 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 22, 2013         Decided December 23, 2014

                       No. 12-1321

         NATURAL RESOURCES DEFENSE COUNCIL,
                     PETITIONER

                            v.

     ENVIRONMENTAL PROTECTION AGENCY AND GINA
                    MCCARTHY,
                   RESPONDENTS


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


    Seth L. Johnson argued the cause for petitioner. With
him on the briefs were David S. Baron, and Robert Yuhnke.

     Christopher Kilian and Caitlin Peale were on the brief
for amicus curiae Conservation Law Foundation in support of
petitioner.

     Kenneth C. Amaditz, Attorney, U.S. Department of
Justice, argued the cause for respondents. With him on the
brief was Jan M. Tierney, Attorney, U.S. Environmental
Protection Agency.

   Before: TATEL and SRINIVASAN, Circuit Judges, and
RANDOLPH, Senior Circuit Judge.
                              2
    Opinion for the Court filed by Circuit Judge SRINIVASAN.

   Dissenting opinion filed by Senior Circuit Judge
RANDOLPH.

     SRINIVASAN, Circuit Judge: Under the Clean Air Act, the
Environmental Protection Agency promulgates National
Ambient Air Quality Standards for air pollutants. The Act
calls for EPA periodically to reconsider those standards. In
2008, EPA revised and strengthened the standards for ozone.

     This case concerns two challenges to EPA’s regulations
implementing the more stringent 2008 ozone standards. First,
EPA allowed affected regions more time to attain the new
ozone standards—roughly a one-third increase in time for
certain areas, amounting to an additional year—as compared
with the previous revision. Second, EPA revoked certain
requirements,    known      as transportation       conformity
requirements, applicable to areas that had yet to attain
governing ozone standards or that had recently come into
attainment but remained under obligations aimed to prevent
any reversion to nonattainment status. The transportation
conformity requirements obligate affected regions to assure
that any proposed project to develop transportation
infrastructure—potentially resulting in significantly increased
emissions—will align with the maximum emissions levels
established in EPA-approved plans for meeting air quality
standards.

     We conclude that both challenged aspects of EPA’s
regulations implementing the 2008 ozone standards exceed
the agency’s authority under the Clean Air Act. First, with
regard to the attainment deadlines, all statutory indications
militate against allowing the agency’s lengthening of the
periods for achieving compliance with revised air quality
                              3
standards. Indeed, the last time EPA issued revised standards
for ozone, EPA itself declined to extend the attainment
periods in essentially the same way it now proposes,
concluding that such an adjustment could not be squared with
the statute. Second, with regard to the revocation of
transportation conformity requirements, the terms of the
statute straightforwardly require maintaining those
requirements for affected areas. If it were otherwise, a region
that had yet to demonstrate an enduring ability to meet ozone
standards would be free to undertake transportation projects
that would increase emissions substantially beyond the levels
permitted in the area’s approved air-quality plan, undercutting
the Act’s objectives. Because we find that the EPA’s
challenged implementation rules exceed the agency’s
authority under the Clean Air Act, we vacate the pertinent
portions of EPA’s regulations.

                              I.

                              A.

     1. The Clean Air Act (CAA), 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.” 42 U.S.C. § 7408(a)(1)(A); see EPA v. EME Homer
City Generation, L.P., 134 S. Ct. 1584, 1594 (2014). For
each such pollutant, EPA must issue a “primary” National
Ambient Air Quality Standard (NAAQS), specifying the level
of air quality “requisite to protect the public health” while
“allowing an adequate margin of safety.”          42 U.S.C.
§ 7409(b)(1). EPA must also promulgate a “secondary”
NAAQS, setting the “level of air quality . . . requisite to
protect the public welfare.” 42 U.S.C. § 7409(b)(2).
                                4
      “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 (ATA III), 283 F.3d 355, 358-59 (D.C. Cir. 2002). EPA,
in coordination with state governments, divides the country
geographically into “[a]ir quality control region[s].” 42
U.S.C. § 7407. EPA then designates each region as either (i)
“attainment,” if the region’s atmospheric concentration of the
pollutant falls below the allowed level; (ii) “nonattainment,”
if it does not; or (iii) “unclassifiable,” if there is insufficient
information. See id. § 7407(d)(1)(A). Each state must then
enact a state implementation plan (SIP) that “provides for
implementation, maintenance, and enforcement of [the]
primary” NAAQS. Id. § 7410(a)(1). States must submit their
SIPs for EPA approval, and the agency can require revisions
or impose a federal implementation plan if a SIP proves
inadequate. See id. § 7410(c)(1).

     When a region’s atmospheric pollutant concentration
changes, EPA may alter the area’s designation. For instance,
EPA can redesignate an attainment area to nonattainment
when the pollutant concentration rises. See id. § 7407(d)(3).
The statute permits redesignation in the other direction, from
nonattainment to attainment, only upon satisfaction of several
additional requirements. See id. § 7407(d)(3)(E). EPA must
determine, inter alia, that “the improvement in air quality is
due to permanent and enforceable reductions in emissions.”
Id. § 7407(d)(3)(E)(iii).      EPA must also approve a
“maintenance plan” to ensure that the area remains in
compliance with the standard. See id. § 7407(d)(3)(E)(iv).
Maintenance requirements remain in effect for twenty years
after redesignation. See id. § 7505a(a)-(b). EPA refers to
former nonattainment areas that have been redesignated to
                             5
attainment, but that remain subject         to   maintenance
requirements, as “maintenance areas.”

     Both maintenance areas and nonattainment areas must
adhere to the Clean Air Act’s transportation conformity
requirements.    See id. § 7506(c).     Those requirements
condition federal funding and approval of an area’s proposed
transportation projects on their compliance with applicable
SIPs. The conformity requirements mandate that emissions
resulting from covered projects will not interfere with
NAAQS attainment. See id.

     The Clean Air Act calls for EPA to conduct a “thorough
review” of each NAAQS every five years and “make such
revisions . . . and promulgate such new standards as may be
appropriate.” Id. § 7409(d)(1). Although EPA may revise a
NAAQS to “relax[]” the standard, the Act contains an anti-
backsliding provision which requires the agency to
“promulgate requirements” that “provide for controls which
are not less stringent than the controls applicable to areas
designated nonattainment before such relaxation.”        Id.
§ 7502(e). The anti-backsliding provision presents one
example of how the Act “reflects Congress’s intent that air
quality should be improved until safe and never allowed to
retreat thereafter.” South Coast Air Quality Mgmt. Dist. v.
EPA, 472 F.3d 882, 900 (D.C. Cir. 2006).

     2. This case concerns the NAAQS for ground-level
ozone (O3), a key component of urban smog. While ozone is
an “essential presence in the atmosphere’s stratospheric
layer,” exposure to ozone at ground level “can cause lung
dysfunction, coughing, wheezing, shortness of breath, nausea,
respiratory infection, and in some cases, permanent scarring
of the lung tissue.” South Coast, 472 F.3d at 887. EPA has
determined that ozone also “has a broad array of effects on
                              6
trees, vegetation, and crops and can indirectly affect other
ecosystem components such as soil, water, and wildlife.”
Mississippi v. EPA, 744 F.3d 1334, 1340 (D.C. Cir. 2013).

     Ground-level ozone does not directly result from human
activity.   It instead arises from the reaction of other
atmospheric pollutants (known as precursors) in the presence
of sunlight. ATA III, 283 F.3d at 359. Ozone’s precursor
pollutants primarily come from emissions produced by cars,
power plants, and chemical solvents. Id.

     EPA classifies areas as having differing “ozone seasons”:
the sunnier months of the year when ozone forms at higher
rates. In most parts of the country, ozone seasons last from
March or April until September or October. Other areas
possess longer, or even year-round, ozone seasons. See 40
C.F.R. pt. 58, app. D, tbl.D-3 (Mar. 18, 2013); see also
Implementation of the 2008 NAAQS for Ozone:
Nonattainment Area Classifications Approach, Attainment
Deadlines and Revocation of the 1997 Ozone Standards for
Transportation Conformity Purposes, 77 Fed. Reg. 30,160,
30,166 nn.16-17 (May 21, 2012).

                             B.

     The history of Congress’s and EPA’s efforts to establish
air quality standards for ozone—and of the judicial responses
to those efforts—provides important context for considering
the issues presented by this case.

    1. In 1979, EPA promulgated primary and secondary
NAAQS for ozone with a limit of 0.12 parts per million
(ppm)—known as the “one-hour” standards, because they
measured average ozone levels over one-hour periods. See
Revisions to the NAAQS for Photochemical Oxidants, 44
                             7
Fed. Reg. 8202, 8202 (Feb. 8, 1979). The Clean Air Act as
amended in 1977 required states to achieve compliance with
the one-hour ozone NAAQS by December 31, 1987. See
South Coast, 472 F.3d at 886. The statute afforded EPA and
the states “broad discretion” as to the means of compliance.
Id. at 886-87.      That discretionary approach ultimately
accomplished “little to reduce the dangers of key
contaminants.” Id. For instance, according to congressional
testimony, the number of regions violating the one-hour
ozone NAAQS actually increased between August 1987 and
February 1989. Id.

