                FOR PUBLICATION

  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT


SANDRA L. BAHR; DAVID                     No. 14-72327
MATUSOW,
                        Petitioners,
                                           OPINION
                v.

U.S. ENVIRONMENTAL PROTECTION
AGENCY; GINA MCCARTHY,
Administrator, United States
Environmental Protection Agency;
JARED BLUMENFELD, Regional
Administrator, EPA Region IX,
                       Respondents,

STATE OF ARIZONA,
            Respondent-Intervenor.


       On Petition for Review of an Order of the
          Environmental Protection Agency

         Argued and Submitted June 17, 2016
              San Francisco, California

               Filed September 12, 2016
2                        BAHR V. U.S. EPA

    Before: Richard R. Clifton and Sandra S. Ikuta, Circuit
       Judges, and William Q. Hayes,* District Judge.

                    Opinion by Judge Ikuta;
    Partial Concurrence and Partial Dissent by Judge Clifton


                            SUMMARY**


                        Environmental Law

    The panel granted in part and denied in part a petition for
review of an order of the United States Environmental
Protection Agency approving Arizona’s Five Percent Plan for
airborne particulate matter around Maricopa County,
promulgated under the Clean Air Act.

    Arizona submitted a new State Implementation Plan
revision on May 25, 2012 – the Five Percent Plan – to
achieve a five percent annual reduction in PM-10, a harmful
air pollutant.

     Petitioners alleged that the EPA acted contrary to law by
failing to require that Arizona include an updated analysis of
best available control measures and most stringent measures
in the Five Percent Plan, excluding 135 exceedances from the
monitoring data as “exceptional events,” and allowing


      *
      The Honorable William Q. Hayes, United States District Judge for
the Southern District of California, sitting by designation.
    **
       This summary constitutes no part of the opinion of the court. It has
been prepared by court staff for the convenience of the reader.
                     BAHR V. U.S. EPA                       3

Arizona to satisfy the “contingency measures” requirement
with previously implemented control measures.

   The panel held that it would apply Chevron U.S.A. Inc. v.
Natural Resources Defense Council, Inc., 467 U.S. 837
(1984), deference to the EPA’s interpretation of the Clean Air
Act issued in connection with a State Implementation Plan
approval.

   The panel upheld the EPA’s determination that the control
measures in Arizona’s Five Percent Plan did not need to be
updated, and that the 135 exceedances were exceptional
events that were excluded from consideration under the
EPA’s regulation and guidance documents.

    The panel did not defer to the EPA’s interpretation of the
contingency measures requirement, however, because under
the plain language of 42 U.S.C. § 7502(c)(9) contingency
measures are measures that will be taken in the future, not
measures that have already been implemented. The panel
remanded to the EPA for further consideration of this portion
of the State Implementation Plan, but otherwise denied the
petition.

    Judge Clifton concurred in sections I-IV of the majority
opinion, and dissented from the majority’s conclusion in
section V that EPA’s approval of the contingency measures
in Arizona’s State Implementation Plan was contrary to the
clear language of the Clean Air Act. In his view, the scope of
the Clean Air Act’s contingency measures requirement was
ambiguous and EPA’s reasonable interpretation of that
requirement was entitled to deference.
4                   BAHR V. U.S. EPA

                        COUNSEL

Joy E. Herr-Cardillo (argued) and Timothy M. Hogan,
Arizona Center for Law in the Public Interest, Tucson,
Arizona, for Petitioners.

Alan D. Greenberg (argued), Attorney; Sam Hirsch, Acting
Assistant Attorney General; Environment & Natural
Resources Division, United States Department of Justice,
Denver, Colorado; Geoffrey Wilcox, Office of General
Counsel; Kara Christenson, Office of Regional Counsel,
Region 9; United States Environmental Protection Agency,
San Francisco, California; for Respondents.

Monique Coady, Assistant Attorney General, Office of the
Attorney General, Phoenix, Arizona, for Respondent-
Intervenor.


