United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued November 6, 2019 Decided July 10, 2020

No. 18-1203

CLEAN WISCONSIN,

PETITIONER

v.

ENVIRONMENTAL PROTECTION AGENCY AND ANDREW 

WHEELER, ADMINISTRATOR, UNITED STATES 

ENVIRONMENTAL PROTECTION AGENCY,

RESPONDENTS

BCCA APPEAL GROUP, ET AL.,

INTERVENORS

Consolidated with 18-1205, 18-1206, 18-1208, 18-1212, 

18-1214

On Petitions for Review of a Final Agency Action 

of the United States Environmental Protection Agency

Daniel I. Rottenberg, Assistant Attorney General, Office 

of the Attorney General for the State of Illinois, Maxine I. 

Lipeles, David R. Baake, Robert Ukeiley, Howard A. Learner, 

and Ann Brewster Weeks argued the causes for petitioners.

With them on the final joint briefs were Kwame Raoul, 

Attorney General, Office of the Attorney General for the State 
2

of Illinois, Jane Elinor Notz, Solicitor General, Benna Ruth 

Solomon, Stephen J. Kane, Susan Hedman, Elena K. 

Saxonhouse, Joshua A. Berman, Scott Strand, Ann Jaworski, 

and Jonathan Evans.

Letitia James, Attorney General, Office of the Attorney 

General for the State of New York, Barbara D. Underwood, 

Solicitor General, Steven C. Wu, Deputy Solicitor General, 

Michael J. Myers, Special Counsel for Air Pollution and 

Climate Change Litigation, Brian Lusignan, Assistant 

Attorney General, Kathleen Jennings, Attorney General, 

Office of the Attorney General for the State of Delaware, Brian 

E. Frosh, Attorney General, Office of the Attorney General for 

the State of Maryland, Keith Ellison, Attorney General, Office 

of the Attorney General for the State of Minnesota, Ellen F. 

Rosenblum, Attorney General, Office of the Attorney General 

for the State of Oregon, Thomas J. Donovan, Jr., Attorney 

General, Office of the Attorney General for the State of 

Vermont, William Tong, Attorney General, Office of the 

Attorney General for the State of Connecticut, Aaron M. Frey, 

Attorney General, Office of the Attorney General for the State 

of Maine, Maura Healy, Attorney General, Office of the 

Attorney General for the Commonwealth of Massachusetts, 

Gurbir S. Grewal, Attorney General, Office of the Attorney 

General for the State of New Jersey, Peter F. Neronha, 

Attorney General, Office of the Attorney General for the State 

of Rhode Island, Karl A. Racine, Attorney General, Office of 

the Attorney General for the District of Columbia, and Robert 

W. Ferguson, Attorney General, Office of the Attorney General 

for the State of Washington, were on the brief for amici curiae

the States of New York, et al. in support of petitioners. 

Sue Chen and Tsuki Hoshijima, Attorneys, U.S. 

Department of Justice, argued the causes for respondent. With 
3

them on the brief was Jeffrey Bossert Clark, Assistant Attorney 

General.

Neil D. Gordon, Assistant Attorney General, Office of the 

Attorney General for the State of Michigan, argued the cause 

for intervenor-respondent State of Michigan. With him on the 

brief were Dana Nessel, Attorney General, and Fadwa A. 

Hammoud, Solicitor General.

Aaron M. Streett argued the cause for intervenors Greater 

El Paso Chamber of Commerce, et al. in support of 

respondents. With him on the brief was Matthew L. Kuryla.

Gabe Johnson-Karp, Assistant Attorney General, Office 

of the Attorney General for the State of Wisconsin, argued the 

cause for intervenor State of Wisconsin in support of 

respondents. With him on the brief was Joshua L. Kaul, 

Attorney General. 

Ken Paxton, Attorney General, Office of the Attorney 

General for the State of Texas, Kyle D. Hawkins, Solicitor 

General, Bill Davis, Deputy Solicitor General, and Linda B. 

Secord, Assistant Attorney General, were on the brief for 

respondent-intervenors the State of Texas, et al.

John R. Jacus, Shannon W. Stevenson, Catherine E. 

Stetson, and Jennifer L. Biever were on the brief for amici 

curiae American Petroleum Institute, et al. in support of 

respondents.
4

Before: TATEL, GRIFFITH, and PILLARD, Circuit Judges.

Opinion for the court filed PER CURIAM.

In these consolidated petitions, several environmental 

organizations, municipal governments, and the State of Illinois

challenge area designations promulgated by the Environmental 

Protection Agency (EPA) for the National Ambient Air Quality 

Standards (NAAQS) applicable to ground-level ozone, i.e., 

smog. Insisting that EPA failed to meet its basic obligation of 

reasoned decisionmaking for many of the designations, 

petitioners ask us to vacate those designations and send EPA 

back to the drawing board. In response, EPA disputes certain 

petitioners’ standing, requests a voluntary remand of some 

designations, and defends other designations on their merits. 

For the following reasons, we find that at least one petitioner 

has standing to challenge each of the designations at issue,

grant several of the petitions, deny one petition, and grant 

EPA’s motion to remand the rest. 

We have previously summarized the governing regulatory

framework, and we draw on those decisions in providing an 

overview of the statutory provisions and the agency 

proceedings relevant to this case. See Miss. Comm’n on Envtl.

Quality v. EPA, 790 F.3d 138 (D.C. Cir. 2015) (per curiam) 

(reviewing area designations for the 2008 ozone NAAQS); 

Catawba Cty. v. EPA, 571 F.3d 20 (D.C. Cir. 2009) (per 

curiam) (same for 1997 particulate matter NAAQS). 

“Congress enacted the Clean Air Act (the Act), 42 U.S.C. 

§§ 7401 et seq., ‘to protect and enhance the quality of the 

Nation’s air resources so as to promote the public health and 

welfare and the productive capacity of its population.’” Miss. 

Comm’n, 790 F.3d at 144 (quoting 42 U.S.C. § 7401(b)(1)). 
5

Under the Act, EPA must establish and periodically revise 

NAAQS for pollutants that “may reasonably be anticipated to 

endanger public health or welfare.” 42 U.S.C. § 7408(a)(1)(A). 

A NAAQS establishes the maximum permissible ambient—

i.e., outdoor—air concentration for these so-called “criteria” 

pollutants. A network of air monitoring stations, known as 

monitors, measure pollutant concentrations and record 

violations of the NAAQS.

After EPA promulgates a new or revised NAAQS, the 

agency must designate each “area” in the United States as 

“attainment,” “nonattainment,” or “unclassifiable.” See id.

§ 7407(d)(1)(A)(i)–(iii). Generally, areas that meet the relevant 

NAAQS are designated as attainment; areas that exceed the 

NAAQS as nonattainment; and areas that “permit no 

determination given existing data” as unclassifiable. Id. Under 

the Act, however, even an area whose ambient air 

concentration complies with the relevant NAAQS must be 

designated as nonattainment if it “contributes” to a NAAQS 

violation in a “nearby area.” Id. § 7407(d)(1)(A)(i). 

“[C]ontributes,” “nearby,” and “area” are undefined in the Act. 

EPA works collaboratively with states and tribes to 

determine the NAAQS attainment status for all areas within a 

respective state’s borders. No later than one year after the 

agency promulgates a new or revised NAAQS, each state must 

recommend “initial designations” to EPA. Id. § 7407(d)(1)(A). 

A state’s initial designations must suggest both the appropriate 

geographic boundaries for each “area” and whether EPA 

should classify the proposed areas as attainment, 

nonattainment, or unclassifiable. See id. § 7407(d)(1)(A)–(B). 

Once EPA receives a state’s initial designations, it may 

either promulgate them as submitted or modify them as it 

“deems necessary.” Id. § 7407(d)(1)(B)(ii). EPA may change a 
6

state’s recommended designation, alter a state’s proposed 

geographic area, or both. See id. Although EPA “has no 

obligation to give any quantum of deference to a designation 

that it ‘deems necessary’ to change,” Catawba, 571 F.3d at 40, 

it must notify the state of any intended change and provide the 

state with at least 120 days “to demonstrate why any proposed 

modification is inappropriate,” 42 U.S.C. § 7407(d)(1)(B)(ii). 

“These notifications are commonly known as the 120-day 

letters.” Air Quality Designations for the 2015 Ozone National 

Ambient Air Quality Standards, 82 Fed. Reg. 54,232, 54,233 

(Nov. 16, 2017) (“2015 Designations Rule”). Under the Act, 

EPA must promulgate designations “as expeditiously as 

practicable, but in no case later than 2 years from” the date it 

revises a NAAQS. 42 U.S.C. § 7407(d)(1)(B)(i). If EPA “has 

insufficient information to promulgate the designations,” it

may extend the deadline “for up to one year,” but no further. 

Id. 

An area’s designation is important because it determines 

the stringency of applicable emission controls. Although EPA 

has ultimate authority to determine each area’s attainment 

status, states have “primary responsibility” for ensuring that the 

geographic areas within their borders either maintain 

attainment status or progress towards it. Id. § 7407(a). 

Accordingly, once EPA finalizes its designations, states must 

submit to EPA “implementation plan[s]” specifying how the 

NAAQS “will be achieved and maintained.” Id. In attainment 

or unclassifiable areas, a state need only implement “emission 

limitations and such other measures as may be necessary . . . to 

prevent significant deterioration of air quality.” Id. § 7471. For 

nonattainment areas, by contrast, the Act imposes more 

stringent requirements, including, for example, requiring states 

to establish permitting programs for pollution sources, see id.

§ 7502(c)(5), and to demonstrate in their plans that they intend 

to implement “all reasonably available control measures” and 
7

“reasonably available control technology” to bring the area into 

attainment, id. § 7502(c)(1). The Act also imposes deadlines, 

or “attainment dates,” on offending areas, which set the time 

period in which an area must reach attainment status. See id.

§ 7502(a)(2)(A). 

This case involves the NAAQS for ozone, “a colorless gas 

that occurs both in the Earth’s upper atmosphere and at ground 

level.” Murray Energy Corp. v. EPA, 936 F.3d 597, 605 (D.C. 

Cir. 2019). Although “ozone is an essential presence in the 

atmosphere’s stratospheric layer, it becomes harmful at ground 

level and can cause lung dysfunction, coughing, wheezing, 

shortness of breath, nausea, respiratory infection, and in some 

cases, permanent scarring of the lung tissue.” Miss. Comm’n, 

790 F.3d at 147 (internal quotation marks omitted). “It also has 

a broad array of effects on trees, vegetation, and crops and can 

indirectly affect other ecosystem components such as soil, 

water, and wildlife.” Id. (internal quotation marks omitted). 

Because ozone forms at ground level when “precursor” 

emissions—nitrogen oxides (NOx) and volatile organic 

compounds (VOCs)—react with sunlight, compliance with the 

ozone NAAQS largely depends on reducing emissions from 

ozone-precursor producers like power plants, motor vehicles, 

and combustion engines. Id.; see also 2015 Designations Rule, 

82 Fed. Reg. at 54,233. “Complicating this task is that ozone 

and ozone precursors travel easily through the atmosphere, 

which can result in NAAQS violations hundreds of miles away 

from the source of the ozone precursors.” Miss. Comm’n, 790 

F.3d at 147. 

