 percent of the 

2015 ozone NAAQS (0.70 ppb) to any 

downwind nonattainment or 

maintenance receptor. As shown in 

Table AR–2, the data 50 indicate that in 

2023, emissions from Arkansas 

contribute greater than 1 percent of the 

standards to nonattainment or 

maintenance-only receptors in Texas: 

Denton County (Monitor ID. 

481210034), Brazoria County (Monitor 

ID. 480391004), Harris County (Monitor 

ID. 482010055, Monitor ID. 482011034, 


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51These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I of this action. 

That modeling showed that Arkansas 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. 

52Allegan County Monitor ID. 260050003 is not 

a receptor in 2023 in the EPA 2016v2 modeling. 

2023 avg DV is 67.3 ppb and 2023 Max. DV is 68.4 

ppb, so the Allegan County monitor is not a 

receptor in 2023 for nonattainment or maintenance. 

53The relative response factor (RRF) is a ratio 

developed using the modeled changes between the 

base case and future case for high ozone modeled 

days. Typically, the 10 highest MDA8 modeled days 

in the base case are found and the maximum value 

from the 3x3 grid centered on the monitor for each 

day is used to calculate a 10-day average base case 

modeled value. Then a similar concentration 

average is developed for same 10 base case days and 

the same grid cell that provided the base case 

concentration to calculate a future year 10-day 

average modeled value using the future year 

modeling results. The RRF is then calculated by 

using this future year 10-day average model value 

divided by the base case year 10-day average model 

value to develop a ratio representing the change in 

modeled ozone. The RRF is then multiplied times 

the base DV value to result in a projected future 

year DV. 

and Monitor ID. 482011035).51 52

Therefore, based on the EPA’s 

evaluation of the information submitted 

by ADEQ, and based on the EPA model 

2016v2 results for 2023, the EPA 

proposes to find that Arkansas 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 AR–2—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS WITH ARKANSAS LINKAGES IN 2023 BASED

ON EPA 2016V2 MODELING

Receptor 

(site ID, county, state) 

Nonattainment/ 

maintenance 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) 

Arkansas 

contribution 

(ppb) 

481210034, Denton, TX ................................ Maintenance ..................................................... 70.4 72.2 0.76 

480391004, Brazoria, TX .............................. Maintenance ..................................................... 70.1 72.3 1.39 

482010055, Harris, TX .................................. Nonattainment .................................................. 71.0 72.0 1.00 

482011034, Harris, TX .................................. Maintenance ..................................................... 70.3 71.6 1.38 

482011035, Harris, TX .................................. Maintenance ..................................................... 68.0 71.6 1.34 

We recognize that the results of the 

EPA modeling released in the March 

2018 memorandum (2011 base year) and 

the EPA 2016v2 modeling (2016 base 

year) identified different receptors and 

linkages at Steps 1 and 2 of the 4-Step 

framework. These differing results about 

receptors and linkages can be affected 

by the varying meteorology from year to 

year, but we do not think the differing 

results mean that the modeling or the 

EPA methodology for identifying 

receptors or linkages is inherently 

unreliable. Rather, these separate 

modeling runs indicated (1) that there 

were receptors that would struggle with 

nonattainment or maintenance in the 

future, and (2) that Arkansas was linked 

to some set of these receptors, even if 

the receptors and linkages differed from 

one another in their specifics (e.g., a 

different set of receptors were identified 

to have nonattainment or maintenance 

problems, or Arkansas was linked to 

different receptors in one modeling run 

versus another). We think this common 

result indicates that Arkansas’s 

emissions were substantial enough to 

generate linkages at Steps 1 and 2 to 

some set of downwind receptors, under 

varying assumptions and meteorological 

conditions, even if the precise set of 

linkages changed between modeling 

runs. Under these circumstances, we 

think it is appropriate to proceed to a 

Step 3 analysis to determine what 

portion of Arkansas’s emissions should 

be deemed ‘‘significant.’’ In doing so, 

we are not considering our own earlier 

modeling results included in EPA’s 

March 2018 memorandum to be of equal 

reliability relative to more recent EPA 

2016v2 modeling. However, where 

alternative or older modeling generated 

linkages, even if those linkages differ 

from linkages in EPA 2016v2 modeling, 

that information provides further 

evidence, not less, in support of a 

conclusion that the state is required to 

proceed to Step 3 to further evaluate its 

emissions. 

4. Evaluation of Information Provided 

by ADEQ Regarding Step 3 

At Step 3 of the 4-Step framework, a 

state’s emissions are further evaluated, 

in light of multiple factors, including air 

quality and cost considerations, to 

determine what, if any, emissions 

contribute significantly to 

nonattainment or interfere with 

maintenance and, thus, must be 

eliminated under CAA section 

110(a)(2)(D)(i)(I). 

ADEQ included in their SIP 

submission a further analysis of its 

modeled linkage to Allegan, MI (the 

only linked receptor it analyzed, based 

on its application of a 1 ppb threshold). 

Arkansas stated that the purpose and its 

conclusion of this analysis was that it 

would not contribute significantly to the 

Allegan, MI monitor because the state’s 

emissions did not result in a consistent 

and persistent pattern of ozone 

contribution. As stated earlier, EPA 

2016v2 modeling projects that the 

Allegan County, MI receptor will be 

attaining and is not expected to have 

difficulty maintaining the standard in 

2023. As such, the EPA is not relying on 

the comparative analysis of emissions 

trends that ADEQ provided in order to 

conclude that Arkansas’s emissions do 

not contribute significantly to a 

nonattainment or maintenance problem 

in Allegan, MI. We note however, that 

ADEQ’s SIP submission and response to 

comments do not clearly define what 

ADEQ considers to be persistent and 

consistent pattern of contribution. 

Rather, the SIP submission simply states 

that contribution should be deemed 

‘‘significant’’ only if there is a persistent 

and consistent pattern of several days 

with elevated ozone. 

To be clear, the modeling establishing 

linkages of Arkansas to downwind 

nonattainment and maintenance 

receptors already establishes that there 

is a consistent and persistent pattern of 

contribution on elevated ozone days 

from Arkansas to other states. That is 

because EPA’s methodology for 

projecting future year ozone 

concentrations accounts for precisely 

these concerns—the relative response 

factor 53 that is applied to historic 

monitored data to generate projections 

is calculated by looking only at days 

with elevated ozone levels (ten days is 

preferred with a minimum of five days). 

The EPA notes that monitored 

attainment with the ozone standard is 

determined by averaging the fourth high 

value recorded each year for three years. 

So, the EPA believes it is important to 


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54The EPA reviewed the ADEQ SIP submission 

and provided comments during the State’s public 

comment period for the proposed SIP action. The 

EPA’s comment letter and ADEQ’s response to 

comments are included in ADEQ’s October 19, 

2019, SIP submission, which is available in the 

Regional docket for this action (Docket ID No. EPA– 

R06–OAR–2021–0801). 

55Concerns included removing of HYSPLIT back 

trajectories based on start height, the start time that 

Arkansas used for the back trajectories and 

removing of back trajectories when the centerline 

passed near but not through Arkansas because 

Arkansas has some very large point sources near the 

Arkansas state line that could be contributing. 

Texas also screened their HYPSLIT back trajectories 

similarly to Arkansas and we have further 

discussed our concerns and why such screening 

invalidates conclusions from the HYSPLIT back 

trajectory analyses. See EPA’s review and 

conclusions in discussion of TCEQ’s HYSPLIT 

analyses in the ‘‘EPA Region 6 2015 8-Hour Ozone 

Transport SIP Proposal Technical Support 

Document’’ (EPA Region 6 2015 Ozone Transport 

SIP TSD.pdf) included in the Regional docket for 

this action (Docket ID No. EPA–R06–OAR–2021– 

0801). 

56ADEQ’s summary of trajectories indicated that 

2011 had three linked back trajectories and 2016 

had one linked back trajectories and the EPA 

calculated the average for 2008–2017 in ADEQ’s 

table was 2.2 linked back trajectories per year. 

estimate impacts on the days with 

highest projected ozone levels. The 

EPA’s approach, as detailed in the Air 

Quality Modeling Technical Support 

Document for 2015 Ozone NAAQS 

Transport SIP Proposed Actions 

included in Docket ID No. EPA–HQ– 

OAR–2021–0663, does this by 

estimating the average fy 2023 impact 

from an upwind state on the days with 

the highest projected ozone levels at the 

downwind nonattainment or 

maintenance receptor. The days chosen 

to analyze the future impacts are chosen 

initially by the selecting the ten highest 

days in the base period modeling that 

are projected to be above 65 ppb in the 

base period. If there are not ten days 

above 65 ppb at a potential receptor, the 

number of days above 65 ppb are used 

as long as there is at least five days 

above 65 ppb in the base period. If the 

air quality modeling shows fewer than 

five days above 65 ppb in the base 

period, then the data for impacts at that 

receptor in fy 2023 are not calculated. 

The base and future year modeling for 

these five to ten days is then used to 

project fy 2023 ozone DVs to determine 

whether it is projected to be a 

nonattainment or maintenance receptor 

in 2023. For the same five to ten days 

identified, the future year modeling 

provides the estimated daily 

contribution at a potential receptor’s 

future year daily MDA8 and these daily 

contributions are averaged for the five to 

ten days to result in the average 

contribution from the upwind area. 

As mentioned previously, ADEQ used 

HYSPLIT back trajectories to assess 

wind patterns on elevated ozone days in 

an attempt to demonstrate that there is 

not persistent and consistent pattern of 

contribution from Arkansas to the 

Allegan County, MI receptor. 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 

concentration at the end point monitor. 

The horizontal and vertical areas that 

potentially contribute to the end point 

concentration 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 

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.’’ Since 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. Chemical 

transport models, such as the one relied 

upon by Arkansas to establish the 

linkage between Arkansas and those 

downwind receptors in the first 

instance, do take these factors into 

account and therefore provide a more 

robust assessment of ozone 

contribution. 

During ADEQ’s public comment 

period, the EPA submitted comments 

noting concerns regarding the 

methodology ADEQ used in their 

HYSPLIT back trajectories analysis.54

While we are not providing a detailed 

evaluation of ADEQ’s HYSPLIT analysis 

in this rulemaking, we do note that our 

review identified a number of concerns 

with how ADEQ screened out a number 

of back trajectories, which invalidates 

ADEQ’s conclusions.55 While we 

disagree with ADEQ’s methodologies 

and conclusions, we note that ADEQ’s 

HYSPLIT back trajectory information 

did not show that the base years used 

in the EPA modeling (2011 and 2016) 

demonstrated an unusual amount of 

transport of air parcels from Arkansas to 

nonattainment or maintenance receptors 

in downwind states (i.e., the modeling 

years used by the EPA do not skew the 

results toward finding linkages).56

Therefore, although Arkansas asserted 

that its additional air quality factor 

analysis using back trajectory analysis is 

a permissible way to interpret which 

contributions are ‘‘significant’’ because 

that analysis examines whether there 

was a ‘‘persistent and consistent pattern 

of contribution on several days with 

elevated ozone,’’ the modeled linkage at 

Step 2 is a superior approach for 

assessing the persistence of a state’s 

contribution. It is superior because it is 

based on the average of the 

contributions on the five to ten highest 

ozone days. Considering the form of the 

standard, this is a sufficient number of 

days to determine if an impact is 

persistent enough to impact an area’s 

ability to attain or maintain the 

standard. The modeling is also a better 

method because it accounts for 

dispersion while back trajectory 

analysis as performed by Arkansas only 

shows the centerline of air parcel travel 

and otherwise will leave out days when 

Arkansas would have contributed to 

downwind problems. Finally, because 

the modeling accounts for dispersion 

and chemical reactions, it can provide a 

quantitative estimate of contribution. 

ADEQ also contested the significance 

of its modeled contribution above 1 ppb 

based on the relatively larger 

contributions of other upwind states to 

the receptor to which it was linked. The 

EPA disagrees that a state’s small 

contribution relative to other upwind 

states is a permissible basis for finding 

no obligation under the interstate 

transport provision. CAA 

110(a)(2)(D)(i)(I) 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 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 


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57 In 2017, National Emission Inventory (NEI) 

NOX emissions from EGU sources represent 56% 

percent of the total NOx emissions categories in 

Arkansas that report emissions to the NEI. See AR 

NOx.xlsx datasheet included in the Regional docket 

for this action (Docket ID No. EPA–R06–OAR– 

2021–0801). 

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 arguments 

suggesting that upwind states should be 

excused from interstate transport 

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 

these arguments as essentially an 

argument ‘‘that an upwind State 

‘contributes significantly’ to downwind 

nonattainment only when its emissions 

are the sole cause of downwind 

nonattainment.’’ 938 F.3d 303 at 324. 

The court explained that ‘‘an upwind 

State can ‘contribute’ to downwind 

nonattainment even if its emissions are 

not the but-for cause.’’ Id. At 324–325. 

See also Catawba County v. EPA, 571 

F.3d 20, 39 (D.C. Cir. 2009) (rejecting 

the argument ‘‘that ‘significantly 

contribute’ unambiguously means 

‘strictly cause’’’ because there is ‘‘no 

reason why the statute precludes EPA 

from determining that [an] addition of 

[pollutant] into the atmosphere is 

significant even though a nearby 

county’s nonattainment problem would 

still persist in its absence’’); Miss. 

Comm’n on Envtl. Quality v. EPA, 790 

F.3d 138, 163 n.12 (D.C. Cir. 2015) 

(observing that the argument that ‘‘there 

likely would have been no violation at 

all . . . if it were not for the emissions 

resulting from [another source]’’ is 

‘‘merely a rephrasing of the but-for 

causation rule that we rejected in 

Catawba County.’’). Therefore, a state is 

not excused from eliminating its 

significant contribution on the basis that 

emissions from other states also 

contribute some amount of pollution to 

the same receptors to which the state is 

linked. 

ADEQ did not provide additional 

analysis for other receptors to which it 

was linked above 1 percent in the air 

quality modeling upon which it relied, 

and to which it continues to be linked 

in EPA 2016v2 modeling. 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 interfere with 

maintenance) in each of its prior 

Federal, regional ozone transport 

rulemakings, and this interpretation of 

the statute has been upheld by the 

Supreme Court. See EME Homer City, 

572 U.S. 489, 519 (2014). While the EPA 

has not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner the EPA has done in its 

prior regional transport rulemakings, 

state implementation plans addressing 

the obligations in CAA section 

110(a)(2)(D)(i)(I) must prohibit ‘‘any 

source or other type of emissions 

activity within the State’’ from emitting 

air pollutants which will contribute 

significantly to downwind air quality 

problems. Thus, states must complete 

something similar to the EPA’s analysis 

(or an alternative approach to defining 

‘‘significance’’ that comports with the 

statute’s objectives) to determine 

whether and to what degree emissions 

from a state should be ‘‘prohibited’’ to 

eliminate emissions that will 

‘‘contribute significantly to 

nonattainment in, or interfere with 

maintenance of’’ the NAAQS in any 

other state. As discussed below, ADEQ 

did not conduct an adequate analysis in 

their SIP submission. We therefore 

propose that ADEQ was required to 

analyze emissions from the sources and 

other emissions activity from within the 

State to determine whether its 

contributions were significant, and we 

propose to disapprove its submission 

because Arkansas failed to adequately 

do so. 

In analyzing potential additional NOX

controls, ADEQ found that additional 

controls on its EGUs would exceed the 

cost-effectiveness thresholds identified 

in the CSAPR and CSAPR Update rules. 

For the cost analysis, Arkansas only 

focused on the potential costs of NOX

controls for EGUs. As stated above, 

Arkansas found that the costs to install 

additional NOX controls (selective 

catalytic reduction, SCR, and selective 

noncatalytic reduction, SNCR) at 

electric generating units (EGUs) exceed 

EPA’s cost thresholds used for the 

CSAPR and CSAPR Update rules. Based 

on the projected cost of these controls 

relative to the thresholds used in those 

two prior EPA rules, Arkansas 

concluded that no new controls beyond 

those Federal and State regulations 

already in existence were cost-effective, 

especially considering that Allegan 

County, MI is projected to be in 

attainment with the 2015 ozone NAAQS 

and Arkansas’s small contribution 

relative to other states potentially linked 

to Allegan County, MI based on EPA’s 

modeling. 

Arkansas’s analysis is inadequate 

because its focus is only on EGUs.57 See 

Wisconsin, 938 F.3d at 318–20. We also 

find Arkansas’s conclusions as to the 

availability of cost-effective controls for 

EGUs to be inadequate. Relying on the 

CSAPR Update’s (or any other CAA 

program’s) determination of costeffectiveness without further Step 3 

analysis is not approvable. Costeffectiveness must be assessed in the 

context of the specific CAA program; 

assessing cost-effectiveness in the 

context of 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 potential air quality impacts of those 

reductions at downwind receptors. 

While the EPA has not established a 

benchmark cost-effectiveness value for 

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 

under this NAAQS to reflect higher 

marginal control costs. ADEQ’s 

submission failed to provide a 

justification for why the $1400/ton 

threshold used in the CSAPR Update is 

appropriate to rely on for the 2015 

ozone NAAQS. ADEQ’s analysis does 

not consider any air quality impacts of 

assessed controls at downwind 

receptors. As stated above, assessing 

cost-effectiveness in the context of 

ozone transport requires more than just 

assessing the cost of controls per ton of 

NOX removed. As such, ADEQ’s 

assessment of the cost of controls and 

reliance on the marginal cost threshold 

of $1,400/ton used for the CSAPR 

Update is inadequate. Furthermore, EPA 

2016v2 modeling captures all existing 

CSAPR trading programs in the baseline 

and confirms that these control 

programs were not sufficient to 

eliminate Arkansas’s linkage at Steps 1 

and 2 under the 2015 ozone NAAQS. 

The State was therefore obligated at 

Step 3 to assess additional control 

measures using a multifactor analysis. 


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58The Louisiana SIP submittal did not provide a 

specific citation to the Simon et al., 2012 reference 

to support this assertion. However, we believe the 

reference is associated with the following article: 

Simon, H., Baker, K.R., Phillips, S., 2012. 

‘‘Compilation and interpretation of photochemical 

model performance statistics published between 

2006 and 2012’’. Atmospheric Environment 61, 

124–139. 

59The five potential nonattainment and 

maintenance receptor monitors identified by LDEQ 

are from the Dallas-Fort Worth and HoustonGalveston-Brazoria, TX nonattainment areas for the 

2015 ozone NAAQS. The Louisiana SIP submittal 

appears to have inadvertently omitted Harris 

County, TX Monitor ID No. 482011034 for analysis. 

EPA’s March 2018 memorandum identified this 

monitor as a maintenance receptor with a 

contribution of 3.38 ppb from Louisiana emissions. 

5. Evaluation of Information Provided 

by ADEQ Regarding Step 4 

Step 4 of the 4-Step interstate 

transport framework calls for the 

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. ADEQ’s 

SIP submission, which looked only at 

additional NOX controls at EGUs and 

dismissed such controls as not costeffective relative to the thresholds 

established in earlier EPA transport 

rules, did not constitute an adequate 

emission reduction analysis at Step 3. 

Based on its conclusions, ADEQ did not 

revise its SIP to include any emission 

reductions. As a result, the EPA 

proposes to disapprove ADEQ’s 

submittal on the separate, additional 

basis that Arkansas has not developed 

or included permanent and enforceable 

emissions reductions in its SIP 

necessary to meet the obligations of 

CAA section 110(a)(2)(D)(i)(I). 

6. Conclusion 

Based on the EPA’s evaluation of 

ADEQ’s SIP submission, the EPA is 

proposing to find that ADEQ’s October 

19, 2019, SIP submission 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 

ozone NAAQS in any other state. 

III. Louisiana SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

and the EPA Evaluation of the SIP 

Submission 

A. Summary of LDEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

On November 13, 2019, the Louisiana 

Department of Environmental Quality 

(LDEQ) made a SIP submission 

addressing the State of Louisiana’s 

interstate transport of air pollution for 

the 2015 ozone NAAQS. The SIP 

submission provided LDEQ’s analysis of 

Louisiana’s impact to downwind states 

and concluded that emissions from 

Louisiana will not contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in other states. 

The LDEQ’s SIP submission provided 

an analysis of Louisiana’s air emissions 

impact to downwind states using a 3- 

Step alternative framework similar to 

the EPA’s 4-Step framework. LDEQ’s 3- 

Step alternative framework includes: 

Step 1: Identify monitors projected to be 

in nonattainment or have maintenance 

issues in a future year; Step 2: Identify 

projected nonattainment and/or 

maintenance monitors in other states 

that might be impacted by emissions 

from Louisiana, tagging them for further 

review; and, Step 3: Determine if 

emissions from Louisiana contribute 

significantly to nonattainment or 

interfere with maintenance at the 

monitors tagged for review in Step 2. 

LDEQ noted that its Step 1 is identical 

to the EPA’s Step 1, and its Steps 2 and 

3 are equivalent to the EPA’s Step 2. 

Louisiana further noted that Steps 3 and 

4 of the EPA’s 4-Step framework are 

relevant only if emissions from 

Louisiana contribute significantly to 

nonattainment or interfere with 

maintenance at downwind monitors in 

another state. 

