FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

SIERRA CLUB; MEDICAL ADVOCATES 

FOR HEALTHY AIR,

Petitioners,

v.

UNITED STATES ENVIRONMENTAL

PROTECTION AGENCY; LISA JACKSON,

Administrator, U.S. Environmental

Protection Agency; JARED  No. 10-71457

BLUMENFELD, Regional

Administrator, Region IX, U.S.

Environmental Protection Agency,

Respondents,

SAN JOAQUIN VALLEY UNIFIED AIR

POLLUTION CONTROL DISTRICT,

Respondent-Intervenor. 

567
COMMITTEE FOR A BETTER ARVIN, a 

California nonprofit corporation;

COMITE RESIDENTES ORGANIZADOS

AL SERVICIO DEL AMBIENTE SANO,

an unincorporated association;

ASSOCIATION OF IRRITATED

RESIDENTS, an unincorporated

association,

Petitioners,

v. No. 10-71458

UNITED STATES ENVIRONMENTAL 

OPINION PROTECTION AGENCY; LISA JACKSON,

Administrator, U.S. Environmental

Protection Agency; JARED

BLUMENFELD, Regional

Administrator, Region IX, U.S.

Environmental Protection Agency,

Respondents,

SAN JOAQUIN VALLEY UNIFIED AIR

POLLUTION CONTROL DISTRICT,

Respondent-Intervenor. 

On Petition for Review of an Order of the

United States Environmental Protection Agency

Argued and Submitted

November 16, 2011—San Francisco, California

Filed January 20, 2012

568 SIERRA CLUB v. UNITED STATES EPA
Before: Sidney R. Thomas, Ronald M. Gould, and

Jay S. Bybee, Circuit Judges.

Opinion by Judge Gould

SIERRA CLUB v. UNITED STATES EPA 569
COUNSEL

Paul R. Cort and Deborah S. Reames, Earthjustice for petitioners Sierra Club and Medical Advocates for Healthy Air;

570 SIERRA CLUB v. UNITED STATES EPA
Sofia L. Parino and Brent Newell, Center on Race, Poverty &

the Environment for Petitioners Committee for a Better Arvin,

Association of Irritated Residents and Comite Residentes

Organizados al Servicio del Ambiente Sano.

Ignacia S. Moreno, John C. Cruden, Monica Derbes Gibson,

Jefferson Wehling, Jeanhee Hong, and Jan M. Tierney for

respondent United States Environmental Protection Agency.

Catherine T. Redmond for respondent-intervenor San Joaquin

Valley Unified Air Pollution Control District.

OPINION

GOULD, Circuit Judge:

Sierra Club and several environmental groups, (collectively, “Petitioners”), petition for review of the United States

Environmental Protection Agency’s (“EPA”) approval of the

2004 State Implementation Plan (“2004 SIP”) for the San Joaquin Valley’s nonattainment area for the one-hour ozone

National Ambient Air Quality Standard (“NAAQS”). Petitioners contend that 1) EPA acted arbitrarily and capriciously,

in violation of the Administrative Procedures Act (“APA”),

by approving the 2004 SIP knowing that the emissions inventory data on which the plan relied were, as an actual matter,

outdated and inaccurate by the time EPA approved the plan

in 2010; 2) EPA violated the Clean Air Act (“CAA”) by

approving the 2004 SIP because the emissions inventory data

on which it relied were outdated and inaccurate within the

meaning of the statute; 3) EPA violated the CAA by approving the 2004 SIP without the inclusion of the State-adopted

regulations on which the plan relied; and 4) EPA violated the

CAA by approving the 2004 SIP knowing that attainment of

the one-hour ozone NAAQS by the 2010 deadline was impossible. We have jurisdiction to review EPA’s action pursuant

SIERRA CLUB v. UNITED STATES EPA 571
to 42 U.S.C. § 7607(b)(1), section 307(b)(1) of the CAA, and

we hold that EPA’s 2010 approval of the 2004 SIP, which

was based on data current only as of 2004, was arbitrary and

capricious. Deciding that issue, we need reach no other.

I

“The CAA makes the States and the Federal Government

partners in the struggle against air pollution.” Jensen Family

Farms, Inc. v. Monterey Bay Unified Air Pollution Control

Dist., 644 F.3d 934, 938 (9th Cir. 2011) (quoting Gen. Motors

Corp. v. United States, 496 U.S. 530, 532 (1990) (internal

quotation marks omitted)). The CAA protects the nation’s air

quality by authorizing EPA to establish NAAQS that apply to

air pollutants that are dangerous to the general health of the

public. 42 U.S.C. § 7409. EPA designates areas that fail to

attain NAAQS as nonattainment areas. Id. §§ 7407(d)(1).

Based on the severity of the pollution problem, nonattainment

areas are further divided into five categories: marginal, moderate, serious, severe, and extreme. Id. § 7511. Central California’s San Joaquin Valley (“the Valley”)1

 has been

designated as an extreme nonattainment area for the pollutant

ozone.2

EPA first established NAAQS for ozone in 1979. This air

quality standard limited the acceptable level of ozone in the

ambient air to a maximum of 0.12 parts per million (“ppm”)

as measured by monitored levels averaged over one hour

1The Valley is made up of eight counties; San Joaquin, Stanislaus, Merced, Madera, Fresno, Kings, Tulare, and the valley portion of Kern.

