Board of County Commissioners of Weld County, Colorado v. Environmental Protection Agency

United States Court of Appeals
FOR THE DISTRICT OF COLUMBIA CIRCUIT
Argued December 7, 2022 Decided June 30, 2023
No. 21-1263
BOARD OF COUNTY COMMISSIONERS OF WELD COUNTY,
COLORADO
PETITIONER
v.
ENVIRONMENTAL PROTECTION AGENCY,
RESPONDENT
BOARD OF COUNTY COMMISSIONERS OF BOULDER COUNTY, ET
AL.,
INTERVENORS FOR RESPONDENT
Consolidated with 21-1013
On Petitions for Review of an Action of the
United States Environmental Protection Agency
Ethan G. Shenkman argued the cause for petitioner Board
of County Commissioners of Weld County, Colorado. With
him on the briefs were Charles Birkel, John R. Jacus, Shannon
Stevenson, and Kathleen Pritchard. Bill Davis, Deputy
Solicitor General, Office of the Attorney General of the State
of Texas, argued the cause for petitioners State of Texas and
Texas Commission on Environmental Quality. With him on

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the briefs were Ken Paxton, Attorney General, Brent Webster,
First Assistant Attorney General, Judd E. Stone II, Solicitor
General, and Michael R. Abrams, Assistant Solicitor General.
Alexandra L. St. Romain, Attorney, U.S. Department of
Justice, argued the cause for respondent. With her on the briefs
were Todd Kim, Assistant Attorney General, Laura J.
Glickman, Attorney, U.S. Department of Justice, and Seth
Buchsbaum, Attorney, U.S. Environmental Protection Agency.
Elliot Higgins, Attorney, U.S. Department of Justice, also
argued the cause for respondent.
David Baake and Ryan Maher argued the cause for Board
of County Commissioners of Boulder County, et al. With them
on the brief were Robert Ukeiley and Joshua D. Smith.
Before: SRINIVASAN, Chief Judge, KATSAS, Circuit Judge,
and ROGERS, Senior Circuit Judge.
Opinion for the Court by Circuit Judge KATSAS.
KATSAS, Circuit Judge: This case involves regulation of
ozone levels under the Clean Air Act. In August 2018, the
Environmental Protection Agency designated northern Weld
County, Colorado and El Paso County, Texas as areas that had
already attained a 2015 ozone pollution standard. But EPA
reversed course after Clean Wisconsin v. EPA, 964 F.3d 1145
(D.C. Cir. 2020), remanded these designations. In November
2021, EPA folded northern Weld and El Paso Counties into
areas previously designated as not having attained the standard.
Weld County contends that EPA improperly relied on data
available in 2018, rather than updated data, and that the data do
not support its adverse designation. We hold that EPA

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reasonably relied on the same data it had used to make the
original designation and that the data support the revised one.
Texas argues that El Paso’s 2021 nonattainment
designation was impermissibly retroactive because EPA made
it effective as of the 2018 attainment designation. As a result,
a statutory deadline for El Paso to attain the governing standard
passed some three months before EPA made the nonattainment
designation. And missing the deadline triggered adverse legal
consequences. We therefore agree with Texas that El Paso’s
revised designation, backdated to the date of the original one,
was impermissibly retroactive.
I
A
The Clean Air Act establishes a comprehensive scheme to
reduce the atmospheric concentration of various air pollutants.
The scheme works in three relevant steps.
First, EPA must establish and periodically revise national
ambient air quality standards (NAAQS) for pollutants that may
endanger public health or welfare. These standards set forth
the maximum permissible concentration of the pollutant in the
atmosphere. 42 U.S.C. §§ 7408(a)(1)(A), 7409(a)–(b).
Second, EPA must divide the country into geographic
areas and designate them according to whether they satisfy the
new standard. 42 U.S.C. § 7407(d)(1)(B)(i)–(iii). EPA marks
an area as “attainment” when local atmospheric concentration
of the pollutant—the area’s so-called “design value”—falls
below the relevant NAAQS. However, an area must be
designated as “nonattainment” if its design value exceeds that