LDEQ’s Step 1 was to identify 

downwind monitors projected to be in 

nonattainment and/or have maintenance 

issues in future year 2023 (fy 2023). At 

this step, LDEQ relied on the EPA’s 

interstate transport modeling results 

that are included as an attachment to 

the March 2018 memorandum. The EPA 

March 2018 modeling results provided: 

(1) Projected average DV and maximum 

DV for 2023 for the ozone monitors (or 

‘‘receptors’’) in the 48 contiguous states 

and (2) the expected contribution of 

state emissions to the projected ozone 

concentrations at each ozone monitor. 

LDEQ used a contribution threshold 

of 1 ppb in LDEQ’s Step 2 to identify 

projected nonattainment and/or 

maintenance receptors in other states 

that might be impacted by emissions 

from Louisiana and tagged them for 

further review. To support a 1 ppb 

contribution threshold, LDEQ’s 

submission stated that a 1 percent 

threshold is inappropriate because that 

value is not detectable by a monitor and 

the value of 1 percent of the 2015 ozone 

NAAQS would be truncated to zero if 

calculated in accordance with the 

method for determining DVs for the 

ozone NAAQS. LDEQ also stated that 

the more stringent threshold of 1 

percent of the NAAQS (0.7 ppb) is an 

order of magnitude smaller than the 

biases and errors typically documented 

for regional photochemical modeling.58

Based on LDEQ’s approach of evaluating 

linkages at the 1 ppb threshold, five 

Texas receptors were identified by 

Louisiana for analysis. The Texas 

receptors and corresponding receptor 

data presented in Louisiana’s SIP are 

summarized further in this notice in 

Table LA–1.59 The March 2018 

memorandum identified monitors in 

Allegan, Michigan and Milwaukee and 

Sheboygan, Wisconsin as potential 

nonattainment and maintenance-only 

receptors linked to emissions from 

Louisiana based on 1 percent of the 

NAAQS threshold. However, Louisiana 

did not include the Allegan, Michigan 

and Milwaukee and Sheboygan, 

Wisconsin receptors in the State’s 

analysis because the March 2018 

memorandum shows that Louisiana’s 

projected modeled contribution values 

to each receptor is less than 1 ppb. 

TABLE LA–1—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS IDENTIFIED BY LOUISIANA BASED ON THE

EPA’S MARCH 2018 MEMORANDUM

Receptor 

(site ID, county, state) 

2023 

Average DV 

(ppb) 60

2023 

Maximum DV 

(ppb) 61

Louisiana 

Contribution 

(ppb) 

480391004, Brazoria, TX ............................................................................................................. 74.0 74.9 3.80 


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60 Information added from the EPA’s March 2018 

memorandum. 61 Id. 62See FN 34. 

63See August 2018 memorandum, at page 4. 

64 ‘‘Air Plan Approval; Iowa; Infrastructure State 

Implementation Plan Requirements for the 2015 

Ozone National Ambient Air Quality Standard’’, 85 

FR 12232 (March 2, 2020). The Agency received 

adverse comments on this proposed approval and 

has not taken final action with respect to this 

proposal. 

TABLE LA–1—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS IDENTIFIED BY LOUISIANA BASED ON THE

EPA’S MARCH 2018 MEMORANDUM—Continued 

Receptor 

(site ID, county, state) 

2023 

Average DV 

(ppb) 60

2023 

Maximum DV 

(ppb) 61

Louisiana 

Contribution 

(ppb) 

482011039, Harris, TX ................................................................................................................ 71.8 73.5 4.72 

484392003, Tarrant, TX .............................................................................................................. 72.5 74.8 1.71 

481210034, Denton, TX .............................................................................................................. 69.7 72.0 1.92 

482010024, Harris, TX ................................................................................................................ 70.4 72.8 4.72 

ForLDEQ’s Step 3, Louisiana stated 

that an air emission contribution from 

the State should only be considered 

significant if there is a persistent and 

consistent pattern of contribution on 

several days with elevated ozone. In 

trying to determine whether there is a 

persistent and consistent pattern of 

contribution, LDEQ analyzed seasonal 

weather patterns, surface wind 

directions, and periodic back 

trajectories. LDEQ used the National 

Oceanic and Atmospheric 

Administration (NOAA) Hybrid Single 

Particle Lagrangian Integrated Trajectory 

(HYSPLIT) 62 model to perform 99 back 

trajectories for exceedances from the 

receptor monitors identified in Table 

LA–1 for 2016, 2017, and 2018. Based 

on an analysis of the HYSPLIT results, 

LDEQ stated that approximately 28% of 

the trajectories travel in or through 

Louisiana, and only 8% of those back 

trajectories originate in the State. The 

SIP submission also stated that a 

comparison of the EPA’s modeled 

contribution between Texas and 

Louisiana monitors indicates that a far 

greater proportion of the total ozone 

detected in Louisiana originates in 

Texas rather than vice versa. Therefore, 

Louisiana concluded that the impact 

from the State’s air emissions was 

insignificant to the overall attainment at 

the receptor monitors identified in 

Table LA–1 and does not significantly 

contribute to nonattainment or interfere 

with maintenance of the 2015 ozone 

NAAQS in other states. 

B. EPA Evaluation of the LDEQ SIP 

Submission 

The EPA is proposing to find that 

LDEQ’s November 13, 2019, SIP 

submission does not meet the State’s 

obligations with respect to prohibiting 

emissions that contribute significantly 

to nonattainment or interfere with 

maintenance of the 2015 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 therefore 

proposing to disapprove Louisiana’s SIP 

submission. 

1. Evaluation of Information Provided 

by LDEQ Regarding Steps 1 and 2 

At Step 1 of the 4-Step interstate 

transport framework, LDEQ relied on 

EPA modeling released in the March 

2018 memorandum to identify 

nonattainment and maintenance 

receptors in 2023. At Step 2 of the 

4-Step interstate transport framework, 

LDEQ relied on the EPA modeling 

released in the March 2018 

memorandum to identify upwind state 

linkages to nonattainment and 

maintenance receptors in 2023. LDEQ 

additionally utilized a 1 ppb threshold 

at Step 2 to identify whether the state 

was ‘‘linked’’ to a projected downwind 

nonattainment or maintenance receptor. 

As discussed in the EPA’s August 2018 

memorandum, with appropriate 

additional analysis it may be reasonable 

for states to use a 1 ppb contribution 

threshold, as an alternative to a 1 

percent threshold, at Step 2 of the 

4-Step interstate transport framework, 

for the purposes of identifying linkages 

to downwind receptors. In any case, the 

State is projected to contribute greater 

than both the 1 percent and the 

alternative 1 ppb thresholds to receptors 

in Texas, regardless of whether we look 

at LDEQ’s analysis (which relied on the 

EPA’s older modeling) or updated 

modeling the EPA has performed in 

advance of this proposal. As seen in the 

tables LA–1 and LA–2, Louisiana 

contributes nearly five times the 1 ppb 

threshold to nonattainment or 

maintenance receptors in Texas. 

Therefore, while the EPA does not, in 

this action, approve of the State’s 

application of the 1 ppb threshold, 

because the State has linkages greater 

than 1 ppb to projected downwind 

nonattainment or maintenance 

receptors, the State’s use of this 

alternative threshold at Step 2 of the 

4-Step interstate framework would not 

alter our review and proposed 

disapproval of this SIP submittal. 

The EPA here shares further 

evaluation of its experience since the 

issuance of the August 2018 

memorandum regarding use of 

alternative thresholds at Step 2. This 

experience leads the Agency to now 

believe it may not be appropriate to 

continue to attempt to recognize 

alternative contribution thresholds at 

Step 2. The August 2018 memorandum 

stated that ‘‘it may be reasonable and 

appropriate’’ for states to rely on an 

alternative threshold of 1 ppb threshold 

at Step 2.63 (The memorandum also 

indicated that any higher alternative 

threshold, such as 2 ppb, would likely 

not be appropriate.) However, the EPA 

also provided that ‘‘air agencies should 

consider whether the recommendations 

in this guidance are appropriate for each 

situation.’’ Following receipt and review 

of 49 interstate transport SIP submittals 

for the 2015 ozone NAAQS, the EPA’s 

experience has been that nearly every 

state that attempted to rely on a 1 ppb 

threshold did not provide sufficient 

information and analysis to support a 

determination that an alternative 

threshold was reasonable or appropriate 

for that state. 

For instance, in nearly all submittals, 

the states did not provide the EPA with 

analysis specific to their state or the 

receptors to which its emissions are 

potentially linked. In one case, the 

proposed approval of Iowa’s SIP 

submittal, the EPA expended its own 

resources to attempt to supplement the 

information submitted by the state, in 

order to more thoroughly evaluate the 

state-specific circumstances that could 

support approval.64 The Agency no 

longer intends to undertake 

supplemental analysis of SIP submittals 

with respect to alternative thresholds at 

Step 2 for purposes of the 2015 ozone 

NAAQS. 

Furthermore, the EPA’s experience 

since 2018 is that allowing for 

alternative Step 2 thresholds may be 


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65We note that Congress has placed on the EPA 

a general obligation to ensure the requirements of 

the CAA are implemented consistently across states 

and regions. See CAA section 301(a)(2). Where the 

management and regulation of interstate pollution 

levels spanning many states is at stake, consistency 

in application of CAA requirements is paramount. 

66See August 2018 memorandum, at page 4. 

67Per the instructions in the Supplementary 

Information section above, all public comments, 

including comments on the EPA’s air quality 

modeling should be submitted in the Regional 

docket for this action, Docket ID No. EPA–R06– 

OAR–2021–0801. Comments are not being accepted 

in Docket No. EPA–HQ–OAR–2021–0663. 68 DVs and contributions at individual monitoring 

sites nationwide are provided in the file: ‘‘2016v2_

DVs_state_contributions.xlsx’’, which is included in 

Docket ID No. EPA–HQ–OAR–2021–0663. 

69These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform, which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I of this action. 

That modeling showed that Louisiana 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 DVs And Contributions 

Revised CSAPR Update.xlsx’’ in Docket No. EPA– 

HQ–OAR–2021–0663. 

impractical or otherwise inadvisable for 

a number of additional policy reasons. 

For a regional air pollutant such as 

ozone, consistency in requirements and 

expectations across all states is 

essential. Based on its review of 

submittals to-date and after further 

consideration of the policy implications 

of attempting to recognize an alternative 

Step 2 threshold for certain states, the 

Agency now believes the attempted use 

of different thresholds at Step 2 with 

respect to the 2015 ozone NAAQS raises 

substantial policy consistency and 

practical implementation concerns.65

The availability of different thresholds 

at Step 2 has the potential to result in 

inconsistent application of interstate 

transport obligations based solely on the 

strength of a state’s SIP submittal at Step 

2 of the 4-Step interstate transport 

framework. From the perspective of 

ensuring effective regional 

implementation of interstate transport 

obligations, the more important analysis 

is the evaluation of the emissions 

reductions needed, if any, to address a 

state’s significant contribution after 

consideration of a multifactor analysis 

at Step 3, including a detailed 

evaluation that considers air quality 

factors and cost. Where alternative 

thresholds for purposes of Step 2 may 

be ‘‘similar’’ in terms of capturing the 

relative amount of upwind contribution 

(as described in the August 2018 

memorandum), nonetheless, use of an 

alternative threshold would allow 

certain states to avoid further evaluation 

of potential emission controls while 

other states must proceed to a Step 3 

analysis. This can create significant 

equity and consistency problems among 

states. 

Further, it is not clear that national 

ozone transport policy is best served by 

allowing for less stringent thresholds at 

Step 2. The EPA recognized in the 

August 2018 memorandum that there 

was some similarity in the amount of 

total upwind contribution captured (on 

a nationwide basis) between 1 percent 

and 1 ppb. However, the EPA notes that 

while this may be true in some sense, 

that is not a compelling basis to move 

to a 1 ppb threshold. Indeed, the 1 ppb 

threshold has the disadvantage of losing 

a certain amount of total upwind 

contribution for further evaluation at 

Step 3 (e.g., roughly seven percent of 

total upwind state contribution was lost 

according to the modeling underlying 

the August 2018 memorandum; 66 in the 

EPA’s updated modeling, the amount 

lost is five percent). Considering the 

core statutory objective of ensuring 

elimination of all significant 

contribution to nonattainment or 

interference of the NAAQS in other 

states and the broad, regional nature of 

the collective contribution problem with 

respect to ozone, there does not appear 

to be a compelling policy imperative in 

allowing some states to use a 1 ppb 

threshold while others rely on a 1 

percent of NAAQS threshold. 

Consistency with past interstate 

transport actions such as CSAPR, and 

the CSAPR Update and Revised CSAPR 

Update rulemakings (which used a Step 

2 threshold of 1 percent of the NAAQS 

for two less stringent ozone NAAQS), is 

also important. Continuing to use a 1 

percent of NAAQS approach ensures 

that as the NAAQS are revised and 

made more stringent, an appropriate 

increase in stringency at Step 2 occurs, 

so as to ensure an appropriately larger 

amount of total upwind-state 

contribution is captured for purposes of 

fully addressing interstate transport. 

Accord 76 FR 48237–38. 

Therefore, notwithstanding the 

August 2018 memorandum’s 

recognition of the potential viability of 

alternative Step 2 thresholds, and in 

particular, a potentially applicable 1 

ppb threshold, the EPA’s experience 

since the issuance of that memorandum 

has revealed substantial programmatic 

and policy difficulties in attempting to 

implement this approach. Nonetheless, 

the EPA is not at this time rescinding 

the August 2018 memorandum. The 

basis for a proposed disapproval of 

LDEQ’s SIP submission with respect to 

the Step 2 analysis we believe is 

warranted under the terms of the August 

2018 memorandum. The EPA invites 

comment on this broader discussion of 

issues associated with alternative 

thresholds at Step 2. Depending on 

public comments received and further 

evaluation of this issue, the EPA may 

determine to rescind the 2018 

memorandum in the future. 

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

2 Modeling and Findings for Louisiana 

As described in Section I of this 

action, the EPA performed air quality 

modeling using the 2016v2 emissions 

platform to project DVs and 

contributions for 2023.67 This data was 

examined to determine if Louisiana 

contributes at or above the threshold of 

1 percent of the 2015 ozone NAAQS 

(0.70 ppb) to any downwind 

nonattainment or maintenance receptor. 

As shown in Table LA–2, the data 68

indicate that in 2023, emissions from 

Louisiana contributed greater than 1 

percent of the standards to 

nonattainment or maintenance-only 

receptors in Texas.69 Therefore, based 

on the EPA’s evaluation of the 

information submitted by LDEQ, and 

based on the EPA’s most recent 

modeling results for 2023, the EPA 

proposes to find that Louisiana 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 LA–2—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS WITH LOUISIANA LINKAGES BASED ON EPA 

2016V2 MODELING

Receptor 

(site ID, county, state) 

Nonattainment/ 

maintenance 

2023 

Average DV 

(ppb) 

2023 

Maximum DV 

(ppb) 

Louisiana 

Contribution 

(ppb) 

482010024, Harris, TX .................................. Nonattainment .................................................. 75.2 76.8 4.31 

482010055, Harris, TX .................................. Nonattainment .................................................. 71.0 72.0 5.39 

480391004, Brazoria, TX .............................. Maintenance ..................................................... 70.1 72.3 7.03 


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70See LDEQ SIP Submission, Appendix A, 

available in the Regional docket for this action 

(Docket ID No. EPA–R06–OAR–2021–0801). 71See FN 53. 

TABLE LA–2—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS WITH LOUISIANA LINKAGES BASED ON EPA 

2016V2 MODELING—Continued 

Receptor 

(site ID, county, state) 

Nonattainment/ 

maintenance 

2023 

Average DV 

(ppb) 

2023 

Maximum DV 

(ppb) 

Louisiana 

Contribution 

(ppb) 

481210034, Denton, TX ................................ Maintenance ..................................................... 70.4 72.2 3.22 

482011034, Harris, TX .................................. Maintenance ..................................................... 70.3 71.6 4.93 

482011035, Harris, TX .................................. Maintenance ..................................................... 68.0 71.6 4.77 

3. Evaluation of Information Provided 

by LDEQ 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 contribute 

significantly 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 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 

interfere with maintenance) in each of 

its prior Federal, regional ozone 

transport rulemakings, and this 

interpretation of the statute has been 

upheld by the Supreme Court. See EME 

Homer City, 572 U.S. 489, 519 (2014). 

While the EPA has not directed states 

that they must conduct a Step 3 analysis 

in precisely the manner the EPA has 

done in its prior regional transport 

rulemakings, state implementation 

plans addressing the obligations in CAA 

section 110(a)(2)(D)(i)(I) must prohibit 

‘‘any source or other type of emissions 

activity within the State’’ from emitting 

air pollutants which will contribute 

significantly to downwind air quality 

problems. Thus, states must complete 

an analysis similar to the EPA’s (or an 

alternative approach to defining 

‘‘significance’’ that comports with CAA 

requirements) to determine whether, 

and to what degree, emissions from a 

state should be ‘‘prohibited’’ to 

eliminate emissions that will 

‘‘contribute significantly to 

nonattainment, or interfere with 

maintenance of’’ the NAAQS in any 

other state. LDEQ did not conduct such 

an analysis in their SIP submission. 

Instead LDEQ interpreted the Act’s 

requirements as only requiring an 

analysis of emission reductions where 

there was a ‘‘consistent and persistent’’ 

pattern of contribution and conducted 

an air-quality-only analysis in order to 

refute such a pattern. We propose to 

find that LDEQ was required to analyze 

emissions from the sources and other 

emissions activity from within 

Louisiana to determine whether its 

contributions were significant, and we 

propose to disapprove its submission 

because LDEQ did not do so. 

As noted, LDEQ stated in its SIP 

submission that emissions from 

Louisiana should not be considered to 

contribute significantly to 

nonattainment or interfere with 

maintenance of the NAAQS in other 

states because there is not a ‘‘persistent 

and consistent’’ pattern of contribution 

from the State. The SIP submission does 

not explain what LDEQ considers to be 

a persistent and consistent pattern of 

contribution, even after the LDEQ 

received a comment during its state 

comment period that requested that the 

LDEQ define ‘‘persistent and 

consistent’’ in terms of impacts on 

downwind states. The LDEQ responded, 

‘‘Louisiana has defined the pattern and 

has provided back trajectories on those 

monitored exceedances for the 2016– 

2018 ozone seasons, which will show 

that the definition is applicable to the 

conclusion.’’ 70 We do not agree that this 

suffices as an explanation as to why 

LDEQ does not need to further analyze 

its potential emission reductions under 

Step 3 before determining it has no 

statutory obligation under the interstate 

transport provision. In the case of 

Louisiana, modeling in the March 2018 

memorandum and the EPA’s more 

recent 2016v2 modeling both project 

that receptors in the Houston-GalvestonBrazoria (HGB) and Dallas-Fort Worth 

(DFW) ozone nonattainment areas in 

Texas will have difficulty attaining or 

maintaining the 2015 ozone NAAQS, 

and Louisiana’s contribution to these 

areas exceed both a 1 percent or a 1 ppb 

threshold. While linkages to specific 

receptors may change with updated 

modeling, both modeling analyses 

consistently show emissions from 

Louisiana impact both downwind 

nonattainment receptors and downwind 

maintenance receptors in Texas. 

The LDEQ SIP submission stated that 

Louisiana’s contribution should be 

deemed ‘‘significant’’ per CAA section 

110(a)(2)(D)(i)(I) only if there is a 

persistent and consistent pattern of 

contribution on several days with 

elevated ozone. LDEQ asserted that its 

linkages to Texas do not warrant further 

analysis because, according to LDEQ, 

emissions from Louisiana do not 

persistently and consistently contribute 

on several days of elevated ozone. 

However, the EPA modeling that LDEQ 

relied upon to demonstrate linkages in 

the first instance already establishes that 

there is a consistent and persistent 

pattern of contribution from Louisiana 

to Texas receptors on elevated ozone 

days. The EPA’s methodology for 

projecting future year ozone 

concentrations accounts for precisely 

these concerns—the relative response 

factor 71 that is applied to historic 

monitored data to generate projections 

is calculated by looking only at days 

with elevated ozone levels. The EPA 

notes that monitored attainment with 

the ozone standard is determined by 

averaging the fourth high value recorded 

each year for three years. So, the EPA 

believes it is important to estimate 

impacts on the days with highest 

projected ozone levels. The days chosen 

to analyze the future impacts are chosen 

initially by the selecting the 10 highest 

days in the base period modeling that 

are projected to be above 65 ppb in the 

base period. If there are not 10 days 

above 65 ppb at a potential receptor, the 

number of days above 65 ppb are used 

so long as there is at least five days 

above 65 ppb in the base period. If the 

air quality modeling shows fewer than 

five days above 65 ppb in the base 

period, then the data for impacts at that 

receptor in 2023 are not calculated. The 

base and future year modeling for these 


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72Simon et al., supra FN 58. 

5–10 days are then used to project 2023 

ozone DVs to determine whether it is 

projected to be a nonattainment or 

maintenance receptor in 2023. For these 

same 5–10 days identified, the future 

year modeling provides the estimated 

daily contribution at a potential 

receptor’s future year daily MDA8 and 

these daily contributions are averaged 

for the 5–10 days to result in the average 

contribution from the upwind area. 