Approval and Promulgation of Implementation Plans, 74 Fed. Reg. 33,933

(July 14, 2009). 

2

“Ozone (O3) is a gas composed of three oxygen atoms. It is not usually

emitted directly into the air, but at ground-level is created by a chemical

reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.” Ground-Level Ozone, U.S.

Envtl. Prot. Agency, http://www.epa.gov/glo/ (last updated Sept. 23,

2011). 

572 SIERRA CLUB v. UNITED STATES EPA
(“the 1-hour ozone standard”). 74 Fed. Reg. at 33,934. In

1997, EPA reset that maximum to 0.08 ppm as measured by

monitored levels over an eight-hour period (“the 8-hour ozone

standard”). Id. Although the 8-hour standard replaced the 1-

hour standard effective June 2005, as an “anti-backsliding”

measure, EPA retained some elements of the 1-hour ozone

standard for certain nonattainment areas that had yet to attain

the 1-hour ozone standard at the time of its revocation. Id. at

33,934-35 (“As a general matter, the planning and control

requirements that remain applicable following the revocation

of the 1-hour ozone standard derive from CAA sections 110,

172, and 182 . . . . Under the [anti-backsliding measure, nonattainment] areas remain subject to the 1-hour requirements

until they attain the 8-hour ozone standard.”) The Valley is

subject to this anti-backsliding provision.

A. State Implementation Plans 

The CAA requires the states to address nonattainment areas

by developing an SIP that sets out how a nonattainment area

will come into compliance with the requisite NAAQS. 42

U.S.C. §§ 7407(a), 7410. Generally, all SIPs for nonattainment areas must include, inter alia, (1) an emissions inventory, important for the required attainment demonstration and

the related “rate of progress” (“ROP”) demonstration, that

“include[s] a comprehensive, accurate, current inventory of

actual emissions from all sources of the relevant pollutant or

pollutants in such area”; (2) an attainment demonstration,

developed from the emissions inventory, consisting of a technical analysis to predict whether the area will attain the

NAAQS by the deadline and a control strategy for how the

State plans to actually meet the standard;3 (3) a means to mea3

“[A]n attainment demonstration [is] a technical analysis that through

air quality modeling demonstrates that the ‘control measures’ proposed by

the SIP will ensure that these nonattainment areas attain the NAAQS by

the applicable deadline.” El Comité para el Bienestar de Earlimart v.

Warmerdam, 539 F.3d 1062, 1066 (9th Cir. 2008) (quoting 42 U.S.C.

§ 7502(c)(1)). 

SIERRA CLUB v. UNITED STATES EPA 573
sure reasonable further progress (“RFP”); (4) nonattainment

area permit requirements for new or modified stationary

sources; and (5) contingency measures to be implemented if

the nonattainment area does not make RFP or does not attain

the NAAQS by the required date. Id. § 7502(c). SIPs for

extreme ozone nonattainment areas, such as the Valley, must

include an attainment demonstration “based on photochemical

grid modeling4 or any other analytical method determined by

the Administrator, in the Administrator’s discretion, to be at

least as effective.” Id. § 7511a(c)(2)(A), (e). 

After public notice and hearings, a state must adopt the SIP

and submit it to EPA for review and approval. Id. § 7410(a).

EPA must then “determine whether a SIP submission is complete within 60 days of receipt . . . . [A]ny plan that has not

been affirmatively determined to be complete or incomplete

shall become complete within 6 months by operation of law.”

74 Fed. Reg. at 33,934 (discussing 42 U.S.C. § 7410(k)(1)).

EPA must then act on the SIP, either approving it in whole or

disapproving it in part or in whole. 42 U.S.C. § 7410(k)(3).

Once approved by EPA, an “SIP bec[o]me[s] federal law . . . ,

4As the Second Circuit stated: 

Photochemical grid modeling is a sophisticated computerized

method of predicting what ozone levels will be in the future. The

model creates a three-dimensional grid over the entire control

region and analyzes how emissions from various sources combine in the atmosphere to create pollutants such as ozone. Photochemical reactions can produce ozone when oxides of nitrogen

(NOx) and volatile organic compounds (VOCs) are released into

the air and combine with sunlight. See 40 C.F.R. pt. 58 app. D.

§ 2.5. Ozone production is affected by a variety of factors such

as temperature, wind, and emissions levels. By manipulating

other variables like meteorology, terrain, predicted population

growth, and the effect of planned emissions reductions, the model

attempts to predict ambient ozone concentrations on the applicable attainment date. See 1000 Friends of Maryland v. Browner,

265 F.3d 216, 220-21 n.4 (4th Cir. 2001). 

Envtl. Def. v. EPA, 369 F.3d 193, 197-98 (2nd Cir. 2004). 

574 SIERRA CLUB v. UNITED STATES EPA
and c[annot] be changed unless and until EPA approve[s] any

change.” Safe Air for Everyone v. EPA, 488 F.3d 1088, 1096

(9th Cir. 2007). The CAA provides a private right of action

for citizens to enforce an SIP by bringing a civil action in federal district court. 42 U.S.C. § 7604.