     2. After nearly a decade of debate, Congress amended
the Clean Air Act in 1990 to “abandon[] the discretion-filled
approach of two decades prior in favor of more
comprehensive regulation” of ozone and five other pollutants.
Id.; see 42 U.S.C. §§ 7511-7514a. The amendments moved
the prior, discretionary approach to Subpart 1 of Part D of
Subchapter I, where it continued to apply as a default matter
to pollutants not specifically addressed in the amended
portions of the Act. See 42 U.S.C. § 7502(a)(1)(C). Congress
enacted Subpart 2 to govern ozone. See id. §§ 7511-7511f
(Subpart 2). Subpart 2 contains “a graduated classification
scheme that prescribe[s] mandatory controls that each state
must incorporate into its SIP.” South Coast, 472 F.3d at 887.
                                        8
     The “backbone” of Subpart 2 is “Table 1,” which
classifies all nonattainment areas for ozone by operation of
law. Whitman, 531 U.S. at 484; see also 42 U.S.C.
§ 7511(a)(1) tbl.1 (Table 1). †      Table 1 includes five
classification categories representing graduated degrees of
non-compliance with the NAAQS: Marginal, Moderate,
Serious, Severe, and Extreme. For each classification, the
table lists a range of “Design value[s],” the range of ground-
level ozone concentrations for that classification. Table 1
expresses design values in terms of the one-hour ozone
NAAQS in effect at the time of the 1990 amendments. For
example, the range for the Marginal category begins at 0.121
ppm, just above the one-hour limit of 0.12 ppm.


†
    Table 1 appears in the statute as follows:


                                   TABLE 1
                                                           Primary standard
Area class                     Design value*
                                                          attainment date**
Marginal................ 0.121 up to 0.138.....      …………3 years after
                                                         November 15, 1990
Moderate...............     0.138 up to 0.160.....   ................6 years after
                                                         November 15, 1990
Serious..................   0.160 up to 0.180.....   ................9 years after
                                                         November 15, 1990
Severe...................   0.180 up to 0.280.....   ..............15 years after
                                                         November 15, 1990
Extreme.................    0.280 and above…...      ..............20 years after
                                                         November 15, 1990

* The design value is measured in parts per million (ppm).
** The primary standard attainment date is measured from
November 15, 1990.
                              9
     Subpart 2 requires all nonattainment areas to achieve the
primary NAAQS “as expeditiously as practicable,” but no
later than the “Primary standard attainment date” in Table 1.
42 U.S.C. § 7511(a)(1) & tbl.1. Higher classifications—more
polluted areas—receive more time to attain compliance.
Unless already classified as Severe or Extreme, areas that fail
to attain the NAAQS by the statutory deadline are
automatically reclassified to the next highest classification,
see id. § 7511(b)(2), allowing more time for compliance but
imposing a harsher set of mandatory controls. See South
Coast, 472 F.3d at 887. The statute imposes additional
penalties on Severe and Extreme areas that fail to meet the
listed deadlines. See 42 U.S.C. § 7511d.

    3.    In 1997, citing new information suggesting a
correlation between prolonged ozone exposure and “a wide
range of health effects,” EPA promulgated new NAAQS for
ground-level ozone. See NAAQS for Ozone, 62 Fed. Reg.
38,856, 38,861 (July 18, 1997). EPA replaced the one-hour,
0.12 ppm standard with a 0.08 ppm standard measured over
an eight-hour period. See id. at 38,856. The agency
explained that an eight-hour, 0.09 ppm standard would have
“generally represent[ed] the continuation of the [old] level of
protection.” Id. at 38,858. The additional reduction from
0.09 ppm to 0.08 ppm manifested a strengthening of the
standard. The regulation stated that the preexisting primary
one-hour NAAQS, governed by Subpart 2 of the statute,
would remain in effect, but only for areas yet to attain the
one-hour standard and only until they did so. Id. at 38,873.
Subpart 1 alone thus would govern the 1997 primary
NAAQS. Id.

    This court rejected EPA’s attempt to apply Subpart 1 to
the new primary 1997 standard, holding that Subpart 2 must
govern any revised primary ozone NAAQS. Am. Trucking
                              10
Ass’ns, Inc. v. EPA (ATA I), 175 F.3d 1027, 1050 (D.C. Cir.
1999), modified on reh’g, 195 F.3d 4 (D.C. Cir. 1999). We
found that conclusion mandated by the plain text of the statute
at step one of the framework prescribed by Chevron, U.S.A.,
Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837
(1984). ATA I, 175 F.3d at 1048. On certiorari, the Supreme
Court agreed that EPA could not rely exclusively on Subpart
1 to implement the 1997 primary NAAQS because Subpart 2
“unquestionably” “provide[s] for classifying nonattainment
ozone areas under the revised standard.” Whitman v. Am.
Trucking Ass’ns, Inc., 531 U.S. 457, 482 (2001). The Court,
however, reached that conclusion at Chevron step two rather
than step one. Id. at 484.

     The Court identified various ways in which the statute,
on its face, was silent as to how Subpart 2 should apply when
EPA revises the ozone standard. See id. at 483-84. Of most
relevance here, the Court noted a timing gap in Table 1
stemming from the table’s use of the enactment date of the
1990 amendments (November 15, 1990) as the starting point
when prescribing the allowable attainment periods. That
approach “seems to make no sense for areas that are first
classified under a new standard after November 15, 1990. If,
for example, areas were classified in the year 2000, many of
the deadlines would already have expired at the time of
classification.” Id. at 483-84.

     In light of the identified gaps in the statute, the Court
found the amended Act “ambiguous concerning the manner in
which Subpart 1 and Subpart 2 interact with regard to revised
ozone standards.” Id. at 484. The Court stated that it
therefore would “defer to the EPA’s reasonable resolution of
that ambiguity,” under Chevron step two.             Id.  But
“emphasiz[ing]” the “narrow scope” of the identified gaps,
South Coast, 472 F.3d at 889, the Court held that “EPA’s
                               11
interpretation making Subpart 2 abruptly obsolete” went
“over the edge of reasonable interpretation.” Whitman, 531
U.S. at 485. “The principal distinction between Subpart 1 and
Subpart 2,” the Court explained, “is that the latter eliminates
regulatory discretion that the former allowed.” Id. at 484.
The agency therefore could “not construe the statute in a way
that completely nullifies textually applicable provisions meant
to limit its discretion.” Id. at 485.

     4. In 2003, with EPA yet to implement the 1997 primary
NAAQS in the aftermath of the Supreme Court decision in
Whitman, several environmental organizations sued the
agency, arguing that it had failed to designate areas (as
attainment or nonattainment) within the statutory deadlines.
See 42 U.S.C. § 7407(d)(1); Air Quality Designations and
Classifications for the 8-Hour Ozone NAAQS; Early Action
Compact Areas With Deferred Effective Dates, 69 Fed. Reg.
23,858, 23,860 (Apr. 30, 2004). EPA entered into a consent
decree requiring it to issue designations by April 15, 2004. 69
Fed. Reg. at 23,860. The agency promulgated designations
on April 30, 2004. See id. at 23,858.

     EPA also contemporaneously issued a new
implementation rule for the 1997 NAAQS. See Final Rule to
Implement the 8-Hour Ozone NAAQS—Phase 1, 69 Fed.
Reg. 23,951, 23,951 (Apr. 30, 2004) (2004 Rule). Of
particular salience, the rule filled the timing gap for Subpart 2
compliance dates by applying the same attainment periods
established in Table 1—e.g., three years for Marginal areas,
six years for Moderate areas, and so on—but measured from
the effective date of EPA’s designations for the 1997
NAAQS, June 15, 2004 (rather than from the November 15,
1990, enactment date of the 1990 amendments). Id. at
23,967. EPA considered an alternative approach under which
it would extend the attainment deadlines by several months, to
                              12
November or December of the relevant year (e.g., to
November or December 2007 for Marginal areas). See id.
The agency determined, however, that it lacked statutory
authority to extend the attainment deadlines in that fashion.
See id.

