 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 6, 2015               Decided July 29, 2016

                       No. 14-1145

              WILDEARTH GUARDIANS, ET AL.,
                     PETITIONERS

                             v.

          ENVIRONMENTAL PROTECTION AGENCY,
                    RESPONDENT

  SAN JOAQUIN VALLEY UNIFIED AIR POLLUTION CONTROL
  DISTRICT AND SOUTH COAST AIR QUALITY MANAGEMENT
                       DISTRICT,
                     INTERVENORS


      On Petition for Review of Final Agency Action of
     the United States Environmental Protection Agency


     Paul Cort argued the cause for petitioners. With him on
the briefs was Colin C. O'Brien.

    Brian H. Lynk, Attorney, U.S. Department of Justice,
argued the cause for respondent. With him on the brief was
John C. Cruden, Assistant Attorney General. Sam Hirsch,
Attorney, entered an appearance.

    Barbara     Baird,   Lauren     Nevitt,  Annette   A.
Ballatore-Williamson, and Jessica E. Hafer Fierro were on
                              2
the brief for respondent-intervenors San Joaquin Valley
Unified Air Pollution Control District and South Coast Air
Quality Management District.

    Before: SRINIVASAN, Circuit Judge, and WILLIAMS and
GINSBURG, Senior Circuit Judges.

    Opinion for the Court filed by Circuit Judge SRINIVASAN.

     Under the Clean Air Act, the Environmental Protection
Agency promulgates national ambient air quality standards.
The Act sets out distinct statutory frameworks of varying
levels of strictness for EPA to apply to air pollutants. This
case involves the agency’s regulation under the Act of a
specific pollutant: fine particulate matter. For some time,
EPA subjected fine particulate matter to a relaxed statutory
framework under the Act. In 2013, however, this court
instructed the agency that the framework it had been applying
to fine particulate matter was incorrect and that a more
onerous statutory framework governed that pollutant.

     In response to our decision, EPA promulgated the
implementation rule at issue here. During the time EPA had
been applying the incorrect (and more relaxed) statutory
framework to fine particulate matter, some of the stricter
compliance deadlines that would have applied under the
correct statutory framework had already elapsed. In its
implementation rule, the agency made certain adjustments to
those deadlines in an effort to avoid treating states as having
already missed deadlines of which they were never aware.

     WildEarth Guardians challenges EPA’s authority to
adjust the deadlines. In WildEarth’s view, after this court
issued its decision holding that EPA had been using the wrong
framework, the agency was required to adopt a rule grounded
                              3
in an assumption that the correct framework had been applied
all along, even though some of the deadlines under that
framework would have already passed.              We reject
WildEarth’s argument.       We hold that, in the novel
circumstances presented here, EPA reasonably acted within
its statutory authority in adopting new deadlines aimed to
avoid imposing retroactive burdens on states seeking to
achieve compliance with governing air quality standards.

                              I.

                              A.

     The Clean Air Act, 42 U.S.C. §§ 7401 et seq., requires
EPA to publish a list of air pollutants that “may reasonably be
anticipated to endanger public health or welfare.” Id. §
7408(a)(1)(A). For each pollutant, EPA must promulgate
national ambient air quality standards (NAAQS). See id. §
7409.

     “Once EPA establishes NAAQS for a particular
pollutant, the standards become the centerpiece of a complex
statutory regime aimed at reducing the pollutant’s
atmospheric concentration.” Am. Trucking Ass’ns, Inc. v.
EPA, 283 F.3d 355, 358-59 (D.C. Cir. 2002). EPA designates
areas of the country as “attainment,” “nonattainment,” or
“unclassifiable,” based on whether the region’s atmospheric
concentration of the pollutant falls below the level permitted
by the NAAQS—in other words, whether the region has
“attained” compliance with the standards. 42 U.S.C. §
7407(d)(1). Each state must then devise and submit to EPA a
state implementation plan (SIP) that explains how any
nonattainment areas will attain the standards.          Id. §
7410(a)(1).
                               4
     EPA first regulated particulate matter in the original set
of NAAQS promulgated in 1971. See National Primary and
Secondary Ambient Air Quality Standards, 36 Fed. Reg. 8186
(Apr. 30, 1971). Particulate matter refers to a mixture of
liquid droplets and extremely small solids, which can be made
up of various components including acids, chemicals, metals,
soil, or dust. Particles can enter deep into the lungs and cause
serious health problems. In 1987, EPA, recognizing that the
size of particles directly affects the health risk, revised the
particulate matter standard to include only “particles with an
aerodynamic diameter less than or equal to a nominal 10
micrometers.” Revisions to the NAAQS for Particulate
Matter, 52 Fed. Reg. 24,634, 24,633-34 (July 1, 1987). Such
particles are referred to as “PM10.”