                        OPINION

IKUTA, Circuit Judge:

    Sandra Bahr and David Matusow petition for review of a
final rule issued by the Environmental Protection Agency
(EPA) approving Arizona’s Five Percent Plan for airborne
particulate matter around Maricopa County. They argue that
the EPA erred in approving this plan because it did not
include best available control measures (BACM) and most
stringent control measures (MSM) as of 2012. The
petitioners also argue that the EPA failed to follow its own
published guidance in approving Arizona’s claim that 135
exceedances of the air emission standard could be excluded
from consideration. See 42 U.S.C. § 7619(b)(1)(A). Finally,
                     BAHR V. U.S. EPA                       5

the petitioners argue that the EPA’s approval of the
contingency measures included in Arizona’s Five Percent
Plan was contrary to 42 U.S.C. § 7502(c)(9) because the
measures had already been implemented. We uphold the
EPA’s determination that the control measures in Arizona’s
Five Percent Plan did not need to be updated, and that the 135
exceedances were exceptional events that are excluded from
consideration under the EPA’s regulation and guidance
documents. We do not defer to the EPA’s interpretation of
the contingency measures requirement, however, because
under the plain language of § 7502(c)(9) contingency
measures are measures that will be taken in the future, not
measures that have already been implemented.

                              I

    We begin by briefly describing the statutory framework.
The Clean Air Act (CAA), 42 U.S.C. § 7401 et seq.,
establishes “cooperative Federal, State, regional, and local
programs to prevent and control air pollution,” id.
§ 7401(a)(4). Under the CAA, the EPA is required to
“publish . . . a list which includes each air pollutant . . .
emissions of which, in [the EPA’s] judgment, cause or
contribute to air pollution which may reasonably be
anticipated to endanger public health or welfare.” Id.
§ 7408(a)(1). The EPA is then required to “prescrib[e] a
national primary ambient air quality standard” (NAAQS) for
that pollutant. Id. § 7409(a).

   One such harmful air pollutant is “PM-10,” defined as
“particulate matter with an aerodynamic diameter less than or
6                    BAHR V. U.S. EPA

equal to a nominal ten micrometers.” Id. § 7602(t).
According to the EPA, “PM-10 causes adverse health
effects by penetrating deep into the lungs, aggravating
the cardiopulmonary system.” Approval and Promulgation
of Implementation Plans—Maricopa County PM-10
Nonattainment Area, 79 Fed. Reg. 7118, 7118 (Feb. 6, 2014).
The EPA established a NAAQS for PM-10 of 150
micrograms per cubic meter, averaged over a 24-hour period.
40 C.F.R. § 50.6(a). This standard, which is sometimes
referred to as the “24-hour PM-10 standard,” is “attained
when the expected number of days per calendar year with a
24-hour average concentration above 150 ìg/m³ . . . is equal
to or less than one.” Id.

     The CAA provides that “[e]ach State shall have the
primary responsibility for assuring air quality” within the
state “by submitting an implementation plan” explaining how
the state will meet and maintain the NAAQS and other
standards. 42 U.S.C. § 7407(a). An area within a state that
does not meet a NAAQS is designated as a “nonattainment”
area, id. § 7407(d). Each state’s implementation plan (called
a State Implementation Plan or SIP) must provide for the
“implementation, maintenance, and enforcement” of the
NAAQS. Id. § 7410(a)(1). The CAA requires each SIP for
a nonattainment area to contain specified information,
including a requirement for reasonable further progress, id.
§ 7502(c)(2), an emissions inventory, id. § 7502(c)(3), and a
list of “contingency measures” to “be undertaken if the area
fails to make reasonable further progress, or to attain the
                          BAHR V. U.S. EPA                            7

national primary ambient air quality standard by the
attainment date applicable under this part,” id. § 7502(c)(9).1

   The CAA sets out a series of deadlines for states to meet
the NAAQS for PM-10, with increasingly stringent
requirements if a state misses a deadline. Id. §§ 7513–7513b.
The sequence is as follows:

    A nonattainment area is initially designated as a
“moderate” area. Id. § 7513(a). A SIP for a “moderate” PM-
10 nonattainment area must explain how that area will meet
the PM-10 NAAQS by the “attainment date,” which for
nonattainment areas designated by Congress was no later than
December 31, 1994. Id. § 7513(c)(1). The SIP must “assure
that reasonably available control measures for the control of
PM-10” are implemented. Id. § 7513a(a).