B. Mobile Source Emissions Regulation

The CAA also regulates mobile source emissions.5“[T]he

regulation of mobile source emissions is a federal responsibility, [and] Congress has expressly preempted states from setting emissions standards for mobile sources.” Jensen Family

Farms, 644 F.3d at 938 (citing 42 U.S.C. § 7543(a)). California, however, is a “notable exception to this general rule,” and

the CAA permits EPA to authorize California “to set its own

mobile source emissions standards so long as it obtains EPA

approval.” Id. at 938 n.3 (citing 42 U.S.C. § 7543(b) (motor

vehicles); id. § 7543(e)(2) (nonroad sources)); see also Nat’l

Ass’n of Home Builders v. San Joaquin Valley Unified Air

Pollution Control Dist., 627 F.3d 730, 733 (9th Cir. 2010)

(“[T]he [CAA] gives the states the job of regulating stationary

sources of pollution, but the EPA, and with the EPA’s permission California, are responsible for regulating emissions from

motor vehicles and other mobile sources.”). California thus

relies on its own mobile source standards in the development

of its SIPs.

C. The San Joaquin Valley 2004 SIP

In 1991, the Valley was classified as a “serious” nonattainment area under the 1-hour ozone standard. Designation of

Areas for Air Quality Purposes, 56 Fed. Reg. 56,694, 56,699

5

“ ‘Mobile sources’ is a term used to describe a wide variety of vehicles,

engines, and equipment that generate air pollution and that move, or can

be moved, from place to place.” What Are Mobile Sources?, U.S. Envtl.

Prot. Agency, http://www.epa.gov/otaq/invntory/overview/examples.htm

(last updated Jan. 3, 2012). 

SIERRA CLUB v. UNITED STATES EPA 575
(Nov. 6, 1991). The State timely submitted its SIP to EPA in

1994, and in January 1997, EPA approved the plan, which set

November 1999 as the attainment deadline. Approval and

Promulgation of Implementation Plans; California—Ozone,

62 Fed. Reg. 1150 (Jan. 8, 1997). 

In 2001, EPA determined that the Valley did not attain the

1-hour ozone standard by the deadline. CAA Reclassification,

San Joaquin Valley Nonattainment Area, 66 Fed. Reg. 56,476

(Nov. 8, 2001). Consistent with the CAA, EPA automatically

reclassified the Valley as a “severe” nonattainment area, set

May 31, 2002 as the deadline by which the State had to submit appropriate revisions to the SIP, and set November 15,

2002 as the Valley’s new attainment deadline. Id. at 56,481;

see also 42 U.S.C. § 7511(a)(1), (b)(2). 

The State did not meet its May 2002 deadline for submission of the SIP revision. Findings of Failure to Submit SIP

Revisions for 1-Hour Standard, 67 Fed. Reg. 61,784 (Oct. 2,

2002). Under the CAA, EPA then issued a finding that the

State did not meet its obligation, which “trigger[ed] the 18-

month clock for mandatory application of sanctions and 2-

year clock for a Federal implementation plan (FIP) under the

[CAA].” Id.; see also 42 U.S.C. § 7509; 40 C.F.R. § 52.31.

The State effectively gained an 18-month extension in which

it was required to revise the SIP before EPA would impose

sanctions. 67 Fed. Reg. 61,784-85. 

Once more, the State did not meet this deadline. To avoid

sanctions, the State requested that EPA reclassify the Valley

as an “extreme” one-hour ozone nonattainment area. CAA

Reclassification, 69 Fed. Reg. 20,550 (Apr. 16, 2004); see

also 42 U.S.C. § 7511(a)(5). This voluntary reclassification

gave the State extra time to submit its revised SIP by resetting

the State’s submission deadline for a revised SIP consistent

with the Valley’s reclassification as an extreme ozone nonattainment area. 69 Fed. Reg. at 20,550. The new deadline was

November 15, 2004, and the new attainment deadline was

576 SIERRA CLUB v. UNITED STATES EPA
November 15, 2010. Id. The State submitted the 2004 SIP for

EPA approval in November 2004. 74 Fed. Reg. at 33,933-34.

In 2003, the State adopted the “State and Federal Strategy

for the California State Implementation Plan,” (“the State

Strategy”), which set forth California’s regulatory agenda to

reduce ozone and specific commitments to reduce emissions

in the Valley. Id. at 33,934. The State Strategy “includes

defined statewide control measures that were to be reflected

in future SIPs and provisions specific to air quality plans for

the San Joaquin Valley.” Id. The 2004 SIP relied on the State

Strategy’s mobile source emissions data, developed by the

State, for its attainment demonstration and rate of progress.

Id.

D. EPA Action on the 2004 SIP

The State amended the 2004 SIP in October 2005 to substitute into the plan a new control strategy and to synchronize

the rulemaking schedule with that in the Valley’s plan for the

NAAQS for particulate matter less than or equal to 10

micrometers in diameter. Id. The State submitted these

amendments to the plan to EPA in March 2006. Id. In September 2008, the State submitted “Clarifications Regarding

the 2004 Extreme Ozone Attainment Demonstration Plan”

(“2008 Clarifications”), which updated information on certain

control programs, ROP demonstration, and contingency measures in the 2004 SIP. Id. By operation of law, the original

2004 SIP was deemed complete on May 15, 2005, the subsequent amendments on September 6, 2006, and the 2008 Clarifications on September 23, 2008. Id.; see also 42 U.S.C.