In 2015, EPA strengthened the ozone NAAQS, reducing 

the maximum allowable “design value”—an average daily

eight-hour level of ozone, see 40 C.F.R. § 50, App. U—from 

0.075 parts per million (ppm) to 0.070 ppm, triggering states’ 

responsibility to submit initial area designations, see 2015 
8

Designations Rule, 82 Fed. Reg. at 54,233. To assist states in 

this process, EPA issued guidance identifying the key factors it 

intended to evaluate in making final nonattainment area 

boundaries, emphasizing that “[b]ecause ozone and its 

precursor emissions are pervasive and readily transported, the 

EPA believes it is important to examine ozone-contributing 

emissions across a relatively broad geographic area associated 

with a monitored violation.” Memorandum from Janet G. 

McCabe, Acting Assistant Adm’r, to Reg’l Adm’rs, Area 

Designations for the 2015 Ozone National Ambient Air Quality 

Standards 5 (Feb. 25, 2016), Joint Appendix (J.A.) 511

(“Guidance Memo”). The agency explained that, “[c]onsistent 

with past designations for ozone NAAQS,” it planned to use a

“weight-of-evidence” approach to designations, evaluating 

information relevant to five factors: (1) air-quality data, i.e., 

whether an area’s monitor reported a NAAQS violation; 

(2) emissions and emissions-related data, including NOx and 

VOC levels, population, degree of urbanization, and traffic and 

commuting patterns; (3) meteorology, which involves 

calculating the effect of things like wind speed and direction, 

temperature, humidity, and pressure on air parcels in order to 

model the transport of ozone and ozone-causing emissions; 

(4) geography/topography, i.e., the effect of physical land 

features on the distribution of ozone; and (5) jurisdictional 

boundaries, which help determine whether a given 

nonattainment area will be able to effectively carry out airquality planning and enforcement functions. Id. at 6, J.A. 512; 

see also id. Attach. 3, at 4-10, J.A. 522-28. 

To evaluate these factors, EPA uses a variety of analytical 

tools and data. We describe just one here in general fashion, 

reserving additional elaboration on other tools for those 

portions of the opinion that require it.
9

Significantly for several of the petitions, when evaluating 

the effect of meteorology, EPA frequently relies on Hybrid 

Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) 

data, which model the three-dimensional paths traveled by air 

parcels to a violating monitor. These paths, known as “back 

trajectories,” Guidance Memo 7, J.A. 525, are depicted on a 

standard map as single lines extending in two-dimensional 

space from a starting point. Each individual trajectory has only 

one starting height, so EPA plots trajectories of different 

starting heights—generally 100, 500, and 1,000 meters—from 

the same point, using different colors to distinguish the heights. 

According to EPA, HYSPLIT maps can “be easily 

misinterpreted as having finer accuracy than the underlying 

model and data” and “[o]ne should avoid concluding a region 

is not along a trajectory’s path” simply because that trajectory’s 

line “missed the region by a relatively small distance.” Id. at 8, 

J.A. 526. 

By October 2016, all states had submitted their proposed 

designations to EPA. Rather than immediately review the 

designations, however, EPA attempted to extend the 

designations process by one year. See Extension of Deadline 

for Promulgating Designations for the 2015 Ozone National 

Ambient Air Quality Standards, 82 Fed. Reg. 29,246, 29,247

(June 28, 2017). After various parties sued, EPA withdrew the 

extension but published designations for only those areas that 

states recommended be designated as attainment or 

unclassifiable. See 2015 Designations Rule, 82 Fed. Reg. at

54,232. Another lawsuit followed, and this time a district court 

ordered EPA to promulgate final designations for all areas of 

the country, except for eight counties in the San Antonio area, 

by April 30, 2018. See In re Ozone Designation Litig., 286 F. 

Supp. 3d 1082, 1091 (N.D. Cal. 2018).
10

In response, EPA sent 120-day letters to states and tribes 

announcing its intended designations, and, although not 

required by the Act, provided a thirty-day public comment 

period. See EPA Responses to Certain State Designation 

Recommendations for the 2015 Ozone National Ambient Air 

Quality Standards: Notice of Availability and Public Comment 

Period, 83 Fed. Reg. 651, 652 (Jan. 5, 2018). In accordance 

with the district-court order, EPA finalized the remaining 

designations—including all those disputed here—in April 

2018. See Additional Air Quality Designations for the 2015 

Ozone National Ambient Air Quality Standards, 83 Fed. Reg. 

25,776 (June 4, 2018) (“Final Designations Rule”). In deciding 

whether to alter the states’ proposed nonattainment boundaries, 

EPA applied the Guidance Memo’s multi-factor, weight-ofthe-evidence test, explaining its findings for each factor in 

technical support documents (TSDs).

After EPA promulgated the final designations, several

public health and environmental groups (“Environmental 

Petitioners”), plus Illinois and three municipalities

(“Government Petitioners”), sought review in this court, 

challenging a subset of the attainment designations. We 

consolidated the petitions and granted the requests of several 

other states and industry groups to intervene on EPA’s behalf, 

and now “review petitioners’ challenges under section 

307(d)(9) of the Clean Air Act.” Catawba, 571 F.3d at 29.

Before proceeding to the merits, we must confirm our 

jurisdiction. The Clean Air Act authorizes judicial review of 

final EPA actions, see 42 U.S.C. § 7607(b)(1), but EPA 

contests petitioners’ Article III standing. First, EPA argues that 

the Environmental Petitioners lack standing because the 

challenged attainment designations cause no harm to members 

who reside in areas that merely “contribut[e]” to NAAQS 
11

violations. EPA Br. 15. Second, EPA claims that Government 

Petitioners may not sue a federal agency as parens patriae, 

invoking the rule articulated in Massachusetts v. Mellon, 262 

U.S. 447 (1923). We reject both arguments. 

Environmental Petitioners

Environmental Petitioners assert associational standing to 

bring this suit on behalf of their members. EPA argues that 

certain Environmental Petitioners lack associational standing 

because none of their members “have standing to sue in their 

own right.” Fund Democracy, LLC v. SEC, 278 F.3d 21, 25 

(D.C. Cir. 2002). For each challenged designation, an 

Environmental Petitioner must identify one member who 

suffers from a “concrete and particularized” injury-in-fact 

caused by the challenged agency conduct and redressable by a 

favorable judicial decision. Nat. Res. Def. Council v. EPA, 755 

F.3d 1010, 1016 (D.C. Cir. 2014) (quoting Ark Initiative v. 

Tidwell, 749 F.3d 1071, 1075 (D.C. Cir. 2014)). As with a 

plaintiff at the summary-judgment stage in district court, a 

petitioner challenging final agency action must “support each 

element of its claim to standing ‘by affidavit or other 

evidence.’” Sierra Club v. EPA, 292 F.3d 895, 899 (D.C. Cir. 

2002) (quoting Lujan v. Defs. of Wildlife, 504 U.S. 555, 561 

(1992)). 

Supreme Court precedent and our own case law identify 

several environmental harms that constitute cognizable 

injuries. First, “environmental plaintiffs adequately allege 

injury in fact when they aver that they use the affected area and 

are persons ‘for whom the aesthetic and recreational values of 

the area will be lessened’ by the challenged activity.” Friends 

of the Earth, Inc. v. Laidlaw Envtl. Servs. (TOC), Inc., 528 U.S. 

167, 183 (2000) (quoting Sierra Club v. Morton, 405 U.S. 727, 

735 (1972)); see also Nat. Res. Def. Council v. EPA, 489 F.3d 

1364, 1371 (D.C. Cir. 2007). Adverse health effects likewise 
12

constitute Article III injuries, even if a petitioner merely asserts 

realistic health concerns instead of providing medical evidence. 

See Nat. Res. Def. Council, 755 F.3d at 1016-17 (finding 

standing based on declarations of members “who are concerned 

about the [challenged EPA action’s] effects on their health and, 

in some cases, spend less time outdoors on that account”); see 

also Sierra Club v. EPA, 926 F.3d 844, 848-49 (D.C. Cir. 2019) 

(similar). Finally, in the Clean Air Act context, “[t]he health 

and economic costs of increased . . . pollution for individuals 

in nonattainment areas constitute injuries in fact that are fairly 

traceable to the EPA’s challenged rule.” WildEarth Guardians 

v. EPA, 830 F.3d 529, 535 (D.C. Cir. 2016). 

EPA contends that Environmental Petitioners have not 

identified members injured by the attainment designations of 

McHenry, Monroe, Lake, Porter, Door, Kenosha, and El Paso 

counties. This argument is correct only as to Monroe County, 

for which no Environmental Petitioner has supplied a member 

affidavit. (We address Government Petitioners’ standing to 

challenge the Monroe County designation in Part II.B, infra.) 

For the remaining designations, Environmental Petitioners 

have submitted declarations that establish concrete injuries-infact caused by EPA’s failure to regulate that are redressable by 

this Court. 

Association members who reside in McHenry, Door, 

Kenosha, and El Paso counties provided affidavits asserting 

concrete health and recreational injuries caused by ozone 

pollution from sources within their home counties. See 

Schindler Decl. ¶¶ 4-6, 10-13; Leline Decl. ¶¶ 5-7, 13-14;

Perloff Decl. ¶¶ 5-7, 15-16; Powers Decl. ¶¶ 5-9, 14-16; 

Antaramian Decl. ¶¶ 5-7, 14-16; De Aztlan Decl. ¶¶ 2-4. For 

example, Clean Wisconsin member Willard Leline, Jr. declares 

that EPA’s designation of much of Door County as attainment 

exposes him to unhealthy ozone concentrations during his 
13

preferred forms of outdoor recreation, including “golfing, 

hiking, boating, tending horses, doing yardwork, grilling, 

running, [and] working outdoors as a Segway tour guide.” 

Leline Decl. ¶¶ 6, 13-14. Association members from Cook 

County declare similar injuries caused by pollution contributed 

by Lake and Porter counties. See Horine Decl. ¶¶ 2, 6-10; 

Lipton Decl. ¶¶ 3-8. 

These concrete injuries are “fairly traceable to the EPA’s 

challenged rule” and redressable by an order setting aside that 

decision as arbitrary and capricious. WildEarth Guardians, 830 

F.3d at 535; see also Sierra Club v. EPA, 699 F.3d 530, 533 

(D.C. Cir. 2012). Put simply, an attainment designation 

amounts to a relaxation of regulatory requirements. State 

regulations in an attainment area need only “prevent significant 

deterioration of air quality.” 42 U.S.C. § 7471. Conversely, a 

nonattainment area must be brought into NAAQS compliance 

prior to a statutory deadline, id. § 7502(a)(2)(A), using “all 

reasonably available control measures,” id. § 7502(c)(1). By 

preserving the status quo rather than demanding stricter 

pollution controls, EPA’s designations increase the likelihood 

that Environmental Petitioners’ members will experience 

ozone-related injuries. 

Faced with these attestations of harm, EPA proposes a new 

categorical rule for standing in area-designation suits. EPA 

argues that petitioners “cannot establish standing by relying on 

members [who reside] in areas that comply with the ozone 

standards,” including “allegedly contributing areas.” EPA Br. 

14-15. This new rule would deny standing to Environmental 

Petitioners whose members live in areas that, in their view, 

contribute to “nearby” ozone violations. See 42 U.S.C. 

§ 7407(d)(1)(A)(i).
14

EPA’s argument requires that we disregard the real-world 

injuries described in petitioners’ declarations and focus instead 

on a formal distinction between “contributing” and “violating” 

areas. But that distinction is a product of the challenged agency 

action, and by relying on it, EPA implicitly assumes that its 

area designations reflect scientific reality. At this stage, 

however, we must assume that petitioners are correct that EPA 

should have designated the contested areas as nonattainment 

zones. See City of Waukesha v. EPA, 320 F.3d 228, 235 (D.C. 

