9838 Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 

1National Ambient Air Quality Standards for 

Ozone, Final Rule, 80 FR 65292 (October 26, 2015). 

Although the level of the standard is specified in 

the units of ppm, ozone concentrations are also 

described in parts per billion (ppb). For example, 

0.070 ppm is equivalent to 70 ppb. 

2SIP revisions that are intended to meet the 

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

of the CAA are often referred to as infrastructure 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Part 52 

[EPA–R05–OAR–2022–0006; EPA–HQ– 

OAR–2021–0663; FRL–9431–01–R5] 

Air Plan Disapproval; Illinois, Indiana, 

Michigan, Minnesota, Ohio, Wisconsin; 

Air Plan Disapproval; Region 5 

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) 

is proposing to disapprove State 

Implementation Plan (SIP) submittals 

from Illinois, Indiana, Michigan, 

Minnesota, Ohio, and Wisconsin 

regarding interstate transport for the 

2015 ozone national ambient air quality 

standards (NAAQS). The ‘‘good 

neighbor’’ or ‘‘interstate transport’’ 

provision requires that each state’s SIP 

contain adequate provisions to prohibit 

emissions from within the state from 

significantly contributing to 

nonattainment or interfering with 

maintenance of the NAAQS in other 

states. This requirement is part of the 

broader set of ‘‘infrastructure’’ 

requirements, which are designed to 

ensure that the structural components of 

each state’s air quality management 

program are adequate to meet the state’s 

responsibilities under the CAA. This 

disapproval, if finalized, will establish a 

2-year deadline for EPA to promulgate 

a Federal Implementation Plan (FIP) to 

address the relevant interstate transport 

requirements, unless EPA approves a 

subsequent SIP submittal that meets 

these requirements. Disapproval does 

not start a mandatory sanctions clock. 

DATES: Comments: Written comments 

must be received on or before April 25, 

2022. 

ADDRESSES: You may send comments, 

identified as Docket No. EPA–R05– 

OAR–2022–0006, by any of the 

following methods: Federal Rulemaking 

Portal at https://www.regulations.gov 

following the online instructions for 

submitting comments or via email to 

arra.sarah@epa.gov. Include Docket ID 

No. EPA–R05–OAR–2022–0006 in the 

subject line of the message. 

Instructions: All submissions received 

must include the Docket ID No. for this 

rulemaking. Comments received may be 

posted without change to https://

www.regulations.gov/, including any 

personal information provided. For 

detailed instructions on sending 

comments and additional information 

on the rulemaking process, see the 

‘‘Public Participation’’ heading of the 

SUPPLEMENTARY INFORMATION section of 

this document. 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 EPA Docket Center 

services and the current status, please 

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

dockets. 

FOR FURTHER INFORMATION CONTACT: 

Olivia Davidson, Environmental 

Scientist, Attainment Planning and 

Maintenance Section, Air Programs 

Branch (AR–18J), Environmental 

Protection Agency, Region 5, 77 West 

Jackson Boulevard, Chicago, Illinois 

60604, (312) 886–0266, 

davidson.olivia@epa.gov. The EPA 

Region 5 office is open from 8:30 a.m. 

to 4:30 p.m., Monday through Friday, 

excluding Federal holidays and facility 

closures due to COVID–19. 

SUPPLEMENTARY INFORMATION: 

Public Participation: Submit your 

comments, identified by Docket ID No. 

EPA–R05–OAR–2022–0006, 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. EPA 

may publish any comment received to 

its public docket. Do not submit to 

EPA’s docket at https://

www.regulations.gov any information 

you consider to be Confidential 

Business Information (CBI) or other 

information whose disclosure is 

restricted by statute. Multimedia 

submissions (audio, video, etc.) must be 

accompanied by a written comment. 

The written comment is considered the 

official comment and should include 

discussion of all points you wish to 

make. EPA will generally not consider 

comments or comment contents located 

outside of the primary submission (i.e., 

on the web, cloud, or other file sharing 

system). 

There are two dockets supporting this 

action, EPA–R05–OAR–2022–0006 and 

EPA–HQ–OAR–2021–0663. Docket No. 

EPA–R05–OAR–2022–0006 contains 

information specific to Illinois, Indiana, 

Michigan, Minnesota, Ohio, and 

Wisconsin, including the notice of 

proposed rulemaking. Docket No. EPA– 

HQ–OAR–2021–0663 contains 

additional modeling files, emissions 

inventory files, technical support 

documents, and other relevant 

supporting documentation regarding 

interstate transport of emissions for the 

2015 ozone NAAQS which are being 

used to support this action. All 

comments regarding information in 

either of these dockets are to be made 

in Docket No. EPA–R05–OAR–2022– 

0006. For additional submission 

methods, please contact Olivia 

Davidson, (312) 886–0266, 

davidson.olivia@epa.gov. For the full 

EPA public comment policy, 

information about CBI or multimedia 

submissions, and general guidance on 

making effective comments, please visit 

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

commenting-epa-dockets. 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. 

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 docket for this action, 

Docket No. EPA–R05–OAR–2022–0006, 

is available electronically at 

www.regulations.gov. While all 

documents in the docket are listed in 

the index, some information may not be 

publicly available due to docket file size 

restrictions or content (e.g., CBI). 

Throughout this document, ‘‘we,’’ 

‘‘us,’’ and ‘‘our’’ means EPA. 

I. Background 

A. Description of Statutory Background 

On October 1, 2015, EPA promulgated 

a revision to the 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 


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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). 

5Cross-State Air Pollution Rule Update for the 

2008 Ozone NAAQS, 81 FR 74504 (Oct. 26, 2016). 

6 In 2019, the D.C. Circuit Court of Appeals 

remanded the CSAPR Update to the extent it failed 

to require upwind states to eliminate their 

significant contribution by the next applicable 

attainment date by which downwind states must 

come into compliance with the NAAQS, as 

established under CAA section 181(a). Wisconsin v. 

EPA, 938 F.3d 303, 313 (D.C. Cir. 2019). The 

Revised CSAPR Update for the 2008 Ozone 

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

to the remand of the CSAPR Update in Wisconsin 

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

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

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

7 In addition to the CSAPR rulemakings, other 

regional rulemakings addressing ozone transport 

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

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

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

8See Notice of Availability of the Environmental 

Protection Agency’s Preliminary Interstate Ozone 

Transport Modeling Data for the 2015 ozone 

National Ambient Air Quality Standard (NAAQS), 

82 FR 1733 (January 6, 2017). 

9 82 FR at 1735. 

10See Information on the Interstate Transport 

State Implementation Plan Submissions for the 

2008 Ozone National Ambient Air Quality 

Standards under Clean Air Act Section 

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

docket EPA–HQ–OAR–2021–0663 or at https://

www.epa.gov/interstate-air-pollution-transport/ 

interstate-air-pollution-transport-memos-andnotices. 

11See Information on the Interstate Transport 

State Implementation Plan Submissions for the 

2015 Ozone National Ambient Air Quality 

Standards under Clean Air Act Section 

110(a)(2)(D)(i)(I), March 27, 2018, available in 

docket EPA–HQ–OAR–2021–0663 or at https://

www.epa.gov/interstate-air-pollution-transport/ 

interstate-air-pollution-transport-memos-andnotices. 

12The March 2018 memorandum, however, 

provided, ‘‘While the information in this 

memorandum and the associated air quality 

analysis data could be used to inform the 

development of these SIPs, the information is not 

a final determination regarding states’ obligations 

under the good neighbor provision. Any such 

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

requirements is found in CAA section 

110(a)(2)(D)(i)(I), otherwise known as 

the ‘‘interstate transport’’ or ‘‘good 

neighbor’’ provision, which generally 

requires SIPs to contain adequate 

provisions to prohibit in-state emissions 

activities from having certain adverse 

air quality effects on other states due to 

interstate transport of pollution. There 

are two so-called ‘‘prongs’’ within CAA 

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

new or revised NAAQS must contain 

adequate provisions prohibiting any 

source or other type of emissions 

activity within the state from emitting 

air pollutants in amounts that will 

significantly contribute to 

nonattainment of the NAAQS in another 

state (prong 1) or interfere with 

maintenance of the NAAQS in another 

state (prong 2). 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 EPA’s Four Step 

Interstate Transport Regulatory Process 

EPA is using the 4-step interstate 

transport framework (or 4-step 

framework) to evaluate the states’ SIP 

submittals addressing the interstate 

transport provision for the 2015 ozone 

NAAQS. EPA has addressed the 

interstate transport requirements of 

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

respect to prior ozone NAAQS in 

several regional regulatory actions, 

including the Cross-State Air Pollution 

Rule (CSAPR), which addressed 

interstate transport with respect to the 

1997 ozone NAAQS as well as the 1997 

and 2006 fine particulate matter 

standards,4 and the 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 EPA, working in 

partnership with states, developed the 

following 4-step interstate transport 

framework to evaluate a state’s 

obligations to eliminate interstate 

transport emissions under the interstate 

transport provision for the ozone 

NAAQS: (1) Identify monitoring sites 

that are projected to have problems 

attaining and/or maintaining the 

NAAQS (i.e., nonattainment and/or 

maintenance receptors); (2) identify 

states that impact those air quality 

problems in other (i.e., downwind) 

states sufficiently such that the states 

are considered ‘‘linked’’ and therefore 

warrant further review and analysis; (3) 

identify the emissions reductions 

necessary (if any), applying a 

multifactor analysis, to eliminate each 

linked upwind state’s significant 

contribution to nonattainment or 

interference with maintenance of the 

NAAQS at the locations identified in 

Step 1; and (4) adopt permanent and 

enforceable measures needed to achieve 

those emissions reductions. 

C. Background on EPA’s Ozone 

Transport Modeling Information 

In general, EPA has performed 

nationwide air quality modeling to 

project ozone design values which are 

used in combination with measured 

data to identify nonattainment and 

maintenance receptors. To quantify the 

contribution of emissions from specific 

upwind states on 2023 ozone design 

values for the identified downwind 

nonattainment and maintenance 

receptors, EPA performed nationwide, 

state-level ozone source apportionment 

modeling for 2023. The source 

apportionment modeling provided 

contributions to ozone at receptors from 

precursor emissions of anthropogenic 

nitrogen oxides (NOX) and volatile 

organic compounds (VOCs) in 

individual upwind states. 

EPA has released several documents 

containing projected ozone design 

values, contributions, and information 

relevant to evaluating interstate 

transport with respect to the 2015 ozone 

NAAQS. First, on January 6, 2017, EPA 

published a notice of data availability 

(NODA) in which we requested 

comment on preliminary interstate 

ozone transport data including projected 

ozone design values and interstate 

contributions for 2023 using a 2011 base 

year platform.8 In the NODA, EPA used 

the year 2023 as the analytic year for 

this preliminary modeling because that 

year aligns with the expected attainment 

year for Moderate ozone nonattainment 

areas for the 2015 ozone NAAQS.9 On 

October 27, 2017, we released a 

memorandum (October 2017 

memorandum) containing updated 2011 

platform-based modeling data for 2023, 

which incorporated changes made in 

response to comments on the NODA, 

and noted that the modeling may be 

useful for states developing SIPs to 

address interstate transport obligations 

for the 2008 ozone NAAQS.10 On March 

27, 2018, we issued a memorandum 

(March 2018 memorandum) noting that 

the same 2023 modeling data released in 

the October 2017 memorandum could 

also be useful for identifying potential 

downwind air quality problems with 

respect to the 2015 ozone NAAQS at 

Step 1 of the 4-step interstate transport 

framework.11 The March 2018 

memorandum also included the then 

newly available contribution modeling 

data for 2023 to assist states in 

evaluating their impact on potential 

downwind air quality problems for the 

2015 ozone NAAQS under Step 2 of the 

4-step interstate transport framework.12

EPA subsequently issued two more 

memoranda in August and October 

2018, providing additional information 

to states developing interstate transport 

SIP submissions for the 2015 ozone 

NAAQS concerning, respectively, 


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13See Analysis of Contribution Thresholds for 

Use in Clean Air Act Section 110(a)(2)(D)(i)(I) 

Interstate Transport State Implementation Plan 

Submissions for the 2015 Ozone National Ambient 

Air Quality Standards, August 31, 2018 (‘‘August 

2018 memorandum’’), and Considerations for 

Identifying Maintenance Receptors for Use in Clean 

Air Act Section 110(a)(2)(D)(i)(I) Interstate 

Transport State Implementation Plan Submissions 

for the 2015 Ozone National Ambient Air Quality 

Standards, October 19, 2018, available in docket 

EPA–HQ–OAR–2021–0663 or at https://

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

14The results of this modeling, as well as the 

underlying modeling files, are included in docket 

ID No. EPA–HQ–OAR–2021–0663. 

15See 85 FR 68964, 68981. 

16See the Air Quality Modeling Technical 

Support Document for the Final Revised Cross-State 

Air Pollution Rule Update, included in the 

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

0663. 

17Additional details and documentation related 

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

www.epa.gov/moves/latest-version-motor-vehicleemission-simulator-moves. 18See Technical Support Document (TSD) 

Preparation of Emissions Inventories for the 2016v2 

North American Emissions Modeling Platform 

included in the Headquarters docket ID No. EPA– 

HQ–OAR–2021–0663. 

19 Detailed information and documentation of 

EPA’s Base Case, including all the underlying 

assumptions, data sources, and architecture 

parameters can be found on EPA’s website at: 

https://www.epa.gov/airmarkets/epas-power-sectormodeling-platform-v6-using-ipm-summer-2021- 

reference-case. 

20The ‘‘Capacity Dropped’’ and the ‘‘Retired 

Through 2023’’ worksheets in NEEDS lists all units 

that are removed from the NEEDS v6 inventory— 

NEEDS v6 Summer 2021 Reference Case. This data 

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

www.epa.gov/airmarkets/national-electric-energydata-system-needs-v6. 

21Ramboll Environment and Health, January 

2021, www.camx.com. 

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.13

Since the release of the modeling data 

shared in the March 2018 

memorandum, EPA has performed 

modeling using a 2016-based emissions 

modeling platform (i.e., 2016v1) This 

emissions platform was developed 

under the EPA/Multi-Jurisdictional 

Organization (MJO)/state collaborative 

project.14 This collaborative project was 

a multi-year joint effort by EPA, MJOs, 

and states to develop a new, more recent 

emissions platform for use by EPA and 

states in regulatory modeling as an 

improvement over the dated 2011-based 

platform that EPA had used to project 

ozone design values and contribution 

data provided in the 2017 and 2018 

memoranda. EPA used the 2016v1 

emissions to project ozone Design 

values and contributions for 2023. On 

October 30, 2020, in the Notice of 

Proposed Rulemaking for the Revised 

CSAPR Update, EPA released and 

accepted public comment on 2023 

modeling that used the 2016v1 

emissions platform.15 See 85 FR 68964, 

68981. Although the Revised CSAPR 

Update addressed transport for the 2008 

ozone NAAQS, the projected design 

values and contributions from the 

2016v1 platform are also useful for 

identifying downwind ozone problems 

and linkages with respect to the 2015 

ozone NAAQS.16

Following the final Revised CSAPR 

Update, EPA made further updates to 

the 2016 emissions platform to include 

mobile emissions from EPA’s Motor 

Vehicle Emission Simulator MOVES3 

model 17 and updated emissions 

projections for electric generating units 

(EGUs) that reflect the emissions 

reductions from the Revised CSAPR 

Update, recent information on plant 

closures, and other sector trends. The 

construct of the updated emissions 

platform, 2016v2, is described in the 

emissions modeling technical support 

document (TSD) for this proposed 

rule.18

EPA’s latest projections of the 

baseline EGU emissions uses the version 

6—Summer 2021 Reference Case of the 

Integrated Planning Model (IPM).19 IPM 

is a multi-regional, dynamic, and 

deterministic linear programming model 

of the U.S. electric power sector. The 

model provides forecasts of least cost 

capacity expansion, electricity dispatch, 

and emission control strategies, while 

meeting energy demand, environmental, 

transmission, dispatch, and reliability 

constraints. 

The IPM version 6—Summer 2021 

Reference Case incorporated recent 

updates through the Summer of 2021 to 

account for updated Federal and State 

environmental regulations for EGUs. 

This projected base case accounts for 

the effects of the finalized Mercury and 

Air Toxics Standards rule (MATS), 

CSAPR, the CSAPR Update, the Revised 

CSAPR Update, New Source Review 

settlements, the final Effluent Limitation 

Guidelines (ELG) Rule, the Coal 

Combustion Residual (CCR) Rule, and 

other on-the-books Federal and State 

rules (including renewable energy tax 

credit extensions from the Consolidated 

Appropriations Act of 2021) through 

early 2021 impacting sulfur dioxide 

(SO2), NOX, directly emitted particulate 

matter, carbon dioxide, and power plant 

operations. It also includes final actions 

EPA has taken to implement the 

Regional Haze Rule and Best Available 

Retrofit Technology (BART) 

requirements. Further, the EPA Platform 

v6 uses demand projections from the 

Energy Information Agency’s (EIA) 

Annual Energy Outlook (AEO) 2020. 

The IPM version 6—Summer 2021 

Reference Case uses the National 

Electric Energy Data System (NEEDS) v6 

database as its source for data on all 

existing and planned-committed units. 

Units are removed from the NEEDS 

inventory only if a high degree of 

certainty could be assigned to future 

implementation of the announced future 

closure or retirement. The available 

retirement-related information was 

reviewed for each unit, and the 

following rules are applied to remove: 

(i) Units that are listed as retired in the 

December 2020 EIA Form 860M 

(ii) Units that have a planned retirement 

year prior to June 30, 2023 in the December 

2020 EIA Form 860M 

(iii) Units that have been cleared by a 

regional transmission operator (RTO) or 

independent system operator (ISO) to retire 

before 2023, or whose RTO/ISO clearance to 

retire is contingent on actions that can be 

completed before 2023 

(iv) Units that have committed specifically 

to retire before 2023 under Federal or state 

enforcement actions or regulatory 

requirements 

(v) And finally, units for which a 

retirement announcement can be 

corroborated by other available information. 

Units required to retire pursuant to 

enforcement actions or state rules on July 1, 

2023 or later are retained in NEEDS v6. 

Retirements or closures taking place 

on or after July 1, 2023 are captured as 

constraints on those units in the IPM 

modeling, and the units are retired in 

future year projections per the terms of 

the related requirements. Any 

retirements excluded from the NEEDS 

v6 inventory can be viewed in the 

NEEDS spreadsheet.20

As highlighted in previous 

rulemakings, the IPM documentation, 

and EPA’s Power Sector Modeling 

website, EPA’s goal is to explain and 

document the use of IPM in a 

transparent and publicly accessible 

manner, while also providing for 

concurrent channels for improving the 

model’s assumptions and representation 

by soliciting constructive feedback to 

improve the model. This includes 

making all inputs and assumptions to 

the model, output files from the model, 

and IPM feedback form publicly 

available on EPA’s website. 

EPA performed air quality modeling 

of the 2016v2 emissions using the most 

recent public release version of the 

Comprehensive Air-quality Model with 

extensions (CAMx) photochemical 

modeling, version 7.10.21 EPA now 

proposes to primarily rely on modeling 


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22 March 2018 memorandum, Attachment A. 

23 Id. at A–1. 

24 Id. 

25For attainment dates for the 2015 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). 

26We note that the court in Maryland did not 

have occasion to evaluate circumstances in which 

EPA may determine that an upwind linkage to a 

downwind air quality problem exists at steps 1 and 

Continued 

based on the updated and newly 

available 2016v2 emissions platform in 

evaluating these submissions with 

respect to Steps 1 and 2 of the 4-step 

interstate transport framework and 

generally referenced within this action 

as 2016v2 modeling for 2023. By using 

the updated modeling results, EPA is 

using the most current and technically 

appropriate information for this 

proposed rulemaking. Section III of this 

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 EPA’s 2016v2 

modeling. In this action, EPA is 

accepting public comment on this 

updated 2023 modeling, which uses a 

2016v2 emissions platform. Comments 

on EPA’s air quality modeling should be 

submitted in the Regional docket for 

this action, docket ID No. EPA–R05– 

OAR–2022–0006. Comments are not 

being accepted in docket ID No. EPA– 

HQ–OAR–2021–0663. 

States may have chosen to rely on the 

results of EPA modeling and/or 

alternative modeling performed by 

states or MJOs to evaluate downwind air 

quality problems and contributions as 

part of their submissions. As most 

Region 5 states have done so, in 

Sections III.A, III.B, III.C, III.D, and III.E, 

we evaluate how Region 5 states used 

air quality modeling information in 

their submissions. 

D. EPA’s Approach to Evaluating 

Interstate Transport SIPs for the 2015 

Ozone NAAQS 

EPA proposes to apply a consistent 

set of policy judgments across all states 

for purposes of evaluating interstate 

transport obligations and the 

approvability of interstate transport SIP 

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 

regional-scale pollution problem 

involving many smaller contributors. 

Effective policy solutions to the problem 

of interstate ozone transport going back 

to the NOx SIP Call have necessitated 

the application of a uniform framework 

of policy judgments to ensure an 

‘‘efficient and equitable’’ approach. See 

EME Homer City, 572 U.S. 489, 519 

(2014). 

In the March, August, and October 

2018 memoranda, 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. 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, 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 potential ‘‘flexibilities’’ as may have 

been identified or suggested in the past, 

EPA will evaluate whether the state 

adequately justified the technical and 

legal basis for doing so. 

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.22 However, 

EPA made clear in that Attachment that 

the list of ideas were not suggestions 

endorsed by the Agency but rather 

‘‘comments provided in various forums’’ 

on which EPA sought ‘‘feedback from 

interested stakeholders.’’ 23 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.’’ 24

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, 

EPA will thoroughly review the 

technical and legal justifications for 

doing so. 

The remainder of this section 

describes EPA’s proposed framework 

with respect to analytic year, definition 

of nonattainment and maintenance 

receptors, selection of contribution 

threshold, and multifactor control 

strategy assessment. 

1. Selection of Analytic Year 

In general, the states and EPA must 

implement the interstate transport 

provision in a manner ‘‘consistent with 

the provisions of [title I of the CAA.]’’ 

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).25 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 EPA must assess the impact 

of interstate transport on air quality at 

the next downwind attainment date, 

including Marginal area attainment 

dates, in evaluating the basis for EPA’s 

denial of a petition under CAA section 

126(b). 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). EPA interprets the court’s 

holding in Maryland as requiring the 

states and the Agency, under the good 

neighbor provision, to assess downwind 

air quality as expeditiously as 

practicable and no later than the next 

applicable attainment date,26 which is 


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2 of the interstate transport framework by a 

particular attainment date, but for reasons of 

impossibility or profound uncertainty the Agency is 

unable to mandate upwind pollution controls by 

that date. See Wisconsin, 938 F.3d at 320. The D.C. 

Circuit noted in Wisconsin that upon a sufficient 

showing, these circumstances may warrant 

flexibility in effectuating the purpose of the 

interstate transport provision. 

27See 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). 

28See North Carolina v. EPA, 531 F.3d at 910– 

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

‘‘independent significance’’ to each prong of CAA 

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

29See 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 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). 

30See 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). 

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, 2024.27 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. 

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 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, EPA must act 

on SIP submittals using the information 

available at the time it takes such action. 

In this circumstance, 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 EPA will 

use the analytical year of 2023 to 

evaluate each state’s CAA section 

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

respect to the 2015 ozone NAAQS. 

2. Step 1 of the 4-Step Interstate 

Transport Framework 

In Step 1, EPA identifies monitoring 

sites that are projected to have problems 

attaining and/or maintaining the 

NAAQS in the 2023 analytic year. 

Where EPA’s analysis shows that a site 

does not fall under the definition of a 

nonattainment or maintenance receptor, 

that site is excluded from further 

analysis under EPA’s 4-step interstate 

transport framework. For sites that are 

identified as a nonattainment or 

maintenance receptor in 2023, we 

proceed to the next step of our 4-step 

interstate transport framework by 

identifying the upwind state’s 

contribution to those receptors. 

EPA’s approach to identifying ozone 

nonattainment and maintenance 

receptors in this action is consistent 

with the approach used in previous 

transport rulemakings. EPA’s approach 

gives independent consideration to both 

the ‘‘contribute significantly to 

nonattainment’’ and the ‘‘interfere with 

maintenance’’ prongs of CAA section 

110(a)(2)(D)(i)(I), consistent with the 

D.C. Circuit’s direction in North 

Carolina v. EPA.28

For the purpose of this proposal, EPA 

identifies nonattainment receptors as 

those monitoring sites that are projected 

to have average design values that 

exceed the NAAQS and that are also 

measuring nonattainment based on the 

most recent monitored design values. 

This approach is consistent with prior 

transport rulemakings, such as the 

CSAPR Update, where EPA defined 

nonattainment receptors as those areas 

that both currently measure 

nonattainment and that EPA projects 

will be in nonattainment in the future 

analytic year (i.e., 2023).29

In addition, in this proposal, EPA 

identifies a receptor to be a 

‘‘maintenance’’ receptor for purposes of 

defining interference with maintenance, 

consistent with the method used in the 

CSAPR and upheld by the D.C. Circuit 

in EME Homer City Generation, L.P. v. 

EPA, 795 F.3d 118, 136 (D.C. Cir. 

2015).30 Specifically, EPA identified 

maintenance receptors as those 

receptors that would have difficulty 

maintaining the relevant NAAQS in a 

scenario that takes into account 

historical variability in air quality at 

that receptor. The variability in air 

quality was determined by evaluating 

the ‘‘maximum’’ future design value at 

each receptor based on a projection of 

the maximum measured design value 

over the relevant period. EPA interprets 

the projected maximum future design 

value to be a potential future air quality 

outcome consistent with the 

meteorology that yielded maximum 

measured concentrations in the ambient 

data set analyzed for that receptor (i.e., 

ozone conducive meteorology). EPA 

also recognizes that previously 

experienced meteorological conditions 

(e.g., dominant wind direction, 

temperatures, air mass patterns) 

promoting ozone formation that led to 

maximum concentrations in the 

measured data may reoccur in the 

future. The maximum design value 

gives a reasonable projection of future 

air quality at the receptor under a 

scenario in which such conditions do, 

in fact, reoccur. The projected 

maximum design value is used to 

identify upwind emissions that, under 

those circumstances, could interfere 

with the downwind area’s ability to 

maintain the NAAQS. 

Recognizing that nonattainment 

receptors are also, by definition, 

maintenance receptors, EPA often uses 

the term ‘‘maintenance-only’’ to refer to 

those receptors that are not 

nonattainment receptors. Consistent 

with the concepts for maintenance 

receptors, as described above, EPA 

identifies ‘‘maintenance-only’’ receptors 

as those monitoring sites that have 

projected average design values above 

the level of the applicable NAAQS, but 

that are not currently measuring 

nonattainment based on the most recent 

official design values. In addition, those 

monitoring sites with projected average 

design values below the NAAQS, but 

with projected maximum design values 

above the NAAQS are also identified as 

‘‘maintenance only’’ receptors, even if 

they are currently measuring 

nonattainment based on the most recent 

official design values. 

3. Step 2 of the 4-Step Interstate 

Transport Framework 

In Step 2, EPA quantifies the 

contribution of each upwind state to 

each receptor in the 2023 analytic year. 

The contribution metric used in Step 2 

is defined as the average impact from 

each state to each receptor on the days 

with the highest ozone concentrations at 

the receptor based on the 2023 

modeling. If a state’s contribution value 

does not equal or exceed the threshold 

of 1 percent of the NAAQS (i.e., 0.70 

ppb for the 2015 ozone NAAQS), the 

upwind state is not ‘‘linked’’ to a 

downwind air quality problem, and 

EPA, therefore, concludes that the state 

does not significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS in the 


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31August 2018 memorandum at 4. 

32Air Plan Approval; Iowa; Infrastructure State 

Implementation Plan Requirements for the 2015 

Ozone National Ambient Air Quality Standard, 85 

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

adverse comment on this proposed approval and 

has not taken final action with respect to this 

proposal. 