LDEQ’s air quality analysis used to 

dismiss its linkages to Texas receptors 

as not ‘‘significant’’ consists of an 

evaluation of seasonal weather patterns, 

surface wind directions, and periodic 

back trajectories. The State’s weather 

pattern analysis relied on large-scale 

weather patterns as they relate to 

commonly observed wind directions 

rather than weather patterns and 

conditions that are specifically 

conducive to ozone formation or tied to 

specific days when high ozone was 

monitored in the downwind areas. 

General weather pattern discussions 

that are not associated with specific 

ozone episodes are not generally 

informative of interstate transport 

decisions. It is necessary to investigate 

specific instances of high ozone, 

because as discussed previously, 

violations of the ozone standard can be 

driven by as few as 4 days per year 

because the compliance with the 

standard is evaluated based on the 

average of the fourth high value 

measured each of three consecutive 

years. 

LDEQ’s wind rose analysis is based on 

surface sites in the Dallas-Fort Worth 

areas, Houston-Galveston-Brazoria 

areas, and other areas in Texas and 

Louisiana, but the analysis does not 

address transport winds between 

Louisiana and the Texas areas with 

receptors on high ozone days at the 

identified receptors. There are several 

limitations associated with LDEQ’s 

wind rose analysis: (1) Wind directions 

measured at the surface are not 

necessarily good indicators of the wind 

direction occurring at higher elevations, 

which tend to have a stronger influence 

on interstate ozone transport; (2) wind 

directions change spatially over the 

range of distance involved in transport 

from Louisiana to Texas; (3) wind 

directions change temporally over the 

range of time involved in ozone 

transport from Louisiana to Texas; and 

(4) the wind roses are based on wind 

data measured throughout the year, not 

just during either ozone season or 

monitored ozone episode days. In 

addition, as discussed previously, 

LDEQ’s wind rose analysis is not 

limited to the wind conditions that are 

conducive to high ozone, so it does not 

provide information directly pertinent 

to when ozone is high at areas in Texas 

and whether Louisiana is a contributing 

area during those specific times. 

LDEQ also included 99 back trajectory 

analyses during the 2016, 2017, and 

2018 years for the dates of ozone 

exceedances at the monitors referenced 

in Table LA–1 of this action. 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.’’ Since 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. LDEQ’s HYSPLIT 

back trajectory analysis for 2016, 2017, 

and 2018 showed that on high ozone 

days in Texas at the receptors identified 

by the EPA in the 2018 memorandum 

that 28% of the trajectories passed 

through Louisiana. LDEQ proffered that 

some of these back trajectories did not 

pass directly over areas with emissions 

but did not consider that the back 

trajectories only represent a centerline 

and there are areas on either side of the 

centerline that would be contributing 

areas. LDEQ’s trajectory analysis 

confirmed that Louisiana is an upwind 

area for the receptors in Texas 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). 

Photochemical modeling simulations 

for ozone interstate transport assessment 

is 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. For the 2015 

ozone NAAQS interstate transport 

analysis, the EPA performed 

nationwide, state-level ozone source 

apportionment modeling using CAMx to 

quantify the contribution of NOX and 

VOC emissions from all sources in each 

state to project 2023 ozone 

concentrations at ozone monitoring 

sites. Detailed information for the EPA’s 

modeling may be found in the Air 

Quality Modeling TSD in Docket No. 

EPA–HQ–OAR–2021–0663. 

LDEQ concluded in the SIP submittal, 

citing an article 72 published in 2012, 

that the use of 1 percent of the standard 

for modeled contribution as the sole 

definition of significant contribution is 

inappropriate for the 2015 ozone 

NAAQS. LDEQ’s reasoning for this 

conclusion is that the more stringent 0.7 

ppb threshold ‘‘is an order of magnitude 

smaller than the biases and errors 

typically documented for regional 

photochemical modeling.’’ First, the 

EPA does not use the 1 percent 

threshold as the sole definition of 

significant contribution; at Step 2 of the 

analysis, the 1 percent threshold is used 

to identify contributions between states 

and downwind problem areas for 

further analysis at Step 3. Second, 

photochemical transport models such as 

CAMx have been extensively peer 

reviewed and used to support SIPs and 

explore relationships between inputs 

and air quality impacts in the U.S. and 

beyond. The EPA works to continually 

develop and update both the guidelines 


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73See ‘‘Modeling Guidance for Demonstrating Air 

Quality Goals for Ozone, PM2.5 and Regional Haze’’, 

Nov. 29, 2018, at 101, available at https://

www.epa.gov/sites/default/files/2020-10/ 

documents/o3-pm-rh-modeling_guidance-2018.pdf 

(‘‘2018 Air Quality Modeling Guidance’’). See also 

‘‘Draft Modeling Guidance for Demonstrating 

Attainment of Air Quality Goals for Ozone, PM2.5,

and Regional Haze’’, Dec. 3, 2014, at 97–98, 

available at https://www.epa.gov/sites/default/files/ 

2020-10/documents/draft-o3-pm-rh-modeling_

guidance-2014.pdf (‘‘2014 Draft Air Quality 

Modeling Guidance’’). 

on using modeling results and the latest 

versions of photochemical model 

platforms to support scientific 

assessments and regulatory 

determinations. Prior to using 

photochemical modeling to support a 

regulatory assessment, a model 

performance evaluation is completed to 

establish a benchmark to assess how 

accurately the model predicts observed 

concentrations and to identify model 

limitations. The model performance 

evaluation provides a better 

understanding of the model’s 

limitations and biases and serves as a 

diagnostic evaluation for further model 

development and improvement. As 

discussed in Section I of this document 

and the Air Quality Modeling TSD in 

Docket No. EPA–HQ–OAR–2021–0663, 

the EPA follows the most recent 

established modeling guidance and 

provides with this action the updated 

modeling analysis based on the recent 

CAMx model update. By using the most 

recent 2016v2 photochemical modeling 

enhancements (EPA 2016v2 modeling) 

results are more representative of the 

projected local and regional air quality 

as it is based on more recent emission 

estimates with fewer years between the 

base case year (2016) and the future year 

(2023). In addition, to reduce the impact 

of any potential biases or errors, the 

EPA uses the modeling results in a 

relative sense rather than rely on 

absolute model predictions.73

Furthermore, it is not appropriate to 

compare the bias/error involved in the 

estimation of total ozone to the potential 

error in the estimation of the subset of 

ozone that is contributed by a single 

state. For example, on a specific day the 

modeled vs. monitored ozone value may 

differ by 2 ppb but that is relatively 

small percentage of the total modeled 

ozone, which for a receptor of interest 

would be on the order of 70 ppb. It 

would be unrealistic to assign all the 2 

ppb, in the above example, to the 

estimated impact from a single state as 

the 2 ppb error would be the 

combination of the error from all 

sources of ozone that contribute to the 

total, including estimated impacts from 

other states, the home state of the 

receptor and natural background 

emissions. 

In sum, the EPA disagrees that the 

estimates of potential error in the 

models estimates of total ozone, call 

into question the use of 1 percent as a 

threshold for linkage. As noted earlier, 

in the case of Louisiana, the difference 

between a 1 percent threshold and a 1 

ppb threshold is irrelevant to the 

decision here because linkages are 

present at both threshold levels. As to 

Louisiana’s conclusion that the impacts 

from Louisiana’s emissions are not 

persistent, the contribution analysis is 

the average impact for at least 5 days 

and up to 10 days for the 2016 base 

period which is sufficiently persistent 

considering the first through fourth high 

monitored values set the monitored DV. 

We recognize that the results of the 

EPA (2011 and 2016 base year) 

modeling indicated different receptors 

and linkages at Steps 1 and 2 of the 

4-Step interstate transport framework. 

These differing results regarding 

receptors and linkages can be affected 

by the varying meteorology from year to 

year, but we do not think the differing 

results means that the modeling or the 

EPA or the state’s methodology for 

identifying receptors or linkages is 

inherently unreliable. Rather, these 

separate modeling runs all indicated: (1) 

That there are receptors that would 

struggle with nonattainment or 

maintenance in the future; and (2) that 

Louisiana is linked to some set of these 

receptors, even if the receptors and 

linkages differed from one another in 

their specifics (e.g., Louisiana was 

linked to a different set of receptors in 

one modeling run versus another). 

These results indicates that Louisiana’s 

emissions were substantial enough to 

generate linkages at Steps 1 and 2 to at 

least some set of downwind receptors, 

under varying assumptions and 

meteorological conditions, even if the 

precise set of linkages changed between 

modeling runs. 

4. Evaluation of Information Provided 

by LDEQ 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, LDEQ’s SIP 

submission did not contain an 

evaluation of additional emission 

control opportunities (or establish that 

no additional controls are required), 

thus, no information was provided at 

Step 4. To the extent that LDEQ 

discussed emissions reductions, the 

State only provided a summary of 

existing already implemented 

enforceable control regulations. The 

EPA’s 2016v2 modeling analyses have 

already accounted for the 

implementation of the regulations cited 

by LDEQ’s submission—including the 

CSAPR rulemakings and prior regional 

rulemakings—and even with those 

reductions in place, the modeling 

results consistently show receptors that 

are projected to be in nonattainment or 

to struggle with maintenance, and 

Louisiana contributing to those 

receptors. Relying only on the existing 

enforceable control regulations is 

insufficient to address the Louisiana air 

emission contributions to linked 

downwind air quality problems. As a 

result, the EPA proposes to disapprove 

LDEQ’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). 

5. Conclusion 

Based on the EPA’s evaluation of 

LDEQ’s SIP submission, the EPA is 

proposing to find that LDEQ’s 

November 13, 2019, SIP submission 

pertaining to interstate transport of air 

pollution 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 ozone NAAQS 

in any other state. 

IV. Oklahoma SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

and the EPA Evaluation of the SIP 

Submission 

A. Summary of ODEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

On October 25, 2018, the Oklahoma 

Department of Environmental Quality 

(ODEQ) made a SIP submission 

addressing interstate transport of air 

pollution for the 2015 ozone NAAQS. 

The SIP submission provided ODEQ’s 

analysis of their impact to downwind 

states using the EPA’s 4-Step framework 

and an analytic year of 2023 and 

concluded that emissions from 

Oklahoma will not contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in other states. 

To identify downwind air quality 

problems that are linked to emissions 


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74Nonattainment receptors are monitoring sites 

that are anticipated to have problems attaining and 

maintaining the 2015 ozone NAAQS (i.e., average 

projected 2023 DV greater than 70.9 ppb). 

Maintenance receptors are monitoring sites that are 

anticipated to have problems maintaining the 2015 

ozone NAAQS (i.e., maximum projected 2023 DV 

greater than 70.9 ppb). 

75See FN 32. 

from Oklahoma and therefore warrant 

further review and analysis (Steps 1 and 

2), ODEQ used EPA interstate transport 

modeling results found in the March 

2018 memorandum. The EPA modeling 

results projected: (1) An average DV and 

a maximum DV for the year 2023 for 

ozone monitors in the 48 contiguous 

States and (2) the expected contribution 

from emissions in each state to the 

ozone concentrations at each ozone 

monitor. 

ODEQ used the information from the 

March 2018 EPA memorandum to 

identify six downwind nonattainment 

and maintenance receptors 74 with a 

contribution from Oklahoma of 1 

percent of the 2015 ozone NAAQS (0.70 

parts ppb) or greater. ODEQ then 

applied a 1 ppb threshold to remove 

from further analysis three receptors 

with a contribution from Oklahoma of 

less than 1 ppb. ODEQ noted that the 

possibility of using an alternative 

contribution threshold was one of the 

areas of flexibility identified in the 

March 2018 EPA memorandum and 

discussed further in the August 2018 

EPA memorandum. To support its 

alternative contribution threshold, 

ODEQ referenced an EPA memorandum 

from April 17, 2018, which 

recommended a Significant Impact 

Level (SIL) for ozone of 1.0 ppb for 

proposed sources subject to the 

Prevention of Significant Deterioration 

(PSD) permitting program.75 Table OK– 

1 provides information on the six 

nonattainment and maintenance 

receptors identified by ODEQ, including 

the three receptors ODEQ identified for 

further analysis. 

TABLE OK–1—NONATTAINMENT AND MAINTENANCE RECEPTORS IDENTIFIED BY ODEQ BASED ON THE EPA’S MARCH

2018 MEMORANDUM

Receptor 

(site ID, county, state) 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) 

Oklahoma 

contribution 

(ppb) 

ODEQ’s step 1 and 2 determination 

260050003, Allegan, MI .................................. 69.0 71.7 1.31 Maintenance receptor identified for further 

analysis. 

481210034, Denton, TX .................................. 69.7 72.0 1.23 Maintenance receptor identified for further 

analysis. 

484392003, Tarrant, TX .................................. 72.5 74.8 1.71 Nonattainment receptor identified for further 

analysis. 

480391004, Brazoria, TX ................................ 74.0 74.9 0.90 Nonattainment receptor with contribution less 

than 1 ppb; no further analysis. 

550790085, Milwaukee, WI ............................ 71.2 73.0 0.76 Nonattainment receptor with contribution less 

than 1 ppb; no further analysis. 

551170006, Sheboygan, WI ........................... 72.8 75.1 0.95 Nonattainment receptor with contribution less 

than 1 ppb; no further analysis. 

ODEQ further evaluated the two 

Texas receptors (Tarrant County and 

Denton County) and the receptor in 

Allegan County, MI. ODEQ did not 

further evaluate the contribution from 

Oklahoma to the receptors in Brazoria 

County, TX, Milwaukee County, WI, 

and Sheboygan County, WI because the 

contributions from Oklahoma to these 

receptors were less than 1 ppb. 

For the two remaining Texas 

receptors, ODEQ returned to Steps 1 and 

2 of the 4-Step interstate transport 

framework using modeling performed 

by the Texas Commission on 

Environmental Quality (TCEQ). The 

TCEQ modeling results are included in 

the Regional docket for this action 

(Docket ID No. EPA–R06–OAR–2021– 

0801). ODEQ stated that the primary 

difference between the EPA modeling 

and the TCEQ modeling is that the 

TCEQ modeling used 2012 as the ‘‘base 

year’’ for assessing interstate transport 

of ozone pollution in 2023 whereas the 

EPA modeling used 2011 as the base 

year for that assessment. In addition, the 

ODEQ stated that TCEQ used a method 

different from the EPA’s method to 

identify whether a monitor would have 

trouble maintaining the 2015 ozone 

NAAQS (i.e., a maintenance receptor). 

To identify maintenance receptors, 

TCEQ calculated a ‘‘maintenance future 

year (fy) DV’’ by projecting to 2023 the 

most recent regulatory DV that contains 

the base year (i.e., the 2012–2014 DV for 

a base year of 2012), whereas the EPA’s 

methodology for identifying 

maintenance receptors uses the 

maximum DV, which is the highest 

monitored DV from among the three 

DVs that contain the base year (i.e., the 

2009–2011, 2010–2012 and 2011–2013 

DVs for a base year of 2011). 

To assess whether Oklahoma is linked 

to nonattainment of the 2015 ozone 

standard at the Denton and Tarrant 

County sites, ODEQ switched to using 

the 2023 average DV projected by TCEQ 

rather than the EPA’s projected average 

DVs. The ODEQ noted that the projected 

2023 average DV was 68 ppb for the 

Denton County site and 66 ppb for the 

Tarrant County site based on the TCEQ 

modeling. ODEQ then claimed that 

these results demonstrate that both of 

these sites are in attainment in 2023. 

To assess whether Oklahoma 

interferes with maintenance of the 2015 

ozone standard at these two sites, ODEQ 

used (1) the Texas method to calculate 

a ‘‘maintenance future year DV’’ for 

2023 and (2) a maximum DV calculated 

using the highest of the three base year 

DVs multiplied by a relative response 

factor derived from TCEQ’s modeling 

(i.e., EPA’s method for identifying 

maintenance receptors but using TCEQ’s 

modeling rather than EPA’s modeling). 

This assessment is summarized in Table 

OK–2. 


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76ODEQ used the EPA’s emissions data shared 

alongside the October 2018 memorandum, ‘‘statesector_annual_emissions_data_1.xlsx’’ available at 

https://www.epa.gov/airmarkets/memo-andsupplemental-information-regarding-interstatetransport-sips-2015-ozone-naaqs. 

77The Southwest Power Pool is a regional electric 

transmission organization regulated by the Federal 

Energy Regulatory Commission whose purpose is 

promoting efficiency and reliability in the operation 

and planning of the electric transmission grid and 

ensuring non-discrimination in the provision of 

electric transmission services. It manages electric 

transmission in portions of fourteen states: 

Arkansas, Iowa, Kansas, Louisiana, Minnesota, 

Missouri, Montana, Nebraska, New Mexico, North 

Dakota, Oklahoma, South Dakota, Texas and 

Wyoming. See 18 CFR 35.34 and https://

www.ferc.gov/electric-power-markets. 

TABLE OK–2—SUMMARY OF TCEQ MODELING (2012 BASE PERIOD) USED BY ODEQ TO ASSESS MAINTENANCE

RECEPTORS

Receptor 

(site ID, county, state) 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) (EPA 

method)* 

Maintenance 

DV 

(ppb)(TCEQ 

method) 

ODEQ’s step 1 and step 2 determination 

481210034 Denton, TX ................................... 68 70.7 65.9 Future DVs project no attainment or maintenance problems. 

484392003 Tarrant, TX ................................... 66 69.9 62.4 Future DVs project no attainment or maintenance problems. 

* These values are not based on calculations made by the EPA. ODEQ calculated these values by using the maximum DV for the 2010–2014 

5-year period (i.e., the highest of the DVs in 2012, 2013, and 2014) multiplied by relative response factor for the receptor obtained from TCEQ’s 

modeling. 

ODEQ noted in their assessment that 

based on the TCEQ modeling and TCEQ 

definition of maintenance receptor, it is 

expected that the Denton and Tarrant 

sites will not experience nonattainment 

or maintenance problems in 2023. 

Because ODEQ claimed that the Denton 

and Tarrant County sites will not be 

nonattainment or maintenance receptors 

in 2023, ODEQ did not analyze potential 

emissions reductions at Step 3 to 

address its contribution to these two 

sites. 

With respect to the remaining 

receptor at Allegan County, MI, ODEQ 

provided an analysis of projected 2023 

DVs for this site and information on 

emissions trends in Oklahoma to assert 

that emissions from Oklahoma do not 

significantly contribute to 

nonattainment or interfere with 

maintenance of the 2015 ozone NAAQS 

at the Allegan County, MI site. 

ODEQ noted that (1) the DV for the 

Allegan County, MI site has had a 

substantial reduction in the last 6 years 

from 84 ppb in 2012 to 73 ppb in 2017, 

a 1.8 ppb per year decrease, on average 

and (2) the Allegan County, MI site is 

substantially influenced by mobile 

sources from the Chicago area and these 

emissions are expected to be greatly 

reduced in the near future, by roughly 

a 1 ppb per year decrease, leading to 

attainment of the 2015 ozone standard. 

The ODEQ then calculated a projected 

2023 maintenance DV for the Allegan 

County, MI site using the EPA’s method, 

but assuming that the base year was 

2016 rather than 2011, as in the EPA’s 

modeling or 2012 as in the TCEQ 

modeling. The ODEQ noted that the 

maximum DV in the 2016-centered base 

period (i.e., 2014–2016, 2015–2017, and 

2016–2018) was 75 ppb at the Allegan 

County, Michigan site. The ODEQ then 

calculated the difference between the 

2011-centered base period maximum 

DV of 86 ppb and the 2023 projected 

maximum DV of 71.7 ppb, using data 

from the EPA’s modeling. The ODEQ 

calculated a ‘‘ppb per year’’ reduction of 

1.1917 ppb per year, based on the 14.3 

ppb difference between the 2011- 

centered and 2023 maximum DVs over 

the 12 years from 2011 to 2023. Finally, 

ODEQ applied the 1.1917 ppb per year 

value to the 2016-centered maximum 

DV of 75 ppb to estimate a 2023 

maximum DV of 66.66 ppb. 

ODEQ also asserted that the relatively 

small contribution from Oklahoma (3% 

of total upwind state contributions) 

combined with the distance between 

Oklahoma sources and the Allegan 

County, Michigan site, warrants a focus 

on nearby states with greater 

proportional contributions as the most 

prudent approach to addressing 

interstate transport of ozone precursors 

for this receptor. 

The ODEQ also provided the 

anthropogenic NOX and VOC data of 

Oklahoma’s emissions from EPA’s 

emission trends and modeling to 

demonstrate an anticipated substantial 

reduction of NOX and VOC from 2011 

to 2023: (1) Reductions of NOX from 

405,000 to 235,000 tons per year and (2) 

reductions of VOC from 414,000 to 

295,000 tons per year.76 ODEQ noted 

these reductions should result in 

considerable reductions in ozone 

concentrations. The ODEQ stated that 

due to the emissions reductions 

required by rules like CSAPR, the 2016 

CSAPR Update, and the regional haze 

requirements, the NOX emissions from 

electric generation in Oklahoma have 

dropped significantly during the ozone 

season from 38,285 tons per year in 

2011 to 10,435 tons per year in 2017. 