    If a moderate nonattainment area fails to meet the PM-10
NAAQS by the attainment date, the EPA must reclassify it as
a “Serious PM-10 nonattainment area.” Id. § 7513(b). After
redesignation, the state must submit a SIP that demonstrates
how the area will meet the PM-10 NAAQS within 10 years of


   1
       42 U.S.C. § 7502(c)(9) states:

          (9) Contingency measures

          Such plan shall provide for the implementation of
          specific measures to be undertaken if the area fails to
          make reasonable further progress, or to attain the
          national primary ambient air quality standard by the
          attainment date applicable under this part. Such
          measures shall be included in the plan revision as
          contingency measures to take effect in any such case
          without further action by the State or the Administrator.
8                          BAHR V. U.S. EPA

the original nonattainment designation, or, for areas originally
designated as nonattainment by Congress, no later than
December 31, 2001. Id. § 7513(c)(2). A SIP for a serious
nonattainment area must also “assure that the best available
control measures [BACM] for the control of PM-10 shall be
implemented.” Id. § 7513a(b).2

    If a state fails to meet the deadline for bringing a Serious
Area into compliance, the EPA may grant the state a single
five-year extension of the deadline to meet the NAAQS for
PM-10, but only if the state submits a SIP that “includes the
most stringent measures [MSM] that are included in the
implementation plan of any State or are achieved in practice
in any State, and can feasibly be implemented in the area.”
Id. § 7513(e).3


    2
        42 U.S.C. § 7513a(b) provides:

           (b) Serious Areas

           (1) Plan provisions. In addition to the provisions
           submitted to meet the requirements of paragraph [1]
           (a)(1) (relating to Moderate Areas), each State in which
           all or part of a Serious Area is located shall submit an
           implementation plan for such area that includes each of
           the following: . . .

           (B) Provisions to assure that the best available control
           measures for the control of PM-10 shall be
           implemented no later than 4 years after the date the area
           is classified (or reclassified) as a Serious Area.
    3
        42 U.S.C. § 7513(e) provides in pertinent part:

           (e) Extension of attainment date for Serious Areas

           Upon application by any State, the Administrator may
                           BAHR V. U.S. EPA                           9

    If a Serious Area fails to achieve compliance by the
attainment date after receiving the one-time five-year
extension under § 7513(e), the CAA requires the state to
“submit within 12 months after the applicable attainment
date, plan revisions which provide for attainment of the PM-
10 air quality standard.” Id. § 7513a(d).4 The SIP revisions



          extend the attainment date for a Serious Area beyond
          the date specified under subsection (c) of this section,
          if attainment by the date established under subsection
          (c) of this section would be impracticable, the State has
          complied with all requirements and commitments
          pertaining to that area in the implementation plan, and
          the State demonstrates to the satisfaction of the
          Administrator that the plan for that area includes the
          most stringent measures that are included in the
          implementation plan of any State or are achieved in
          practice in any State, and can feasibly be implemented
          in the area. . . . The Administrator may not approve an
          extension until the State submits an attainment
          demonstration for the area. The Administrator may
          grant at most one such extension for an area, of no
          more than 5 years.
   4
       42 U.S.C. § 7513a(d) states:

          (d) Failure to attain

          In the case of a Serious PM-10 nonattainment area in
          which the PM-10 standard is not attained by the
          applicable attainment date, the State in which such area
          is located shall, after notice and opportunity for public
          comment, submit within 12 months after the applicable
          attainment date, plan revisions which provide for
          attainment of the PM-10 air quality standard and, from
          the date of such submission until attainment, for an
          annual reduction in PM-10 or PM-10 precursor
          emissions within the area of not less than 5 percent of
10                     BAHR V. U.S. EPA

must provide for an annual five percent reduction in PM-10
within the Serious Area from the date the SIP revision was
submitted to the EPA until the state attains the NAAQS in
that area. Id.

     States are required to conduct ambient air quality
monitoring to determine whether a geographical region or
area in the state is meeting the NAAQS for PM-10. Id.
§ 7410(a)(2)(B)(i). State air quality monitoring systems must
use the criteria and methodology established by the EPA. Id.
§ 7619(a). Congress recognized that air quality monitoring
data could be affected by exceptional events that could not
reasonably be controlled by the states, and directed the EPA
to promulgate regulations “governing the review and
handling of air quality monitoring data influenced by
exceptional events.” Id. § 7619(b)(2)(A). The statute defines
an “exceptional event” as an event that “(i) affects air quality;
(ii) is not reasonably controllable or preventable; (iii) is an
event caused by human activity that is unlikely to recur at a
particular location or a natural event; and (iv) is determined
by the Administrator through the process established in the
regulations promulgated under paragraph (2) to be an
exceptional event.” Id. § 7619(b)(1)(A).

