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 antibacksliding 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 groundlevel 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

Area class Design value* Primary standard 

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-HQOAR-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 agencyselected 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 ozoneseason 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 antibacksliding 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 oneyear “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-thelesser-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

The Clean Air Act provides that areas designated nonattainment 1

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.

There is evidence, as EPA notes, that “Congress did not invariably 2

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. 

 As the majority notes, EPA has published a proposed rule that would 3

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. The Clean Air Act 5

contains an explicit exhaustion requirement: “Only an objection

NRDC’s opening brief argues that the removal of the transportation

4

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)).

During the comment period, the group Earthjustice stated that “EPA 5

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). We have repeatedly 6

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.

 It is of no moment whether 42 U.S.C. § 7607(d)(7)(B) is 6

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 “antibacksliding” 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.