ODEQ also stated that changes in the 

Southwest Power Pool 77, building of 

additional windfarms, and electric 

utilities installing solar generation 

facilities have led to Oklahoma NOX

emissions reductions; and that any 

additional NOX reductions from the 

electric generation section would 

require more costly emissions controls. 

ODEQ concluded that the existing 

controls in Oklahoma have resulted in 

significant decreases in ozone DVs in 

Oklahoma and that additional controls 

would not be cost-effective. Given their 

conclusions, ODEQ did not adopt 

additional controls to reduce ozone 

precursor emissions (Step 4). 

B. EPA Evaluation of the ODEQ SIP 

Submission 

The EPA is proposing to find that 

ODEQ’s October 25, 2018, SIP 

submission does not demonstrate that 

the State’s obligations with respect to 

prohibiting emissions that contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in any other state based 

on the EPA’s evaluation of the SIP 

submission using the 4-Step interstate 

transport framework have been met. The 

EPA is therefore proposing to 

disapprove ODEQ’s submission. 

1. Evaluation of Information Provided 

by ODEQ Regarding Steps 1 and 2 

As noted earlier, ODEQ first used the 

information from the EPA’s March 2018 

memorandum to identify nonattainment 

and maintenance receptors with a 

contribution from Oklahoma of 0.70 ppb 

or greater (i.e., ODEQ identified 

receptors that would be deemed 

nonattainment and maintenance 

receptors under the EPA’s methodology 

for Steps 1 and 2). ODEQ then utilized 

a 1 ppb threshold and elected not to 

further analyze any receptors to which 

it did not contribute greater than 1 ppb. 


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78See FN 32. 

79 ‘‘Air Plan Approval; Iowa; Infrastructure State 

Implementation Plan Requirements for the 2015 

Ozone National Ambient Air Quality Standard’’, 85 

FR 12232 (March 2, 2020). The agency received 

adverse comments on this proposed approval and 

has not taken final action with respect to this 

proposal. 

80We note that Congress has placed on the EPA 

a general obligation to ensure the requirements of 

the CAA are implemented consistently across states 

and regions. See CAA section 301(a)(2). Where the 

management and regulation of interstate pollution 

levels spanning many states is at stake, consistency 

in application of CAA requirements is paramount. 81See August 2018 memorandum, at page 4. 

ODEQ provided further evaluation of 

the State’s emissions to those receptors 

to which Oklahoma contributes greater 

than 1 ppb (i.e., Allegan County, MI, 

Denton County, TX and Tarrant County, 

TX). 

As discussed in the EPA’s August 

2018 memorandum, with appropriate 

additional analysis it may be reasonable 

for states to use a 1 ppb contribution 

threshold, as an alternative to a 1 

percent threshold, at Step 2 of the 4- 

Step interstate transport framework, for 

the purposes of identifying linkages to 

downwind receptors. However, the 

EPA’s August 2018 memorandum 

provided that whether or not a 1 ppb 

threshold is appropriate must be based 

on an evaluation of state-specific 

circumstances, and no such evaluation 

was included in the ODEQ’s submittal. 

Instead, ODEQ’s SIP submission 

justified the State’s use of a 1 ppb 

threshold based on the threshold’s use 

in the SILs Guidance.78 ODEQ did not 

explain the relevance of the SILs 

Guidance to Oklahoma’s statutory 

obligation under the interstate transport 

provision. The SILs Guidance relates to 

a different provision of the CAA 

regarding implementation of the 

prevention of significant deterioration 

(PSD) permitting program, i.e., a 

program that applies in areas that have 

been designated attainment of the 

NAAQS, and it is not applicable to the 

interstate transport provision, which 

requires states to eliminate emissions 

that contribute significantly or interfere 

with maintenance of the NAAQS at 

known, ongoing, or projected air quality 

problem areas in other states. The EPA 

does not, in this action, agree that the 

State has justified its application of the 

1 ppb threshold. 

Additionally, the EPA here shares 

further evaluation of its experience 

since the issuance of the August 2018 

memorandum regarding use of 

alternative thresholds at Step 2. This 

experience leads the Agency to now 

believe it may not be appropriate to 

continue to attempt to recognize 

alternative contribution thresholds at 

Step 2. The August 2018 memorandum 

stated that ‘‘it may be reasonable and 

appropriate’’ for states to rely on an 

alternative threshold of 1 ppb threshold 

at Step 2. (The memorandum also 

indicated that any higher alternative 

threshold, such as 2 ppb, would likely 

not be appropriate.) However, the EPA 

also provided that ‘‘air agencies should 

consider whether the recommendations 

in this guidance are appropriate for each 

situation.’’ Following receipt and review 

of 49 interstate transport SIP submittals 

for the 2015 ozone NAAQS, the EPA’s 

experience has been that nearly every 

state that attempted to rely on a 1 ppb 

threshold did not provide sufficient 

information and analysis to support a 

determination that an alternative 

threshold was reasonable or appropriate 

for that state. 

For instance, in nearly all submittals, 

the states did not provide the EPA with 

analysis specific to their state or the 

receptors to which its emissions are 

potentially linked. In one case, the 

proposed approval of Iowa’s SIP 

submittal, the EPA expended its own 

resources to attempt to supplement the 

information submitted by the state, in 

order to more thoroughly evaluate the 

state-specific circumstances that could 

support approval. 79 It was at the EPA’s 

sole discretion to perform this analysis 

in support of the state’s submittal, and 

the Agency is not obligated to conduct 

supplemental analysis to fill the gaps 

whenever it believes a state’s analysis is 

insufficient. The Agency no longer 

intends to undertake supplemental 

analysis of SIP submittals with respect 

to alternative thresholds at Step 2 for 

purposes of the 2015 ozone NAAQS. 

Furthermore, the EPA’s experience 

since 2018 is that allowing for 

alternative Step 2 thresholds may be 

impractical or otherwise inadvisable for 

a number of additional policy reasons. 

For a regional air pollutant such as 

ozone, consistency in requirements and 

expectations across all states is 

essential. Based on its review of 

submittals to-date and after further 

consideration of the policy implications 

of attempting to recognize an alternative 

Step 2 threshold for certain states, the 

Agency now believes the attempted use 

of different thresholds at Step 2 with 

respect to the 2015 ozone NAAQS raises 

substantial policy consistency and 

practical implementation concerns.80

The availability of different thresholds 

at Step 2 has the potential to result in 

inconsistent application of interstate 

transport obligations based solely on the 

strength of a state’s SIP submittal at Step 

2 of the 4-Step interstate transport 

framework. From the perspective of 

ensuring effective regional 

implementation of interstate transport 

obligations, the more important analysis 

is the evaluation of the emissions 

reductions needed, if any, to address a 

state’s significant contribution after 

consideration of a multifactor analysis 

at Step 3, including a detailed 

evaluation that considers air quality 

factors and cost. Where alternative 

thresholds for purposes of Step 2 may 

be ‘‘similar’’ in terms of capturing the 

relative amount of upwind contribution 

(as described in the August 2018 

memorandum), nonetheless, use of an 

alternative threshold would allow 

certain states to avoid further evaluation 

of potential emission controls while 

other states must proceed to a Step 3 

analysis. This can create significant 

equity and consistency problems among 

states. 

Further, it is not clear that national 

ozone transport policy is best served by 

allowing for less stringent thresholds at 

Step 2. The EPA recognized in the 

August 2018 memorandum that there 

was some similarity in the amount of 

total upwind contribution captured (on 

a nationwide basis) between 1 percent 

and 1 ppb. However, the EPA notes that 

while this may be true in some sense, 

that is hardly a compelling basis to 

move to a 1 ppb threshold. Indeed, the 

1 ppb threshold has the disadvantage of 

losing a certain amount of total upwind 

contribution for further evaluation at 

Step 3 (e.g., roughly seven percent of 

total upwind state contribution was lost 

according to the modeling underlying 

the August 2018 memorandum; 81 in 

EPA 2016v2 modeling, the amount lost 

is five percent). Considering the core 

statutory objective of ensuring 

elimination of all significant 

contribution to nonattainment or 

interference of the NAAQS in other 

states and the broad, regional nature of 

the collective contribution problem with 

respect to ozone, there does not appear 

to be a compelling policy imperative in 

allowing some states to use a 1 ppb 

threshold while others rely on a 1 

percent of NAAQS threshold. 

Consistency with past interstate 

transport actions such as CSAPR, and 

the CSAPR Update and Revised CSAPR 

Update rulemakings (which used a Step 

2 threshold of 1 percent of the NAAQS 

for two less stringent ozone NAAQS), is 

also important. Continuing to use a 1 

percent of NAAQS approach ensures 

that as the NAAQS are revised and 

made more stringent, an appropriate 

increase in stringency at Step 2 occurs, 

so as to ensure an appropriately larger 

amount of total upwind-state 

contribution is captured for purposes of 


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82See FN 73. 

83See FN 53. 

84While it is not critical to this discussion, for 

purposes of explanation, the relative response factor 

is a fractional change that represents how ozone at 

a given receptor responds to changes in emissions 

when all other variables are constant. For more 

explanation of the RRF, please see 2018 Air Quality 

Modeling Guidance or 2014 Draft Air Quality 

Modeling Guidance. 

fully addressing interstate transport. 

Accord 76 FR 48237–38. 

Therefore, notwithstanding the 

August 2018 memorandum’s 

recognition of the potential viability of 

alternative Step 2 thresholds, and in 

particular, a potentially applicable 1 

ppb threshold, the EPA’s experience 

since the issuance of that memorandum 

has revealed substantial programmatic 

and policy difficulties in attempting to 

implement this approach. Nonetheless, 

the EPA is not, at this time, rescinding 

the August 2018 memorandum. The 

basis for the EPA’s proposed 

disapproval of ADEQ’s SIP submission 

with respect to the Step 2 analysis is, in 

the Agency’s view, warranted even 

under the terms of the August 2018 

memorandum. The EPA invites 

comment on this broader discussion of 

issues associated with alternative 

thresholds at Step 2. (See 

Supplementary Information section 

above for details and docket to submit 

comments). Depending on public 

comments received in relation to this 

action and further evaluation of this 

issue, the EPA may determine to rescind 

the 2018 memorandum in the future. 

In any case, as discussed in the 

following subsection, based on the 

EPA’s most recent modeling, the State is 

projected to contribute greater than both 

the one percent and alternative 1 ppb 

thresholds at the Denton County, TX 

receptor, (Monitor ID. 481210034). 

Based on the EPA’s modeling results 

included in the March 2018 

memorandum, Oklahoma was also 

projected to contribute 1.23 ppb to the 

Denton County, TX receptor. (In the 

EPA 2016v2 modeling the Allegan 

County, MI and Tarrant County, TX 

receptors are not projected to have 

problems attaining or maintaining the 

2015 ozone NAAQS). Even under 

ODEQ’s own analysis, the State was 

linked to receptors with contributions 

exceeding 1 ppb. Therefore, based on 

Oklahoma’s linkages greater than 1 ppb 

to projected downwind nonattainment 

or maintenance receptors, the State’s 

use of this alternative threshold at Step 

2 of the 4-Step interstate framework is 

inconsequential to our proposed action 

on the state’s SIP. 

In the remainder of this section, EPA 

evaluates ODEQ’s conclusions that 

emissions from Oklahoma do not 

contribute to nonattainment or interfere 

with maintenance at receptors in 

Tarrant County, TX (Monitor ID. 

484392003) and Denton County, TX 

(Monitor ID. 481210034). We evaluate 

ODEQ’s conclusions as to the Allegan, 

MI (Monitor ID. 260050003) in Section 

IV.B.3 of this action. 

With regard to the Denton County and 

Tarrant County, TX receptors cited in 

ODEQ’s submission, ODEQ chose to rely 

on the TCEQ’s modeling and 

methodology, instead of the EPA 

modeling, and trends in ozone DVs and 

emissions to conclude that these 

monitoring sites will be in attainment 

by 2023 and will not have a problem 

maintaining the 2015 ozone NAAQS. As 

noted in Section IV.A of this action, 

ODEQ used modeling results from the 

TCEQ along with the TCEQ alternative 

method for identifying maintenance 

receptors to claim that using the TCEQ 

modeling and methods, the Denton 

County and Tarrant County monitors 

would not have a problem maintaining 

the NAAQS in 2023. The ODEQ 

supplemented that analysis by citing the 

downward trend in NOX and VOC 

emissions in Oklahoma. ODEQ also 

provided TCEQ modeling and emissions 

data for the Dallas-Fort Worth 

nonattainment area to show that mobile 

sources represent the largest emissions 

category in this area and that emissions 

from this sector have declined since 

2005 and are expected to continue to 

decline in the future. As described in 

Table OK–2, ODEQ (1) provided the 

average 2023 DV for the Denton County, 

TX receptor from the TCEQ modeling 

and (2) used TCEQ modeling data with 

a 2012 base year to calculate a 2023 

maintenance DV of 65.9 ppb (using the 

TCEQ methodology for identifying 

maintenance receptors) and a 2023 

maximum DV of 70.7 ppb (using the 

EPA methodology for identifying 

maintenance receptors, combined with 

TCEQ’s modeling results). ODEQ relied 

on this information, which is based on 

TCEQ modeling with a 2012 base year, 

to conclude that the Denton County, TX 

and Tarrant County, TX monitors would 

not have problems attaining and 

maintaining the 2015 ozone NAAQS. 

ODEQ’s SIP submission (or TCEQ, to 

the extent that Oklahoma is merely 

incorporating and relying on Texas’ 

submission) does not adequately 

explain or justify how relying on 

TCEQ’s method for identifying 

maintenance receptors reasonably 

identifies areas that will have difficulty 

maintaining the NAAQS. EPA proposes 

to find that ODEQ has provided no 

sound technical basis (either on its own 

or through reliance on Texas) for how 

its chosen methodology gives meaning 

to the CAA’s instruction that states 

submit interstate transport SIPs that 

prohibit their states’ emissions from 

interfering with the maintenance of the 

NAAQS in another state. 

In North Carolina v. EPA, 531 F.3d 

896, 909–11 (D.C. Cir. 2008), the D.C. 

Circuit rejected the EPA’s CAIR on the 

basis that the EPA had not adequately 

given meaning to the phrase ‘‘interfere 

with maintenance’’ in the interstate 

transport provision. Specifically, North 

Carolina argued that it had counties that 

were projected to attain the NAAQS in 

the future analytic year, but were at risk 

of falling back into nonattainment due 

to interference from upwind sources, 

particularly given year-to-year 

variability in ozone levels. The court 

agreed, holding that the EPA’s rule did 

not adequately protect ‘‘[a]reas that find 

themselves barely meeting attainment.’’ 

Id. at 910. Consequently, the EPA has 

developed a methodology, used in its 

2011 CSAPR and its 2016 CSAPR 

Update and Revised CSAPR Update, for 

identifying areas that may struggle to 

maintain the NAAQS. See 76 FR at 

48227–28. EPA’s approach to addressing 

maintenance receptors was upheld in 

the EME Homer City litigation. See 795 

F.3d 118, 136–37. It was also upheld in 

Wisconsin. 938 F.3d at 325–26. In 

Wisconsin, the court noted that four 

upwind states were linked only to 

maintenance receptors and rejected the 

argument that application of the same 

control level as EPA imposes for those 

states linked to nonattainment receptors 

was unreasonable or unlawful absent a 

particularized showing of overcontrol. 

Id. at 327. 

In order to explain the differences 

between TCEQ’s and the EPA’s 

methodology for identifying 

maintenance receptors, it is helpful to 

provide some additional context of how 

the EPA projects future air quality. 

The EPA’s air quality modeling 

guidance has long recommended 

developing a base DV (i.e., the DV that 

will be used as a starting point to model 

and analyze for purposes of projecting 

future air quality concentrations) that is 

the average of three DVs spanning a 

five-year period, centered around one 

year for which an emissions inventory 

will be submitted (e.g., if 2011 was the 

base emissions inventory year, a state 

would use monitored values from 2009– 

2011, 2010–2012, 2011–2013 as the 

starting point for projecting air quality 

concentrations in future years).82 The 

average of these three DVs is then 

multiplied by a relative response 

factor 83 to generate an average DV for 

the future year.84 If a receptor’s average 


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85TCEQ submission at 3–39 to 3–40, available in 

the Regional docket for this action (Docket ID No. 

EPA–R06–OAR–2021–0801). 

86 ‘‘EPA Region 6 2015 8-Hour Ozone Transport 

SIP Proposal Technical Support Document’’ (EPA 

Region 6 2015 Ozone Transport SIP TSD.pdf) 

included in Docket ID No. EPA–R06–OAR–2021– 

0801. 

87 Id. 

88Monitoring data from the EPA’s Air Quality 

System (AQS) (https://www.epa.gov/aqs). 2021 

monitoring data is preliminary and still has to 

undergo Quality Assurance/Quality Control 

analysis and be certified by the State of Texas, 

submitted to EPA, and reviewed and concurred on 

by EPA. 2018–2020 DVs are 72 ppb and 73 ppb at 

Continued 

future year DV is greater than or equal 

to the level of the NAAQS, and the 

receptor has recent monitored data that 

violates the NAAQS, that receptor is 

considered a ‘‘nonattainment’’ receptor 

at Step 1. To identify maintenance 

receptors, the EPA’s methodology looks 

to the highest DV of the three DVs used 

to calculate the 5-year weighted average 

DV (e.g., in the 2011 example, if 2009– 

2011 had the highest DV of 2009–2011, 

2010–2012, and 2011–2013). The EPA 

then applies the same relative response 

factor to that highest DV to generate a 

projected future maximum DV. Where a 

receptor’s maximum DV exceeds the 

level of the NAAQS, the EPA has 

deemed those receptors to be 

‘‘maintenance’’ receptors. This 

methodology was designed to address 

the D.C. Circuit’s holding that the CAA’s 

‘‘interference with maintenance’’ prong 

requires states and the EPA to protect 

areas that may struggle with 

maintaining the standard in the face of 

variable conditions. 

In its modeling, TCEQ adopted an 

identical approach to the EPA’s for 

identifying nonattainment receptors—it 

looked at three sets of DVs over a fiveyear period and averaged those DVs to 

generate a base year DV. TCEQ then 

applied a relative response factor to that 

base year DV to project a receptor’s 

average DV in the future year. For 

maintenance receptors, however, TCEQ 

elected not to examine variability in 

DVs over a five-year period by using the 

highest DV of the three DVs making up 

the base year DV. Instead, TCEQ (and by 

extension, ODEQ), used only the most 

recent DV of the three DVs, regardless 

of whether the most recent DV was 

highest or lowest. TCEQ’s proffered 

explanation for using the most recent 

DV to identify maintenance receptors 

was that the latest DV ‘‘takes into 

consideration . . . any emissions 

reductions that might have occurred.’’ 85

TCEQ in its submission does not 

explain why or how this methodology 

identifies those areas that may be 

meeting the NAAQS or that may be 

projected to meet the NAAQS but may 

nevertheless struggle to maintain the 

NAAQS, given meteorological 

variability. In fact, because TCEQ’s 

stated purpose in using the most recent 

DV was to capture more recent 

emissions reductions, Texas’ 

methodology appears to be aimed at 

limiting receptors which could be 

identified as maintenance receptors, 

compared to the EPA’s methodology, 

which was designed to identify those 

areas that might struggle to maintain the 

NAAQS in particularly ozone conducive 

conditions. 

As discussed further in the EPA 

Region 6 TSD 86 for this action, the EPA 

has reviewed the set of 21 receptors for 

which Texas had contributions of 0.7 

ppb or more in the EPA’s 2016 base year 

modeling analyses, or TCEQ’s modeling 

(2012 base year), and evaluated the 

results of using TCEQ’s alternate 

maintenance methodology. For these 21 

receptors, TCEQ’s method resulted in 15 

of the 21 2023 maintenance DVs 

predicted to be lower than the 2023 

nonattainment DVs from the 

nonattainment methodology that uses 

the 5-year center weighted average. Of 

these 15 receptors, three receptors have 

2023 maintenance DVs that are 3 ppb 

lower, five receptors have 2023 

maintenance DVs that are 2 ppb lower, 

and seven receptors have 2023 

maintenance DVs that are 1 ppb lower. 

In comparison, using the EPA’s 

maintenance methodology results in all 

21 2023 maintenance DVs being equal 

or up to 4 ppb higher than the 2023 

nonattainment DVs. Again, the EPA 

uses the average of the three DVs that 

contain the base year modeled for the 

nonattainment methodology and the 

maximum of these three DVs for the 

maintenance methodology. Because 

TCEQ’s maintenance methodology of 

just using the most recent DV (2012– 

2014 DV) often results in maintenance 

DVs lower than the 2023 nonattainment 

DVs methodology results, the EPA finds 

that the TCEQ methodology is not 

adequately identifying conditions when 

a receptor would have more difficulty 

maintaining the standard. In fact, the 

TCEQ’s method also identified one 

receptor in their SIP submission as a 

nonattainment receptor in 2023 that 

would not have been identified as a 

maintenance receptor, which further 

highlights the concern that TCEQ’s 

method did not adequately identify 

areas that may struggle to maintain the 

standard. TCEQ did not address 

whether the three years that comprise 

the most recent design value (i.e., 2012, 

2013, and 2014) had meteorological 

conditions highly conducive for 

formation of high ozone concentrations 

and thus would be an appropriate time 

period to assess whether area could 

have difficulty maintaining the standard 

and the EPA’s analysis confirms that 

this time period is not highly conducive 

to ozone formation, at least for many 

receptors. The consequence of TCEQ’s 

maintenance method is that it often 

results in lower DVs than the 

nonattainment method as demonstrated 

by our analysis, which indicates that it 

is often not considering conditions 

when an area would have difficulty 

maintaining the standard. Further, it is 

unreasonable to have a method that 

would not identify nonattainment 

receptors also as maintenance receptors. 