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level or if the area “contributes” to nonattainment in a “nearby
area.” Id. § 7407(d)(1)(A)(i).
EPA works with the States to make these designations.
Within a year of a new NAAQS, each State must make “initial
designations” suggesting appropriate areas and attainment
designations. 42 U.S.C. § 7407(d)(1)(A)–(B). If EPA
proposes to modify an initial designation, it must notify the
State in advance and allow it to contest the proposal. Id.
§ 7407(d)(1)(B)(ii). EPA must finalize its designations within
two years of promulgating the new standard—a deadline
extendable for at most one year. Id. § 7407(d)(1)(B)(i).
Third, States must ensure that their designated areas
achieve or maintain attainment status. To that end, a State must
prepare a State Implementation Plan (SIP) specifying how each
of its areas will do so. 42 U.S.C. § 7407(a). EPA sets the SIP
deadline, which must be within three years of any
nonattainment designation. Id. § 7502(b).
A nonattainment designation triggers more stringent
regulation. For attainment areas, the SIP need only set forth
measures “to prevent significant deterioration of air quality.”
42 U.S.C. § 7471. But for nonattainment areas, the SIP must
impose “all reasonably available” measures to achieve
attainment “as expeditiously as practicable.” Id. § 7502(c)(1).
In addition, the Clean Air Act imposes deadlines for
nonattainment areas to achieve attainment, which are called
“attainment dates.” For ozone standards, EPA must designate
nonattainment areas as marginal, moderate, serious, severe, or
extreme. Areas designated as marginal nonattainment have
three years to attain, while areas with worse designations have
correspondingly longer deadlines. 42 U.S.C. § 7511(a)(1).

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A worse nonattainment designation triggers more stringent
regulation. For moderate nonattainment areas, SIPs must
undertake to significantly reduce emissions. 42 U.S.C.
§ 7511a(b)(1)(A)(i). And for serious, severe, or extreme
nonattainment areas, SIPs must undertake even more. Id.
§ 7511a(c)–(e).
Failing to achieve attainment by the attainment date also
has consequences. Within six months of that deadline, EPA
must determine whether the area achieved attainment. 42
U.S.C. § 7511(b)(2)(A). In general, an area that missed the
deadline—i.e., failed to timely achieve attainment—must be
“reclassified by operation of law” into a worse nonattainment
status. Id. § 7511(b)(2)(A)(i)–(ii). And in some
circumstances, EPA may sanction a State by taking away
federal highway funds or by imposing further environmental
regulations. Id. § 7509(a)–(b). So as a practical matter, States
with nonattainment areas must “implement potentially
expensive technology or expensive process changes to reduce
pollution levels over a relatively short period of time.” Miss.
Comm’n on Env’t Quality v. EPA, 790 F.3d 138, 146 (D.C. Cir.
2015) (per curiam).
B
Ground-level ozone forms when its precursors, nitrogen
oxides and volatile organic compounds, react with sunlight.
Power plants, motor vehicles, and combustion engines emit the
precursors. Because ozone and its precursors travel easily
through the atmosphere, nonattainment can occur hundreds of
miles away from where the precursors were emitted.
In 2015, EPA reduced the NAAQS for ozone from 0.075
to 0.070 parts per million. National Ambient Air Quality
Standards for Ozone, 80 Fed. Reg. 65,292 (Oct. 26, 2015). For

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this standard, EPA calculates design values based on three
years of certified data. Id. at 65,294.
At the same time, EPA issued a guidance memo on how to
designate areas under the 2015 ozone standard. The memo
flagged five primary considerations: air quality, emissions,
weather, topography, and jurisdictional boundaries. J.A. 152.
In 2018, EPA promulgated its designations. Additional
Air Quality Designations for the 2015 Ozone National Ambient
Air Quality Standards, 83 Fed. Reg. 25,776 (June 4, 2018).
The agency relied primarily on data from 2014 to 2016, which
was “the most recent data that states were required to certify at
the time the EPA notified the states of its intended
modifications to their recommendations.” Id. at 25,779. The
designations went into effect in August 2018, starting the clock
for nonattainment areas to attain. See 40 C.F.R. § 51.1303(a).
C
In Clean Wisconsin, this Court held that EPA had acted
arbitrarily in designating northern Weld County and El Paso
County as attainment areas.
Weld is a large Colorado county located north of Denver.
EPA concluded that the southern part of Weld County, but not
the northern part, contributed to ozone pollution in the Denver
metropolitan area. So it folded the southern part into a
nonattainment area encompassing greater Denver, and it
designated the northern part as a standalone attainment area.
We were skeptical because northern Weld County produced
emissions that “approached or exceeded those of several entire
counties in the nonattainment area.” 964 F.3d at 1168. And
we found EPA’s analysis of the local weather and topography
to be shallow and inconsistent. Id. at 1169.