Cir. 2003). It does not matter whether EPA should have done 

so because these areas contain violations or because they 

contribute to “nearby” violations—more ozone is more ozone, 

and there is no “threshold concentration below which” groundlevel ozone is “known to be harmless.” Am. Trucking Ass’ns, 

Inc. v. EPA, 283 F.3d 355, 360 (D.C. Cir. 2002) (citing 

National Ambient Air Quality Standards for Ozone, 62 Fed. 

Reg. 38,856, 38,863 (July 18, 1997)). Either way, petitioners 

attest that EPA’s decision to forgo stricter controls on emitters 

in these areas will expose them to higher ozone levels. 

Government Petitioners

Illinois and the City of Chicago challenge the designations 

of Monroe, McHenry, Kenosha, Lake, and Porter counties. 

Illinois and Chicago are the only petitioners that filed an 

affidavit alleging injuries caused by emissions from Monroe 

County. The City of Sunland Park, New Mexico challenges the 

designation of El Paso County, but because we have already 

concluded that Familias Unidas del Chamizal has standing to 

challenge the El Paso designation, see Part II.A, supra, we need 

not consider Sunland Park’s independent assertion of direct 

harm. See Massachusetts v. EPA, 549 U.S. 497, 518 (2007) 

(explaining that only one petitioner must demonstrate 

standing).
15

EPA argues that Government Petitioners such as Illinois 

and Chicago may not bring a parens patriae suit against an arm 

of the federal government. EPA Br. 15-17. In Massachusetts v. 

Mellon, the Supreme Court held that “a state, as parens patriae, 

may [not] institute judicial proceedings to protect citizens of 

the United States from the operation of the statutes thereof.” 

262 U.S. at 485. Petitioners disclaim parens patriae status and 

assert standing based on “direct” injuries. Reply Br. 7-10. 

Alternatively, they claim that various Clean Air Act provisions 

supplant the Mellon rule.

Mellon established that states lack “standing as parens 

patriae to bring an action against the Federal Government.” 

Alfred L. Snapp & Son, Inc. v. P.R. ex rel. Barez, 458 U.S. 592, 

610 n.16 (1982). In other words, state governments cannot sue 

the federal government on behalf of their injured state citizens. 

The same rule presumably applies to municipal governments. 

City of Olmsted Falls, Ohio v. FAA, 292 F.3d 261, 268 (D.C. 

Cir. 2002). However, Mellon does not apply when a state sues 

in its capacity as a state rather than a representative of its 

citizens. See West Virginia v. EPA, 362 F.3d 861, 868 (D.C. 

Cir. 2004). A government that demonstrates direct harm to its 

economic, environmental, or administrative interests as a result 

of federal action may have standing to sue the federal 

government. In Massachusetts v. EPA, for instance, the 

Supreme Court found standing based on the Commonwealth’s 

assertion of “a particularized injury in its capacity as a 

landowner.” 549 U.S. at 522; see also Nat’l Ass’n of Clean Air 

Agencies v. EPA, 489 F.3d 1221, 1228 (D.C. Cir. 2007); City 

of Olmsted Falls, 292 F.3d at 268 (finding standing where, on 

a “generous reading of the petitioner’s materials,” the City 

“alleged harm to its own economic interests based on the 

environmental impacts of the approved [agency] project”).
16

Illinois and Chicago submitted their first standing 

declaration with petitioners’ opening brief. The declaration 

conveys expert testimony from Dr. Stephen Zemba, who 

described “[t]he purpose of [his] declaration” as “provid[ing] 

information to the Court relating to the following question: 

whether residents of the State of Illinois and City of Chicago 

will suffer injuries as a result of” EPA’s designations. First 

Zemba Decl. ¶ 4 (emphasis added). Although the declaration 

notes that ozone “damages vegetation,” id. ¶ 7, it does not 

contain direct evidence of harm to the City or State in their 

governmental capacities. Instead, it sounds in the language of 

parens patriae.

EPA noted this omission. EPA Br. 15-16. Illinois and 

Chicago responded by alleging a direct injury in their reply 

brief: property damage to government parks. Reply Br. 7-9. 

They support this claim with another declaration from Dr. 

Zemba. Through that declaration, Illinois and Chicago refocus 

their injury-in-fact claim on “harm [to] trees and other 

vegetation on state and city owned-and-operated lands,” 

including “Illinois State Parks and Chicago Park District 

parks.” Second Zemba Decl. ¶ 5, Reply Br. Attach. B. Dr. 

Zemba declares that emissions from the challenged areas 

traveled over state parks in northeast Illinois, id. ¶ 7, and city 

parks in Chicago, id. ¶ 8, and degraded government-owned 

flora. 

On its own terms, this declaration demonstrates an Article

III injury. As in Massachusetts v. EPA, Illinois and Chicago

have alleged “particularized injur[ies] in [their] capacit[ies] 

as . . . landowner[s],” 549 U.S. at 522, so Mellon poses no 

barrier to suit. EPA, however, urges us not to consider any 

standing declaration submitted in the first instance with 

petitioners’ reply brief. EPA Br. 17 n.10. 
17

Our decision in Sierra Club v. EPA instructs “a petitioner 

whose standing is not self[-]evident” to “establish its standing 

by the submission of its arguments and any affidavits or other 

evidence appurtenant thereto at the first appropriate point in the 

review proceeding”—typically its opening brief. 292 F.3d at

900. But Sierra Club is not “a ‘gotcha’ trap,” Am. Library Ass’n 

v. FCC, 401 F.3d 489, 493 (D.C. Cir. 2005), and we retain 

“discretion to look beyond the opening brief and consider 

material submitted later if the petitioner reasonably believed its 

standing was self-evident,” Del. Dep’t of Nat. Res. & Envtl. 

Control v. EPA, 785 F.3d 1, 8 (D.C. Cir. 2015) (alteration 

omitted) (internal quotation marks omitted). 

Illinois and Chicago claim they reasonably believed their 

standing was self-evident. Reply Br. 8. They point to our 

observation in Mississippi Commission that ozone “has a broad 

array of effects on trees, vegetation, and crops,” 790 F.3d at 

147, as well as statements in the administrative record by EPA 

and commenters reciting ozone’s harmful effects on forests, 

crops, and “park ecosystems around the country,” Comments 

of National Parks Conservation Association 1-2, J.A. 721-22; 

see also Guidance Memo 1, J.A. 507. Although we reiterate 

that litigants risk forfeiture by submitting standing declarations 

on reply, these petitioners could reasonably believe “that the 

initial filings before the court had sufficiently demonstrated 

standing,” Ctr. for Sustainable Econ. v. Jewell, 779 F.3d 588, 

599 (D.C. Cir. 2015), and that the administrative record 

contained ample evidence supporting standing, Am. Library 

Ass’n, 401 F.3d at 494.

At oral argument, EPA raised for the first time a second 

objection to petitioners’ standing, namely that Monroe County 

emissions originate too far away from northeastern Illinois to 

harm the parks identified in petitioners’ second declaration. 

Oral Arg. Tr. 7:5-10. However, EPA has been aware since 
18

January 25, 2019, that petitioners’ standing theory relied on 

ozone that was carried from Monroe County to northeastern 

Illinois. See First Zemba Decl. ¶¶ 22-23. Dr. Zemba’s initial 

expert declaration—submitted alongside petitioners’ opening 

brief—made that clear. Id. His second declaration relied on the 

same ozone-transport theory and simply identified a second 

injury: one to government-owned vegetation rather than 

Chicago-area residents. See Second Zemba Decl. ¶¶ 4-8. 

EPA has never directed us to evidence in the 

administrative record refuting petitioners’ factual claims. Nor 

has it submitted a rebuttal affidavit. Thus, applying Sierra 

Club’s summary-judgment analogy, 292 F.3d at 899, EPA has 

not generated a factual dispute regarding injury-in-fact, 

causation, or redressability. See Massachusetts v. EPA, 415 

F.3d 50, 66 (D.C. Cir. 2005) (Tatel, J., dissenting) (“[I]f EPA 

wants to challenge the facts petitioners have set forth in their 

affidavits, it has an obligation to respond to the petitioners 

[with affidavits or record evidence].”), rev’d on other grounds, 

549 U.S. 497. And even if summary judgment is an imperfect 

comparison, we are ill-equipped to conduct a rigorous sua 

sponte inquiry into the scientific soundness of an unrebutted 

expert declaration. See id. at 55 (majority opinion) (declining

to “refer the standing issues to a special master for a factual 

determination” or “remand to EPA for a factual determination 

of causation and redressability”). 

We therefore accept Dr. Zemba’s affidavits for purposes 

of establishing Government Petitioners’ standing to challenge 

the Monroe County designation. On their own uncontested 

terms, these expert declarations—relying in part on evidence 

from the administrative record—show that emissions from 

sources in Monroe County increase ozone concentrations in 

northeast Illinois. First Zemba Decl. ¶¶ 22-23. These 

heightened concentrations in turn damage vegetation in state-
19

and city-owned parks. Second Zemba Decl. ¶¶ 4-8. This injury 

is sufficiently concrete and particularized, see Massachusetts, 

549 U.S. at 522, and the unrebutted declarations demonstrate a 

“substantial probability” that air quality in northeastern Illinois 

would be improved if EPA subjected Monroe County emitters 

to stricter regulation, Sierra Club, 292 F.3d at 899. Finally, 

EPA’s eleventh-hour assertion that Monroe County is simply 

too far away to affect the relevant area is belied by its statement 

that “[o]zone and ozone precursors can be transported to an 

area from sources in nearby areas or from sources located 

hundreds of miles away.” Final Designations Rule, 83 Fed. 

Reg. at 25,777.

Alternatively, Government Petitioners argue that the 

Clean Air Act displaces the Mellon bar and permits parens 

patriae suits. Reply Br. 10-12. We have held that “the Mellon

bar speaks to prudential, not Article III, standing,” and that 

Congress may therefore enact a statute “authoriz[ing] a State to 

sue the federal government in its parens patriae capacity.” 

Gov’t of Manitoba v. Bernhardt, 923 F.3d 173, 180 (D.C. Cir. 

2019). Because we conclude that Government Petitioners have 

adequately demonstrated standing based on direct injuries 

rather than parens patriae status, we leave this question for 

another day. 

Having established that at least one petitioner has standing 

to challenge each designation, we turn to the merits. In 

reviewing EPA’s area designations, we apply “the same 

standard we use in reviewing a challenge brought under the 

Administrative Procedure Act,” Miss. Comm’n, 790 F.3d at 

150, “which requires the court to set aside EPA’s final actions 

when they are in excess of the agency’s statutory authority or 

otherwise arbitrary and capricious,” Catawba, 571 F.3d at 29 

(citing 42 U.S.C. § 7607(d)(9)). “[W]e must give an extreme 
20

degree of deference to the EPA’s evaluation of scientific data 

within its technical expertise,” Maryland v. EPA, 958 F.3d 

1185, 1196 (D.C. Cir. 2020) (per curiam) (internal quotation 

marks omitted), and will “uphold its action if the record shows 

that the EPA considered all relevant factors and articulated a 

rational connection between the facts found and the choice 

made,” Miss. Comm’n, 790 F.3d at 150 (internal quotation 

marks omitted). “That requirement is satisfied when the 

agency’s explanation is clear enough that its path may 

reasonably be discerned.” Encino Motorcars, LLC v. Navarro, 

136 S. Ct. 2117, 2125 (2016) (internal quotation marks 

omitted). We “may not,” however, “accept . . . counsel’s post 

hoc rationalizations” for EPA’s decisions; rather, the 

designations “must be upheld, if at all, on the basis articulated 

by the agency itself.” Motor Vehicle Mfrs. Ass’n v. State Farm 

Mut. Auto. Ins. Co., 463 U.S. 29, 50 (1983). 

