United States Court of Appeals
              FOR THE DISTRICT OF COLUMBIA CIRCUIT

                          –————
No. 03–1084                            September Term, 2003

                  Filed On: April 16, 2004


                        SIERRA CLUB,
                         PETITIONER

                               v.

ENVIRONMENTAL PROTECTION AGENCY AND MICHAEL O. LEAVITT,
  ADMINISTRATOR, US ENVIRONMENTAL PROTECTION AGENCY,
                     RESPONDENTS

                  STATE OF MARYLAND, ET AL.,
                         INTERVENORS



                 Consolidated with
             03–1103, 03–1115, 03–1152
                     –————
  BEFORE: Sentelle, Henderson and Garland, Circuit
Judges

                         ORDER
  Upon consideration of petitioner Sierra Club’s petition for
rehearing, it is
  ORDERED that the Opinion filed herein on February 3,
2004, is hereby amended as follows:
   On Page 18, footnote 9: Delete the last two sentences
(beginning with, ‘‘But Sierra Club’s reading TTT ’’) and the
final citation and insert in lieu thereof: ‘‘But no statute or
regulation requires such a demonstration.’’
    2

Per Curiam
              FOR THE COURT:
              Mark J. Langer, Clerk
        BY:
              Deputy Clerk
  Notice: This opinion is subject to formal revision before publication in the
Federal Reporter or U.S.App.D.C. Reports. Users are requested to notify
the Clerk of any formal errors in order that corrections may be made
before the bound volumes go to press.




       United States Court of Appeals
                  FOR THE DISTRICT OF COLUMBIA CIRCUIT




Argued November 20, 2003                    Decided February 3, 2004

                               No. 03-1084

                              SIERRA CLUB,
                               PETITIONER

                                     v.

             ENVIRONMENTAL PROTECTION AGENCY AND
              MICHAEL O. LEAVITT, ADMINISTRATOR,
            U.S. ENVIRONMENTAL PROTECTION AGENCY,
                         RESPONDENTS

                      STATE OF MARYLAND, ET AL.,
                             INTERVENORS



                         Consolidated with
                      03–1103, 03–1115, 03–1152



         On Petitions for Review of Final Actions of the
               Environmental Protection Agency
                           –————
 Bills of costs must be filed within 14 days after entry of judgment.
The court looks with disfavor upon motions to file bills of costs out
of time.
                              2

  David S. Baron argued the cause and filed the briefs for
petitioner.
  Cynthia J. Morris, Attorney, U.S. Department of Justice,
argued the cause for respondents. With her on the brief
were Howard J. Hoffman and Sara Schneeberg, Attorneys.
  Kathy M. Kinsey, Assistant Attorney General, State of
Maryland, argued the cause for intervenors State of Mary-
land, et al. With her on the brief were J. Joseph Curran, Jr.,
Attorney General, Robert J. Spagnoletti, Corporation Coun-
sel, District of Columbia, Edward E. Schwab, Acting Deputy
Corporation Counsel, Donna M. Murasky, Senior Litigation
Counsel, Jerry Kilgore, Attorney General, Commonwealth of
Virginia, Roger L. Chaffe, Senior Assistant Attorney General,
and Carl Josephson, Assistant Attorney General.

  Before: SENTELLE, HENDERSON, and GARLAND, Circuit
Judges.
  Opinion for the Court filed by Circuit Judge GARLAND.
  GARLAND, Circuit Judge: In these consolidated cases, Sier-
ra Club challenges two final actions of the Environmental
Protection Agency (EPA) regarding ozone control plans for
the Washington, D.C. area. Those plans were designed to
bring the area into compliance with ozone standards pre-
scribed by the Clean Air Act and EPA regulations. Sierra
Club contends that EPA violated the Act by giving condition-
al approval to the plans notwithstanding that they lacked
required statutory elements. Sierra Club also challenges the
substance of two elements that were included in the plans, as
well as EPA’s extension of the deadlines by which revised
plans must be submitted for final approval.
  We agree with Sierra Club’s principal contention that EPA
was not authorized to grant conditional approval to plans that
did nothing more than promise to do tomorrow what the Act
requires today. We therefore vacate the conditional approval
and remand the matter to EPA for further action consistent
with this opinion. In other respects we deny the petitions for
review.
                                  3

                                  I
   The Clean Air Act (CAA), 42 U.S.C. § 7401 et seq., directs
EPA to establish National Ambient Air Quality Standards
(NAAQS) that set maximum permissible concentration levels
for air pollutants that endanger the public health and welfare.
42 U.S.C. §§ 7408, 7409. Pursuant to that direction, the
agency has adopted NAAQS for ozone. 40 C.F.R. § 50.9.1
Under the Act, EPA designates areas of the country as
‘‘attainment’’ or ‘‘nonattainment’’ (or as ‘‘unclassifiable’’)
based on whether they comply with the ozone NAAQS. 42
U.S.C. § 7407(d). Nonattainment areas are further classified
as ‘‘marginal,’’ ‘‘moderate,’’ ‘‘serious,’’ ‘‘severe,’’ or ‘‘extreme,’’
depending upon the severity and duration of their noncompli-
ance. 

The Act establishes air quality planning
and control requirements that increase in stringency as the
classification increases in severity. See 

It also
establishes deadlines for attainment of the NAAQS depending
upon an area’s nonattainment classification. 

7502, 7511(a). If an area fails to attain the NAAQS by the
applicable deadline, EPA must reclassify it to a higher classi-
fication. 

Generally, reclassification grants
the area a later attainment deadline, but requires it to comply
with the more stringent pollution control measures applicable
to the higher classification. 