§ 7410(k)(1)(B). Together, the SIP as submitted in 2004, the

2006 Amendments, and the 2008 Clarifications constitute the

2004 SIP, now at issue. 

In October 2008, EPA issued a proposed rule to approve

the 2004 SIP and sought public comment. Approval and Promulgation of Implementation Plans, 73 Fed. Reg. 61,381

SIERRA CLUB v. UNITED STATES EPA 577
(Oct. 16, 2008). After comments were received, EPA withdrew its proposed rule approving the 2004 SIP, and instead

proposed to approve the 2004 SIP in part, disapproving only

the attainment contingency measures. 74 Fed. Reg. at 33,947.

In response, the State submitted supplemental technical information, and EPA withdrew its proposed disapproval and proposed to approve the attainment contingency measures, and

thus the 2004 SIP in full, in October 2009. Approval and Promulgation of Implementation Plans, 74 Fed. Reg. 50,936

(Oct. 2, 2009). EPA finally approved the 2004 SIP on March

8, 2010, six years after its original submission and just eight

months before the plan’s November 15, 2010 attainment

deadline. Approval and Promulgation of Implementation

Plans, 75 Fed. Reg. 10,420 (Mar. 8, 2010). 

II

“We have jurisdiction to review EPA’s approval of the

[2004 SIP] under 42 U.S.C. § 7607(b)(1).” Vigil v. Leavitt,

381 F.3d 826, 833 (9th Cir. 2004). Because the CAA “does

not specify a standard of review, . . . we apply the general

standard of review for agency actions set forth in the [APA],

5 U.S.C. §§ 701-06.” Latino Issues Forum v. EPA, 558 F.3d

936, 941 (9th Cir. 2009) (citing Vigil, 381 F.3d at 833)).

“Under the APA, we consider whether the EPA’s action was

‘arbitrary, capricious, an abuse of discretion, or otherwise not

in accordance with law.’ ” Id. (quoting 5 U.S.C. § 706(2)(A)).

“This standard requires the EPA to ‘articulate[ ] a rational

connection between the facts found and the choice made.’ ”

Id. (quoting Ariz. Cattle Growers’ Ass’n v. U.S. Fish & Wildlife, 273 F.3d 1229, 1236 (9th Cir. 2001)). “[We] review the

record to ensure that agency decisions are founded on a reasoned evaluation of the relevant factors, and may not rubberstamp . . . administrative decisions that [are] inconsistent with

a statutory mandate or that frustrate the congressional policy

underlying a statute . . . .” Id. (quoting Friends of Yosemite

Valley v. Norton, 348 F.3d 789, 793 (9th Cir. 2003)). 

578 SIERRA CLUB v. UNITED STATES EPA
Chevron, U.S.A., Inc. v. Natural Res. Def. Council, Inc.,

467 U.S. 837 (1984) generally sets forth the framework by

which we review an agency’s interpretation of a statute. Id. at

842-44. Under this framework at the first step we determine

“whether Congress has directly spoken to the precise question

at issue. If 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.” Id.

at 842-43. “[I]f the statute is silent or ambiguous with respect

to the specific issue, the question for the court is whether the

agency’s answer is based on a permissible construction of the

statute.” Id. at 843. 

Not all agency statutory interpretations are entitled to Chevron deference, however. Rather, Chevron deference is appropriate where “the agency can demonstrate that it has the

general power to ‘make rules carrying the force of law’ and

that the challenged action was taken ‘in the exercise of that

authority.’ ” The Wilderness Soc’y v. U.S. Fish & Wildlife

Serv., 353 F.3d 1051, 1067 (9th Cir. 2003) (quoting United

States v. Mead Corp., 533 U.S. 218, 226-27 (2001)); see also

Vigil, 381 F.3d at 834 (finding that Chevron deference did not

apply to EPA statutory interpretations where “EPA has not, in

fact, exercised its general rulemaking authority to define these

terms”). “A very good indicator of delegation meriting Chevron treatment [is] express congressional authorizations to

engage in the process of rulemaking or adjudication that produces regulations or rulings for which deference is claimed.”

Mead, 533 U.S. at 229. 

[1] With respect to the CAA, “Congress has given EPA

general rulemaking authority, 42 U.S.C. § 7601(a)(1), which,

when exercised, requires our deference in accordance with

Chevron.” Vigil, 381 F.3d at 834. But EPA has not exercised

its authority to make rules carrying the force of law to fill in

the meaning of “current” and “accurate,” the terms in the

CAA we now ponder. Instead, EPA’s interpretation of the

CAA in its material terms here is largely based on a policy

SIERRA CLUB v. UNITED STATES EPA 579
guidance document issued by the Office of Air Quality Planning and Standards and the Office of Transportation and Air

Quality in 2002 along with the agency’s past practices. See

Memorandum from John S. Seitz, Office of Air Quality Planning & Standards, and Margo Tsirigotis Oge, Office of

Transp. & Air Quality, Policy Guidance on the Use of

MOBILE6 for SIP Development and Transportation Conformity (Jan. 18, 2002), available at http://www.epa.gov/otaq/

models/mobile6/m6policy.pdf (“Seitz Memo”). “ ‘Interpretations such as those . . . contained in policy statements, agency

manuals, and enforcement guidelines, all of which lack the

force of law—do not warrant Chevron-style deference.’ ”

Vigil, 381 F.3d at 835 (quoting Christensen v. Harris Cnty.,

529 U.S. 576, 587 (2000). Indeed, “[t]hey are beyond the

Chevron pale.” Mead, 533 U.S. at 234. 