In framing their challenges to the final designations, 

petitioners rely on comparisons to EPA’s intended 

designations, which do not bind EPA. Quite the opposite. The 

Act prescribes an iterative process in which EPA may revise its 

intended designations in response to states’ comments. See 42 

U.S.C. § 7407(d)(1)(B)(ii) (requiring EPA to allow states 120 

days “to demonstrate why any proposed modification[s]” to the 

states’ recommendations are “inappropriate”). Nonetheless, we 

have held that where EPA “rhetorical[ly] revis[es] . . . its 

characterization” of data between intended and final 

designations, that may be evidence of arbitrariness. Catawba, 

571 F.3d at 51. As we recently explained, the relevant inquiry 

asks not simply whether EPA changed position, but whether 

the agency satisfied “its obligation ‘to enable’ a reviewing 

court to conclude that the agency’s action ‘was the product of 

reasoned decisionmaking.’” Physicians for Soc. Responsibility 

v. Wheeler, 956 F.3d 634, 646 (D.C. Cir. 2020) (quoting State 

Farm, 463 U.S. at 52). 
21

Jefferson County

We first consider petitioners’ challenge to EPA’s 

attainment designation for Jefferson County, Missouri, located 

in the St. Louis, Missouri-Illinois metropolitan area. EPA 

initially announced that it would designate Jefferson as 

nonattainment based on its contributions to area violations,

citing 2014-2016 data showing five violating monitors in the 

St. Louis area. Although Jefferson itself lacks a monitor, EPA 

concluded that the violating monitors’ back trajectories 

“highlight[] the potential impacts to the violating monitors 

from the counties to the south” of St. Louis, including 

Jefferson. St. Louis, MO-IL Nonattainment Area, Intended 

Area Designations for the 2015 Ozone National Ambient Air 

Quality Standards Technical Support Document 24, J.A. 715. 

The agency also emphasized that Jefferson has “among the 

highest NOx and VOC emissions” and “among the highest 

[vehicle miles traveled]” for the area. Id. at 23-24, J.A. 714-15.

Despite this, EPA ultimately designated Jefferson as 

attainment, explaining that between Jefferson’s initial and final 

designations, the number of violating St. Louis-area monitors 

dropped from five to one. See St. Louis, MO-IL Nonattainment 

Area, Final Area Designations for the 2015 Ozone National 

Ambient Air Quality Standards Technical Support Document 

8-9, J.A. 1167-68 (“St. Louis Final TSD”). Petitioners contend 

that Jefferson’s attainment designation is arbitrary and 

capricious because (1) EPA designated similarly-situated 

Boles Township, located in nearby Franklin County, as 

nonattainment; and (2) EPA failed to address data supporting 

its earlier conclusion that Jefferson contributes to St. Louisarea violations. We agree on both points. 

We have explained that EPA engages in “arbitrarily 

disparate treatment . . . if it treat[s] genuinely similar counties 

dissimilarly.” Miss. Comm’n, 790 F.3d at 169 (emphasis 
22

omitted) (internal quotation marks omitted). That is precisely 

what EPA did here. Both Jefferson and Boles Township

contain stationary sources that emit approximately 6,000 tons 

of NOx annually, and Jefferson fares worse than Franklin 

County on almost every other metric discussed in the final 

TSD: Jefferson has twice the population, is growing more than

twice as fast, has more vehicle miles travelled, and has more

residents that commute to St. Charles County, where the stillviolating West Alton monitor is located. See St. Louis Final 

TSD 10, 11, 13, 15, J.A. 1169, 1170, 1172, 1174; Memo from 

Denise Scott, U.S. EPA, Office of Air Quality Planning and 

Standards, Air Quality Policy Division, to EPA Docket No. 

EPA-HQ-OAR-2017-0548, EPA Datasets Used for 2015 

Ozone NAAQS Designations, J.A. 1367-68. For Franklin, 

moreover, EPA addressed the back-trajectory data for all 

seventeen of the West Alton monitor’s exceedance days, but 

for Jefferson, the agency addressed only the back-trajectory 

data for the monitor’s three highest exceedance days and took 

into account only 100-meter trajectories, while ignoring data 

from other exceedance days and trajectory heights. See St. 

Louis Final TSD 22-23, J.A. 1181-82.

Making matters worse, EPA treated Jefferson’s data 

inconsistently between the intended and final designations. See 

Catawba, 571 F.3d at 52 (finding that EPA acted arbitrarily and 

capriciously where the agency’s “rhetorical revision of its 

characterization” of data “[was] not justified by any change in 

the underlying data”). As explained above, EPA’s initial TSD

emphasized Jefferson’s emissions and vehicle miles travelled 

as reasons for the agency’s finding that Jefferson contributes to 

St. Louis-area violations. It also observed, citing HYSPLIT 

back-trajectory modeling, that ozone transport to the St. Louis 

area is predominately from the south, where Jefferson is 

located. EPA presented the same data in support of Jefferson’s 

final designation but said nothing about its significance, 
23

leaving us to conclude that EPA believed this unchanged data 

supports Jefferson’s attainment designation—a belief that 

makes no sense given the agency’s earlier emphasis on this 

very same data as evidence of contribution.

It is true, as EPA points out, that when the agency 

announced Jefferson’s intended designation, five St. Louisarea monitors were violating the ozone NAAQS—West Alton, 

Maryland Heights, Orchard Farm, Clara Barton School, and 

Water Plant—whereas by the time of Jefferson’s final 

designation, only the West Alton monitor continued to show 

violations. This, however, does nothing to bolster EPA’s 

conclusion, implicit in Jefferson’s attainment designation, that 

the county no longer contributes to West Alton’s violations. Put 

differently, we cannot see how the four other air-quality 

monitors’ compliance relates to the question of whether 

Jefferson contributes to the West Alton monitor’s violations, 

and EPA does not tell us.

In any event, EPA points to nothing in the record that 

justifies its differing treatment of Boles Township and 

Jefferson County. Instead, it argues in its brief that Jefferson’s 

largest point sources are forty-eight and fifty-one miles from 

the West Alton monitor, whereas Boles’s major source is just 

thirty-nine miles away. See EPA Br. 24. The agency, however, 

nowhere specified these distances in the final TSD, and “we 

cannot accept . . . counsel’s post hoc rationalizations for 

agency action.” Nat’l Ass’n of Clean Water Agencies v. EPA, 

734 F.3d 1115, 1138 (D.C. Cir. 2013) (internal quotation marks 

omitted). To be sure, the point sources’ locations are indicated 

on a map in the final TSD. See St. Louis Final TSD at 11 fig.3,

J.A. 1170. Even were this sufficient to specify the distances, 

however, EPA nowhere explains why nine- to twelve-mile 

differences between the point sources’ distances to the monitor 

justify the two areas’ differential treatment. 
24

In sum, EPA has, without explanation, treated similarlysituated areas—Jefferson and Boles—differently and drawn 

conflicting conclusions from the same data. “Such inconsistent 

treatment is the hallmark of arbitrary agency action,” and 

requires further explanation from EPA. Catawba, 571 F.3d at 

51.

Monroe County

Petitioners next challenge EPA’s attainment designation 

of Monroe County, Illinois, which, like Jefferson, is located in 

the St. Louis metropolitan area. Also as with Jefferson, EPA 

initially announced its intent to designate Monroe as 

nonattainment based on its contributions to St. Louis-area

violations. Although Monroe itself lacks a monitor, EPA 

concluded that it contributed to violations in nearby counties 

based on back-trajectory data from nearby violating monitors 

and data regarding Monroe’s emissions and vehicle miles 

travelled. Illinois concurred in EPA’s intended nonattainment 

designation.

Five days before EPA was due to promulgate Monroe’s 

final designation, however, an EPA employee emailed Illinois 

EPA director Alec Messina, seeking “5 min[ute]s . . . for a 

quick call about ozone” and explaining that then-EPA director 

Scott Pruitt had asked him to “reach out.” Email from Clint 

Woods, Deputy Assistant Adm’r, Office of Air and Radiation, 

U.S. EPA to Alex Messina, Dir., Ill. Envtl. Prot. Agency

(Apr. 25, 2018), J.A. 1452. The next day, Messina sent a onepage letter to Pruitt, stating, in relevant part:

I appreciate the recent opportunity for 

discussion of impending air quality 

designations for ozone as part of the 120-day 

consultation process. Indeed, Illinois EPA 

would be comfortable in an approach to such 
25

designations that ensures national and regional 

consistency by considering the 2014 emissions 

data that evidences the county-by-county 

contributions of nitrogen oxides and volatile 

organic material. As such, it would seem 

appropriate to consider a designation of 

attainment for . . . Monroe. 

Email from Alex Messina, Dir., Ill. Envtl. Prot. Agency, to 

Scott Pruitt, Adm’r, U.S. EPA (Apr. 26, 2018), J.A. 1371 

(“Messina Letter”). A few days later, EPA designated Monroe 

as attainment. In the final TSD, EPA cited the Messina Letter 

three times, noting that Monroe’s designation was “consistent 

with Illinois’ communication” in the letter. St. Louis Final TSD 

25, J.A. 1184; see also id. at 1 n.1, 2 n.3, J.A. 1160, 1161.

Petitioners argue that EPA (1) “inadequately explained”

Monroe’s attainment designation; (2) “employed an illogical 

process in switching [the] designation at the last minute based 

principally on the Messina Letter”; and (3) violated the Act’s 

requirement that the agency provide Illinois with 120 days’ 

notice before modifying its recommended designation.

Petitioners’ Br. 94. We need address only the first argument

because it alone demonstrates that Monroe’s designation must 

be remanded to EPA for further explanation.

As petitioners point out, the final TSD repeatedly cites the 

Messina Letter, which in turn cites the 2014-2016 monitoring 

data—the very data that EPA previously found supported a 

nonattainment designation. This incongruity, about which EPA 

says nothing in the final TSD, renders EPA’s explanation 

suspect. Like with Jefferson County, moreover, nothing in the 

final TSD disavows EPA’s initial reason for designating 

Monroe as nonattainment—namely, the county’s location to 

the south of the violating monitors, which, together with back-
26

trajectory data, supported EPA’s finding that Monroe 

contributes to St. Louis-area violations. To the contrary, the 

West Alton monitor continued to violate the NAAQS between 

2015 and 2017 and EPA itself noted that “HYSPLIT 

trajectories show the potential for air masses to traverse . . . 

Monroe.” St. Louis Final TSD 26, J.A. 1185.

It is true, as EPA points out, that the final TSD notes that 

air trajectories that traverse Monroe on their way to the West 

Alton monitor subsequently pass through other, higheremitting counties before reaching the monitor and, 

accordingly, that Monroe is “less likely to contribute to [West 

Alton’s] violations. Id. This statement, however, leaves open 

the possibility that Monroe contributes to West Alton’s 

violations—albeit, perhaps, at lower rates or less often than do 

neighboring counties. Cf. Sierra Club v. EPA, 884 F.3d 1185, 

1196 (D.C. Cir. 2018) (“[D]escribing [emissions] as ‘low,’

even ‘extremely low,’ . . . implies that [they] have not been 

entirely eliminated.”). In other words, we have no way of 

telling whether EPA means that, compared to neighboring

counties, Monroe has a lower probability of contributing on 

any given day, contributes less ozone to West Alton’s

violations, or does not contribute at all. This is a problem 

because, as EPA itself explains, the agency “must designate as 

nonattainment any area that monitors a violation of the 

NAAQS and any nearby areas that contribute to the violation.” 