(b)(2); 

   The Act also prescribes the process by which areas must
arrive at and maintain compliance with the NAAQS. Each
state must adopt and submit for approval to EPA a state
implementation plan (SIP) that provides for ‘‘implementation,
maintenance, and enforcement’’ of applicable NAAQS in each
air quality region (or portion thereof) within the state. 

In addition to the general SIP requirements,
states in ozone nonattainment areas must submit SIPs meet-
  1Ozone, a principal component of urban smog, can cause acute
respiratory problems. It presents a special health risk to people
with lung ailments, and to children and adults who are active
outdoors. Control of Air Pollution from New Motor Vehicles, Final
Rule, 66 Fed. Reg. 5002, 5012 (Jan. 18, 2001); see infra note 4.
                               4

ing additional requirements that depend upon the severity of
the ozone problem. 

7511a. Each SIP must
contain an ‘‘attainment demonstration’’ that shows that the
area will achieve the NAAQS by the area’s statutory attain-
ment deadline. 

40 C.F.R. § 51.112.
The attainment demonstration is based on the state’s control
strategy for ozone-precursor emissions, which must ‘‘include
enforceable emissions limitations, and such other control mea-
sures TTT as may be necessary or appropriate to provide for
attainment of such standard in such area by the applicable
attainment date.’’ 42 U.S.C. § 7502(c)(6).
  Particularly relevant for this case, SIPs from states in
nonattainment areas must also: (1) provide for ‘‘the imple-
mentation of all reasonably available control measures
[RACM] as expeditiously as practicable,’’ 

and, for serious and severe areas, (2) contain a rate of
progress (ROP) plan that demonstrates an average reduction
of baseline emissions of 3% per year for each consecutive
three-year period commencing in 1996 until the attainment
deadline for the classification area, 

(d);
and (3) ‘‘provide for the implementation of specific [contingen-
cy] measures to be undertaken if the area fails’’ to meet any
ROP milestone or to attain the NAAQS by the statutory
deadline, 

7511a(c)(9) & (d).
   The Washington, D.C. Metropolitan Area (‘‘D.C. area’’) is
made up of the District of Columbia and several Maryland
and Virginia counties. Pursuant to the Clean Air Act, EPA
classified the D.C. area as a ‘‘serious’’ nonattainment area for
ozone in 1991. Designation of Areas for Air Quality Planning
Purposes, Final Rule, 56 Fed. Reg. 56,694, 56,738, 56,772,
56,841 (Nov. 6, 1991). The statutory deadline for ozone
attainment by serious areas was November 15, 1999, and
revised SIPs including the attainment demonstration and
ROP plans were due by November 15, 1994. 42 U.S.C.
§§ 7511(a)(1), 7511a(c)(2).
  Maryland, Virginia, and the District of Columbia (‘‘the
States’’) did not submit their attainment demonstration and
other plan provisions for the D.C. area until 1997-1998; they
                              5

amended and supplemented those submissions during 1998-
2000. See Approval and Promulgation of Air Quality Imple-
mentation Plans, Final Rule, 66 Fed. Reg. 586 (Jan. 3, 2001).
Those SIPs lacked the three statutory elements noted above:
(1) the RACM analysis; (2) ROP plans for post-1999 emis-
sions reductions; and (3) contingency measures. 

608, 615. Moreover, the SIPs did not demonstrate that the
area would reach attainment by the statutory deadline of
November 15, 1999. 

Instead, the States asked
EPA to extend the attainment deadline by six years to
November 15, 2005, without reclassifying the area as ‘‘severe’’
as the Act would otherwise have required. 

On
January 3, 2001, EPA fully approved the SIPs and granted
the States’ requests to extend the attainment deadline with-
out reclassifying the D.C. area. 

608, 605, 630-31.
   Sierra Club petitioned this court for review, contending
(inter alia) that EPA could not approve the SIPs without the
missing statutory elements, and that it had no authority to
extend the statutory attainment deadline without reclassify-
ing the area as severe. We agreed, vacated EPA’s approval
of the SIPs, and remanded the matter to the agency. Sierra
Club v. EPA, 

, 160-65 (D.C. Cir. 2002) [Sierra
Club I]. On November 13, 2002, Sierra Club filed a new
action in the U.S. District Court for the District of Columbia,
seeking an injunction to compel EPA to reclassify the D.C.
area as severe and to take final action either approving or
disapproving the previously submitted SIPs. The district
court granted those requests and ordered EPA: (1) by Janu-
ary 27, 2003, to determine whether the D.C. area had attained
the NAAQS for ozone by the applicable attainment date of
November 15, 1999, and, if not, to reclassify the area; and (2)
by April 17, 2003, to approve or disapprove the SIP submit-
tals that had been remanded in Sierra Club I. Sierra Club v.
Whitman, No. 02-2235 (D.D.C. Dec. 18, 2002).
   EPA responded to the district court’s order with the two
actions that are at issue in this case. On January 24, 2003, in
a decision known as the ‘‘bump-up’’ action, EPA determined
that the D.C. area had not attained the NAAQS for serious
ozone nonattainment areas by the statutory deadline and
                              6

therefore reclassified the area as severe. Determination of
Nonattainment as of November 15, 1999 and Reclassification
of the Metropolitan Washington, DC Ozone Nonattainment
Area, Final Rule, 68 Fed. Reg. 3410 (Jan. 24, 2003) [hereinaf-
ter Reclassification]. A statutory consequence of the reclassi-
fication was the extension of the ozone attainment deadline to
‘‘as expeditiously as possible but not later than’’ November
15, 2005. 42 U.S.C. § 7511(a)(1). At the same time, EPA
extended until March 1, 2004 the deadline for submitting
revised SIPs to comply with the requirements for severe
nonattainment areas. Reclassification, 68 Fed. Reg. at 3410.
   In a second decision issued on April 17, 2003, the so-called
‘‘conditional approval’’ action, EPA granted conditional ap-
proval to the existing SIPs, notwithstanding the absence of
the three elements that Sierra Club I had identified as
precluding final approval. Approval and Promulgation of Air
Quality Implementation Plans, Final Rule, 68 Fed. Reg. 19,-
106, 19,107 (Apr. 17, 2003) [hereinafter Conditional Approval].
The agency based its conditional approval on letters submit-
ted by the States that committed to cure those deficiencies
and to comply with the additional requirements of the severe
area classification by April 17, 2004. 