                              B.

     In 1990, Congress enacted the Clean Air Act
Amendments. See Pub. L. No. 101-549, 104 Stat. 2399. The
Amendments revised the procedures for implementing
NAAQS. The new Part D established five Subparts, each of
which sets out a different framework of deadlines and
requirements. Whereas Subparts 2 through 5 each pertain to a
particular pollutant, Subpart 1 serves as a catch-all category,
establishing the requirements for all remaining pollutants.
Subpart 4 specifically governs particulate matter. See 42
U.S.C. §§ 7513-7513b. Because PM10 was the only kind of
particulate matter regulated by EPA at the time of the
Amendments, Subpart 4 expressly referred to PM10. Id.

     Subpart 4’s requirements are stricter than the default
requirements set forth in Subpart 1. Compare id. §§ 7501-
7509a (Subpart 1), with id. §§ 7513-7513b (Subpart 4). For
instance, Subpart 1 gives EPA greater discretion to establish
deadlines for states to submit SIPs following a nonattainment
                               5
designation, see id. § 7502(b), whereas Subpart 4 mandates
specific deadlines under which SIPs are due within “18
months after the designation as nonattainment.” Id. §
7513a(a)(2)(B).

     Additionally, Subpart 1 does not require EPA to classify
nonattainment areas based on the severity of their air control
problem or the length of time for which an area has failed to
achieve attainment. See id. § 7502(a)(1). By contrast,
Subpart 4 establishes a rigorous set of classification
procedures. Nonattainment areas are initially classified as
“moderate areas.” See id. § 7513(a). Such areas are expected
to attain the requisite standards by “the end of the sixth
calendar year after the area’s designation as nonattainment.”
Id. § 7513(c)(1). Areas unable to achieve attainment by that
deadline are reclassified as “serious areas” subject to
heightened obligations. Id. § 7513(b).

     Reclassification from moderate to serious can occur
through one of two routes. Should an area fail to attain the
requisite standard by the moderate-area attainment date, it is
“reclassified by operation of law as a [s]erious [a]rea.” Id. §
7513(b)(2)(A). Alternatively, an area can be reclassified as
serious in advance of the moderate-area attainment deadline if
EPA determines that the area cannot achieve attainment by
that date. Id. § 7513(b)(1). That determination often arises
out of an area’s request to EPA for reclassification. See, e.g.,
Intervenor Br. 7. Such voluntary reclassification must occur
“within 18 months after the required date for the State’s
submission of a SIP for the [m]oderate [a]rea.” 42 U.S.C. §
7513(b)(1)(B). Voluntary reclassification is advantageous
because, if an area is reclassified as serious voluntarily rather
than by operation of law, the state will have additional time
(four years instead of eighteen months) to submit a SIP for
that area. See id. § 7513a(b)(2).
                              6
    Nonattainment areas can suspend further attainment
planning obligations altogether (such as SIP submission) upon
obtaining a “clean data determination” from EPA. An area
qualifies for a clean data determination if it attains the
NAAQS for three consecutive years. After securing a clean
data determination, an area becomes eligible to seek
redesignation as an attainment area. See Identification of
Nonattainment Classification and Deadlines for Submission
of SIP Provisions for the 1997 Fine Particle (PM2.5) NAAQS
and 2006 PM2.5 NAAQS, 78 Fed. Reg. 69,806, 69,809-10
(proposed Nov. 21, 2013) (to be codified at 40 C.F.R. pt. 51).

                              C.

     In 1997, EPA revised the particulate matter standard.
EPA acted in response to evidence that particles with an
aerodynamic diameter less than or equal to 2.5 micrometers
(PM2.5) posed serious health risks even at levels permitted
under the existing PM10 regulations. See NAAQS for
Particulate Matter, 62 Fed. Reg. 38,652, 38,665-68 (July 18,
1997). The agency thus established separate standards for
two distinct types of particulate matter: fine (PM2.5) and
coarse (PM10). Id. at 38,665. EPA set a stricter standard for
fine particulate matter than for coarse particulate matter.

     Since its initial adoption in 1997 of a separate standard
for PM2.5, EPA has revised that standard twice—in 2006 and
again in 2012. See NAAQS for Particulate Matter, 71 Fed.
Reg. 61,144 (Oct. 17, 2006); NAAQS for Particulate Matter,
78 Fed. Reg. 3086 (Jan. 15, 2013). This case implicates only
the 1997 and 2006 standards. The 1997 standard took effect
in 2005, when EPA published the initial air quality
designations for most areas in the country. See Air Quality
Designations and Classifications for the Fine Particles (PM2.5)
NAAQS, 70 Fed. Reg. 944 (Jan. 5, 2005). The 2006 standard
                               7
took effect in 2009 through an analogous rulemaking. See Air
Quality Designations for the 2006 24-Hour Fine Particle
(PM2.5) NAAQS, 74 Fed. Reg. 58,688 (Nov. 13, 2009).