St. Louis Final TSD 6, J.A. 1165 (emphasis added).

“The outcome the EPA ultimately reached [for Monroe]

may be reasonable; however, [n]ot only must an agency’s 

decreed result be within the scope of its lawful authority, but 

the process by which it reaches that result must be logical and 

rational.” U.S. Sugar Corp. v. EPA, 830 F.3d 579, 652 (D.C. 

Cir. 2016) (internal quotation marks omitted). By failing to

“provide[] an explanation” for designating Monroe as 
27

attainment “that is not premised on inconsistencies in the 

record,” id., EPA fell short of that requirement here.

Ottawa County

Petitioners next challenge EPA’s attainment designation 

of Ottawa County, Michigan. They focus on EPA’s failure to 

conduct a five-factor analysis of Ottawa County’s potential 

contributions to NAAQS violations in two adjacent counties. 

Petitioners’ Br. 85-90. They contend further that any 

reasonable analysis would have identified ozone contributions 

and resulted in a nonattainment designation.

Ottawa County sits on the eastern shore of Lake Michigan 

between two nonattainment counties: Muskegon to the north 

and Allegan to the south. Although Ottawa County contains no 

violating monitors, the violating monitor in neighboring 

Allegan County is quite literally next door. See Mich. Dep’t of 

Envtl. Quality, Recommended Area Designations in Michigan 

for the Ozone National Ambient Air Quality Standard 3-5

(Sept. 30, 2016), J.A. 293-95 (“Michigan Recommended 

TSD”); Michigan: Detroit, Muskegon County, Allegan 

County, and Berrien County Nonattainment Areas, Final Area 

Designations for the 2015 Ozone National Ambient Air Quality 

Standards Technical Support Document 23 fig.7, J.A. 1130 

(“Michigan Final TSD”); Oral Arg. Tr. 23:15-18 (“[T]he city 

of Holland where the Allegan monitor is located . . . [is] 

partially in Allegan and partially in Ottawa, in fact, I believe 

the violating monitor is actually across the street from Ottawa 

County.”). Michigan recommended an attainment designation 

for Ottawa County but nonattainment designations for 

Muskegon and Allegan, both of which contained violating 

monitors. Michigan Recommended TSD 6-9, 42-43, J.A. 296-

99, 332-33. The State attributed these violations to emissions 

from across Lake Michigan. See id. at 14-15, J.A. 304-05. EPA 

adopted Michigan’s approach in its intended designation, see 
28

Michigan: Allegan County, Berrien County, Muskegon County 

and Detroit Nonattainment Areas, Intended Area Designations 

for the 2015 Ozone National Ambient Air Quality Standards 

Technical Support Document 20-22, J.A. 576-78, but did not 

include a five-factor analysis of Ottawa County. 

Petitioner Sierra Club submitted comments challenging 

EPA’s intended attainment designation of Ottawa County, 

arguing that because the county was clearly “nearby” the 

violating areas of Muskegon and Allegan counties, EPA was 

required to evaluate its contribution potential. Comments of 

Sierra Club on EPA’s Intended Ozone Nonattainment Areas for 

Michigan 1 n.2 (Feb. 5, 2018), J.A. 757; see also 42 U.S.C. 

§ 7407(d)(1)(A)(i) (requiring nonattainment designations for 

areas that “contribute[] to ambient air quality in a nearby area 

that does not meet” the NAAQS). Sierra Club also identified 

Ottawa County’s J.H. Campbell Generating Complex as a 

significant source of ozone precursors based on a 2011 study 

showing that emissions from the plant “exceeded 1 percent of 

the 2015 primary ozone [NAAQS] on forty-three distinct 

days . . . at monitors in Muskegon, Allegan and/or Berrien 

Counties.” Id. at 4, J.A. 760. 

EPA made no relevant changes in its final TSD and did not 

add an Ottawa County contribution analysis. Michigan Final 

TSD 20-41, J.A. 1127-48. In response to Sierra Club’s 

comment, EPA stated that (1) according to air-transport 

modeling, violations in western Michigan were caused 

primarily by Chicago-area emissions; (2) the Club’s 2011 

study was outdated because the J.H. Campbell plant had 

installed new pollution controls since then; and (3) the Club 

offered no evidence connecting the plant’s emissions to “days 

when the relevant monitors . . . exceed[ed] the 2015 ozone 

NAAQS.” EPA, Responses to Significant Comments on the 

State and Tribal Designation Recommendations for the 2015 
29

Ozone National Ambient Air Quality Standards (NAAQS) 20

(Apr. 2018), J.A. 1206 (“EPA, Response to Comments”).

Petitioners characterize EPA’s treatment of Ottawa 

County as arbitrary and capricious. They point out that EPA 

declined to conduct the holistic contribution analysis 

prescribed by the Guidance Memo and contend that any such 

analysis would confirm that Ottawa contributes to violations in 

neighboring counties. EPA defends its decision not to conduct 

a five-factor analysis of Ottawa County and contests 

petitioners’ evidence of ozone contributions. 

The Clean Air Act requires EPA to designate an area as 

nonattainment if it “contributes to ambient air quality in a 

nearby area that does not meet” the ozone NAAQS. 42 U.S.C. 

§ 7407(d)(1)(A)(i) (emphasis added). By implication, EPA 

need not evaluate the contribution potential of an area without 

a “nearby” violation. Here, EPA announced its intention to 

“examine ozone-contributing emissions across a relatively 

broad geographic area” and to consider data “associated with 

the counties in the Combined Statistical Area (CSA) or, where 

appropriate, the Core Based Statistical Area (CBSA) in which 

the violating monitor(s) are located.” Guidance Memo 5, J.A. 

511.

As a general matter, we see nothing wrong with EPA’s 

chosen approach. Because “nearby,” as used in this portion of 

the Act, is an ambiguous term, “Chevron requires that we defer 

to the agency’s reasonable interpretation.” Pa. Dep’t of Envtl. 

Prot. v. EPA, 429 F.3d 1125, 1130 (D.C. Cir. 2005) (citing 

Chevron U.S.A. Inc. v. Nat. Res. Def. Council, Inc., 467 U.S. 

837, 843 (1984)). And in Mississippi Commission, we upheld 

an interpretation of “nearby” akin to that in EPA’s Guidance 

Memo. 790 F.3d at 152. In that case, petitioner Delaware 

“proposed a nonattainment area that would stretch across 16 
30

upwind states and . . . as far west as Missouri.” Id. at 151.

Connecticut volunteered a similar transcontinental 

nonattainment zone. Id. The states argued that any area “near 

enough to contribute to nonattainment” was sufficiently 

“nearby” for Clean Air Act purposes. Id. (internal quotation 

marks omitted). We declined to adopt that interpretation and 

deferred to EPA’s “definitional presumption,” id. at 153, that a 

“nearby” area comprises only those “counties within the same 

metropolitan area as the violating county,” id. at 152. EPA’s 

approach comported with dictionary definitions of “nearby,” 

past agency practice, and the Act’s tendency to refer to 

metropolitan areas as significant geographic units. Id. 

Elsewhere in Mississippi Commission we clarified that the 

colloquial term “metropolitan area” may describe either a 

Combined Statistical Area (CSA) or a Core Based Statistical 

Area (CBSA). Id. at 147 n.3. Each CBSA is composed of one 

Metropolitan or Micropolitan Statistical Area. Guidance Memo 

5, J.A. 511. Each CSA “is formed by two or more adjacent 

CBSAs if there is sufficient employment interchange between 

them.” Miss. Comm’n, 790 F.3d at 147 n.3 (internal quotation 

marks omitted); see also Guidance Memo 6, J.A. 512. 

According to the Office of Management and Budget’s 

2013 list, which EPA used for its 2015 designations, see 

Guidance Memo 5 n.8, J.A. 511, Ottawa County is in the same 

CSA as Muskegon and Allegan counties, see Office of Mgmt. 

& Budget, Exec. Office of the President, OMB Bull. No. 13-01,

Revised Delineations of Metropolitan Statistical Areas, 

Micropolitan Statistical Areas, and Combined Statistical 

Areas, and Guidance on the Uses of the Delineations of These 

Areas 33, 101 (2013). But because Muskegon and Allegan 

counties are both stand-alone CBSAs, Ottawa County does not 

share a CBSA with either violating monitor. See id. at 41, 72. 

As a result, the choice between analyzing the Muskegon and 
31

Allegan violations on a CSA or CBSA level meant the 

difference between considering or ignoring Ottawa County. 

EPA took the latter route. 

Petitioners protest that Ottawa “is directly adjacent to, and 

in the same metropolitan area as, a county with a violating 

monitor.” Reply Br. 40. The second assertion inaccurately 

conflates “metropolitan area” with CSA; as explained, 

Ottawa’s Metropolitan Statistical Area does not contain a 

violation. But petitioners understandably balk at the notion that 

Ottawa County, sandwiched between two violating monitors,

is not “nearby” for contribution purposes. Petitioners’ Br. 84 

n.10. 

EPA responds with two explanations. First, it reasons that 

a CBSA-based approach makes good sense when a lakeshore 

county is primarily affected by cross-lake, out-of-state sources. 

According to EPA’s support document, “the meteorological 

data strongly indicates that the violating monitors in” 

Muskegon and Allegan counties “are predominantly affected 

by the transport of emissions over Lake Michigan.” Michigan 

Final TSD 31, J.A. 1138; see also id. at 20, 29, 39, J.A. 1127, 

1136, 1146 (showing that exceedance-day air masses 

originated almost exclusively in the areas around Milwaukee 

and Chicago). Second, EPA points out that its treatment of 

Muskegon and Allegan counties is consistent with past agency 

practice. EPA Br. 44; see also Air Quality Designations and 

Classifications for the 8-Hour Ozone National Ambient Air 

Quality Standards, Early Action Compact Areas with Deferred 

Effective Dates, 69 Fed. Reg. 23,858, 23,910 (Apr. 30, 2004) 

(listing Muskegon and Allegan separately rather than as part of 

a multi-county CSA). 

These justifications strike us as plausible, but they arrive 

too late. “[T]he soundness of an agency’s decision must rest on 
32

the reasoning contained therein, and not on any post hoc 

justifications offered by counsel.” Oglala Sioux Tribe v. NRC, 

896 F.3d 520, 536 n.11 (D.C. Cir. 2018) (citing SEC v. Chenery 

Corp., 318 U.S. 80, 87-88 (1943)). EPA failed to offer either 

explanation in its final rule or support documents. In fact, EPA 

never explained its decision to evaluate the Muskegon and 

Allegan violations on the CBSA level and therefore to 

disregard Ottawa County’s emissions. Meanwhile, petitioner 

Sierra Club marshalled significant evidence of Ottawa 

County’s contribution potential in its public comment, 

including the 2011 study showing that emissions from 

Ottawa’s J.H. Campbell facility contributed ozone in excess of 

0.7 parts per billion (ppb) at the Muskegon and Allegan 

monitors. See Comments of Sierra Club 4, J.A. 760. Petitioners 

also emphasized Ottawa County’s large population and high 

levels of precursor emissions, categories in which Ottawa 

surpasses its northern and southern neighbors. See Michigan 

Recommended TSD at 42, J.A. 332; Comments of Sierra Club 

2, J.A. 758; Michigan Final TSD 27 fig.10, 36 fig.16, J.A. 