“Such views, however, even if not authoritative for purposes of Chevron, are entitled to so-called Skidmore deference insofar as they ‘constitute a body of experience and

informed judgment to which courts and litigants may properly

resort for guidance.’ ” Vigil, 381 F.3d at 835 (quoting Skidmore v. Swift & Co., 323 U.S. 134, 140 (1944)). “Under Mead

and Skidmore, the weight that we are to give an administrative

interpretation not intended by an agency to carry the general

force of law is a function of that interpretation’s thoroughness, rational validity, and consistency with prior and subsequent pronouncements.” Wilderness Soc’y, 353 F.3d at 1068

(citing Skidmore, 323 U.S. at 140). Moreover, “ ‘[c]ogent

administrative interpretations . . . not the products of formal

rulemaking . . . nevertheless warrant respect.’ ” Vigil, 381

F.3d at 835 (quoting Alaska Dep’t of Envtl. Conservation v.

EPA, 540 U.S. 461, 488 (2004). 

[2] Here, the Seitz Memo was not issued by EPA under

rulemaking authority, but, in promulgating the memo, EPA

followed “relatively formal administrative procedure tending

to foster the fairness and deliberation that should underlie a

pronouncement of . . . force.” Mead, 533 U.S. at 230. Before

580 SIERRA CLUB v. UNITED STATES EPA
its issuance, “EPA provided state, local, and tribal agencies an

opportunity to comment on the draft policy guidance in the

fall of 2001.” Seitz Memo at 1. To the extent that EPA relies

on the Seitz Memo, along with EPA’s past practices that have

not been codified through EPA’s rulemaking authority, to

support its interpretation of the statute, we give limited deference to EPA’s interpretation of the statutory provisions at

issue here. See Mead, 533 U.S. at 234 (“Chevron did nothing

to eliminate Skidmore’s holding that an agency’s interpretation may merit some deference whatever its form, given the

‘specialized experience and broader investigations and information’ available to the agency and given the value of uniformity in its administrative and judicial understandings of what

a national law requires.” (citations omitted) (quoting Skidmore, 323 U.S. at 140)).

III

The crux of the case for us is whether EPA’s actions were

arbitrary and capricious, for if so, EPA needs to conduct its

process anew and other issues concerning statutory interpretation need not be reached. In assessing whether EPA’s agency

action approving the SIP is arbitrary and capricious, we consider whether EPA “examine[d] the relevant data and articulate[d] a satisfactory explanation for its action including a

‘rational connection between the facts found and the choice

made.’ ” Motor Vehicle Mfrs. Ass’n of the U.S., Inc. v. State

Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983) (quoting

Burlington Truck Lines, Inc. v. United States, 371 U.S. 156,

168 (1962)). 

[3] Section 172(c)(3) of the CAA requires that nonattainment plans “include a comprehensive, accurate, current

inventory of actual emissions from all sources of the relevant

pollutant or pollutants in such area .” 42 U.S.C. § 7502(c)(3)

(emphasis added). Petitioners contend that by the time EPA

approved the 2004 SIP in 2010, the mobile source data on

which the emissions inventory in the 2004 SIP relied were

SIERRA CLUB v. UNITED STATES EPA 581
neither “accurate” nor “current,” as shown by the emissions

inventory data that the State submitted in connection with its

2007 SIP for the 8-hour ozone standard. Petitioners further

contend that EPA’s approval of the 2004 SIP was arbitrary

and capricious because, in approving the 2004 SIP, EPA did

not reconcile the differences in the emissions data produced

by the two models. EPA also did not explain why it chose not

to consider the 2007 data in approving the 2004 SIP. We

agree that the use of stale data was not adequately reconciled

with the more current data. 

The State based the 2004 SIP emissions inventory data on

mobile source data estimated through the use of the State’s

computer modeling tool released in 2002, called

EMFAC2002. EMFAC2002 estimated emissions from heavyduty diesel trucks based on where the trucks were registered.

The 2007 SIP emissions inventory data were compiled using

mobile source data estimated using the next generation of that

computer modeling tool, EMFAC2007, released in November

2006 and approved by EPA in January 2008. Official Release

of EMFAC2007 Motor Vehicle Emission Factor Model, 73

Fed. Reg. 3464, 3465-66 (Jan. 18, 2008). EMFAC2007

improved mobile source emission estimates by basing them

on where the trucks were being driven, which better

accounted for the amount of pollution from trucks driven in,

but not necessarily registered in, the Valley. The change in

computer modeling tool resulted in disparities in NOx emissions estimates between the 2004 SIP emissions inventory and

the 2007 SIP emissions inventory. For example, the 2004 SIP

emission inventory estimate for total Valley NOx emissions

in 2008 was 429.1 tons per day, but the same estimate for

2008 rose to 597.8 tons per day in the 2007 SIP. 

Although it knew of the disparities in mobile source emissions estimates between the 2004 SIP and the 2007 SIP, EPA

did not address those differences or their likely impact, if any,

on the validity of the 2004 SIP in its final rule approving the

2004 SIP. Instead, EPA relied on its past practice and inter582 SIERRA CLUB v. UNITED STATES EPA
pretation of the CAA, as stated in the Seitz Memo, in which

emissions inventory data are considered “current” and “accurate” within the meaning of § 172(c)(3) as long as the data are

current and accurate when the State submits an SIP to EPA

for approval and not necessarily when EPA approves an SIP.6

See Seitz Memo at 2. 