The
agency also made determinations regarding, inter alia, two
elements that were contained in the existing SIPs: it conclud-
ed that the attainment demonstration showed that implemen-
tation of already adopted control measures would result in
attainment of the ozone NAAQS by the statutory deadline,
and it conditionally approved the States’ ROP plans for 1996-
1999.
  Sierra Club now petitions for review of both actions pursu-
ant to the jurisdictional grant of 42 U.S.C. § 7607(b)(1). In
Part II, we consider Sierra Club’s attack on the April 17, 2003
conditional approval action, based on EPA’s acceptance of the
States’ commitments to cure the SIPs’ deficiencies by April
17, 2004. In Part III, we consider petitioner’s additional
attack on the conditional approval action, based on EPA’s
acceptance of the two elements that were contained in the
SIPs. Finally, in Part IV, we consider petitioner’s challenge
                                7

to the deadline extension in the January 24, 2003 bump-up
action.

                                 II
   We begin with Sierra Club’s primary challenge: its conten-
tion that EPA violated the Clean Air Act by conditionally
approving the concededly deficient D.C. area SIPs on the
basis of the States’ commitment letters. EPA’s response is
that conditional approval was authorized by CAA § 110(k)(4),
which provides:
      (4) Conditional approval
         The administrator may approve a plan revision based
      on a commitment of the State to adopt specific enforce-
      able measures by a date certain, but not later than 1
      year after the date of approval of the plan revision. Any
      such conditional approval shall be treated as a disapprov-
      al if the State fails to comply with such commitment.
42 U.S.C. § 7410(k)(4) (emphasis added). We review EPA’s
construction of that section under the framework set forth in
Chevron U.S.A. Inc. v. Natural Resoures Defense Council,

(1984). Under Chevron’s first step, we ask
‘‘whether Congress has directly spoken to the precise ques-
tion at issue.’’ If we determine that ‘‘the intent of Congress
is clear, that is the end of the matter; for the court, as well as
the agency, must give effect to the unambiguously expressed
intent of Congress.’’ 

Only ‘‘if the statute is
silent or ambiguous with respect to the specific issue,’’ do we
proceed to Chevron’s second step, and ask ‘‘whether the
agency’s answer is based on a permissible construction of the
statute.’’ 

We conclude that EPA’s construction of
the conditional approval provision is contrary to Congress’
unambiguous intent, and we therefore reject it.
   EPA acknowledges that the SIPs did not contain a number
of the elements required for full approval. Indeed, it con-
cedes that the SIPs were missing the same three features
that led this court to vacate EPA’s full approval in Sierra
Club I: (1) RACM analyses and the implementation of rea-
sonable available control measures necessitated by such anal-
                                  8

yses; (2) post-1999 ROP plans with control measures ade-
quate to ensure emission reductions of 3% per year; and (3)
contingency control measures to be implemented automatical-
ly if the area fails to meet a ROP milestone or to reach
attainment by the deadline. Conditional Approval, 68 Fed.
Reg. at 19,106, 19,109, 19,119–20; see 42 U.S.C. §§ 7502(c)(1)
& (c)(9), 7511a(c)(2)(B) & (c)(9).2 As the agency further
concedes, the plans were also missing the additional elements
required for severe nonattainment areas, including specific
enforceable measures to offset growth in vehicle emissions
and reasonably available control technology for additional
major sources. Conditional Approval, 68 Fed. Reg. at 19,-
106–07; see 42 U.S.C. § 7511a(d).
   Notwithstanding these deficiencies, EPA argues that the
SIPs qualified for conditional approval — rather than disap-
proval — because they contained other required elements
(including the attainment demonstration and control strate-
gy), and because the States submitted letters committing to
remedy the deficiencies within one year, by April 17, 2004.
But while the States did file commitment letters, those letters
identified no ‘‘specific enforceable measures’’ of any kind.
Maryland’s commitment regarding contingency measures and
RACM is typical of the States’ submissions, and reads in full:
     We also commit to submit to EPA, not later than April
     17, 2004, adopted contingency measures to be imple-
     mented if the D.C. area does not attain the one-hour
     ozone NAAQS by November 15, 2005. Additionally, by
     April 17, 2004, we commit to submitting to EPA an
     appropriate RACM analysis for the D.C. Area, along
     with any revisions to the attainment demonstration SIP
     necessitated by such analysis, including adopted mea-
     sures to demonstrate timely attainment and to meet
     RACM requirements, should there be any.
  2 Sierra Club contends that, due to these deficiencies, the decision
in Sierra Club I alone bars conditional approval. That case is not
dispositive, however, because it addressed only the question of full
approval: at that time, EPA had not sought to employ its condition-
al approval authority under CAA § 110(k)(4).
                              9