Again, EPA also assessed a number of 

monitored DV trends that were provided 

in TCEQ’s SIP and previous TCEQ 

attainment demonstration SIPs 

indicating that there are at times large 

annual fluctuations upward from year to 

year in monitored DVs (sometimes 2–3 

ppb increase in one year) that are due 

to variations in meteorology. Neither 

TCEQ nor ODEQ addressed in their SIP 

submissions whether the three years 

that comprise the most recent DV (i.e., 

2012, 2013, and 2014) had 

meteorological conditions conducive for 

formation of high ozone concentrations. 

On the other hand, the EPA 

methodology can identify variations in 

ozone levels that might result in 

difficulty in maintaining the standard 

over a longer period of time. The TCEQ 

method will only identify areas that 

have difficulty maintaining the standard 

for a single design value period and, as 

a result, does not address the 

meteorological variability issue 

sufficiently. 

In its SIP submittal, ODEQ contended 

that, based on TCEQ’s use of a 2012 base 

year, and using TCEQ’s air quality 

modeling, even if Texas had used the 

EPA’s method of identifying 

maintenance receptors, the projected 

maximum DV for the Denton County 

and Tarrant County receptors would be 

70.7 ppb and 69.9 ppb, respectively, 

which are considered to be in 

attainment of the 2015 ozone NAAQS in 

2023. However, this conclusion relied 

upon a relative response factor derived 

from the TCEQ modeling and TCEQ’s 

modeling results, which are discussed 

in more detail in Section V of this action 

and in the EPA Region 6 TSD.87 TCEQ’s 

modeled projections for 2023 including 

nonattainment and maintenance values 

(using either TCEQ’ or EPA’s 

methodology) are much lower than 

recent monitored values (2018–2020 DV 

and preliminary 2019–2021 DVs) 88 for 


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the Denton County and Tarrant County monitors/ 

receptors respectively. Preliminary 2019–2021 DVs 

are 74 ppb and 72 ppb at the Denton County and 

Tarrant County monitors/receptors respectively. 

89EPA also analyzed trends using AQS data, See 

EPA Region 6 TSD. 

90 DVs 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. 

91These 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 above. That modeling 

showed that Oklahoma 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 

‘‘Ozone DVs And Contributions Revised CSAPR 

Update.xlsx’’ in Docket ID No. EPA–HQ–OAR– 

2021–0663. 

many monitors and the amount of 

further DV reductions needed to match 

TCEQ’s modeling is more than is 

reasonably expected to occur for many 

monitors/receptors. This 

underestimation of future DVs results in 

mis-identifying these two receptors and 

other receptors as not being 

nonattainment or maintenance 

receptors. Specifically, these two 

receptors would need to have at least a 

3–4 ppb decrease in the next 2–3 years 

just to attain the 2015 Ozone NAAQS in 

2023. As discussed in the EPA Region 

6 TSD, TCEQ’s previous DFW 

Attainment Demonstration SIP includes 

long-term DV trends analysis that 

indicates that DFW DVs decrease 

approximately 1 ppb per year.89

Moreover, as discussed in Section 

IV.B.2 of this action, the EPA’s updated 

modeling, which relies upon more 

recent data and the latest information on 

emissions reductions, indicates that the 

maximum design value in 2023 for the 

Denton County receptor is 72.2 ppb. 

Recent monitored air quality data at the 

Denton receptor are consistent with the 

EPA’s projections that this is an area 

that will struggle to maintain the 2015 

ozone NAAQS in 2023; the 2020 DV for 

Denton was 72 ppb.90

Finally, in its submittal, ODEQ 

pointed to the significant reductions in 

emissions that have occurred in the 

State, but the EPA believes these 

reductions have already been accounted 

for in the most recent modeling; 

therefore, even with these reductions, 

the Denton County, TX receptor is 

projected to struggle with maintenance 

of the 2015 ozone NAAQS in 2023. 

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

2 Modeling and Findings for Oklahoma 

As described in Section I of this 

action, the EPA performed air quality 

modeling using the 2016v2 platform to 

project DVs and contributions for 2023. 

This data was examined to determine if 

Oklahoma contributes at or above the 

threshold of 1 percent of the 2015 ozone 

NAAQS (0.70 ppb) to any downwind 

nonattainment or maintenance receptor. 

As shown in Table OK–3, the most 

recent modeling data 91 indicate that in 

2023, emissions from Oklahoma 

contribute greater than one percent of 

the standard to maintenance-only 

receptors in Denton County, TX and in 

Cook County, IL. Oklahoma is not 

linked to any nonattainment receptors 

in EPA’s most recent modeling (EPA 

2016v2 modeling). Therefore, based on 

the EPA’s evaluation of the information 

submitted by ODEQ and based on the 

EPA’s most recent modeling results for 

2023, the EPA proposes to find that 

Oklahoma 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 OK–3—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS WITH OKLAHOMA LINKAGES IN 2023 BASED

ON EPA 2016V2 MODELING

Receptor 

(site ID, county, state) Nonattainment/maintenance 2020 DV 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) 

Oklahoma 

contribution 

(ppb) 

481210034, Denton, TX ............................... Maintenance ......................... 72 70.4 72.2 1.19 

170310032, Cook, IL .................................... Maintenance ......................... 74 69.8 72.4 0.75 

3. Evaluation of Information Provided 

by ODEQ 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 contribute 

significantly 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 interfere with 

maintenance) in each of its prior 

Federal, regional ozone transport 

rulemakings, and this interpretation of 

the statute has been upheld by the 

Supreme Court. See EME Homer City, 

572 U.S. at 519. 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. ODEQ did not conduct such 

an analysis in their SIP submission. 

As noted earlier, ODEQ provided 

some data on emissions and already 

implemented emissions reductions for 

sources in Oklahoma and stated that the 

2016 CSAPR Update is the only 

reasonable control warranted based on 

Oklahoma’s limited contributions to the 

Michigan and Texas receptors. Thus, 

Oklahoma relied on its EGUs being 

subject to the CSAPR Update (which 

reflected a stringency at the nominal 

marginal cost threshold of $1400/ton 

(2011$) for the 2008 ozone NAAQS) to 

argue that it had already implemented 

all cost-effective emissions reductions, 

and had no additional statutory 


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obligation to prohibit emissions under 

CAA section 110(a)(2)(D)(i)(I) with 

respect to the 2015 ozone NAAQS. 

The EPA disagrees with ODEQ’s 

conclusions for the following reasons: 

First, the CSAPR Update did not 

regulate non-electric generating units, 

and thus this analysis is incomplete. See 

Wisconsin, 938 F.3d at 318–20. Second, 

relying on the CSAPR Update’s (or any 

other CAA program’s) determination of 

cost-effectiveness without further Step 3 

analysis is not approvable. Costeffectiveness must be assessed in the 

context of the specific CAA program; 

assessing cost-effectiveness in the 

context of 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 established a benchmark costeffectiveness value for 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 under this 

NAAQS to reflect higher marginal 

control costs. As such, the marginal cost 

threshold of $1,400/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. 

In addition, the most recent EPA 

modeling captures all existing CSAPR 

trading programs in the baseline, and 

that modeling confirms that these 

control programs were not sufficient to 

eliminate Oklahoma’s linkage at Steps 1 

and 2 under the 2015 ozone NAAQS. 

The State was therefore obligated at 

Step 3 to assess additional control 

measures using a multifactor analysis. 

Finally, relying on a FIP at Step 3 is 

per se not approvable if the state has not 

adopted that program into its SIP and 

instead continues to rely on the FIP. 

States may not rely on FIP measures to 

meet SIP requirements. See CAA section 

110(a)(2)(D) (‘‘Each such [SIP] shall . . . 

contain adequate provisions . . . .’’). 

See also CAA section 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). 

In addition, ODEQ’s submission 

included a weight of evidence 

evaluation of its contribution to the 

Allegan County, MI receptor to 

conclude that it does not contribute 

significantly to nonattainment or 

maintenance at the receptor. 

The EPA disagrees with respect to 

ODEQ’s assertion regarding the 

relatively small contribution of 

emissions from Oklahoma to the 

Allegan County, MI receptor compared 

to emissions from other upwind states 

such as Illinois. Whether emissions 

from other states or 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 contributing significantly 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. 

Further, the court in Wisconsin 

explained 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. Indeed, the 

D.C. Circuit in Wisconsin specifically 

rejected petitioner arguments suggesting 

that upwind states should be excused 

from interstate transport obligations on 

the basis that some other sources of 

emissions (whether international or 

another upwind state) could be 

considered the ‘‘but-for’’ cause of 

downwind air quality problem. 938 

F.3dat 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 at 324. The 

court explained that ‘‘an upwind state 

can ‘contribute’ to downwind 

nonattainment even if its emissions are 

not the but-for cause.’’ Id.at 324–325. 

See also Catawba County v. EPA, 571 

F.3d 20, 39 (D.C. Cir. 2009) (rejecting 

the argument ‘‘that ‘significantly 

contribute’ unambiguously means 

‘strictly cause’’’ because there is ‘‘no 

reason why the statute precludes EPA 

from determining that [an] addition of 

[pollutant] into the atmosphere is 

significant even though a nearby 

county’s nonattainment problem would 

still persist in its absence’’); Miss. 

Comm’n on Envtl. Quality v. EPA,790 

F.3d 138, 163 n. 12 (D.C. Cir. 2015) 

(observing that the argument the ‘‘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 

other upwind states also contribute 

some amount of pollution to the same 

receptors to which the state is linked. 

As explained in Section IV.A of this 

action, ODEQ’s weight of evidence also 

concluded that the Allegan receptor 

would be attaining the NAAQS in 2023 

based on an analysis that assumed a 

projection of a linear reduction in DVs 

across a 12-year period (2011 to 

projected 2023 values), and then 

applied that annual reduction (1.1917 

ppb/year) to the receptor’s 2016- 

centered base period maximum DV (75 

ppb). The EPA does not necessarily 

agree that the assumptions made in 

Oklahoma’s weight-of-evidence analysis 

are reasonable; however, because the 

updated modeling also shows that 

Allegan County, MI is no longer a 

receptor in 2023, we propose to find 

such assumptions are inconsequential to 

our action on Oklahoma’s SIP. 

We recognize that the results of the 

EPA (2011 and 2016 base year) 

modeling indicated different receptors 

and linkages at Steps 1 and 2 of the 4- 

Step interstate transport framework. 

These differing results regarding 

receptors and linkages can be affected 

by the varying meteorology from year to 

year, but we do not think the differing 

results mean that the modeling or the 

EPA methodology for identifying 

receptors or linkages is inherently 

unreliable. Rather, these separate 

modeling runs all indicated: (1) That 

there are receptors that would struggle 

with nonattainment or maintenance in 

the future; and (2) that Oklahoma was 

linked to some set of these receptors, 

even if the receptors and linkages 

differed from one another in their 

specifics (e.g., Oklahoma was linked to 

a different set of receptors in one 

modeling run versus another). These 

results indicate that emissions from 

Oklahoma are substantial enough to 

generate linkages at Steps 1 and 2 to at 

least some downwind receptors, under 

varying assumptions and meteorological 

conditions, even if the precise set of 

linkages changed between modeling 

runs. 

We therefore propose that ODEQ was 

required to analyze emissions from the 

sources and other emissions activity 

from within the State to determine 


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92 In ODEQ v. EPA, the D.C. Circuit held that 

under the CAA, a state has the authority to 

implement a SIP in non-reservation areas of Indian 

country in the state, where there has been no 

demonstration of tribal jurisdiction. Under the D.C. 

Circuit’s decision, the CAA does not provide 

authority to states to implement SIPs in Indian 

reservations. ODEQ did not, however, substantively 

address any request under the separate authority in 

Indian country provided specifically to Oklahoma 

under SAFETEA. That separate authority was not 

invoked until the State submitted its request under 

SAFETEA, and was not approved until the EPA’s 

decision, described in this section, on October 1, 

2020. 

93The EPA’s prior approvals relating to 

Oklahoma’s SIP frequently noted that the SIP was 

not approved to apply in areas of Indian country 

(consistent with the D.C. Circuit’s decision in 

ODEQ v. EPA) located in the state. See, e.g., 85 FR 

20178, 20180 (April 10, 2020). Such prior expressed 

limitations are superseded by the EPA’s approval of 

Oklahoma’s SAFETEA request. 

94On December 22, 2021, the EPA proposed to 

withdraw and reconsider the October 1, 2020 

SAFETEA approval. See https://www.epa.gov/ok/ 

proposed-withdrawal-and-reconsideration-andsupporting-information. The EPA is engaging in 

further consultation with tribal governments and 

expects to have discussions with the State of 

Oklahoma as part of this reconsideration. The EPA 

also notes that the October 1, 2020 approval is the 

subject of a pending challenge in Federal court. 

Pawnee Nation of Oklahoma v Regan, No. 20–9635 

(10th Cir.). The EPA may make further changes to 

the approval of Oklahoma’s program to reflect the 

outcome of the proposed withdrawal and 

reconsideration of the October 1, 2020 SAFETEA 

approval. To the extent any change occurs in the 

scope of Oklahoma’s SIP authority in Indian 

country before the finalization of this proposed 

rule, such a change may affect the scope of the 

EPA’s final action on the proposed rule. 

whether its contributions were 

significant. Because ODEQ failed to 

perform this analysis, we propose to 

disapprove its submission. 

4. Evaluation of Information Provided 

by ODEQ 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, ODEQ’s SIP 

submission did not contain an 

evaluation of additional emission 

control opportunities (or establish that 

no additional controls are required), 

thus, no information was provided at 

Step 4. As a result, EPA proposes to 

disapprove ODEQ’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). 

5. Conclusion 

Based on the EPA’s evaluation of 

ODEQ’s SIP submission, the EPA is 

proposing to find that the portion of 

ODEQ’s SIP submission 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 which will interfere with 

maintenance of the 2015 ozone NAAQS 

in any other state. 

C. Impact on Areas of Indian Country 

Following the U.S. Supreme Court 

decision in McGirt v Oklahoma, 140 S 

Ct. 2452 (2020), the Governor of the 

State of Oklahoma requested approval 

under Section 10211(a) of the Safe, 

Accountable, Flexible, Efficient 

Transportation Equity Act of 2005: A 

Legacy for Users, Public Law 109–59, 

119 Stat. 1144, 1937 (August 10, 2005) 

(‘‘SAFETEA’’), to administer in certain 

areas of Indian country (as defined at 18 

U.S.C. 1151) the State’s environmental 

regulatory programs that were 

previously approved by the EPA for 

areas outside of Indian country. The 

State’s request excluded certain areas of 

Indian country further described below. 

In addition, the State only sought 

approval to the extent that such 

approval is necessary for the State to 

administer a program in light of 

Oklahoma Dept. of Environmental 

Quality v. EPA, 740 F.3d 185 (D.C. Cir. 

2014).92

On October 1, 2020, the EPA 

approved Oklahoma’s SAFETEA request 

to administer all of the State’s EPAapproved environmental regulatory 

programs, including the Oklahoma SIP, 

in the requested areas of Indian country. 

As requested by Oklahoma, the EPA’s 

approval under SAFETEA does not 

include Indian country lands, including 

rights-of-way running through the same, 

that: (1) Qualify as Indian allotments, 

the Indian titles to which have not been 

extinguished, under 18 U.S.C. 1151(c); 

(2) are held in trust by the United States 

on behalf of an individual Indian or 

Tribe; or (3) are owned in fee by a Tribe, 

if the Tribe (a) acquired that fee title to 

such land, or an area that included such 

land, in accordance with a treaty with 

the United States to which such Tribe 

was a party, and (b) never allotted the 

land to a member or citizen of the Tribe. 

The EPA’s approval under SAFETEA 

expressly provided that to the extent the 

EPA’s prior approvals of Oklahoma’s 

environmental programs excluded 

Indian country, any such exclusions are 

superseded for the geographic areas of 

Indian country covered by the EPA’s 

approval of Oklahoma’s SAFETEA 

request.93 The approval also provided 

that future revisions or amendments to 

Oklahoma’s approved environmental 

regulatory programs would extend to 

the covered areas of Indian country 

(without any further need for additional 

requests under SAFETEA).94

As explained earlier, the EPA is 

proposing to find that the portion of 

Oklahoma’s SIP submission 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 which will contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in any other state. 

Consistent with the D.C. Circuit’s 

decision in ODEQ v. EPA and the EPA’s 

October 1, 2020, SAFETEA approval, 

this disapproval if finalized as proposed 

will extend to areas of Indian country in 

Oklahoma where the State has SIP 

planning authority. 

V. Texas SIP Submission Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS and the EPA 

Evaluation of the SIP Submission 

A. Summary of TCEQ SIP Submission 

Addressing Interstate Transport of Air 

Pollution for the 2015 Ozone NAAQS 

On August 17, 2018, the Texas 

Commission on Environmental Quality 

(TCEQ) made a SIP submission 

addressing interstate transport of air 

pollution for the 2015 ozone NAAQS. 

The SIP submission provided TCEQ’s 

analysis of their impact to downwind 

states using a framework similar to 

EPA’s 4-Step framework and concluded 

that emissions from Texas will not 

contribute significantly to 

nonattainment or interfere with 

maintenance of the 2015 ozone NAAQS 

in other states. 

In the submittal, TCEQ provided the 

steps they used to assess whether 

emissions from Texas contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS in other States: (1) 

Identify monitors projected to be in 

nonattainment or have maintenance 

issues in future year 2023; (2) identify 

for further review projected 

nonattainment and/or maintenance 

monitors in other states that are 

impacted by emissions from Texas; and 

(3) determine if emissions from Texas 

contribute significantly to 

nonattainment or interfere with 

maintenance at the monitors identified 

in TCEQ Step 2. TCEQ stated that their 

Step 1 is the same as EPA’s Step 1 and 

that their Steps 2 and 3 are equivalent 

to EPA’s Step 2. TCEQ used a 


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95 ‘‘EPA Region 6 2015 8-Hour Ozone Transport 

SIP Proposal Technical Support Document’’ (EPA 

Region 6 2015 Ozone Transport SIP TSD.pdf) 

included in Docket ID No. EPA–R06–OAR–2021– 

0801. 

96The NODA and the October 2017 modeling are 

discussed in Section I.C of this action. 

contribution threshold of one percent of 

the NAAQS (0.7 ppb) in their Step 2 

analysis to identify nonattainmentand/ 

or maintenance monitors in other states 

that are impacted by emissions from 

Texas. TCEQ further stated that EPA’s 

Steps 3 and 4 are relevant only if 

emissions from Texas contribute 

significantly to nonattainment or 

interfere with maintenance at 

downwind monitors in another state. 

Because Texas TCEQ concluded that it 

has no such emissions, EPA’s Steps 3 

and 4 are not addressed in the SIP 

submission. 

To identify monitors projected to be 

in nonattainment or have maintenance 

issues in 2023, (EPA Step 1 and TCEQ 

Step 1), TCEQ conducted its own 

regional photochemical modeling using 

a 2012 base year. TCEQ’s modeling and 

EPA’s modeling differ in significant 

respects, which are discussed in detail 

in the EPA Region 6 2015 8-Hour Ozone 

Transport SIP Proposal Technical 

Support Document (EPA Region 6 

TSD).95 In particular, TCEQ used a 2012 

base year, stating that (1) the year 2012 

had above average temperatures across 

most of the U.S., except in some states 

in the southeast and (2) the year 2011, 

(which was used by the EPA in the 

NODA published on January 6, 2017 

and the October 2017 updated modeling 

data for 2023),96 was a meteorologically 

anomalous year for Texas and 

surrounding states as it was the hottest 

year on record and the single-worst 

drought year recorded in Texas since 

1895. TCEQ’s modeling also used some 

different emissions estimates for the 

base year and future year 2023 

emissions, including different future 

year emissions for EGUs. There were 

also some differences in methods used 

in the model results analysis and the 

model performance evaluation. TCEQ 

also used a different methodology than 

the EPA to identify monitors projected 

to be maintenance receptors in 2023. 