     As a general matter, the 2004 Rule addressed the
transition from the old one-hour NAAQS to the new 1997
standard by “revok[ing] the 1-hour standard in full, including
the associated designations and classifications.” Id. at 23,954.
Interpreting the statute’s anti-backsliding provision to apply
to the 1997 NAAQS, see 42 U.S.C. § 7502(e), the rule
mandated that all “controls” from the one-hour standard
remain in place after revocation of the one-hour NAAQS. See
Final Rule To Implement the 8-Hour Ozone NAAQS—Phase
1, 69 Fed. Reg. at 23,972. EPA concluded, however, that
certain requirements in Subpart 2 failed to qualify as
“controls” within the meaning of the anti-backsliding
provision.    Those requirements, including transportation
conformity obligations and attainment contingency plans,
would not be retained after the NAAQS transition. See id. at
23,984-87.

     This court invalidated several aspects of the 2004 Rule.
See South Coast, 472 F.3d at 890-905. We partially rejected
the rule’s approach to the prior, one-hour NAAQS. See id. at
899-905. We initially explained that EPA does possess power
to revoke a superseded standard in full: because EPA may
make “appropriate” “revisions” to an ozone NAAQS, see 42
U.S.C. § 7409(d)(1), the agency “retains the authority to
revoke the one-hour standard so long as adequate . . .
provisions are introduced” to satisfy the anti-backsliding
requirements of 42 U.S.C. § 7502(e). South Coast, 472 F.3d
at 899. But we rejected as “impermissible backsliding”
EPA’s failure to maintain several requirements associated
                            13
with the one-hour NAAQS on the flawed theory that they
were not “controls” (including “rate-of-progress milestones,
contingency plans, and motor vehicle emissions budgets”).
Id. at 900.

     5. In 2000, while the litigation concerning the 1997
NAAQS was underway, EPA initiated a new round of ozone
NAAQS review. See Mississippi, 744 F.3d at 1340. EPA
promulgated a revised NAAQS in 2008, in compliance with a
schedule adopted by consent decree. See id. Citing new data
concerning ozone’s health effects, EPA lowered the primary
and secondary standards to 0.075 ppm, measured using the
same eight-hour average as the 1997 NAAQS. See NAAQS
for Ozone, 73 Fed. Reg. 16,436, 16,436 (Mar. 27, 2008).

     EPA originally intended to designate areas under the
2008 NAAQS within two years of their issuance, by March
12, 2010. See Air Quality Designations for the 2008 Ozone
NAAQS, 77 Fed. Reg. 30,088, 30,090 (May 21, 2012). The
agency extended the designation deadline by one year, to
March 12, 2011, citing its authority under 42 U.S.C.
§ 7407(d)(1)(B). See 77 Fed. Reg. at 30,090-91. When the
extended deadline passed, an environmental organization sued
to compel EPA to issue the designations. EPA entered into a
consent decree requiring it to sign a final rule designating
areas by May 31, 2012. See id. The agency issued a final
rule on May 21, 2012, designating most areas of the United
States effective July 20, 2012. See id. at 30,088.

     On the same day, EPA promulgated the implementation
rule at issue in this case. See Implementation of the 2008
NAAQS for Ozone: Nonattainment Area Classifications
Approach, Attainment Deadlines and Revocation of the 1997
Ozone Standards for Transportation Conformity Purposes, 77
Fed. Reg. 30,160 (May 21, 2012) (Implementation Rule).
                             14
The Implementation Rule applies Subpart 2 to all
nonattainment areas for the 2008 NAAQS, translating the
one-hour design values in Table 1 to correspond to the new
0.075 ppm, eight-hour standard. See id. at 30,161-64. The
rule contains two additional actions challenged here. First,
rather than setting attainment deadlines based on the
attainment periods in Table 1 measured from the effective
date of the new designations—as EPA had done in the 2004
Rule—the agency extended the attainment deadlines by
several months to “December 31 of the [corresponding]
calendar year.” Id. at 30,166. Second, EPA revoked the 1997
NAAQS for purposes of, and only for purposes of, the
transportation conformity requirements. See id. at 30,167-68.
The rule otherwise leaves the 1997 NAAQS in place to
operate in parallel with the 2008 NAAQS.

                             II.

     NRDC petitions for review of the Implementation Rule,
challenging its schedule of attainment deadlines and its
revocation of the 1997 NAAQS for transportation conformity
purposes. We have jurisdiction to hear the petition under 42
U.S.C. § 7607(b)(1). NRDC contends that the challenged
portions of the rule are “arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with law.” See 42
U.S.C. § 7607(d)(9)(A). We apply the same standard of
review for arbitrary-and-capricious challenges under the
Clean Air Act as we do for similar challenges under the
Administrative Procedure Act, 5 U.S.C. § 706(2)(A). See
Catawba County v. EPA, 571 F.3d 20, 41 (D.C. Cir. 2009).

    We review EPA’s interpretation of the Clean Air Act
pursuant to the two-step Chevron framework. See Util. Air
Regulatory Grp. v. EPA, 134 S. Ct. 2427, 2439 (2014).
“Under Chevron, we presume that when an agency-
                               15
administered statute is ambiguous with respect to what it
prescribes, Congress has empowered the agency to resolve the
ambiguity. The question for a reviewing court is whether in
doing so the agency has acted reasonably and thus has ‘stayed
within the bounds of its statutory authority.’” Id. (quoting
City of Arlington v. FCC, 133 S. Ct. 1863, 1868 (2013)).
Applying that standard, we agree with both challenges to the
Implementation Rule, and we therefore vacate the rule in
relevant part.

                               A.

     NRDC first challenges the Implementation Rule’s
schedule of Subpart 2 attainment deadlines as contrary to the
statute and as an arbitrary and capricious change from prior
agency practice. Because we conclude that the rule’s
deadlines cannot be squared with the statute, we do not reach
the arbitrary-and-capricious challenge.

     Subpart 2 of the statute requires all nonattainment areas
to achieve compliance with the ozone NAAQS “as
expeditiously as practicable,” but in all events “not later than”
the “Primary standard attainment date” set forth in Table 1.
42 U.S.C. § 7511(a)(1). The Table 1 attainment dates, which
operated as initial deadlines for complying with the primary
NAAQS under the 1990 Clean Air Act amendments, specify a
set number of years for each classification measured from the
date of “enactment” of the amendments. 104 Stat. 2423(a)(1)
(1990). After the statute’s enactment, the actual enactment
date—November 15, 1990—was substituted into each row of
the table. So, for example, Table 1 expresses the attainment
deadline for Marginal areas as “3 years after November 15,
1990,” for Moderate areas as “6 years after November 15,
1990,” and so forth. 42 U.S.C. § 7511(a)(1) tbl.1. Under the
parties’ nomenclature, Table 1 thus prescribes an “attainment
                              16
period” of three years for Marginal areas, commencing on a
“trigger date” of November 15, 1990. Importantly, the
November 15, 1990, trigger date is the date on which
Congress specified that the initial designations/classifications
(e.g., nonattainment/Marginal) under the 1990 amendments
would take effect. See id. §§ 7407(d)(1)(C), 7511(a)(1).
Congress set the trigger date, that is, as the designation date—
i.e., the effective date of the designations/classifications.

     In addition to prescribing the attainment deadlines for
areas initially designated as nonattainment under the 1990
amendments, Congress also specified the attainment deadlines
for any area subsequently redesignated from attainment to
nonattainment.       An area redesignated to nonattainment
receives its classification as Marginal, Moderate, and the like,
on the date of the redesignation. See id. § 7511(b)(1). With
regard to the attainment deadlines for such an area, Congress
applied the same approach as it did for areas initially
designated as nonattainment: the statute mandates using the
same attainment periods in Table 1, with a trigger date of the
designation—or in this case, redesignation—date. See id. An
area redesignated to nonattainment and classified as Marginal
thus would be required to attain the governing NAAQS within
three years of the redesignation date, an area redesignated to
nonattainment and classified as Moderate would be required
to attain the applicable NAAQS within six years of the
redesignation date, and so on.

     While Subpart 2 specifies the attainment deadlines both
for initial designations under the 1990 amendments and for
subsequent redesignations of areas from attainment to
nonattainment, the statute does not, on its face, prescribe the
calculation of attainment deadlines when EPA promulgates a
revised NAAQS, as it did here. See Whitman, 531 U.S. at
483-84. As the Supreme Court recognized in Whitman, it
                              17
would “make no sense” strictly to apply the attainment
deadlines from Table 1 to areas newly designated under a
revised NAAQS. Id. at 483. In the case of a revised standard
issued years after the Table 1 trigger date of November 15,
1990, “many of the deadlines would already have expired.”
Id. at 484. The Act therefore contains a “timing gap,” South
Coast, 472 F.3d at 889, requiring application of Subpart 2 to a
revised NAAQS but without directly setting forth the
attainment deadlines. See Whitman, 531 U.S. at 483-85.