Letter from Md. Dep’t of Env’t to EPA 2 (Apr. 7, 2003) (J.A.
at 784) (emphasis added); see also Letter from D.C. Dep’t of
Health to EPA 2 (Apr. 7, 2003) (J.A. at 791); Letter from Va.
Dep’t of Env’t Quality to EPA 2 (Apr. 8, 2003) (J.A. at 798).
   EPA does not dispute that the States’ letters failed to
identify specific measures that the States committed to adopt.
Indeed, EPA argues that the States do not yet know what
those measures will be because they have not yet completed
the necessary analyses. As the agency explains: ‘‘It is true
that the States have not yet identified the specific measures
that could ultimately be adopted, however, it would be impos-
sible for them to do so in advance of conducting the requisite
RACM and modeling analyses.’’ Conditional Approval, 68
Fed. Reg. at 19,109. EPA’s contention is that all that the
States need do to qualify for conditional approval is to
commit to adopt specific enforceable measures by a date
certain; they do not need to tell EPA what those measures
are — or even know what they are. Oral Arg. Tr. at 35. The
agency frames the issue as follows:
     EPA interprets the provision to require that the States
     commit to adopt specific enforceable measures by a date
     certain, but does not require that the individual measures
     be identified in the commitment. Petitioner, on the
     other hand, interprets the provision to require the States
     to identify, in their commitments, the individual enforce-
     able measures that will be adopted by a date certain.
Resp’ts’ Br. at 33.
   EPA’s interpretation cannot be squared with the unambig-
uous statutory language. The statute requires that the
States commit to adopt specific enforceable measures. Here,
the agency has accepted as sufficient a commitment to adopt
what it concedes are unspecified measures — with the specif-
ics to be named later. Moreover, the statute requires a
commitment to adopt specific enforceable measures by a date
certain. Here, not only are the measures unspecified, but the
States have committed to adopt them only ‘‘if such measures
are determined to be needed based on further analysis.’’
Conditional Approval, 68 Fed. Reg. at 19,109 (emphasis add-
                                10

ed); see 

have clarified in [their] letters their
intent to submit specific measures in support of the demon-
strations, if appropriate.’’) (emphasis added).             Section
110(k)(4), however, contains no such qualifier.
   This is not the first time that EPA has defended a con-
struction of § 110(k)(4) that we have found plainly inconsis-
tent with the statute. Indeed, in Natural Resources Defense
Council v. EPA, 

(D.C. Cir. 1994) [NRDC], we
rejected the agency’s use of that section to permit states to
meet statutory SIP deadlines by submitting ‘‘commital’’ SIPs
that contained nothing ‘‘more than a mere promise to take
appropriate but unidentified measures in the future.’’ 

That ‘‘construction of the conditional approval provi-
sion,’’ we held, ‘‘is contrary to Congress’s unambiguous intent
and must therefore be rejected.’’ 

EPA insists
that NRDC is distinguishable because the commital SIPs at
issue there contained no specific enforceable measures at all,
while the SIPs here — though concededly lacking several
important measures — do contain an attainment demonstra-
tion that includes adopted control measures and that shows
attainment by the 2005 deadline.
   Although NRDC is factually distinguishable on the ground
suggested by EPA, it is not legally distinguishable. It is true
that the SIPs in NRDC were bereft of substantive provisions.
But we did not suggest that the States’ failure to submit the
specific elements of concern to the petitioner there — vehicle
inspection and maintenance programs, and emission controls
at stationary emission sources — would have been cured had
substantive provisions addressing different requirements
been included. Nor is there anything in the language of
§ 110(k)(4) to support EPA’s contention that it may condi-
tionally approve a SIP ‘‘as long as the SIP submittal contains
substantive provisions but not all of them.’’ Oral Arg. Tr. at
36; see also Virginia v. EPA, 

, 1404 n.7 (D.C.
Cir. 1997) (holding, where SIPs contained many specific con-
trol measures, that EPA could not use § 110(k)(4) to grant
conditional approval based on ‘‘other measures that may be
promulgated thereafter’’ to make up an emission reduction
shortfall).
                               11

   EPA’s retreat to an argument based on legislative purpose
is no more successful. ‘‘EPA does not require the commit-
ments to identify the specific measures the States will adopt,’’
the agency argues, ‘‘because that would defeat the purpose of
the conditional approval which is, in large part, to allow the
States additional time to identify the measures needed.’’
Resp’ts’ Br. at 34. But as we explained in NRDC, the
purpose of the conditional approval provision is not to permit
states more time to identify control measures, but rather to
give EPA the opportunity to determine whether a SIP,
‘‘although not approvable in its present form, can be made so
by adopting specific EPA-required changes within the pre-
scribed conditional period.’’ 

(empha-
sis added). As we further explained, ‘‘[s]uch a determination
cannot reasonably be made unless the conditionally approved
submittal contains something more than a mere promise to
take appropriate but unidentified measures in the future.’’

requires that the States complete the analyses
necessary to identify appropriate measures before, rather
than after, conditional approval is granted.
   At bottom, NRDC rejected EPA’s construction of
§ 110(k)(4) because it turned conditional approval into a
‘‘means of circumventing’’ SIP deadlines. 

The same is true here. In the absence of conditional approv-
al, EPA would have been required to determine, by April 17,
2003, whether the SIPs demonstrated that the States had
already adopted the required specifically enforceable mea-
sures. Conditional Approval, 68 Fed. Reg. at 19,107 (reciting
deadline issued by district court order); see 42 U.S.C.
§ 7410(a)(1), (a)(2)(A) (requiring states to ‘‘adopt and submit’’
SIPs including ‘‘enforceable TTT control measures’’ by rele-
vant deadlines). Instead, EPA’s conditional approval pur-
ports to grant the States an additional year to adopt such
measures — without specifying what they will be. In short,
as EPA conceded at oral argument, the agency’s position is
that it may grant conditional approval on nothing more than
the States’ promise to do next year what the Clean Air Act
requires them to have already done. Oral Arg. Tr. at 34-35.
And that amounts to nothing more than the use of § 110(k)(4)
                               12

‘‘to postpone SIP deadlines,’’ a power that the section does
not confer. 