    Pursuant to this direction, the EPA promulgated the
“Exceptional Events Rule,” 40 C.F.R. § 50.14. The rule
repeats the statute’s definition of “exceptional event,” id.




        the amount of such emissions as reported in the most
        recent inventory prepared for such area.
                            BAHR V. U.S. EPA                            11

§ 50.1(j),5 and allows a state to “request EPA to exclude data
showing exceedances or violations of the national ambient air
quality standard that are directly due to an exceptional event,”
id. § 50.14(a)(1). In order to obtain EPA approval to exclude
exceptional event data, a state must provide evidence that
“[t]he event satisfies the criteria set forth in 40 C.F.R 50.1(j)”
and meets other criteria. Id. § 50.14(c)(3)(iv). If a state
makes the required showing, the “EPA shall exclude [the
exceptional event] data from use in determinations of
exceedances and NAAQS violations.” Id. § 50.14(b)(1).

    The EPA has recognized that PM-10 levels can be
affected by natural events such as dust storms, see id.
§ 50.1(j)–(k), and has therefore developed guidance for
applying the Exceptional Events Rule to high wind events.
See Treatment of Data Influenced by Exceptional Events,
72 Fed. Reg. 13560 (Mar. 22, 2007) (Treatment of Data
Guidance). The Treatment of Data Guidance states that
increased particulate matter concentrations “raised by
unusually high winds will be treated as due to uncontrollable
natural events where (1) the dust originated from
nonanthropogenic sources, or (2) the dust originated from


    5
        40 C.F.R. § 50.1(j) provides:

           (j) Exceptional event means an event that affects air
           quality, is not reasonably controllable or preventable, is
           an event caused by human activity that is unlikely to
           recur at a particular location or a natural event, and is
           determined by the Administrator in accordance with
           40 CFR 50.14 to be an exceptional event. It does not
           include stagnation of air masses or meteorological
           inversions, a meteorological event involving high
           temperatures or lack of precipitation, or air pollution
           relating to source noncompliance.
12                       BAHR V. U.S. EPA

anthropogenic sources within the State, that are determined
to have been reasonably well-controlled at the time that the
event occurred, or from anthropogenic sources outside the
State.” Id. at 13576 (emphasis added).

   In May 2013, the EPA published additional “guidance
and interpretation” explaining how the Exceptional Events
Rule and the Treatment of Data Guidance applies to high
wind events. See EPA, Interim Guidance on the Preparation
of Demonstrations in Support of Requests to Exclude
Ambient Air Quality Data Affected by High Winds Under the
Exceptional Events Rule (May 2013) (Interim Guidance).6
The Interim Guidance addresses when an anthropogenic
source within the State is “reasonably well-controlled at the
time that the event occurred,” 72 Fed. Reg. at 13576, as
required by the Treatment of Data Guidance.

                                    II

    We now provide the background of this case. Congress
designated Maricopa County, Arizona, as a “moderate” PM-
10 nonattainment area in 1990. 42 U.S.C. § 7407(d)(4)(B);
PM10 Group I and Group II Areas, 52 Fed. Reg. 29383,
29384 (Aug. 7, 1987). The designated nonattainment area,
termed the “Maricopa County PM-10 Nonattainment Area”
(Maricopa Area), covers the eastern portion of Maricopa
County, including the cities of Phoenix, Mesa, Scottsdale,

     6
       The Interim Guidance is available at: https://www.epa.gov/
sites/production/files/2015-09/documents/exceptevents_highwinds_
guide_130510.pdf; see also Draft Guidance To Implement Requirements
for the Treatment of Air Quality Monitoring Data Influenced by
Exceptional Events, 77 Fed. Reg. 39959, 39960 (July 6, 2012)
(announcing the availability of a draft version of the Interim Guidance on
the EPA’s website).
                     BAHR V. U.S. EPA                      13

Tempe, Chandler, and Glendale. 79 Fed. Reg. at 7118. It
also covers unincorporated parts of Maricopa County and
portions of Pinal County. Id.