TCEQ used only the most recent DV 

containing the base year 2012, (i.e., the 

monitored DV for 2012–2014), to project 

a 2023 ‘‘maintenance DV’’ for assessing 

whether a monitor would have 

maintenance issues. The EPA’s 

methodology uses the maximum of the 

three consecutive regulatory DVs 

containing the base year, which is the 

highest monitored DV from among the 

three DVs that contain the 2011 base 

year (i.e., the 2009–2011 DV, 2010–2012 

DV or and 2011–2013 DV that all 

contain modeled base year of 2011), to 

project a 2023 maximum DV for 

assessing whether a monitor would have 

maintenance issues. Texas explained 

that it chose to define maintenance 

receptors in this way to capture more 

recent emission reductions. The SIP 

submittal also included a discussion of 

why TCEQ believes their approach for 

identifying maintenance receptors is 

appropriate. The TCEQ modeling and 

differences with the EPA modeling is 

discussed in detail in the EPA Region 6 

TSD for this action. 

Based on their modeling, TCEQ 

provided: (1) A table of downwind 

receptors projected to be in 

nonattainment of the 2015 ozone 

NAAQS in 2023 and have a contribution 

from Texas emissions at a threshold of 

0.7 ppb or greater and (2) a table of 

downwind maintenance receptors 

projected to have problems attaining 

and maintaining the 2015 ozone 

NAAQS in 2023 and have a contribution 

from Texas emissions at a threshold of 

0.7 ppb or greater. TCEQ identified 

these receptors for further analysis. The 

nonattainment and maintenance 

receptors provided by TCEQ are listed 

in Table TX–1. TCEQ noted that except 

for Arapahoe County, CO (Monitor ID. 

80050002) all the maintenance receptors 

are also nonattainment receptors. 

TABLE TX–1—PROJECTED 2023 NONATTAINMENT AND MAINTENANCE RECEPTORS IDENTIFIED BY TCEQ MODELING

USING 2012 BASE YEAR

Receptor 

(site ID, county, state) 

2023 

average DV 

(ppb) 

2023 

maintenance 

DV 

(ppb) (TCEQ 

method) 

Texas 

contribution 

(ppb) 

80350004, Douglas, CO .............................................................................................................. 73 72 1.42 

80590006, Jefferson, CO ............................................................................................................ 72 73 1.26 

80590011, Jefferson, CO ............................................................................................................ 71 71 1.26 

80690011, Larimer, CO ............................................................................................................... 72 71 1.22 

80050002, Arapahoe, CO ............................................................................................................ *70 71 1.15 

40038001, Cochise, AZ ............................................................................................................... 71 **69 1.06 

60371201, Los Angeles, CA ........................................................................................................ 80 78 0.76 

60371701, Los Angeles, CA ........................................................................................................ 80 82 0.72 

60376012, Los Angeles, CA ........................................................................................................ 87 86 0.9 

60658001, Riverside, CA ............................................................................................................. 88 85 0.73 

60658005, Riverside, CA ............................................................................................................. 84 83 0.71 

60710001, San Bernardino, CA .................................................................................................. 71 72 0.84 

60710306, San Bernardino, CA .................................................................................................. 76 77 0.81 

60711004, San Bernardino, CA .................................................................................................. 91 90 0.88 

60714001, San Bernardino, CA .................................................................................................. 82 79 0.86 

60714003, San Bernardino, CA .................................................................................................. 94 91 0.74 

* TCEQ did not include this value in their SIP narrative (this cell was blank). The EPA obtained this value from data that was in TCEQ’s 

spreadsheet of future 2023 DVs with state contributions. 

** TCEQ did not provide this calculation. The EPA used TCEQ’s modeling information to calculate this value using the Relative Response Factor in TCEQ spreadsheet of future 2023 DVs with state contributions and the monitor’s 2012–2014 DV (0.983 X 71 ppb, truncation applied). 

TCEQ also noted that in the EPA’s 

2017 Transport NODA, the EPA’s 

modeling linked Texas to six receptors 

based on the receptors being identified 

as nonattainment or maintenance 

receptors and based on a 0.7 ppb 

contribution threshold. TCEQ provided 

a table of those monitors along with the 

EPA and TCEQ modeling results for 


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97TCEQ SIP Submission, at page 3–49 (Table 3– 

12). 

98See FN 8. 99See FN 73. 

those receptors (Table TX–2).97 TCEQ 

stated that the differences are due to 

changes the TCEQ made to modeling 

inputs (primarily the different base year 

of 2012 versus the EPA’s 2011), 

analysis, and methodologies (primarily 

TCEQ’s alternate maintenance receptor 

methodology), see the EPA Region 6 

TSD included in the Regional docket for 

this action (Docket ID No. EPA–R06– 

OAR–2021–0801) for more details. With 

exception of the Jefferson County, CO 

receptor (Monitor ID. 80590011) TCEQ 

did not further review its linkages to 

any of the receptors in Table TX–2. 

TABLE TX–2—TCEQ INFORMATION ON RECEPTORS LINKED TO TEXAS BY EPA MODELING IN THE TRANSPORT NODA 

PUBLISHED ON JANUARY 6, 2017 

Receptor 

(site ID, county, state) 

EPA 2023 

average DV 

(ppb) 

EPA Texas 

contribution 

(ppb) 

TCEQ 2023 

average DV 

(ppb) 

TCEQ Texas 

contribution 

(ppb) 

260050003, Allegan, MI ................................................................................... 68.8 2.49 71 0.59 

551170006, Sheboygan, WI ............................................................................ 71.0 1.92 70 0.73 

240251001, Harford, MD ................................................................................. 71.3 0.91 65 0.69 

360850067, Richmond, NY ............................................................................. 71.2 0.77 62 0.67 

361030002, Suffolk, NY ................................................................................... 71.3 0.71 67 0.63 

80590011, Jefferson, CO ................................................................................ 69.7 1.03 71 1.26 

TCEQ then used a weight of evidence 

approach to assess whether emissions 

from Texas contribute significantly to 

nonattainment or interfere with 

maintenance at the receptors listed in 

Table TX–1. TCEQ stated that the Texas 

contribution to a receptor should be 

deemed ‘‘significant’’ only if there is a 

persistent and consistent pattern of 

contribution on several days with 

elevated ozone. Consideration was given 

to factors such as DV trends, number of 

elevated ozone days, back trajectory 

analysis on elevated ozone days, 

modeled concentrations on future 

expected elevated ozone days, total 

interstate contributions at tagged 

monitors, and responsiveness of ozone 

to emissions from Texas. Based on their 

assessment, TCEQ concluded that 

emissions from Texas do not contribute 

significantly to nonattainment or 

interfere with maintenance of the 2015 

ozone NAAQS at any downwind 

monitors. Our evaluation of the TCEQ 

submission is further discussed in 

Section V.B and in the EPA Region 6 

TSD for this action. 

B. EPA Evaluation of the TCEQ SIP 

Submission 

Based on the EPA’s evaluation of the 

SIP submission, the EPA is proposing to 

find that TCEQ’s August 17, 2018, SIP 

submission does not meet the State’s 

obligations with respect to prohibiting 

emissions that contribute significantly 

to nonattainment or interfere with 

maintenance of the 2015 ozone NAAQS 

in any other state. 

1. Evaluation of Information Provided 

by TCEQ Regarding Step 1 

As explained in Section I of this 

action, at Step 1 of the 4-Step interstate 

transport framework, the EPA identifies 

monitoring sites that are projected to 

have problems attaining and/or 

maintaining the NAAQS (i.e., 

nonattainment and maintenance 

receptors). In executing this step, TCEQ 

elected to rely on their own modeling 

and methodology for identifying 

receptors. The EPA is evaluating the 

TCEQ’s modeling and methodology here 

at Step 1. 

i. Evaluation of TCEQ’s Methodology for 

Identifying Maintenance Receptors 

As discussed in Section V.A of this 

action, in addition to the use of an 

alternative modeling platform, TCEQ 

also created its own method for 

identifying maintenance receptors. 

TCEQ has not adequately explained or 

justified how its method for identifying 

maintenance receptors reasonably 

identifies areas that will have difficulty 

maintaining the NAAQS. The EPA 

proposes to find that TCEQ has not 

provided a sufficient technical basis for 

how its chosen methodology gives 

meaning to the CAA’s instruction that 

states submit good neighbor SIPs that 

prohibit their states’ emissions from 

interfering with the maintenance of the 

NAAQS in another state. 

In North Carolina v. EPA, 531 F.3d 

896, 909–11 (D.C. Cir. 2008), the D.C. 

Circuit rejected the EPA’s CAIR on the 

basis that the EPA had not adequately 

given meaning to the phrase ‘‘interfere 

with maintenance’’ in the good neighbor 

provision. Specifically, North Carolina 

argued that it had counties that were 

projected to attain the NAAQS in the 

future analytic year but were at risk of 

falling back into nonattainment due to 

interference from upwind sources, 

particularly given year-to-year 

variability in ozone levels. The court 

agreed, holding that the EPA’s rule did 

not adequately protect ‘‘[a]reas that find 

themselves barely meeting attainment.’’ 

Id. at 910. Consequently, the EPA has 

developed a methodology, as described 

elsewhere in this action and used in its 

2011 CSAPR and its 2016 CSAPR 

Update and Revised CSAPR Update, for 

identifying areas that may struggle to 

maintain the NAAQS. See 76 FR at 

48227–28. The EPA’s approach to 

addressing maintenance receptors was 

upheld in the EME Homer City 

litigation. See 795 F.3d 118, 136–37. It 

was also upheld in Wisconsin. 938 F.3d 

at 325–26. In Wisconsin, the court noted 

that four upwind states were linked 

only to maintenance receptors and 

rejected the argument that application of 

the same control level as the EPA 

imposes for those states linked to 

nonattainment receptors was 

unreasonable or unlawful absent a 

particularized showing of overcontrol. 

Id. at 327. 

To explain the differences between 

TCEQ’s and the EPA’s methodology for 

identifying maintenance receptors, it is 

helpful to provide some additional 

context of how the EPA projects future 

air quality. The EPA’s air quality 

modeling guidance has long 

recommended developing a base design 

value (DV) 98 (i.e., the design value that 

will be used as a starting point to model 

and analyze for purposes of projecting 

future air quality concentrations) that is 

the average of three DVs spanning a 

five-year period, centered around one 

year for which an emissions inventory 

will be submitted (e.g., if 2011 was the 

base emissions inventory year, a state 

would use monitored values from 2009– 

2011, 2010–2012, 2011–2013 as the 

starting point for projecting air quality 

concentrations in future years).99 The 


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100See FN 53. 

101TCEQ SIP submission at 3–39 to 3–40. 

102See EPA Region 6 TSD, included in Docket ID 

No. EPA–R06–OAR–2021–0801. 

103 Id. 

average of these three DVs is then 

multiplied by a relative response factor 

(RRF) 100 to generate an average DV for 

the future year. If a receptor’s average 

future year DV is greater than or equal 

to the level of the NAAQS, and the 

receptor has recent monitored data that 

violates the NAAQS, that receptor is 

considered a ‘‘nonattainment’’ receptor 

at Step 1. To identify maintenance 

receptors, the EPA’s methodology looks 

to the highest DV of the three DVs used 

to calculate the 5-year weighted average 

design value (e.g., in the 2011 example, 

if 2009–2011 had the highest design 

value of 2009–2011, 2010–2012, and 

2011–2013). The EPA then applies the 

same relative response factor to that 

highest design value to generate a 

projected future maximum design value. 

Where a receptor’s maximum design 

value exceeds the level of the NAAQS, 

the EPA has deemed those receptors to 

be ‘‘maintenance’’ receptors. This 

methodology was designed to address 

the D.C. Circuit’s holding that the CAA’s 

‘‘interference with maintenance’’ prong 

requires states and the EPA to protect 

areas that may struggle with 

maintaining the standard in the face of 

inter-annual variability in ozoneconducive conditions. 

In its modeling, TCEQ adopted an 

identical approach to the EPA’s for 

identifying nonattainment receptors—it 

looked at three sets of DVs over a fiveyear period and averaged those DVs to 

generate a base year DV. TCEQ then 

applied a relative response factor to that 

base year design value to project a 

receptor’s average design value in the 

future year. For its maintenance 

receptors, however, TCEQ used only the 

most recent design value of the set of 

three DVs, regardless of whether the 

most recent design value was highest or 

lowest, instead of considering 

variability in conditions over a five-year 

period, or using the highest DV of the 

three DVs making up the base year 

design value. TCEQ’s proffered 

explanation for using the most recent 

DV to identify maintenance receptors 

was that the latest DV ‘‘takes into 

consideration . . . any emissions 

reductions that might have 

occurred.’’ 101 However, TCEQ in its 

submission does not explain how this 

methodology takes into account 

meteorological variability in identifying 

those areas that may be meeting the 

NAAQS or that may be projected to 

meet the NAAQS but may nevertheless 

struggle to maintain the NAAQS. 

TCEQ argued that the 3-year DV used 

includes some meteorological 

variability. Unfortunately, the three 

years of variation that TCEQ accounted 

for is already built into the structure of 

the standard. Thus, the TCEQ method 

gave no consideration to the variability 

between calculated DVs which provides 

a direct indication of the difficulty a 

receptor will have in maintaining the 

standard. In other words, to determine 

whether a receptor will have difficulty 

maintaining the standard, one must 

consider the variation in the metric that 

will be used to determine compliance 

with the standard. An indication of the 

variability of a metric cannot be 

determined by only considering a single 

estimate of that metric. 

TCEQ’s stated purpose in using the 

most recent DV was to capture more 

recent emissions reductions. TCEQ’s 

methodology, however, limits receptors 

which could be identified as 

maintenance receptors, compared to the 

EPA’s methodology largely because it 

only looks at one design value period 

rather than selecting the maximum of 

the three DV periods EPA’s 

methodology considers. Thus, TCEQ’s 

methodology greatly reduces the 

probability that meteorological 

conditions which make it difficult to 

maintain the standard will be 

considered. As discussed further below, 

the effects of emissions trends are 

already captured through other aspects 

of the methodology to identify 

receptors. So, in trying to give more 

weight to emission reductions, by 

selecting only one design value (2012– 

2014) for its base year, TCEQ’s 

methodology did not give any 

consideration to interannual variability 

in ozone-conducive meteorology as does 

the EPA’s method. 

The EPA’s methodology, using the 

maximum DV which accounts for the 

variability in ozone concentrations and 

DVs due to changes in meteorology over 

the five years of the base year DV 

period, was designed to identify those 

areas that might struggle to maintain the 

NAAQS in particularly ozone conducive 

conditions. TCEQ claimed that the 

EPA’s method undervalues changes in 

air quality due to emission reductions 

and overvalues changes due to variation 

in meteorology. TCEQ pointed out that 

emissions nationwide are generally 

trending downward as a result of 

Federal motor vehicle standards and 

other technological improvements. The 

EPA agrees that ozone levels generally 

trend downward, but there is not a 

steady decline from year to year in 

ozone concentrations. Rather, ozone 

levels tend to vary from year to year 

with some years showing an increase 

instead of a decrease mainly due to 

inter-annual variability in ozoneconducive meteorology.102 The 

variation of DVs at individual monitors 

from year to year can be significant, 

even where emissions trend 

downwards. The EPA also assessed a 

number of monitored DV trends that 

were provided in TCEQ’s SIP 

submission and previous TCEQ 

attainment demonstration SIPs 

indicating that there are at times large 

annual fluctuations upward from year to 

year in monitored DVs (sometimes 2–3 

ppb increase in one year) that are due 

to variations in meteorology.103 This is 

precisely why it is important to consider 

highly variable meteorology and its 

influence on DVs—the issue at the heart 

of the D.C. Circuit’s finding on 

‘‘interference with maintenance’’ in 

North Carolina. Areas that are required 

under the Act to attain by an attainment 

date may fail to attain because of a 

combination of both local emissions, 

upwind emissions, and ozone 

conducive meteorology, among other 

factors. The North Carolina decision 

made clear that in interpreting the good 

neighbor provision, upwind state and 

the EPA obligations to reduce emissions 

must account for variable conditions 

that could cause an area that is 

sometimes attaining the NAAQS to fall 

out of attainment. See also Wisconsin, 

938 F.3d at 327 (‘‘Variations in 

atmospheric conditions and weather 

patterns can bring maintenance 

receptors into nonattainment even 

without elevated emissions.’’). 

In addition, TCEQ claimed that its use 

of the 2012–2014 DV (i.e., the most 

recent in the 5-year base period it 

examined) is more reliable than the 

EPA’s method, because that more recent 

DV accounts for both emission 

reductions and because there is a 

shorter interval between the monitored 

DV and the projected DV. As we note 

elsewhere, the TCEQ’s base year 

modeled inventory is 2012 emissions 

and the TCEQ’s model projections for 

2023 include the expected emission 

reductions from 2012 thru 2014 and to 

2023. By just using the 2012–2014 DV 

data, TCEQ claimed they are giving 

weight to emission reductions during 

the final base years where EPA’s method 

does not. The effect of emission 

reductions, however, is already factored 

in the method since the modeling 

projection to 2023 is explicitly designed 

to project the changes in ozone due to 

emission reductions from the 2012 base 

year emission levels. So, in fact, the 

EPA method does give weight to 

emission reductions. Furthermore, since 


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9828 Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 

104 ‘‘EPA Region 6 2015 8-Hour Ozone Transport 

SIP Proposal Technical Support Document’’ (EPA 

Region 6 2015 Ozone Transport SIP TSD.pdf) 

included in Docket ID No. EPA–R06–OAR–2021– 

0801. 

TCEQ agrees that the average of the DVs 

based on 2010–2014 ozone levels are 

reliable enough to use in the 

identification of nonattainment 

receptors, it is unclear how the 2012– 

2014 period is deemed more reliable for 

the maintenance test since the modeled 

emissions are still for 2012. We also 

note, as discussed throughout this 

action, the EPA has updated its 

modeling to use a 2016 base year—that 

is, a five year period spanning 2014– 

2018, and applied its methodology for 

defining maintenance receptors using 

that five year base period. Using a more 

recent base period (EPA’s 2016v2) 

provides the most recent design values, 

shorter period of projection (2016 to 

2023 versus a 2011 or 2012 base year) 

and a more accurate basis for 

projections of future air quality. We note 

that the EPA undertook a large 

collaborative multi-year effort with 

states (including TCEQ) and other 

stakeholders input and review in 

developing the 2016v2 emission 

inventories. By virtue of this update, 

any monitored DV used by the EPA to 

identify maintenance receptors in this 

action accounts for more recent 

emission reductions and provides a 

shorter interval between base year 

monitored DV and the projected future 

analytic year. 

As discussed further in the EPA 

Region 6 TSD 104 for this action, the EPA 

has reviewed the set of 21 receptors for 

which Texas had contributions of 0.7 

ppb or more in the EPA’s 2016 base year 

modeling analyses, or TCEQ’s modeling 

(2012 base year), and evaluated the 

results of using TCEQ’s alternate 

maintenance methodology. For these 21 

receptors, TCEQ’s method resulted in 15 

of the 21 2023 maintenance DVs 

predicted to be lower than the 2023 

nonattainment DVs from the 

nonattainment methodology that uses 

the 5-year center weighted average. Of 

these 15 receptors, three receptors have 

2023 maintenance DVs that are 3 ppb 

lower, five receptors have 2023 

maintenance DVs that are 2 ppb lower, 

and seven receptors have 2023 

maintenance DVs that are 1 ppb lower. 

In comparison, using the EPA’s 

maintenance methodology results in all 

21 2023 maintenance DVs being equal 

or up to 4 ppb higher than the 2023 

nonattainment DVs. Again, the EPA 

uses the average of the three DVs that 

contain the base year modeled for the 

nonattainment methodology and the 

maximum of these three DVs for the 

maintenance methodology. Because 

TCEQ’s maintenance methodology of 

just using the most recent DV (2012– 

2014 DV) often results in maintenance 

DVs lower than the 2023 nonattainment 

DVs methodology results, the EPA finds 

that the TCEQ methodology is not 

adequately identifying conditions when 

a receptor would have more difficulty 

maintaining the standard. In fact, the 

TCEQ’s method also identified one 

receptor in their SIP submission as a 

nonattainment receptor in 2023 that 

would not have been identified as a 

maintenance receptor, which further 

highlights the concern that TCEQ’s 

method did not adequately identify 

areas that may struggle to maintain the 

standard. TCEQ did not address 

whether the three years that comprise 

the most recent design value (i.e., 2012, 

2013, and 2014) had meteorological 

conditions highly conducive for 

formation of high ozone concentrations 

and thus would be an appropriate time 

period to assess whether area could 

have difficulty maintaining the standard 

and the EPA’s analysis confirms that 

this time period is not highly conducive 

to ozone formation, at least for many 

receptors. The consequence of TCEQ’s 

maintenance method is that it often 

results in lower DVs than the 

nonattainment test as demonstrated by 

our analysis, which indicates that it is 

often not considering conditions when 

an area would have difficulty 

maintaining the standard. It is also 

unreasonable to have a test that would 

not identify nonattainment receptors 

also as maintenance receptors. 

TCEQ also made several additional 

assertions in support of their conclusion 

that their method for identifying 

maintenance receptors was the better 

reading of the CAA, compared to the 

EPA’s. TCEQ claimed that its approach 

was more consistent with the CAA’s 

concept of maintenance as areas that 

were formerly nonattainment and that 

have since attained and will continue to 

maintain by accounting for: (1) 

Emissions reductions occurring in the 

later design values of the base DV 

period; (2) ‘‘commitments regarding 

contingency measures to address future 

emission reductions;’’ and (3) the 

impact of any maintenance plans that 

are in place. TCEQ also asserted that the 

EPA’s approach conflates the likelihood 

of attaining the standard in a future year 

and the ability of an attainment monitor 

to maintain that attainment status. 