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El Paso County lies in western Texas and borders New
Mexico. In 2018, EPA designated it as an attainment area. But
when the Clean Wisconsin petitioners argued that El Paso
contributed to nonattainment in Doña Ana County, New
Mexico, EPA asked us to remand the designation for further
explanation. We obliged but instructed the agency to revise its
analysis “as expeditiously as practicable.” 964 F.3d at 1176.
We remanded the northern Weld and El Paso designations
without vacating either one. In declining to vacate, we
perceived a “realistic possibility” that EPA would be able to
justify the original designations on remand. 964 F.3d at 1177.
EPA promulgated the revised designations in November
2021. Additional Revised Air Quality Designations for the
2015 Ozone National Ambient Air Quality Standards: El Paso
County, Texas and Weld County, Colorado, 86 Fed. Reg.
67,864 (Nov. 30, 2021) (Final Rule). These designations rest
only on data that was available to EPA when it promulgated its
original designations. Id. at 67,868–69. Based on this data, the
agency concluded that the disputed areas contribute to nearby
nonattainment. So EPA folded northern Weld County into the
greater Denver marginal nonattainment area, and it folded El
Paso County into a marginal nonattainment area that includes
Doña Ana County. Id. at 67,873.
In doing so, EPA declined to recognize new attainment
dates running from the date of the revised designations. Final
Rule, 86 Fed. Reg. at 67,869. Because EPA designated Doña
Ana County as a marginal nonattainment area in August 2018,
its attainment date passed in August 2021—three months
before the Final Rule folded El Paso County into that area. And
because EPA recognized no new deadline, El Paso had no
opportunity to meet its attainment date and thus timely attain.

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EPA did extend one other deadline: Because Texas “had no
notice that it should prepare a marginal area SIP submission”
for the expanded nonattainment area, EPA gave Texas one
more year to prepare a SIP with planning requirements for a
marginal nonattainment area. Id. EPA later concluded that the
El Paso-Doña Ana County area had not attained the 2015 ozone
standard as of its August 2021 attainment date. Determination
of Attainment by the Attainment Date But for International
Emissions for the 2015 Ozone National Ambient Air Quality
Standard; El Paso-Las Cruces, Texas-New Mexico, 88 Fed.
Reg. 14,095 (Mar. 7, 2023).
Weld County and Texas seek review of the revised
designations. We have jurisdiction under 42 U.S.C.
§ 7607(b)(1).
II
Weld County offers two reasons for why EPA acted
arbitrarily in designating the entire county as marginal
nonattainment. First, EPA failed to consider the most current
available data. Second, the older data do not support the
designation. Neither argument persuades.
A
On remand, EPA faced a choice about what data to use in
considering whether northern Weld County contributes to
nonattainment in Denver. One option was to use only the
certified data, gathered primarily from 2014 to 2016, that EPA
had considered in making the original designations. Another
option was to consider the most recent certified data, which
was gathered from 2015 to early 2021. Weld contends that it
was arbitrary for EPA to use the original data. We disagree.

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The Clean Air Act allows us to reverse rules that are
“arbitrary, capricious, an abuse of discretion, or otherwise not
in accordance with law.” 42 U.S.C. 7607(d)(9)(a). Under this
familiar standard, copied from the Administrative Procedure
Act, we uphold a rule if the agency “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
(cleaned up).
EPA reasonably explained its decision to use only the data
at its disposal while making the original designations. First,
using the same data for localized redesignations would
standardize its analysis and thus facilitate consistent treatment
of all affected counties. J.A. 676. Second, using the original
data would streamline the process and thus comply with our
instruction in Clean Wisconsin to make any redesignations “as
expeditiously as practicable.” 964 F.3d at 1176; see J.A. 677.
Mississippi Commission bolsters EPA’s choice. In that
case, EPA used older data to designate a tristate nonattainment
area despite possessing more recent, certified data from two of
the three states. We declined “to declare irrational the EPA’s
conclusion that comparing data from the same time period
would be more appropriate than analyzing data from different
time periods in the same evaluation process.” 790 F.3d at 160.
So too here. If EPA could choose a matched dataset to classify
a nonattainment area spanning multiple states, then it can also
choose a matched dataset to classify a nonattainment area
spanning multiple counties within a state.
We recognize that an agency generally must base its
decisions on the best available data. But the question here is
whether EPA was required to use one data set (the most recent
certified data) in assessing northern Weld County’s
contribution to ozone pollution in greater Denver even though