.

                              III
  In the previous Part, we considered Sierra Club’s attack on
EPA’s conditional approval action insofar as it was based on
mere promises to adopt SIP elements in the future. In this
Part, we consider petitioner’s attacks on the substance of two
elements that the States did submit as part of their SIPs.

                               A
  The Clean Air Act requires that each SIP contain an
attainment demonstration showing that the area will achieve
the ozone NAAQS by the statutory deadline. 42 U.S.C.
§ 7511a(c). After reclassification as a severe nonattainment
area, the D.C. area’s outside attainment deadline became
November 15, 2005. 

In the conditional
approval action, EPA determined that the States had demon-
strated that the D.C. area would in fact meet the NAAQS by
that date. Sierra Club contends that EPA should not have
approved the States’ attainment demonstration, because it
was inconsistent with statutory requirements.
  Under CAA § 182(c)(2)(A), each state in a serious or severe
nonattainment area must submit a SIP that includes:
      A demonstration that the plan, as revised, will provide
      for attainment of the ozone national ambient air quality
      standard by the applicable attainment date. This attain-
      ment demonstration must be based on photochemical
      grid modeling or any other analytical method determined
      by the Administrator, in the Administrator’s discretion,
      to be at least as effective.
42 U.S.C. § 7511a(c)(2)(A) (emphasis added). Sierra Club
argues that, in this case, the States’ attainment demonstra-
tion3 was insufficient because it was neither ‘‘based on photo-
  3Although Maryland, Virginia, and the District of Columbia each
submitted a separate SIP, they jointly conducted modeling and
                                13

chemical grid modeling,’’ nor based on ‘‘any other analytical
method that the Administrator had determined TTT to be at
least as effective.’’ EPA defends solely on the first ground:
that the attainment demonstration was in fact ‘‘based on’’
photochemical grid modeling.
  A photochemical grid model is a mathematical model that
predicts ozone levels on the attainment date based on moni-
toring data, meteorology, planned emission reductions, the
area’s projected growth, and other factors. BCCA Appeal
Group v. EPA, 

, 106 n.12 (5th Cir. 2003).4 Both
parties agree that the States’ demonstration began with a
photochemical grid model known as the Urban Airshed Model
(UAM). The model was used to predict ozone levels in 2005
by assuming implementation of the control strategy adopted
in the SIPs and extrapolating from data collected on three
high-ozone summer days in 1991: July 16, 19, and 20. Both
parties also agree that use of the model alone showed peak
ozone concentrations exceeding the NAAQS on those three
days in 2005; the model indicated that on those days the
area’s SIPs would result in daily one-hour maximum ozone
levels of 139, 150, and 178 parts per billion (ppb) — all in
excess of the 120 ppb NAAQS for ozone.5 EPA, First

other analyses and submitted the same attainment demonstration.
Resp’ts’ Br. at 11 n.7.
   4 Ozone is formed in the atmosphere when oxides of nitrogen

(NOx) and volatile organic compounds (VOCs) are emitted into the
air in the presence of sunlight. Clean Air Act Amendments of 1990,
H.R. Rep. No. 101-490, at 202 (1990). The ‘‘photochemical’’ reac-
tions that produce ozone from these pollutants are complex, and
sophisticated computer models are required to accurately predict
future ozone levels. 40 C.F.R. pt. 51, app. W §§ 6.1, 6.2.1 (2002).
The photochemical grid model makes a grid on the geographic area
in question and simulates emissions and ozone concentrations in
each ‘‘cell’’ of the grid. 

n.12; see 1000
Friends of Maryland v. Browner, 

, 220 n.4 (4th Cir.
2001); 40 C.F.R. pt. 51, app. W at app. A, § A.6 (2002).
  5EPA’s regulations set the one-hour NAAQS for ozone at 0.12
parts per million (ppm), which is 120 parts per billion (ppb).
However, EPA uses a rounding convention under which 0.124 ppm
                              14

Amendment to Technical Support Document for Approval and
Promulgation of Air Quality Implementation Plans 5 (Table
IV G-1) (Apr. 10, 2003) (J.A. at 808) [hereinafter Amendment
to Technical Support Document].
   EPA did not, however, conclude its analysis with these
exceedances. Rather, it adjusted the model’s extrapolations
in light of the agency’s concerns about the model’s reliability
and uncertainty. In particular, the agency noted that, when
it applied the model to the three days in the base year (1991)
and compared the model’s results to the actual monitored
results for those days, the model over-predicted known ozone
concentrations by an average of 19%. 

(J.A. at
813-14). It therefore adjusted the model’s calculations, using
a variety of supplemental statistical techniques, to correct
‘‘average modeled peak over-prediction’’ and ‘‘day-specific
over-prediction.’’ 

resulted in demon-
strated attainment for two out of the three modeled days. 

adjustments still did not indicate attainment for
July 20, 2005, EPA determined that the base-year data used
to model that day was too anomalous to demonstrate nonat-
tainment. July 20, 1991, the agency found, had been the 13th
most severe ozone-producing day in 44 years. 