    Because the Maricopa Area was designated as a
nonattainment area by Congress, Arizona’s first deadline for
meeting the NAAQS for PM-10 was December 31, 1994.
42 U.S.C. § 7513(c). The Maricopa Area was not in
attainment by 1994, so the EPA reclassified the Maricopa
Area as a Serious PM-10 nonattainment area. 42 U.S.C.
§ 7513(b)(2); Clean Air Act Reclassification; Arizona-
Phoenix Nonattainment Area; PM-10, 61 Fed. Reg. 21372,
21373 (May 10, 1996). This required Arizona to submit a
SIP that would demonstrate how the Maricopa Area would
meet the NAAQS for PM-10 by December 31, 2001, see
42 U.S.C. § 7513(c)(2), and to explain how it would
implement the best available control measures for PM-10, id.
§ 7513a(b).

    The Maricopa Area did not meet the NAAQS for PM-10
by the end of 2001. Rather, in 2000 Arizona preemptively
applied for a five-year extension (until December 2006) under
§ 7513(e) and at the same time submitted a SIP for the
Maricopa Area (the 2000 SIP). The 2000 SIP proposed to
implement over 70 best available control measures for major
dust sources, Approval and Promulgation of Implementation
Plans; Arizona—Maricopa County PM-10 Nonattainment
Area; Serious Area Plan for Attainment of the Annual PM-10
Standard, 65 Fed. Reg. 19964, 19972–83 (Apr. 13, 2000), and
stated that its measures were “the most stringent measures
that are included in the implementation plan of any State, or
are achieved in practice in any State,” id. at 19984. In 2002,
the EPA issued a final rule approving the 2000 SIP and
granting the requested extension. Approval and Promulgation
14                       BAHR V. U.S. EPA

of Implementation Plans; Arizona–Maricopa County PM-10
Nonattainment Area; Serious Area Plan for Attainment of the
PM-10 Standards, 67 Fed. Reg. 48718, 48718–19 (July 25,
2002) (the 2002 Final Rule). The 2002 Final Rule stated that
the control measures in the 2000 SIP met the BACM and
MSM standards. Id.7

    By December 2006, the Maricopa Area had still failed to
meet the NAAQS for PM-10. See Findings of Failure To
Attain; State of Arizona, Phoenix Nonattainment Area; State
of California, Owens Valley Nonattainment Area; Particulate
Matter of 10 Microns or Less, 72 Fed. Reg. 31183, 31184–85
(June 6, 2007). At that point, the CAA gave Arizona 12
months to submit revisions to the SIP that would achieve
attainment of the NAAQS for PM-10, provide for an annual
five percent reduction in PM-10 in the Maricopa Area,
42 U.S.C. § 7513a(d), and contain appropriate contingency
measures, id. § 7502(c)(9).

    Arizona submitted revisions to the Maricopa Area SIP in
December 2007 (the 2007 SIP). In addition to proposing 53
control measures, the 2007 SIP proposed revising a
previously approved agricultural control measure, namely an
agricultural general permit specifying best management
practices for reducing PM-10 from agricultural activities. See
Approval and Promulgation of Implementation Plans—
Maricopa County (Phoenix) PM-10 Nonattainment Area;

     7
      We considered a challenge to EPA’s 2002 Final Rule approving the
2000 SIP and held that the EPA’s approval of Arizona’s rejection of a
measure requiring the use of a reformulated diesel fuel as BACM was
arbitrary and capricious. Vigil v. Leavitt, 381 F.3d 826, 841–46 (9th Cir.
2004). We upheld the rest of the EPA’s approval, including its approval
of Arizona’s general permit rule for agricultural emissions of PM-10 as
BACM. Id. at 836–38, 847.
                      BAHR V. U.S. EPA                       15

Serious Area Plan for Attainment of the 24-Hour PM-10
Standard; Clean Air Act Section 189(d), 75 Fed. Reg. 54806,
54810, 54812–13 (Sept. 9, 2010) (the 2010 Proposed Rule).
The 2010 Proposed Rule stated that the EPA would
disapprove this revision to the agricultural general permit rule
on the ground that other states and local agencies had
“acquired additional expertise about how to control emissions
from these sources,” and as a result the EPA no longer
believed that the requirements in the agricultural general
permit rule in the 2000 SIP were best available control
measures. Id. To avoid a partial disapproval, Arizona
withdrew the plan in 2011. 79 Fed. Reg. at 7119. As a result
of this withdrawal of the 2007 SIP, the EPA found that
Arizona had failed to make a required SIP submittal. Finding
of Failure To Submit State Implementation Plan Revisions for
Particulate Matter, PM-10, Maricopa County (Phoenix) PM-
10 Nonattainment Area, AZ, 76 Fed. Reg. 8300, 8300–01
(Feb. 14, 2011). This finding required Arizona to submit
another SIP by 2013. Id. at 8301; 42 U.S.C. § 7509(a).