     Because “the statute is silent or ambiguous with respect
to the specific issue,” we do not conclude, at Chevron step
one, that “Congress has directly spoken to the precise
question.” City of Arlington, 133 S. Ct. at 1868 (quoting
Chevron, 467 U.S. at 842-43); see also South Coast, 472 F.3d
at 892-93 (noting ambiguity from related statutory gaps). We
thus proceed to Chevron step two and assess “whether the
agency’s answer is based on a permissible construction of the
statute.” South Coast, 472 F.3d at 891 (quoting Chevron, 467
U.S. at 843). We conclude it is not.

     In assessing the permissibility under the statute of EPA’s
attainment deadlines for the 2008 NAAQS, it is instructive to
consider the agency’s current approach against the backdrop
of its contrasting methodology when implementing the 1997
NAAQS. When it implemented the 1997 NAAQS, the
agency, consistent with the Supreme Court’s explanation in
Whitman, acknowledged that rote application of the Table 1
deadlines to the revised NAAQS “would produce absurd
results.” See Final Rule To Implement the 8-Hour Ozone
NAAQS—Phase 1, 69 Fed. Reg. at 23,967. EPA nonetheless
did “not believe” that Subpart 2’s timing gap “allows broad
authority to re-write the statute.” Id. Instead, looking “to the
legislative history and other provisions of the CAA to discern
Congressional intent,” id., EPA applied the same approach to
                              18
the revised 1997 NAAQS as Congress had prescribed for the
initial designations and subsequent redesignations. The
agency set the trigger date as the designation date—June 15,
2004, the “effective date of designations and classifications
for the 8-hour ozone NAAQS”—and applied the attainment
periods established in Table 1 for each classification. Id. The
deadlines accordingly fell on June 15 of the attainment year:
June 15, 2007, for Marginal areas, June 15, 2010, for
Moderate areas, and so on.

     EPA materially altered course in its Implementation Rule
for the 2008 NAAQS, at issue here. See Implementation
Rule, 77 Fed. Reg. at 30,160. The agency considered
adhering to the 2004 Rule’s approach: using the attainment
periods from Table 1 and a trigger date equaling “the effective
date of designation[s]” under the revised NAAQS. Id. at 30,
165. Because designations and classifications for the 2008
NAAQS became effective on July 20, 2012, see Air Quality
Designations for the 2008 Ozone NAAQS, 77 Fed. Reg. at
30,088, that option would have yielded an attainment deadline
for Marginal areas of three years from that date, or July 20,
2015. The agency rejected that approach, however, in favor
of an alternative one under which it set attainment deadlines
to “December 31 of the calendar year that is the number of
years specified for each classification in Table 1 with the
number of years running from” the year of designation, or
2012. Implementation Rule, 77 Fed. Reg. at 30,166. In other
words, instead of setting the trigger date as the designation
date under the revised NAAQS—July 20, 2012—EPA
deferred the trigger date to December 31, 2012, giving
Marginal areas, for instance, a later attainment deadline of
December 31, 2015, instead of July 20, 2015.

    EPA identifies no statutory basis in Subpart 2 for
deferring the trigger date to December 31 of the designation
                              19
year. To the contrary, all textual indications point to
triggering commencement of the attainment periods on the
designation date, rather than delaying the trigger date to some
other date selected by the agency. Congress, as explained,
prescribed the initial attainment deadlines under the 1990
amendments based on a trigger date set as the designation
date—November 15, 1990, the date on which initial
designations and classifications took effect. See 42 U.S.C.
§§ 7407(d)(1)(C), 7511(a)(1). For areas redesignated to
nonattainment under an existing NAAQS, Congress likewise
required measuring the attainment deadlines based on a
trigger date equaling the redesignation date.           See id.
§ 7511(b)(1). Congress’s decision to run the Table 1
attainment periods starting from the designation date in each
part of Subpart 2 that directly addresses that issue strongly
suggests that the same trigger date should apply when
adapting Table 1 to the analogous situation of a revised
NAAQS. The trigger date (and attainment deadlines) for the
2008 NAAQS then would fall on July 20 rather than
December 31.

     Indeed, EPA itself previously concluded that it lacked
statutory authority to establish precisely the same December
attainment deadlines it now adopts in the Implementation
Rule. When the agency implemented the 1997 NAAQS, it
prescribed a trigger date of the designation date—June 15,
2004—resulting in attainment deadlines of June 15 of the
attainment year for each classification. EPA specifically
rejected an alternative approach, supported by several
commentators, under which it would have extended the
attainment deadlines by several months to “November or
December of the attainment year.” 2004 Rule, 69 Fed. Reg.
at 23,967. “We do not believe we have [that] authority,” EPA
explained, because “Congress would have intended for areas
designated nonattainment and classified under subpart 2 for
                              20
the [1997] NAAQS to have attainment periods consistent with
those in Table 1 (e.g., 3 years for marginal areas . . . ).” Id.

     There is thus a notable contrast between EPA’s
implementation of the 2008 NAAQS and its implementation
of the 1997 NAAQS. Designations under the 1997 revised
NAAQS became effective in June of the designation year (on
June 15, 2004), and designations under the 2008 revised
NAAQS became effective at a similar point during the
designation year (on July 20, 2012). But whereas EPA
explicitly declined to adopt an attainment deadline of
December of the attainment year for the 1997 NAAQS,
concluding that it had no statutory authority to do so, EPA
nonetheless prescribed an attainment deadline of December
31 of the attainment year for the 2008 NAAQS. EPA did so,
moreover, even though it expressly continues to believe that it
lacks statutory authority to extend the Table 1 attainment
periods—the same view it held when implementing the 1997
NAAQS.          See EPA, Response to Comments on
Implementation of the 2008 NAAQS for Ozone:
Nonattainment Area Classifications Approach, Attainment
Deadlines, and Revocation of the 1997 Ozone Standards for
Transportation Conformity Purposes, Dkt. No. EPA-HQ-
OAR-2010-0885 41 (2012).

     In its briefing in this court, the agency suggests that the
seeming discrepancy can be explained based on an ostensible
distinction between (i) extending the trigger date at the front
end of the attainment period by several months (the current
approach) and (ii) extending the attainment deadline at the
back end of the attainment period by several months (the
rejected approach). That distinction in form, however, is not
one in substance: in either case, the effect is to extend the
resulting attainment deadlines by several months to December
of the attainment year. Those “attainment deadlines,” we
                             21
have explained, “are central to the regulatory scheme.” Sierra
Club v. EPA (Sierra Club-delay), 294 F.3d 155, 161 (D.C.
Cir. 2002) (internal quotation marks and ellipsis omitted).
And because the substantive effect for the attainment
deadlines is precisely the same, there is no reason to suppose
that Congress (concededly) disallowed the latter approach but
nevertheless allowed the former one.

     That is all the more apparent in light of additional
constraints on EPA’s authority to extend the attainment
deadlines for the 2008 NAAQS. EPA does not dispute that
Subpart 2 dramatically limited the agency’s authority to
extend attainment deadlines after promulgating them:
Subpart 2 permits the agency to extend the deadlines for at
most two years under limited circumstances, see 42
U.S.C. § 7511(a)(5), unlike Subpart 1, which gives the agency
much broader discretion to extend attainment dates for as long
as 12 years, see id. § 7502(a)(2)(A), (C). See Whitman, 531
U.S. at 485. EPA also lacked authority to delay the
attainment deadlines indirectly, either by deferring
designation of nonattainment areas or by withholding
issuance of a revised NAAQS in the first place: The Clean
Air Act requires EPA periodically to promulgate revised
NAAQS according to specified deadlines, 42 U.S.C.
§ 7409(d)(1), and generally requires EPA to issue
designations within two years of promulgating a revised
NAAQS, id. § 7407(d)(1)(B)(i). EPA entered into consent
decrees aimed to enforce those provisions under which it
agreed to promulgate revised NAAQS by March 2008 and to
issue designations by May 2012. See Air Quality
Designations for the 2008 Ozone NAAQS, 77 Fed. Reg. at
30,090; NAAQS for Ozone, 73 Fed. Reg. at 16,438.

    In short, EPA correctly concedes that it had no authority
to extend overall attainment deadlines to the end of the
                               22
calendar year by either (i) delaying its promulgation of the
revised NAAQS; (ii) deferring its designation of areas under
the revised NAAQS; (iii) granting a blanket deadline
extension following its initial establishment of attainment
deadlines for the revised NAAQS; or (iv) extending the
attainment periods set forth in Table 1. The agency
nonetheless seeks to achieve precisely the same result simply
by introducing a nominally distinct category of extension—
namely, by delaying the trigger date for the start of the
(concededly fixed) attainment periods until some agency-
selected date, rather than measuring the attainment periods
from the designation date, as EPA did for the 1997 NAAQS
and as the statute prescribes both for the initial
designations/classifications under the 1990 amendments and
for subsequent redesignations.