     From 1997 onward, EPA implemented all of the PM2.5
standards pursuant to the more relaxed provisions of Subpart
1, rather than the more prescriptive and onerous requirements
of Subpart 4. See NAAQS for Particulate Matter, 62 Fed.
Reg. at 38,695. The agency maintained that, under the plain
terms of the statute, Subpart 4 referred specifically to PM10.
See id.

     In Natural Resources Defense Council v. EPA, 706 F.3d
428 (D.C. Cir. 2013) (NRDC), we rejected EPA’s approach of
enforcing PM2.5 standards under Subpart 1 rather than Subpart
4. By definition, we concluded, PM2.5 is PM10. Id. at 435.
And under the statute, the agency was required to “implement
all standards applicable to PM10—including its PM2.5
standards—pursuant to Subpart 4.” Id. at 436. We therefore
“remand[ed] to EPA to re-promulgate the[] [implementation]
rules” for fine particulate matter under the correct framework.
Id. at 437.

                              D.

     In an effort to shift its implementation of PM2.5 standards
to Subpart 4 per our direction in NRDC, the agency
promulgated the Implementation Rule at issue here. The Rule
set forth new nonattainment designations and revised
deadlines for states to submit SIPs for nonattainment areas.
Identification of Nonattainment Classification and Deadlines
for Submission of SIP Provisions for the 1997 Fine Particle
(PM2.5) NAAQS and 2006 PM2.5 NAAQS, 79 Fed. Reg.
31,566 (June 2, 2014) (to be codified at 40 C.F.R. ch. I). In
fashioning the Rule, EPA sought to account for the fact that,
                              8
had it applied Subpart 4 to PM2.5 from the outset, some of the
original Subpart 4 deadlines would have already passed by the
time of the Rule (and of our decision in NRDC).

     One set of those deadlines pertains to the submission of
SIPs. For the 1997 standard, EPA issued nonattainment area
designations in 2005, with an effective date of April 5 of that
year. See Air Quality Designations and Classifications for the
Fine Particles (PM2.5) NAAQS, 70 Fed. Reg. at 944. Under
the Subpart 1 framework applied to PM2.5 at the time, EPA
had discretion to establish the deadline for submission of SIPs
for those areas. See 42 U.S.C. § 7502(b). But if Subpart 4
had been applied, states would have been statutorily required
to submit SIPs within eighteen months of the nonattainment
designation, see id. § 7513a(a)(2)(B), or by October 5, 2006.
For the 2006 standard, correspondingly, the deadline for the
submission of SIPs for nonattainment areas would have been
June 14, 2011, because nonattainment designations under that
standard became effective on December 14, 2009. See Air
Quality Designations for the 2006 24-Hour Fine Particle
(PM2.5) NAAQS, 74 Fed. Reg. at 58,688. A failure to submit
SIPs by the deadline results in a renewed requirement to
submit plans and the possibility of penalties. See 42 U.S.C. §
7509.

     Another set of affected deadlines pertains to
reclassification of nonattainment areas. In particular, if the
Subpart 4 framework had applied all along, moderate
nonattainment areas for the 1997 standard would have already
been reclassified as serious. That is because, under the April
5, 2005, effective date for EPA’s nonattainment designations
for the 1997 standard, the deadline for moderate areas to
achieve attainment would have been December 31, 2011. See
id. § 7513(c)(1). And if EPA did not find that an area
achieved attainment within six months of that deadline, or by
                               9
June 30, 2012, Subpart 4 would have called for
reclassification of the area to serious by operation of law as of
that date. See id. § 7513(b)(2)(A). Affected states then would
have been subject to the accelerated deadline for submission
of SIPs for serious areas, under which the SIPs would have
been due within eighteen months, or by December 31, 2013.
See id. § 7513a(b)(2).

     In its Implementation Rule, EPA declined to impose
deadlines on states that rested on a counterfactual assumption
that Subpart 4 had been applied to fine particulate matter from
the outset. Such an approach, in EPA’s view, would have
carried unfair, retroactive implications for states that had been
operating under the Subpart 1 framework (erroneously)
applied by EPA.