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it had used another data set (the certified data available at the
time of the original designations) in assessing the contribution
of at least eight other counties in the same area. In these
circumstances, EPA plausibly explained why the benefits of a
matched dataset—greater parity among counties and faster
turnaround—make the original data a better choice than partial
updating.
Weld County objects that EPA failed to act consistently.
Weld notes that EPA refused to consider certain air quality data
from 2014 to 2016. But this data was not made available to
EPA until 2020 and 2021, so its exclusion was consistent with
EPA’s overall approach to stay within the record available
when it made the original designations. Weld further notes that
EPA, in making those designations, did consider some data
from as late as 2017. EPA did so to the extent that some States
chose to certify air quality data ahead of schedule. But because
Colorado did not avail itself of this option, EPA based its 2018
designation on Colorado’s certified data from 2014 to 2016.
None of this suggests that EPA acted inconsistently or
otherwise arbitrarily.
B
Weld County further argues that the certified data do not
support including its northern part in the greater Denver
nonattainment area. As noted above, EPA uses five factors to
define areas and determine their attainment status. The last
four factors (emissions, weather, topography, and jurisdictional
boundaries) bear on the appropriate boundaries for a particular
area. The first factor (air quality) bears on its appropriate
designation. If EPA determines that one area contributes to
another’s nonattainment, it will combine those areas into a
single nonattainment area.

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Weld County does not challenge this overall framework
for making the designations. Instead, it contends that EPA
unreasonably applied the framework to conclude that northern
Weld County contributes to Denver’s nonattainment. We see
no reason to disturb this highly technical judgment.
1. Air Quality. EPA assesses air quality by considering
whether local monitors report NAAQS violations—i.e., a
design value above 0.070 parts per million. The presence of a
single violating monitor justifies a nonattainment designation.
In the Denver metropolitan area, EPA found five of them.
Weld contends that EPA erred by using outlier data
associated with wildfires and stratospheric intrusions. EPA
may disregard data that arises from an “exceptional event.” 40
C.F.R. § 50.14. But to exclude data on this ground, a State
must prove to EPA that an exceptional event “caused a specific
air pollution concentration at a particular air quality monitoring
location.” Id. § 50.14(a)(1)(ii). Colorado made no effort to
link the Denver monitor readings to any exceptional event. To
the contrary, in opposing the Final Rule, Colorado
acknowledged its failure to submit any “exceptional event
demonstrations.” J.A. 631.
2. Emissions. This factor primarily considers the origin
and quantity of precursor emissions. Data showed that Weld
County produces more than three times the emissions of the
next-highest-emitting county in the Denver metropolitan area.
And although northern Weld County accounts for only a small
fraction of the County’s overall emissions, we noted in Clean
Wisconsin that a small fraction of a large number can still be a
large number. 964 F.3d at 1168.
On remand, EPA concluded that northern Weld County
produces significant emissions because (a) the County

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produces far more emissions than any nearby county, (b) most
of its emissions come from oil-and-gas wells, and (c) nearly
eight percent of the County’s 36,682 wells are in its northern
portion. Furthermore, northern Weld County has three
individual sources that each emit over 100 tons of ozone
precursors per year. In our view, these facts support EPA’s
revised conclusion.
Weld again claims inconsistency. It objects that EPA
failed to reevaluate emissions from the northern part of nearby
Larimer County, which EPA excluded from the Denver
nonattainment area. But no data in the relevant set compares
the emissions of northern Weld and northern Larimer counties.
Instead, Weld flags data comparing the combined emissions of
northern Weld and northern Larimer counties to emissions
from the Denver nonattainment area. We cannot infer from this
that northern Larimer’s emissions exceed northern Weld’s.
Weld continues that northern Larimer County is a stronger
candidate for inclusion in the nonattainment area because it has
a higher population density and more vehicle miles travelled
than does northern Weld County. But as EPA explained,
northern Larimer and northern Weld Counties differ in other
important respects, such as topography, that cut in favor of
designating only northern Weld County. To establish
arbitrariness based on inconsistency, Weld must show that
EPA “treated genuinely similar counties dissimilarly.” Miss.
Comm’n, 790 F.3d at 169 (cleaned up). Given the various
cross-cutting considerations we have noted, Weld has not made
that showing.
3. Weather. EPA considers how meteorological
conditions affect the movement of ozone and its precursors
through the atmosphere. EPA uses a model to determine this
movement from data about wind speed and direction,