Specifically, TCEQ argued that because 

any remedies devised to address 

nonattainment monitors would have to 

apply to maintenance monitors, a 

practical consequence of the EPA’s 

approach is that it could lead to overcontrol and that it might require upwind 

states to consider or implement controls 

when the downwind state in which the 

monitor is located does not have any 

obligations to control local emissions. 

TCEQ argued that this ‘‘conflation’’ of 

nonattainment and maintenance results 

in there being no independent meaning 

to ‘‘maintenance.’’ 

With respect to the first of these 

assertions from TCEQ, we note that 

TCEQ’s methodology for identifying 

receptors (like the EPA’s) is entirely 

distinct from ozone designations under 

the Clean Air Act; neither TCEQ nor the 

EPA take current or presumed future 

designations of areas into account, and 

any implementation requirements like a 

maintenance plan under CAA section 

175A, in identifying receptors. TCEQ’s’ 

discussion, therefore, of maintenance 

plan contingency measures or 

maintenance plans generally is 

irrelevant and misplaced. None of the 

areas to which Texas is linked in the 

EPA 2016v2 modeling has been 

redesignated to attainment for the 2015 

ozone NAAQS, and none of the areas to 

which Texas is linked in its own 

modeling has been redesignated to 

attainment for that NAAQS. We also fail 

to see how TCEQ’s approach to 

identifying maintenance receptors 

differs in any relevant respect from the 

EPA’s approach with regard to the 

alleged ‘‘conflation’’ of projecting 

attainment in a future year rather than 

the ability of an attainment receptor to 

maintain attainment. Both TCEQ and 

the EPA identify maintenance receptors 

based on projections of air quality in a 

future year to determine whether the 

receptor will have difficulty attaining or 

maintaining the standard. TCEQ’s 

arguments about overcontrol based on 

the application of a uniform remedy to 

states linked to both nonattainment and 

maintenance receptors were also not 

germane; in this case, TCEQ had 

identified no remedy to apply 

whatsoever because it had failed to 

identify that the emissions from Texas 

cause a problem in the first instance. 

The D.C. Circuit has already rejected the 

idea that the application of a uniform 

control to both nonattainment and 

maintenance receptors is on its face 

overcontrol or impermissible under the 

interstate transport provision. See 

Wisconsin, 938 F.3d at 327. Based on 

our evaluation of TCEQ’s approach to 

identify maintenance receptors for 2023, 

we propose to find the State’s approach 

is inadequate as it does not sufficiently 

identify maintenance receptors. Further, 

TCEQ had not explained how its 


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Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 9829 

105We note that for two of the Wisconsin 

receptors, TCEQ’s modeling does not provide 

information to generate 2023 DVs, so only 5 of the 

7 monitors can be compared. 

approach meets the statutory 

requirement to address areas that, even 

if meeting the NAAQS, may struggle to 

maintain the standard in years where 

conditions are conducive to ozone 

formation. Rather, the TCEQ had created 

its own approach to identify these areas 

that they describe as designed to 

account for the most emission 

reductions possible—i.e., the most 

recent DV of the three under analysis; 

an approach that likely under-identifies 

areas that will struggle to maintain the 

NAAQS and that certainly is not 

designed to capture potential air quality 

problems. 

ii. Evaluation of the TCEQ Modeling 

As discussed in Section V.A of this 

action, TCEQ conducted regional 

photochemical modeling to identify 

nonattainment and maintenance 

receptors in 2023 using a 2012 base 

year. As discussed further in the EPA 

Region 6 TSD, we have several concerns 

with the reliability of TCEQ’s modeling 

results. States are free to develop their 

own modeling, but that modeling must 

be technically supportable, and the EPA 

is obligated to assess and evaluate the 

reliability of that technical 

demonstration when determining 

whether the Act’s requirements are met. 

The TCEQ’s modeling underestimates 

future ozone levels. When the TCEQ 

2023 projected concentrations are 

compared to 2020 and preliminary 2021 

monitor values, it is clear that the TCEQ 

modeling is projecting an unusual 

decline in ozone levels without there 

being an unusual level of emission 

reductions to support the decline. The 

EPA compared recent monitoring values 

and reasonably anticipated decreases in 

DVs by 2023 both within Texas and in 

other parts of the country. These 

underestimations likely result in 

TCEQ’s modeling not adequately 

identifying nonattainment and/or 

maintenance receptors in 2023. These 

underestimations also result in smaller 

projected contributions from Texas 

emissions to downwind states. See EPA 

Region 6 TSD for full analysis details. 

One analysis included in the EPA 

Region 6 TSD examined the average 

amount of improvement that would 

have to occur for the 9 monitors with 

the highest measured design values in 

the Dallas-Ft. Worth and HoustonGalveston-Brazoria nonattainment areas 

(those with an observed 2018–2020 DV 

of 74 ppb or greater) to reach the level 

of ozone projected by the TCEQ 

modeling. The average decrease needed 

by 2023 to meet TCEQ’s 2023 projected 

DVs is 7.56 ppb. Improvements of this 

magnitude do not occur in three years 

unless there is an unusually large 

change in emissions or a large change in 

meteorological conduciveness for ozone 

generation. TCEQ did not identify any 

large emission reductions not already 

accounted for in the modeling to be 

implemented in the 2021–2023 

timeframe nor is the EPA aware of such 

a change. This information supports our 

finding that that TCEQ’s modeling is 

underestimating future ozone levels in 

the two nonattainment areas in Texas 

that make up a large proportion of the 

total ozone and a large portion of 

emissions of ozone pre-cursors that 

transport to downwind areas. This 

underestimation of future year ozone 

levels from Texas emissions can cause 

both an underestimation of ozone in 

downwind areas and also an 

underestimation of Texas’s impact on 

downwind State’s ozone nonattainment 

and maintenance receptors. 

TCEQ’s modeling also underestimates 

2023 ozone levels outside of the State of 

Texas including areas of interest in 

California, Colorado and the Midwest 

Region (Illinois, Wisconsin, and 

Michigan). The EPA discusses this 

underprediction for all of these areas in 

the EPA Region 6 TSD. In Table TX–3, 

we present only the results for the 

Midwest Region along with the EPA’s 

modeling prediction. We note that 

TCEQ’s 2023 modeled DVs are 

significantly lower than the EPA’s 2023 

modeled DVs. The table also provides 

recent monitored 2020 DVs and 

preliminary 2021 DVs, which shows 

that recent monitored ozone 

concentrations are significantly higher 

than TCEQ’s modeling projected for 

2023. TCEQ’s ozone DVs for these 

receptors would need to drop on the 

order of 7–15 ppb in two to three years 

for TCEQ’s projections to bear out. As 

noted previously, this would require an 

unusual amount of emission reductions 

without any control measures identified 

of sufficient magnitude. We note that 

the EPA’s projected 2023 ozone DVs 

based on EPA 2016v2 modeling show 

ozone DVs that are also lower than 

recent monitoring data. However, EPA 

2016v2 modeling projections are much 

closer to anticipated 2023 ozone levels 

as compared to TCEQ’s modeling. This 

indicates that the EPA’s modeling is 

more accurate in identifying 

nonattainment and/or maintenance 

receptors in the Midwest Region. While 

the TCEQ modeling projects much 

lower overall ozone levels for the 

Midwest Region in 2023, the modeling 

does tend to corroborate the projected 

amount emissions that Texas may be 

contributing to projected ozone levels at 

5 of the 7 nonattainment and 

maintenance receptors identified in the 

EPA’s modeling.105 Thus, despite the 

differences in identification of 

nonattainment and maintenance 

receptors, both sets of modeling indicate 

that Texas’s contribution to receptors in 

the Midwest Region are greater than 0.7 

ppb (i.e., 1 percent of the 2015 ozone 

NAAQS). Table TX–3 provides 

information on those receptors, 

including the amount of contribution 

attributed to emissions from Texas 

based on EPA’s 2016v2 modeling and 

TCEQ’s modeling. Despite the 

differences in identification of 

nonattainment and maintenance 

receptors, both sets of modeling indicate 

that Texas’s contribution to receptors in 

the Midwest are greater than 0.7 ppb 

(i.e., 1 percent of the 2015 ozone 

NAAQS). 

TABLE TX–3—EPA AND TCEQ MODELING RESULTS FOR DOWNWIND RECEPTORS IDENTIFIED BY EPA 2016V2 MODELING

Receptor 

(site ID, county, state) 

2023 nonattainment/ 

maintenance 

(EPA 2016v2) 

EPA: 2023 

average DV/ 

maximum DV 

(ppb) 

TCEQ: 2023 

average DV/ 

maintenance 

DV 

(ppb)* 

Monitored 

2018–2020 

DV/preliminary 

2019–2021 

DV** 

(ppb) 

EPA: Texas 

contribution 

(ppb) 

TCEQ: Texas 

contribution 

(ppb) 

170310001, Cook, IL ........... Maintenance ......................... 69.6/73.4 60/58 ............ 75/71 0.86 1.6. 

170310032, Cook, IL ........... Maintenance ......................... 69.8/72.4 68/66 ............ 74/75 1.46 1.31. 

170314201, Cook, IL ........... Maintenance ......................... 69.9/73.4 64/62 ............ 77/74 1.15 1.25. 

170317002, Cook, IL ........... Maintenance ......................... 70.1/73.0 66/65 ............ 75/73 1.58 1.22. 


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106 Design values and contributions at individual 

monitoring sites nationwide are provided in the 

file: ‘‘2016v2_DVs_state_contributions.xlsx’’, which 

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

0663. 

TABLE TX–3—EPA AND TCEQ MODELING RESULTS FOR DOWNWIND RECEPTORS IDENTIFIED BY EPA 2016V2 

MODELING—Continued 

Receptor 

(site ID, county, state) 

2023 nonattainment/ 

maintenance 

(EPA 2016v2) 

EPA: 2023 

average DV/ 

maximum DV 

(ppb) 

TCEQ: 2023 

average DV/ 

maintenance 

DV 

(ppb)* 

Monitored 

2018–2020 

DV/preliminary 

2019–2021 

DV** 

(ppb) 

EPA: Texas 

contribution 

(ppb) 

TCEQ: Texas 

contribution 

(ppb) 

550590019, Kenosha, WI .... Nonattainment ....................... 72.8/73.7 67/66 ............ 74/74 1.72 1.44. 

550590025, Kenosha, WI .... Maintenance ......................... 69.2/72.3 No data*** .... 74/72 1.81 No data.*** 

551010020, Racine, WI ....... Nonattainment ....................... 71.3/73.2 No data*** .... 73/73 1.34 No data.*** 

* TCEQ did not provide sufficient data and analysis of the meteorology for the 2010–2014 period to support their claim that 2012–2014 period 

was a worst-case combination of meteorology compared to the 2010–2012 and 2011–2013 periods. If the future DV projected from this highest 

value is below the standard, one can be reasonably certain the receptor will not have difficulty maintaining the standard and, as such, upwind 

states will not interfere with maintenance in downwind states. Because the TCEQ method only looks at one DV and does not account for the variability in DVs due to meteorological conditions, it is less likely to identify maintenance receptors than the EPA method. See https://www.epa.gov/ 

air-trends/air-quality-design-values 

** Preliminary 2019–2021 DVs. Monitoring data from the EPA’s Air Quality System (AQS) (https://www.epa.gov/aqs). 2021 monitoring data is 

preliminary and still has to undergo Quality Assurance/Quality Control analysis and be certified by the State of Texas, submitted to the EPA, and 

reviewed and concurred on by EPA. 2018–2020 DVs are 72 ppb and 73 ppb at the Denton County and Tarrant County monitors/receptors respectively. Preliminary 2019–2021 DVs are 74 ppb and 72 ppb at the Denton County and Tarrant County monitors/receptors respectively. 

*** Kenosha, WI Monitor ID. 550590025 was installed and began operating May 13, 2013, so the first three year DV available is 2013–2015. 

Racine, WI Monitor ID. 551010020 was installed in April 14, 2014 so the first three year DV available is 2015–2017. TCEQ’s modeling used 

monitored DV data for 2010–2012, 2011–2013, and 2012–2014 to project to the future year. Since these monitors do not have valid DVs for 

these periods, TCEQ’s modeling can’t be used to project 2023 values and identify if they would be nonattainment or maintenance receptors. 

The EPA investigated TCEQs 

modeling and the underestimation for 

the future year. See the EPA Region 6 

TSD for further information on our 

review. Our review indicated some 

underestimation bias in the base case 

and general model performance 

concerns but nothing that was a clear 

cause of the much lower 2023 DVs that 

TCEQ’s modeling is projecting. For the 

EPA’s 2016 base year modeling, the EPA 

undertook a large collaborative multiyear effort with states (including Texas) 

and other stakeholder input in 

developing the 2016 emission 

inventories including 2016v2, so that 

the EPA’s modeling would be based on 

the best data available. Using a 2016 

base year also provides a more recent 

platform that shortens the number of 

years to project emission changes, 

reducing uncertainties in the 2023 

projection compared to TCEQ’s 

projection from a 2012 base to 2023 or 

the EPA’s earlier 2011 base year 

modeling. Use of a more recent 2016 

base year also allows for the use of 

monitored DVs from a more recent 

period. The combination of these and 

other issues discussed in the EPA 

Region 6 TSD result in less model 

uncertainty compared to TCEQ’s 2012 

base year modeling and has provided a 

better estimate of 2023 ozone levels and 

therefore, we believe a more reliable 

tool for predicting which areas of the 

country will be nonattainment or have 

difficulty maintaining the standard as 

well as assessing contributions from 

upwind states. 

The EPA’s modeling using both 2011 

and 2016 base year periods identified 

that Texas was linked to nonattainment 

and/or maintenance receptors in 2023 in 

the Midwest Region (Illinois, 

Wisconsin, and Michigan), while 

TCEQ’s modeling using a 2012 base year 

indicated only linkages to western 

receptors. As discussed above and in the 

EPA Region 6 TSD, the TCEQ’s 

modeling is underestimating projected 

ozone levels in the Midwest Region for 

2023. If TCEQ’s 2023 modeled DVs were 

closer to recent observed monitoring 

data and anticipated 2023 monitored 

DVs, TCEQ would likely have also 

identified nonattainment and/or 

maintenance receptors in the Midwest 

Region. 

To summarize, TCEQ did its own 

modeling at Step 1. Our analysis shows 

that TCEQ’s modeling likely 

underestimates ozone levels at potential 

receptors and that TCEQ’s methodology 

for identifying maintenance receptors 

used to identify maintenance receptors 

fails to reasonably identify areas that 

will have difficulty maintaining the 

NAAQS. 

2. Evaluation of Information Provided 

by TCEQ Regarding Step 2 

TCEQ, like the EPA, used a 1 percent 

of the ozone NAAQS (or 0.7 ppb) as the 

‘‘linkage’’ threshold to identify states as 

‘‘linked’’ for contributions it made to 

areas with projected air quality 

problems. Although TCEQ asserted that 

the EPA treats the 1 percent threshold 

as the threshold by which the EPA 

determines ‘‘significant contribution’’ 

this is in fact incorrect. The EPA, like 

TCEQ, uses the 1 percent contribution 

threshold to identify those linkages 

between a contributing upwind state 

and a receptor projected to have air 

quality problems that warrant further 

review and additional analysis. We 

therefore endorse TCEQ’ use of the 1 

percent contribution threshold to 

identify linkages requiring further 

analysis. However, because we propose 

to disapprove TCEQ’s identification of 

nonattainment and/or maintenance 

receptors (at Step 1) due to 

underestimations in TCEQ’s modeling 

and their unsupported methodology of 

identifying maintenance receptors, their 

submission as to Step 2 is also flawed. 

We note, however, that even in its own 

modeling, TCEQ has identified 

nonattainment and/or maintenance 

receptors to which it contributed more 

than 1 percent of the NAAQS (i.e., 

identified linkages warranting 

additional analysis at Step 3). 

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

2 Modeling and Findings for Texas 

As described in Section I and 

elsewhere in this action, the EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. This data was examined to 

determine if Texas contributes at or 

above the threshold of 1 percent of the 

2015 ozone NAAQS (0.70 ppb) to any 

downwind nonattainment or 

maintenance receptor. As shown in 

Table TX–4, the data 106 indicate that in 

2023, emissions from Texas are 

projected to contribute greater than 1 

percent of the standard to both 


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107These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I of this action. 

That modeling showed that Texas 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 No. EPA– 

HQ–OAR–2021–0663. 

nonattainment and maintenance-only 

receptors in the Chicago, IL–IN–WI 

nonattainment area (4 Cook County, IL 

receptors and 2 Kenosha County, WI 

receptors) and the Milwaukee, WI 

nonattainment area (one Racine County 

receptor).107

TABLE TX–4—PROJECTED NONATTAINMENT AND MAINTENANCE RECEPTORS WITH TEXAS LINKAGES BASED ON EPA 

2016V2 

Receptor 

(site ID, county, state) Nonattainment/maintenance 

2023 

average DV 

(ppb) 

2023 

maximum DV 

(ppb) 

Texas 

contribution 

(ppb) 

170310001, Cook, IL ...................................... Maintenance ................................................... 69.6 73.4 0.86 

170310032, Cook, IL ...................................... Maintenance ................................................... 69.8 72.4 1.46 

170314201, Cook, IL ...................................... Maintenance ................................................... 69.9 73.4 1.15 

170317002, Cook, IL ...................................... Maintenance ................................................... 70.1 73.0 1.58 

550590019, Kenosha, WI ............................... Nonattainment ................................................ 72.8 73.7 1.72 

550590025, Kenosha, WI ............................... Maintenance ................................................... 69.2 72.3 1.81 

551010020, Racine, WI .................................. Nonattainment ................................................ 71.3 73.2 1.34 

We recognize that the results of the 

EPA (2011 and 2016 base year) and 

TCEQ (2012 base year) modeling 

indicated different receptors and 

linkages at Steps 1 and 2 of the 4-Step 

interstate transport framework. These 

differing results regarding receptors and 

linkages can be affected by the varying 

meteorology from year to year, but we 

do not think the differing results mean 

that the modeling or the EPA or the 

State’s methodology for identifying 

receptors or linkages is inherently 

unreliable. Rather, the three separate 

modeling runs all indicated: (1) There 

were receptors that would struggle with 

nonattainment or maintenance in the 

future; and (2) Texas was linked to some 

set of these receptors, even if the 

receptors and linkages differed from one 

another in their specifics (e.g., a 

different set of receptors were identified 

to have nonattainment or maintenance 

problems, or Texas was linked to 

different receptors in one modeling run 

versus another). These results indicate 

that emissions from Texas were 

substantial enough to generate linkages 

at Steps 1 and 2 to some downwind 

receptors, under varying assumptions 

and meteorological conditions, even if 

the precise set of linkages changed 

between modeling runs. Under these 

circumstances, we think it is 

appropriate to proceed to a Step 3 

analysis to determine what portion of 

emissions from Texas should be deemed 

‘‘significant.’’ In doing so, we are not 

agreeing with the methods and 

assumptions contained in TCEQ’s 

modeling (see previous discussion and 

the EPA Region 6 TSD included in the 

docket for this proposal for further 

discussion on evaluation of that 

modeling), or that we consider our own 

earlier modeling to be of equal 

reliability relative to more recent 

modeling. However, where alternative 

or older modeling generated linkages, 

even if those linkages differ from 

linkages in the EPA’s most recent set of 

modeling (EPA 2016v2), that 

information provides further evidence, 

not less, in support of a conclusion that 

the State is required to proceed to Step 

3 to further evaluate its emissions. 

Therefore, based on the EPA’s 

evaluation of the information submitted 

by TCEQ and based on the EPA 2016v2 

modeling results for 2023, the EPA 

proposes to find that Texas 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. 

4. Evaluation of Information Provided 

by TCEQ Regarding Step 3 

At Step 3 of the 4-Step interstate 

transport framework, a state’s emissions 

are further evaluated, considering 

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 approach (i.e., 

Step 3 of the 4-Step interstate transport 

framework) when identifying emissions 

contributions that the Agency has 

determined to be ‘‘significant’’ 

(contribution to nonattainment or 

interfere with maintenance) in each of 

its prior Federal, regional ozone 

transport rulemakings, and this 

interpretation of the statute has been 

upheld by the Supreme Court. See EME 

Homer City, 572 U.S. 489, 519 (2014). 

While the EPA has not directed states 

that they must conduct a Step 3 analysis 

in precisely the manner the EPA has 

done in its prior regional transport 

rulemakings, state implementation 

plans addressing the obligations in CAA 

section 110(a)(2)(D)(i)(I) must prohibit 

‘‘any source or other type of emissions 

activity within the State’’ from emitting 

air pollutants which will contribute 

significantly to downwind air quality 

problems. Thus, states must complete 

something similar to the EPA’s analysis 

(or an alternative approach to defining 

‘‘significance’’ that comports with the 

statute’s objectives) to determine 

whether and to what degree emissions 

from a state should be ‘‘prohibited’’ to 

eliminate emissions that will 

‘‘contribute significantly to 

nonattainment in, or interfere with 

maintenance of’’ the NAAQS in any 

other state. TCEQ did not demonstrate 

such an analysis in their SIP 

submission. We therefore propose that 

TCEQ was required to analyze 

emissions from the sources and other 

emissions activity from within the State 

to determine whether its contributions 

were significant, and we propose to 

disapprove its submission because 

Texas failed to do so. 