     EPA instead adopted an approach in which it made two
adjustments to offset the fact that some Subpart 4 deadlines
would have already passed had that framework been used all
along. First, rather than find that states had already missed
SIP submission deadlines for moderate areas, EPA established
a deadline of December 31, 2014, for states to submit plans in
accordance with Subpart 4.            See Identification of
Nonattainment Classification and Deadlines for Submission
of SIP Provisions for the 1997 Fine Particle (PM2.5) NAAQS
and 2006 PM2.5 NAAQS, 79 Fed. Reg. at 31,570. That
revised deadline superseded all deadlines previously set under
the Subpart 1 framework (as well as any deadlines that would
have applied under the Subpart 4 framework). Second, rather
than treat certain nonattainment areas as if they had already
been reclassified as serious by operation of law, EPA
classified all nonattainment areas under both the 1997 and
2006 standards as moderate. See id. at 31,567-70.
                               10
     The areas primarily affected by the new Rule are those
that were previously designated as nonattainment under the
Subpart 1 regime and that had not yet submitted a SIP to the
agency or received a clean data determination suspending
attainment planning obligations altogether. Such areas, rather
than being treated as having already missed the submission
deadline for SIPs or as having been already reclassified as
serious by operation of law, were classified as moderate areas
and given until December 31, 2014, to submit SIPs. Three
nonattainment areas fell into that category for the 1997
standard. Five areas did so for the 2006 standard.

                               II.

     WildEarth filed a petition for review in this court in
which it challenges EPA’s approach in the Implementation
Rule as inconsistent with Subpart 4’s statutory deadlines.
Before addressing the merits of WildEarth’s claim, we must
resolve the threshold question of whether we have
jurisdiction. This court generally has jurisdiction to review
final actions taken by EPA under the Clean Air Act. See 42
U.S.C. § 7607(b)(1). EPA argues, however, that WildEarth
lacks Article III standing to bring this challenge or, in the
alternative, that intervening events have rendered the case
moot. We disagree and conclude that WildEarth has standing
to bring this petition for review and that the case is not moot
with respect to WildEarth’s challenge to the 2006 standard.

     First, with regard to the question of standing, WildEarth,
as the party filing suit, bears the burden of establishing its
standing. See Kokkonen v. Guardian Life Ins. Co., 511 U.S.
375, 377 (1994). To do so, WildEarth must demonstrate it
has suffered an “injury in fact” that is “fairly traceable” to the
defendant’s action and that can likely be “redressed by a
favorable decision.” Lujan v. Defs. of Wildlife, 504 U.S. 555,
                               11
560-61 (1992) (quotations, internal alterations, and quotation
marks omitted). The health and economic costs of increased
PM2.5 pollution for individuals in nonattainment areas
constitute injuries in fact that are fairly traceable to the EPA’s
challenged rule, satisfying the first two standing requirements.
The dispute here is primarily about the last prong,
redressability.

     EPA argues that a ruling in favor of WildEarth would not
redress WildEarth’s injury, because, while a favorable ruling
would invalidate EPA’s Implementation Rule, it would then
have the effect of reinstating the preexisting (and more
relaxed) Subpart 1 rules. That argument is unpersuasive. The
necessary consequence of vacating the Implementation Rule
on the ground that it failed adequately to adhere to Subpart 4
would be some kind of corrective EPA action strictly
implementing that Subpart, e.g., immediate findings
reclassifying nonattainment areas as serious (rather than
moderate). And even if EPA were to fail to initiate that sort
of remedial response, WildEarth could then file a mandamus
petition to compel agency action.          WildEarth’s injury
therefore could be redressed if it were to prevail in this
challenge.

     This brings us to the question of mootness. “[I]f an event
occurs while a case is pending on appeal that makes it
impossible for the court to grant any effectual relief whatever
to a prevailing party, the appeal must be dismissed.” Church
of Scientology of Cal. v. United States, 506 U.S. 9, 12 (1992)
(quotation and internal quotation marks omitted). EPA
asserts that intervening events have rendered this case moot.
Although we agree with EPA as to the 1997 PM2.5 standard,
we conclude that WildEarth continues to have a legally
cognizable interest in the outcome of the case with respect to
the 2006 standard.
                              12
     Understanding why the case is moot with regard to the
1997 standard, but not moot with regard to the 2006 standard,
requires a highly technical review of the effect of WildEarth’s
prevailing in this case on the nonattainment areas affected by
the Implementation Rule. For the 1997 standard, the Rule, as
previously noted, affected three nonattainment areas. (All
other areas that had been classified as nonattainment had
already obtained a clean data determination by the time of the
Rule’s promulgation and accordingly had no ongoing
planning obligations.) The three affected nonattainment areas
were: Libby, Montana; Los Angeles-South Coast Air Basin,
California; and San Joaquin Valley, California.            See
Identification of Nonattainment Classification and Deadlines
for Submission of SIP Provisions for the 1997 Fine Particle
(PM2.5) NAAQS and 2006 PM2.5 NAAQS, 78 Fed. Reg. at
69,809.      Due to developments following the Rule’s
promulgation, the challenged aspects of the Rule no longer
have any effect on those three areas.