EPA relies principally on the Seitz Memo and on the D.C.

Circuit’s holding in Sierra Club v. EPA, 356 F.3d 296 (D.C.

Cir. 2004), to support its interpretation that emissions inventory data satisfy the “accurate” and “current” requirements of

the CAA as long as they are accurate and current when the

SIP is submitted for EPA approval, and that no further review

of the 2007 data was necessary.

The Seitz Memo, published in 2002, gives policy guidance

issued to address “when to use MOBILE67 in [SIP] develop6As a preliminary matter, Respondent-Intervenor, San Joaquin Valley

Unified Air Pollution Control District (“the District”) argues that this court

cannot give relief to petitioners because the 1-hour ozone standard was

supplanted by the 8-hour ozone standard, and EPA has no obligation “to

take affirmative action to demonstrate attainment of the [1-hour ozone

standard].” As petitioners point out, however, EPA does not similarly

argue that the court may not provide a remedy here. Moreover, 40 C.F.R.

§§ 51.900(f), 51.905 set forth the ongoing obligations for 1-hour ozone

nonattainment areas after the adoption of the 8-hour standards. They

include ROP reductions and, here, a 1-hour NAAQS ozone attainment

demonstration because the State apparently has not met the other alternatives for compliance with the regulations. See 40 C.F.R. § 51.905(a)(ii);

Adequacy Status of Motor Vehicle Emissions: 8-hour Ozone, 74 Fed. Reg.

4032, 4033 (Jan. 22, 2009) (stating that the Valley’s 8-hour ozone plan did

not show reasonable further progress for the year 2008). 

Ultimately, the State is still required to submit, and EPA to approve, a

valid plan including measures relating to 1-hour NAAQS, although

revoked. To that end, the 2004 SIP contained these measures, “including

control measures, rate-of-progress (ROP) and attainment demonstrations,

and contingency measures,” and the panel may remand the matter to EPA

for action consistent with its holding. 74 Fed. Reg. at 33,934. 

7

“MOBILE is EPA’s highway vehicle emissions [computer modeling

tool] for predicting gram-per-mile emissions of hydrocarbons (HC), carSIERRA CLUB v. UNITED STATES EPA 583
ment and transportation conformity determinations.” Seitz

Memo at 1. In 2001, MOBILE6 supplanted MOBILE5 as the

most current iteration of EPA’s federally-developed computer

modeling tool for highway vehicle emissions for “state and

local agencies outside of California.” Id. In the Seitz Memo,

EPA expressed its “belie[f] that the [CAA] would not 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.” Seitz Memo at 2. In coming to this conclusion, EPA

seemed concerned that requiring States to rework previously

submitted or recently approved SIPs whenever a new model

becomes available would pose “an obstacle to EPA approval.”

Id. EPA stated that, “[i]t would be unreasonable to require the

States to revise [recently submitted or approved] SIPs with

MOBILE6 since significant work has already occurred, and

EPA intends to act on these SIPs in a timely manner.” Id.

(emphasis added). 

[4] Contrary to EPA’s argument, however, the Seitz Memo

does not appear unequivocally to stand for the proposition

that EPA action comports as a matter of course with

§ 172(c)(3) whenever EPA approves an SIP based on an old

emissions computer modeling tool as long as the modeling

tool was the most current and accurate at the time of SIP submission. That is because at least part of the Seitz Memo’s

rationale on not requiring an updated SIP when a new computer modeling tool arrives appears to rest on the timeliness

of EPA’s subsequent action on the SIP in question. See id.

Indeed, the latter part of the statement quoted above suggests

that while there is some period of time after the release of a

bon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO2), particulate matter (PM), and toxics from cars, trucks, and motorcycles under

various conditions.” Mobile, Office of Planning, Env’t, & Realty, http://

www.fhwa.dot.gov/environment/air_quality/conformity/methodologies/

mobile.cfm (last updated July 6, 2011). The most current version is

MOBILE6.2, released in 2004. Id.

584 SIERRA CLUB v. UNITED STATES EPA
new computer modeling tool in which the CAA does not

require a finding that SIPs based on the previous version are

not current and accurate, there comes a time after which reliance on outdated models and data is inconsistent with requisite guidelines for ensuring that agency action is timely and

responsive to current public needs. The Seitz Memo itself further supports this perspective as it states the following:

EPA also believes that the legal basis for approving

a MOBILE5-based SIP is less clear the longer that

MOBILE6 is in place and available for use. Since

SIPs must be based on applicable models and data

inputs, it could be difficult for EPA to approve a SIP

developed with MOBILE5 significantly after

MOBILE6 becomes available. 

Id. at 2-3 (emphasis added). It appears then that EPA, when

it promulgated its guidance, contemplated, not surprisingly,

that relying on an outdated computer modeling tool after a

significant amount of time has passed since the introduction

of a newer model would pose a problem. Here, EMFAC2007

was released to the public in November 2006, submitted to

EPA for approval in April 2007, and approved by EPA in January 2008. See 73 Fed. Reg. at 3465. EPA, however, did not

approve the 2004 SIP until 2010, more than 3 years after the

release of EMFAC2007. By March of 2010, EPA knew that

a new computer modeling tool was available and had access

to data compiled through the use of the more current tool.