33EPA notes that Congress has placed on 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. 

downwind states. However, if a state’s 

contribution equals or exceeds the 1 

percent threshold, the state’s emissions 

are further evaluated in Step 3, 

considering both air quality and cost as 

part of a multi-factor analysis, to 

determine what, if any, emissions might 

be deemed ‘‘significant’’ and, thus, must 

be eliminated under CAA section 

110(a)(2)(D)(i)(I). EPA is proposing to 

rely in the first instance on the 1 percent 

threshold for the purpose of evaluating 

a state’s contribution to nonattainment 

or maintenance of the 2015 ozone 

NAAQS (i.e., 0.70 ppb) at downwind 

receptors. This is consistent with the 

Step 2 approach that EPA applied in 

CSAPR for the 1997 ozone NAAQS, 

which has subsequently been applied in 

the CSAPR Update when evaluating 

interstate transport obligations for the 

2008 ozone NAAQS. EPA continues to 

find 1 percent to be an appropriate 

threshold. For ozone, as EPA found in 

the Clean Air Interstate Rule (CAIR), 

CSAPR, and the CSAPR Update, a 

portion of the nonattainment problems 

from anthropogenic sources in the U.S. 

result from the combined impact of 

relatively small contributions from 

many upwind states, along with 

contributions from in-state sources and, 

in some cases, substantially larger 

contributions from a subset of particular 

upwind states. EPA’s analysis shows 

that much of the ozone transport 

problem being analyzed in this 

proposed rule is still the result of the 

collective impacts of contributions from 

many upwind states. Therefore, 

application of a consistent contribution 

threshold is necessary to identify those 

upwind states that should have 

responsibility for addressing their 

contribution to the downwind 

nonattainment and maintenance 

problems to which they collectively 

contribute. Continuing to use 1 percent 

of the NAAQS as the screening metric 

to evaluate collective contribution from 

many upwind states also allows EPA 

and states to apply a consistent 

framework to evaluate interstate 

emissions transport under the interstate 

transport provision from one NAAQS to 

the next. 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). 

i. EPA’s Experience With Alternative 

Step 2 Thresholds 

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, 

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. 

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 2.31 (The memorandum also 

indicated that any higher alternative 

threshold, such as 2 ppb, would likely 

not be appropriate.) However, EPA also 

provided that ‘‘air agencies should 

consider whether the recommendations 

in this guidance are appropriate for each 

situation.’’ Following receipt and review 

of 49 good neighbor SIP submittals for 

the 2015 ozone NAAQS, 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 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, 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.32 It was at 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, 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.33 The 

availability of different thresholds at 

Step 2 has the potential to result in 

inconsistent application of good 

neighbor obligations based solely on the 

strength of a state’s implementation 

plan submittal at Step 2 of the 4-Step 

interstate transport framework. From the 

perspective of ensuring effective 

regional implementation of good 

neighbor obligations, the more 

important analysis is the evaluation of 

the emissions reductions needed, if any, 

to address a state’s significant 

contribution after consideration of a 

multifactor analysis at Step 3, including 

a detailed evaluation that considers air 

quality factors and cost. Where 

alternative thresholds for purposes of 

Step 2 may be ‘‘similar’’ in terms of 

capturing the relative amount of upwind 

contribution (as described in the August 

2018 memorandum), nonetheless, use of 

an alternative threshold would allow 

certain states to avoid further evaluation 

of potential emission controls while 

other states must proceed to a Step 3 

analysis. This can create significant 

equity and consistency problems among 

states. 

Further, it is not clear that national 

ozone transport policy is best served by 

allowing for less stringent thresholds at 

Step 2. 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, 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 


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34See August 2018 memorandum at 4. 

35As examples of general approaches for how 

such an analysis could be conducted for their 

sources, states could look to the CSAPR Update, 81 

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

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

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

CSAPR Update, 86 FR 23054, 23086–23116. 

Consistently across these rulemakings, EPA has 

developed emissions inventories, analyzed different 

levels of control stringency at different cost 

thresholds, and assessed resulting downwind air 

quality improvements. 

36LADCO works collaboratively with state 

governments, tribal governments, and various 

Federal agencies in Illinois, Indiana, Michigan, 

Minnesota, Ohio, and Wisconsin. 

37https://www.marama.org/2013-ertac-eguforecasting-tool-documentation. 

38Technical Support Document (TSD) Additional 

Updates to Emissions Inventories for the Version 

6.3, 2011 Emissions Modeling Platform for the Year 

2023 https://www.epa.gov/air-emissions-modeling/ 

additional-updates-2011-and-2023-emissionsversion-63-platform-technical. 

39See EPA’s 2014 Draft Guidance Document, 

‘‘Draft Modeling Guidance for Demonstrating 

Attainment of Air Quality Goals for Ozone, PM2s, 

and Regional Haze’’, https://www.epa.gov/sites/ 

default/files/2020–10/documents/draft-o3-pm-rhmodeling_guidance-2014.pdf. 

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; 34 in 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 the 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. See 

76 FR 48208, 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, EPA’s experience since 

the issuance of that memorandum has 

revealed substantial programmatic and 

policy difficulties in attempting to 

implement this approach. Nonetheless, 

EPA is not at this time rescinding the 

August 2018 memorandum. EPA invites 

comment on this broader discussion of 

issues associated with alternative 

thresholds at Step 2. Depending on 

comment and further evaluation of this 

issue, EPA may determine to rescind the 

August 2018 memorandum in the 

future. 

4. Step 3 of the 4-Step Interstate 

Transport Framework 

Consistent with EPA’s longstanding 

approach to eliminating significant 

contribution or interference with 

maintenance at Step 3, states linked at 

Steps 1 and 2 are generally expected to 

prepare a multifactor assessment of 

potential emissions controls. EPA’s 

analysis at Step 3 in prior Federal 

actions addressing interstate transport 

requirements has primarily focused on 

an evaluation of the cost-effectiveness of 

potential emissions controls (on a 

marginal cost-per-ton basis), the total 

emissions reductions that may be 

achieved by requiring such controls (if 

applied across all linked upwind states), 

and an evaluation of the air quality 

impacts such emissions reductions 

would have on the downwind receptors 

to which a state is linked; other factors 

may potentially be relevant if 

adequately supported. In general, where 

EPA’s or alternative air quality and 

contribution modeling establishes that a 

state is linked at Steps 1 and 2, it will 

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 emissionsreducing 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 still to be linked to one or 

more downwind receptor(s), the state 

must provide a well-documented 

evaluation determining whether their 

emissions constitute significant 

contribution or interference with 

maintenance by evaluating additional 

available control opportunities by 

preparing a multifactor assessment. 

While EPA has not prescribed a 

particular method for this assessment, 

EPA expects states at a minimum to 

present a sufficient technical evaluation. 

This would typically include 

information on emissions sources, 

applicable control technologies, 

emissions reductions, costs, cost 

effectiveness, and downwind air quality 

impacts of the estimated reductions, 

before concluding that no additional 

emissions controls should be required.35

5. Step 4 of the 4-Step Interstate 

Transport Framework 

At Step 4, states (or EPA) develop 

permanent and federally enforceable 

control strategies to achieve the 

emissions reductions determined to be 

necessary at Step 3 to eliminate 

significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. For a state 

linked at Steps 1 and 2 to rely on an 

emissions control measure at Step 3 to 

address its interstate transport 

obligations, that measure must be 

included in the state’s SIP so that it is 

permanent and federally enforceable. 

See CAA section 110(a)(2)(D) (‘‘Each 

such [SIP] shall . . . contain adequate 

provisions . . .’’). See also CAA 

110(a)(2)(A); Committee for a Better 

Arvin v. U.S. E.P.A., 786 F.3d 1169, 

1175–76 (9th Cir. 2015) (holding that 

measures relied on by state to meet CAA 

requirements must be included in the 

SIP). 

II. SIP Submissions Addressing 

Interstate Transport of Air Pollution for 

the 2015 Ozone NAAQS 

Five of the six states within EPA 

Region 5 that are included in this multistate proposed disapproval have chosen 

to use air quality modeling performed 

by the Lake Michigan Air Directors 

Consortium (LADCO) 36 as an 

alternative to or in addition to EPA’s 

modeling for the purpose of identifying 

downwind receptors and upwind state 

contributions to these receptors relevant 

to their submissions. The LADCO 

modeling consisted of ozone season 

(May 1–September 30, 2011) model 

simulations using the Comprehensive 

Air Quality Model with Extensions, 

CAMx version 6.4 for a 2011 base year 

and 2023 as the future analytic year. In 

their modeling, LADCO used EPA’s 

2011-based ‘‘EN’’ emissions modeling 

platform, except for emissions from 

EGU’s in 2023. In their modeling, 

LADCO used the Eastern Regional 

Technical Advisory Committee (ERTAC) 

EGU Tool version 2.7 37 to provide EGU 

emissions for 2023, whereas EPA used 

projected EGU emissions based on 

engineering analytics.38

LADCO provided projected 2023 

future year average and maximum 

design values using the methodology in 

EPA’s 2014 modeling guidance.39

Although projected design values were 

presented based on the 3x3 approach 

and the ‘‘no water cell’’ approach, 

described in the March 2018 

memorandum, LADCO relied upon 

design values from the 3x3 approach for 


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40 IEPA’s SIP submission, Table CC, page 15. 41 Id. 42 Illinois’ SIP submission at 16. 

calculating contribution metric values at 

each receptor. 

Source apportionment modeling was 

performed by LADCO using the CAMx 

Anthropogenic Precursor Culpability 

Assessment (APCA) tool to calculate 

contributions from individual states to 

ozone at downwind monitoring sites. In 

their modeling, LADCO tracked ozone 

contributions from only those states that 

contributed at or above a 1 percent of 

the NAAQS threshold to nonattainment 

and maintenance monitors in the EPA 

2023 modeling provided in the March 

2018 memorandum. 

A. Illinois 

Illinois Environmental Protection 

Agency (IEPA) submitted a SIP revision 

to address CAA Section 110(a)(2)(D)(i)(I) 

on May 21, 2019. The submission 

utilized LADCO modeling results 

previously mentioned. IEPA followed 

the 4-step interstate transport 

framework using an analytic year of 

2023 to identify receptors, Illinois’ 

linkages to receptors, and assess some 

emission reduction considerations. The 

following sections will discuss IEPA’s 

submission and the information 

provided for each step in the process. 

1. Information Provided by Illinois 

Regarding Step 1 

For Step 1 in the 4-step framework, 

the IEPA relied on LADCO modeling to 

identify monitoring sites that are 

projected to have problems attaining or 

maintaining the 2015 ozone NAAQS in 

2023. As previously mentioned, the 

LADCO modeling used the same 

technique as EPA to calculate future 

year design values which were used to 

identify maintenance and 

nonattainment receptors but used the 

ERTAC EGU Tool for EGU emissions. 

IEPA noted they believed that the 

ERTAC EGU tool provides better 

estimates of growth and forecasts than 

EPA’s EGU emission projections. IEPA 

identified two maintenance receptors in 

the Great Lakes area (Sheboygan, 

Wisconsin and Allegan County, 

Michigan) as well as and three 

nonattainment receptors and five 

additional maintenance receptors in the 

Northeast. Across the continental U.S., 

IEPA identified a total of twelve 

nonattainment or maintenance 

receptors: seven nonattainment 

receptors and five maintenance 

receptors.40

2. Information Provided by Illinois 

Regarding Step 2 

IEPA’s submittal at Step 2 presented 

Illinois’ projected 2023 contribution of 

ozone emissions to the downwind 

maintenance and nonattainment 

receptors and based on LADCO’s ‘‘with 

water’’ modeling 41 IEPA used a 

contribution threshold of 1 ppb to 

define linkage as opposed to one 

percent of the 2015 ozone NAAQS 

standard (0.70 ppb). Illinois, relying on 

the August 2018 memorandum, argued 

that the state’s reliance on the 1 ppb 

threshold to identify linkages was 

justified. First, IEPA asserted that the 

one percent of the NAAQS contribution 

threshold is arbitrary because it is not in 

the CAA. Second, IEPA claimed that a 

one percent of the NAAQS contribution 

threshold is inappropriate for the 2015 

ozone NAAQS because 0.70 ppb is an 

order of magnitude smaller than the 

biases and errors typically documented 

for regional photochemical modeling. 

IEPA noted that 1 ppb is very small 

compared to the allowable error in peak 

performance and bias and in IEPA’s 

view is a conservative Step 2 

contribution threshold. 

IEPA identified that Illinois is 

projected to contribute 14.93 ppb and 

19.25 ppb, respectively, in 2023 to two 

maintenance receptors: Sheboygan, 

Wisconsin (Site ID: 55–117–0006), and 

Allegan, Michigan (Site ID: 26–005– 

0003). IEPA concluded that Illinois is 

linked above a contribution threshold of 

1 ppb to these receptors. 

3. Information Provided by Illinois 

Regarding Step 3 

IEPA provided several arguments to 

justify their conclusion that no 

additional emission reductions are 

necessary to satisfy Illinois’ ozone 

transport obligations. The submittal 

stated that Illinois is developing new 

NOX RACT standards for the Chicago 

area that, in conjunction with existing 

NOX reductions, are estimated to reduce 

NOX emissions by up to 20,000 tons/ 

year relative to existing state and 

Federal regulations impacting non-EGUs 

since 2014. IEPA claims that these 

reductions were not included in any 

2023 modeling. Illinois claims these 

expected emissions reductions, 

alongside future Federal rules, will be 

enough to meet Illinois’ good neighbor 

obligations. 

IEPA asserted that Illinois’ 

contributions to the Sheboygan, 

Wisconsin and Allegan County 

maintenance monitors, which are both 

in the LADCO domain, can be addressed 

in a manner that is fair and equitable for 

all involved states through Illinois’ 

participation in LADCO. Illinois has 

been a member of LADCO since 1991. 

IEPA says that since the inception of 

LADCO in 1991, ambient ozone 

concentrations have drastically 

decreased in the Lake Michigan region. 

IEPA stated that it is working through 

LADCO to refine ozone forecasting for 

air quality and ozone receptors in the 

Midwest through various means 

including updating emissions 

inventories and a base case for the 

modeling that LADCO is preparing. 

IEPA stated that it expects field data 

from the 2017 Lake Michigan Ozone 

Study will inform the LADCO states to 

better simulate the meteorology and 

chemistry of ozone in the Lake 

Michigan area. 

IEPA also attempted to rely on a 

concept related to international 

emissions, which was developed by 

outside parties and listed in Attachment 

A to the March 2018 memorandum. 

IEPA noted that if international 

emissions and offshore contributions to 

receptors from LADCO’s modeling, 1.40 

ppb to the Sheboygan, Wisconsin 

receptor and 0.98 ppb at the Allegan 

County receptor, were subtracted off the 

top of the receptors’ maximum design 

values, Allegan, Michigan receptor’s 

maximum design value would be below 

71 ppb and the Sheboygan, Wisconsin 

receptor’s maximum design value 

would drop down to 71.4 ppb. IEPA 

noted that subtracting international and 

offshore contributions would result in 

Allegan County no longer qualifying as 

a maintenance receptor. 

4. Information Provided by Illinois 

Regarding Step 4 

IEPA did not consider any new 

permanent and enforceable measures to 

reduce emissions in the SIP submission. 

IEPA instead noted they would continue 

to assist LADCO with future modeling 

and analysis and would work with EPA 

to identify additional ‘‘flexibilities’’ to 

define maintenance, quantify transport, 

and demonstrate attainment.42

B. Indiana 

On November 2, 2018 the Indiana 

Department of Environmental 

Management (IDEM) submitted a 

revision to the Indiana SIP addressing 

interstate transport of air pollution for 

the 2015 ozone NAAQS. The 

submission contained what the state 

characterized as a weight of evidence 

analysis of Indiana’s ozone transport 

receptors utilizing LADCO modeling 

results previously mentioned. Indiana 

did not explicitly follow the 4-Step 

interstate transport framework but did 

examine downwind air quality and 

Indiana’s contributions using an 

analytic year of 2023 to describe 


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9846 Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 

43 Indiana’s SIP submission Attachment 1, Table 

7, page 30. 

44 Indiana’s SIP submission Attachment 1, Table 

1, page 12. 

45 IDEM acknowledged that both the Harford, 

Maryland and Richmond, New York receptors are 

nonattainment receptors in EPA’s modeling. 

Indiana’s SIP submission Attachment 1, page 12. 

46 Indiana’s SIP submission Attachment 1, page 

38, 53. 

47 Indiana’s SIP submission, page 6. 

48 Indiana’s SIP submission, Attachment 1, Table 

9. 49 Indiana’s SIP submission, Attachment 1, Table 

11. 50Based on the reference to the potential 

flexibilities in Attachment A to the March 2018 

memorandum on page 2 of Attachment 1 to 

Indiana’s SIP submission, EPA assumes the 

reference to ‘‘flexibilities’’ on page 38 of 

Attachment 1 likewise references Attachment A to 

the March 2018 memorandum. 

Indiana’s linkages to receptors. The 

following sections will describe IDEM’s 

submission, and the information 

provided for each step in the process. 

1. Information Provided by Indiana 

Regarding Step 1 

For Step 1 of the 4-Step framework, 

IDEM identified monitoring sites that 

are projected to have problems attaining 

and/or maintaining the 2015 ozone 

NAAQS in 2023. As previously 

mentioned, the LADCO modeling used 

the same technique as EPA to calculate 

future year design values which were 

used to identify maintenance and 

nonattainment receptors but used the 

ERTAC EGU Tool for EGU emissions. 

IDEM noted they believed that the 

ERTAC EGU tool provides better 

estimates of growth and forecasts than 

EPA’s EGU emission projections. IDEM 

presented the LADCO results, based on 

the ‘‘3x3’’ approach, which identified 

ten receptors: Seven monitors with 2023 

maximum design values above the 

NAAQS, or maintenance receptors, and 

three monitors with 2023 average design 

values greater than the 2015 ozone 

standard, or nonattainment receptors.43

2. Information Provided by Indiana 

Regarding Step 2 

IDEM’s submittal presented Indiana’s 

projected 2023 ozone contributions to 

maintenance and nonattainment 

receptors projected by the LADCO 

modeling. IDEM relied primarily on the 

August 2018 memorandum to justify the 

State’s reliance on a 1 ppb contribution 

threshold to identify linkages, as 

opposed to the 1 percent of the 2015 

ozone NAAQS standard (0.70 ppb) 

contribution threshold. IDEM noted that 

(1) the tolerance level of ozone monitors 

is 1 ppb and (2) model run results may 

contain biases larger than 1 percent of 

the NAAQS (0.70 ppb). Using a 1 ppb 

threshold, IDEM identified that Indiana 

is linked to three maintenance 

receptors: Sheboygan, Wisconsin 

(Monitor ID: 551170006), Allegan, 

Michigan (Monitor ID: 260050003), and 

Richmond, New York (Monitor ID: 

360850067), and one nonattainment 

receptor: Harford, Maryland (Monitor 

ID: 240251001).44 45 However, IDEM 

concluded on the basis of its weight of 

evidence analysis, summarized in the 

following subsection, that the monitors 

at Sheboygan, Wisconsin and Allegan, 

Michigan would be in attainment in 

2023.46

3. Information Provided by Indiana 

Regarding Step 3 

IDEM cited Indiana’s rule 

amendments under CSAPR to conclude 

the State was already satisfying its good 

neighbor obligations for the ozone 

NAAQS.47 In support, IDEM provided a 

weight-of-evidence analysis to justify 

their conclusion that no additional 

emission reductions as necessary to 

satisfy Indiana’s ozone transport 

obligations. The evidence consisted of 

an ozone monitoring data analysis, 

emissions analysis, and photochemical 

modeling analyses, including a back 

trajectory analysis. 

IDEM provided information ozone 

and emissions trends. They cited a 

general decline in monitored ozone 

concentrations across Indiana from 2007 

through 2017, a decrease in Indiana’s 

overall statewide NOX emissions and 

VOC emissions from 2005 to 2014, a 

decrease in Indiana EGU NOX emissions 

from 2011 to 2016, and projected 

decreases in Region 5 EGU NOX and 

VOC emissions through 2023 relative to 

a 2011 base year (based on both the 

ERTAC EGU Tool and EPA’s EGU 

projections using the Integrated 

Planning Model (IPM)). IDEM credited 

the downward emissions trends to 

permanent and enforceable control 

measures. IDEM made the argument that 

overall decreasing ozone concentration 

and emissions trends in the State, and 

in the LADCO states, correlate with 

reduced contributions to nonattainment 

and maintenance receptors outside of 

Indiana. IDEM also identified air quality 

trends at the four downwind receptors 

to which IDEM determined Indiana was 

linked. IDEM asserted that a declining 

trend in three-year design values at the 

receptors in Harford, Maryland and 

Richmond, New York in combination 

with national emission reduction 

regulations in place for EGUs, tighter 

mobile source emission controls and 

other transport related emission 

reduction measures, would result in 

those two receptors reaching attainment 

over time. For the Sheboygan, 

Wisconsin and Allegan, Michigan 

receptors, IDEM concluded that if ozone 

design value trends continued as 

expected then those two receptors 

would reach attainment before 2023. 

IDEM also asserted they believed that 

the receptors in the Northeast (Harford, 

Maryland and Richmond, New York) 

received a greater contribution from 

local sources. IDEM represented EPA 

had reached the same conclusion, citing 

a May 14, 2018 presentation. 

Next, IDEM presented an analysis of 

NOx controls for EGUs and non-EGUs in 

the state from 2008 to 2017. IDEM 

considered current NOx control 

measures, consent decree requirements, 

and future fuel switches and retirements 

for large EGUs and non-EGUs. IDEM 

reported that the State has seen a 

downward trend in annual NOX

emissions from both EGUs and nonEGUs due a combination of state and 

Federal rules targeting fossil fueled 

EGUs and other large sources of NOX. 

IDEM argued that it would not be costeffective for non-EGUs to implement 

further NOX controls because in 2017 

there were more than 93% fewer NOX

emissions from non-EGUs when 

compared to EGUs. IDEM stated they 

expect to see continued future NOX

emission reductions through 2028 from 

implementation of Federal rules, the 

expected shutdown of nine EGUs, 

planned fuel switches to natural gas for 

three EGUs, and enforceable consent 

decree caps on NOX emissions at eleven 

EGUs. IDEM noted that future 

retirements or retrofits to coal fired 

EGUs in Indiana were expected to 

reduce NOX emissions by several 

thousand tons beyond those projected 

by either LADCO or EPA. IDEM argued 

that the non-modeled emission 

reductions would further assure future 

year attainment of the ozone NAAQS at 

downwind receptors. 

For their photochemical analyses, 

IDEM presented LADCO modeling 

results to show contributions from 

individual states to 12 monitors,48 as 

well as contributions from individual 

sectors to the same 12 monitors.49 IDEM 

noted that Indiana contributed above 1 

ppb to monitors that would be receptors 

based on EPA’s definitions. IDEM used 

these data as the backdrop to several 

arguments related to potential 

flexibilities identified in Attachment A 

to the March 2018 memorandum.50

IDEM stated that additional emissions 

reductions from EGU and non-EGU 

sources in Indiana are becoming more 

difficult to require because of reduced 

effectiveness of controls to make 

significant decreases in ozone values, 

operational concerns, and increased 


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Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 9847 

51 Indiana’s SIP submission, Attachment 1 at 42. 

52https://www.epa.gov/sites/default/files/2018- 

04/documents/sils_policy_guidance_document_

final_signed_4-17-18.pdf. 

costs for customers. IDEM asserted that 

emission reductions from local mobile 

and nonroad sources would be more 

beneficial to the receptors than 

additional reductions from EGUs and 

non-EGUs in Indiana because EGUs and 

non-EGUs contribute less to the 

receptors than either the mobile or 

nonroad sectors. IDEM also argued that 

EPA should address contributions from 

Canada and Mexico as well as 

contributions from offshore commercial 

marine vessels. In addition, IDEM 

compared 2012–2017 monitoring data to 

LADCO’s and EPA’s modeling and 

concluded that all four linked receptors 

were already below the projected 2023 

design values. IDEM also calculated 

Indiana’s portion of contribution to the 

Harford, Maryland receptor as 0.077 ppb 

(based on a contribution threshold of 1 

ppb) and determined that Indiana 

would need to reduce its contribution 

by 0.0077 ppb to bring the Harford, 

Maryland receptor into attainment. 

IDEM argued that 0.0077 ppb is well 

within the error of the model and it 

would be ‘‘difficult’’ to translate into an 

emission reduction requirement.51

Finally, IDEM provided a back 

trajectory analysis to evaluate 

contributions from Indiana to the 

Harford, Maryland and Richmond, New 

York receptors on exceedance days from 

2015 to 2017. These back trajectories 

were conducted at 10 meters and 750 

meters and initialized at 18Z Greenwich 

Mean Time over a three-year period 

from 2015–2017. The trajectories were 

run backwards over a 72-hour period 

from the exceedance day measured at 

each of the monitors. Considering the 

Harford, Maryland receptor, Indiana 

measured one back trajectory at 10 

meters and six back trajectories at 750 

meters that passed through Indiana. For 

the Richmond, New York receptor, out 

of 27 exceedance days, Indiana 

measured three back trajectories at 10 

meters and eight trajectories at 750 

meters that passed through Indiana. 

Indiana argues that only a fraction of the 

exceedance days at the Harford and 

Richmond receptors has back 

trajectories that pass-through Indiana. 

Based on this analysis, IDEM concluded 

that those receptors are more likely to be 

impacted by local emissions and 

suggested that emission reductions 

should come first from the surrounding 

areas in the Northeast before from 

Indiana. 

4. Information Provided by Indiana 

Regarding Step 4 

IDEM did not provide a Step 4 

analysis. 

C. Michigan 

Environment, Great Lakes and Energy 

(EGLE) (formerly Michigan Department 

of Environmental Quality) submitted a 

SIP revision to address CAA Section 

110(a)(2)(D)(i)(I) on March 5, 2019. 

EGLE utilized EPA modeling released 

with the March 2018 memorandum and 

LADCO modeling and followed the 

4-step interstate transport framework 

using an analytic year of 2023 to 

describe Michigan’s linkages to 

receptors in other states. The 

submission also contained a weight-ofevidence analysis to support EGLE’s 

conclusions. The following sections will 

discuss EGLE’s submission and the 

information provided for each step in 

the process. 

1. Information Provided by Michigan 

Regarding Step 1 

For Step 1 in the 4-step framework, 

EGLE identified monitoring sites that 

are projected to have problems attaining 

and/or maintaining the 2015 ozone 

NAAQS in 2023. EGLE presented the 

results from both the EPA modeling 

from the March 2018 memorandum and 

LADCO (using both the with- and 

without-water approaches). EGLE noted 

that they believe the ERTAC EGU Tool 

uses a more transparent and state-driven 

data gathering mechanism for EGU 

emissions and control projects. EPA’s 

modeling projected 16 receptors to 

which Michigan is projected to 

contribute in 2023. LADCO modeling 

(with and without water) identified 15 

receptors to which Michigan is 

projected to contribute in 2023. 

2. Information Provided by Michigan 

Regarding Step 2 

The first part of EGLE’s submittal’s 

step 2 analysis presented Michigan’s 

projected contributions to maintenance 

and nonattainment receptors in 2023 

from both the LADCO modeling (with 

and without water) and the EPA 

modeling released with the March 2018 

memorandum. The submittal noted 

similar contribution concentrations 

between the two, but found that the 

LADCO results often yielded slightly 

lower contribution from Michigan 

sources than the EPA modeling for some 

receptors. The state claimed this is 

attributed to LADCO’s use of the ERTAC 

EGU tool which they stated includes 

information on EGU shutdowns and 

facility-specific information not 

included in the EPA modeling. EGLE 

stated they had more confidence in the 

LADCO modeling. 

Further, EGLE attempted to rely on 

the 1 ppb Significant Impact Level (SIL) 

threshold from Guidance on Significant 

Impact Levels for Ozone and Fine 

Particles in the Prevention of Significant 

Deterioration Permitting Program (April 

17, 2018) 52 for the Prevention of 

Significant Deterioration (PSD) 

permitting program as an appropriate 

contribution threshold to determine 

whether Michigan was linked to any 

receptors at step 2. Michigan argued that 

if a stationary source’s contributions are 

insignificant below 1 ppb, then a state’s 

interstate transport contributions below 

1 ppb are likewise insignificant. EGLE 

performed an analysis on contribution 

thresholds to analyze whether a 1 ppb 

threshold is appropriate for identifying 

Michigan’s linkages. The analysis 

looked at all 15 receptors to which 

Michigan contributes, based on the 

LADCO with water modeling, and 

plotted potential contribution 

thresholds ranging from 0.5 to 1.4 ppb 

against collective upwind contributions 

from all source categories and states 

with contribution above 0.5 ppb. EGLE 

plotted the collective contribution as a 

function of contribution threshold and 

concluded the first inflection point 

occurred in a majority of the collective 

contribution at a contribution threshold 

between 0.9 and 1 ppb, correlating with 

the PSD permitting SIL of 1 ppb. Based 

on this analysis, EGLE concluded that a 

1 ppb contribution threshold was 

appropriate for use in the good neighbor 

context. EGLE also mentioned the 

August 2018 memorandum, described 

in Section I of this proposal, as 

additional support for the use of a 1 ppb 

contribution threshold.53

EGLE’s Step 2 conclusion was that, 

based on LADCO with water modeling, 

Michigan was projected to be linked 

above a 1 ppb contribution threshold in 

2023 to three maintenance receptors; 

1.85 ppb to Sheboygan, Wisconsin (Site 

ID: 36–081–0124), 1.22 ppb to Queens, 

New York (Site ID: 36–085–0067), and 

1.03 ppb to Richmond, New York (Site 

ID: 55–117–0006). 