1134, 1143. This evidence is “relevant to our assessment of the 

reasonableness” of EPA’s unexplained failure to consider 

Ottawa County’s contribution potential. Miss. Comm’n, 790 

F.3d at 153.

On this record, we agree with Petitioners that EPA’s 

unexplained decision to disregard Ottawa County was arbitrary 

and capricious. We do not question EPA’s general discretion 

to choose between CSA- and CBSA-level contribution 

analyses. Nor do we require EPA to provide an explanation in 

every case. No challenger could fault EPA for failing to explain 

why a county located hundreds of miles from any violating 

monitor is not a “nearby area.” But Ottawa is not such a county. 

It sits directly between two violating monitors—one of which 

lies extremely close to the county line. And the record evidence 

renders plausible petitioners’ assertion that Ottawa County 
33

contributes to these ozone exceedances. On remand, EPA 

should either analyze Ottawa County’s contribution potential 

or explain why that analysis is unnecessary. 

Weld County

Petitioners challenge EPA’s decision to designate the 

northern portion of Weld County, Colorado, a large county 

located north of Denver, as attainment. Petitioners advance two 

theories. First, they argue that, in analyzing northern Weld’s 

contribution to Denver area exceedances, EPA improperly read 

a “significant contribution” requirement into the Clean Air 

Act’s instructions to designate as nonattainment areas that 

“contribute[] to” exceedances. 42 U.S.C. § 7407(d)(1)(A)(i); 

see Petitioners’ Br. 109-13. Second, petitioners argue that EPA 

arbitrarily drew a nonattainment boundary based on flawed 

analysis of topographical features and emissions data. We need 

not resolve the first argument because we accept the second. 

(That is not to suggest that EPA on remand could require a 

“significant contribution,” but it is not apparent that EPA in 

fact did so, nor that petitioners have shown that the flaws in the 

challenged Weld County designation flow from an erroneous 

use of a “significant contribution” standard.) We must, in any 

event, remand to EPA for further consideration because we 

conclude that, on the record before it, EPA’s nonattainment 

decision was inadequately supported and reasoned. 

EPA analyzed Weld County as a potential contributor to 

the ozone exceedances recorded in the Denver nonattainment 

area. EPA proposed, then finalized, a nonattainment area that 

excluded the northern part of Weld County, bordered to the 

north by Wyoming. EPA explained that it excluded northern 

Weld because “emissions from the northern portions of Weld 

County were approximately 25% of NOx and 18% of VOC total 

emissions in Weld County” in 2011 (the year for which 

Colorado provided data), and because the Denver Basin’s 
34

“unique meteorological conditions and topographical 

features . . . indicate that emissions in Northern Weld . . . are 

not likely to contribute to violating monitors.” Colorado: 

Denver Metro/North Front Range Nonattainment Area, Final 

Area Designations for the 2015 Ozone National Ambient Air 

Quality Standards Technical Support Document 36, J.A. 1080 

(“Denver Final TSD”). Neither explanation satisfies.

Given that Weld County sources generate exceptionally 

high amounts of VOCs and NOx—mostly from oil and gas 

operations—the fact that northern Weld contributed only a 

quarter of those emissions does not support EPA’s decision not 

to consider them. In 2011, Weld County produced 

approximately six times as many VOCs and substantially more 

NOx than the next-highest county included in the Denver 

nonattainment area; compared to the lowest-emitting county, 

Weld produced about sixty times as many VOCs and twenty 

times more NOx. Colo. Dep’t of Pub. Health & Env’t, 

Technical Support Document for Recommended 8-Hour 

Ozone Designations 13 tbl.1-2 (Sept. 15, 2016), J.A. 103. 

And, even if the northern part of Weld County were viewed 

separately, the volume of its emissions alone in 2011 

approached or exceeded those of several entire counties in the 

nonattainment area that year. See id. at 12-13, J.A. 102-03. 

EPA could not rationally rule out contribution from northern 

Weld by downplaying the area’s proportionate contribution to 

the county’s overall, excessive emissions.

Nor does EPA coherently explain its conclusion that local 

topography and meteorology prevent northern Weld from 

contributing to Denver exceedances. EPA contends that a 

topographical feature called the Cheyenne Ridge “restrict[s] 

contributions from sources on the upper reaches of and beyond 

the feature[], including” northern Weld County, from reaching 

monitors further south in the Denver Basin. Denver Final TSD 
35

33, J.A. 1077; see also id. at 27, J.A. 1071. EPA similarly 

asserts that the Cheyenne Ridge “roughly coincide[s] with” the 

boundary of the nonattainment area bisecting Weld County. 

Id. at 33, J.A. 1077.

But, in Colorado’s initial submission, cited by EPA in the 

final TSD, the state accurately located the Cheyenne Ridge 

farther north, “along Colorado’s border with Wyoming.” Id. at 

27, J.A. 1071. EPA’s initial TSD nonetheless mistakenly 

located northern Weld outside the Denver Basin, requiring it to 

clarify in its final TSD that northern Weld “include[s] the 

elevated terrain which forms the norther[n] boundary of the 

Denver Basin,” namely, “the southern aspect [of] Cheyenne 

Ridge.” Id. at 36, J.A. 1080. Moreover, maps included in the 

final TSD reflect no topographical basis at all for northern 

Weld’s exclusion; they show no distinctly elevated terrain 

walling off northern Weld from the rest of the Denver basin, 

see id. at 34 fig.15, J.A. 1078, and show northern Weld only at 

an elevation comparable to that of Denver itself, where 

violating monitors are located, see id. at 37 fig.16, J.A. 1081.

In light of EPA’s conflicting characterizations of the 

topographical and meteorological data, its central reliance on 

one—apparently mistaken—interpretation of those data to 

justify the Weld County nonattainment boundary is arbitrary 

and capricious.

Lake County

Petitioners also challenge Lake County, Indiana’s

attainment designation. EPA initially planned to designate all 

of Lake as nonattainment based on its contributions to 

Chicago-area violations, but ultimately confined the 

nonattainment designation to the county’s northern portion. 

Petitioners contend that Lake’s designation is unreasonable 

because EPA failed to identify any “material change” justifying 

its change of position, Reply Br. 13, and treated Lake 
36

dissimilarly from similarly-situated neighboring Kane County, 

which has lower emissions but was designated as 

nonattainment in its entirety. We disagree.

Lake’s designation is consistent with the Guidance Memo 

and reasonably explained in the final TSD. As EPA explained 

in the Guidance Memo, “[w]hile the EPA generally believes it 

is appropriate to include the entire violating or contributing 

county in an ozone nonattainment area, . . . in some cases, an 

assessment of relevant information may support inclusion of 

only part of a county.” Guidance Memo 7, J.A. 513. Here, 

northern Lake not only contains all of the county’s point 

sources, but also accounts for 88% of the county’s population, 

98% of its NOx point-source emissions, and 99% of its VOC

emissions. See Chicago, IL-IN-WI Nonattainment Area Final 

Area Designations for the 2015 Ozone National Ambient Air 

Quality Standards Technical Support Document 25, J.A. 1293.

The differing designations thus reflect the differences in the 

two areas’ data and, accordingly, a “rational connection 

between the facts found and the choice made.” Encino 

Motorcars, 136 S. Ct. at 2125 (internal quotation marks 

omitted); see also Catawba, 571 F.3d at 42 (finding “no fault” 

in EPA’s decision to designate as nonattainment a township 

with an emitting source rather than the entire county). 

Petitioners’ two arguments to the contrary are 

unpersuasive. First, our court has never said that EPA 

automatically acts unreasonably when it alters a nonattainment 

boundary with no “material change” in data. Reply Br. 13. To 

the contrary, we have emphasized that “EPA’s burden to justify 

[a] change in policy . . . [does] not require[] [it] to refute the 

factual underpinnings of its prior policy with new factual data.” 

U.S. Sugar Corp., 830 F.3d at 626. Rather, “[EPA] only need[s]

to provide a reasoned explanation for discounting the 

importance of the facts that it had previously relied upon,”
37

id.—a requirement EPA satisfied here by explaining that 

Lake’s final nonattainment area encompasses the vast majority 

of the county’s emitting sources. Second, the agency’s 

differing treatment of Lake and Kane was perfectly appropriate

given that Kane’s point sources are scattered across the county, 

whereas Lake’s sources, as noted above, are clustered in its 

northern half. 

Sheboygan and Door Counties

Petitioners challenge as arbitrary and capricious EPA’s 

decision to designate nine Wisconsin counties located along or 

near the Lake Michigan shoreline as either in partial or full 

attainment. EPA does not defend its designations for seven of 

those counties: Milwaukee, Ozaukee, Waukesha, Washington, 

Racine, Manitowoc, and Kenosha. See EPA Br. 59 & nn.29-30. 

As discussed below, we treat EPA’s decision not to defend 

those designations as a concession that they are arbitrary and 

capricious. As for the two Wisconsin designations EPA 

defends, Sheboygan and Door, our review persuades us that 

those designations, too, are arbitrary and capricious.

1. Sheboygan County 

According to Wisconsin’s certified 2014-2016 data, a 

monitoring site located on Sheboygan County’s Lake Michigan 

shorefront exceeded the 2015 NAAQS by nine ppb. See Wis. 

Dep’t of Nat. Res., 2015 Ozone National Ambient Air Quality 

Standards Area Designations: Technical Support Document 7 

tbl.2.1 (Apr. 2017), J.A. 422 (“Wisconsin Recommended 

TSD”). A second monitor in Sheboygan County, located 3.2 

miles inland, reported air quality in attainment with the 

NAAQS. See id. EPA initially proposed a 3.2-mile-wide 

nonattainment boundary along the shoreline of Sheboygan 

County, observing that Sheboygan’s emissions were 

“relatively low” but “not trivial,” and concluding that violating 
38

design values extended no further inland than the attaining 

monitor. Wisconsin: Milwaukee Area, Sheboygan County 

Area, Manitowoc County Area, Door County Area, Intended 

Area Designations for the 2015 Ozone National Ambient Air 

Quality Standards Technical Support Document 43, J.A. 652 

(“Wisconsin Intended TSD”). EPA therefore drew the 

boundary of the nonattainment area to encompass sources 

accounting for a substantial percentage of the county’s 

emissions, and to “conservatively capture the likely spatial 

extent of the violating area.” Id. 

In comments on the proposed designation, Wisconsin 

objected that EPA had not sufficiently credited the state’s 

technical analyses. Wisconsin attributed all violations within 

the counties EPA initially designated as nonattainment to 

pollution from other counties or states, including ozone 

transported to Wisconsin over Lake Michigan via “lake breeze” 

meteorology. Letter from Gail Hood, Director, Air Mgmt. 

Program, Wis. Dep’t of Nat. Res., to Robert Kaplan, Acting 

Reg’l Adm’r, EPA - Region 5, at 2 (Apr. 20, 2017), J.A. 405-

06; see also Wisconsin Recommended TSD 10-14, J.A. 425-

29. 

EPA’s final designation reduced the Sheboygan 

nonattainment area to extend only 2.3 rather than 3.2 miles 

inland from the lakeshore. Wisconsin: Northern 

Milwaukee/Ozaukee Shoreline Area, Sheboygan County Area, 

Manitowoc County Area, Door County Area, Final Area 

Designations for the 2015 Ozone National Ambient Air Quality 

Standards Technical Support Document 41, J.A. 1334 

(“Wisconsin Final TSD”). Reversing its initial assessment of 

data and analyses submitted by Wisconsin, EPA concluded that 

it lacked “sufficient evidence” of any contribution from 

Sheboygan-area emissions. Id. EPA’s final designation also 

adopted Wisconsin’s tightly confined estimate of the violating 
39

area, rather than EPA’s prior, “conservative[]” estimate. 