     Even assuming EPA could adequately justify choosing a
trigger date other than the designation date, it has failed to do
so here. EPA attempts to explain its departure from the
designation date by reference to the number of “ozone
seasons” within which nonattainment areas must achieve the
revised standard. See Implementation Rule, 77 Fed. Reg. at
30,166. The 2008 NAAQS, like its predecessors, requires
averaging three calendar years of ozone concentration data to
determine an area’s ozone levels. Under the applicable
regulations, each calendar year of data must include
measurements spanning that year’s full ozone season. As a
result, if the attainment deadline falls during an ozone season,
the data for that entire year cannot be used in the calculation.
For instance, an area classified as Marginal under the 2008
NAAQS would, if the trigger date were set to the designation
date, face an attainment deadline of July 20, 2015. If the
area’s ozone season runs from April to September, ozone
measurements from 2015 could not be used to determine
compliance because the 2015 ozone season would not have
                               23
ended by the July attainment deadline. Data from calendar
years 2012, 2013, and 2014 thus would be used. Under the
Implementation Rule, by contrast, the attainment deadline
would be delayed until December 31, 2015—after completion
of the 2015 ozone season—allowing use of measurements
from 2013, 2014, and 2015. Because all ozone seasons end
after July 20, the Implementation Rule’s delay of attainment
deadlines effectively allows one additional year to achieve the
2008 standard as compared with EPA’s approach for the 2004
NAAQS, a roughly one-third increase in compliance time for
Marginal areas.

      EPA suggests that allowing an additional ozone season
for compliance serves an interest in establishing achievable
attainment deadlines. It is not our role to question the
agency’s policy judgment in that regard. It is our role,
however, to determine whether the statute authorizes EPA to
base Subpart 2 attainment deadlines on that policy judgment.
As the Supreme Court recently explained, “EPA must ‘ground
its reasons for action or inaction in the statute,’ rather than on
‘reasoning divorced from the statutory text.’” Util. Air
Regulatory Grp., 134 S. Ct. at 2411 (emphasis and citation
omitted) (quoting Massachusetts v. EPA, 549 U.S. 497, 532,
535 (2007)). EPA identifies no statutory provision giving it
free-form discretion to set Subpart 2 compliance deadlines
based on its own policy assessment concerning the number of
ozone seasons within which a nonattainment area should be
expected to achieve compliance. Cf. Whitman, 531 U.S. at
484 (“The principal distinction between Subpart 1 and
Subpart 2 is that the latter eliminates regulatory discretion that
the former allowed.”). To the contrary, the “attainment
deadlines . . . leave no room for claims of technological or
economic infeasibility.” Sierra Club-delay, 294 F.3d at 161
(internal quotation marks and brackets omitted).
                               24
     EPA’s ozone-season explanation lacks any grounding in
the statute. The agency itself previously recognized as much.
When EPA declined to extend the attainment deadlines to
“November or December” of the attainment year in its
implementation of the 1997 NAAQS, the reason cited in
support of the proposed extension was to enable “areas [to]
use the ozone season air quality data from the attainment year
to demonstrate attainment”—precisely the same explanation
now invoked by the agency. At the time, however, EPA did
“not believe [it] ha[d] authority to change the attainment dates
to November or December of the attainment year.” 2004
Rule, 69 Fed. Reg. at 23,967. The agency points to no
intervening statutory change that would now give it authority
to extend the attainment deadlines for reasons having to do
with ozone seasons. The point here is not that an agency is
barred from changing its mind. See FCC v. Fox Television
Stations, Inc., 556 U.S. 502, 515 (2009). The point instead is
that EPA had it right the first time in concluding that the
statute gave it no authority to extend the attainment deadlines
based on ozone seasons.

     EPA’s current ozone-season rationale in fact runs counter
to all indicia of congressional intent. As an initial matter, the
agency itself acknowledges Congress’s awareness that ozone
measurements are taken over a three-year period and require
full ozone seasons of data. Congress even used the concept of
“high ozone season” in other parts of the statute. See, e.g., 42
U.S.C. § 7545(h)(1). Yet despite that awareness, Congress
chose to measure the initial Table 1 attainment deadlines in
“years,” not “ozone seasons.”

     Notably, moreover, Congress set December 31
attainment dates elsewhere in the 1990 amendments in
analogous provisions governing pollutants other than ozone,
see, e.g., id. § 7512(a)(1) (Subpart 3 attainment deadlines for
                              25
carbon monoxide); id. § 7513(c)(1)-(2) (Subpart 4 attainment
deadlines for particulate matter), as well as in a special rule
pertaining to Subpart 2 “[t]ransitional areas,” see id. § 7511e.
By contrast, Congress tellingly declined to use a December 31
attainment date in Table 1, instead opting for a specific period
of years triggered on the designation date. Cf. Dean v. United
States, 556 U.S. 568, 573 (2009) (“Where Congress includes
particular language in one section of a statute but omits it in
another section of the same Act, it is generally presumed that
Congress acts intentionally and purposely in the disparate
inclusion or exclusion.” (brackets omitted)).          Congress
likewise declined to take ozone seasons into account when
setting attainment deadlines for areas redesignated to
nonattainment, see 42 U.S.C. § 7511(b)(1), even though
redesignation by nature can occur at any point during the
year. Most newly redesignated “Marginal” areas therefore
would not receive three new ozone seasons to achieve
compliance.

     Those statutory choices indicate rejection of the ozone-
season justification that underlies the Implementation Rule’s
attainment deadlines. EPA responds that, because the
enactment date of the 1990 amendments turned out to be
November 15—after the end of ozone season in most
regions—most Marginal areas under those amendments in
practice had three new ozone seasons to come into
compliance. That may be so, but there is no indication
Congress intended (or even contemplated) that result. The
statutory language linking Table 1 deadlines to the enactment
date appeared in the bill reported out of committee in May
1990, substantially before the end of ozone season. See H.R.
Rep. No. 101-490 pt. 1, at 17 (1990). The fact that enactment
ultimately occurred late in the calendar year, after completion
of most ozone seasons, thus was happenstance.
                              26
    At any rate, while ozone season had ended by November
15 in many areas (such as all parts of Colorado), it had yet to
end in areas with year-round seasons (including all of
California). See 40 C.F.R. pt. 58, app. D, tbl.D-3. EPA offers
no explanation for why, if Congress had ozone seasons in
mind, it would have granted three ozone seasons to Marginal
areas in Colorado but only two to Marginal areas in
California. Had Congress desired in Subpart 2 to grant an
equal number of ozone seasons to all areas, it presumably
would have set the attainment date to December 31, just as it
contemporaneously did in Subparts 3 and 4. Congress did not
do so.

     EPA’s delay of the trigger date for the fixed attainment
periods to the end of the calendar year, based on the agency’s
ozone-season rationale, is “untethered to Congress’s
approach” and thus fails at Chevron step two. South Coast,
472 F.3d at 894. This is not the first time a court has rejected
certain aspects of EPA’s implementation of a revised NAAQS
for ozone at Chevron step two. The Supreme Court did so in
Whitman, 531 U.S. at 481-86, and we later did the same,
South Coast, 472 F.3d at 894-95. We do so again here with
regard to the Implementation Rule’s attainment deadlines.

                              B.

    NRDC’s second challenge concerns the Implementation
Rule’s revocation of the 1997 NAAQS for purposes of the
Clean Air Act’s transportation conformity requirements.
According to NRDC, the partial revocation both exceeds
EPA’s statutory authority and violates the Act’s anti-
backsliding provision. We agree that EPA lacks authority to
revoke the conformity requirements, and we therefore do not
address the applicability of the anti-backsliding provision.
                             27
     The transportation conformity requirements, 42 U.S.C.
§ 7506(c), “integrate[] the Clean Air Act with the
transportation planning process by conditioning federal
approval and funding of transportation activities on their
demonstrated compliance with applicable SIPs.” Sierra Club
v. EPA (Sierra Club-conformity), 129 F.3d 137, 138 (D.C.
Cir. 1997). “In enacting the 1990 Amendments, Congress
was particularly concerned with pollution arising from
automobile emissions.” South Coast, 472 F.3d at 904.
Congress accordingly strengthened the transportation
conformity requirements. As amended, the statute requires
transportation planners to estimate emissions from covered
projects to assure that new emissions will not thwart NAAQS
attainment. See id. EPA implemented that statutory mandate
through motor vehicle emissions budgets, which “identif[y]
those vehicle emissions that can be produced without
jeopardizing an area’s attainment status.” BCCA Appeal Grp.
v. EPA, 355 F.3d 817, 842 (5th Cir. 2003). Emissions
associated with proposed transportation projects must stay
within “the motor vehicle emissions budget(s) established in
the applicable implementation plan.” 40 C.F.R. § 93.118(a).