(J.A. at 815-16). EPA reasoned that ‘‘[t]his type of day is not
likely to occur often enough to be a major causative factor for
nonattainment because the [NAAQS] allows up to three moni-
tored exceedances in any three year period.’’ Conditional
Approval, 68 Fed. Reg. at 19,114. It therefore concluded
‘‘that attainment of the 1-hour ozone standard has been
successfully demonstrated for the Washington area by no
later than 2005.’’ Amendment to Technical Support Docu-
ment at 13 (J.A. at 816).
  Sierra Club insists that once the model showed exceedances
on three days, that should have been the end of the matter.

is rounded to 0.12 ppm (120 ppb) and is therefore in compliance
with the standard. Conditional Approval, 68 Fed. Reg. at 19,111.
Sierra Club argues that EPA cannot relax the standard in that
manner. We need not resolve this issue, since the difference
between 120 and 124 ppb is not dispositive in this case.
                                15

In petitioner’s view, because the SIPs did not demonstrate
attainment without EPA’s adjustments, the demonstration
was not ‘‘based on’’ photochemical grid modeling within the
meaning of § 182(c)(2)(A). According to Sierra Club, the
Clean Air Act ‘‘uses ‘based on’ to identify the sole basis for a
decision.’’ Reply Br. at 13.
   We disagree. Under Chevron, ‘‘when the statute ‘is silent
or ambiguous’ we must defer to a reasonable construction by
the agency charged with its implementation.’’ Barnhart v.
Thomas, 

, 382 (2003) (quoting 

).6 There is no question that the phrase ‘‘based on’’ is
ambiguous. As EPA points out, it does not necessarily
require that attainment demonstrations rest solely on grid
modeling. See 

(holding that ‘‘the
statute is ambiguous’’ and ‘‘does not require that an attain-
ment demonstration be based solely TTT on photochemical
grid modeling’’); cf. McDaniel v. Chevron Corp., 

, 1111 (9th Cir. 2000) (noting that, in the context of
various statutes, courts have held that the phrase ‘‘ ‘based on’
is synonymous with ‘arising from’ and ordinarily refers to a
‘starting point’ or a ‘foundation’ ’’); United States ex rel.
Kreindler & Kreindler v. United Tech. Corp., 

,
1158 (2d Cir. 1993) (holding that ‘‘based upon’’ in the False
Claims Act does not mean based ‘‘solely’’ upon).7 On the
other hand, as the agency also properly concedes, the phrase
would not permit an attainment demonstration that wholly
abandoned the results of a model by using a supplemental
analysis that effectively supplanted the model’s calculations.
Oral Arg. Tr. at 26-27.
   Because the statute is ambiguous, the question is whether
the results obtained by adjusting the model can still reason-
ably be described as ‘‘based on’’ that model. We agree with
  6 As the Supreme Court indicated in United States v. Mead Corp.,

(2001), Chevron deference is required (inter alia)
where, as here, an agency’s interpretation is ensconced in the
product of notice-and-comment rulemaking. See 

  7 Indeed, the only ground on which the phrase might be labeled
unambiguous is that it unambiguously grants EPA at least some
discretion to apply a supplemental analysis to the results generated
by a photochemical grid model.
                                 16

EPA that in this case they can, because here ‘‘photochemical
modeling is the primary basis for the attainment demonstra-
tion,’’ while the supplementary analysis ‘‘is merely an adjunct
for assessing the photochemical grid modeling.’’ Conditional
Approval, 68 Fed. Reg. at 19,113. As explained above, that
analysis was employed to ensure that the model achieved its
statutory purpose: determining whether the SIPs actually
‘‘provide for attainment of the ozone national ambient air
quality standard by the applicable attainment date.’’ 42
U.S.C. § 7511a(c)(2)(A). And the adjustments appear well-
suited to that end, as they do no more than correct for the
model’s over-prediction of ozone levels as compared to actual
observations, and for its reliance on a base day that appears
to be a statistical outlier.
   Sierra Club further contends that, even if supplemental
adjustments of the model are not statutorily barred, the
agency was nonetheless arbitrary and capricious in applying
them in this case. See 5 U.S.C. § 706(2)(A).8 But to with-
stand such an attack, the agency need only demonstrate that
it ‘‘examine[d] the relevant data and articulate[d] a satisfacto-
ry explanation for its action including a rational connection
between the facts found and the choice made.’’ Motor Vehi-
cle Mfg. Ass’n v. State Farm Mut. Auto. Ins. Co., 

,
43 (1983) (internal quotation marks omitted). The agency has
done so.
   The relevant data here are the agency’s findings that the
model ‘‘systematically over-predict[ed] ozone concentration’’
in comparison to actual observed results, and that it over-
weighted conditions on a single day that were ‘‘not likely to
occur often enough to be a major causative factor for nonat-
tainment.’’ Conditional Approval, 68 Fed. Reg. at 19,114-15.
Sierra Club offers no evidence to dispute either finding. The
rational connection between these data and the agency’s
  8 Petitioner also argues that, even if the adjustments are statuto-
rily permissible, EPA cannot use them without first subjecting the
methodology to notice-and-comment rulemaking. But even if that
were correct, Sierra Club does not explain why the notice-and-
comment rulemaking that specifically approved the supplemental
adjustments in this case, see Conditional Approval, 68 Fed. Reg. at
19,111-19, was procedurally insufficient.
                               17

choice is that which we have just described: the adjustments
were necessary to ensure consistency with real-world obser-
vations and thus to ensure reliable prognostications about the
future. Making adjustments in such circumstances is hardly
arbitrary; indeed, the failure to make such adjustments may
itself incur such a charge. Cf. Columbia Falls Aluminum
Co. v. EPA, 

, 923 (D.C. Cir. 1998) (‘‘An agency’s
use of a model is arbitrary if that model bears no rational
relationship to the reality it purports to represent’’) (internal
quotation marks omitted); Chemical Mfrs. Ass’n v. EPA, 

, 1265 (D.C. Cir. 1994) (noting that ‘‘we must
reverse the agency’s application of [a] model as arbitrary and
capricious if there is simply no rational relationship between
the model and the known behavior of the hazardous air
pollutant to which it is applied’’). Accordingly, ‘‘[to] reject
the EPA’s conclusion under these circumstances would be to
substitute our judgment concerning mathematical modeling
techniques for that of the AgencyTTTT This we cannot do.’’
Connecticut v. EPA, 

