Court Opinion

ID: 4547488
Source: CourtListenerOpinion
Date Created: 2020-07-10 16:00:35.894817+00
Date Added: 2024-06-11T12:53:36.805413
License: Public Domain

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 air-
quality 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-of-
the-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-in-
fact 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” ground-
level 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. Louis-
area 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 still-
violating 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. Louis-
area 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 similarly-
situated 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 one-
page 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, higher-
emitting 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 ten-
percent 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 non-
arbitrary, 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 inland-
penetration 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 well-
understood,” 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 emissions-
reduction 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 “episode-
specific” 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 source-
apportionment 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.