That data told a different story than that told by the earlier

data about projected emissions of NOx from mobile sources

in the Valley, which undermined the accuracy and currency

of the 2004 SIP emissions inventory data. EPA, in its final

rule approving the 2004 SIP, however, did not analyze this

new data or explain why it chose not to analyze the data in

considering the 2004 SIP. EPA did not “cogently explain why

it . . . exercised its discretion” not to consider the new and

available data. State Farm, 463 U.S. at 48. Instead, EPA

merely repeated its mantra that, despite availability of the

SIERRA CLUB v. UNITED STATES EPA 585
2007 emissions inventory data three years before the 2010

approval of the 2004 SIP, EPA had no duty under the CAA

to consider new data so long as the data relied upon was current and accurate when submitted. But we think the agency in

the public’s interest should have considered that six years had

passed between the State’s submission of the 2004 SIP and its

approval by the agency.

EPA also relies on the D.C. Circuit’s holding in Sierra

Club to support its contention that reliance on the currency

and accuracy of the data at the time of submission renders its

approval of the 2004 SIP valid. In Sierra Club, the D.C. Circuit considered, inter alia, the significance of EPA’s introduction of MOBILE6 on EPA’s approval of the D.C.-area’s 1-

hour ozone SIPs, which were based on emissions data complied using MOBILE5. 356 F.3d at 308. The ROP plans for

the years 1996-1999, based on MOBILE5, were originally

submitted for EPA approval in 1999 and then resubmitted as

part of the D.C.-area SIPs in February 2002, even though

MOBILE6 was made available one month earlier in January

2002. EPA approved the SIPs one year later in April 2003. Id.

The Sierra Club argued that EPA’s approval of the SIPs

there was arbitrary and capricious in light of the availability

of the newer modeling tool. Id. at 307-08. Relying on the

Seitz Memo, the D.C. Circuit concluded, however, that “[t]o

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.” Id. at 308.

Accordingly, the D.C. Circuit held that “EPA’s decision to

. . . accept the use of MOBILE5 . . . was neither arbitrary nor

capricious.” Id.

Contrary to EPA’s argument, our conclusion today is not

inconsistent with the D.C. Circuit’s holding in Sierra Club

because the facts here are not alike; there are marked differences between the plans’ submissions and their approvals. In

Sierra Club, the new modeling tool was available for only one

586 SIERRA CLUB v. UNITED STATES EPA
year before EPA approved the D.C.-area SIPs that were based

on the previous iteration. Here, EMFAC2007 was available to

the public as early as November 2006, and the 2004 SIP was

not approved until more than three years later in March 2010.

Thus as the Seitz Memo itself suggests, EPA stands on shaky

legal ground relying on significantly outdated data, given the

amount of time that EMFAC2007 was available and authorized for use before EPA approved the 2004 SIP. See Seitz

Memo at 2-3.8

Second, in Sierra Club, the D.C. Circuit noted the Seitz

Memo’s argument that requiring a new compilation of emission data based on the new computer modeling tools would

place an onerous burden on and unnecessarily prolong

approval. Indeed, in that case, no new data based on

MOBILE6 were available and a new study just to collect the

raw data would have had to be commissioned. Here, by contrast, the State had already collected the emissions data using

EMFAC2007, and had presented to EPA the 2007 SIP that

relied on that updated data. There was no need for a new

study to be commissioned, and the burden and expense of

commissioning a new study was minimized as one had

already been completed with respect to the 8-hour ozone standard 2007 SIP. Based on these factual differences then, the

D.C. Circuit’s opinion in Sierra Club does not support EPA’s

8

In another example cited by EPA to support the proposition that it is

EPA’s policy to approve SIPs based on MOBILE5, although MOBILE6

was available for use, there was also a relatively short period of time

between the plan submission and EPA approval. Specifically, in July 2001

the state of Georgia submitted a revised attainment demonstration for the

1-hour ozone nonattainment area in Atlanta which replaced the previous

attainment demonstration submitted in October 1999. Approval and Promulgation of Georgia SIPs, 67 Fed. Reg. 30,574 (May 7, 2002). EPA

approved the attainment demonstration effective June 2002, about 6

months after MOBILE6 was available. Id. This example does not speak

to the Seitz Memo’s own observation regarding the legality of relying on

an old version of the computer modeling tool when a new version has

been available for a significant amount of time. 

SIERRA CLUB v. UNITED STATES EPA 587
contention that its approval of the 2004 SIP here, without consideration of the long available updated emissions inventory

data, was not arbitrary or capricious. 

Moreover, EPA’s decision not to evaluate the EMFAC2007

data, of which EPA had actual knowledge and which could

reasonably be described as relevant to the efficacy of the 2004

SIP, is also inconsistent with our holding in Ass’n of Irritated

Residents v. EPA (A.I.R.), 632 F.3d 584 (9th Cir. 2011). In

A.I.R., we considered whether EPA’s decision not to evaluate

the adequacy of California’s South Coast nonattainment

area’s previously approved 1997/1999 SIP was arbitrary and

capricious after the State, upon new modeling, submitted a

2003 attainment demonstration revision that made clear that

the attainment demonstration in the 1997/1999 SIP was inaccurate. 632 F.3d at 589-90. EPA ultimately disapproved the

2003 SIP revision’s attainment demonstration, but took no

further action to evaluate the 1997/1999 SIP’s extant attainment demonstration. We held that EPA’s “failure to evaluate

the adequacy of the existing [1997/1999] SIP was arbitrary

and capricious,” because California, in its 2003 SIP revision,

acknowledged that the attainment demonstration in EPAapproved 1997/1999 SIP was inaccurate. Id. at 590. We stated

that, “[t]hrough the 2003 SIP Revision, EPA knew, or should

have known, of the inadequacy of the 1997/1999 SIP,” and

that “even if EPA did not actually know the extent to which

the new modeling undermined the existing SIP, it has a duty

to evaluate the adequacy of the existing SIP as a whole when

approving SIP revisions.” Id. at 590-91.