     San Joaquin Valley has since been reclassified as serious
and has submitted its serious-area SIP. See Designation of
Areas for Air Quality Planning Purposes, 80 Fed. Reg. 18,528
(Apr. 7, 2015) (to be codified at 40 C.F.R. pts. 52, 81). San
Joaquin thus would be unaffected by a decision in
WildEarth’s favor—i.e., a decision vacating the
Implementation Rule on the ground that EPA lacked authority
to set a revised deadline for submission of SIPs of December
31, 2014. Because the extended timeline for submission of
serious-area SIPs is the only consequential result of that
revision, vacatur of the Rule’s December 31, 2014, deadline
would have no effect on an area (like San Joaquin) that has
already submitted its plan. Even if EPA were to reject San
Joaquin’s plan, that rejection, regardless of the outcome in
this case, would result in the imposition of a twelve-month
period within which to submit plan revisions because of San
                               13
Joaquin’s failure to achieve the 1997 standard by the serious-
area attainment date of December 31, 2015. See 42 U.S.C. §
7513a(d). WildEarth’s sole response is that EPA conceivably
could do more than merely reject San Joaquin’s plan, in that
EPA might separately revoke the 1997 standard altogether
and end any unmet planning obligations under that standard.
That possibility is highly (and unduly) speculative, too much
so to resuscitate an otherwise-moot controversy as to San
Joaquin.

     There is also no remaining practical effect for the other
two affected areas—Libby and South Coast—under the 1997
NAAQS. Since the Rule’s promulgation, Libby and South
Coast have both received clean data determinations
suspending all planning obligations under that standard. See
Determinations of Attainment of the 1997 Annual Fine
Particulate Matter Standard for the Libby, Montana
Nonattainment Area, 80 Fed. Reg. 40,911 (July 14, 2015) (to
be codified at 40 C.F.R. pt. 52); Clean Data Determination for
1997 PM2.5 Standards; California-South Coast (signed July 8,
2016) (to be codified at 40 C.F.R. pt. 52). WildEarth
responds that, if it were to prevail in this challenge, Libby and
South Coast would be subject to the stricter requirements
applicable to serious areas, and those additional restrictions
would remain in effect notwithstanding a clean data
determination. See 42 U.S.C. § 7513a(b). At this point,
however, Libby and South Coast no longer face the possibility
of reclassification as serious: it is clear from the clean data
determinations for those areas that they, as of today, have
attained the 1997 standard, precluding their reclassification as
serious. For Libby and South Coast, then—as with San
Joaquin—there is no continuing controversy as to the 1997
standard.
                              14
     WildEarth’s challenge is not moot, however, with regard
to the 2006 standard. That standard took effect on December
14, 2009. See Air Quality Designations for the 2006 24-Hour
Fine Particle (PM2.5) NAAQS, 74 Fed. Reg. at 58,688. At the
time, five areas were designated as nonattainment. Had the
Act been implemented correctly from the outset, i.e., under
Subpart 4, moderate-area SIPs for those areas would have
been due on June 14, 2011. See 42 U.S.C. § 7513a(a)(2)(B).

     WildEarth thus contends that, as relief, we should compel
EPA to make an immediate finding under section
7410(k)(1)(B) that the states failed to submit their plans. See
WildEarth Opening Br. 21. EPA claims the issue is now
moot, because, under its approach in the Rule, such failure-to-
submit findings have been ongoing in any event: the Rule
called for states to submit moderate-area plans in compliance
with Subpart 4 no later than December 31, 2014. Based on
the statutory framework, EPA then had until June 30, 2015, to
determine whether any state had failed to submit. See 42
U.S.C. § 7410(k)(1)(B). Were we now to compel EPA to
make failure-to-submit findings based on the Rule’s supposed
departure from Subpart 4, the agency contends, it would
merely delay the time of its failure-to-submit findings.

     Even assuming that to be true, however, the schedule
pursuant to which the failure-to-submit findings are
conducted and imposed—i.e., the timeline under Subpart 4
versus the modified schedule under the Rule—would have a
significant effect: it would determine whether areas can elect
voluntary reclassification as serious. Under the statutory
scheme, EPA can voluntarily reclassify an area only if (i) the
moderate-area attainment deadline has yet to pass, and (ii)
reclassification occurs within eighteen months of the deadline
for submitting a SIP. See id. § 7513(b)(1). According to
WildEarth, because moderate-area SIPs would have been due
                              15
by June 14, 2011, had Subpart 4 been applied all along,
EPA’s ability to grant voluntary reclassification expired
eighteen months later, on December 14, 2012. The Rule, by
contrast, set a new plan submission deadline of December 31,
2014, which had the effect of triggering a new window for
voluntary reclassification. As a result, EPA was able to grant
requests for voluntary reclassification until the December 31,
2015, moderate-area attainment deadline (which was
unchanged by the Rule).