     The Implementation Rule “revoke[s] the 1997 ozone
NAAQS one year after the effective date of designations for
the 2008 ozone NAAQS for transportation conformity
purposes only.” Implementation Rule, 77 Fed. Reg. at
30,167. The rule leaves the 1997 NAAQS in place for all
other purposes. See id. at 30,168. As a result, all 1997
designations and maintenance requirements remain in effect.
This case thus raises no questions concerning EPA’s authority
to revoke the 1997 NAAQS in full, as EPA has now proposed
to do in a proposed rule published during the pendency of this
case. See Implementation of the 2008 NAAQS for Ozone:
State Implementation Plan Requirements, 78 Fed. Reg.
34,178 (June 6, 2013). If that proposed rule were to develop
                               28
into a final rule, it might itself become the subject of a future
challenge. The sole question now before us in this case,
however, is whether EPA can revoke the transportation
conformity requirements alone.

     Under the plain terms of the statute, EPA lacks authority
to revoke the 1997 transportation conformity requirements
alone, while otherwise preserving the 1997 NAAQS. The
conformity      provision’s     “Applicability”     paragraph
straightforwardly prescribes that the conformity requirements
“shall apply” to:

         (A) a nonattainment area . . . ; and

        (B) an area that was designated as a
    nonattainment       area   but    that   was   later
    redesignated . . . as an attainment area and that is
    required to develop a maintenance plan . . . .

42 U.S.C. § 7506(c)(5). EPA does not deny the mandatory
nature of the conformity requirements for all nonattainment
and maintenance areas. Nor does EPA dispute that all
nonattainment designations and maintenance requirements
remain in place for the 1997 NAAQS. Rather, EPA seeks to
turn off the conformity requirements—and those requirements
alone—for areas that remain in nonattainment or maintenance
status under the 1997 NAAQS. The statute forbids that result.
It mandates application of the conformity requirements,
without exception, for “a nonattainment area” and for a
former nonattainment area “redesignated” as “an attainment
area” and “required to develop a maintenance plan.” Id.

    Our precedent compels that understanding. As we have
previously explained: “The Clean Air Act categorically
mandates that the transportation conformity requirements
                              29
shall apply to nonattainment and maintenance areas.” Sierra
Club-conformity, 129 F.3d at 138 (citing 42 U.S.C.
§ 7506(c)(5)). We therefore held that the plain text of the
statute “does not authorize the EPA to limit the applicability
of the conformity requirements by exempting some
nonattainment areas, even for a limited period of time.” Id. at
142. The regulation at issue in that case established a one-
year “grace period” during which newly designated
nonattainment areas received an exemption from the
conformity requirements. See id. at 139. We invalidated the
regulation at Chevron step one, concluding that the “grace
period impermissibly creates an exception to the unqualified
requirement in the statute.” Id. at 140. And although the
plain text resolved the issue, we also found it “instructive”
that Congress strengthened the conformity requirements in the
1990 amendments based on concerns that the prior version
had been “largely ignored by the agencies required to apply”
it. Id. at 140 (ellipsis omitted) (quoting legislative history).
The amended statute left no discretion for EPA to suspend the
conformity requirements alone for even a year. For the same
reason, EPA lacks authority permanently to suspend (i.e.,
revoke) the conformity requirements, as it would do here.

     EPA identifies no statutory provision specifically
authorizing revocation of the conformity requirements alone
in that fashion. The agency instead relies on its power to
revoke a NAAQS in its entirety. In South Coast, we
recognized EPA’s power to revoke a prior NAAQS in toto
when it promulgates a revised standard. In implementing the
1997 NAAQS, the agency had “revoke[d]” the prior one-hour
NAAQS “in full, including the associated designations.”
South Coast, 472 F.3d at 898. Noting EPA’s statutory
obligation to conduct a periodic “review” of existing NAAQS
and to “make such revisions . . . as may be appropriate,” we
held that the statute authorized EPA’s total revocation of the
                             30
prior NAAQS, subject to the anti-backsliding provision. See
id. at 899 (quoting 42 U.S.C. § 7409(d)(1)). EPA contends
that, because it has authority to revoke the 1997 NAAQS
entirely, it also possesses authority to revoke the conformity
requirements alone while leaving the rest of the 1997 NAAQS
in place. We are unpersuaded by EPA’s greater-includes-the-
lesser-power argument.

     We have no occasion to consider whether EPA can effect
a partial revocation of other parts of a NAAQS besides the
transportation conformity requirements. With regard to the
transportation conformity requirements, however, the statute
disallows the agency to eliminate those requirements alone.
Our decision in South Coast does not suggest otherwise.
Because the 2004 Rule considered in South Coast revoked the
prior NAAQS “in full, including the associated designations,”
id. at 898, there remained no nonattainment areas or
maintenance areas for purposes of the previous, fully revoked
standard. By contrast, the Implementation Rule at issue here
leaves all 1997 designations and maintenance requirements in
place. Any area that qualified as a nonattainment area or a
maintenance area before the Implementation Rule took effect
retains that status thereafter. The statute “categorically
mandates that the transportation conformity requirements
shall apply to” those “nonattainment and maintenance areas.”
Sierra Club-conformity, 129 F.3d at 138. We therefore
conclude, at Chevron step one, see id. at 140, that EPA lacks
authority to eliminate the conformity requirements alone—
whether it characterizes the elimination as a partial
“revocation” or otherwise.

     It is true, as our dissenting colleague observes, see
Dissent Op. at 4-6, that NRDC made no argument in its
opening brief in this Court (or previously before the agency)
in favor of distinguishing between revocation of a NAAQS in
                              31
its entirety and revocation of the transportation conformity
requirements alone. But NRDC was not required to do so.
NRDC argued in its opening brief (and before the agency)
that the plain language of the statute prohibits elimination of
the transportation conformity requirements. See Pet’r’s
Opening Br. 21-23, 35-38; Comments by David S. Baron,
Atty, EarthJustice, on Proposal at 77 Fed. Reg. 8197 (Feb. 14,
2012), Dkt. No.: EPA-HQ-OAR-2010-0885 3 (Mar. 15,
2012). NRDC was not obligated to go further and anticipate
in its opening brief that EPA would later respond by invoking
its power under South Coast to eliminate a NAAQS in full.
See Resp’t Br. 43-45. Only after EPA did so did it become
salient to draw a distinction between revoking a NAAQS in
its entirety and revoking the transportation conformity
requirements alone. See Pet’r’s Reply Br. 18-20. The
statutory terms, as explained, forbid the latter approach.

     Because the plain terms of the statute resolve the matter,
EPA’s rationale for eliminating the conformity requirements
cannot justify its action. The agency’s explanation falls short
in any event. EPA asserts that revocation of the 1997
conformity requirements “makes the most sense because it
would result in only one ozone NAAQS—the 2008 ozone
NAAQS—applying for purposes of transportation
conformity.” Implementation Rule, 77 Fed. Reg. at 30,167.
That would avoid the “unnecessary complexity” that state and
local planning authorities would confront if “required to
implement the transportation conformity program for both
ozone NAAQS concurrently.” Id. at 30,168. EPA reasons
that, because any area initially designated as nonattainment
for purposes of the 2008 NAAQS would thereby become
subject to the conformity requirements, the Implementation
Rule “provides a seamless transition” to the “more protective”
2008 NAAQS, “leav[ing] no gap in conformity’s application
                               32
in any 2008 ozone nonattainment area.”           Id. (emphasis
added).

    The rule, however, leaves a gap in conformity coverage
for certain 2008 ozone attainment areas—specifically, any
area initially designated as attainment under the 2008
NAAQS but still designated as nonattainment or subject to
maintenance requirements for the 1997 NAAQS. An area
could remain designated as nonattainment under the 1997
NAAQS despite having attained the more stringent 2008
standard if it had yet to satisfy the additional requirements for
redesignation from nonattainment to attainment for the 1997
standard—for instance, if EPA had yet to determine that the
area’s “improvement in air quality is due to permanent and
enforceable reductions in emissions.” 42 U.S.C.
§ 7407(d)(3)(E)(iii). An area also could remain subject to
maintenance requirements under the 1997 NAAQS even if it
gains an initial designation as attainment for the 2008
standard. See id. § 7505a(a)-(b) (maintenance requirements
remain in effect for twenty years). For both of those
categories of areas, the Implementation Rule would revoke
the 1997 conformity requirements without putting in place
any conformity requirements for the 2008 NAAQS.

     The parties agree that more than seventy such “orphan
areas” existed at the time of the 2008 NAAQS effective date,
including most of Massachusetts, as well as Detroit,
Michigan, and Richmond, Virginia. In such areas, the
conformity requirements would work to maintain ozone
concentrations at safe levels in the face of new automobile
emissions associated with proposed transportation projects.
Under the Implementation Rule’s approach, however, the
statute would no longer require any inquiry into whether
proposed transportation projects square with an orphan area’s
plans for maintaining safe ozone levels, notwithstanding
                              33
Congress’s concern that motor vehicles present the largest
source of ozone pollution.