3. Information Provided by Michigan 

Regarding Step 3 

EGLE provided what it characterized 

as a ‘‘weight-of-evidence analysis’’ in 

step 3 to justify their conclusion that no 

additional emissions reductions are 

necessary to satisfy Michigan’s ozone 

transport obligations for the ozone 

NAAQS. The evidence presented in 

EGLE’s submittal consisted primarily of 

an argument that upwind states should 

have a lower responsibility to other 

states when the upwind state is only 

linked to maintenance-only receptors. 


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54Michigan’s SIP submission at 27. 

EGLE’s analysis focused in large part on 

the Sheboygan, Wisconsin maintenance 

receptor (Site ID: 36–081–0124), as 

EGLE concluded it was the receptor to 

which Michigan was projected to 

contribute the most in 2023 at 1.85 ppb. 

On the issue of maintenance 

receptors, the state referenced a concept 

identified by outside parties and listed 

in Attachment A to the March 2018 

memorandum, which was to consider 

whether the remedy for upwind states 

linked to maintenance receptors could 

be less stringent than those linked to 

nonattainment receptors. EGLE 

reasoned that because the CAA incudes 

different SIP development requirements 

for nonattainment and maintenance 

areas, that likewise nonattainment and 

maintenance areas should be treated 

differently in good neighbor SIPs. EGLE 

argued that because the CAA does not 

require emission reductions from 

maintenance areas, then upwind states 

can potentially make a sufficient 

showing they have no obligation to 

reduce emissions to monitors in other 

states projected to be maintaining the 

NAAQS. EGLE said they believe the 

reduction of projected contributions to 

projected maintenance receptors is not 

required in certain circumstances, such 

as when: (1) The projected maintenance 

exceedance is very small in magnitude, 

(2) the projected contribution is very 

small, especially compared to other 

states’ contributions, (3) sector 

contributions demonstrate the majority 

of contribution is from either sources 

already federally regulated or sources 

without the possibility of additional 

regulation, (4) there are large impacts of 

international emissions, and (5) there 

are downward emission trends. 

Applying this logic to the Sheboygan, 

Wisconsin receptor, EGLE argued that 

because the projected maximum design 

value at that receptor is 72.8 ppb, the 

projected exceedance above the 2015 

ozone NAAQS of 1.9 ppb is very small 

(values are truncated to ppb, thus 70.9 

ppb would be considered in attainment 

of the NAAQS). Based on this number, 

EGLE argued that further emissions 

reductions from Michigan would be 

overly burdensome. Secondly, citing a 

potential flexibility in Attachment A to 

the March 2018 memorandum related to 

sector contribution, EGLE claimed that 

77% of contribution to the Sheboygan 

receptor is from federally regulated 

sources or sources that cannot be 

regulated by Michigan. EGLE noted that 

the other sectors of contribution it 

identified—oil and gas, EGUs, and nonEGUs—are already controlled by 

Michigan. EGLE argued Michigan 

sources should not be required to 

implement additional contribution 

reductions in light of the relative size of 

their contribution. 

Citing Attachment A to the March 

2018 memorandum, again, EGLE argued 

that the three receptors to which 

Michigan is linked are heavily 

influenced by international emissions 

and other states. EGLE shared that 

LADCO modeling projects Michigan’s 

contribution to the Sheboygan receptor 

(1.85 ppb) is less than projected 

contributions from international 

contributions and boundary conditions 

(16.61 ppb), Illinois (14.93 ppb), 

Wisconsin (9.1 ppb), biogenic sources 

(7.19 ppb), and Indiana (6.19 ppb). 

EGLE used these numbers and an 

apportionment of contributions from 

states and other sources to the amount 

the Sheboygan monitor exceeds the 

2015 ozone NAAQS to conclude that 

additional emissions reductions from 

Michigan should not be required. 

Additionally, EGLE argued that 

international contributions to the 

Sheboygan receptor, which is 

geographically close to Canada, should 

be eliminated from the projected DV, in 

which case the Sheboygan monitor 

would be in attainment. 

EGLE also cited Attachment A to the 

March 2018 memorandum along with 

an interpretation of EME Homer City to 

argue that EPA cannot require 

reductions that would result in a 

reduction greater than an upwind state’s 

portion of the difference between the 

NAAQS and a maintenance receptor’s 

projected maximum design value. EGLE 

claimed that Michigan’s apportioned 

contribution to three linked receptors, 

when distributed proportionally among 

other states that also contribute more 

than 1 ppb to those receptors, is less 

than 0.12 ppb, but that Michigan’s 

responsibility to the exceedance is 

actually substantially less than that 

amount when the home state’s 

responsibility is considered. EGLE also 

stated Michigan’s apportioned 

contribution is at less than 0.05 ppb of 

the projected maintenance exceedance 

at the Sheboygan receptor, which is less 

than the variation among the modeled 

maximum design values by both 

LADCO and EPA. EGLE concluded that 

because of built-in modeling noise it 

would be ‘‘difficult’’ to either verify that 

Michigan contributed 0.05 ppb to the 

Sheboygan, Wisconsin receptor or for 

Michigan to require any additional 

reduction from sources in the state.54

EGLE speculated it was quite likely that 

the three linked receptors would 

maintain the NAAQS without any 

emissions reductions from Michigan at 

all because the projected exceedances 

were small. 

The state also stated that Michigan 

has downward emissions trends (44% 

and 18% reduction in industrial point 

source NOX and VOC emissions from 

2008 through 2016, respectively) that 

are expected to continue to decline or 

stay consistent over time due to 

projected reductions in emissions from 

point sources of NOX and VOCs EGUs, 

mobile sources and through other 

Federal measures. For EGUs, EGLE 

pointed out that the shutdown of the 

Marquette Board of Light & Power 

Shiras Steam Plant shut down was not 

included in either EPA or LADCO 

modeling and that the U.S. Energy 

Information Agency’s Annual Energy 

Outlook anticipates growth in 

renewable energy in Michigan in 2019 

and a decline in coal beginning in 2022. 

EGLE also provided a list of additional 

coal-fired EGUs that they stated were 

expected to retire by 2023. To support 

the conclusion that emissions will 

further be reduced from mobile sources 

and through other Federal action, EGLE 

listed several on the books and on the 

way Federal regulations. Finally, EGLE 

noted existing controls on the oil and 

gas sector (applicable Federal 

standards), non-EGUs (subject to the 

NOX SIP Call), and EGUs (subject to 

CSAPR Update). 

4. Information Provided by Michigan 

Regarding Step 4 

EGLE concluded it would be 

unreasonable for Michigan to take 

further actions to address its obligations 

under the good neighbor provisions for 

the ozone NAAQS, and so at Step 4 

EGLE determined that no permanent 

and enforceable measures to reduce 

emissions were necessary. 

D. Minnesota 

Minnesota Pollution Control Agency 

(MPCA) submitted a SIP revision to 

address CAA Section 110(a)(2)(D)(i)(I) 

on October 1, 2018. The submission 

utilized both EPA modeling released 

with the March 2018 memorandum and 

LADCO modeling results previously 

mentioned. Minnesota followed the 

4-step interstate transport framework 

and used an analytic year of 2023 to 

describe Minnesota’s lack of 

contributions to out of state receptors 

and assess emission reduction 

considerations. The following sections 

will summarize MPCA’s submission and 

the information provided for each step 

in the process. 


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55MPCA’s SIP submittal at Tables 2 and 3, pages 

8–9. 

56See Minnesota’s SIP submittal Figures 1–3, 

pages 10–11. 

1. Information Provided by Minnesota 

Regarding Step 1 

For Step 1 in the 4-step framework, 

MPCA identified monitoring sites that 

are projected to have problems attaining 

and/or maintaining the 2015 ozone 

NAAQS in 2023 according to LADCO 

modeling and EPA modeling released in 

the March 2018 memorandum.55 As 

previously mentioned, the LADCO 

modeling efforts used the same 

technique as EPA to calculate future 

year design values which are used to 

identify maintenance and 

nonattainment receptors. The submittal 

expressed MPCA’s opinion that the 

ERTAC EGU tool used in LADCO’s 

modeling is superior to EPA’s 2023en 

modeling platform because the ERTAC 

EGU tool addresses economic factors, 

preserves system reliability, and 

includes controls or emissions 

reductions measure justified through 

some legal framework. 

2. Information Provided by Minnesota 

Regarding Step 2 

MPCA’s submittal at Step 2 presented 

Minnesota’s projected 2023 ozone 

contributions to maintenance and 

nonattainment receptors identified by 

both LADCO modeling and EPA 

modeling released in the March 2018 

memorandum.55 The submittal noted 

there were differences in identified 

receptors between the two modeling 

results, and that the LADCO results 

overall yielded slightly lower projected 

contributions to downwind receptors 

from Minnesota sources than EPA 

modeling. 

Minnesota relied on a contribution 

threshold of 1 percent of the ozone 

NAAQS (0.70 ppb) to define linkages for 

a state’s contribution to downwind air 

quality problems. Both the LADCO 

modeling and the EPA modeling 

released in the March 2018 

memorandum projected that Minnesota 

contributes less than 1 percent of the 

NAAQS to all downwind receptors. 

MPCA showed in Table 2 of its 

submission that the highest projected 

contribution to a receptor in 2023 was 

0.40 ppb, based on EPA modeling 

released in the March 2018 

memorandum, or 0.45 ppb, based on 

LADCO ‘‘no water’’ modeling, to the 

Milwaukee, Wisconsin receptor (Site ID: 

55–079–0085). Based on this analysis, 

MPCA concluded that Minnesota was 

not linked above 1 percent of the 

NAAQS to any downwind receptor, and 

therefore would not contribute to 

nonattainment or interference with 

maintenance in other state with respect 

to the ozone NAAQS. 

3. Information Provided by Minnesota 

Regarding Step 3 

Despite concluding Minnesota was 

not linked at Step 2, MPCA proceeded 

with a Step 3 analysis. MPCA provided 

air quality data in Step 3 to justify that 

no additional emissions reductions are 

necessary to satisfy their transport 

obligations. MPCA provided evidence of 

decreasing ambient ozone 

concentrations in Minnesota from the 

mid-1990s through 2017 as well as 

decreasing NOX and VOC emissions 

from the state from 2002 through 2015 

to further support their conclusion.56

The state concluded that decreasing 

ambient ozone concentrations in the 

state point to Minnesota contributing 

less to ozone in downwind states as 

time goes on. Minnesota provided an 

analysis of NOX and VOC emissions 

levels in the state from 2002 through 

2015 to further support this point. 

According to MPCA, NOX emissions 

have been steadily declining in the state 

from all sectors and especially from 

EGUs due to emission limits and 

reductions required in that category. 

MPCA also asserted that VOCs 

emissions have also seen a similar 

decline in Minnesota in the years 

reported. MPCA concluded that 

decreasing emissions in the state would 

make it unlikely for the state to 

contribute significantly to 

nonattainment or interference with 

maintenance in downwind states. 

4. Information Provided by Minnesota 

Regarding Step 4 

Citing declining emissions and their 

conclusion that Minnesota was not 

projected to contribute above 1 percent 

of the NAAQS to any receptor, MPCA 

concluded that no additional permanent 

or enforceable measures would be 

needed to address ozone transport 

contribution from Minnesota sources. 

MPCA determined the existing emission 

controls would be sufficient to maintain 

Minnesota’s continued contribution of 

less than 1 percent of the NAAQS to 

downwind receptors. In support of this 

argument, Minnesota provided a list of 

several Federal and State emissions 

regulations applicable to sources in 

Minnesota, including Part 70 permits, 

the CSAPR NOX trading programs, 

Mercury and Air Toxics Standards, and 

various state standards for SO2, 

particulate matter, NOX, NO2, and VOC. 

Hence, Minnesota declined to consider 

any new permanent and enforceable 

measures to reduce emissions as part of 

the Step 4 analysis. 

E. Ohio 

On September 28, 2018 the Ohio 

Environmental Protection Agency 

(OEPA) submitted a revision to the Ohio 

SIP addressing interstate transport of air 

pollution for the 2015 ozone NAAQS. 

OEPA stated that its submittal, which 

relied on an analytic year of 2023, 

conforms with EPA’s four-step 

framework. The following sections will 

describe what OEPA provided for each 

step. 

1. Information Provided by Ohio 

Regarding Step 1 

For Step 1 in the 4-step framework, 

OEPA first identified 10 monitoring 

sites in the Northeast and Midwest that 

are projected to be nonattainment, 

nonattainment/maintenance, or 

maintenance-only receptors for the 2015 

ozone NAAQS in 2023 based on 

LADCO’s modeling and EPA’s method 

for defining nonattainment and 

maintenance receptors (See Table 1 

below, reproduced from OEPA’s 

submission). 

TABLE 1—OHIO’S PROJECTED 2023 NONATTAINMENT AND MAINTENANCE MONITORS

Site ID County State 2015–2017 

DV 

2023 

Average DV 

2023 Max 

DV 

OH 

contribution Status 

2023 

Maintenance 

DV (TX 

approach) 

Status 

(TX approach) 

36–103–0002 ..... Suffolk ............... NY 76 71.6 73.1 1.75 Nonattainment/ 

Maintenance.

69.7 Nonattainment. 

09–001–9003 ..... Fairfield .............. CT 83 71.4 74.2 1.58 Nonattainment/ 

Maintenance.

73.7 Nonattainment/ 

Maintenance. 


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57Ohio’s SIP submission, Table 1. 

TABLE 1—OHIO’S PROJECTED 2023 NONATTAINMENT AND MAINTENANCE MONITORS—Continued 

Site ID County State 2015–2017 

DV 

2023 

Average DV 

2023 Max 

DV 

OH 

contribution Status 

2023 

Maintenance 

DV (TX 

approach) 

Status 

(TX approach) 

24–035–1001 ..... Harford .............. MD 75 71.0 73.3 2.83 Nonattainment/ 

Maintenance.

67.0 Nonattainment. 

36–085–0067 ..... Richmond .......... NY 76 70.9 72.4 2.24 Maintenance ......... 68.0 Attainment. 

55–117–0006 ..... Sheyboygan ...... WI 80 70.5 72.8 1.17 Maintenance ......... 71.1 Maintenance. 

09–009–9002 ..... New Haven ........ CT 82 69.9 72.6 1.12 Maintenance ......... 72.6 Maintenance. 

09–001–3007 ..... Fairfield .............. CT 83 69.8 73.7 1.84 Maintenance ......... 73.7 Maintenance. 

36–081–0124 ..... Queens .............. NY 74 69.2 71.0 1.88 Maintenance ......... 70.1 Attainment. 

09–001–0017 ..... Fairfield .............. CT 79 68.9 71.2 1.05 Maintenance ......... 71.2 Maintenance. 

26–005–0003 ..... Allegan .............. MI 73 68.8 71.5 0.19 Maintenance—Not 

Linked.

71.5 Maintenance—Not 

Linked. 

OEPA then claimed that EPA’s 

methodology for determining 

maintenance-only receptors is 

inappropriate because it is more 

stringent than EPA’s methodology for 

identifying nonattainment monitors and 

is inconsistent with the CAA. In OEPA’s 

view, EPA’s methodology results in 

greater emissions reduction 

requirements to address maintenance 

receptors than nonattainment receptors. 

Citing stakeholder-identified potential 

flexibilities that were listed in an 

attachment to EPA’s March 2018 

memorandum, OEPA used an 

alternative method developed by the 

Texas Commission for Environmental 

Quality (TCEQ) to identify maintenance 

receptors. This method determines a 

future year design value (DV) for 

purposes of identifying maintenanceonly receptors by applying the modelpredicted relative response factor (RRF) 

to the most recent 3-year design value 

(i.e., 2011–2013 design value) within 

the five-year base period (i.e., 2009– 

2013), rather than the highest 3-year DV 

in the same 5-year base period, which 

is used in EPA’s approach. OEPA stated 

that using the TCEQ approach accounts 

for emissions reductions over the fiveyear period, while also accounting for 

meteorological variability, since the 

design value is calculated based on 

monitoring data from a three-year 

period. By using the TCEQ approach, 

Ohio concluded that four monitors 

which would be either nonattainment/ 

maintenance receptors under EPA’s 

method would, under the TCEQ 

method, actually be nonattainment-only 

receptors (i.e., sites 261030002 in 

Suffolk, New York, 240251001 in 

Harford, Maryland) or monitors in 

attainment (i.e., sites 360850067 in 

Richmond, New York, and 360810124 

in Queens, New York) in 2023.57

OEPA’s submittal provided 

information on inter-annual 

meteorological variability, ozone design 

value trends at the four monitoring sites 

that the state eliminated as receptors, as 

well as recent and projected trends in 

NOX and VOC emissions to support the 

use of an alternative definition of 

maintenance receptors. The 

meteorological information provided in 

OEPA’s submission included 

nationwide maps showing the 

maximum temperature anomaly (i.e., 

departure from the long-term average or 

‘‘normal’’) for the period May through 

October in the years 2011, 2012, and 

2013. OEPA concluded from these maps 

that temperatures in the Northeast and 

Midwest were above or much above 

average during May through October in 

each of the years 2011, 2012, and 2013. 

OEPA examined trends in ozone design 

values at each of the four monitoring 

sites in question and concluded that 

design values at these sites have 

declined substantially from 2000 

through 2017 and that although there 

has been a slight leveling off or increase 

in recent years, this is no greater than 

the normal year to year variability. In 

addition, based on the emissions trends 

data, OEPA stated that NOX emissions 

have declined in concert with these 

trends in design values and are 

projected to continue to decline through 

2028 for the continental U.S. as well as 

those states that were projected to be 

linked to the four monitors eliminated 

under the TCEQ approach. Based on 

their analysis of the meteorological and 

ozone and emissions trends data, OEPA 

concluded that the four monitoring sites 

identified previously are not reasonably 

expected to have difficulty maintaining 

the standards in 2023. 

2. Information Provided by Ohio 

Regarding Step 2 

OEPA’s submittal presented the 

projected 2023 ozone contributions from 

Ohio to ten maintenance and 

nonattainment receptors in the 

Northeast and Midwest using data from 

the LADCO source apportionment 

modeling. LADCO’s contribution data 

identified a total of nine receptors in 

2023 (3 nonattainment and 6 

maintenance-only) with modeled 

contributions from emissions in Ohio 

that exceed both a one percent of the 

2015 NAAQS threshold (i.e., 0.70 ppb) 

and a 1 ppb contribution threshold 

(Table 1). Despite acknowledging Ohio 

was linked to the same number of 

receptors under either a 1 percent of the 

NAAQS or 1 ppb, OEPA maintained 

they had concerns about both thresholds 

being too stringent, noting that Ohio 

would have two linkages if the 

threshold were 3 percent and zero 

linkages if the threshold were 4 percent. 

As noted above, OEPA applied the 

TCEQ method for identifying 

maintenance-only receptors, and 

concluded that four of the receptors to 

which Ohio was linked would not have 

difficulty maintaining the NAAQS in 

2023. However, after eliminating these 

four monitoring sites as having 

maintenance issues, OEPA 

acknowledged that Ohio would still be 

linked to seven receptors in 2023. 

3. Information Provided by Ohio 

Regarding Step 3 

OEPA provided what they 

characterize as a weight of evidence 

analysis in Step 3 to justify their 

conclusion that no additional emissions 

reductions are necessary to satisfy 

Ohio’s interstate transport obligations 

under the 2015 ozone NAAQS. OEPA 

argued that their analysis demonstrated 

that any additional controls beyond 

those on the books or already planned 

would result in overcontrol of sources 

in Ohio, likely at cost-prohibitive levels. 

First, OEPA attempted to show that 

NOX and VOC 2023 emissions from 

EGU, nonEGU, and onroad sectors had 

been overestimated by 21,761 tons of 

NOX and 3,240 tons of VOC annually, 

and 7,040 tons of NOX and 878 tons of 

VOC per ozone season. The submittal 

performed an evaluation of the ERTAC 

EGU Tool, emphasizing that projected 

2023 EGU emissions from eight facilities 


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58Ohio’s SIP submission at 43. 59Wisconsin’s SIP submission at 4. 

60See Wis. Admin. Code NR 285.15 (2021), 

available at https://docs.legis.wisconsin.gov/ 

statutes/statutes/285/ii/11. 

were overestimated, analyzed through a 

comparison of actual emissions data 

obtained from EPA’s Clean Air Markets 

Database (CAMD) and CSAPR/CSAPR 

Update allocations obtained from EPA’s 

CSAPR website with projected 

emissions in 2023 obtained from ERTAC 

EGU tool. OEPA also based its 

conclusions on an expected increase of 

natural gas sources projected by the U.S. 

Energy Information’s Annual Energy 

Outlook 2018 and recently permitted 

natural gas facilities not reflected in the 

ERTAC EGU tool. Further, OEPA 

evaluated emissions from nine non-EGU 

sources to claim EPA’s 2023 projected 

emissions were overestimated. For this 

analysis, OEPA compared actual 

emissions data trends from Ohio’s 

Emissions Inventory System18 with 

projected emissions from EPA’s Air 

Emissions Modeling Platform 

2011v6.319 to conclude EPA’s 

projections overestimated non-EGU 

emissions. OEPA also asserted that 

EPA’s projections of vehicle miles 

traveled (VMT) were higher than the 

local projections. 

OEPA’s submittal also looked at NOX

and VOC emissions trends, and asserted 

that from 2011 to 2016, NOX emissions 

declined while VOC emissions 

remained steady. Additionally, based on 

state-specific data, OEPA posited that 

Ohio’s VOC emissions will decrease 

even more rapidly than predicted by 

EPA because the large growth in the 

State’s oil and gas sector had begun to 

level off. OEPA attributed the trends it 

identified to several Federal and State 

programs, including SIP approved state 

programs, non-SIP approved programs 

such as NOX RACT, Architectural and 

Industrial Maintenance (AIM) Coatings 

rules, Ohio’s Beneficiary Mitigation 

Plan for the Volkswagen settlement, and 

Federal programs such as CAIR and 

CSAPR, NOX SIP Call, the National 

Emission Standards for Hazardous Air 

Pollutants (NESHAP), the Regional Haze 

Rule, BART, SO2 Data Requirements 

Rule, and MATS. 

In addition to emissions trends data, 

OEPA noted LADCO modeling projected 

downwind trends in design values at 

the ten receptors from 2000 through 

2017 and included a reference to ‘‘a May 

14, 2018 presentation, U.S. EPA Office 

of Air Quality Planning and Standards 

(OAQPS)’’.58 OEPA also stated that 

LADCO sector-based source 

apportionment modeling indicates a 

significant contribution from onroad 

sources at nine receptors. OEPA argued 

that local onroad emissions should be 

addressed by EPA before EPA requires 

additional reductions from upwind 

states. OEPA also suggested EPA should 

take into account the impact on ozone 

of the proposed Safer Affordable FuelEfficient (SAFE) Vehicles Rule before 

taking final action on SAFE. Further, 

OEPA asserted that Ohio’s contribution 

to nine monitors is in the 1 or 2 ppb 

range while the home state and initial 

and boundary contributions are each up 

to 19 ppb, and the contribution from 

Canada/Mexico is in the same range as 

Ohio’s. OEPA argued that ignoring 

international emissions sources and 

placing all responsibility for addressing 

receptors on upwind states would result 

in overcontrol. 

OEPA concluded that there were no 

cost-effective measures to be taken for 

EGU or non-EGU sources in Ohio. To 

support this claim, OEPA pointed to the 

cost effectiveness threshold of $1400/ 

ton from the CSAPR Update (81 FR 

74508, October 26, 2016), and while 

OEPA recognized that it was developed 

for the 2008 ozone standard, OEPA 

stated they believed it is a reasonable 

cost-effectiveness level for the 2015 

ozone NAAQS. As for non-EGUs, OEPA 

asserted that those sectors were 

adequately controlled by the Boiler 

MACT and numerous other MACT 

categories, BART, SO2 Data 

Requirements Rule and other Federal 

regulations. 

4. Information Provided by Ohio 

Regarding Step 4 

OEPA concluded, based on its weight 

of evidence analysis, that no additional 

emissions reduction measures beyond 

existing and planned controls are 

necessary to address ozone transport 

contribution from Ohio sources for the 

2015 ozone NAAQS. 

F. Wisconsin 

Wisconsin Department of Natural 

Resources (WDNR) submitted a SIP 

revision to address CAA Section 

110(a)(2)(D)(i)(I) on September 14, 2018. 

The submittal notes state and Federal 

rules applicable to sources in Wisconsin 

that are relevant to interstate transport, 

as well as Wisconsin’s participation in 

LADCO. WDNR identified CAIR, 

CSAPR, CSAPR Update, Wisconsin’s 

regional haze SIP applicable for the 

2008–2018 planning period, and state 

PSD programs. Further, WDNR cited 

continued consultation with LADCO, 

three Wisconsin Administrative Code 

statutes that could be relied on ‘‘[i]f 

needed’’ to address disagreements for 

SIP development in other states’ 

nonattainment areas, and an adequate 

PSD program.59 The statues mentioned 

include Wisconsin Administrative 

Code, Natural Resources (Wis. Admin. 

Code, NR), subsections 285.11, 285.13 

and 285.15. The first two address air 

pollution control department duties and 

powers. The third, Wisconsin Statute 

285.15, entitled Interstate Agreement, 

gives the governor the authority to enter 

an agreement to solve interstate 

pollution transport with Illinois, 

Indiana and Michigan if the area 

includes portions of both Wisconsin and 

Illinois.60 WDNR does not explicitly 

reach the conclusion that Wisconsin has 

satisfied the good neighbor provision for 

the 2015 ozone NAAQS, but it is 

implied. WDNR did not reference the 4- 

step framework. WDNR did not rely on 

any modeling, identify any receptors, or 

determine whether Wisconsin 

contributes any amount of ozone 

precursor emissions to downwind 

states. The submittal does not include 

an analysis of potential NOX controls. 

WDNR did not rely on any EPA 

memoranda. No supporting 

documentation was provided. Apart 

from the cited rules and LADCO 

membership, WDNR provided no 

discussion or analysis to determine 

whether they have any obligations 

under the 2015 ozone NAAQS. 

III. EPA Evaluation 

EPA is proposing to find that the 

Illinois, Indiana, Michigan, Minnesota, 

Ohio, and Wisconsin’s SIP submissions 

do not meet the states’ 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 EPA’s 

evaluation of the SIP submissions using 

the 4-step interstate transport 

framework, and EPA is therefore 

proposing to disapprove Illinois, 

Indiana, Michigan, Minnesota, Ohio, 

and Wisconsin’s SIP submissions. 

A. Illinois 

1. Evaluation of Information Provided 

by Illinois Regarding Step 1 

For Step 1 in the 4-step framework, 

IEPA identified monitoring sites that are 

projected to have problems attaining or 

maintaining the 2015 ozone NAAQS in 

2023. As previously mentioned, the 

LADCO modeling efforts used the same 

technique as EPA to calculate future 

year design values used to identify 

maintenance and nonattainment 

receptors. IEPA presented the LADCO 

results with water cells, which 

identified five monitors with 2023 

maximum design values greater than the 


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61 Illinois’ SIP submission at 8, 14. 

62See Section 4.1’ ‘‘Overview of Modeled 

Attainment Test in EPA Modeling Guidance for 

Demonstrating Air Quality Goals for Ozone, PM2.5, 

and Regional Haze. November 2018. EPA 454–R– 

18–009. https://www.epa.gov/scram/sip-modelingguidance-documents. 

63Christopher Emery, Zhen Liu, Armistead G. 

Russel, M. Talat Odman, Greg Yarwood and Naresh 

Kumar (2017). Recommendations on statistics and 

benchmarks to assess photochemical model 

performance, Journal of the Air & Waste 

Management Association, 67:5,582–598, DOI: 

10.1080/10962247.2016.1265027. 

2015 ozone standard, or maintenance 

receptors, and seven monitors with 2023 

average design values greater than the 

2015 ozone standard, or nonattainment 

receptors. Since new modeling has been 

performed by EPA with updated 

emission data, EPA proposes to rely on 

the most recent modeling to identify 

nonattainment and maintenance 

receptors with linkage to Wisconsin in 

2023. Nonetheless, the alternative 

modeling relied on by IEPA also 

identified a number of nonattainment 

and maintenance receptor sites in 2023. 

See Table CC on page 15 of IEPA’s 

submittal. Thus, even under the 

alternative modeling of 2023, IEPA 

acknowledges in its submittal the 

existence of several nonattainment and 

maintenance receptors. 

2. Evaluation of Information Provided 

by the State Regarding Step 2 

Although Illinois relied on alternative 

modeling to EPA’s modeling, Illinois 

acknowledged in their SIP submission 

that it is linked to one or more 

downwind receptors above either a 1 

percent of the NAAQS or a 1 ppb 

contribution threshold in 2023. Because 

the alternative modeling relied on by 

the state also demonstrates that a 

linkage exists between the state and 

downwind receptors at step 2, EPA need 

not conduct a comparative assessment 

of the alternative modeling; the state 

concedes that it is linked. IEPA’s 

analysis corroborates the conclusion in 

EPA’s most recent modeling in that the 

modeling demonstrates the State to be 

linked above 1 percent of the NAAQS to 

a downwind receptor, as described in 

the next section. 