     The opportunity to obtain voluntary reclassification in
turn affects the deadline for a state’s submission of a serious-
area SIP: whereas voluntarily reclassified areas receive four
years to submit a plan, areas reclassified by operation of law
receive only eighteen months. See id. § 7513a(b)(2). If, as
WildEarth suggests, areas can no longer obtain voluntary
reclassification with respect to the 2006 standard, those areas
would have to submit serious-area plans within eighteen
months of their reclassification by operation of law—i.e., by
December 31, 2017 (assuming, as WildEarth submits,
reclassification occurred by operation of law within six
months of the December 31, 2015, moderate-area attainment
deadline). See id. § 7513(b)(2). By contrast, under EPA’s
approach, states with voluntarily-reclassified areas would
have four years from the date of reclassification within which
to submit serious-area plans—i.e., until December 31, 2019.

     The difference between SIP submission deadlines of
December 2017 and December 2019 would affect areas for
which EPA has issued Proposed Rules granting voluntary
reclassification as serious with regard to the 2006 standard.
See Approval and Promulgation of Implementation Plans, 80
Fed. Reg. 1816 (proposed Jan. 13, 2015) (to be codified at 40
C.F.R. pts. 52, 81) (San Joaquin Valley); Approval and
Promulgation of Implementation Plans, 80 Fed. Reg. 69172
                               16
(proposed Nov. 9, 2015) (to be codified at 40 C.F.R. pt. 52)
(Logan, Utah/Franklin County, Idaho). Because of the Rule’s
impact on the serious-area SIP deadline for those areas, the
case presents a live controversy as to the 2006 standard.

     At oral argument, EPA counsel contended that the case
nonetheless was moot even with respect to the 2006 standard.
That contention was based on an EPA regulation promulgated
long ago (and apparently since dormant) under which,
according to counsel, the agency has leeway to disregard
section 7513(b)(1)(B)’s requirement that voluntary
reclassification be sought “within 18 months after the required
date for the State’s submission of a SIP for the Moderate
Area.” 42 U.S.C. § 7513(b)(1)(B); see State Implementation
Plans for Serious PM-10 Nonattainment Areas, and
Attainment Date Waivers for PM-10 Nonattainment Areas
Generally, 59 Fed. Reg. 41,998 (Aug. 16, 1994). As a result
of that regulation, EPA counsel contended at argument,
voluntary reclassification of the affected three areas is
permissible regardless of the outcome of WildEarth’s
challenge. We are unpersuaded.

     Initially, it is unclear from the text of the regulation
whether it necessarily even provides support for the
interpretation suggested at oral argument. The regulation
appears to reserve some discretion for EPA in allowing
voluntary reclassification by clarifying that the statutory
“directive does not restrict EPA’s general authority”; but it
otherwise affirms the statute’s mandate that “[a]ppropriate
areas are . . . reclassified as serious within 18 months after the
required date for the State’s submission of a moderate area
PM-10 SIP.” Id. at 41,999 & n.4. Moreover, we are unaware
of any indication that the agency has previously invoked the
authority purportedly established by the regulation to
overcome the statute’s eighteen-month timeframe, much less
                              17
of any judicial interpretation of that supposed authority.
Lastly, the agency failed to note the regulation, even while
arguing mootness, at any prior point in the proceedings. In
these circumstances, the government has failed to carry its
“heavy” burden to demonstrate mootness based on the
regulation (or otherwise). Cty. of L.A. v. Davis, 440 U.S. 625,
631 (1979) (quotation omitted).

                              III.

     Having concluded that WildEarth has established its
standing to challenge the Rule and that EPA has failed to
demonstrate mootness with regard to the 2006 standard, we
turn to the merits of WildEarth’s challenge to EPA’s
implementation of that standard. (In light of our conclusion
that WildEarth’s challenge concerning the 1997 standard is
moot, we have no occasion to consider WildEarth’s argument
that EPA lacked authority to classify all nonattainment areas
under the 1997 standard as moderate.) With respect to the
2006 standard, WildEarth argues that the plan submission
deadline established by the Implementation Rule is
incompatible with Subpart 4. We conclude that, in the unique
circumstances presented here, the Rule constitutes a
reasonable exercise of EPA’s rulemaking authority.

                              A.

     The Act grants the EPA Administrator general authority
“to prescribe such regulations as are necessary to carry out his
functions under this chapter.” 42 U.S.C. § 7601(a)(1). Of
course, “EPA cannot rely on its gap-filling authority to
supplement the Clean Air Act’s provisions when Congress
has not left the agency a gap to fill”—i.e., “when there is
statutory language on point.” Nat. Res. Def. Council v. EPA,
749 F.3d 1055, 1063-64 (D.C. Cir. 2014). Here, in response
                              18
to our remand in NRDC, see 706 F.3d at 437, EPA relied on
its gap-filling authority to issue the Implementation Rule in an
effort to bring its enforcement of the PM2.5 standards into
alignment with the Subpart 4 framework. In doing so,
WildEarth argues, EPA exceeded its authority by overriding
specific statutory requirements prescribed by Subpart 4.