Instead, as noted in Section V.A of 

this action, TCEQ interpreted the Act’s 

requirements as only requiring an 


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108See FN 34. 

analysis of emission reductions where 

‘‘there is a persistent and consistent 

pattern of contribution on several days 

with elevated ozone.’’ TCEQ asserted 

that it would make the determination of 

whether such pattern existed based on 

a weight-of-evidence approach that 

takes into consideration air quality 

factors such as: Current attainment 

status of the monitors, design value 

trends, the meteorological conditions 

that lead to high ozone formation at the 

monitor, the number of days with 

elevated observed ozone, back 

trajectories, Texas’ relative contribution 

on modeled high ozone days, Texas’ 

contribution as part of the collective 

interstate contribution to future 

modeled DVs, alternate contribution 

method analysis, and model sensitivity 

runs to reductions of Texas’ emissions 

on receptors. However, TCEQ stated that 

it did not consider or analyze all factors 

for every monitor. Thus, different 

factors were analyzed for the receptors 

in different regions (Colorado, Arizona, 

and Southern California). The EPA has 

reviewed the different factors that TCEQ 

provided for each of the regions in the 

EPA Region 6 TSD, but we will provide 

a brief summary of the evaluation 

below. TCEQ also asserted that use of 

the 1 percent threshold as the ‘‘sole’’ 

definition of significant contribution for 

the 2015 ozone NAAQS is 

inappropriate. Based on the application 

of selected factors for each of the 

monitors to which TCEQ’s modeling 

found that it was linked, TCEQ 

concluded that none of its contributions 

to any other states were significant. 

As explained above, TCEQ has 

mischaracterized the EPA’s 

interpretation of the CAA in stating that 

the EPA defines significant contribution 

‘‘solely’’ using a 1 percent threshold. 

The EPA, like TCEQ, uses the 1 percent 

threshold to identify areas for further 

analysis. The difference is that the EPA 

in past analyses has examined potential 

emission reductions in linked upwind 

states and the air quality impacts at 

downwind receptors that would result 

from the implementation of those 

reductions to assess which 

contributions are ‘‘significant.’’ This 

interpretation of significant 

contribution, as discussed above, has 

been upheld by the Supreme Court and 

the D.C. Circuit. 

As an initial matter, the EPA believes 

source apportionment modeling, as 

performed by the EPA and also by 

TCEQ, to determine which states are 

linked is an appropriate tool to identify 

impacts that are persistent enough to 

impact a downwind receptors ability to 

attain or maintain the standard. This 

approach is described in more detail 

above in Section II.B.4 of this action, 

but, in summary, averages the 

contributions from an upwind state for 

up to 10 days, which is preferred, (but 

a minimum 5 days) at a given receptor. 

Given the ozone standard is an average 

of the fourth high value from each of 

three years, the EPA technique, also 

used by Texas, is appropriate to identify 

impacts of sufficient persistence to 

impact a downwind receptor’s ability to 

attain or maintain the standard. 

The EPA reviewed TCEQ’s evaluation 

of the current attainment status of the 

monitors and design value trends, and 

concludes, as described in more detail 

in the EPA Region 6 TSD, that the 

provided information does not support 

the large decreases in ozone levels that 

TCEQ’s modeling projects will occur by 

2023. The analysis for California and 

Colorado receptors provides evidence 

that TCEQ’s photochemical modeling is 

overestimating the ozone reductions 

expected at these receptors between 

2012 and 2023 and actually presents 

evidence that more nonattainment and/ 

or maintenance receptors should have 

been identified. 

The EPA also reviewed the trends in 

the number of high ozone days per year 

provided by TCEQ for Colorado and 

California. While this data supports that 

the number of ozone exceedance days is 

improving, neither the analysis of the 

number of high ozone days in Colorado 

or California provide any evidence to 

refute the TCEQ’s photochemical 

modeling results that show these areas 

should be considered nonattainment 

and/or maintenance receptors. TCEQs 

modeling overestimates ozone 

reductions yet still shows Texas linked 

to receptors at both nonattainment and 

maintenance levels in 2023. 

The TCEQ cited a conceptual model 

of ozone formation for areas in Southern 

California. TCEQ indicated that 

Southern California is isolated and 

transport into the basin is unlikely on a 

frequent basis, but this information does 

not refute the TCEQ’s modeling. As 

discussed in Section III.B.3 of this 

action, 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. TCEQ’s 

modeling showed transport at 10 

monitors having contributions greater 

than 0.7 ppb on average for the 5–10 

days used in the modeling analyses. 

Considering the form of the standard, 

this is a sufficient number of days to 

determine if an impact is persistent 

enough to impact an area’s ability to 

attain or maintain the standard. 

TCEQ used the National Oceanic and 

Atmospheric Administration (NOAA) 

HYSPLIT108 model to produce back 

trajectories for all the monitored ozone 

exceedance days (2007–2016) for the 

five receptors in Colorado and 10 

receptors in Southern California to 

evaluate how many of the back 

trajectories went through Texas. TCEQ 

also used data from these back 

trajectories to do an endpoint count 

analysis. We note that we have several 

concerns with how TCEQ performed the 

back trajectories including start time 

and heights, length (number of hours) of 

the back trajectory, inappropriate 

removal of some back trajectories based 

on start height, center-line height touch 

down, and trajectory center-line height 

when over Texas, and inappropriate 

counting of trajectories by not 

considering that the center-line 

represents the centerline of a much 

wider area of air parcels that could have 

reached the monitor/receptor. Due to 

these concerns, as discussed in more 

detail in the EPA Region 6 TSD, the EPA 

finds the results of TCEQ’s back 

trajectory and endpoint analysis flawed 

(underestimates back trajectories that 

reach Texas) and do not provide 

evidence that refutes the TCEQ 

photochemical modeling analysis 

results. 

We note that even valid back 

trajectories are of limited use as 

HYSPLIT simply estimates the path 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 (monitor) 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 


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109TCEQ also identified a monitor in Cochise 

County, Arizona (ID 40038001), but the monitor’s 

recent DVs are below the NAAQS. From AQS, the 

2014–2016 and 2015–2017 DVs are each 65 ppb; 

2016–2018, 2017–2019, and 2018–2020 DVs are 66 

ppb; and preliminary 2019–2021 DV is 66 ppb. 

‘‘linkages.’’ Since 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. It is interesting to 

note that TCEQ’s analysis of the back 

trajectories indicates that the 2012 

meteorology used by TCEQ seemed to 

yield more back trajectories that reach 

Texas than most years for many of the 

Colorado monitors. This seems to be 

consistent with TCEQ identifying 

linkages to Colorado when the EPA’s 

modeling of 2016 does not. 

TCEQ performed an alternate 

contribution analysis for the ten 

California receptors and the five 

Colorado receptors using all days 

modeled in 2023 that had values over 70 

ppb rather than focus on just the 5–10 

highest values under the EPA’s 

technique. Particularly for California, 

this meant many more days could be 

included in the average which had the 

effect of showing a smaller estimated 

contribution. We believe it is 

appropriate to focus on the highest 

values as these are the ones that 

ultimately will have to be reduced for 

the standard to be attained. As 

discussed in the EPA Region 6 TSD, the 

EPA’s review of TCEQ’s alternate 

contribution method analysis for 

California and Colorado receptors is that 

it does not provide substantial evidence 

that refutes the TCEQ’s photochemical 

modeling analysis results, including the 

contribution analysis using the EPA’s 

contribution methodology. 

TCEQ provided an analysis of 

collective interstate contribution to the 

2023 DV for the five Colorado and ten 

California receptors. The collective 

interstate contribution at tagged 

Colorado receptors ranges from 9.32% 

to 10.27%. The collective interstate 

contribution at tagged California 

receptors ranges from 3.2% to 4.58%. 

TCEQ argues that these are small 

percentages (Colorado and California) 

and not as high as the collective 

interstate contribution percentages the 

EPA calculated for monitors in Eastern 

States, which ranged from 17% to 67%. 

TCEQ also notes that a significant 

portion of the tagged Colorado monitors’ 

2023 modeled DVs is due to background 

emissions (sum of contributions from to 

biogenic, fires, and boundary 

conditions). For the California receptors 

TCEQ argues that these percentages are 

small compared to Intra-State 

contribution. 

As an initial matter, the EPA is not 

solely relying on TCEQ’s findings of 

linkages to Colorado and California but 

is also relying on its own findings of 

linkages to areas in the Midwest Region. 

As such, TCEQ’s analysis of relative 

contributions to Colorado and California 

does not provide justification for not 

addressing downwind impacts. 

Nonetheless, EPA has found in the past 

that certain California receptors are so 

heavily impacted by local emissions, 

and total upwind contribution is so low, 

that those receptors may not be 

considered to be affected by interstate 

ozone transport. See 81 FR 15200 (Mar. 

22, 2016). However, this is a narrow 

circumstance that does not apply in the 

vast majority of cases and has never 

been applied outside of California. EPA 

has previously found, for instance, that 

receptors in Colorado are heavily 

impacted by upwind-state contribution. 

See 82 FR 9155 (Feb. 3, 2017); 81 FR 

71991 (Oct. 19, 2016). EPA need not 

draw any conclusions here regarding 

whether the California sites TCEQ 

identified should or should not be 

considered receptors for ozone-transport 

purposes. EPA affirms, contrary to 

TCEQ’s suggestion, that the Colorado 

receptors TCEQ analyzed are impacted 

by upwind state contributions. 

However, the EPA’s finding that Texas 

is linked to receptors in other states is 

based on still other linkages found in 

EPA’s modeling to receptors in other 

states, which are clearly impacted by 

the collective contribution of multiple 

upwind states, including Texas. Under 

CAA section 110(a)(2)(D)(i)(I) 

downwind states are not obligated to 

reduce emissions on their own to 

resolve 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. 

TCEQ also performed photochemical 

modeling analysis using the Direct 

Decoupled Method (DDM) tool for 

receptors in Colorado. DDM provides a 

first derivative of the changes in ozone 

(linear relationship where the DDM 

value is the slope of the line for changes 

in ozone) resulting from changes in NOX

emissions from all Texas’ NOX

emissions. The DDM modeling does 

show some response to Texas NOX

emissions but from the scale it is hard 

to discern the level of response but it 

appears to be in the 0–2 ppb range in 

general with some values in the 0.2 –2 

ppb range for modeled values over 60 

ppb. Since the modeling has 

underprediction and underestimation 

issues, these values could be higher. Not 

surprisingly, the DDM tool shows that 

monitors in Colorado are much more 

responsive to intra-state reductions than 

reductions in Texas. That said, the 

results of the DDM tool showing only a 

relatively small response to reductions 

is not inconsistent with the finding that 

Texas emissions contribute significantly 

to elevated readings in Colorado. As has 

been discussed elsewhere, the EPA 

believes a contribution of 1 percent of 

the standard is an appropriate threshold 

such that further analysis is warranted. 

Overall, these additional analyses 

performed by TCEQ do not provide 

sufficient evidence to refute the 

modeling results that TCEQ’s modeling 

indicates downwind nonattainment 

and/or maintenance receptors in 

Colorado and Southern California are 

impacted by Texas emissions and Texas’ 

contribution is 0.7 ppb or greater.109 In 

fact, the monitored ozone design value 

trends provide evidence that future year 

modeled ozone levels are 

underestimated by TCEQ’s modeling 

and there are likely more receptors that 

should have been identified with 

additional potential linkages. Although 

Texas asserted that its additional air 

quality factor analysis is a permissible 

way to interpret which contributions are 

‘‘significant’’ because that analysis 

examines whether there was a 

‘‘persistent and consistent pattern of 

contribution on several days with 

elevated ozone’’ we find that such 

pattern is already established by a 

modeled linkage at Step 2. 

In addition, EPA 2016v2 modeling 

using 2016 base year meteorology 

indicates linkages from Texas to 

receptors in the Midwest Region but 

does not indicate impacts from Texas 

emissions on the Colorado and other 

western receptors identified by TCEQ. 

With a different base period such as 

TCEQ’s 2012 base period meteorology 

and the EPA’s 2016 base period 

meteorology, it is not uncommon that 

the potential downwind nonattainment 

or maintenance receptors could change. 

These differing results about receptors 

and linkages can be affected by the 

varying meteorology from year to year 

and the selection of different base years, 

but we do not think the differing results 

mean that the modeling or the EPA 

methodology for identifying receptors or 

linkages is inherently unreliable. Rather, 

these separate modeling runs indicated 

(1) that there were receptors that would 


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110Pointing to anticipated upcoming emission 

reductions, even if they were not included in the 

analysis at Steps 1 and 2, is not sufficient as a Step 

3 analysis, for the reasons discussed in Section 

V.B.4 of this action. In this section, we explain that 

to the extent such anticipated reductions are not 

included in the SIP and rendered permanent and 

enforceable, reliance on such anticipated reductions 

is also insufficient at Step 4. 

struggle with nonattainment or 

maintenance in the future, and (2) that 

Texas was linked to some set of these 

receptors, even if the receptors and 

linkages differed from one another in 

their specifics (e.g., a different set of 

receptors were identified to have 

nonattainment or maintenance 

problems, or Texas was linked to 

different receptors in one modeling run 

versus another). We think this common 

result indicates that Texas’s emissions 

were substantial enough to generate 

linkages at Steps 1 and 2 to some set of 

downwind receptors, under varying 

assumptions and meteorological 

conditions, even if the precise set of 

linkages changed between modeling 

runs. 

In sum, the EPA’s more recent and 

robust 2016 base year modeling 

platform indicates that Texas is linked 

to several receptors in the Midwest 

Region as does the EPA’s earlier 2011 

base year modeling. TCEQ’s 2012 base 

case modeling showed linkages to states 

in the west. As discussed, the EPA does 

not find the additional weight of 

evidence evaluations conducted by 

TCEQ provide compelling reasons to 

discount the impacts indicated in 

Colorado and California by the TCEQ 

modeling. In fact, we think TCEQ’s 

modeling likely underestimates these 

issues. We therefore propose that Texas 

was required to analyze emissions from 

the sources and other emissions activity 

from within the State to determine 

whether its contributions were 

significant, and we propose to 

disapprove its submission because 

Texas failed to do so. 

5. Evaluation of Information Provided 

by TCEQ 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. Texas 

indicated that because a number of 

counties in its state had been designated 

nonattainment for the 2015 ozone 

NAAQS, there could be attainment 

demonstration and potential controls 

contemplated in association with those 

nonattainment designations.110

However, the State’s interstate transport 

submission did not revise its SIP to 

identify any specific emission 

reductions, nor did it include a revision 

to its SIP to ensure any such reductions 

were permanent and enforceable. The 

other control measures identified in 

TCEQ’s submission are, as noted by 

TCEQ, already adopted and 

implemented measures and do not 

contain an evaluation of additional 

emission control opportunities (or 

establish that no additional controls are 

required). As a result, the EPA proposes 

to disapprove TCEQ’s submittal on the 

separate, additional basis that the Texas 

has not included permanent and 

enforceable emissions reductions in its 

SIP as necessary to meet the obligations 

of CAA section 110(a)(2)(d)(i)(I). 

6. Conclusion 

Based on the EPA’s evaluation of 

TCEQ’s SIP submission, the EPA is 

proposing to find that the Texas August 

17, 2018, SIP submission pertaining to 

interstate transport of air pollution 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 

ozone NAAQS in any other state. 

VI. Proposed Action 

We are proposing to disapprove the 

SIP submissions from Arkansas, 

Louisiana, Oklahoma, and Texas 

pertaining to interstate transport of air 

pollution which will significantly 

contribute to nonattainment or interfere 

with maintenance of the 2015 ozone 

NAAQS in other states. Under CAA 

section 110(c)(1), the disapprovals 

would establish a 2-year deadline for 

the EPA to promulgate FIPs for these 

states 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 ozone NAAQS in other states, 

unless the EPA approves SIPs that meet 

these requirements. Disapproval does 

not start a mandatory sanctions clock for 

Arkansas, Louisiana, Oklahoma, or 

Texas. 

VII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

This action is not a significant 

regulatory action and was therefore not 

submitted to the Office of Management 

and Budget for review 

B. Paperwork Reduction Act (PRA) 

This proposed action does not impose 

an information collection burden under 

the PRA because it does not contain any 

information collection activities 

C. Regulatory Flexibility Act (RFA) 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the RFA. This action merely 

proposes to disapprove a SIP 

submission as not meeting the CAA. 

D. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain any 

unfunded mandate as described in 

UMRA, 2 U.S.C. 1531–1538, and does 

not significantly or uniquely affect small 

governments. The action imposes no 

enforceable duty on any state, local or 

tribal governments or the private sector. 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the states, on the 

relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This proposed action disapproving 

the portion of Oklahoma’s SIP 

submission addressing the State’s 

interstate transport obligations under 

CAA section 110(a)(2)(D)(i)(I) for the 

2015 ozone NAAQS will apply to 

certain areas of Indian country as 

discussed in Section IV.C of this action, 

and therefore, has tribal implications as 

specified in E.O. 13175 (65 FR 67249, 

November 9, 2000). However, this 

proposed action will neither impose 

substantial direct compliance costs on 

federally recognized tribal governments, 

nor preempt tribal law. This proposed 

action will not impose substantial direct 

compliance costs on federally 

recognized tribal governments because 

no actions will be required of tribal 

governments. This proposed action will 


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Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 9835 

111 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. 

112A 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. 

113The 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. 

also not preempt tribal law as no 

Oklahoma tribe implements a regulatory 

program under the CAA, and thus does 

not have applicable or related tribal 

laws. Consistent with the EPA Policy on 

Consultation and Coordination with 

Indian Tribes (May 4, 2011), the EPA 

will offer consultation to tribal 

governments whose lands are located 

within the exterior boundaries of the 

State of Oklahoma that may be affected 

by this action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

The EPA interprets Executive Order 

13045 as applying only to those 

regulatory actions that concern 

environmental health or safety risks that 

the EPA has reason to believe may 

disproportionately affect children, per 

the definition of ‘‘covered regulatory 

action’’ in section 2–202 of the 

Executive Order. This action is not 

subject to Executive Order 13045 

because it merely proposes to 

disapprove a SIP submission as not 

meeting the CAA. 

H. Executive Order 13211, Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not subject to Executive 

Order 13211, because it is not a 

significant regulatory action under 

Executive Order 12866. 

I. National Technology Transfer and 

Advancement Act 

This rulemaking does not involve 

technical standards. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

The EPA believes the human health or 

environmental risk addressed by this 

action will not have potential 

disproportionately high and adverse 

human health or environmental effects 

on minority, low-income or indigenous 

populations. This action merely 

proposes to disapprove a SIP 

submission as not meeting the CAA. 

K. CAA Section 307(b)(1) 

Section 307(b)(1) of the CAA governs 

judicial review of final actions by the 

EPA. This section provides, in part, that 

petitions for review must be filed in the 

D.C. Circuit: (i) When the agency action 

consists of ‘‘nationally applicable 

regulations promulgated, or final actions 

taken, by the Administrator,’’ or (ii) 

when such action is locally or regionally 

applicable, if ‘‘such action is based on 

a determination of nationwide scope or 

effect and if in taking such action the 

Administrator finds and publishes that 

such action is based on such a 

determination.’’ For locally or regionally 

applicable final actions, the CAA 

reserves to the EPA complete discretion 

whether to invoke the exception in 

(ii).111

The EPA anticipates that this 

proposed rulemaking, if finalized, 

would be ‘‘nationally applicable’’ 

within the meaning of CAA section 

307(b)(1) because it would take final 

action on SIP submittals for the 2015 

ozone NAAQS for four states, which are 

located in three different Federal 

judicial circuits. It would apply 

uniform, nationwide analytical 

methods, policy judgments, and 

interpretation with respect to the same 

CAA obligations, i.e., implementation of 

interstate transport requirements under 

CAA section 110(a)(2)(D)(i)(I) for the 

2015 ozone NAAQS for states across the 

country, and final action would be 

based on this common core of 

determinations, described in further 

detail below. 

If the EPA takes final action on this 

proposed rulemaking[, in the 

alternative,] 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 interstate transport 

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 the 4- 

Step framework. Further, the EPA 

proposes to determine and apply a set 

of nationally consistent policy 

judgments to apply the 4-Step 

framework. The EPA has selected a 

nationally uniform analytic year (2023) 

for this analysis and is applying a 

nationally uniform approach to 

nonattainment and maintenance 

receptors and a nationally uniform 

approach to contribution threshold 

analysis.112 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).113

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: February 1, 2022. 

Earthea Nance, 

Regional Administrator, Region 6. 

[FR Doc. 2022–02961 Filed 2–18–22; 8:45 am] 

BILLING CODE 6560–50–P 


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