     It is true that orphan areas would have attained the 2008
standard, indicating an improvement in air quality. But those
areas would be in either maintenance or nonattainment status
for purposes of the still-operative 1997 standard. For any
such areas in maintenance status, they would have already
attained the 1997 standard—likewise indicating an
improvement in air quality—yet Congress determined that
they must continue to satisfy maintenance obligations, id.
§ 7505a, and must also remain subject to the conformity
requirements for the same period, id. § 7506(c)(5)(B). For
orphan areas in nonattainment status under the 1997 standard,
there presumably would have been no showing of a
permanent improvement of a kind that would have justified
redesignation to attainment. See id. § 7407(d)(3)(E)(iii).

     To be sure, an orphan area might again become subject to
the conformity requirements if its air quality deteriorated and
it were redesignated to nonattainment under the 2008
NAAQS. But by that point, it would be too late to prevent the
adverse health and welfare effects associated with the interim
increase in ozone levels—effects that might have been
avoided if the conformity requirements had remained in place
under the still-operative 1997 standard. And it may be
markedly more difficult to reverse such an increase after the
completion of new transportation projects than to avert the
increase in the first place. See S. Rep. No. 101-228, at 28
(1989) (“By evaluating air quality impacts of proposed
activities before they are undertaken, future pollution
problems can be prevented.” (emphasis added)).

   EPA does not dispute that the Implementation Rule
would wholly release orphan areas from otherwise-applicable
                             34
conformity obligations in that manner. The plain terms of the
statute foreclose that result.

                       * * * * *

     We hold that the Implementation Rule’s schedule of
attainment deadlines exceeds EPA’s authority under the
statute. We further hold that the Implementation Rule’s
revocation of the 1997 NAAQS for transportation conformity
purposes alone is contrary to the statute. Accordingly, we
vacate the rule “to the extent that the court has sustained
challenges to it.” South Coast Air Quality Mgmt. Dist. v.
EPA, 489 F.3d 1245, 1248 (D.C. Cir. 2007).

                                                 So ordered.
     RANDOLPH, Senior Circuit Judge, dissenting: The majority
opinion treats this case as if it were facing a linguistic puzzle.
The court’s solution, we are told, is better than EPA’s. And so
the court sets aside two important aspects of EPA’s regulations
implementing its 2008 ozone standards. The court’s decision
and its reasoning are, I believe, mistaken.

     The first subject of the majority opinion is EPA’s judgment
about when future attainment deadlines begin running. The
Clean Air Act says nothing about when EPA should start the
clock after the agency has issued new, stricter National Ambient
Air Quality Standards (NAAQS) for ozone. The Supreme Court
identified this timing gap in Whitman v. American Trucking
Ass’ns, 531 U.S. 457, 483-84 (2001), and concluded that a
reviewing court must defer to EPA’s reasonable resolution of
the statutory uncertainty. Id. at 484; see also South Coast Air
Quality Mgmt. Dist. v. EPA, 472 F.3d 882, 896 (D.C. Cir. 2006),
clarified on denial of reh’g, 489 F.3d 1245 (D.C. Cir. 2007).

     The majority disagrees with EPA’s decision to run the
deadlines from December 31 rather than from July 20, 2012,
when 2008 NAAQS designations became effective. Exactly
why the majority disagrees with EPA is hard to discern. In large
part, the majority’s theory appears to rely on EPA’s statement a
decade ago that it had no “authority to change the attainment
dates to November or December of the attainment year.” See
Maj. Op. at 24. As the majority sees it, EPA should be held to
the concession that it lacked any statutory authority to run the
deadlines from a date other than the designation date.1


1
  The Clean Air Act provides that areas designated nonattainment
areas for ozone “shall be classified at the time of such designation,
under table 1 . . ..” 42 U.S.C. § 7511(a)(1). Table 1 establishes
classifications and attainment dates for nonattainment areas with a
“design value” – an area’s ozone pollution level over three
consecutive calendar years – that exceeded the one-hour NAAQS.
Table 1 also establishes five classifications that correspond to design
                                   2

     Regardless of what EPA meant by its decade-old remark, it
has made no such concession in this case. EPA has thoroughly
explained why the end-of-year date is not only preferable but
also perfectly consistent with the Clean Air Act. See Resp’t Br.
at 31-34. EPA’s judgment is in harmony with Congress’s intent
as expressed in Table 1. The majority’s view is not. EPA
justified its decision on the basis that “it provides nonattainment
areas with the same number of post-designation ozone seasons
to demonstrate attainment as similarly-classified areas had under
the 1990 Amendments.” Resp’t Br. at 31 (citing 77 Fed. Reg.
30,160, 30,166 (May 21, 2012)). Pursuant to the 1990
Amendments, “marginal nonattainment areas had three full
ozone seasons (1991-1993) between the date of enactment and
the November 15, 1993 deadline to attain the NAAQS.” Id. at
31-32. I therefore disagree with the majority’s conclusion that
this interpretation “lacks any grounding in the statute,” and
“runs counter to all indicia of congressional intent.” Maj. Op.
at 24. What is not grounded in the statute is the majority’s


value ranges, and it establishes attainment dates for each classification
running from the date of enactment of the 1990 Amendments
(November 15, 1990), not from the date of designation of the
nonattainment areas. Id.

     When Congress adopted the 1990 Amendments, EPA had already
designated large areas of the country comprising half the population
  as nonattaining, that is, as having unacceptably high levels of ozone.
See S. Rep. No. 101-228 (1989), as reprinted in 1990 U.S.C.C.A.N.
3385, 3389. These nonattaining areas had already been engaged in
pollution control efforts to comply with environmental deadlines that
had already passed. See South Coast, 472 F.3d at 886-87.
Accordingly, those nonattaining areas had more time to reach
attainment than the three years stipulated for even marginally
nonattaining areas under the 1990 Amendments. This history shows
that EPA’s deadlines were never as firm or limited as the majority
would like to believe.
                                  3

notion that once EPA issues its designations of areas, the
attainment deadlines must begin running.2

     As to what may appear to be EPA’s contrary stance years
ago, an “initial agency interpretation is not instantly carved in
stone. On the contrary, the agency, to engage in informed
rulemaking, must consider varying interpretations and the
wisdom of its policy on a continuing basis.” Chevron, U.S.A.,
Inc. v. Natural Res. Def. Council, Inc., 467 U.S. 837, 863-64
(1984). Here, EPA was faced with the task of adapting
Table 1’s timeframes, which were effective in November 1990,
to the July 2012 designations for the new ozone NAAQS. Those
designations fell in the middle of an ozone season “a variable
Congress did not face at the time it enacted the [Clean Air Act]
Amendments in late 1990.” Resp’t Br. at 33. As EPA notes in
its brief, in the early 1990s, EPA allowed marginal locations
with year-round ozone seasons to extend the deadline until the
end of the calendar year so that those areas could submit data for
three full ozone seasons. See Resp’t Br. at 32 n.8. Because the
statute is ambiguous about this subject and because
“[a]ttainment can only be demonstrated with monitoring data
from three full and consecutive ozone seasons, which, for areas
with year-round ozone seasons, means three full calendar years
of monitoring data,” Resp’t Br. at 34 n.9 (citing 77 Fed. Reg.


2
  There is evidence, as EPA notes, that “Congress did not invariably
consider a designation or classification as the trigger date for NAAQS
attainment deadlines” regardless of circumstances. Resp’t Br. at 29.
First, Table 1 could have simply measured the attainment periods from
the date of designation “but Congress instead chose to run the
attainment periods from the date of enactment of the 1990
Amendments.” Resp’t Br. at 27. Second, other NAAQS attainment
deadlines are pegged to different events. See, e.g., 42 U.S.C. § 7511e
(giving “transitional areas” designated nonattainment as of November
15, 1990 until December 31, 1991 to attain ozone standard).
                                 4

8,197, 8,204 (Feb. 14, 2012)), EPA’s accommodation is a
reasonable interpretation of the Clean Air Act entitled to judicial
deference. See, e.g., Envtl. Def. v. EPA, 489 F.3d 1320, 1329
(D.C. Cir. 2007). In short, EPA’s explanation (and its
longstanding practice for areas with year-round ozone seasons)
is more than sufficient. An agency “need not demonstrate to a
court’s satisfaction that the reasons for the new policy are better
than the reasons for the old one; it suffices that the new policy
is permissible under the statute, that there are good reasons for
it, and that the agency believes it to be better . . ..” FCC v. Fox
Television Stations, Inc., 556 U.S. 502, 515 (2009); see also
White Stallion Energy Ctr., LLC v. EPA, 748 F.3d 1222, 1235
(D.C. Cir. 2014) (per curiam).