     WildEarth’s argument is grounded in the understanding
that, once EPA has established a particulate matter standard
and identified nonattainment areas, Subpart 4 constrains the
agency’s discretion over implementation of the standard in
certain ways. See generally Whitman v. Am. Trucking Ass’ns,
531 U.S. 457, 484-85 (2001); Nat. Res. Def. Council v. EPA,
777 F.3d 456, 464 (D.C. Cir. 2014).            For instance,
classification of an area as moderate occurs “by operation of
law”—without any further action by EPA—“at the time of [an
area’s] designation” as nonattainment. 42 U.S.C. § 7513(a).
Moreover, the statute sets the moderate-area plan submission
deadline, again without any action by EPA, as eighteen
months after the date of nonattainment designation. See 42
U.S.C. § 7513a(a)(2)(B). And while a failure-to-submit
finding by EPA is necessary to trigger sanctions for a state’s
failure to meet the plan submission deadline, see id. §
7509(a), the statute limits EPA’s control over the timing of
that finding, calling for the agency to make the finding “no
later than 6 months after” the (statutorily mandated)
submission deadline. Id. § 7410(k)(1)(B).

    Under the language of those provisions, WildEarth
argues, EPA was required to issue immediate, failure-to-
submit findings for any state that had yet to submit a
moderate-area plan for the 2006 standard. WildEarth reasons
as follows: because Subpart 4 prescribes the date of
moderate-area classification and the resulting deadlines for
moderate-area plan submission and failure-to-submit findings,
                              19
and because all of those dates would have long lapsed had
Subpart 4 been correctly applied to PM2.5 from the outset, see
supra p. 8, EPA was obligated to issue immediate, failure-to-
submit findings to comply with the terms of Subpart 4.
Instead of doing so, EPA adopted a Rule giving states the
opportunity to submit moderate-area SIPs by a new deadline
of December 31, 2014. That approach, WildEarth contends,
is foreclosed by the statute.

     We find that EPA acted within its authority under the
statute. It is true that Subpart 4 sets the schedule for plan
submission and failure-to-submit findings once the agency
issues nonattainment designations, and that the agency
generally lacks discretion to modify that schedule. But the
statute does not address what should happen if, as in the novel
circumstances of this case, all affected parties have been long
acting on the mistaken assumption that a different
framework—and hence a different schedule—controls. Of
particular significance, the statute is silent about the
appropriate course when, as here, issuing immediate, failure-
to-submit findings (as WildEarth urges) in a purported effort
to adhere to Subpart 4’s plan submission schedule would have
the effect of nullifying another feature of Subpart 4.

     That is because, if EPA had found that states had already
failed to submit moderate-area plans by the applicable
deadline (WildEarth’s approach) rather than establish a
modified deadline for the submission of moderate-area plans
(the Rule’s approach), the agency would have eliminated a
state’s ability to request voluntary reclassification of a
moderate area as serious. Ordinarily, states can receive
voluntary reclassification within eighteen months of the plan
submission deadline. See 42 U.S.C. § 7513(b)(1)(B). By
pursuing that course, states gain an additional 2.5 years to
submit a SIP for the area. See id. § 7513a(b)(2). Under
                              20
WildEarth’s approach, however, the moderate-area plan
submission deadline would have passed without states’
awareness on June 14, 2011, such that states would have
(unknowingly) lost the opportunity to seek voluntary
reclassification within eighteen months of that (unknown)
deadline. That result would effectively read out of the statute
the voluntary-reclassification option afforded by Subpart 4,
which presumably exists to encourage states to be proactive
about meeting their obligations under the Act. EPA’s
approach in the Rule avoids that consequence.

     In addition, the Rule thereby avoids a situation in which
the agency’s action would impose retroactive consequences
on states, a result we have sought to avoid in our decisions.
In Sierra Club v. Whitman, 285 F.3d 63 (D.C. Cir. 2002)
(Sierra Club I), for instance, we ordered EPA to make a
mandatory ozone NAAQS determination for the St. Louis
area, which, under the statute, the agency should have done
several years earlier. We refused, however, to order EPA to
backdate its determination “to the date the statute envisioned,
rather than the actual date of EPA’s action.” Id. at 68. We
saw no basis for concluding that “Congress intended to give
EPA the unusual ability to implement rules retroactively.” Id.
“Although EPA failed to make the nonattainment
determination within the statutory time frame,” we explained,
“Sierra Club’s proposed solution [would] only make[] the
situation worse” because states “would face fines and suits”
for not having timely implemented plans “even though they
were not on notice at the time” of the various requirements
and deadlines. Id.