    Arizona submitted a new SIP revision on May 25, 2012.
See 79 Fed. Reg. at 7119. Because this SIP was prepared
pursuant to § 7513a(d), which requires a state to achieve a
five percent annual reduction in PM-10, we adopt the EPA’s
term and refer to it as the “Five Percent Plan.” The Five
Percent Plan proposed to achieve the five percent annual
reduction required by § 7513a(d) by implementing many of
the 53 control measures previously proposed in the 2007 SIP,
as well as adopting a new emissions control measure, the
“Dust Action General Permit.” Unlike the 2007 SIP, the Five
Percent Plan did not propose any changes to the agricultural
general permit.
16                      BAHR V. U.S. EPA

    As required by § 7502(c)(9), the Five Percent Plan
proposed a number of contingency measures. 79 Fed. Reg. at
7124. Four of the five measures were permanent changes to
infrastructure that had been completed in the years 2008
through 2011, namely, paving existing dirt roads and alleys,
paving and stabilizing unpaved shoulders, repaving or
overlaying paved roads with rubberized asphalt, and lowering
speed limits on dirt roads and alleys. Id. The fifth
contingency measure required the purchase of PM-10
certified sweepers (which had already been accomplished by
the end of 2009), and ongoing sweeping of ramps, freeways,
and frontage roads. Id.

    Arizona also acknowledged that there had been a number
of exceedances of the 24-hour PM-10 standard during 2011
and 2012, but claimed they should be deemed “exceptional
events” and excluded from a determination of whether the
Maricopa Area met the NAAQS for PM-10. In support of
this claim, Arizona submitted documentation to the EPA to
demonstrate that 137 exceedances of the NAAQS for PM-10
on 27 days during the period from 2011 to 2012 were the
result of “exceptional events,” namely high wind dust events.
79 Fed. Reg. at 7122.8 The EPA wrote separate reports on
each of Arizona’s submissions. The reports analyzed the data
and concluded that the flagged exceedances met “the
definition of an exceptional event: the exceedances affected
air quality, were not reasonably controllable or preventable,
and meet the definition of a natural event.”

   In February 2014, the EPA published its proposed
decision regarding Arizona’s Five Percent Plan. 79 Fed. Reg.

    8
      Though the EPA’s proposed rulemaking mentioned 133
exceedances, the parties agree that Arizona requested approval of 137.
                      BAHR V. U.S. EPA                        17

at 7118. The EPA noted that the monitoring data for the
Maricopa Area showed 133 exceedances of the 24-hour PM-
10 NAAQS during the 2011–2012 time period, but stated that
131 of those exceedances “were caused by high wind
exceptional events,” and “should not be used for regulatory
purposes,” including for evaluation of the Five Percent Plan.
Id. at 7122. Excluding these exceedances, the EPA proposed
to determine that the Maricopa Area had attained the NAAQS
for 24-hour PM-10 by December 31, 2012. Id. at 7125. The
EPA also proposed to approve Arizona’s contingency
measures. Id. at 7124. In doing so, the EPA explained that
it had previously “interpreted [42 U.S.C. § 7502(c)(9)] to
allow states to implement contingency measures before they
are triggered by a failure of . . . attainment as long as those
measures are intended to achieve emission reductions over
and beyond those relied on in the attainment and [reasonable
further progress] demonstrations.” 79 Fed. Reg. at 7124.

    The EPA issued a final rule on June 10, 2014, approving
the Five Percent Plan “as meeting all relevant statutory and
regulatory requirements.” Approval and Promulgation of
Implementation Plans—Maricopa County PM-10
Nonattainment Area; Five Percent Plan for Attainment of the
24-Hour PM-10 Standard, 79 Fed. Reg. 33107, 33107 (June
10, 2014) (2014 Final Rule). The 2014 Final Rule stated that
the EPA was excluding 135 exceedances9 from the
monitoring data as exceptional events. Id. at 33111. For each
of the events that EPA determined was exceptional, “EPA
found that the event was not reasonably controllable or
preventable,” and that “reasonable controls” were in place for


    9
      The EPA’s final rule clarified that the number of approved
exceedances was 135, which occurred on 25 days over the period of
2010–12. 79 Fed. Reg. at 33110–11.
18                   BAHR V. U.S. EPA

anthropogenic sources of dust. Id. The EPA reached its
conclusion that “reasonable controls” were in place by relying
on its 2002 Final Rule approving Arizona’s 2000 SIP as
including best available control measures and most stringent
control measures. Id. at 33112. The EPA also determined
that Arizona sufficiently demonstrated that dust sources
outside the Maricopa Area were reasonably controlled. Id. at
33113.