     This brings me to the second aspect of the implementing
regulations the majority sets aside EPA’s revocation of the
1997 ozone standard for transportation conformity purposes. In
South Coast Air Quality Management District v. EPA, 472 F.3d
882, the court upheld EPA’s revocation of a prior NAAQS for
ozone when it issued a new, stricter ozone NAAQS, as it did
here. Id. at 899-900; see also Natural Res. Def. Council v. EPA,
643 F.3d 311, 315-16 (D.C. Cir. 2011). But in this case, the
majority holds that EPA lacked the authority to revoke the prior
NAAQS for transportation conformity purposes because it did
not revoke the NAAQS in its entirety. See Maj. Op. at 28-30.3

    NRDC never advanced this argument in its opening brief.


3
 As the majority notes, EPA has published a proposed rule that would
revoke the 1997 ozone standard for all purposes. See Maj. Op. at 27-
28; Implementation of the 2008 National Ambient Air Quality
Standards for Ozone: State Implementation Plan Requirements, 78
Fed. Reg. 34,178 (June 6, 2013). While the rule is not yet final, it
could very well undercut the distinction between revoking in whole
and revoking in part on which the majority relies.
                                  5

EPA, in its responsive brief, never mentioned it either.
Although NRDC’s reply brief disputes EPA’s interpretation of
South Coast and argues that South Coast does not authorize the
EPA to revoke a NAAQS in part, that terse remark did not
preserve the argument. See Reply Br. at 20. For good reason,
we do not permit issues to “be raised for the first time in a reply
brief.” Rollins Envtl. Servs. v. EPA, 937 F.2d 649, 652 n.2 (D.C.
Cir. 1991) (citing McBride v. Merrell Dow & Pharm., Inc., 800
F.2d 1208, 1210-11 (D.C. Cir. 1986)). In addition, the argument
in NRDC’s reply brief is that EPA misinterpreted South Coast,
not that the statute bars EPA from revoking the transportation
conformity requirements because it did not revoke the entire
1997 NAAQS. See Reply Br. at 20.4

     The majority’s theory was never presented to EPA during
the rulemaking either. For that reason, EPA said not a word
about it in the lengthy preamble to the rule.5 The Clean Air Act
contains an explicit exhaustion requirement: “Only an objection


4
  NRDC’s opening brief argues that the removal of the transportation
conformity requirements for the 1997 NAAQS contravenes the Clean
Air Act and that “under the guise of a ‘partial revocation’ of the 1997
NAAQS, EPA is claiming the power to pick and choose which parts
of the Clean Air Act apply to nonattainment and maintenance areas.”
Pet. Br. at 37. But this argument is quite different from the argument
NRDC advances in its reply brief, which NRDC doubtless waived
anyway. See Cement Kiln Recycling Coal. v. EPA, 255 F.3d 855, 869
(D.C. Cir. 2001) (per curiam) (“A litigant does not properly raise an
issue by addressing it in a ‘cursory fashion’ with only ‘bare-bones
arguments.’”) (quoting Wash. Legal Clinic for the Homeless v. Barry,
107 F.3d 32, 39 (D.C. Cir. 1997)).
5
 During the comment period, the group Earthjustice stated that “EPA
has no authority to suspend [transportation conformity requirements]
by fiat,” but as this conclusory comment itself indicates, this did not
present the majority’s theory with any specificity. J.A. 314.
                                    6

to a rule or procedure which was raised with reasonable
specificity during the period for public comment (including any
public hearing) may be raised during judicial review.” 42
U.S.C. § 7607(d)(7)(B) (emphasis added).6 We have repeatedly
applied a “strict[]” interpretation of the exhaustion requirement
“to ensure that EPA has an opportunity to respond to every
challenge” and so that “the court enjoys the benefit of the
agency’s expertise and possibly avoids addressing some of the
challenges unnecessarily.” Motor & Equip. Mfrs. Ass’n v.
Nichols, 142 F.3d 449, 462 (D.C. Cir. 1998); see also Util. Air
Regulatory Grp. v. EPA, 744 F.3d 741, 747 (D.C. Cir. 2014);
Natural Res. Def. Council v. EPA, 559 F.3d 561, 563 (D.C. Cir.
2009).

     In this case we have not had the benefit of EPA’s response
to the argument the majority sponsors, not in EPA’s rulemaking
proceedings and, I should add, not in the briefs in this court,
or in the oral argument before us. I therefore believe the
majority erred in reaching out to decide the case on a theory of
its own devising.

    Even if the majority’s revocation-in-part argument were
properly before us, I believe EPA’s interpretation of the Clean
Air Act is permissible. The majority relies upon Sierra Club v.


6
   It is of no moment whether 42 U.S.C. § 7607(d)(7)(B) is
jurisdictional, contra EPA v. EME Homer City Generation, L.P., 134
S. Ct. 1584, 1602 (2014), or quasi-jurisdictional, see EEOC v. Fed.
Labor Relations Auth. (“FLRA”), 476 U.S. 19, 24 (1986). The
designation jurisdictional would be significant if the agency could
have, but did not, raise the objection (that is, waived the objection) but
the court nevertheless invoked it, as in FLRA. Here EPA did not
interpose a § 7607(d)(7)(B) objection to the sort of argument the
majority embraces, but it could not have waived the objection for the
quite apparent reason that petitioner’s brief never made the argument.
                               7

EPA (Sierra Club-conformity), 129 F.3d 137, 138, 140 (D.C.
Cir. 1997), to support its conclusion that EPA lacks authority to
revoke the conformity requirements alone. See Maj. Op. at 30.
Sierra Club-conformity held impermissible a grace period
during which time newly designated nonattainment areas were
exempt from conformity requirements. The Clean Air Act, the
court wrote, “does not provide for any grace periods or other
exemptions from the conformity requirements for areas
designated as nonattainment areas, nor does it authorize the EPA
to create such exemptions.” Sierra Club-conformity, 129 F.3d
at 138. The opinion is silent about revoking transportation
conformity requirements themselves.

     On that score, the Act indicates that conformity
requirements apply “only” in nonattainment and maintenance
areas, not in areas that have been removed from such status by
the revocation of the underlying NAAQS. 42 U.S.C.
§ 7506(c)(5). We recognized EPA’s authority to revoke existing
NAAQS when it adopts a new standard for the same pollutant in
South Coast, 472 F.3d at 899. The implication of South Coast
is that transportation conformity requirements are not graved in
stone. EPA can alter them, and the existence of the “anti-
backsliding” provision of the Clean Air Act shows that Congress
anticipated the revocation of some restrictions and their
replacement with weaker controls. See id. at 899-900, 42 U.S.C.
§ 7502(e). Because I believe, as South Coast determined, that
the Act does not prohibit EPA’s revocation of the transportation
conformity requirements, I disagree with the majority’s view
that an objective observer, looking only at the statutory
language, would conclude that EPA violated the Act.

     Beyond this, EPA had good reason for revoking the 1997
transportation conformity requirements. EPA explains that “[i]f
the 1997 ozone NAAQS were to remain in place after
conformity applies for the 2008 ozone NAAQS, . . . areas that
                                8

are currently nonattainment or maintenance for the 1997 ozone
NAAQS and will be designated nonattainment for the 2008
ozone NAAQS would be required to implement the
transportation conformity program for both ozone NAAQS
concurrently.” 77 Fed. Reg. at 30,167-68. “This could lead to
unnecessary complexity,” id. at 30,168, whereas revoking the
1997 ozone NAAQS “would bring certainty to the transportation
planning process in ozone nonattainment and maintenance
areas.” Id. at 30,167. The majority claims that the 2008 rule
leaves a gap in conformity coverage for some 2008 attainment
areas. Maj. Op. at 32. But, as EPA notes, either these so-called
“orphan areas” had attained the 1997 standard at the time EPA
promulgated the 2008 standard, or, even if they had not, they
would not create a concern because “the new NAAQS does not
represent a ‘relaxation’ of an existing NAAQS” and is, in fact,
stricter than the 1997 requirements. Resp’t Br. at 48 n.15. If
these areas are in attainment for new, stricter standards, there is
no reason to force them to conform to prior, weaker ones to
prevent backsliding. See South Coast, 472 F.3d at 900.

     A few final points. Notably absent from the majority
opinion is any discussion of how exactly EPA and the States are
expected to implement the majority’s decision. We do not know
whether the majority intends that attainment deadlines set years
ago must now be retroactively shortened as a result of court
order. Still less do we know what sort of disarray this will cause
throughout the country. And we do not know whether State and
local transportation plans plans approved and implemented,
presumably after large effort and at great expense must be
retroactively disapproved. These and other problems are bound
to arise as a result of what I consider a mistake in judicial
analysis.

    For these reasons, I respectfully dissent.