In reasoning that EPA’s “obligation to take further action

[could] be derived from the statutory requirement that the

Administrator issue a SIP call upon a finding that the existing

SIP is substantially inadequate,” we noted:

[T]he question is not whether EPA has discretion in

determining substantial inadequacies exist, but

whether EPA has unlimited discretion to ignore evi588 SIERRA CLUB v. UNITED STATES EPA
dence indicating an existing SIP might be substantially inadequate and choose to do nothing. We

believe EPA’s failure to act in light of the strong evidence provided in the 2003 SIP Revision demonstrating the substantial inadequacies of the 1997/

1999 Plan is arbitrary and capricious. 

Id. at 593 (emphasis added). While A.I.R. addressed EPA’s

obligation to monitor the accuracy and efficacy of approved

SIPs, its holding is relevant because it supports the proposition that if new information indicates to EPA that an existing

SIP or SIP awaiting approval is inaccurate or not current,

then, viewing air quality and scope of emissions with public

interest in mind, EPA should properly evaluate the new information and may not simply ignore it without reasoned explanation of its choice. 

The District argues that it was appropriate for EPA to

ignore the 2007 8-hour ozone emissions inventory data

because they were not relevant to the 2004 1-hour ozone

emissions inventory data, and that any comparison would not

be “an apples to apples comparison.” We are not persuaded

for several reasons. First, while the conclusions presented

within the two SIPs might vary, the undisputed fact is that the

EMFAC2007 revealed a significantly different measurement

of expected NOx emissions (e.g., 597.8 tons per day in 2008)

than did the data observed with EMFAC2002 (e.g., 429.1 tons

per day in 2008). Given NOx’s integral role in the creation of

harmful ground-level ozone, this disparity should have alerted

EPA to the possibility that the 2004 Plan may have been significantly flawed. 

Second, although the District characterizes the 8-hour

ozone NAAQS as being so different from the 1-hour ozone

standard as to make any comparison unreasonable, there is

evidence that the two standards are significantly close enough

that the above-noted differences in NOx emissions should

have reasonably alerted EPA to the real possibility that the

SIERRA CLUB v. UNITED STATES EPA 589
data on which the 2004 SIP was based were inaccurate and

ineffective. National Ambient Air Quality Standards for

Ozone, 62 Fed. Reg. 38,856, 38,858 (July 18, 1997) (stating

that the 8-hour standard (0.09 ppm) “generally represents the

continuation of the present level of protection”); see also S.

Coast Air Quality Mgmt. Dist., 472 F.3d 882, 888 (D.C. Cir.

2006) (citing 62 Fed. Reg. at 38,858 for the proposition that

the 8-hour ozone standard is only “marginally more stringent”

than the 1-hour ozone standard it replaced). 

Third, and most importantly, EPA, in its final rule, did not

bother to address substantively the apparent disparities

between the 2007 emissions inventory data and the 2004

emissions inventory data, leaving us with no means of determining whether there is any merit to the District’s argument.

In this light, the District’s conclusions in its briefing that the

2007 emissions inventory data should have had no bearing on

EPA’s approval of the 2004 SIP are unpersuasive insofar as

they are presented for the purposes of litigation rather than

framed objectively and scientifically. See State Farm, 463

U.S. at 50 (“[T]he courts may not accept appellate counsel’s

post hoc rationalizations for agency action.”). EPA did not

“bring its expertise to bear on” the updated emissions inventory data in the 2004 SIP approval process. Id. at 54. The

agency did not adequately address the staleness of its data and

availability of more current data before reaching its conclusion. 

[5] Our role is not to substitute our conclusions based on

the facts presented for those of the agency, and we express no

opinion as to what conclusion EPA should have reached, with

respect to the validity of the 2004 SIP, upon consideration of

the 2007 data. But we should not silently rubber stamp agency

action that is arbitrary and capricious in its reliance on old

data without meaningful comment on the significance of more

current compiled data. We hold that EPA’s failure to even

consider the new data and to provide an explanation for its

choice rooted in the data presented was arbitrary and capri590 SIERRA CLUB v. UNITED STATES EPA
cious. Indeed, it was unreasonable for EPA summarily to rely

on the point of view taken under markedly different circumstances in the Seitz Memo and in Sierra Club, without

advancing an explanation for its action based on “the facts

found and the choice made.” Burlington Truck Lines, 371

U.S. at 168. 

[6] Concluding that the agency’s action in approving the

challenged SIP was arbitrary and capricious under the APA,

we need not and do not reach petitioners’ other arguments.

We grant the petition for review and remand the matter to

EPA for further proceedings consistent with our decision.9

PETITION GRANTED.

9Upon EPA’s reconsideration and subsequent final action, petitioners

may again seek review of that EPA action, but shall file a new petition in

order to do so. In such an instance, petitioners, where appropriate, may

raise the issues not reached by us today. 

SIERRA CLUB v. UNITED STATES EPA 591