     Similar considerations animated our decision in Sierra
Club v. EPA, 356 F.3d 296 (D.C. Cir. 2004) (Sierra Club II).
In that case, EPA reclassified the District of Columbia from
“serious” to “severe” nonattainment (a classification not at
                              21
issue here). The agency established a new deadline for the
District’s submission of a severe-area nonattainment plan
because the original statutory deadline for such plans had long
since passed. We rejected Sierra Club’s argument that the
original statutory deadline should govern, reasoning that such
a result “would give the reclassification retroactive effect by
holding the States in default of their submission obligations
before the events necessary to trigger that obligation
(reclassification) occurred.” Id. at 309 (quotation and
alteration omitted).

     Those cases differ from this one in certain respects, as
WildEarth emphasizes. The petitioners in Sierra Club I
requested backdated findings, for instance, whereas
WildEarth urges present findings of noncompliance. And in
Sierra Club II, EPA had specific authority under Subpart 2 to
“adjust any applicable deadlines (other than attainment dates)
to the extent such adjustment is necessary or appropriate to
assure consistency among the required submissions.” 42
U.S.C. § 7511a(i). Subpart 4 includes no such specific
authority to adjust deadlines.

     At a more fundamental level, though, invalidating the
Rule before us on the rationale that EPA should have
immediately found that states had already missed the plan
submission deadline would impose retroactive consequences
of the kind that raised concerns in Sierra Club I and Sierra
Club II. States would be held to long-passed deadlines of
which they were unaware, with meaningful legal
consequences. EPA emphasized those sorts of concerns in its
rulemaking: “Because of the complexity of the [Act]’s SIP
provisions and the interrelationship between federal and state
action,” EPA explained, “it is inappropriate to impose
retroactive effect on decisions in a manner that would create
deadlines that have long passed.”           Identification of
                             22
Nonattainment Classification and Deadlines for Submission
of SIP Provisions for the 1997 Fine Particle (PM2.5) NAAQS
and 2006 PM2.5 NAAQS, 79 Fed. Reg. at 31,568.

     For those reasons, we reject WildEarth’s argument that
the Rule’s establishment of a new plan submission date under
the 2006 standard is foreclosed by Subpart 4. In the novel
circumstances confronting EPA here, we conclude that EPA
did not exceed its statutory authority under section 7601 in
promulgating the Rule.

                             B.

    We also find that the Rule was a reasonable exercise of
EPA’s gap-filling authority. In devising the new plan
submission deadline, the agency took into account the
“amount of time that ha[d] passed since the NRDC decision,
when this rulemaking will be finalized, and the amount of
time remaining before the 2006 PM2.5 NAAQS attainment
deadline under [S]ubpart 4 for most areas of December 31,
2015.” Id. at 31,569. EPA sought to ensure that “all states
with PM2.5 nonattainment areas have a reasonable amount of
time to develop any additional SIP elements that may be
required under [S]ubpart 4 in response to the NRDC
decision.” Id. at 31,570.

    Viewed in light of those understandable considerations,
the Rule constitutes a reasonable effort to implement the
Subpart 4 requirements in a timely fashion. Significantly, the
agency retained the attainment deadline of December 31,
2015, for the 2006 standard, even though that date was fast
approaching by the time of the Rule’s promulgation in June
2014. Moderate areas thus received no additional time within
which to achieve attainment with the 2006 NAAQS.
Moreover, the agency’s plan submission deadline of
                             23
December 31, 2014, was less than two years after the NRDC
decision and some six months after the Rule. That short
timeframe suggests a reasonable effort to expedite compliance
with the Subpart 4 framework without imposing unfair
obligations on states.

     Additionally, in our decision in NRDC, we specifically
declined NRDC’s request to set a deadline for EPA to re-
promulgate the rules governing PM2.5 plans. See NRDC, 706
F.3d at 437 n.10. And rather than vacate the preexisting
implementation rules, we chose to remand them, presumably
in an effort to avoid “substantial disruption” and “at least
temporarily preserve the environmental values covered by
[the existing rules] until [they] could be replaced by a rule
consistent with [the court’s] opinion.” EME Homer City
Generation, L.P. v. EPA, 795 F.3d 118, 132, 134 (D.C. Cir.
2015) (quotation and internal quotation marks omitted).
EPA’s approach in the Rule is in keeping with those concerns.
The Rule, in short, is a reasonable exercise of EPA’s general
rulemaking authority to bring its enforcement of PM2.5
standards into alignment with the Subpart 4 framework.

                     *   *   *    *   *

     For the foregoing reasons, we dismiss the petition for
review insofar as it concerns the 1997 standard, and we
otherwise deny the petition.

                                                 So ordered.