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temperature, humidity, and air pressure. The model predicts
the paths, known as “back trajectories,” traveled by air parcels
that reach a violating monitor. Clean Wisc., 964 F.3d at 1155.
If the model’s projections show air parcels moving from a
region to the violating monitor, that supports including the
region in the nonattainment area. According to EPA, the model
indicates that emissions from northern Weld County move into
the Denver nonattainment area.
Weld contends that EPA ignored certain model projections
suggesting otherwise. But EPA did consider these projections,
and it offered three sound reasons for discounting them: First,
the projections missed all back trajectories from one of the
violating monitors. Second, they missed back trajectories from
2016. Third, they missed back trajectories from each day when
a monitor registered an above-NAAQS design value. In any
event, EPA further explained that even the County’s preferred
data suggest that air flow from northern Weld County “affect[s]
violating monitors” in the Denver area. J.A. 682.
Weld also highlights supposed flaws in EPA’s data. First,
it complains that the data tracks air parcels arriving at a monitor
only during a single hour of the day. But Weld did not raise
this argument below, and we thus need not consider it. In any
event, EPA sensibly focused on the time of day when ozone
concentrations were highest. Plus, data from other times could
only expand the possible source regions; they could not change
the critical fact that some air parcels travel from northern Weld
County to greater Denver. Second, Weld objects that EPA did
not try to predict how terrain near violating monitors might
influence particle trajectory. But the model seeks to measure
the paths traveled by air parcels to a violating monitor; it is not
concerned with how terrain near violating monitors impacts
particle trajectory.

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4. Topography. In originally excluding northern Weld
County from the Denver nonattainment area, EPA claimed that
its boundary line tracked an elevation called the Cheyenne
Ridge, which assertedly blocks local emissions from reaching
Denver. But in Clean Wisconsin, we found that the ridge was
in the northernmost part of Weld County, along the boundary
between Colorado and Wyoming. See 964 F.3d at 1168.
On remand, EPA agreed with our assessment and then
concluded that the local topography funneled, rather than
impeded, the flow of air from Northern Weld County to greater
Denver. Weld objects that EPA again misplaced the ridgeline,
but it provides no convincing response to the evidence already
credited by this Court.
5. Jurisdictional boundaries. The last main factor that
EPA considers is existing jurisdictional boundaries. All else
equal, administration is easier when area designations track
preexisting boundaries such as county lines. So here, EPA
reasoned, such boundaries support combining northern and
southern Weld County.
The County objects that EPA failed to quantify the
emissions contributions of its northern portion. But we have
allowed EPA to designate nonattainment areas without
isolating and quantifying the exact emissions from each subpart
of a jurisdiction. Catawba Cnty., N.C. v. EPA, 571 F.3d 20, 40
(D.C. Cir. 2009). Weld notes considerations such as its size
and uneven elevation. But we cannot conclude that EPA was
legally compelled to subdivide the County, particularly given
some affirmative evidence that northern Weld County does
contribute to Denver’s nonattainment.

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III
Texas argues that the Final Rule is impermissibly
retroactive because, in December 2021, it folded El Paso
County into a nonattainment area for which the August 2021
attainment date had already passed. We agree.
A
Agencies cannot promulgate retroactive rules without
express statutory authorization. See, e.g., Bowen v.
Georgetown Univ. Hosp., 488 U.S. 204, 224 (1988) (Scalia, J.,
concurring). And this Court has held that nothing in the Clean
Air Act gives EPA “the unusual ability to implement rules
retroactively.” Sierra Club v. Whitman, 285 F.3d 63, 68 (D.C.
Cir. 2002). So, if the Final Rule operates retroactively as
applied to El Paso, then it cannot stand.
A rule operates retroactively when it “would impair rights
a party possessed when he acted, increase a party’s liability for
past conduct, or impose new duties with respect to transactions
already completed.” Landgraf v. USI Film Prods., 511 U.S.
244, 280 (1994). In other words, “retroactive rules alter the
past legal consequences of past actions.” Arkema, Inc. v. EPA,
618 F.3d 1, 7 (D.C. Cir. 2010) (quoting Bowen v. Georgetown
Univ. Hosp., 488 U.S. 204, 219 (1988) (Scalia, J., concurring))
(cleaned up); see Landgraf, 511 U.S. at 269–70 (“the court
must ask whether the new provision attaches new legal
consequences to events completed before its enactment”). On
the other hand, it is not enough that a rule “upsets expectations
based in prior law,” id. at 269, or “draws upon antecedent facts
for its operation,” id. at n.24 (cleaned up).