, 159 (2d Cir. 1982).
   Finally, we note that our conclusion, that EPA was reason-
able in interpreting § 182(c)(2)(A) to permit the supplementa-
tion of the photochemical grid model, is in accord with that
reached by the other circuits that have considered the issue.
See 

(holding that, although ‘‘Texas’
modeled control strategy alone did not demonstrate attain-
ment of the NAAQS for ozone,’’ the state’s supplemental
analysis did and EPA’s approval of the attainment demon-
stration on that basis was ‘‘reasonable and TTT entitled to
deference’’); 1000 Friends of Maryland v. Browner, 

, 234 (4th Cir. 2001) (upholding as neither arbitrary nor
capricious EPA’s ‘‘use of supplemental analysis TTT to demon-
strate attainment in cases where the modeling shows ozone
levels exceeding the NAAQS’’). Our conclusion certainly
does not mean that EPA has unlimited authority to adjust the
calculations of a photochemical grid model. Neither the
government nor this court disputes Sierra Club’s contention
that EPA is not free to apply any ‘‘adjustment’’ that it wishes
and still claim that the results are ‘‘based on’’ the model. In
this case, however, the agency has offered both substantial
                                 18

(and uncontradicted) evidence and a reasonable explanation
for the alterations that it has made. Nothing more is re-
quired.9

                              B
  Sierra Club also takes issue with another component of the
States’ SIPs that EPA conditionally approved in its order of
April 17, 2003: the rate-of-progress (ROP) plans for 1996-
1999.10 Petitioner contends that the agency’s approval of
those plans was arbitrary and capricious because the plans
relied on an outdated emissions model.
  As we have discussed, the Clean Air Act requires that SIPs
for serious and severe nonattainment areas include ROP
plans that demonstrate an average reduction of baseline
emissions of 3% per year for each consecutive three-year
period from 1996 to the attainment deadline. 42 U.S.C.
§ 7511a(c)(2)(B), (d). To facilitate compliance with the Act,
EPA makes available computer programs that determine
baseline emissions and predict future emissions from motor
vehicles. At issue here is the computer model on which the
States’ ROP plans for 1996-1999 were based: the so-called
MOBILE5 model. Sierra Club contends that the ROP plans
  9 Sierra Club also challenges EPA’s conditional approval of the
attainment demonstration on the ground that it relied on modeling
solely for 2005, rather than over the three-year period of 2003-2005.
According to an EPA regulation, an area has not achieved actual
attainment if monitoring data shows that the average number of
annual exceedances at any monitor over a three-year period is
greater than one. 40 C.F.R. pt. 50, app. H. Sierra Club reads this
to say that the States should have demonstrated predicted attain-
ment over three years — from 2003 to 2005 — and not just in 2005.
But no statute or regulation requires such a demonstration.
  10The SIPs did contain 1996–1999 ROP plans, although — as

noted above — they did not contain ROP plans for the post-1999
period.
                              19

were inaccurate because they were not based on the updated
MOBILE6 model.
  EPA explained that it accepted the States’ use of MO-
BILE5 because it was the most recent model available at the
time the plans were prepared. Conditional Approval, 68 Fed.
Reg. at 19,121. The States originally submitted their ROP
plans in 1999, and then resubmitted them as part of the D.C.
area SIPs in February 2002. EPA did not make MOBILE6
available until January 29, 2002 — just one month before the
States submitted their SIPs and long after the modeling had
been completed and the ROP plans prepared. See Official
Release of the MOBILE6 Motor Vehicle Emissions Factor
Model, Notice of Availability, 67 Fed. Reg. 4254 (Jan. 29,
2002); see also Conditional Approval, 68 Fed. Reg. at 19,120-
21.
  Sierra Club argues that the States should nonetheless have
revised the D.C. area ROP plans to incorporate the advances
of MOBILE6, for two reasons. First, MOBILE6 was avail-
able, albeit for only one month, before the States submitted
their plans. Second, EPA did not approve the plans until
April 17, 2003, over a year after MOBILE6’s release.
   EPA responds that, although it requires that states use the
latest model available at the time a plan is developed, see 42
U.S.C. § 7502(c)(3); 40 C.F.R. § 51.112(a)(1), its policy was
not to ‘‘require states that have already submitted SIPs or
will submit SIPs shortly after MOBILE6’s release to revise
these SIPs simply because a new motor vehicle emissions
model is now available.’’ Conditional Approval, 68 Fed. Reg.
at 19,121; see also Memorandum from EPA Office of Air
Quality Planning & Standards 2 (Jan. 18, 2002) (J.A. at 530)
(same). As the agency explains, ‘‘emissions factors, as well as
inventory calculation methodologies, are continually being
improved.’’ 68 Fed. Reg. at 19,120. Indeed, as its name
suggests, MOBILE5 is the fifth generation of this particular
model; MOBILE6 is the sixth. To require states to revise
completed plans every time a new model is announced would
lead to significant costs and potentially endless delays in the
approval processes. EPA’s decision to reject that course, and
                              20

to accept the use of MOBILE5 in this case, was neither
arbitrary nor capricious.