IEPA, relying on a concept listed in 

Attachment A to the March 2018 

memorandum, attempted to justify the 

use of a 1 ppb threshold at step 2 to 

identify whether the state was ‘‘linked’’ 

to a projected downwind nonattainment 

or maintenance receptor. As explained 

in Section I above, the concepts 

presented in Attachment A to the March 

2018 memorandum were developed by 

outside parties; they are neither 

guidance nor determined by EPA to be 

consistent with the CAA. However, 

EPA’s August 2018 memorandum also 

addressed possible alternative 

thresholds and suggested that, with 

appropriate additional analysis, it may 

be reasonable for states to use a 1 ppb 

contribution threshold, as an alternative 

to a 1 percent of the NAAQS threshold, 

at Step 2 of the 4-Step interstate 

transport framework for the purposes of 

identifying linkages to downwind 

receptors. 

As an initial matter, EPA does not 

accept Illinois’ argument that a 1 

percent of the NAAQS contribution 

threshold at Step 2 is ‘‘inappropriate’’ 

for the 2015 ozone NAAQS due to 

modeling biases and errors.61 The 

explanation for how the 1 percent 

contribution threshold was originally 

derived is available in the 2011 CSAPR 

rulemaking. See 76 FR 48208, 48237–38. 

Further, in the CSAPR Update, EPA reanalyzed the threshold for purposes of 

the 2008 ozone NAAQS and determined 

it was appropriate to continue to apply 

this threshold. EPA compared the 1 

percent threshold to a 0.5 percent of 

NAAQS threshold and a 5 percent of 

NAAQS threshold. EPA found that the 

lower threshold did not capture 

appreciably more upwind state 

contribution compared to the 1 percent 

threshold, while the 5 percent threshold 

allowed too much upwind state 

contribution to drop out from further 

analysis. See Final CSAPR Update Air 

Quality Modeling TSD, at 27–30 (EPA– 

HQ–OAR–2015–0596–0144). If EPA 

were to apply this analysis to the 2015 

ozone NAAQS using the updated 

modeling based on the 2016v2 

emissions platform, a 5 percent of the 

NAAQS contribution threshold (i.e., 3.5 

ppb) only captures approximately 50 

percent of the total upwind 

contribution. Compared to a 1 percent 

threshold, a 5 percent threshold would, 

on average, forgo 27 percent of the total 

upwind contribution. As EPA noted in 

the August 2018 memorandum, the use 

of even a 2 ppb contribution threshold 

under the modeling released with the 

March 2018 memorandum would only 

capture about 55 percent of all upwind 

contributions, and therefore ‘‘emission 

reductions from states linked at that 

higher threshold may be insufficient to 

address collective upwind state 

contribution to downwind air quality 

problems.’’ 31

With these figures in mind, IEPA’s 

claims that the contribution threshold 

should be substantially higher than 1 or 

even 2 ppb solely on the basis of 

modeling uncertainty cannot be 

accepted. First, both LADCO’s and 

EPA’s modeling techniques are 

sufficiently reliable and fit for the 

purpose to measure upwind 

contribution levels down to at least one 

percent of the NAAQS. EPA’s 

recommended model attainment test is 

based on application of the model in a 

relative sense rather than relying upon 

absolute model predictions.62 All 

models have limitations resulting from 

uncertainties in inputs and scientific 

formulation. To minimize the effects of 

these uncertainties, the modeling is 

anchored to base period measured data 

in EPA’s guidance approach for 

projecting design values. Notably, EPA 

also uses our source apportionment 

modeling in a relative sense when 

calculating the average contribution 

metric (used to identify linkages). In this 

method the magnitude of the 

contribution metric is tied to the 

magnitude of the projected average 

design value which is tied to the base 

period average measured design value. 

EPA’s guidance has not established a 

bright line criteria for judging whether 

or not statistical measures of model 

performance constitute acceptable or 

unacceptable model performance. So, 

contrary to what Illinois appears to be 

claiming with regards to modeling 

biases, there are no EPA recommended 

measures of allowable error. Although 

EPA does not typically focus on using 

particular benchmarks as the sole 

criteria for model performance, EPA 

notes that the model performance for 

the updated modeling based on the 

2016v2 emissions platform is generally 

within the benchmarks recommended 

by Emery.63

EPA has successfully applied a 1 

percent of NAAQS threshold to identify 

linked upwind states in three prior 

rulemakings. And the D.C. Circuit has 

declined to establish bright line criteria 

for model performance. In upholding 

EPA’s approach to evaluating interstate 

transport in CSAPR, the D.C. Circuit 

held that they would not ‘‘invalidate 

EPA’s predictions solely because there 

might be discrepancies between those 

predictions and the real world. That 

possibility is inherent in the enterprise 

of prediction.’’ EME Homer City 

Generation, L.P. v. EPA, 795 F.3d 118, 

135 (2015). The court continued to note 

that ‘‘the fact that a ‘model does not fit 

every application perfectly is no 

criticism; a model is meant to simplify 

reality in order to make it tractable.’’’ Id. 

at 135–36 (quoting Chemical 

Manufacturers Association v. EPA, 28 

F.3d 1259, 1264 (D.C. Cir. 1994). 

Finally, EPA’s August 2018 

memorandum provided that whether 

use of a 1 ppb threshold is appropriate 

must be based on an evaluation of statespecific circumstances, and no such 

evaluation was included in the 


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64 Design values and contributions at individual 

monitoring sites nationwide are provide in the file:’’ 

2016v2_DVs_state_contributions.xlsx’’ which is 

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

0663. 

65These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I. That 

modeling showed that Illinois 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 ID No. 

EPA–HQ–OAR–2021–0663. 

submission. IEPA did not conduct such 

an analysis. EPA’s experience with the 

alternative Step 2 thresholds is further 

discussed in Section I.D.3.i. As 

discussed there, EPA is considering 

withdrawing the August 2018 

memorandum. 

However, based on both the state’s 

alternative modeling and EPA’s updated 

modeling, the state is projected to 

contribute greater than both the 1 

percent and alternative 1 ppb 

thresholds. While EPA does not, in this 

action, approve of the IEPA’s 

application of the 1 ppb threshold, 

based on Illinois’ 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 this SIP submittal. 

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

Modeling and Findings for Illinois 

As described in Section I, EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. These data were examined to 

determine if Illinois 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 2, the data 64 indicate that in 2023, 

emissions from Illinois contribute 

greater than 1 percent of the standard to 

nonattainment or maintenance-only 

receptors in Wisconsin.65

TABLE 2—ILLINOIS LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location Nonattainment/maintenance 

2023 Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Illinois 

contribution 

(ppb) 

550590025 ............................. Kenosha, WI ......................... Maintenance ......................... 69.2 72.3 18.55 

550590019 ............................. Kenosha, WI ......................... Nonattainment ...................... 72.8 73.7 18.13 

551010020 ............................. Racine, WI ............................ Nonattainment ...................... 71.3 73.2 13.86 

Therefore, based on EPA’s evaluation 

of the information submitted by IEPA, 

and based on EPA’s most recent 

modeling results for 2023, EPA proposes 

to find that Illinois 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. EPA 

will proceed to Step 3 of the 4-step 

interstate transport framework to assess 

the arguments the state presented as to 

why, despite this linkage, the state 

should not be considered to 

significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS in any other 

state. 

4. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

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

To effectively evaluate which 

emissions in the state should be deemed 

‘‘significant’’ and therefore prohibited, 

states generally should prepare an 

accounting of sources and other 

emissions activity for relevant 

pollutants and assess potential, 

additional emissions reduction 

opportunities and resulting downwind 

air quality improvements. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner 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 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. IEPA did not conduct such 

an analysis in Illinois’ SIP submission. 

IEPA argued that Illinois’ 

contributions to the nonattainment 

monitors in the LADCO domain, namely 

the Allegan and Sheboygan receptors, 

EPA can be fairly and adequately 

addressed through Illinois’ participation 

in LADCO. Though IEPA suggested that 

LADCO may be partially responsible for 

decreases in ambient ozone 

concentrations in the Lake Michigan 

area, LADCO is not a regulatory agency 

responsible for implementing emissions 

controls. Furthermore, Illinois did not 

provide any information on any planned 

emission reductions, or evaluation of 

control strategies that the LADCO states 

intend to implement within their 

domain, that would reduce either the 

ozone concentrations or Illinois’ 

contributions at the nonattainment or 

maintenance monitors to which IEPA 

identified Illinois as linked. IEPA’s basis 

for concluding that LADCO 

participation may relieve Illinois of any 

good neighbor obligations to downwind 

receptors is entirely unsubstantiated 

and does not present any basis on which 

EPA can approve IEPA’s SIP submittal. 

As such, EPA proposes to find Illinois’ 

LADCO participation as inadequate to 

resolve Illinois’ good neighbor 

obligations for the 2015 ozone NAAQS. 


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The submittal also stated that Illinois 

is developing new NOX RACT standards 

for the Chicago area that, in conjunction 

with existing regulations and future 

Federal reductions, are estimated to 

reduce NOX emissions by up to 20,000 

tons/year which had not yet been 

reflected in modeling to 2023. However, 

Illinois failed to provide any 

information on the control measures or 

implementation schedule that would 

achieve the estimated 20,000 tons/year 

in reductions. IEPA did not quantify 

how the emissions reductions they 

estimated would impact air quality at 

downwind receptors or Illinois’ 

contributions. In fact, Illinois has not 

yet finalized the NOX RACT rule for 

Chicago. In general, any changes in the 

emissions inventory and on-the-books 

controls relevant to emissions in 2023 

have now been incorporated into EPA’s 

modeling using the 2016v2 emissions 

platform, which projects a continuing 

contribution from Illinois to out of state 

receptors above a threshold of 1 percent 

of the NAAQS (at Steps 1 and 2) despite 

these measures. Therefore, IEPA’s SIP 

submission should have evaluated the 

availability of additional air quality 

controls to improve downwind air 

quality at nonattainment and 

maintenance receptors at Step 3. 

Additionally, states may not rely on 

non-SIP 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). IEPA did not 

attempt to revise Illinois’ SIP to include 

these measures in order to implement 

its good neighbor obligations. Further, 

the listing of existing or on-the-way 

control measures, whether approved 

into the State’s SIP or not, does not 

substitute for a complete Step 3 analysis 

under EPA’s 4-step framework to define 

‘‘significant contribution.’’ IEPA did not 

identify the control measures, provide 

an assessment of the overall effects of 

these measures, note when the 

reductions would be achieved, or 

explain what the overall resulting air 

quality effects would be at identified out 

of state receptors. IEPA did not evaluate 

additional, potential emissions control 

opportunities, or their costs or impacts, 

or attempt to analyze whether, if 

applied more broadly across linked 

states, the emissions reductions would 

constitute the elimination of significant 

contribution on a regional scale. IEPA 

did not offer an explanation as to 

whether any faster or more stringent 

emissions reductions that may be 

available were prohibitively costly or 

infeasible. Although EPA acknowledges 

states are not necessarily bound to 

follow its own analytical framework at 

Step 3, IEPA did not attempt to 

determine or justify an appropriate 

uniform cost-effectiveness threshold for 

the more stringent 2015 ozone NAAQS. 

This would have been similar to the 

approach to defining significant 

contribution that EPA has applied in 

prior rulemakings such as CSAPR and 

or the CSAPR Update, even if such an 

analysis is not technically mandatory. 

Finally, under the Wisconsin 

decision, states and EPA may not delay 

implementation of measures necessary 

to address good neighbor requirements 

beyond the next applicable attainment 

date without a showing of impossibility 

or necessity. See 938 F.3d at 320. The 

IEPA’s submittal is insufficient to the 

extent the implementation timeframes 

for identified control measures were left 

unidentified, unexplained, or too 

uncertain to permit EPA to form a 

judgment as to whether the timing 

requirements for good neighbor 

obligations have been met. 

IEPA also attempted to rely on a 

concept related to international 

emissions identified in Attachment A to 

the March 2018 memorandum—a 

concept that apparently had its origins 

outside EPA and was not endorsed or 

recommend by EPA at the time or since. 

IEPA noted that Illinois would be linked 

to only one receptor if international and 

offshore emissions were simply 

subtracted from the receptor’s maximum 

design values. As explained in Section 

I.D above, the concepts presented in 

Attachment A to the March 2018 

memorandum were neither guidance 

nor determined by EPA to be consistent 

with the CAA. EPA made clear at the 

time that it would thoroughly review 

the technical and legal justifications 

states put forward in relying on any 

concepts from Attachment A to the 

March 2018 memorandum. In this case, 

what IEPA proposes is clearly 

unacceptable. 

The state’s reasoning related to 

international and offshore emissions is 

inapplicable to the requirements of CAA 

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

neighbor provision requires states and 

EPA to address interstate transport of air 

pollution that contributes to downwind 

states’ ability to attain and maintain 

NAAQS. Whether emissions from other 

states or other countries also contribute 

to the same downwind air quality issue 

is irrelevant in assessing whether a 

downwind state has an air quality 

problem, or whether an upwind state is 

significantly contributing to that 

problem. States are not obligated under 

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

emissions sufficient on their own to 

resolve downwind receptors’ 

nonattainment or maintenance 

problems. Rather, states are obligated to 

eliminate their own ‘‘significant 

contribution’’ or ‘‘interference’’ with the 

ability of other states to attain or 

maintain the NAAQS. 

Indeed, the D.C. Circuit in Wisconsin 

specifically rejected petitioner 

arguments suggesting that upwind states 

should be excused from good neighbor 

obligations on the basis that some other 

source of emissions (whether 

international or another upwind state) 

could be considered the ‘‘but-for’’ cause 

of downwind air quality problem. See 

938 F.3d at 323–324. The court viewed 

petitioners’ arguments as essentially an 

argument ‘‘that an upwind state 

‘contributes significantly’ to downwind 

nonattainment only when its emissions 

are the sole cause of downwind 

nonattainment.’’ Id. at 324. The court 

explained that ‘‘an upwind state can 

‘contribute’ to downwind 

nonattainment even if its emissions are 

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

See also Catawba County v. EPA, 571 

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

the argument ‘‘that ‘significantly 

contribute’ unambiguously means 

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

reason why the statute precludes EPA 

from determining that [an] addition of 

[pollutant] into the atmosphere is 

significant even though a nearby 

county’s nonattainment problem would 

still persist in its absence’’); Miss. 

Comm’n on Envtl. Quality v. EPA, 790 

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

(observing that the argument that ‘‘there 

likely would have been no violation at 

all . . . if it were not for the emissions 

resulting from [another source]’’ is 

‘‘merely a rephrasing of the but-for 

causation rule that we rejected in 

Catawba County’’). Therefore, a state is 

not excused from eliminating its 

significant contribution on the basis that 

international emissions also contribute 

some amount of pollution to the same 

receptors to which the state is linked. 

Illinois’ argument related to 

international and offshore emissions 

fails to change the status of any receptor 

at Step 1, to eliminate Illinois’ linkages 

at Step 2, or to provide sufficient 

evidence that Illinois does not 

contribute significantly to receptors at 

Step 3. 

We therefore propose that Illinois was 

required to analyze emissions from the 

sources and other emissions activity 

from within the state to determine 

whether its contributions were 


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66Pointing to anticipated upcoming emission 

reductions is not sufficient as a step 3 analysis, for 

the reasons discussed in Section 4. 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. 

67Modeling Guidance for Demonstrating Air 

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

November 2018. EPA 454–R–18–009. https://

www.epa.gov/scram/sip-modeling-guidancedocuments. 

significant, and we propose to 

disapprove its submission because 

Illinois failed to do so. 

5. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-step interstate 

transport framework calls for 

development of permanent and 

federally enforceable control strategies 

to achieve the emissions reductions 

determined to be necessary at Step 3 to 

eliminate significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. IEPA 

identified future NOX RACT standards 

for the Chicago area and unnamed 

Federal rules were sufficient to resolve 

Illinois’ good neighbor obligations for 

the 2015 ozone NAAQS. However, 

Illinois did not revise its SIP to include 

these emission reductions in a revision 

to its SIP to ensure the reductions were 

permanent and enforceable. As a result, 

EPA proposes to disapprove Illinois’ 

submittal on the separate,66 additional 

basis that the Illinois 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 EPA’s evaluation of Illinois 

SIP submission, EPA is proposing to 

find that the portion of Illinois’ May 21, 

2018 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. 

B. Indiana 

1. Evaluation of Information Provided 

by Indiana Regarding Step 1 

IDEM relied on LADCO modeling 

released in 2018 to identify 

nonattainment and maintenance 

receptors in 2023. As described 

previously in this action, LADCO 

performed a modeling demonstration 

like that of EPA’s 2018 transport 

modeling efforts, except with use of the 

ERTAC EGU Tool for EGU emissions. 

LADCO identified nonattainment and 

maintenance receptors using EPA 

methodology identified in Section I. 

IDEM elected to rely on LADCO’s 

modeling results, which identified 

similar receptors to EPA’s modeling 

included in the March 2018 memo. 

Since new modeling has been 

performed by EPA which includes 

updated emission data using the 2016v2 

platform, EPA proposes to primarily 

rely on the most recent modeling to 

identify nonattainment and 

maintenance receptors in 2023 (see 

Table 3 further in this action). 

Nonetheless, the LADCO modeling 

relied on by IDEM also identified a 

number of nonattainment and 

maintenance receptor sites in 2023. See 

Table 7 on page 30 of Attachment 1 to 

Indiana’s submittal. Thus, even under 

the LADCO modeling for 2023, IDEM 

acknowledges in its submittal the 

existence of several nonattainment and 

maintenance receptors. 

IDEM essentially argues that two of 

the receptors to which Indiana was 

linked would not actually be receptors 

in 2023. Based on updated modeling of 

EPA’s 2016v2 emissions platform, EPA 

agrees with IDEM that that the Allegan, 

Michigan monitor is not expected to be 

a receptor in 2023, but not the receptor 

in Sheboygan, Wisconsin. Regardless, 

EPA disagrees with the line of reasoning 

IDEM put forward to argue that those 

two monitors would not be receptors to 

the extent such reasoning could be 

applied to Indiana’s linkages in EPA’s 

modeling using the 2016v2 emissions 

platform. First, IDEM concluded that if 

ozone design value trends continued as 

expected then those two receptors 

would reach attainment before 2023. In 

addition, IDEM compared 2012–2017 

monitoring data with LADCO’s and 

EPA’s modeling and concluded that the 

Sheboygan, Wisconsin and Allegan, 

Michigan receptor monitors were 

already below the 2023 projections. 

Additionally, EPA’s updated modeling, 

which considers more recent design 

values and emissions, continues to find 

that Indiana is linked to downwind 

nonattainment and maintenance 

receptors, despite downward trends in 

emissions and design values 

2. Evaluation of Information Provided 

by the State Regarding Step 2 

Although Indiana relied on alternative 

modeling to EPA’s modeling, Indiana 

acknowledged in its SIP submission that 

it is linked to one or more downwind 

receptors above either a 1 percent of the 

NAAQS or 1 ppb contribution threshold 

in 2023. Because the alternative 

modeling relied on by IDEM also 

demonstrates that a linkage exists 

between the state and one or more 

downwind receptors at Step 2, EPA 

need not conduct a comparative 

assessment of the alternative modeling; 

the State concedes that it is linked. 

IDEM’s analysis corroborates the 

conclusion in EPA’s most recent 

modeling, described in the next section. 

IDEM additionally utilized a 1 ppb 

threshold at Step 2 to identify whether 

it was linked to a projected downwind 

nonattainment or maintenance receptor. 

As discussed in 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, IDEM’s 

submission did not contain any 

additional analysis of contributions at 

the receptors to which they were linked 

to support their claim that a 1 ppb 

threshold was an appropriate Step 2 

screening threshold. Rather, IDEM 

claimed that a threshold of 1 percent 

was too low because it is less than the 

ozone monitoring ‘‘tolerance level’’ of 1 

ppb (i.e., precision) used for reporting 

measured ozone concentrations. In its 

submittal IDEM failed to provide any 

basis for asserting that the precision of 

an ozone monitor is applicable to the 

precision of ozone contributions which 

are not a directly measured quantity. 

Regardless, total upwind contributions 

are well above 1 ppb at all receptors to 

which Indiana is linked based on 

modeling by LADCO and by EPA. In 

addition, Indiana alone contributes 

above 1 ppb to several downwind 

receptors. Because contributions are not 

directly measured, modeling is used to 

apportion projected ozone design values 

into contributions from individual states 

and other sources of ozone precursors 

(e.g., fires and biogenic sources). The 

projected ozone design values are 

calculated using the method 

recommended in EPA’s modeling 

guidance.67 As part of this method, 

projected design values are reported 

with a precision of a tenth of a ppb. 

Consistent with our modeling guidance, 

ozone contributions are evaluated with 

a precision of a tenth of a ppb. For 

example, a contribution of 0.6999 . . . 

ppb is reported as 0.69 ppb and 

evaluated as 0.6 ppb which is below the 

1 percent threshold. 

Indiana seemingly conflates the 

contribution threshold at Step 2 with a 

Step 3 determination of ‘‘significance’’ 

(which is reached only after the 

application of a multi-factor analysis), 

regardless EPA does not accept 


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68 Indiana’s SIP submission, Attachment 1 at 4. 69August 2018 memorandum at 4. 70See Section 4.1’ ‘‘Overview of Modeled 

Attainment Test in EPA Modeling Guidance for 

Demonstrating Air Quality Goals for Ozone, PM2.5, 

and Regional Haze. November 2018. EPA 454–R– 

18–009. https://www.epa.gov/scram/sip-modelingguidance-documents. 

71Christopher Emery, Zhen Liu, Armistead G. 

Russel, M. Talat Odman, Greg Yarwood and Naresh 

Kumar (2017). Recommendations on statistics and 

benchmarks to assess photochemical model 

performance, Journal of the Air & Waste 

Management Association, 67:5,582–598, DOI: 

10.1080/10962247.2016.1265027. 

72 Design values and contributions at individual 

monitoring sites nationwide are provide in the file: 

2016v2_DVs_state_contributions.xlsx which is 

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

0663. 

73These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I. That 

modeling showed that Indiana 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 ID No. 

EPA–HQ–OAR–2021–0663. 

Indiana’s argument that a 1 percent of 

the NAAQS contribution threshold at 

Step 2 is ‘‘not appropriate’’ for the 2015 

ozone NAAQS on the basis of 

unwarranted modeling reliability 

concerns.68 The explanation for how the 

1 percent contribution threshold was 

originally derived is available in the 

2011 CSAPR rulemaking. See 76 FR 

48208, 48237–38. Further, in the CSAPR 

Update, EPA re-analyzed the threshold 

for purposes of the 2008 ozone NAAQS 

and determined it was appropriate to 

continue to apply this threshold. EPA 

compared the 1 percent threshold to a 

0.5 percent of NAAQS threshold and a 

5 percent of NAAQS threshold. EPA 

found that the lower threshold did not 

capture appreciably more upwind state 

contribution compared to the 1 percent 

threshold, while the 5 percent threshold 

allowed too much upwind state 

contribution to drop out from further 

analysis. See Final CSAPR Update Air 

Quality Modeling TSD, at 27–30 (EPA– 

HQ–OAR–2015–0596–0144). If EPA 

were to apply this analysis to the 2015 

ozone NAAQS using the updated 

modeling based on the 2016v2 

emissions platform, a 5 percent of the 

NAAQS contribution threshold would 

forgo nearly 30 percent of the total 

upwind contribution, on average, for 

those receptors to which Indiana is 

linked using a 1 percent threshold. As 

EPA noted in the August 2018 

memorandum, the use of even a 2 ppb 

contribution threshold under the 

modeling released with the March 2018 

memorandum would only capture about 

55 percent of all upwind contributions, 

and therefore ‘‘emission reductions from 

states linked at that higher threshold 

may be insufficient to address collective 

upwind state contribution to downwind 

air quality problems.’’ 69

With these figures in mind, IDEM’s 

claims that the contribution threshold 

should be substantially higher than 1 or 

even 2 ppb solely on the basis of 

modeling uncertainty cannot be 

accepted. First, both IDEM’s and EPA’s 

modeling techniques are sufficiently 

reliable and fit for the purpose to 

measure upwind contribution levels 

down to at least one percent of the 

NAAQS. EPA’s recommended model 

attainment test is based on application 

of the model in a relative sense rather 

than relying upon absolute model 

predictions.70 All models have 

limitations resulting from uncertainties 

in inputs and scientific formulation. To 

minimize the effects of these 

uncertainties, the modeling is anchored 

to base period measured data in EPA’s 

guidance approach for projecting design 

values. Notably, EPA also uses our 

source apportionment modeling in a 

relative sense when calculating the 

average contribution metric (used to 

identify linkages). In this method the 

magnitude of the contribution metric is 

tied to the magnitude of the projected 

average design value which is tied to the 

base period average measured design 

value. EPA’s guidance has not 

established a bright-line criteria for 

judging whether or not statistical 

measures of model performance 

constitute acceptable or unacceptable 

model performance. So, contrary to 

what Indiana appears to be claiming 

with regards to modeling biases, there 

are no EPA recommended measures of 

allowable error. Although EPA does not 

typically focus on using particular 

benchmarks as the sole criteria for 

model performance, EPA notes that the 

model performance for the updated 

modeling based on the 2016v2 

emissions platform is generally within 

the benchmarks recommended by 

Emery, et al., (2017).71

EPA has successfully applied a 1 

percent of NAAQS threshold to identify 

linked upwind states in three prior 

rulemakings. And the D.C. Circuit has 

declined to establish bright line criteria 

for model performance. In upholding 

EPA’s approach to evaluating interstate 

transport in CSAPR, the Supreme Court 

held that they would not ‘‘invalidate 

EPA’s predictions solely because there 

might be discrepancies between those 

predictions and the real world. That 

possibility is inherent in the enterprise 

of prediction.’’ EME Homer City 

Generation, L.P. v. EPA, 795 F.3d 118, 

135 (2015). The court continued to note 

that ‘‘the fact that a ‘model does not fit 

every application perfectly is no 

criticism; a model is meant to simplify 

reality in order to make it tractable.’ ’’ 

Id. at 135–36 (quoting Chemical 

Manufacturers Association v. EPA, 28 

F.3d 1259, 1264 (D.C. Cir. 1994). 

EPA’s August 2018 memorandum 

provided that whether use of a 1 ppb 

threshold is appropriate must be based 

on an evaluation of state-specific 

circumstances, and no such evaluation 

was included in the submission. EPA’s 

experience with the alternative Step 2 

thresholds is further discussed in 

Section I.D.3.i. As discussed there, EPA 

is considering withdrawing the August 

2018 memorandum. 

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

Modeling and Findings for Indiana 

As described in Section I, EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. These data were examined to 

determine if Indiana 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 3, the data 72 indicate that in 2023, 

emissions from Indiana contribute 

greater than 1 percent of the standard to 

nonattainment or maintenance-only 

receptors in Wisconsin, Illinois, 

Connecticut, and Pennsylvania.73

TABLE 3—INDIANA LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location Nonattainment/maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Indiana 

contribution 

(ppb) 

550590025 ............................. Kenosha, WI ......................... Maintenance ......................... 69.2 72.3 7.10 

170310032 ............................. Cook, IL ................................ Maintenance ......................... 69.8 72.4 7.03 


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TABLE 3—INDIANA LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING—Continued 

Receptor ID Location Nonattainment/maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Indiana 

contribution 

(ppb) 

550590019 ............................. Kenosha, WI ......................... Nonattainment ...................... 72.8 73.7 6.60 

551010020 ............................. Racine, WI ............................ Nonattainment ...................... 71.3 73.2 6.60 

170317002 ............................. Cook, IL ................................ Maintenance ......................... 70.1 73.0 6.33 

170310076 ............................. Cook, IL ................................ Maintenance ......................... 69.3 72.1 6.21 

170310001 ............................. Cook, IL ................................ Maintenance ......................... 69.6 73.4 5.44 

170314201 ............................. Cook, IL ................................ Maintenance ......................... 69.9 73.4 4.65 

90099002 ............................... New Haven, CT .................... Nonattainment ...................... 71.8 73.9 0.87 

90019003 ............................... Fairfield, CT .......................... Nonattainment ...................... 76.1 76.4 0.76

90013007 ............................... Fairfield, CT .......................... Nonattainment ...................... 74.2 75.1 0.75

420170012 ............................. Bucks, PA ............................. Maintenance ......................... 70.7 72.2 0.73 

Therefore, based on EPA’s evaluation 

of the information submitted by IDEM, 

and based on EPA’s most recent 

modeling results for 2023, EPA proposes 

to find that Indiana 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. EPA 

therefore will proceed to Step 3 of the 

4-Step interstate transport framework to 

assess the arguments the State presented 

as to why, despite this linkage, the state 

should not be considered to 

significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS in any other 

state. 

4. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-Step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

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

To effectively evaluate which 

emissions in the state should be deemed 

‘‘significant’’ and therefore prohibited, 

states generally should prepare an 

accounting of sources and other 

emissions activity for relevant 

pollutants and assess potential, 

additional emissions reduction 

opportunities and resulting downwind 

air quality improvements. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner 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 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. IDEM did not conduct such 

an analysis in their SIP submission. 

IDEM first asserted that Indiana’s rule 

amendments under CSAPR meant that 

Indiana was already meeting the good 

neighbor requirements for the 2015 

ozone NAAQS. The submittal, however, 

did not contain a demonstration at Step 

3 that the State was adequately 

controlling its emissions for purposes of 

the good neighbor provision, 

particularly because the State conceded 

in its submission that it was potentially 

significantly contributing to one or more 

receptors in 2023 at Steps 1 and 2. The 

SIP submittal pointed to the state’s 

existing NOX control measures, consent 

decree requirements, and future fuel 

switches and retirements for large EGUs 

and non-EGUs for the years 2008 

through 2017 to conclude Indiana is 

already meeting its good neighbor 

obligations for the 2015 ozone NAAQS. 

However, the state’s submittal does 

not include a sufficient examination or 

a technical justification that could 

support the conclusion that the state has 

no further good neighbor obligations for 

the 2015 ozone NAAQS. In particular, 

the state did not conduct in its submittal 

an analysis of potential additional 

emissions reductions measures to 

further reduce its impact on the 

identified downwind receptors. For 

example, although Indiana did include 

in its submission a list of controls at 

individual emissions units at facilities 

in the state, IDEM did not analyze 

additional potential NOX emissions 

control technologies, their associated 

costs, estimated emissions reductions, 

and downwind air quality 

improvements. Nor does the submittal 

include an analysis of whether such 

potential, additional control 

technologies or measures could reduce 

the impact of Indiana’s emissions on out 

of state receptors. Though there is not a 

prescribed method for a Step 3 analysis, 

EPA has consistently applied Step 3 of 

the good neighbor framework through a 

more rigorous evaluation of potential 

additional control technologies or 

measures than what Indiana provided in 

its submission. Identifying a range of 

various emissions control measures that 

have been or may be enacted at the state 

level, without analysis of the impact of 

those measures on the out of state 

receptors, is not analytically sufficient. 

In general, the air quality modeling that 

EPA has conducted (as well the 

modeling relied on by Indiana in its 

submittal) already accounts for ‘‘on-thebooks’’ emissions control measures. 

Both sets of modeling clearly establish 

continued linkage from Indiana to 

downwind receptors in 2023 at Steps 1 

and 2, despite those emissions control 

efforts. 

IDEM provided what they 

characterized as a weight of evidence 

analysis consisting of monitoring data, 

emissions data, and photochemical 

modeling to justify their conclusion that 

no additional emission reductions 

would be necessary to satisfy Indiana’s 

ozone transport obligations. First, IDEM 


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74 Indiana’s SIP submission, Attachment 1 at 37. 

75See 81 FR 74504 at 74517; 85 FR 68964 at 

68979. 

76The ‘‘Capacity Dropped’’ and the ‘‘Retired 

Through 2023’’ worksheets in NEEDS lists all units 

that are removed from the NEEDS v6 inventory— 

NEEDS v6 Summer 2021 Reference Case. This data 

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

www.epa.gov/airmarkets/national-electric-energydata-system-needs-v6. 

presented evidence of downward trends 

of statewide ozone concentrations and 

emissions, as well as a decrease in 

projected EGU emissions in 2023 

relative to 2011. Despite these trends, 

however, the LADCO modeling that 

Indiana depended on for its submittal 

still identified that Indiana would 

contribute over 1 ppb to one or more 

receptors in 2023. 

As for downwind design value trends, 

EPA disagrees that IDEM’s reliance on 

trends data to conclude that the Harford, 

Maryland and Richmond, New York 

monitors would reach attainment ‘‘over 

time’’ is sufficient to support a 

conclusion that Indiana has no good 

neighbor obligations. The states and 

EPA are to address interstate transport 

obligations ‘‘as expeditiously as 

practicable’’ and no later than the 

attainment schedule set in accordance 

with CAA section 181(a). See North 

Carolina, 531 F.3d at 911–13; 

Wisconsin, 938 F.3d at 313–20; 

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

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

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

4, 6–7 (D.C. Cir. 2019). IDEM asserted 

that EGUs are well controlled in Indiana 

and cited several state and Federal 

regulations that EGUs may be subject to 

in Indiana. In general, however, the 

listing of existing or on-the-way control 

measures, whether approved into the 

state’s SIP or not, does not substitute for 

a complete Step 3 analysis under EPA’s 

4-Step framework to define ‘‘significant 

contribution.’’ IDEM did not provide an 

assessment of the overall effects of the 

identified control measures or explain 

what the overall resulting air quality 

effects would be at identified out of 

State receptors. IDEM did not perform 

an analysis of all large NOX emitting 

EGU for factors that may affect the 

facilities’ emissions, including but not 

limited to allowance prices, fuel prices, 

and enforceable limits. IDEM did not 

evaluate additional, potential emissions 

control opportunities, or their costs or 

impacts, or attempt to analyze whether, 

if applied more broadly across linked 

states, the emissions reductions would 

constitute the elimination of significant 

contribution on a regional scale. IDEM 

did not offer an explanation as to 

whether any faster or more stringent 

emissions reductions that may be 

available were prohibitively costly or 

infeasible. Although EPA acknowledges 

states are not necessarily bound to 

follow its own analytical framework at 

Step 3, IDEM did not attempt to 

determine or justify an appropriate 

uniform cost-effectiveness threshold. 

This would have been similar to the 

approach to defining significant 

contribution that EPA has applied in 

prior rulemakings such as CSAPR and 

or the CSAPR Update, even if such an 

analysis is not technically mandatory. 

As discussed previously, both the 

LADCO modeling relied on by the state 

and EPA’s updated modeling indicates 

sources in Indiana are linked to 

downwind air quality problems for the 

2015 ozone standard. However, 

Indiana’s SIP submittal did not include 

an analysis of potential NOX emissions 

control technologies, associated costs, 

estimated emissions reductions, and 

downwind air quality in order to 

determine whether the State had 

eliminated the State’s downwind 

contribution in amounts which will 

significantly contribute to 

nonattainment or interfere with 

maintenance. Thus, EPA proposes to 

disapprove Indiana’s SIP submission on 

the separate, additional basis that the 

SIP submittal did not assess additional 

emission control opportunities. 

IDEM concluded it is not costeffective to evaluate and implement 

controls on non-EGUs in the state on the 

sole basis that the majority of NOX

emissions in the state come from EGUs. 

EPA cannot accept the assertion as it is 

insufficiently supported. 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. EPA notes that 

there are as many as two dozen nonEGU facilities in Indiana with more than 

300 tons per year of NOX emissions 

each, but IDEM did not analyze control 

opportunities at these sources at all in 

the SIP submission. 

IDEM also argued that additional 

emissions reductions from EGU and 

non-EGU sources in Indiana ‘‘are getting 

more difficult to mandate’’ because of 

reduced effectiveness of controls to 

make significant decreases in ozone 

values, operational concerns, and 

increased costs for customers.74 Again, 

the SIP submission does not contain 

sufficient evidence to support that 

conclusion. IDEM did not identify 

controls that had reduced effectiveness 

or explain why they believed they had 

reduced effectiveness. IDEM did not 

describe what any operational concerns 

were for any controls, nor did IDEM 

provide any information to support their 

claim that controls would increase costs 

for consumers. While Indiana’s existing 

control measures have undoubtedly 

reduced the amount of transported 

ozone pollution to other states and have 

contributed to the downward emissions 

trends and improving air quality in the 

State as shown in the state’s SIP 

submittal, in the Revised CSAPR 

Update, EPA’s analysis found that 

despite Indiana’s existing control 

programs, additional emissions 

reductions were achievable from EGUs 

in the state, even under the level of 

control stringency EPA determined 

appropriate to eliminate significant 

contribution for the 2008 ozone 

NAAQS. In any case, EPA has not 

established a benchmark costeffectiveness threshold for good 

neighbor obligations for the 2015 ozone 

NAAQS, and IDEM in its submittal has 

not conducted an analysis to establish 

one for EPA to evaluate. 

IDEM also identified several planned 

retirements or retrofits to coal fired 

EGUs in Indiana that were not included 

in any modeling available at the time of 

Indiana’s submission and stated they 

would reduce emissions several 

thousand tons beyond the modeling. 

Further, EPA’s assessment of future air 

quality conditions generally accounts 

for on-the-books emission reductions 

and the most up-to-date forecast of 

future emissions in the absence of the 

transport policy being evaluated (i.e., 

base case conditions).75 As described in 

more detail in Section I, EPA’s latest 

projections of the baseline EGU 

emissions uses the version 6—Summer 

2021 Reference Case of the IPM. The 

IPM version 6—Summer 2021 Reference 

Case uses the NEEDS v6 database as its 

source for data on all existing and 

planned-committed units. Units are 

removed from the NEEDS inventory 

only if a high degree of certainty could 

be assigned to future implementation of 

the announced future closure or 

retirement. Any retirements excluded 

from the NEEDS v6 inventory can be 

viewed in the NEEDS spreadsheet.76

EPA looked into the upcoming 

retirements cited by IDEM and 

following the guidelines regarding 

retirements for the IPM version—6 

Summer 2021 Reference Case certain 

units are not excluded from the NEEDS 

v6 inventory. There are other 

retirements that were not included in 

the SIP submission that were excluded 


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77Based on the reference to the potential 

flexibilities in Attachment A to the March 2018 

memorandum on page 2 of Attachment 1 to 

Indiana’s SIP submission, EPA assumes the 

reference to ‘‘flexibilities’’ on page 38 of 

Attachment 1 likewise references Attachment A to 

the March 2018 memorandum. 78 Indiana’s SIP submission, Appendix E at 4, 17. 

from the NEEDS v6 inventory for the 

2023 projections. This includes 

retirements at AES Petersburg, Merom, 

and RM Schahfer. In other words, in 

general, any changes in the emissions 

inventory and on-the-books controls 

relevant to emissions in 2023 have now 

been incorporated into the EPA’s 

modeling using the 2016v2 emissions 

platform, which projects a continuing 

contribution from Indiana to out of state 

receptors above a threshold of 1 percent 

of the NAAQS (at Steps 1 and 2) despite 

these measures. Therefore, in light of 

continuing contribution to out of state 

receptors from Indiana notwithstanding 

these identified retirements, IDEM’s SIP 

submission should have evaluated the 

availability of additional air quality 

controls to improve downwind air 

quality at nonattainment and 

maintenance receptors at Step 3 

Furthermore, under the Wisconsin 

decision, states and EPA may not delay 

implementation of measures necessary 

to address good neighbor requirements 

beyond the next applicable attainment 

date without a showing of impossibility 

or necessity. See 938 F.3d at 320. The 

IDEM’s submittal is insufficient to the 

extent the implementation timeframes 

for several claimed expected shutdowns 

were left unidentified, unexplained, or 

too uncertain to permit EPA to form a 

judgment as to whether the timing 

requirements for good neighbor 

obligations have been met. 

Additionally, IDEM explained in only 

the most general terms how the 

unaccounted emissions reductions 

would influence downwind air quality 

or Indiana’s contributions to other state. 

IDEM also did not quantify how the 

emissions reductions they estimated 

would impact air quality at downwind 

receptors or Indiana’s contributions. 

IDEM did not demonstrate that the 

downwind improvements from these 

regulations and programs would be 

sufficient to eliminate Indiana’s linkages 

or prohibit the State’s emissions in 

amounts that will contribute 

significantly to nonattainment or 

interfere with maintenance of the 

NAAQS in any other state. 

IDEM also made several arguments 

related to potential flexibilities 

identified in Attachment A to the March 

2018 memorandum.77 As explained 

previously in Section I, the concepts 

presented in Attachment A to the March 

2018 memorandum were neither 

guidance nor determined by EPA to be 

consistent with the CAA. EPA will 

thoroughly review the technical and 

legal justifications IDEM put forward in 

their attempt to use a potential 

flexibility from Attachment A to the 

March 2018 memorandum. 

IDEM suggested that local emissions 

reductions from the jurisdiction where 

downwind receptors are located should 

first be implemented and accounted for 

before imposing obligations on upwind 

states under the interstate transport 

provision. IDEM represented that EPA 

had concluded that monitors in the 

Northeast ‘‘are impacted from more 

local emissions’’ by citing a May 14, 

2018 presentation. The purpose of that 

presentation was to share a technical, 

exploratory analysis of ozone trends. 

IDEM misrepresented the contents of 

the presentation, which labeled the 

results as ‘‘preliminary’’ and indicated 

that ‘‘[f]urther exploration of the relative 

contribution from various source sectors 

within the NE Corridor and in nearby 

upwind states might also be 

informative.’’ 78 These preliminary 

results of that analysis are generally 

consistent with EPA’s updated 

modeling using the 2016v2 emissions 

platform. Although EPA’s modeling 

shows that a large portion of the 

transport problem affecting the 

receptors in Coastal Connecticut is 

indeed from sources within the Ozone 

Transport Region (OTR), a substantial 

portion of the transport problem at these 

receptors, on the order of 25 percent, is 

the result of transport from states 

outside the OTR. However, the 

relevance of that presentation to the 

evaluation of Indiana’s good neighbor 

obligations is not clear. As already 

discussed, the statute and the case law 

(particularly the holdings in Wisconsin 

and Maryland) make clear that good 

neighbor obligations are not merely 

supplementary to or deferable until after 

local emission reductions are achieved. 

Further, based on EPA’s modeling 

released with the March 2018 

memorandum, nearly all of the 

receptors to which Indiana is linked are 

also heavily impacted by distant 

upwind state emissions in addition to 

local sources and sources in neighboring 

states. The Wisconsin decision’s holding 

in regard to international contribution 

(discussed in more detail later) is 

equally applicable to an upwind state’s 

claims that some other state’s emissions, 

or local emissions, are more to blame 

than its own emissions. See 938 F.3d 

303 at 323–25 (‘‘an upwind state can 

‘contribute’ to downwind 

nonattainment even if its emissions are 

not the but-for cause’’). 

There is nothing in the CAA that 

supports Indiana’s position on local 

sources, and Indiana does not provide 

grounds on which to approve its SIP 

submission. The D.C. Circuit has held 

on five different occasions that the 

timing framework for addressing 

interstate transport obligations must be 

consistent with the downwind areas’ 

attainment schedule. In particular, for 

the ozone NAAQS, the states and EPA 

are to address interstate transport 

obligations ‘‘as expeditiously as 

practicable’’ and no later than the 

attainment schedule set in accordance 

with CAA section 181(a). See North 

Carolina, 531 F.3d at 911–13; 

Wisconsin, 938 F.3d at 313–20; 

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

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

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

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

Wisconsin explained its reasoning in 

part by noting that downwind 

jurisdictions often may need to heavily 

rely on emissions reductions from 

upwind states in order to achieve 

attainment of the NAAQS, 938 F.3d at 

316–17; such states would face 

increased regulatory burdens including 

the risk of bumping up to a higher 

nonattainment classification if 

attainment is not reached by the 

relevant deadline, Maryland, 958 F.3d at 

1204. The statutory framework of the 

CAA and these cases establish clearly 

that states and EPA must address 

interstate transport obligations in line 

with the attainment schedule provided 

in the CAA in order to timely assist 

downwind states in attaining and 

maintain the NAAQS, and this schedule 

is ‘‘central to the regulatory scheme.’’ 

Wisconsin, 938 F.3d at 316 (quoting 

Sierra Club v. EPA, 294 F.3d 155, 161 

(D.C. Cir. 2002)). 

IDEM similarly suggested that 

international and offshore emissions 

contributions should be part of the good 

neighbor calculus. IDEM’s reasoning 

related to international and offshore 

emissions is inapplicable to the 

requirements of CAA section 

110(a)(2)(D)(i)(I). The good neighbor 

provision requires states and EPA to 

address interstate transport of air 

pollution that contributes to downwind 

states’ ability to attain and maintain 

NAAQS. Whether emissions from other 

states or other countries also contribute 

to the same downwind air quality issue 

is irrelevant in assessing whether a 

downwind state has an air quality 

problem, or whether an upwind state is 

significantly contributing to that 

problem. States are not obligated under 

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


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79 Indiana’s SIP submission, Attachment 1 at 42. 

emissions sufficient on their own to 

resolve downwind receptors’ 

nonattainment or maintenance 

problems. Rather, states are obligated to 

eliminate their own ‘‘significant 

contribution’’ or ‘‘interference’’ with the 

ability of other states to attain or 

maintain the NAAQS. 

Indeed, the D.C. Circuit in Wisconsin 

specifically rejected petitioner 

arguments suggesting that upwind states 

should be excused from good neighbor 

obligations on the basis that some other 

source of emissions (whether 

international or another upwind state) 

could be considered the ‘‘but-for’’ cause 

of downwind air quality problem. See 

938 F.3d at 323–324. The court viewed 

petitioners’ arguments as essentially an 

argument ‘‘that an upwind state 

‘contributes significantly’ to downwind 

nonattainment only when its emissions 

are the sole cause of downwind 

nonattainment.’’ Id. at 324. The court 

explained that ‘‘an upwind state can 

‘contribute’ to downwind 

nonattainment even if its emissions are 

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

See also Catawba County v. EPA, 571 

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

the argument ‘‘that ‘significantly 

contribute’ unambiguously means 

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

reason why the statute precludes EPA 

from determining that [an] addition of 

[pollutant] into the atmosphere is 

significant even though a nearby 

county’s nonattainment problem would 

still persist in its absence’’); Miss. 

Comm’n on Envtl. Quality v. EPA, 790 

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

(observing that the argument that ‘‘there 

likely would have been no violation at 

all . . . if it were not for the emissions 

resulting from [another source]’’ is 

‘‘merely a rephrasing of the but-for 

causation rule that we rejected in 

Catawba County’’). Therefore, a state is 

not excused from eliminating its 

significant contribution on the basis that 

international emissions also contribute 

some amount of pollution to the same 

receptors to which the state is linked. 

IDEM also calculated Indiana’s 

portion of contribution to the Harford, 

Maryland receptor was 0.077 ppb, and 

determined that Indiana would need to 

reduce its contribution by 0.0077 ppb 

(based on a contribution threshold of 1 

ppb) to bring the Maryland receptor into 

attainment. IDEM argued that 0.0077 

ppb is well within the error of the 

model and would be ‘‘difficult’’ to 

translate into an emission reduction 

requirement.79 We first note that this 

approach is a deviation from EPA’s 

traditional approach of apportioning 

upwind-state responsibility at Step 3 

using a uniform cost of control metric 

set at a level that maximizes costeffectiveness of emissions reductions in 

relation to downwind state impacts 

across all linked states. Thus, this is not 

how EPA has interpreted the statutory 

term ‘‘significant’’ in the past, and EPA 

does not reach a conclusion whether 

this approach would be approvable, had 

IDEM had imposed emissions 

reductions in line with this logic. 

We do not need to reach that point in 

the analysis, however, because, having 

selected that approach to defining its 

obligations, IDEM proceeded to ignore 

the result. IDEM’s submission identified 

Indiana’s proportional contribution as 

0.077 ppb to the Harford, Maryland 

receptor. Having acknowledged Indiana 

was responsible for eliminating up to 

0.0077 ppb of contribution, IDEM 

claimed that because that amount was 

within the ‘‘error of the model’’ that it 

would be ‘‘difficult’’ to require that 

amount of reductions from Indiana 

sources. 

This argument does not rise to the 

level of acceptable proof. EPA has 

routinely been capable of successfully 

implementing good neighbor obligations 

through the CSAPR framework, and 

achieving significant downwind air 

quality improvements through upwindstate reductions, at levels of ‘‘significant 

contribution’’ comparable or even less 

than those found in Indiana’s 

submission, irrespective of alleged 

modeling errors. See Wisconsin, 938 

F.3d at 322–23 (rejecting Wisconsin’s 

argument that it should not face good 

neighbor obligations on the basis that its 

emission reductions would only 

improve a downwind receptor by two 

ten-thousandths of a part per billion). 

After measuring Indiana’s significant 

contribution, IDEM suggested that 

modeling uncertainty was too great to 

either require emissions reductions. But 

IDEM had measured the state’s 

significant contribution and was 

therefore identifying the measurable 

amount of significant contribution the 

state was legally responsible for 

eliminating. See Michigan v. EPA, 213 

F.3d 663, 683–84 (D.C. Cir. 2000) 

(significant contribution must be 

‘‘measurable’’). Further, scientific 

uncertainty may only be invoked to 

avoid comporting with the requirements 

of the CAA when ‘‘the scientific 

uncertainty is so profound that it 

precludes . . . reasoned judgment’’ 

Massachusetts v. EPA, 127 S.Ct. 1438 

(2007). See Wisconsin, 938 F.3d at 318– 

19 (‘‘Scientific uncertainty, however, 

does not excuse EPA’s failure to align 

the deadline for eliminating upwind 

States’ significant contributions with the 

deadline for downwind attainment of 

the NAAQS.’’). See also EME Homer 

City, 795 F.3d 118, 135–36 (‘‘We will 

not invalidate EPA’s predictions solely 

because there might be discrepancies 

between those predictions and the real 

world. That possibility is inherent in the 

enterprise of prediction.’’). IDEM’s 

arguments related to modeling 

uncertainty do not establish a level of 

uncertainty so high as to preclude 

reasoned judgement. 

IDEM provided an analysis of back 

trajectories from the Harford and 

Richmond receptors to support its 

contention that Indiana does not 

contribute significantly to 

nonattainment or maintenance at those 

monitors, and that the receptors are 

more impacted by local emissions 

anyway. IDEM also relied on an EPA 

presentation from 2018 to support this 

conclusion. 

As already discussed, the statute and 

the case law (particularly the holdings 

in Wisconsin and Maryland) make clear 

that good neighbor obligations are not 

merely supplementary to or deferable 

until after local emission reductions are 

achieved. Further, all of the receptors to 

which Indiana is linked are heavily 

impacted by upwind state emissions in 

addition to local sources and 

conditions. The Wisconsin decision’s 

holding in regard to international 

contribution (discussed previously) is 

equally applicable to an upwind state’s 

claims that some other state’s emissions, 

or local emissions, are more to blame 

than its own emissions. See 938 F.3d 

303 at 323–25 (‘‘an upwind state can 

‘contribute’ to downwind 

nonattainment even if its emissions are 

not the but-for cause’’). 

Further, EPA finds Indiana’s back 

trajectory analysis to be deficient in 

proving that Indiana does not contribute 

significantly to nonattainment or 

maintenance at the Harford and 

Richmond monitors that the State was 

linked to in the LADCO modeling. 

Indiana’s back trajectory analysis shows 

a linkage between Indiana and the 

monitors when evaluating two altitudes, 

10 meters and 750 meters, on several of 

the exceedance days at these monitoring 

sites. By only evaluating two altitudes, 

Indiana neglects to consider the wide 

range of heights that might show back 

trajectories leading back to Indiana, 

potentially further tying the state to 

more exceedance events. Furthermore, 

10 meters is too low of an altitude to 

measure long range transport and it 

would have been appropriate for 

Indiana to analyze several higher 

altitudes to bolster its back trajectory 

analysis. 


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80Pointing 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 

II.B.4. 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. 

Back trajectories alone are not 

sufficient to disconnect upwind States 

from downwind receptors. Relying 

solely on back trajectories for 

establishing linkages neglects the 

myriad of factors, most importantly 

photochemical reactions, that are 

important for determining the 

magnitude of ozone and precursor 

transport from upwind states to 

downwind receptors. In this regard, 

EPA and LADCO modeling which 

accounts for 3 dimensional 

meteorological conditions, regional 

emissions, and photochemical reactions 

is the most complete, and technically 

sound method to establish linkages 

between upwind states and downwind 

nonattainment and maintenance 

receptors. 

The information and claims presented 

by IDEM did not provide sufficient 

evidence to support alternative 

conclusions that EPA is proposing to 

make in this action: Namely, that 

several receptors exist, Indiana 

contributes to those receptors above a 1 

percent of the NAAQS contribution 

threshold, and that Indiana continues to 

have good neighbor obligations that 

need to be addressed for the 2015 ozone 

NAAQS. We therefore propose that 

Indiana 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 

Indiana failed to do so. 

5. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-Step interstate 

transport framework calls for 

development of permanent and 

federally enforceable control strategies 

to achieve the emissions reductions 

determined to be necessary at Step 3 to 

eliminate significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. IDEM 

identified the State’s existing NOX

control measures, consent decree 

requirements, and future fuel switches 

and retirements for large EGUs and nonEGUs for the years 2008 through 2017 80

States may not rely on non-SIP 

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). However, the state did not 

revise its SIP to include these emission 

reductions in a revision to its SIP to 

ensure the reductions were permanent 

and enforceable. As a result, EPA 

proposes to disapprove Indiana’s 

submittal on the separate, additional 

basis that Indiana 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. 

6. Conclusion 

Based on EPA’s evaluation of 

Indiana’s SIP submission, EPA is 

proposing to find that the portion of 

Indiana’s November 12, 2018 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. 

C. Michigan 

1. Evaluation of Information Provided 

by Michigan Regarding Step 1 

At Step 1 of the 4-step interstate 

transport framework, Michigan relied 

primarily on the LADCO modeling 

released in 2018 to identify 

nonattainment and maintenance 

receptors in 2023. As described 

previously in this action, LADCO 

performed a modeling demonstration 

like that of EPA modeling released with 

the March 2018 memorandum, except 

with use of the ERTAC EGU Tool to 

replace specific EGU information. 

LADCO identified nonattainment and 

maintenance receptors using EPA 

methodology. EGLE elected to rely on 

LADCO’s ‘‘water only’’ modeling 

results, but also presented results from 

EPA’s modeling released with the 

March 2018 memorandum. EGLE noted 

that in general, design values in the 

LADCO modeling were lower. However, 

since new modeling has been performed 

by EPA which includes updated 

emission data using the 2016v2 

platform, EPA proposes to primarily 

rely on the most recent modeling to 

identify nonattainment and 

maintenance receptors in 2023. 

Nonetheless, the alternative modeling 

relied on by EGLE also identified a 

number of nonattainment and 

maintenance receptor sites in 2023. See 

Table 2 on page 14 of EGLE’s submittal. 

Thus, even under its alternative 

modeling of 2023, EGLE acknowledges 

in its submittal the existence of several 

nonattainment and maintenance 

receptors. 

2. Evaluation of Information Provided 

by the State Regarding Step 2 

Although Michigan relied on 

alternative modeling to EPA’s modeling, 

EGLE acknowledged in their SIP 

submission that Michigan is linked 

above either a 1 percent of the NAAQS 

or 1 ppb or threshold to one or more 

downwind receptors in 2023 (1.85 ppb 

to Sheboygan, Wisconsin (Site ID: 36– 

081–0124), 1.22 ppb to Queens, New 

York (Site ID: 36–085–0067), and 1.03 

ppb to Richmond, New York (Site ID: 

55–117–0006)). Because the alternative 

modeling relied on by the state also 

demonstrates that a linkage exists 

between the state and downwind 

receptors at Step 2, EPA need not 

conduct a comparative assessment of 

the alternative modeling; the state 

concedes that it is linked. EGLE’s 

analysis corroborates the conclusion in 

EPA’s most recent modeling, described 

in the next section. 

EGLE, relying on a concept from 

outside parties listed in Attachment A 

to the March 2018 memorandum, 

attempted to justify the use of a 1 ppb 

threshold at Step 2 to identify whether 

the state was ‘‘linked’’ to a projected 

downwind nonattainment or 

maintenance receptor. In part, EGLE 

attempted to justify the use of a 1 ppb 

contribution threshold based on the 

2018 PSD SIL guidance document. 

EGLE also referenced EPA’s August 

2018 memorandum, which said that 

with appropriate additional analysis it 

may be reasonable for states to use a 1 

ppb contribution threshold, as an 

alternative to a one percent threshold, at 

Step 2 of the 4-Step interstate transport 

framework for the purposes of 

identifying linkages to downwind 

receptors. As explained in Section I 

above, the concepts presented in 

Attachment A to the March 2018 

memorandum were neither guidance 

nor determined by EPA to be consistent 

with the CAA. Further, EGLE did not 

explain the relevance of the SILs 

Guidance to which it referred. This 

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, 

and it is not applicable to the good 

neighbor provision, which requires 


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9862 Federal Register / Vol. 87, No. 35 / Tuesday, February 22, 2022 / Proposed Rules 

81Michigan’s SIP submission at 16. 