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The Final Rule is impermissibly retroactive. Recall that a
marginal nonattainment designation gives rise to an attainment
date that is three years after the designation. 42 U.S.C.
§ 7511(a). If a State misses the deadline, EPA must reclassify
the designated area to a worse nonattainment status “by
operation of law.” Id. § 7511(b)(2)(A)(i)–(ii). And the
reclassification triggers the various additional burdens that
come with the downgraded status. Id. § 7511a(b)(1). By
design, this scheme provides strong incentives for States with
nonattainment areas to use the three-year runway to achieve
attainment. And 17 of the 36 areas designated as marginal
nonattainment for the 2008 ozone NAAQS did timely attain.
See Determinations of Attainment by the Attainment Date, 81
Fed. Reg. 26,697, 26,700 (May 4, 2016).
Here, Texas never had the requisite opportunity to reach
timely attainment. In August 2018, EPA classified El Paso as
an attainment area. Texas thus had no reason to plan for
improving El Paso’s air quality at that time. Yet in November
2021, EPA folded El Paso into an existing nonattainment
area—three months after that area’s August 2021 attainment
deadline had passed. And despite considering the question at
length, EPA refused to recognize an attainment date for El Paso
running from the date of its new nonattainment designation.
J.A. 661–63. Thus, despite designating El Paso as a
nonattainment area in November 2021, EPA effectively
backdated to August 2018 the start of its three-year runway for
reaching attainment. The Final Rule thereby imposed
liabilities on Texas’s inaction between August 2018 and
August 2021—i.e., it imposed on Texas the consequences of
missing a compliance deadline that passed before the
underlying legal obligation was imposed.
Our caselaw confirms that the Final Rule operates
retroactively. We have made clear that because EPA lacks

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statutory authority to promulgate retroactive rules, it cannot
impose on States new obligations with compliance deadlines
already in the past. Three decisions prove this point.
Start with Sierra Club v. Whitman. In 1991, EPA
designated St. Louis as a moderate nonattainment area,
triggering a 1996 attainment date. After EPA missed its own
1997 deadline for determining whether St. Louis had timely
attained, the agency refused to backdate to that deadline its
later determination that the city had not timely attained.
Upholding the refusal to backdate, we invoked the principle
that EPA cannot engage in “retroactive rulemaking.” 285 F.3d
at 68. Likewise, we explained that the requested backdating
would have “likely impose[d] large costs on the States, which
would face fines and suits for not implementing air pollution
prevention plans in 1997, even though they were not on notice
at the time” of any legal obligation to do so. Id.
The same logic guided our decision in Sierra Club v. EPA,
356 F.3d 296 (D.C. Cir. 2004). In that case, EPA downgraded
the District of Columbia from serious to severe nonattainment.
But because the deadline for submitting a severe nonattainment
SIP had already passed, EPA gave the District a new deadline
for doing so. We again rejected a contention that the original
deadline should control—which, we said, would make the
reclassification retroactive “by holding the States in default of
their submission obligations before the events necessary to
trigger that obligation (reclassification) occurred.” Id. at 309
(cleaned up).
Last is WildEarth Guardians v. EPA, 830 F.3d 529 (D.C.
Cir. 2016). After we held that EPA had been using the wrong
statutory scheme to regulate fine particulate matter, the agency
adjusted the SIP and attainment deadlines under the correct
scheme “to avoid treating states as having already missed