Wisconsin Intended TSD 43, J.A. 652.

Petitioners contend that EPA has not adequately justified 

its final nonattainment boundary in Sheboygan County. 

Petitioners point out that the TSD supporting EPA’s final 

designations relied on multiple analyses submitted by 

Wisconsin that, in response to comments and in the final TSD, 

EPA had said it could not fully assess or had determined to be 

seriously flawed. For example, EPA observed that Wisconsin’s 

source-apportionment modeling—which extrapolated the 

origins of emissions contributing to ozone at Sheboygan’s 

violating monitor—attributed only twelve percent of the 

detected ozone to sources within Wisconsin. See Wisconsin 

Final TSD 40, J.A. 1333. However, as EPA itself

acknowledged, Wisconsin “did not provide any details on the 

source apportionment modeling” or any information “on 

specific methods used to calculate design value contributions 

from the source apportionment modeling outputs,” which 

would be necessary to meaningfully assess the modeling 

analysis. Id. at 40 n.32, J.A. 1333; see also id. at 25 n.23, J.A. 

1318. Without further explanation, therefore, EPA could not 

reasonably rely on Wisconsin’s source-apportionment 

modeling.

EPA also cited Wisconsin’s modeling of a hypothetical 

ten-percent reduction in emissions from stationary and nonroad 

sources “across a 10-county area that includes Sheboygan 

County,” which purported to show that such emission 

reductions would produce no “meaningful reduction in ozone 

design values” at the Sheboygan County monitor. Id. at 40, J.A. 

1333. But, as far as EPA was able to analyze the model, it was 

critically flawed: The “base case” omitted certain sources 

accounting for between twenty-five and forty percent of 

emissions in the studied area. Id. at 40 n.33, J.A. 1333. In 
40

addition, in response to comments encouraging EPA to rely on 

Wisconsin’s ten-percent reduction scenario, which EPA had 

not mentioned in its proposed designations, EPA cautioned that 

a partial-emissions cut scenario was not particularly 

informative. EPA specifically noted that, “[d]ue to the 

nonlinear nature of ozone chemistry,” the impact of specific 

emissions changes “cannot be used to infer the overall impact 

that results from total emissions.” EPA, Response to 

Comments 27, J.A. 1213. 

Further, without explanation, EPA excluded from its final 

contribution analysis HYSPLIT trajectories of air parcels 

shown to arrive at the Sheboygan violating monitor at heights 

of 500 and 1,000 meters. Looking to trajectories only at the 

100-meter level, which pass predominantly over Lake 

Michigan, rather than the 500- and 1,000-meter trajectories, 

which pass predominantly over Sheboygan County, EPA 

concluded that “air parcels traveled almost exclusively” over 

the lake to reach the monitor. Wisconsin Final TSD 38 & fig.6, 

J.A. 1331. In its proposed designations, however, EPA had 

analyzed trajectories at all three heights, and in response to 

comments EPA affirmed that trajectories originating at all three 

heights “are relevant in assessing transport of air parcels for 

potential contribution.” EPA, Response to Comments 28, 

J.A. 1214. EPA has not explained its failure to account for 

admittedly pertinent data in its final designation for 

Sheboygan. 

In defending its decision to this court, EPA disclaims 

reliance on the selective HYSPLIT consideration, or the tenpercent emissions-cut photochemical sensitivity modeling or 

source-apportionment modeling. See EPA Br. 35-36; Oral Arg. 

Tr. 55:14-25. EPA instead asserts that its shift between the 

proposed and final designations turned on two other analyses 

submitted by Wisconsin: (1) an “inland penetration analysis,” 
41

which showed a correspondence between inland penetration of 

a lake breeze and ozone levels recorded at the lakeshore and 

inland Sheboygan monitors; and (2) photochemical modeling 

of an emissions “zero-out” scenario, which predicted that 

elimination of all man-made emissions in Sheboygan County 

would result in no improvement in design values at the 

violating monitor. EPA Br. 30-34. EPA argues that, on closer 

inspection, those analyses persuaded it that lake breeze was the 

sole culprit for ozone violations in Sheboygan County.

“EPA ‘retains a duty to examine key assumptions as part 

of its affirmative burden of promulgating and explaining a nonarbitrary, noncapricious rule.’” Columbia Falls Aluminum Co. 

v. EPA, 139 F.3d 914, 923 (D.C. Cir. 1998) (quoting Small 

Refiner Lead Phase-Down Task Force v. EPA, 705 F.2d 506, 

534 (D.C. Cir. 1983)). With respect to both the inlandpenetration analysis and the zero-out modeling, EPA’s 

explanation falls short, rendering the Sheboygan designation 

decision arbitrary and capricious. 

First, even as EPA now says the inland-penetration 

analysis suggests that ozone travels across Lake Michigan to 

Sheboygan monitors, the agency did not account for additional 

contribution from local emissions. Wisconsin’s own data show 

that local Wisconsin pollution “contribut[es] to the Sheboygan 

County violating monitor.” Wisconsin Final TSD 40, J.A. 

1333. As EPA explained in response to comments questioning 

its initial nonattainment designation of portions of the 

Milwaukee area subject to lake breeze: “That an area 

experiences lake breeze meteorology is alone not mutually 

exclusive with a determination that an area may also contribute 

to its own ozone violations.” EPA, Response to Comments 31, 

J.A. 1217. Moreover, EPA seriously equivocated as to its 

understanding of the effects of the lake breeze, admitting that 

“many details of the various factors regarding how the local 
42

lake breeze . . . influences ozone production and transport 

around Lake Michigan are episode-specific and not wellunderstood,” and that “[t]here are gaps in the peer-reviewed 

scientific literature on this topic.” Wisconsin Final TSD 11-12,

J.A. 1304-05. EPA insists that it nevertheless understands the 

“basic concepts of lake breeze meteorology and its potential to 

influence ozone production and transport . . . well enough to 

place significant weight on the effects of lake area meteorology 

on the analysis of both the area determined to violate the ozone 

NAAQS and emissions contribution from nearby sources.” Id. 

at 12, J.A. 1305. But EPA has failed to explain how, even 

assuming the propriety of placing “significant weight” on lake 

breeze effects, that analysis could justify disregarding 

emissions originating inside Sheboygan County. See Columbia 

Falls, 139 F.3d at 923. After all, Sheboygan emissions need not 

be a “but-for cause of a violation,” Miss. Comm’n, 790 F.3d at 

163, but may “contribute” even if they “simply exacerbate [the] 

problem rather than cause it,” Catawba, 571 F.3d at 39. 

In court EPA contends that the zero-out modeling provides 

the missing link for its sole-causation conclusion—i.e., that 

elimination of emissions that originate in Sheboygan County 

would result in no improvement in measured ozone. See EPA 

Br. 36. But it is far from clear from the record that EPA fulfilled 

its responsibility to verify the zero-out modeling, let alone that 

the agency intended the modeling to bear critical weight in 

supporting the Sheboygan designation. EPA introduced the 

zero-out modeling only at the end of its analysis of Sheboygan 

County, in the last sentence of an addendum to its five-factor 

contribution analysis titled “Additional Information.” See 

Wisconsin Final TSD 39-40, J.A. 1332-33. EPA there noted 

that Wisconsin had “provided information suggesting 

emissions levels from sources in the immediate area are not 

highly correlated with high ozone values.” Id. at 40, J.A. 1333. 

EPA cautioned, however, that “[s]ome aspects of 
43

[Wisconsin’s] claims are difficult to fully evaluate because 

EPA does not have the details necessary to fully review the 

emissions reduction modeling analyses that these claims are 

based on.” Id. EPA then briefly described the results of 

Wisconsin’s ozone source-apportionment modeling, its 

hypothetical ten-percent emissions-reduction modeling, and “a 

‘zero-out Sheboygan run’” under Wisconsin’s model. Id.

EPA’s counsel now argues that the agency’s qualification 

that it could not “fully review [Wisconsin’s] emissions 

reduction modeling analyses” meant to temper only its reliance 

on the source-apportionment and ten-percent emissionsreduction modeling, and that EPA was, in contrast, confident 

in the model and the assumptions to which it was applied in the 

zero-out scenario. Oral Arg. Tr. 55:20-56:23 (citing J.A. 455-

56). The record does not reflect grounds for EPA’s differential 

crediting of those data; it contains no explanation why, for 

example, EPA relies on the zero-out analysis even as it 

disavows the ten-percent emissions-reduction data derived 

from the same model.

In sum, we cannot say that, on the record assembled by 

EPA, “the evidence supports the [attainment] designations 

EPA promulgated.” Catawba, 571 F.3d at 52. We remand the 

designation to EPA to provide explanation how the evidence 

supports its attainment designation, or to make a different 

designation if it concludes on re-examination that the evidence 

so requires. 

2. Door County 

Door County’s single monitor, perched at the tip of a 

roughly seventy-five-mile-long peninsula extending into Lake 

Michigan, reported design values exceeding the ozone NAAQS 

by two ppb. EPA proposed to designate as nonattainment the 

upper half of the Door County peninsula, where the violating 
44

monitor and most of the sources producing Door County’s “not 

trivial” but “relatively low” emissions are located. Wisconsin 

Intended TSD 72-73, J.A. 681-82. Arguing that exceedances at 

the monitor resulted from pollution in other counties and states, 

and that NAAQS exceedances were unlikely to occur anywhere 

in Door County except at the violating monitor, Wisconsin 

advocated instead for a 3.7-square-mile nonattainment area 

limited to the state park where the monitor is located. 

Ultimately, as with Sheboygan County, EPA concluded that it 

did not have “sufficient evidence that these other portions of 

Door County contribute to air quality at the violating monitor” 

and accepted Wisconsin’s proposed drastic diminution of the 

nonattainment area. Wisconsin Final TSD 70, J.A. 1363. 

Petitioners challenge Door County’s constricted final 

nonattainment area, arguing that EPA did not adequately 

explain why it ignored local emissions it previously identified 

as contributing to Door County violations. Our review of the 

record confirms the inadequacy of the agency’s explanation. 

Without explaining its extrapolation from the Sheboygan 

lake-breeze analysis, EPA applied the conclusions of the 

Sheboygan study to Door County. It did so in the face of its 

own description of lake-breeze meteorology as “episodespecific” and its warning that the Sheboygan study was not 

generalizable. Wisconsin Intended TSD 43, J.A. 652; see also 

EPA, Response to Comments 31, J.A. 1217. In particular, EPA 

had noted that a reason it was able to justify a narrow 

nonattainment area in Sheboygan “is the existence of a second 

ozone monitor in Sheboygan [C]ounty which is attaining the 

standard coupled with [Wisconsin Department of Natural 

Resources’] lake breeze inland penetration distance analysis.” 