82See Section 4.1’ ‘‘Overview of Modeled 

Attainment Test in EPA Modeling Guidance for 

Demonstrating Air Quality Goals for Ozone, PM2.5, 

and Regional Haze. November 2018. EPA 454–R– 

18–009. https://www.epa.gov/scram/sip-modelingguidance-documents. 

83Christopher Emery, Zhen Liu, Armistead G. 

Russel, M. Talat Odman, Greg Yarwood and Naresh 

Kumar (2017). Recommendations on statistics and 

benchmarks to assess photochemical model 

performance, Journal of the Air & Waste 

Management Association, 67:5,582–598, DOI: 

10.1080/10962247.2016.1265027. 

states to eliminate significant 

contribution or interference with 

maintenance of the NAAQS at known 

and ongoing air quality problem areas in 

other states. Further, it is not correct to 

conflate the use of the term 

‘‘significance’’ as used in the SIL 

guidance, with the term ‘‘contribution,’’ 

which is the appliable statutory term 

that EPA applies at Step 2 of the 4-step 

interstate transport framework. 

(‘‘Significance’’ within the 4-step 

framework is evaluated at Step 3 

through a multifactor analysis, for those 

states that are determined to 

‘‘contribute’’ to downwind receptors at 

Steps 1 and 2. See Section I.D.4 above.) 

Given the fundamentally different 

statutory objectives and context, EPA 

disagrees with EGLE’s contention that 

the SIL guidance is applicable in the 

good neighbor context. 

EGLE’s attempt to show ‘‘inflection 

points’’ through collectively presenting 

contribution data at each linked 

receptor and its claim that 1 ppb reflects 

the most meaningful inflection point are 

likewise not compelling. The presented 

data show a range of upwind 

contribution levels captured by different 

contribution thresholds depending on 

which receptor is analyzed. Certain 

receptors show a substantial downward 

trend in captured total upwind 

contribution well before a threshold of 

1 ppb. Therefore, EPA does not find this 

evidence supportive of a 1 ppb 

threshold. 

EPA does not accept Michigan’s 

position that a 1 percent of the NAAQS 

contribution threshold at Step 2 ‘‘may 

not be appropriate’’ for the 2015 ozone 

NAAQS due to modeling biases and 

errors.81 The explanation for how the 1 

percent contribution threshold was 

originally derived is available in the 

2011 CSAPR rulemaking. See 76 FR 

48208, 48237–38. Further, in the CSAPR 

Update, EPA re-analyzed the threshold 

for purposes of the 2008 ozone NAAQS 

and determined it was appropriate to 

continue to apply this threshold. EPA 

compared the 1 percent threshold to a 

0.5 percent of NAAQS threshold and a 

5 percent of NAAQS threshold. EPA 

found that the lower threshold did not 

capture appreciably more upwind state 

contribution compared to the 1 percent 

threshold, while the 5 percent threshold 

allowed too much upwind state 

contribution to drop out from further 

analysis. See Final CSAPR Update Air 

Quality Modeling TSD, at 27–30 (EPA– 

HQ–OAR–2015–0596–0144). If EPA 

were to apply this analysis to the 2015 

ozone NAAQS using the updated 

modeling based on the 2016v2 

emissions platform, a 5 percent of the 

NAAQS contribution threshold (i.e., 3.5 

ppb) only captures approximately 50 

percent of the total upwind 

contribution. Compared to a 1 percent 

threshold, a 5 percent threshold would, 

on average, forgo 27 percent) of the total 

upwind contribution. As EPA noted in 

the August 2018 memorandum, the use 

of a 2 ppb contribution threshold under 

the modeling released with the March 

2018 memorandum would only capture 

about 55 percent of all upwind 

contributions, and therefore ‘‘emission 

reductions from states linked at that 

higher threshold may be insufficient to 

address collective upwind state 

contribution to downwind air quality 

problems.’’31

With these figures in mind, EGLE’s 

claim based on unwarranted concerns 

over modeling uncertainty cannot be 

accepted. Both LADCO’s and EPA’s 

modeling techniques are sufficiently 

reliable and fit for the purpose to 

measure upwind contribution levels 

down to at least one 1 percent of the 

NAAQS. EPA’s recommended model 

attainment test is based on application 

of the model in a relative sense rather 

than relying upon absolute model 

predictions.82 All models have 

limitations resulting from uncertainties 

in inputs and scientific formulation. To 

minimize the effects of these 

uncertainties, the modeling is anchored 

to base period measured data in EPA’s 

guidance approach for projecting design 

values. Notably, EPA also uses our 

source apportionment modeling in a 

relative sense when calculating the 

average contribution metric (used to 

identify linkages). In this method the 

magnitude of the contribution metric is 

tied to the magnitude of the projected 

average design value which is tied to the 

base period average measured design 

value. EPA’s guidance has not 

established a bright-line criteria for 

judging whether or not statistical 

measures of model performance 

constitute acceptable or unacceptable 

model performance. So, contrary to 

what Michigan appears to be claiming 

with regards to modeling biases, there 

are no EPA recommended measures of 

allowable error. Although EPA does not 

typically focus on using particular 

benchmarks as the sole criteria for 

model performance, EPA notes that the 

model performance for the updated 

modeling based on the 2016v2 

emissions platform is generally within 

the benchmarks recommended by 

Emery.83

EPA has successfully applied a 1 

percent of the NAAQS threshold to 

identify linked upwind states in three 

prior rulemakings. And the D.C. Circuit 

has also declined to establish bright line 

criteria for model performance. In 

upholding EPA’s approach to evaluating 

interstate transport in CSAPR, the D.C. 

Circuit held that they would not 

‘‘invalidate EPA’s predictions solely 

because there might be discrepancies 

between those predictions and the real 

world. That possibility is inherent in the 

enterprise of prediction.’’ EME Homer 

City Generation, L.P. v. EPA, 795 F.3d 

118, 135 (2015). The court continued to 

note that ‘‘the fact that a ‘model does 

not fit every application perfectly is no 

criticism; a model is meant to simplify 

reality in order to make it tractable.’ ’’ 

Id. at 135–36 (quoting Chemical 

Manufacturers Association v. EPA, 28 

F.3d 1259, 1264 (D.C. Cir. 1994). 

EPA’s August 2018 memorandum 

provided that whether use of a 1 ppb 

threshold is appropriate must be based 

on an evaluation of state-specific 

circumstances, and no such evaluation 

was included in the submission. EPA’s 

experience with the alternative Step 2 

thresholds is further discussed in 

Section I.D.3.i. As discussed there, EPA 

is considering withdrawing the August 

2018 memorandum. 

Based on EPA’s updated modeling (as 

well as the LADCO’s 2018 modeling 

(with water) the state elected to rely on 

in its SIP submission), the state is 

projected to contribute greater than both 

the 1 percent and alternative 1 ppb 

thresholds. While EPA does not, in this 

action, approve of the state’s application 

of the 1 ppb threshold, based on its 

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 action on this 

SIP submission. 

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

Modeling and Findings for Michigan 

As described in Section I, EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. These data were examined to 

determine if Michigan contributes at or 

above the threshold of 1 percent of the 


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84 Design values and contributions at individual 

monitoring sites nationwide are provide in the file 

‘‘2016v2_DVs_state_contributions.xlsx’’ which is 

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

0663. 

85These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I. That 

modeling showed that Illinois 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 & Contributions Revised 

CSAPR Update.xlsx’’ in docket ID No. EPA–HQ– 

OAR–2021–0663. 

2015 ozone NAAQS (0.70 ppb) to any 

downwind nonattainment or 

maintenance receptor. As shown in 

Table 4, the data 84 indicate that in 2023, 

emissions from Michigan contribute 

greater than one percent of the standard 

to nonattainment or maintenance-only 

receptors in Illinois, Connecticut, 

Wisconsin, and Pennsylvania.85

TABLE 4—MICHIGAN LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location 

(county, state) 

Nonattainment/ 

maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Michigan 

contribution 

(ppb) 

170314201 ........................... Cook, IL ............................... Maintenance ........................... 69.9 73.4 1.67

170310076 ........................... Cook, IL ............................... Maintenance ........................... 69.3 72.1 1.54

90099002 ............................. New Haven, CT ................... Nonattainment ......................... 71.8 73.9 1.27 

170317002 ........................... Cook, IL ............................... Maintenance ........................... 70.1 73.0 1.26

170310032 ........................... Cook, IL ............................... Maintenance ........................... 69.8 72.4 1.21

550590025 ........................... Kenosha, WI ........................ Maintenance ........................... 69.2 72.3 1.17 

550590019 ........................... Kenosha, WI ........................ Nonattainment ......................... 72.8 73.7 1.07 

90010017 ............................. Fairfield, CT ......................... Nonattainment ......................... 73.0 73.7 1.07

551010020 ........................... Racine, CT ........................... Nonattainment ......................... 71.3 73.2 1.02 

90013007 ............................. Fairfield, CT ......................... Nonattainment ......................... 74.2 75.1 0.94

170310001 ........................... Cook, IL ............................... Maintenance ........................... 69.6 73.4 0.93

90019003 ............................. Fairfield, CT ......................... Nonattainment ......................... 76.1 76.4 0.92

420170012 ........................... Bucks, PA ............................ Nonattainment ......................... 70.7 72.2 0.75 

Therefore, based on EPA’s evaluation 

of the information submitted by EGLE, 

and based on EPA’s most recent 

modeling results for 2023, EPA proposes 

to find that Michigan 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. EPA 

therefore will proceed to Step 3 of the 

4-step interstate transport framework to 

assess the arguments the state presented 

as to why, despite this linkage, the state 

should not be considered to 

significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS in any other 

state. 

4. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

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

To effectively evaluate which 

emissions in the state should be deemed 

‘‘significant’’ and therefore prohibited, 

states generally should prepare an 

accounting of sources and other 

emissions activity for relevant 

pollutants and assess potential, 

additional emissions reduction 

opportunities and resulting downwind 

air quality improvements. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner 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 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. 

EGLE did not conduct a sufficient 

step 3 analysis in Michigan’s SIP 

submission. As explained previously, at 

Step 3 EGLE instead applied a weight of 

evidence analysis to argue that the state 

needed no additional emission 

reductions despite concluding Michigan 

was linked to three receptors at Step 2. 

The evidence presented in EGLE’s 

submittal consisted primarily of support 

for the argument that upwind states 

should have a lower responsibility to 

other states when the upwind state is 

only linked to maintenance receptors. 

EGLE’s analysis focused on the 

Sheboygan, Wisconsin maintenance 

receptor (Site ID: 36–081–0124), as 

EGLE concluded it was the receptor to 

which Michigan was projected to 

contribute the most in 2023 at 1.85 ppb. 

EGLE also relied on several ideas in 

Attachment A to the March 2018 

memorandum to further discount the 

importance of its own emissions. As 

noted in Section I, the ideas listed in 

Attachment A to the March 2018 

memorandum were not agency guidance 

nor had EPA determined them to be 

consistent with the requirements of the 

CAA. EPA will thoroughly review the 

technical and legal justifications ELGE 

made put forward in their attempt to use 

them as flexibilities. 

In its submittal, EGLE cited a concept 

in Attachment A to the March 2018 

memorandum to ‘‘[c]onsider whether 

the remedy for upwind states linked to 

maintenance receptors could be less 


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86See Michigan SIP submission p. 20. 87See Michigan SIP submission p. 32. 

stringent than those linked to 

nonattainment receptors’’ and argued 

that because the CAA incudes different 

SIP development requirements for 

nonattainment and maintenance areas, 

that likewise nonattainment and 

maintenance areas should be treated 

differently in good neighbor SIPs. EGLE 

posited that because the CAA does not 

require emission reductions from 

maintenance areas, then upwind states 

can potentially make a sufficient 

showing they have no obligation to 

reduce emissions to monitors in other 

states projected to be maintaining the 

NAAQS. EGLE specifically noted that 

(1) the projected exceedance at the 

Sheboygan, Wisconsin receptor is very 

small, (2) the majority of the projected 

contribution to the Sheboygan, 

Wisconsin receptor is from federally 

regulated sources or sources Michigan 

cannot otherwise regulate, (3) 

Michigan’s projected contribution to all 

three linked receptors is small 

compared to the projected contribution 

from other states and sources, (4) there 

are large projected contributions to the 

Sheboygan, Wisconsin receptor from 

international emissions, (5) Michigan’s 

contributions to projected exceedance at 

the three maintenance receptors are 

small relative to other sources that also 

contribute more than 1 ppb to those 

receptors, (6) the modeling variability is 

greater than Michigan’s contributions to 

the amount of the projected exceedance 

at each linked receptor and (7) there is 

a downward emissions trend in 

Michigan. 

As a general matter, EPA disagrees 

with EGLE’s premise that if no emission 

reductions are needed for the receptor to 

which Michigan contributes the most, 

that automatically no emission 

reductions are needed for the other 

receptors to which Michigan is linked. 

EGLE unreasonably failed to analyze 

receptor-specific circumstances present 

at other receptors to which it was 

linked, and this is particularly the case 

because EGLE chose to rely so heavily 

on receptor-specific information to 

support their conclusions with respect 

to the Sheboygan receptor. Further, 

while the set of receptors to which 

Michigan is linked has changed in the 

most recent modeling (and now 

includes nonattainment receptors), EPA 

disagrees with Michigan’s arguments to 

the extent such reasoning could be 

applied to Michigan’s linkages 

identified in EPA’s 2016v2 emissions 

platform modeling. 

EGLE argued that because the 

Sheboygan, Wisconsin receptor, had a 

small projected exceedance over the 

NAAQS, requiring additional emission 

reductions in Michigan would be 

‘‘premature’’ and ‘‘burdensome.’’ 86

EGLE’s premise goes beyond the 

concept in Attachment A to the 2018 

memorandum that emission-reduction 

obligations as to maintenance receptors 

may be different; rather, EGLE argues 

that not only should Michigan have 

lower obligations with respect to 

maintenance receptors, but no 

obligations at all. Under the D.C. 

Circuit’s decision in North Carolina, 

states and EPA are required to give 

independent significance to the 

‘‘interference with maintenance’’ prong 

of section 110(a)(2)(D)(i)(I). 531 F.3d at 

910. Since CSAPR, EPA’s nationally 

consistent policy framework for 

addressing interstate ozone transport 

has given meaning to this prong through 

a separate definition of maintenance 

receptors at step 1 of the 4-step 

interstate transport framework. For 

states linked only to those receptors, 

EPA has found it appropriate to apply 

an emissions control solution that is 

uniform with the strategy applied for 

states that are linked to nonattainment 

receptors. See 76 FR at 48271. EPA’s 

approach to addressing interference 

with maintenance under prong 2 for 

ozone NAAQS has been upheld twice, 

including on remand from the Supreme 

Court decision EGLE cited. See EME 

Homer City Generation, L.P., 795 F.3d at 

136; Wisconsin, 938 F.3d at 325–27. See 

also 86 FR at 23074. Particularly given 

this context, Michigan’s SIP does not 

provide sufficient evidence to support 

less stringent or even no standards of 

emissions reductions relative to what 

would result from EPA’s historical 

approach of addressing emissions 

activities from upwind states that are 

linked to maintenance-only receptors. 

Further, EPA believes it would be 

inconsistent with the CAA for EPA to 

identify receptors that are at risk of 

NAAQS violations given certain 

conditions due to transported upwind 

emissions and then not prohibit the 

emissions that place the receptor at risk. 

The Supreme Court held that it was a 

permissible interpretation of the statute 

to apportion responsibility for states 

linked to nonattainment receptors 

considering ‘‘both the magnitude of 

upwind states’ contributions and the 

cost associated with eliminating them.’’ 

EME Homer City, 572 U.S. at 518–19. It 

is equally reasonable and permissible to 

use these factors to apportion 

responsibility among upwind states 

linked to maintenance receptors because 

the goal in both instances is to prohibit 

the ‘‘amounts’’ of pollution that will 

either significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS downwind. 

See Id. 515 n.18 (finding EPA’s uniformcost approach reasonable as to both 

prongs of the good neighbor provision). 

EPA’s updated modeling indicates that 

Michigan will remain linked to 

downwind nonattainment and 

maintenance receptors for the 2015 

ozone standard at least through 2023. 

Consequently, EPA believes EGLE’s 

assertion that upwind states linked to 

maintenance-only receptors should be 

held to less stringent standards of 

emissions reductions (as compared to 

states linked to a nonattainment 

receptor) is inappropriate, whether 

applied to its downwind linkages in 

either the modeling EGLE relied on or 

in EPA’s more recent modeling. 

EGLE also claimed that Attachment A 

to the March 2018 memorandum 

suggested states linked only to 

maintenance receptors should consider 

whether emissions reduction factors 

should be influenced by high 

international contributions and high 

contributions from other states and 

sources. As a concept presented by 

outside parties, Attachment A to the 

March 2018 memorandum listed an idea 

that states may consider whether air 

quality, cost, or emission reduction 

factors should be weighted differently in 

areas where international contributions 

are relatively high. EPA did not at the 

time endorse this concept, nor does it 

do so now. However, EGLE did not 

present an approach or explain how 

international contributions to the linked 

receptors should influence the 

weighting of air quality, cost, or 

emission reductions at Step 3. Rather, 

EGLE suggested that if a receptor is near 

an international border, then 

international contribution could simply 

be removed from that monitor’s 

projected design value. This is neither 

appropriate nor acceptable under the 

good neighbor provision or any other 

provision of the Clean Air Act. 

Michigan’s approach effectively takes 

the position that no air quality problem 

should be deemed to exist at a 

downwind receptor location under the 

false assumption that the international 

portion of emissions affecting that area 

simply do not exist. EPA categorically 

rejects this approach as an entirely 

unacceptable form of air quality 

planning. 

EGLE further cited contributions from 

other states and sources to the linkages 

it identified to conclude it would be 

‘‘unreasonable’’ for linked states with 

relatively low contributions to reduce 

their contributions.87 The Step 2 

threshold (whether at 1 percent or 1 


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ppb) is intended to reflect the 

‘‘collective contribution’’ nature of the 

interstate ozone transport problem and 

the complexity of the various linkages 

among states. Cf. EME Homer City, 572 

U.S. at 515–16. The threshold functions 

as a screening step toward a more 

detailed analysis of emission-reduction 

opportunities across all of the states that 

contribute to some extent (i.e., above the 

threshold) to a downwind air quality 

problem. To simply conclude that 

nothing need be done regarding 

emissions that exceed the step 2 

threshold because those emissions can 

be characterized as ‘‘small’’ compared to 

others’ emissions (by the upwind state’s 

lights at least) is an attempt to simply 

move the ‘‘contribution’’ threshold at 

Step 2 and is clearly insufficient at Step 

3. 

Whether emissions from other states 

or other countries also contribute to the 

same downwind air quality issue is 

irrelevant in assessing whether a 

downwind state has an air quality 

problem, or whether an upwind state is 

significantly contributing to that 

problem. States are not obligated under 

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

emissions sufficient on their own to 

resolve downwind receptors’ 

nonattainment or maintenance 

problems. Rather, states are obligated to 

eliminate their own ‘‘significant 

contribution’’ or ‘‘interference’’ with the 

ability of other states to attain or 

maintain the NAAQS. 

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 good neighbor obligations on the 

basis that some other source of 

emissions (whether international or 

another upwind state) could be 

considered the ‘‘but-for’’ cause of 

downwind air quality problem. 938 F.3d 

at 323–324. The court viewed 

petitioners’ arguments as essentially an 

argument ‘‘that an upwind state 

‘contributes significantly’ to downwind 

nonattainment only when its emissions 

are the sole cause of downwind 

nonattainment.’’ 938 F.3d 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 sources also 

contribute some amount of pollution to 

the same receptors to which the state is 

linked. Thus, the state’s arguments 

related to contributions from other 

sources, including removing 

international emissions from projected 

design values at the Sheboygan, 

Wisconsin monitor, are insufficient at 

Step 3 of the analysis. 

EGLE’s submission included an 

apportionment analysis to quantify 

individual states’ relative responsibility 

of the projected exceedances at the three 

linked receptors. EGLE cited 

Attachment A to the March 2018 

memorandum as well as EME Homer 

City Generation to suggest Michigan 

could be found to be only responsible 

for eliminating its share of the projected 

exceedances relative to other states that 

also contribute more than 1 ppb to the 

same receptors. We first note that this 

approach is a deviation from EPA’s 

traditional approach of apportioning 

upwind-state responsibility at Step 3 

using a uniform cost of control metric 

set at a level that maximizes costeffectiveness of emissions reductions in 

relation to downwind state impacts 

across all linked states. Thus, this is not 

how EPA has interpreted the statutory 

term ‘‘significant’’ in the past, and EPA 

does not reach a conclusion whether 

this approach would be approvable, had 

EGLE had imposed emissions 

reductions in line with this logic. We do 

not need to reach that point in the 

analysis, however, because, having 

selected that approach to defining its 

obligations, EGLE proceeded to ignore 

the result. 

EGLE’s submission identified 

Michigan’s proportional contribution as 

less than 0.12 ppb to the three linked 

receptors and .05 ppb to the Sheboygan, 

Wisconsin receptor. Having 

acknowledged Michigan was 

responsible for eliminating up to 0.12 

ppb of contribution to the downwind 

receptors, EGLE claimed that modeling 

‘‘noise’’ made it ‘‘difficult’’ to require 

that amount of reductions from 

Michigan sources. EGLE further opined 

that the downwind jurisdiction’s share 

of responsibilities likely made 

Michigan’s contributions even lower 

and the projected exceedances were so 

small that those three receptors were 

likely to not have difficulty attaining the 

NAAQS anyway. EPA has routinely 

been capable of successfully 

implementing good neighbor obligations 

through the CSAPR framework, and 

achieving significant downwind air 

quality improvements through upwindstate reductions, at levels of ‘‘significant 

contribution’’ comparable or even less 

than those found in Michigan’s 

submittal, irrespective of alleged 

‘‘modeling noise.’’ See Wisconsin, 938 

F.3d at 322–23 (rejecting Wisconsin’s 

argument that it should not face good 

neighbor obligations on the basis that its 

emission reductions would only 

improve a downwind receptor by two 

ten-thousandths of a part per billion). 

After measuring Michigan’s 

significant contribution, EGLE suggested 

that modeling uncertainty was too great 

to either require emissions reductions or 

demonstrate that EGLE had any linkages 

to maintenance receptors at all. But 

EGLE had measured the state’s 

significant contribution and was 

therefore identifying the measurable 

amount of significant contribution the 

state was legally responsible for 

eliminating. See Michigan v. EPA, 213 

F.3d 663, 683–84 (D.C. Cir. 2000) 

(significant contribution must be 

‘‘measurable’’). Further, scientific 

uncertainty may only be invoked to 

avoid comporting with the requirements 

of the CAA when ‘‘the scientific 

uncertainty is so profound that it 

precludes . . . reasoned judgment’’ 

Massachusetts v. EPA, 127 S.Ct. 1438 

(2007). See Wisconsin, 938 F.3d at 318– 

19 (‘‘Scientific uncertainty, however, 

does not excuse EPA’s failure to align 

the deadline for eliminating upwind 

States’ significant contributions with the 

deadline for downwind attainment of 

the NAAQS.’’). See also EME Homer 

City, 795 F.3d 118, 135–36 (‘‘We will 

not invalidate EPA’s predictions solely 

because there might be discrepancies 

between those predictions and the real 

world. That possibility is inherent in the 

enterprise of prediction.’’). EGLE’s 

arguments related to modeling 

uncertainty or ‘‘noise’’ do not establish 

a level of uncertainty so high as to 


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88See 81 FR 74504 at 74517; 85 FR 68964 at 

68979. 

89 Detailed information and documentation of 

EPA’s Base Case, including all the underlying 

assumptions, data sources, and architecture 

parameters can be found on EPA’s website at: 

https://www.epa.gov/airmarkets/epas-power-sectormodeling-platform-v6-using-ipm-summer-2021- 

reference-case. 

90We note that for one of the units EGLE listed 

as projected to retire, Wyandotte—Unit 5, this 

facility was still included in the NEEDS as 

operating. Additionally, the unit IDs listed by EGLE 

in the SIP submittal may be different from those 

listed in EPA’s NEEDS v6 inventory—NEEDS v6 

Summer 2021 Reference Case, however we have 

verified that these emissions decreases have been 

accounted for in our most recent modeling. 

91See Technical Support Document (TSD) 

Preparation of Emissions Inventories for the 2016v2 

North American Emissions Modeling Platform 

included in the Headquarters docket ID No. EPA– 

HQ–OAR–2021–0663. 

92Notably, in focusing its Step 3 analysis only on 

a single receptor, EGLE gave no weight to the scope 

of its contribution to downwind air quality 

problems. Linkages to thirteen receptor sites in 

EPA’s most recent modeling indicate that 

Michigan’s emissions have widespread effects in 

other states—effects that the State’s SIP submittal 

would do nothing to address. 

preclude reasoned judgement. EGLE 

argued that the three maintenance 

receptors at issue could maintain the 

NAAQS without further emissions 

reductions from any linked upwind 

state. In support, EGLE’s submission 

provided a list of on-the-way and onthe-books emission reductions measures 

to argue that Michigan’s good neighbor 

obligations were already satisfied. EGLE 

provided references to certain facility 

retirements in Michigan, Federal mobile 

source rules, Federal rules reducing 

NOX and VOCs such as MATS and the 

Oil and Natural Gas Industry Standards, 

the NOX SIP Call, and CSAPR Update. 

EPA’s assessment of future air quality 

conditions generally already accounts 

for on-the-books emission reductions 

and the most up-to-date forecast of 

future emissions in the absence of the 

transport policy being evaluated (i.e., 

base case conditions).88 As described in 

more detail in Section I, EPA’s latest 

projections of the baseline EGU 

emissions uses the version 6—Summer 

2021 Reference Case of the IPM.89 The 

IPM version 6—Summer 2021 Reference 

Case uses the NEEDS v6 database as its 

source for data on all existing and 

planned-committed units. Units are 

removed from the NEEDS inventory 

only if a high degree of certainty could 

be assigned to future implementation of 

the announced future closure or 

retirement. Any retirements excluded 

from the NEEDS v6 inventory can be 

viewed in the NEEDS spreadsheet.90

The inventory for these projections 

takes account of the retirement of the 

Marquette Board of Light & Power 

Shiras Steam Plant, Lansing Board of 

Water and Light, Eckert Station, Units 1 

and 3–6; DTE, River Rouge, Unit 3; We 

Energies, Presque Isle Power Plant, 

Units 5–9; DTE St. Clair, Units 1–4 and 

6–7; DTE Trenton Channel, Unit 9; 

Wyandotte, Unit 5; Consumers Energy 

Karn, Units 1–2. 

Additionally, EPA’s modeling using 

the 2016v2 emissions platform accounts 

for the onroad and nonroad rules that 

Michigan identified, such as the Tier 3 

Motor Vehicle Emission and Fuel 

Standards, to the extent still on the 

books and projected to have ozoneprecursor emissions consequences.91

In other words, changes in the 

emissions inventory and on-the-books 

controls relevant to emissions in 2023 

that EGLE claims EPA missed in its 

prior modeling have now been 

incorporated into EPA’s most recent 

modeling of 2023 using the 2016v2 

emissions platform. This modeling 

projects a continuing contribution from 

Michigan to thirteen out-of-state 

receptors above a threshold of 1 percent 

of the NAAQS (at Steps 1 and 2) despite 

these measures—nine of which have 

contribution from Michigan above 1 ppb 

and seven of which are nonattainment 

receptors (see Table 4).92 Therefore, in 

light of continuing contribution to out of 

state receptors from Michigan 

notwithstanding these identified on-thebooks control measures, EGLE’s SIP 

submission should have evaluated the 

availability of additional air quality 

controls to improve downwind air 

quality at nonattainment and 

maintenance receptors at Step 3. 

Nor does EGLE’s listing of existing 

control measures or overall emission 

trends serve as an adequate substitute 

for a Step 3 analysis of additional 

potential emission reductions. In 

general, the listing of existing or on-theway control measures, whether 

approved into the State’s SIP or not, 

does not substitute for a complete step 

3 analysis under EPA’s 4-step 

framework to define ‘‘significant 

contribution.’’ ELGE did not provide an 

assessment of the overall effects of these 

measures, when the emissions 

reductions would be achieved, and what 

the overall resulting air quality effects 

would be at identified out of state 

receptors. EGLE did not identify which 

portion of ongoing emissions trends 

were not already accounted for in steps 

1 and 2 of the analysis (EPA addresses 

specific identified changes in emissions 

inventory in the discussion above). 

EGLE did not evaluate additional, 

potential emissions control 

opportunities, or their costs or impacts, 

or attempt to analyze whether, if 

applied more broadly across linked 

states, the emissions reductions would 

constitute the elimination of significant 

contribution on a regional scale. The 

state did not offer an explanation as to 

whether any faster or more stringent 

emissions reductions that may be 

available were prohibitively costly or 

infeasible. Although EPA acknowledges 

states are not necessarily bound to 

follow its own analytical framework at 

step 3, we note that the state did not 

attempt to determine or justify an 

appropriate uniform cost-effectiveness 

threshold for the more stringent 2015 

ozone NAAQS. This would have been 

similar to the approach to defining 

significant contribution that EPA has 

applied in prior rulemakings such as 

CSAPR and or the CSAPR Update, even 

if such an analysis is not technically 

mandatory. 