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deadlines of which they were never aware.” Id. at 531. We
rejected a contention that EPA should have assumed the correct
framework had been applied all along. In doing so, we
described the adjustments as necessary to avoid imposing
“retroactive consequences on states.” Id. at 540. We also
rejected a proposed distinction between “present findings of
noncompliance” and the sort of “backdated findings” in the
Sierra Club cases. Id. In either instance, we reasoned, “States
would be held to long-passed deadlines of which they were
unaware, with meaningful legal consequences.” Id. at 541.
B
EPA’s responses are unpersuasive. EPA objects that
Texas did not preserve its retroactivity argument below. But in
opposing the proposed Final Rule, Texas could not have been
much clearer. It argued that the El Paso County area “should
not be tied retroactively to implementation deadlines that
existed prior to the area being designated as nonattainment.”
J.A. 614. And it warned that “[a]ny attempt to ‘link’ El Paso
County to the [Doña Ana] nonattainment designation
implementation dates would exceed” EPA’s statutory
authority. J.A. 615. The Texas Association of Manufacturers
echoed these concerns. It objected that EPA lacked statutory
authority to eliminate Texas’s three-year attainment runway
“[b]y retroactively attaching Dona Ana County’s attainment
date to El Paso.” J.A. 411. This comment also preserved the
retroactivity issue. See Ne. Md. Waste Disposal Auth. v. EPA,
358 F.3d 936, 948 n.12 (D.C. Cir. 2004) (“It is sufficient that
an issue was raised by any commenter; the party petitioning for
judicial review need not have done so itself.”).
On the merits, EPA contends that its classification of El
Paso as a nonattainment area imposed only prospective
obligations on Texas—the requirement to submit a new SIP,

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for which EPA imposed a new deadline, and the future
planning requirements associated with nonattainment status.
But as discussed, EPA refused to set an attainment date keyed
to the November 2021 designation. Texas thus found itself in
the unenviable position of learning in November 2021 that El
Paso County either had to have reached attainment by its
August 2021 attainment date or would suffer the consequences
flowing “by operation of law” from having missed that
deadline. 42 U.S.C. § 7511(b)(2)(A). To be sure, a downgrade
does not happen by itself, but only after EPA determines that
the area has missed the deadline. See id. But EPA’s decision
to backdate El Paso’s nonattainment designation retroactively
adjusted Texas’s legal rights by increasing the State’s exposure
to the harsh consequences that follow from failing to meet an
already past deadline.
We recognize that El Paso may yet avoid the additional
burdens flowing from a downgraded attainment classification.
The Clean Air Act provides an exception to the mandatory
downgrade if a State proves to EPA that the nonattainment area
would have met its deadline “but for emissions emanating from
outside of the United States.” 42 U.S.C. § 7509a(a)(2); see
Implementation of the 2015 National Ambient Air Quality
Standards for Ozone, 83 Fed. Reg. 62,998, 63,009 & n.24 (Dec.
6, 2018). Several months after oral argument, EPA proposed a
rule reflecting its tentative judgment that the El Paso-Doña Ana
area would have timely attained but for emissions from
Mexico. Determination of Attainment by the Attainment Date
But for International Emissions for the 2015 Ozone National
Ambient Air Quality Standard, 88 Fed. Reg. at 14,101. This
proposed rule does not change our analysis. For one thing, it
is not final and thus currently lacks the force of law. In any
event, the nonattainment designation still created substantial
legal exposure for Texas based on its inaction between August
2018 and August 2021. The fact that a distinct affirmative

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defense might extinguish it does not change the retroactive
character of the rule creating the exposure in the first place.
The intervenors press two further retroactivity points,
which we reject. First, they claim Texas knew all along that El
Paso’s status could change. For support, they invoke Treasure
State Resource Industry Association v. EPA, 805 F.3d 300
(D.C. Cir. 2015), which held that a NAAQS designation is not
impermissibly retroactive just because it relies on old data. Id.
at 305–06; see also Landgraf, 511 U.S. at 269 n.24. But Texas
does not contend that El Paso’s nonattainment designation in
November 2021 was impermissibly retroactive because it was
based on air quality data from earlier years. Instead, Texas
contends that the designation was impermissibly backdated to
August 2018. Moreover, agencies always may prospectively
change their regulations, just as legislatures always may
prospectively amend their statutes. If that possibility were
enough to vitiate retroactivity concerns, the presumption
against retroactive statutes and rules would amount to nothing.
Second, the intervenors object that Texas failed to identify
any different steps it would have taken if EPA had designated
El Paso as a nonattainment area in 2018. But Texas need not
make that showing. As explained above, a rule that “attaches
new legal consequences to events completed before its
enactment” is retroactive, see Landgraf, 511 U.S. at 269–70,
and thus “invalid unless specifically authorized.” Nat’l
Petrochemical & Refiners Ass’n v. EPA, 630 F.3d 145, 159
(D.C. Cir. 2010). On the other hand, a rule that merely “upsets
expectations based in prior law” is not retroactive in the same
way; it is instead only “secondarily retroactive,” and thus
“invalid only if arbitrary and capricious.” Id. (quoting
Landgraf, 511 U.S. at 269); see Nat’l Cable & Telecomm.
Ass’n v. FCC, 567 F.3d 659, 670–71 (D.C. Cir. 2009). In
reviewing a rule with such secondary retroactivity, we must