EPA, Response to Comments 31, J.A. 1217. Door County had 

no second, inland monitor, nor had Wisconsin conducted any 

inland-penetration analysis there.
45

As it did for Sheboygan County, EPA cited Wisconsin’s 

source-apportionment analysis and ten-percent emissions cut 

simulation, and looked exclusively to 100-meter HYSPLIT 

trajectories, most of which pass over Lake Michigan. See 

Wisconsin Final TSD 67 & fig.6, J.A. 1360. (Wisconsin did not 

submit modeling of a zero-out scenario for Door County.) EPA 

suggests in its briefing that the agency considered HYSPLIT 

trajectories at all heights, “as memorialized by a map in the 

final designations showing trajectories at all three levels.” EPA 

Br. 41. But that is belied by EPA’s own record statement 

immediately following the map, adopting Wisconsin’s position 

that only 100-meter paths are relevant and pointing out the 

position of those paths on the map. As it had in the Sheboygan 

final designation, EPA cautioned with respect to Door County

that it “d[id] not have the details necessary to fully review the 

emissions reduction modeling analyses” underlying 

Wisconsin’s claim that “emissions reductions from sources in 

Door County and other upwind lakeshore counties [would] 

have little to no impact on high ozone values at the violating 

monitor.” Wisconsin Final TSD 69, J.A. 1362. Wisconsin 

simply “did not provide any details” on either the sourceapportionment or ten-percent emissions cut modeling, id. at 69 

n.48 & 49, J.A. 1362, which prevented EPA from fulfilling its 

“duty to examine key assumptions as part of its affirmative 

burden of promulgating and explaining a non-arbitrary, 

noncapricious rule,” Columbia Falls, 139 F.3d at 923 (quoting 

Small Refiner, 705 F.2d at 534). As we have explained above, 

EPA cannot reasonably rely on those analyses, at least without 

explaining how EPA could rely on their conclusions without 

validating, or even understanding, their methodology.

Rather than countering petitioners’ arguments, EPA 

contends that the initial Door County designation, like the final 

designation, reflected a judgment that no sources within the 

county contributed to Door County’s recorded violation; the 
46

final designations merely narrowed EPA’s estimation of the 

area shown to be in violation due to external sources. Even 

assuming that to be the case, EPA’s final reasons for 

disavowing local contribution are flawed for the reasons just 

identified. EPA also utterly failed to explain why it believes 

violating design values vanish at the boundaries of a state park 

rather than extending further inland or along the coastline of 

the peninsula, as EPA believed to be the case for Sheboygan 

and Manitowoc Counties.

We therefore remand to “give EPA another opportunity to 

provide a coherent explanation for its designation.” Catawba, 

571 F.3d at 52.

McHenry, Porter, El Paso, Manitowoc, and

Milwaukee-area Counties

In response to the petitions challenging EPA’s 

designations for McHenry County, Illinois; Porter County, 

Indiana; El Paso County, Texas; Manitowoc County, 

Wisconsin, and several Milwaukee, Wisconsin-area counties, 

EPA offers no defense. Instead, it asks us to remand those 

designations for further explanation. 

“We generally grant an agency’s motion to remand so long 

as the agency intends to take further action with respect to the 

original agency decision on review.” Util. Solid Waste 

Activities Grp. v. EPA, 901 F.3d 414, 436 (D.C. Cir. 2018) 

(internal quotation marks omitted). That said, “[w]e have broad 

discretion to grant or deny [such a motion],” and in exercising 

that discretion “we consider whether remand would unduly 

prejudice the non-moving party” and the reasons for the 

remand. Id. We have explained, for example, that remand may 

be appropriate to “allow[] agencies to cure their own mistakes 

rather than wasting the courts’ and the parties’ resources 

reviewing a record that both sides acknowledge to be incorrect 
47

or incomplete,” or “in response to intervening events outside of 

the agency’s control,” including “a new legal decision or the 

passage of new legislation.” Id. (internal quotation marks 

omitted); see also, e.g., id. at 426 (explaining that statutory

revisions authorized new regulatory options and EPA sought 

an opportunity to “decide[] whether or not to alter some of its 

regulatory choices” (internal quotation marks omitted)); Ethyl 

Corp. v. Browner, 989 F.2d 522, 523 (D.C. Cir. 1993) (noting 

“[t]he Administrator[’s] acknowledg[ment] that evidence 

developed since denial of the waiver has undermined the stated 

basis for denial”); Nat’l Fuel Gas Supply Corp. v. FERC, 899 

F.2d 1244, 1249-50 (D.C. Cir. 1990) (remanding to afford 

agency “the first word on how an intervening change in law 

affects an agency decision pending review”). 

Here, EPA has neither conceded the record’s infirmity nor 

identified any intervening events. Indeed, EPA told us virtually 

nothing about why it has chosen not to defend the designations 

or how it would proceed upon remand. Instead, expressly 

disclaiming any “error or impropriety,” EPA informs us that

in hindsight (including after considering 

Petitioners’ brief), EPA believes that the Court 

could benefit from additional explanations of 

the remaining designations. . . . [Remand] could 

potentially entail a range of actions, such as 

supplementing the record, additional 

communications with states, . . . undertaking the 

120-day notice process[,] . . . or modify[ing] the 

remaining designations in ways that could moot 

Petitioners’ challenges or at least narrow issues 

for judicial review. 

EPA Br. 59-60. Although an agency need not “confess error or 

impropriety in order to obtain a voluntary remand,” Limnia, 
48

Inc. v. U.S. Dep’t of Energy, 857 F.3d 379, 387 (D.C. Cir. 

2017), we have found no cases where, as here, an agency has 

neither explained why it seeks remand nor committed to a 

course of action. Cf. Util. Solid Waste, 901 F.3d at 436 (noting 

that “even if there are no intervening events, the agency may 

request a remand . . . to reconsider its previous position” 

(emphasis added) (internal quotation marks omitted)).

Emphasizing that EPA has not “identif[ied] any new 

evidence or intervening events that undermine the . . .

designations,” intervenors oppose EPA’s motion, urging us to 

reach the petitions’ merits. Wisconsin Br. 35. But they identify 

no “undu[e] prejudice” from granting the motion. Util. Solid 

Waste, 901 F.3d at 436. True, as intervenors argue, remand

could require them “to submit new or updated evidence and 

analysis, participate in another round of notice and comment, 

and re-litigate the next petition for review.” Wisconsin Br. 36; 

see also, e.g., El Paso Chamber of Commerce Br. 13-14. As

this court has explained, however, “[b]y statute EPA can at any 

time redesignate [areas designated as attainment]”—like those 

in question here—“with nothing more than a reasoned 

explanation.” Masias v. EPA, 906 F.3d 1069, 1074 (D.C. Cir. 

2018) (citation omitted) (internal quotation marks omitted); see 

also 42 U.S.C. § 7407(d)(3)(A) (authorizing the Administrator 

to “at any time” redesignate areas designated as attainment or 

unclassifiable, using the same general process as for initial 

designations). Intervenors, in other words, face these potential 

costs regardless of whether we grant EPA’s motion and, given 

this, such costs hardly constitute “undu[e] prejudice,” Util.

Solid Waste, 901 F.3d at 436. 

Petitioners argue that simply remanding the designations 

will permit “uncabined agency delay” and, accordingly, that we 

should also vacate the designations so that EPA will again be 

“subject . . . to a firm deadline for corrective actions it claims 
49

it wants to undertake.” Reply Br. 58, 60. In support, they 

invoke the Act’s requirement that EPA issue designations “as 

expeditiously as practicable, but in no case later than 2 years 

from the date of promulgation of [a] . . . revised [NAAQS].” 42 

U.S.C. § 7407(d)(1)(B)(i). 

Based on the particular circumstances of this case—EPA’s 

failure to give any reason for the remand and the absence of 

any undue prejudice—and given the “extreme degree of 

deference [we owe] to the EPA’s evaluation of scientific data 

within its technical expertise,” Maryland, 958 F.3d at 1197

(internal quotation marks omitted), we think the best course of 

action is to treat EPA’s motion as a concession that its 

explanations fall short of the Clean Air Act’s requirement of 

reasoned decisionmaking, grant the motion to remand, and

impose a deadline on the issuance of revised designations. 

Regarding the timing requirement, both circuit precedent and 

the Clean Air Act support our authority to direct EPA to 

complete the remand “as expeditiously as practicable.” 42 

U.S.C. § 7407(d)(1)(B)(i);see also, e.g., Nat. Res. Def. Council 

v. EPA, 22 F.3d 1125, 1136-37 (D.C. Cir. 1994) (per curiam) 

(imposing “accelerat[ed] deadlines for EPA’s approval or 

disapproval of state implementation plans “because the EPA, 

by ignoring the statutory deadline for promulgating guidance, 

was responsible for the tardy submission of [those plans]”); 

Envtl. Def. Fund v. EPA, 852 F.2d 1316, 1331 (D.C. Cir. 1988) 

(“EPA’s history of delay and missed deadlines with respect to 

its statutory obligations . . . indicates that a court-imposed 

schedule is necessary here.”); Sierra Club v. EPA, 719 F.2d

436, 469-70 (D.C. Cir. 1983) (directing EPA to promulgate 

new Clean Air Act regulations within six months, i.e., “the 

period originally specified by Congress”). EPA offers no 

reason, nor can we think of one, why it should be permitted to 

evade the Clean Air Act’s statutory deadline through a 

voluntary remand. 
50

To be sure, as EPA points out, in Vermont Yankee Nuclear 

Power Corp. v. Nat. Res. Def. Council, 435 U.S. 519 (1978), 

the Supreme Court cautioned that, “[a]t least in the absence of 

substantial justification for doing otherwise, a reviewing court 

may not, after determining that additional evidence is requisite 

for adequate review, proceed by dictating to the agency the . . . 

time dimension of the needed inquiry and ordering the results 

to be reported to the court without opportunity for further 

consideration on the basis of the new evidence by the agency.” 

Id. at 544-45 (quoting Fed. Power Comm’n v. Transcon. Gas 

Pipe Line Corp., 423 U.S. 326, 333 (1976)). Here, however, 

Congress, not “the reviewing court,” has “dictat[ed] . . . the 

time dimension” for EPA action. Id. Far from “propelling the 

court into the domain which Congress has set aside exclusively 

for the administrative agency,” id. at 545 (quoting SEC v. 

Chenery Corp., 332 U.S. 194, 196 (1947))—the Court’s 

concern in Vermont Yankee—imposing a deadline is in keeping 

with Congress’s purpose of ensuring timely area designations.

We therefore require EPA to issue revised designations as 

expeditiously as practicable.

This brings us, finally, to petitioners’ argument that with 

respect to the petitions we grant, we should vacate the area 

designations rather than simply remand them to EPA for 

further explanation. As we have recognized, the Act “confers

broad discretionary authority” on EPA to determine when an 

area “contributes” to a “nearby” violation. Catawba, 571 F.3d 

at 39. Given this, despite the deficiencies in EPA’s current 

explanations, we think there is at least a realistic possibility that 

EPA will be able to substantiate the relevant designations on 

remand. See Allied-Signal, Inc. v. NRC, 988 F.2d 146, 150-51 

(D.C. Cir. 1993) (explaining that “[t]he decision whether to 

vacate depends on the seriousness of the order’s deficiencies 

(and thus the extent of doubt whether the agency chose 
51

correctly) and the disruptive consequences of an interim 

change that may itself be changed” (internal quotation marks 

omitted)). Petitioners concede, moreover, that vacatur would 

leave areas “undesignated” for purposes of the 2015 ozone 

NAAQS, Reply Br. 60, and nothing about that status will 

immediately subject the areas to more stringent environmental 

controls. We accordingly remand the unlawful attainment 

designations to EPA without vacating them.

For the foregoing reasons, we (1) grant EPA’s motion to 

remand the designations of McHenry County, Porter County, 

El Paso County, Manitowoc County, and the Milwaukee-area 

counties for further explanation; (2) grant the petitions for 

review of the designations of Jefferson County, Monroe 

County, Ottawa County, Weld County, Door County, and 

Sheboygan County; and (3) deny the petition for review of 

Lake County’s designation. The defective designations are 

remanded to EPA with directions to complete the remand as 

expeditiously as practicable.