Further, the state’s attempt to 

categorize certain sectors of emissions 

as beyond its regulatory control is 

unpersuasive. Clearly the state 

possesses regulatory authority over its 

EGU and non-EGU large stationary 

sources as well as authority over other 

types of ‘‘emissions activity within the 

state,’’ see CAA section 110(a)(2)(D)(i). 

And while mobile sources are generally 

regulated at the Federal level under title 

II of the Clean Air Act, the state also has 

the authority to undertake any number 

of measures to reduce emissions from 

mobile sources through means and 

techniques that are not preempted by 

title II. See, e.g., CAA sections 182(b)(3), 

182(b)(4), 182(c)(3), 182(c)(4), 182(c)(5), 

182(d)(1), 182(e)(3), and 182(e)(4) 

(identifying programs to control mobile 

source emissions that states are required 

to implement depending on the degree 

of ozone nonattainment). Specifically 

with respect to EGUs, EPA notes that no 

EGU NOX control program has yet been 

established to implement good neighbor 

requirements for the 2015 ozone 

NAAQS. Thus reliance on prior 

programs, such as the CSAPR Update or 

Revised CSAPR Update, is misplaced, 

since those programs only addressed 

good neighbor obligations under the less 

stringent 2008 ozone NAAQS. 

Finally, under the Wisconsin 

decision, states and EPA may not delay 

implementation of measures necessary 

to address good neighbor requirements 

beyond the next applicable attainment 

date without a showing of impossibility 

or necessity. See 938 F.3d at 320. In 

those cases where the measures 

identified by Michigan had 

implementation timeframes beyond the 

next relevant attainment dates, the 

submission did not offer a 

demonstration of impossibility of earlier 

implementation of those control 

measures that would go into effect after 


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2024. Similarly, the State’s submittal is 

insufficient to the extent the 

implementation timeframes for 

identified control measures were left 

unidentified, unexplained, or too 

uncertain to permit EPA to form a 

judgment as to whether the timing 

requirements for good neighbor 

obligations have been met. 

For the reasons listed above, EPA 

proposes to find that Michigan has not 

satisfied its obligations of the good 

neighbor SIP provisions at Step 3 of the 

4-step transport framework. We propose 

that Michigan was required to analyze 

emissions more fully 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 Michigan failed to 

do so. 

5. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-step interstate 

transport framework calls for 

development of permanent and 

federally enforceable control strategies 

to achieve the emissions reductions 

determined to be necessary at Step 3 to 

eliminate significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. EGLE 

provided references to on the books and 

on the way Federal mobile source rules, 

MATS and the Oil and Natural Gas 

Industry Standards, the NOX SIP Call, 

and CSAPR Update. As an initial matter, 

pointing to or listing existing state or 

Federal control measures is not what is 

called for at Step 4. Rather Step 4 

requires the development of permanent 

and enforceable measures to implement 

those measures determined to be 

required at Step 3. EGLE claimed that 

nothing was required of Michigan at 

Step 3 and thus EGLE stated that it did 

not believe anything was required at 

Step 4. Therefore, we do not interpret 

the list of existing state or Federal 

measures to be EGLE’s attempt at 

implementation at Step 4. 

Because Michigan’s SIP submission 

did not contain an evaluation of 

additional emission control 

opportunities (or establish that no 

additional controls are required), no 

information was provided at Step 4. As 

a result, EPA proposes to disapprove 

Michigan’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). 

6. Conclusion 

Based on EPA’s evaluation of EGLE’s 

SIP submission, EPA is proposing to 

find that the portion of Michigan’s 

March 5, 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. 

D. Minnesota 

1. Evaluation of Information Provided 

by Minnesota Regarding Steps 1 and 2 

At Step 1 of the 4-step interstate 

transport framework, Minnesota relied 

on both LADCO modeling and EPA 

modeling released in the March 2018 

memorandum and to identify 

nonattainment and maintenance 

receptors in 2023. As described 

previously, LADCO performed a 

modeling demonstration like that of 

EPA’s 2018 transport modeling, except 

with use of the ERTAC EGU Tool to 

supplement state specific EGU 

information. LADCO identified 

nonattainment and maintenance 

receptors using EPA methodology. 

MPCA presented several nonattainment 

and maintenance receptors identified by 

both LADCO modeling, showing ‘‘no 

water’’ and ‘‘with water’’ results and 

EPA modeling released with the March 

2018 memorandum. Since new 

modeling has been performed by EPA 

with updated emission data, EPA 

proposes to primarily rely on the most 

recent modeling to identify 

nonattainment and maintenance 

receptors in 2023. MPCA made several 

criticisms of EPA’s method for 

projecting EGU emissions in EPA’s 

modeling released with the March 2018 

memorandum. Although EPA does not 

agree with those criticisms, we note that 

EPA is relying on a different method for 

projecting emissions from EGUs in the 

updated modeling using the 2016v2 

emissions platform as explained in more 

detail in Section I. 

Nonetheless, the alternative modeling 

relied on by MPCA also identified a 

number of nonattainment and 

maintenance receptor sites in 2023. See 

Tables 2 and 3 on pages 8 and 9 of 

MPCA’s submittal. Thus, even under the 

alternative modeling of 2023, MPCA 

acknowledges in its submittal the 

existence of several nonattainment and 

maintenance receptors. 

At Step 2 of the 4-Step interstate 

transport framework, MPCA relied on 

both LADCO modeling and EPA 

modeling released in the March 2018 

memorandum to identify upwind state 

linkages to nonattainment and 

maintenance receptors in 2023. Based 

on both modeling results, MPCA 

concluded that Minnesota would 

contribute below 1 percent of the 

NAAQS to receptors in 2023. However, 

in this proposal, EPA relies on the 

Agency’s most recently available 

modeling, which uses a more recent 

base year and more up-to-date emissions 

inventories, to identify upwind 

contributions and ‘‘linkages’’ to 

downwind air quality problems in 2023 

using a threshold of 1 percent of the 

NAAQS. As shown in Table 5 

(explained in the next section), the 

updated EPA modeling identifies 

Minnesota’s maximum contribution to a 

downwind nonattainment or 

maintenance receptor is greater than 1 

percent of the standard (i.e., 0.70 ppb). 

Although the state did not rely on a 1 

ppb contribution threshold in its SIP 

submittal, EPA recognizes that the 

modeling the MPCA used relied on the 

most recently available EPA modeling at 

the time the state submitted its SIP 

submittal (EPA modeling released in the 

March 2018 memorandum as well as the 

LADCO modeling). The 2018 modeling 

indicated the state was not projected to 

contribute above one 1 percent of the 

NAAQS to a projected downwind 

nonattainment or maintenance receptor. 

Therefore, the state may not have 

considered analyzing the reasonableness 

and appropriateness of a 1 ppb 

threshold at Step 2 of the 4-step Step 

interstate transport framework per the 

August 2018 memorandum. EPA’s 

August 2018 memorandum provided 

that whether use of a 1 ppb threshold 

is appropriate must be based on an 

evaluation of state-specific 

circumstances, and no such evaluation 

was included in the submission. EPA’s 

experience with the alternative Step 2 

thresholds is further discussed in 

Section I.D.3.i. As discussed there, EPA 

is considering withdrawing the August 

2018 memorandum. 

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

Modeling and Findings for Minnesota 

As described in Section I, EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. These data were examined to 

determine if Minnesota 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 


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93 Design values and contributions at individual 

monitoring sites nationwide are provide in the file: 

2016v2_DVs_state_contributions.xlsx which is 

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

0663. 

94These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I. That 

modeling showed that Minnesota 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. 

95See Minnesota’s SIP submittal Figures 1–3, 

pages 10–11. 

96For a complete explanation of air quality 

modeling of the 2016v2 emissions platform 

modeling, please see ‘‘AQ Modeling TSD_2016v2 

Platform.pdf’’ included in docket ID No. EPA–HQ– 

OAR–2021–0663. 

Table 5, the data 93 indicate that in 2023, 

emissions from Minnesota contribute 

greater than 1 percent of the standards 

to two maintenance-only receptors in 

Illinois. These modeling results are 

consistent with the results of a prior 

round of 2023 modeling using the 

2016v1 emissions platform that became 

available to the public in the fall of 2020 

in the Revised CSAPR Update, as noted 

in Section I, which showed that 

Minnesota had a maximum contribution 

of 0.86 ppb to a nonattainment or 

maintenance receptor in 2023.94

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

Receptor ID Location Nonattainment/maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Minnesota 

contribution 

(ppb) 

170310001 ............................. Cook ..................................... Maintenance ......................... 69.6 73.4 0.97 

170310076 ............................. Cook ..................................... Maintenance ......................... 69.3 72.1 0.79 

Based on EPA’s evaluation of the 

information submitted by MPCA, and 

based on EPA’s most recent modeling 

results for 2023 using the 2016v2 

emissions platform, EPA proposes to 

find that Minnesota 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. Despite 

the linkage EPA determines exists at 

Step 2, the state concluded in its 

submission based on other factors that 

it should not be considered to 

significantly contribute to 

nonattainment or interfere with 

maintenance of the NAAQS in other 

states. Therefore, EPA will proceed to 

evaluate MPCA’s additional analyses at 

Step 3 of the 4-Step interstate transport 

framework. 

4. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-Step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

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

To effectively evaluate which 

emissions in the state should be deemed 

‘‘significant’’ and therefore prohibited, 

states generally should prepare an 

accounting of sources and other 

emissions activity for relevant 

pollutants and assess potential, 

additional emissions reduction 

opportunities and resulting downwind 

air quality improvements. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner 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 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. MPCA did not conduct such 

an analysis in their SIP submission. 

Neither the LADCO modeling nor 

EPA modeling released with the March 

2018 memorandum indicated that 

Minnesota would contribute over 1 

percent of the NAAQS to any 

nonattainment or maintenance receptor 

in 2023. Therefore, MPCA stated they 

did not consider it necessary to consider 

further emission reductions because 

Minnesota was not projected to 

contribute to downwind air quality 

issues above the contribution threshold. 

Despite this, Minnesota provided 

supporting analysis to strengthen the 

conclusions of the modeling results. 

MPCA presented evidence that ambient 

ozone concentrations in Minnesota had 

been at or below the NAAQS from the 

late 1990s to 2017, and that NOX and 

VOCs emissions had been steadily 

decreasing from 2002 through 2015. 

MPCA asserted that these trends would 

translate to continued reductions in 

ozone being transported from the state 

to nonattainment or maintenance 

receptors. Additionally, MPCA listed 

several state and Federal regulatory 

programs that control or incentivize 

NOX and VOC limits, including the 

CSAPR NOX trading program. 

EPA does not dispute the evidence 

about ambient ozone concentrations and 

NOX and VOC emissions trends or 

existence of the NOX and VOC controls 

presented by Minnesota.95 However, as 

explained in Section I.C, the most recent 

EPA modeling captures numerous 

updates to the 2016 emissions platform, 

including all existing CSAPR trading 

programs, in the baseline,96 and that 

modeling confirms that most these 

control programs were not sufficient to 

eliminate Minnesota’s linkage at Steps 1 

and 2 under the 2015 ozone NAAQS. 

The state therefore has good neighbor 

obligations under the 2015 8-hour 

NAAQS and is obligated at Step 3 to 

assess additional control measures using 

a multifactor analysis. 

MPCA identified state permitting 

programs, rules, voluntary programs, 

and the CSAPR NOX trading program, 


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among others, as NOX and VOC control 

measures which satisfy Minnesota’s 

good neighbor obligations under the 

2015 ozone NAAQs. In general, 

however, the listing of existing or onthe-way control measures, whether 

approved into the state’s SIP or not, 

does not substitute for a complete Step 

3 analysis under EPA’s 4-step 

framework to define ‘‘significant 

contribution.’’ Minnesota’s submission 

does not include an assessment of the 

overall effects of these measures, when 

the reductions would be achieved, and 

what the overall resulting air quality 

effects would be observed at identified 

out-of-state receptors. Minnesota’s 

submission does not include an 

evaluation of additional potential 

emissions control opportunities, or their 

costs or impacts, or attempt to analyze 

whether, if applied more broadly across 

linked states, the emissions reductions 

would constitute the elimination of 

significant contribution on a regional 

scale. The state’s submission did not 

contain an explanation as to whether 

any faster or more stringent emissions 

reductions that may be available were 

prohibitively costly or infeasible. 

Furthermore, states may not rely on 

non-SIP measures to meet SIP 

requirements, and Minnesota has not 

revised its SIP to contain the CSAPR 

NOX trading program or the non-SIP 

approved rules MPCA identified. 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). 

As mentioned above, EPA has newly 

available information that indicates 

sources in Minnesota are linked to 

downwind air quality problems for the 

2015 ozone standard. Therefore, EPA 

proposes to disapprove Minnesota’s 

August 20, 2018 interstate transport SIP 

submission on the separate, additional 

basis that the SIP submittal did not 

assess additional emissions control 

opportunities. 

5. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-Step interstate 

transport frameworks 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. MPCA 

identified state permitting programs, 

rules, voluntary programs, and the 

CSAPR NOX trading program, among 

others, as NOX and VOC control 

measures which are not all part of 

Minnesota’s SIP. Although the state has 

since incorporated some of these control 

measures into their SIP, Minnesota did 

not revise its SIP to include all these 

emission reductions in a revision to its 

SIP to ensure the reductions were 

permanent and enforceable and 

eliminate their significant contribution 

to nonattainment or interference with 

maintenance of the NAAQS. As a result, 

EPA proposes to disapprove 

Minnesota’s submittal on the separate, 

additional basis that the Minnesota has 

not developed permanent and 

enforceable emissions reductions 

necessary to meet the obligations of 

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

6. Conclusion 

Based on EPA’s evaluation of 

Minnesota’s SIP submission and after 

consideration of updated EPA modeling 

using the 2016-based emissions 

modeling platform, EPA is proposing to 

find that the portion of Minnesota’s 

October 1, 2018 SIP submission 

addressing CAA section 

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

state’s interstate transport obligations 

for 2015 ozone NAAQS, because it fails 

to contain the necessary provisions to 

eliminate emissions that will contribute 

significantly to nonattainment or 

interfere with maintenance of the 

NAAQS in any other state. 

E. Ohio 

1. Evaluation of Information Provided 

by Ohio Regarding Steps 1 

At Step 1 of the 4-step interstate 

transport framework, OEPA relied on 

LADCO modeling released in 2018 to 

identify nonattainment and 

maintenance receptors in 2023. As 

described previously in this action, 

LADCO performed modeling similar to 

EPA’s modeling released in the March 

2018 memorandum, except with use of 

ERTAC for projecting future year EGU 

emissions. LADCO identified 

nonattainment and maintenance 

receptors using EPA methodology. 

OEPA elected to rely on LADCO’s ‘‘3x3’’ 

modeling results, which identified 

similar receptors to EPA’s modeling 

included in the March 2018 

memorandum. 

However, OEPA elected to use an 

alternative method developed by TCEQ 

for identifying maintenance receptors at 

Step 1 of the 4-step framework. Using 

the TCEQ method to identify 

maintenance receptors OEPA claimed 

that four maintenance receptors based 

on EPA’s approach would not have 

difficulty maintaining the NAAQS in 

2023. OEPA relied on the potential 

flexibilities in Attachment A to the 

March 2018 in support of its use of the 

TCEQ method. As explained in Section 

I.C above, the concepts presented in 

Attachment A to the March 2018 

memorandum were neither guidance 

nor determined by EPA to be consistent 

with the CAA. OEPA submitted Ohio’s 

SIP submission before EPA released its 

October 2018 memorandum discussing 

maintenance receptors. Regardless, EPA 

will examine the legal and technical 

merits of OEPA’s arguments related to 

the use of an alternative maintenanceonly definition in light of the October 

2018 memorandum. OEPA has not 

adequately explained or justified how 

TCEQ’s method for identifying 

maintenance receptors reasonably 

identifies areas that will have difficulty 

maintaining the NAAQS. That is, EPA 

proposes to find that OEPA has 

provided no sound technical basis for 

how TCEQ’s 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 EPA’s CAIR on the basis 

that 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 EPA’s rule did not 

adequately protect ‘‘[a]reas that find 

themselves barely meeting attainment.’’ 

Id. at 910. Consequently, 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 


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97TCEQ submission at 3–39 to 3–40. 

98See ‘‘2010 Thru 2020 Ozone Design 

Values.xlsx’’ in docket ID No. EPA–HQ–OAR– 

2021–0663. 

particularized showing of overcontrol. 

Id. at 327. 

In order to explain the differences 

between TCEQ’s and EPA’s 

methodology for identifying 

maintenance receptors, it is helpful to 

provide some additional context for 

how EPA projects future air quality. 

EPA’s air quality modeling guidance has 

long recommended developing a base 

design value (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 design values 

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). 

The average of these three design values 

is then multiplied by a relative response 

factor to generate an average design 

value for the future year. If a receptor’s 

average future year design value 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, EPA’s 

methodology looks to the highest design 

value 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). 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, 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 EPA to protect areas 

that may struggle with maintaining the 

standard in the face of variable 

conditions. 

For its maintenance receptors, TCEQ 

elected not to use the highest design 

value of the three DVs making up the 

base period average design value. 

Instead, Texas (and by extension, Ohio), 

used the most recent design value of the 

three DVs, regardless of whether the 

most recent design value was highest or 

lowest. OEPA’s proffered explanation 

for using the most recent design value 

to identify maintenance receptors was 

that the latest design value ‘‘takes into 

consideration . . . any emissions 

reductions that might have occurred.’’ 97

OEPA in its submission did 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 

interannual variability in ozone 

conducive meteorology. In fact, because 

the TCEQ’s methodology adopted by 

OEPA uses the most recent design value 

to capture more recent emissions 

reductions rather than capture variable 

conditions, the methodology appears to 

be aimed at limiting receptors which 

could be identified as maintenance 

receptors, compared to EPA’s 

methodology, which was designed to 

identify those areas that might struggle 

to maintain the NAAQS in ozone 

conducive conditions. 

EPA disagrees that the use of latest 

three years for calculating a DV properly 

accounts for the effects of 

meteorological variability for the 

purpose of identifying projected 

maintenance receptors. Rather, the use 

of a three-year average is intended to 

average out, not account for, the effects 

of inter-annual variability in ozone 

conducive meteorology. EPA reviewed 

the information provided by OEPA and 

proposes to find that the information is 

insufficient to support the use of an 

alternative approach. OEPA analysis of 

meteorological information did not 

discuss or consider how other 

meteorological factors that are typically 

associated with high ozone episodes 

such as humidity, solar radiation, 

vertical mixing, and/or other 

meteorological indicators such as 

cooling-degree days to confirm whether 

conditions affecting these monitors may 

have been conducive to ozone formation 

during the 2009 through 2013 base 

period. In addition, the ozone trends 

data provided in OEPA submittal 

indicate that several of the receptors in 

Coastal Connecticut to which Ohio is 

linked by more than 1 ppb continue to 

measure ozone design values close to or 

exceeding 80 ppb with no overall 

downward trend in the most recent data 

in the submittal.98 In any event, OEPA’s 

use of an alternative approach to 

identifying maintenance receptors does 

not result in a dispositive change in 

receptor status for purposes of EPA’s 

evaluation of OEPA’s SIP submittal at 

Step 1 because OEPA did not reach the 

conclusion that there were no receptors 

in 2023 or claim at Step 2 that Ohio was 

not linked to any receptor on the basis 

of the use of an alternative definition of 

maintenance receptor. 

In conclusion, the modeling relied on 

by OEPA identified a number of 

nonattainment and maintenance 

receptor sites in the Midwest and 

Northeast in 2023. See Table 1 on page 

8 of OEPA’s submittal. Under EPA’s 

approach to defining nonattainment and 

maintenance receptors, Ohio was shown 

to be linked to three ‘‘nonattainment/ 

maintenance’’ receptors and six 

‘‘maintenance’’ receptors. Under an 

alternative approach to defining 

receptors (discussed below), OEPA 

concluded that Ohio was shown to be 

linked to two ‘‘nonattainment’’ 

receptors, one ‘‘nonattainment/ 

maintenance’’ receptor, and four 

‘‘maintenance’’ receptors. Thus, based 

on using the LADCO’s 2023 modeling 

and even under an alternative approach 

to defining ‘‘maintenance’’ receptors, 

OEPA acknowledges in its submittal the 

existence of several nonattainment and 

maintenance receptors in the Midwest 

and Northeast. EPA further evaluates 

Ohio’s linkage to these receptors in the 

following section. 

2. Evaluation of Information Provided 

by the State Regarding Step 2 

Although OEPA relied on alternative 

modeling to EPA’s modeling, OEPA 

acknowledged in their SIP submission 

that Ohio is linked above either a 1 

percent of the NAAQS or a 1 ppb 

contribution threshold to one or more 

downwind receptors in 2023. Because 

the LADCO modeling relied on by the 

state also demonstrates that a linkage 

exists between the state and downwind 

receptors at Step 2, EPA need not 

conduct a comparative assessment of 

the alternative modeling; the state 

concedes that it is linked above either 

1 percent of the NAAQS or 1 ppb. 

The state additionally evaluated the 

use of an alternative threshold 

exceeding 1 ppb at Step 2 to identify 

whether the state was ‘‘linked’’ to a 

projected downwind nonattainment or 

maintenance receptor. EPA’s August 

2018 memorandum provided that 

whether use of a 1 ppb threshold is 

appropriate must be based on an 

evaluation of state-specific 

circumstances, but that the use of a 

threshold greater than 1 ppb at Step 2 

would likely not be appropriate because 

higher thresholds would not capture a 

sufficient amount of total upwind state 

contribution to allow for the 

development of effective remedies at 

Step 3.31 In particular, EPA found that 

a 2 ppb threshold would cause 45% of 

total upwind contribution to be 

removed from further analysis across all 


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99Michigan’s SIP submission at 16. 

receptors as compared to a 1 percent of 

NAAQS threshold. 

EPA does not accept Ohio’s position 

that a 1 percent of the NAAQS 

contribution threshold at Step 2 is 

‘‘impractical and infeasible’’ for the 

2015 ozone NAAQS because ‘‘it results 

in very small contributions having 

substantial consequences.’’ 99 This 

argument conflates the contribution 

threshold at Step 2 with a determination 

of ‘‘significance’’ reached at Step 3 after 

a multi-factor analysis. In its submittal, 

OEPA justified a higher threshold than 

either 1 percent or 1 ppb by noting that, 

if applied, these alternative thresholds 

(3 or 4 percent of the NAAQS) would 

progressively de-link the State from an 

increasing number of identified 

downwind receptors. EPA likewise 

disagrees with this reasoning; selecting 

progressively higher contribution 

thresholds simply on the basis that they 

would excuse an ever greater number of 

upwind states from having any good 

neighbor obligations lacks any 

persuasive technical justification and is 

inconsistent with the purposes of the 

Act. 

The explanation for how the 1 percent 

contribution threshold was originally 

derived is available in the 2011 CSAPR 

rulemaking. See 76 FR 48208, 48237–38. 

Further, in the CSAPR Update, EPA reanalyzed the threshold for purposes of 

the 2008 ozone NAAQS and determined 

it was appropriate to continue to apply 

this threshold. EPA compared the 1 

percent threshold to a 0.5 percent of 

NAAQS threshold and a 5 percent of 

NAAQS threshold. EPA found that the 

lower threshold did not capture 

appreciably more upwind state 

contribution compared to the 1 percent 

threshold, while the 5 percent threshold 

allowed too much upwind state 

contribution to drop out from further 

analysis. See Final CSAPR Update Air 

Quality Modeling TSD, at 27–30 (EPA– 

HQ–OAR–2015–0596–0144). If EPA 

were to apply this analysis to the 2015 

ozone NAAQS using the updated 

modeling based on the 2016v2 

emissions platform, a 5 percent of the 

NAAQS contribution threshold (i.e., 3.5 

ppb) only captures approximately 50 

percent of the total upwind 

contribution. Compared to a 1 percent 

threshold, a 5 percent threshold would, 

on average, forgo 27 nearly 30 percent) 

of the total upwind contribution. As 

EPA noted in the August 2018 

memorandum, the use of a 2 ppb 

contribution threshold under the 

modeling released with the March 2018 

memorandum would only capture about 

55 percent of all upwind contributions, 

and therefore ‘‘emission reductions from 

states linked at that higher threshold 

may be insufficient to address collective 

upwind state contribution to downwind 

air quality problems.’’31

Based on EPA’s updated modeling 

and the LADCO modeling, the state is 

projected to contribute greater than both 

the 1 percent and alternative 1 ppb 

thresholds. While EPA does not, in this 

action, approve of the state’s application 

of the 1 ppb threshold, based on its 

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. 

TABLE 6—OHIO LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location 

(county, state) Nonattainment/maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Ohio 

Contribution 

(ppb) 

90099002 ............................... New Haven, CT .................... Nonattainment ...................... 71.8 73.9 1.94 

90019003 ............................... Fairfield, CT .......................... Nonattainment ...................... 76.1 76.4 1.90

420170012 ............................. Bucks, PA ............................. Maintenance ......................... 70.7 72.2 1.88 

90013007 ............................... Fairfield, CT .......................... Nonattainment ...................... 74.2 75.1 1.87

170317002 ............................. Cook, IL ................................ Maintenance ......................... 70.1 73.0 1.69 

550590019 ............................. Kenosha, WI ......................... Nonattainment ...................... 72.8 73.7 1.67 

550590025 ............................. Kenosha, WI ......................... Maintenance ......................... 69.2 72.3 1.33 

170310032 ............................. Cook, IL ................................ Maintenance ......................... 69.8 72.4 1.26 

170314201 ............................. Cook, IL ................................ Maintenance ......................... 69.9 73.4 1.23 

170310076 ............................. Cook, IL ................................ Maintenance ......................... 69.3 72.1 1.23 

90010017 ............................... Fairfield, CT .......................... Nonattainment ...................... 73.0 73.7 1.18

551010020 ............................. Racine, WI ............................ Nonattainment ...................... 71.3 73.2 1.00 

170310001 ............................. Cook, IL ................................ Maintenance ......................... 69.6 73.4 0.82 

4. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

110(a)(2)(D)(i)(I). As explained in 

Section II.E, Ohio relied on a 

combination of both cost and air quality 

factors to determine that there were no 

further reductions necessary for Ohio to 

meet its obligations under the interstate 

transport provision. In this subsection, 

we have evaluated the information 

provided by the state at Step 3 to 

support this conclusion. 

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. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner EPA has done in its prior 

regional transport rulemakings, state 

implementation plans addressing the 


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100See https://www.eia.gov/outlooks/archive/ 

aeo18/, last accessed 1/18/2022. 

101See 81 FR 74504 at 74517; 85 FR 68964 at 

68979. 102 Detailed information and documentation of 

EPA’s Base Case, including all the underlying 

assumptions, data sources, and architecture 

parameters can be found on EPA’s website at: 

https://www.epa.gov/airmarkets/epas-power-sectormodeling-platform-v6-using-ipm-summer-2021- 

reference-case. 103The ‘‘Capacity Dropped’’ and the ‘‘Retired 

Through 2023’’ worksheets in NEEDS lists all units 

that are removed from the NEEDS v6 inventory— 

NEEDS v6 Summer 2021 Reference Case. This data 

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

www.epa.gov/airmarkets/national-electric-energydata-system-needs-v6. 

104Technical Support Document (TSD) 

Preparation of Emissions Inventories for the 2016v2 

North American Emissions Modeling Platform, 

section 4.3.2. Available in the Headquarters docket 

ID No. EPA–HQ–OAR–2021–0663. 

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 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. OEPA did not conduct such 

an analysis in their SIP submission. 

OEPA’s submission concluded that 

projected emissions were overestimated 

for the EGU, non-EGU, and onroad 

sectors. OEPA claimed that the ERTAC 

EGU tool’s emissions inventories were 

overestimated for eight specific sources 

for various reasons, including adoption 

of rules in late 2016 and early 2017, 

CSAPR and CSAPR Update allocations, 

and substantive changes in plant 

operation. The submission also asserted 

that ERTAC EGU tool version 2.7 does 

not consider that future energy 

generation sources will likely be a 

steady level of coal with increasing 

natural gas and renewable fuels, citing 

an un-enumerated number of natural gas 

source permits issued by Ohio and 

projected trends identified in the US 

Energy Information Administration’s 

Annual Energy Outlook (AEO) 2018.100

Similarly, the submission claimed 

projected emissions from EPA’s Air 

Emissions Modeling Platform 2011v6.3 

were overestimated for nine non-EGU 

point sources, primarily based on actual 

emissions trends from 2010 to 2017. 