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balance the harm of “upsetting prior expectations” against any
benefits of applying the rule “to those preexisting interests.”
Nat’l Cable & Telecomm. Ass’n, 567 F.3d at 670; see also
Bowen, 488 U.S. at 220 (Scalia, J., concurring) (“A rule that
has unreasonable secondary retroactivity—for example,
altering future regulation in a manner that makes worthless
substantial past investment incurred in reliance upon the prior
rule—may for that reason be ‘arbitrary’ or ‘capricious.’”). For
this inquiry, the extent of any reliance or expectation interests
is obviously critical. But the intervenors cite no case
suggesting that a statute or regulation exhibiting primary
retroactivity, by changing the past legal consequences of past
actions, is presumptively valid absent a showing of case-
specific reliance by adversely affected parties.
By backdating El Paso’s 2021 nonattainment designation
to 2018, EPA changed the legal consequences of Texas’s
inaction over that past period. The designation thus exhibited
primary retroactivity—and was invalid for that reason.
IV
Our final task is to determine the appropriate remedy. The
Clean Air Act permits us to “reverse” any EPA “action” found
to be arbitrary. 42 U.S.C. 7607(d)(9). Texas asks us to reverse
the Final Rule itself. But regulations—like statutes—are
presumptively severable: If parts of a regulation are invalid
and other parts are not, we set aside only the invalid parts unless
the remaining ones cannot operate by themselves or unless the
agency manifests an intent for the entire package to rise or fall
together. This is true for agency rules in general, e.g., Finnbin,
LLC v. CPSC, 45 F.4th 127, 136 (D.C. Cir. 2022); Carlson v.
Postal Regul. Comm’n, 938 F.3d 337, 351 (D.C. Cir. 2019),
and for EPA rules in particular, e.g., Virginia v. EPA, 116 F.3d
499, 500–01 (D.C. Cir. 1997); Davis Cnty. Solid Waste Mgmt.

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v. EPA, 108 F.3d 1454, 1459 (D.C. Cir. 1997). Moreover,
judicial remedies should be “no more burdensome to the
defendant than necessary to provide complete relief” to the
plaintiffs or petitioners. Califano v. Yamasaki, 442 U.S. 682,
702 (1979); see also California v. Texas, 141 S. Ct. 2104, 2115
(2021) (remedies “operate with respect to specific parties”
rather than “on legal rules in the abstract”) (cleaned up).
Under these standards, the revised Weld County and El
Paso designations are clearly severable. They adjust the
geographic boundaries, and thereby the attainment status, of
areas hundreds of miles apart. Each revised designation
functions perfectly well on its own, and we have no reason to
think that EPA would want both the revised designations to fall
simply because one of them is invalid. For these reasons, we
decline to disturb the Weld County designation.
A distinct severability question relates to the El Paso
nonattainment designation. As we have explained, its
impermissible retroactivity arises not from the designation
itself, but from the designation combined with EPA’s refusal
to recognize a new attainment date. We could cure the legal
violation by reversing either decision. But since EPA has
strenuously argued that a new attainment date would create
both fairness and administrability concerns, J.A. 662–64, we
are reluctant to force that option on EPA. Instead, we think it
more prudent simply to reverse the nonattainment designation,
leaving EPA free on remand to decide whether to make a new
designation with its own attainment date or simply to let well
enough alone.
V
For these reasons, we deny Weld County’s petition for
review, grant Texas’s petition for review, and reverse the Final

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Rule insofar as it designates El Paso County to be a marginal
nonattainment area.
So ordered.

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