                                IV
   Finally, we consider Sierra Club’s attack on EPA’s bump-
up action of January 24, 2003, which reclassified the D.C. area
from ‘‘serious’’ to ‘‘severe’’ nonattainment. Reclassification,
68 Fed. Reg. at 3410. Needless to say, Sierra Club does not
contest the decision to reclassify D.C. as a severe nonattain-
ment area: to the contrary, petitioner’s suit in Sierra Club I
sought just that result. What Sierra Club does dispute is
EPA’s concomitant decision to extend the States’ final dead-
line for submitting revised SIPs complying with the Act’s
requirements for severe areas, including post-1999 ROP
plans, to March 1, 2004. 

the reclassification of the D.C. area extended
the attainment deadline from November 1999 (the deadline
for serious nonattainment areas) to November 2005 (the
deadline for severe areas), and required the States to revise
their SIPs to comply with the additional requirements appli-
cable to the new classification. 42 U.S.C. § 7511(a)(1), (b)(2).
By extending the attainment deadline from 1999 to 2005, the
reclassification also obligated the States to submit ROP plans
for    the     2000-2002    and     2003-2005    periods.   

see Reclassification, 68 Fed. Reg. at 3414
(‘‘[O]nly an area with an attainment date of 2005 has a legal
obligation to provide for post-1999 ROP.’’). However, the
deadline for filing severe area SIP components including
post–1999 ROP plans had already passed long before reclassi-
fication took place. Indeed, the statutory deadline for such
submittals was November 15, 1994.                   42 U.S.C.
§ 7511a(c)(2)(B), (d), (i); Reclassification, 68 Fed. Reg. at
3413; see Pet’r’s Br. at 44; Resp’ts’ Br. at 56.
  Section 182(i) of the CAA gives EPA the authority to
adjust applicable statutory deadlines, other than attainment
dates, when it reclassifies an attainment area:
    Each State containing an ozone nonattainment area re-
    classified under section 7511(b)(2) of this title shall meet
                              21

     the requirements of subsections (b) through (d) of this
     section as may be applicable to the area as reclassified,
     according to the schedules prescribed in connection with
     such requirements, except that the Administrator may
     adjust any applicable deadline (other than attainment
     dates) to the extent such adjustment is necessary or
     appropriate to assure consistency among the required
     submissions.
42 U.S.C. § 7511a(i) (emphasis added). EPA exercised this
authority to extend the States’ deadlines for submitting their
revised SIPs, based on the following rationale:
     [I]n light of the fact that the original submission dates
     for severe areas have elapsed prior to the time that we
     issued the proposed reclassfication for the Washington
     area, it is a reasonable exercise of EPA’s discretion to
     adjust the applicable submission deadlines in order to
     ensure consistency among the new requirementsTTTT
     Because the States must now meet newly imposed re-
     quirements such as post-1999 ROP and additional severe
     area control requirements, EPA must set prospective
     dates, and has authority under section 182(i) to make
     these dates consistent.
Reclassification, 68 Fed. Reg. at 3413. Moreover, extending
the SIP and ROP deadlines to one year from the date of
reclassification (i.e., to March 1, 2004) would ‘‘assure consis-
tency among’’ all of the ‘‘required submissions,’’ including the
severe area and post-1999 ROP requirements. 42 U.S.C.
§ 7511a(i). The agency’s rationale was reasonable.
  Sierra Club contends that EPA should have retained the
original submission deadlines, and that its decision not to do
so was an unlawful application of § 182(i). As EPA noted,
however, to adopt petitioner’s suggestion ‘‘would give the
reclassification retroactive effect by holding the States in
default of their submission obligations before the events
necessary to trigger that obligation (reclassification) TTT oc-
curred.’’ Reclassification, 68 Fed. Reg. at 3413. That result,
EPA concluded, would be both unfair and inconsistent with
the agency’s past practice. 

(‘‘Where a submission
                                22

date has passed and is therefore impossible to meet, EPA has
concluded that the Administrator may establish a later date.
EPA has applied this interpretation in its prior reclassifica-
tion rulemaking actions.’’) (citing reclassification rulemakings
for Santa Barbara, Phoenix, and Dallas-Fort Worth).
   This circuit, too, has previously rejected Sierra Club’s
suggestion. In Sierra Club v. Browner, petitioner had simi-
larly urged the district court to require a reclassified area (St.
Louis) to comply with the new classification’s statutory dead-
line for SIP submittals, even though that deadline had passed
long before the reclassification, and thus to declare the area
in default. Sierra Club v. Browner, 

, 87
(D.D.C. 2001). The district court refused that request, 

and we affirmed, Sierra Club v. Whitman, 

,
68 (D.C. Cir. 2002). ‘‘The relevant provisions of the Clean Air
Act,’’ we said, ‘‘contain no language suggesting that Congress
intended to give EPA the unusual ability to implement rules
retroactively.’’ 

argument in this case is
indistinguishable, and we therefore reject it.11

                                V
  For the foregoing reasons, we vacate and remand EPA’s
conditional approval action insofar as it granted conditional
approval based on the States’ commitment letters. In all
other respects, we deny the petition for review of that action,
as well as the petition to review the bump-up action.
  11 For the same reasons, we deny Sierra Club’s challenge to
EPA’s decision to permit the States to meet the already passed
November 2002 ROP milestone ‘‘as expeditiously as practicable’’ but
‘‘no later than’’ the attainment deadline of November 15, 2005.
Reclassification, 68 Fed. Reg. at 3422. This, too, has been the
agency’s policy in past situations in which it has had to establish
new ROP deadlines for compliance with already passed ROP mile-
stones. See Approval and Promulgation of Implementation Plans,
Final Rule, 65 Fed. Reg. 31,485 (May 18, 2000) (Missouri); Approv-
al and Promulgation of Implementation Plans, Final Rule, 63 Fed.
Reg. 28,898 (May 27, 1998) (Phoenix); Approval and Promulgation
of Air Quality Implementation Plans, Notice of Final Rulemaking,
62 Fed. Reg. 31,343 (June 8, 1997) (Philadelphia).