OEPA also claimed that EPA over 

projected onroad emissions using 2023 

vehicle miles traveled (VMT). However, 

OEPA did not explain how accounting 

for changed projected emissions from 

those 17 sources or the onroad sector 

would have resulted in different 

outcomes with regards to the 

identification of downwind receptors or 

Ohio’s contributions or linkages in the 

2023 analytic year. Furthermore, 

nationwide trends and an unspecified 

number of state permits are insufficient 

by themselves to support a conclusion 

that EGUs in Ohio would not be affected 

by generation shifting. EPA notes the 

information presented from the AEO is 

related to nationwide trends and OEPA 

did not explain what the nationwide 

trends revealed about Ohio’s level of 

contribution or good neighbor 

obligations to downwind receptors. 

Merely claiming that the modeling used 

to project receptors and contributions 

relies on overestimated emissions 

projections without an explanation of 

how the inputs would affect the 

outcome is not enough to draw a 

conclusion at Step 2 that Ohio is not 

linked to any downwind receptor or a 

conclusion at Step 3 that Ohio does not 

contribute significantly or interfere with 

maintenance in any other state. 

Considered individually or in the 

context of the other information and 

arguments put forward by OEPA, select 

EGU, non-EGU, and onroad emissions 

evaluations and nation-wide projections 

of fuel types fail to show that additional 

emissions reductions are either not costeffective or permanent and federally 

enforceable. OEPA did not demonstrate 

that the downwind improvements from 

these regulations and programs would 

be sufficient to eliminate the state’s 

significant contribution or interference 

with maintenance. 

Further, EPA’s assessment of future 

air quality conditions generally 

accounts for on-the-books emission 

reductions and the most up-to-date 

forecast of future emissions in the 

absence of the transport policy being 

evaluated (i.e., base case conditions).101

As described in more detail in Section 

I, EPA’s latest projections of the baseline 

EGU emissions uses the version 6— 

Summer 2021 Reference Case of the 

IPM.102 The IPM version 6—Summer 

2021 Reference Case uses the NEEDS v6 

database as its source for data on all 

existing and planned-committed units. 

Units are removed from the NEEDS 

inventory only if a high degree of 

certainty could be assigned to future 

implementation of the announced future 

closure or retirement. Any retirements 

excluded from the NEEDS v6 inventory 

can be viewed in the NEEDS 

spreadsheet.103 The inventory for these 

projections contains various Ohio EGUs 

including the Avon Lake Power Plant in 

Lorain County (Facility ID 0247030013), 

Painesville Municipal Electric Plant in 

Lake County (Facility ID 0243110008), 

and the Department of Public Utilities, 

City of Orrville in Wayne County 

(Facility ID 0285010188). Mingo 

Junction Energy Center in Jefferson 

County (Facility ID 0641090234) and the 

Conesville Power Plant (Facility ID 

0616000000) retired in 2020. 

Also, EPA’s non-EGU emissions 

inventory in the updated modeling 

using the 2016v2 emissions platform 

does not include either Carmeuse Lime 

Inc Millersville Operations (Facility ID 

0372000081) or RockTenn CP, LLC 

(Facility ID 0616010001). EPA’s latest 

modeling also uses emissions 

inventories that incorporate Ohio’s 

submitted 2023 VMT data.104 In other 

words, in general, any changes in the 

emissions inventory and on-the-books 

controls relevant to emissions in 2023 

have now been incorporated into EPA’s 

modeling using the 2016v2 emissions 

platform, which projects a continuing 

contribution from Ohio to out of state 

receptors above a threshold of 1 percent 

of the NAAQS (at Steps 1 and 2) despite 

these measures. Therefore, in light of 

continuing contribution to out of state 

receptors from Indiana notwithstanding 

these identified retirements, OEPA’s SIP 

submission should have evaluated the 

availability of additional air quality 

controls to improve downwind air 

quality at nonattainment and 

maintenance receptors at Step 3. 

Ohio’s projected contribution to 

downwind receptors in EPA’s updated 

modeling is lower relative to the 

LADCO modeling results presented in 

OEPA’s submission; it could be 

assumed that these decreases are due to 

overestimation of sources that were 

corrected in the updated modeling. 

These results could also be attributed to 

Federal programs in place (NOX RACT, 

AIM Coatings Rules, CSAPR, NOx SIP 

Call, NESHAPs, RHR, BART, SO2 Data 

Requirements rule, and MATS) as OEPA 

suggests. Regardless, despite the 

lessened projected contributions, Ohio’s 

contributions continue to be projected 

to be above 1 percent of the NAAQS to 

one or more receptors in 2023 as shown 

in Table 6. 

OEPA’s assessment of actual and 

projected NOX and VOC emissions 

trends and listing of various regulations 

likewise do not support a conclusion 

that existing controls in Ohio 

adequately address the state’s good 

neighbor obligations for the 2015 ozone 

NAAQS. For one, OEPA listed 

numerous non-SIP measures and states 

may not rely on non-SIP measures to 


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105EPA notes that OEPA submitted a source 

specific NOX emission limit contained in the Ohio 

NOX RACT Rules for approval into the Ohio SIP, 

approved by EPA on September 8, 2017 (82 FR 

42451). 106 Indiana’s SIP submission, Appendix E at 4, 17. 

107The Safer Affordable Fuel-Efficient (SAFE) 

Vehicles Rule for Model Years 2021–2026 Passenger 

Cars and Light Trucks, 85 FR 24174 (April 30, 2020) 

(SAFE Vehicles Rule). 

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). OEPA did not 

attempt to revise Ohio’s SIP to include 

all these measures.105 In general, the 

listing of existing or on-the-way control 

measures, whether approved into the 

state’s SIP or not, does not substitute for 

a complete Step 3 analysis under EPA’s 

4-step framework to define ‘‘significant 

contribution.’’ OEPA’s submittal does 

not include an assessment of the overall 

effects of these measures, when the 

reductions would be achieved, and what 

the overall resulting air quality effects 

would be observed at identified out-ofstate receptors. The state’s submission 

does not include an evaluation of 

additional potential emissions control 

opportunities, or their costs or impacts, 

or attempt to analyze whether, if 

applied more broadly across linked 

states, the emissions reductions would 

constitute the elimination of significant 

contribution on a regional scale. The 

state’s submission did not contain an 

explanation as to whether any faster or 

more stringent emissions reductions 

that may be available were prohibitively 

costly or infeasible. Second, the 

information and claims presented by 

OEPA did not provide sufficient 

evidence to support alternative 

conclusions that EPA is proposing to 

make in this action: Namely, that 

several receptors exist, Ohio contributes 

to those receptors above a 1 percent of 

the NAAQS contribution threshold, and 

that Ohio continues to have good 

neighbor obligations that need to be 

addressed for the 2015 ozone NAAQS. 

OEPA also pointed to declining 

design values at the ten receptors 

identified by LADCO to support their 

conclusion that no further emissions 

reductions were required from Ohio to 

meet their interstate transport 

obligations. They additionally reference 

a May 14, 2018 EPA presentation, 

stating that EPA indicated remaining 

ozone air quality problems were 

becoming more local and less regional 

in nature. While it is true that since 

2011, design values have generally 

declined, air quality problems at some 

locations are projected to continue out 

to 2023 and beyond, based on EPA’s 

2018 modeling provided in the March 

2018 memorandum, LADCO’s modeling 

completed in 2018, EPA’s modeling 

results used in the Revised CSAPR 

Update, and EPA’s updated modeling 

results. In addition, each of these 

modeling analyses show that Ohio will 

contribute to the air quality problems in 

excess of 1 percent of the 2015 ozone 

standards in 2023. Regarding the May 

14, 2018 presentation, EPA assumes the 

state is referencing a presentation given 

by an EPA air quality modeler, which 

Indiana attached to their SIP 

submission. The purpose of that 

presentation was to share a technical, 

exploratory analysis of ozone trends. 

The results of that presentation, which 

were labeled as ‘‘preliminary’’ indicated 

that ‘‘[f]urther exploration of the relative 

contribution from various source sectors 

within the NE Corridor and in nearby 

upwind states might also be 

informative.’’ 106 The preliminary 

results of that analysis are generally 

consistent with EPA’s updated 

modeling using the 2016v2 emissions 

platform. Although EPA’s modeling 

shows that a large portion of the 

transport problem affecting the 

receptors in Coastal Connecticut is 

indeed from sources within the Ozone 

Transport Region (OTR), a substantial 

portion of the transport problem at these 

receptors, on the order of 25 percent, is 

the result of transport from states 

outside the OTR. However, the 

relevance of that presentation to the 

evaluation of Ohio’s good neighbor 

obligations is not clear. As already 

discussed, the statute and the case law 

(particularly the holdings in Wisconsin 

and Maryland) make clear that good 

neighbor obligations are not merely 

supplementary to or deferable until after 

local emission reductions are achieved. 

Further, based on EPA’s modeling 

released with the March 2018 

memorandum, nearly all of the 

receptors to which Ohio is linked are 

also heavily impacted by distant 

upwind state emissions in addition to 

local sources and sources in neighboring 

states. The Wisconsin decision’s holding 

in regard to international contribution 

(discussed in more detail later) is 

equally applicable to an upwind state’s 

claims that some other state’s emissions, 

or local emissions, are more to blame 

than its own emissions. See 938 F.3d 

303 at 323–25 (‘‘an upwind state can 

‘contribute’ to downwind 

nonattainment even if its emissions are 

not the but-for cause’’). 

OEPA also put forward an argument 

that onroad mobile sources in 

downwind states should be more 

stringently controlled before any 

additional sources in upwind states. 

This is equivalent to the claim that local 

emissions reductions from the 

jurisdiction where the downwind 

receptor is located must first be 

implemented and accounted for before 

imposing obligations on upwind states 

under the interstate transport provision. 

However, there is nothing in the CAA 

that supports that position, and it does 

not provide grounds on which to 

approve OEPA’s SIP submission. The 

D.C. Circuit has held on five different 

occasions that the timing framework for 

addressing interstate transport 

obligations must be consistent with the 

downwind areas’ attainment schedule. 

In particular, for the ozone NAAQS, the 

states and EPA are to address interstate 

transport obligations ‘‘as expeditiously 

as practicable’’ and no later than the 

attainment schedule set in accordance 

with CAA section 181(a). See North 

Carolina, 531 F.3d at 911–13; 

Wisconsin, 938 F.3d at 313–20; 

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

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

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

4, 6–7 (DC Cir. 2019). 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. The statutory framework of the 

CAA and these cases establish clearly 

that states and EPA must address 

interstate transport obligations in line 

with the attainment schedule provided 

in the Act in order to timely assist 

downwind states in attaining and 

maintain the NAAQS, and this schedule 

is ‘‘central to the regulatory scheme.’’ 

Wisconsin, 938 F.3d at 316 (quoting 

Sierra Club v. EPA, 294 F.3d 155, 161 

(D.C. Cir. 2002)). 

As for the suggestion that EPA should 

assess the SAFE Vehicles Rule’s impact 

on ozone before finalizing, EPA and the 

National Highway Traffic Safety 

Administration finalized the revisions 

to the greenhouse gas (GHG) and CAFE 

standards for light duty vehicles in 

2020.107 However, that final action is 

not expected to have a meaningful 

impact on ozone-precursor emissions. 

Because the vehicles affected by the 


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108Revised 2023 and Later Model Year Light-Duty 

Vehicle Greenhouse Gas Emissions Standards, 86 

FR 74434 (December 30, 2021). 

109See Technical Support Document (TSD) 

Preparation of Emissions Inventories for the 2016v2 

North American Emissions Modeling Platform 

included in the Headquarters docket ID No. EPA– 

HQ–OAR–2021–0663. 

2017–2025 GHG standards would still 

need to meet applicable criteria 

pollutant emissions standards (e.g., the 

Tier 3 emissions standards; see 79 FR 

23414), the SAFE Vehicles Rule 

anticipated that any impacts of the 

SAFE Vehicles Rule on ozone precursor 

emissions ‘‘would most likely be far too 

small to observe.’’ See 85 FR 25041. On 

December 30, 2021, EPA revised the 

GHG light duty standards for model 

years 2023 and later to make them more 

stringent.108 The impacts of the SAFE 

Vehicles Rule are included in the 

2016v2 onroad emissions as described 

in the emissions modeling TSD in 

Section 4.3.2.109

Further, OEPA makes the argument 

that assigning all responsibility to Ohio 

and other upwind states for downwind 

air quality problems despite home state 

and international contributions would 

result in overcontrol of Ohio sources. 

OEPA’s reasoning related to emissions 

in downwind states and international 

emissions is inapplicable to the 

requirements of CAA section 

110(a)(2)(D)(i)(I). As an initial matter, 

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

requires that upwind states prohibit 

those emissions that ‘‘contribute 

significantly to nonattainment’’ or 

‘‘interfere with maintenance of the 

NAAQS.’’ It does not require that the 

upwind states bear the full burden of 

bringing downwind states into 

attainment or that a threshold ppb 

improvement from upwind states 

emission reductions be met in order for 

them to be required (once the 1 percent 

threshold has been satisfied). However, 

the good neighbor provision does 

require states and EPA to address 

interstate transport of air pollution that 

contributes to downwind states’ ability 

to attain and maintain NAAQS. Whether 

emissions from other states or other 

countries also contribute to the same 

downwind air quality issue is irrelevant 

in assessing whether a downwind state 

has an air quality problem, or whether 

an upwind state is significantly 

contributing to that problem. States are 

not obligated under CAA section 

110(a)(2)(D)(i)(I) to reduce emissions 

sufficient on their own to resolve 

downwind receptors’ nonattainment or 

maintenance problems. Rather, states 

are obligated to eliminate their own 

‘‘significant contribution’’ or 

‘‘interference’’ with the ability of other 

states to attain or maintain the NAAQS. 

Indeed, after OEPA submitted Ohio’s 

SIP submission, the D.C. Circuit in 

Wisconsin specifically rejected 

petitioner arguments suggesting that 

upwind states should be excused from 

good neighbor obligations on the basis 

that some other source of emissions 

(whether international or another 

upwind state) could be considered the 

‘‘but-for’’ cause of downwind air quality 

problem. 938 F.3d 303 at 323–324. The 

court viewed petitioners’ arguments as 

essentially an argument ‘‘that an 

upwind state ‘contributes significantly’ 

to downwind nonattainment only when 

its emissions are the sole cause of 

downwind nonattainment.’’ 938 F.3d 

303 at 324. The court explained that ‘‘an 

upwind state can ‘contribute’ to 

downwind nonattainment even if its 

emissions are not the but-for cause.’’ Id. 

at 324–325. See also Catawba County v. 

EPA, 571 F.3d 20, 39 (D.C. Cir. 2009) 

(rejecting the argument ‘‘that 

‘significantly contribute’ unambiguously 

means ‘strictly cause’’’ because there is 

‘‘no reason why the statute precludes 

EPA from determining that [an] addition 

of [pollutant] into the atmosphere is 

significant even though a nearby 

county’s nonattainment problem would 

still persist in its absence’’); Miss. 

Comm’n on Envtl. Quality v. EPA, 790 

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

(observing that the argument that ‘‘there 

likely would have been no violation at 

all . . . if it were not for the emissions 

resulting from [another source]’’ is 

‘‘merely a rephrasing of the but-for 

causation rule that we rejected in 

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

not excused from eliminating its 

significant contribution on the basis that 

international emissions, or emissions 

from other sources, also contribute some 

amount of pollution to the same 

receptors to which the state is linked. 

Finally, as part of its costeffectiveness evaluation, OEPA 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 8-hour 

ozone NAAQS) to argue that it has 

already implemented all cost-effective 

emissions reductions. For non-EGUs, 

OEPA did not identify a costeffectiveness threshold, but rather listed 

a few regulations (the Boiler MACT and 

other MACT categories, BART, SO2 Data 

Requirements Rule and other 

unidentified Federal regulations) to 

draw the conclusion that emissions 

reductions had been achieved from nonEGUs in Ohio. 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. Cost-effectiveness must be 

assessed in the context of the specific 

CAA program; assessing costeffectiveness 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 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. The lack of a sufficient costeffectiveness evaluation also means that 

Ohio’s claims that requiring additional 

emissions reductions would result in 

overcontrol is premature. Ohio’s 

submission does present sufficient 

evidence to support that conclusion. 

In addition, the updated EPA 

modeling captures all existing CSAPR 

trading programs in the baseline, and 

that modeling confirms that these 

control programs were not sufficient to 

eliminate Ohio’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 non-SIP 

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). We therefore propose that Ohio 

was required to analyze emissions from 

the sources and other emissions activity 

from within the state to determine 

whether its contributions were 


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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 

[Ohio step 3 analysis section]. 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. 

111 Design values and contributions at individual 

monitoring sites nationwide are provide in the file 

‘‘2016v2_DVs_state_contributions.xlsx’’ which is 

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

0663. 112These modeling results are consistent with the 

results of a prior round of 2023 modeling using the 

2016v1 emissions platform which became available 

to the public in the fall of 2020 in the Revised 

CSAPR Update, as noted in Section I. That 

modeling showed that Wisconsin 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 ID No. 

EPA–HQ–OAR–2021–0663. 

significant, and we propose to 

disapprove its submission because Ohio 

failed to do so. 

5. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-step interstate 

transport framework calls for 

development of permanent and 

federally enforceable control strategies 

to achieve the emissions reductions 

determined to be necessary at step 3 to 

eliminate significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. OEPA 

identified NOX RACT rules limiting 

NOX emissions from new and existing 

sources, VOC reduction measures 

through control of architectural and 

industrial maintenance coatings, and 

reallocation of funds received through a 

settlement with Volkswagen to be 

applied to on-road and off-road mobile 

emissions reductions through 

replacements and infrastructure 

updates.110 However, OEPA did not 

revise Ohio’s SIP to include these 

emission reductions in a revision to its 

SIP to ensure the reductions were 

permanent and enforceable.105 As a 

result, EPA proposes to disapprove 

OEPA submittal on the separate, 

additional basis that the Ohio 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 EPA’s evaluation of Ohio’s 

SIP submission, EPA is proposing to 

find that the portion of Ohio’s 

September 28, 2018 SIP submission 

addressing CAA section 

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

state’s interstate transport obligations 

for the 2015 ozone NAAQS, 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. 

F. Wisconsin 

1. Evaluation of Information Provided 

by Wisconsin Regarding Steps 1 and 2 

WDNR did not perform an analysis 

under the 4-step framework to assess 

Wisconsin’s good neighbor obligations. 

The submission did not identify areas in 

other states that may have trouble 

attaining or maintaining the 2015 ozone 

NAAQS. Nor did WDNR perform a Step 

2 analysis to identify Wisconsin’s 

contribution to areas that are projected 

to have difficulty attaining or 

maintaining the NAAQS or reach a 

conclusion about whether Wisconsin is 

linked to any receptors. 

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

Modeling and Findings for Wisconsin 

As described in Section I, EPA 

performed air quality modeling using 

the 2016v2 emissions platform to 

project design values and contributions 

for 2023. These data were examined to 

determine if Wisconsin contributes at or 

above the threshold of one percent of 

the 2015 ozone NAAQS (0.70 ppb) to 

any downwind nonattainment or 

maintenance receptor. As shown in 

Table 7, the data 111 indicate that in 

2023, emissions from Wisconsin 

contribute greater than one percent of 

the standard to nonattainment or 

maintenance-only receptors in 

Illinois.112 Therefore, based on EPA’s 

evaluation of the information submitted 

by WDNR, and based on EPA’s most 

recent modeling results for 2023, EPA 

proposes to find that Wisconsin 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 7—WISCONSIN LINKAGE RESULTS BASED ON EPA UPDATED 2023 MODELING

Receptor ID Location 

(county, state) Nonattainment/maintenance 

2023 

Average 

design value 

(ppb) 

2023 

Maximum 

design value 

(ppb) 

Wisconsin 

contribution 

(ppb) 

170310032 ............................. Cook, IL ................................ Maintenance ......................... 69.8 72.4 2.61 

170314201 ............................. Cook, IL ................................ Maintenance ......................... 69.9 73.4 2.55 

170310076 ............................. Cook, IL ................................ Maintenance ......................... 69.3 72.1 2.47 

170310001 ............................. Cook, IL ................................ Maintenance ......................... 69.6 73.4 2.41 

170317002 ............................. Cook, IL ................................ Maintenance ......................... 70.1 73.0 1.47 

As shown in Table 7, the updated 

EPA modeling identifies Wisconsin’s 

maximum contribution Because the 

entire technical basis for the state’s 

submittal is that the state has satisfied 

good neighbor obligations through 

implementation of various rules, 

including CSAPR Update, EPA proposes 

to disapprove the SIP submission based 

on EPA’s finding that WDNR has not 

provided adequate information to allow 

EPA to assess whether Wisconsin has 

adequate provisions to prohibit 

emissions in amounts which will 

contribute significantly to 

nonattainment or interfere with 

maintenance in any other state. Though 

this deficiency would be sufficient on 

its own to disapprove Wisconsin’s good 

neighbor submission, EPA will proceed 

to evaluate the additional points raised 

by WDNR at Step 3 of the 4-step 

interstate transport framework. 

3. Evaluation of Information Provided 

Regarding Step 3 

At Step 3 of the 4-step interstate 

transport framework, a state’s emissions 

are further evaluated, in light of 

multiple factors, including air quality 

and cost considerations, to determine 

what, if any, emissions significantly 

contribute to nonattainment or interfere 

with maintenance and, thus, must be 

eliminated under CAA section 

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


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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. 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 EPA has 

not directed states that they must 

conduct a Step 3 analysis in precisely 

the manner 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 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. The state did not conduct 

such an analysis in their SIP 

submission. 

WDNR listed several rules relevant to 

interstate transport and seemingly relied 

on its participation in LADCO to suggest 

sources in Wisconsin are adequately 

controlled for purposes of the good 

neighbor provision for the 2015 ozone 

NAAQS. WDNR mentioned Wisconsin’s 

FIPs under CSAPR and CSAPR Update. 

EPA disagrees that this is a sufficient 

approach for assessing good neighbor 

obligations. 

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 (or any 

other CAA program) without further 

Step 3 analysis is not approvable. While 

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. As such, the CSAPR 

Update Rule is not an appropriate 

analysis and cannot be approved to 

satisfy interstate transport obligations 

for the 2015 ozone NAAQS. 

In addition, the updated EPA 

modeling captures all existing CSAPR 

trading programs in the baseline, and 

that modeling confirms that these 

control programs were not sufficient to 

eliminate the Wisconsin’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 non-SIP 

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). 

WDNR cited continued consultation 

with LADCO, three Wis. Admin. Code 

subsections that could be relied on ‘‘if 

needed’’ to address disagreements for 

SIP development in other states’ 

nonattainment areas, and an adequate 

PSD program. WDNR did not attempt to 

revise Wisconsin’s SIP to include to 

include all these measures. In general, 

the listing of existing or on-the-way 

control measures, including potential 

future emissions reductions obtained 

through participation in LADCO, 

whether approved into the state’s SIP or 

not, does not substitute for a complete 

Step 3 analysis under EPA’s 4-step 

framework to define ‘‘significant 

contribution.’’ WDNR did not identify 

control measures, provide an 

assessment of the overall effects of these 

measures, note when the reductions 

would be achieved, or explain what the 

overall resulting air quality effects 

would be at identified out of state 

receptors. WDNR did not evaluate 

additional, potential emissions control 

opportunities, or their costs or impacts, 

or attempt to analyze whether, if 

applied more broadly across linked 

states, the emissions reductions would 

constitute the elimination of significant 

contribution on a regional scale. WDNR 

did not offer an explanation as to 

whether any faster or more stringent 

emissions reductions that may be 

available were prohibitively costly or 

infeasible. Although EPA acknowledges 

states are not necessarily bound to 

follow its own analytical framework at 

Step 3, WDNR did not attempt to 

determine or justify an appropriate 

uniform cost-effectiveness threshold. 

This would have been similar to the 

approach to defining significant 

contribution that EPA has applied in 

prior rulemakings such as CSAPR and 

or the CSAPR Update, even if such an 

analysis is not technically mandatory. 

As mentioned previously, Wis. 

Admin. Code NR 285.15, entitled 

Interstate Agreement, gives the governor 

the authority to enter an agreement to 

solve interstate pollution transport with 

Illinois, Indiana, and Michigan if the 

area includes portions of both 

Wisconsin and Illinois. Furthermore, 

Wis. Admin. Code, NR 285.1560 does 

not provide for emission reductions 

toward resolving good neighbor 

obligations, as while the statute allows 

for consultation, there is no indication 

this rule has been exercised to resolve 

good neighbor obligations or explain 

how the rule would impact areas in 

Illinois to which Wisconsin is linked. 

Under the Wisconsin decision, states 

and EPA may not delay implementation 

of measures necessary to address good 

neighbor requirements beyond the next 

applicable attainment date without a 

showing of impossibility or necessity. 

See 938 F.3d at 320. Wisconsin’s 

submittal is insufficient to the extent the 

implementation timeframes for the cited 

control measures were left unidentified, 

unexplained, or too uncertain to permit 

EPA to form a judgment as to whether 

the timing requirements for good 

neighbor obligations have been met. 

We therefore propose that Wisconsin 

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 

Wisconsin failed to do so. 

4. Evaluation of Information Provided 

Regarding Step 4 

Step 4 of the 4-step interstate 

transport framework calls for 

development of permanent and 

federally enforceable control strategies 

to achieve the emissions reductions 

determined to be necessary at Step 3 to 

eliminate significant contribution to 

nonattainment or interference with 

maintenance of the NAAQS. As 

mentioned previously, Wisconsin’s SIP 

submission did not contain an 

evaluation of additional emission 


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113 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. 

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 Wisconsin’ 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 EPA’s evaluation of 

Wisconsin’s SIP submission, EPA is 

proposing to find that the portion of 

Wisconsin’s September 14, 2018 SIP 

submission addressing CAA section 

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

Wisconsin’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. Proposed Action 

We are proposing to disapprove the 

portions of Illinois, Indiana, Michigan, 

Minnesota, Ohio, and Wisconsin’s SIP 

submissions 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), disapproval would establish a 

2-year deadline for EPA to promulgate 

a FIP for 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 EPA approves a 

SIP that meets these requirements. 

Disapproval does not start a mandatory 

sanctions clock for Illinois, Indiana, 

Michigan, Minnesota, Ohio, or 

Wisconsin. The remaining elements of 

the states’ submissions are not 

addressed in this action and either have 

been or will be acted on in a separate 

rulemaking. 

V. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review and Executive 

Order 13563: Improving Regulation and 

Regulatory Review 

This action is not a significant 

regulatory action and was therefore not 

submitted to the Office of Management 

and Budget for review. 

B. Paperwork Reduction Act (PRA) 

This proposed action does not impose 

an information collection burden under 

the PRA because it does not contain any 

information collection activities. 

C. Regulatory Flexibility Act (RFA) 

I certify that this action will not have 

a significant economic impact on a 

substantial number of small entities 

under the RFA. This action merely 

proposes to disapprove a SIP 

submission as not meeting the CAA. 

D. Unfunded Mandates Reform Act 

(UMRA) 

This action does not contain any 

unfunded mandate as described in 

UMRA, 2 U.S.C. 1531–1538, and does 

not significantly or uniquely affect small 

governments. The action imposes no 

enforceable duty on any state, local or 

tribal governments or the private sector. 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the states, on the 

relationship between the national 

government and the states, or on the 

distribution of power and 

responsibilities among the various 

levels of government. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have tribal 

implications as specified in Executive 

Order 13175. This action does not apply 

on any Indian reservation land, any 

other area where EPA or an Indian tribe 

has demonstrated that a tribe has 

jurisdiction, or non-reservation areas of 

Indian country. Thus, Executive Order 

13175 does not apply to this action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

EPA interprets Executive Order 13045 

as applying only to those regulatory 

actions that concern environmental 

health or safety risks that 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 

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 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 EPA complete discretion 

whether to invoke the exception in 

(ii).113

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 six 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 good neighbor 

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 EPA takes final action on this 

proposed rulemaking, in the alternative, 


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114A 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. 

115EPA 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 EPA take a single final action on 

all such disapprovals, this action would be 

nationally applicable, and 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. 

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), 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, EPA is applying here 

(and in other proposed actions related to 

the same obligations) the same, 

nationally consistent 4-step framework 

for assessing good neighbor obligations 

for the 2015 ozone NAAQS. EPA relies 

on a single set of updated, 2016-base 

year photochemical grid modeling 

results of the year 2023 as the primary 

basis for its assessment of air quality 

conditions and contributions at steps 1 

and 2 of that framework. Further, EPA 

proposes to determine and apply a set 

of nationally consistent policy 

judgments to apply the 4-step 

framework. 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.114 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).115

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: January 31, 2022. 

Debra Shore, 

Regional Administrator, Region 5. 

[FR Doc. 2022–02953 Filed 2–18–22; 8:45 am] 

BILLING CODE 6560–50–P 


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