    The 2014 Final Rule included the EPA’s response to
comments made by petitioners Sandra Bahr and David
Matusow, two residents of Phoenix. First, the EPA addressed
petitioners’ argument that the EPA should have required
Arizona to update its control measures to ensure that they
were BACM and MSM, rather than letting Arizona rely on
the EPA’s approval of the 2000 SIP. 79 Fed. Reg. at
33108–10. The EPA rejected this claim, explaining that “the
requirement for BACM is triggered by a specific event: The
reclassification of a moderate PM-10 nonattainment area to
serious.” Id. at 33108. Likewise, the EPA explained that
§ 7513(e) provides “that the requirement for MSM is
triggered by a particular event: EPA’s granting of a state’s
request for an extension of the attainment deadline for a
serious nonattainment area.” Id. The EPA then stated that
§ 7513(d) (the section requiring the submission of a five
percent plan) “does not contain a specific requirement that the
state update the previously approved requirements for BACM
and MSM as a consequence of failing to reach attainment by
the applicable deadline for serious PM-10 nonattainment
areas.” Id. at 33109. Because there was no statutory trigger
requiring Arizona to update its demonstration that its control
measures were best available and most stringent, the EPA
explained, the Five Percent Plan could rely on the EPA’s
approval of BACM and MSM in the 2000 SIP. Id. The EPA
                     BAHR V. U.S. EPA                      19

also disagreed with petitioner’s comment that the Five
Percent Plan was inadequate because the agricultural control
measures were not BACM, as indicated by the EPA’s 2010
Proposed Rule disapproving of Arizona’s proposed revision
to its agricultural general permit. In response to this
comment, the EPA stated that the Five Percent Plan satisfied
the requirements in § 7513a(d) “without relying on additional
emissions reductions from agricultural sources.” Id. at
33109.

    Second, the EPA addressed petitioners’ argument that its
determination that the 135 exceedances constituted
exceptional events was contrary to the Interim Guidance.
Petitioners interpreted the Interim Guidance as preventing the
EPA from concurring that best available control measures
were in place unless the EPA had determined control
measures for windblown dust to be BACM within the past
three years. 79 Fed. Reg. at 33111–12. The EPA disagreed
with this interpretation and concluded that Arizona’s controls
were reasonable. Id. at 33112. The EPA also rejected
petitioners’ argument that Arizona had failed to provide an
adequate description of upwind sources and control measures
as required by the Interim Guidance. Id. at 33113–14.

     Finally, the EPA dismissed petitioners’ comment that the
EPA erred in accepting Arizona’s “contingency measures” in
its Five Percent Plan because the measures had already been
implemented. Id. at 33114–15. The petitioners argued that
because § 7502(c)(9) requires contingency measures that “are
automatically and immediately implemented if a milestone
for reasonable further progress or attainment is not met,” the
previously implemented measures in the Five Percent Plan
could not qualify. Id. at 33114. In rejecting this comment,
the EPA stated it interpreted § 7502(c)(9) as requiring only
20                   BAHR V. U.S. EPA

that “[c]ontingency measures must provide for additional
emission reductions” that were not otherwise included “in the
attainment demonstration,” and that “[n]othing in the statute
precludes a state from implementing such measures before
they are triggered,” relying on the Fifth Circuit decision in
Louisiana Environmental Action Network v. EPA, 382 F.3d
575 (5th Cir. 2004). 79 Fed. Reg. at 33114.

   The plaintiffs filed a petition for review of the 2014 Final
Rule on July 29, 2014.

                              III

    Under 42 U.S.C. § 7607(b)(1), we have jurisdiction over
“[a] petition for review of the Administrator’s action in
approving or promulgating any implementation plan.”

    In reviewing a challenge to the EPA’s approval of a SIP
under § 7607(b)(1), we apply “the general standard of review
for agency actions set forth in the Administrative Procedure
Act (APA).” Latino Issues Forum v. EPA, 558 F.3d 936, 941
(9th Cir. 2009); see also Vigil v. Leavitt, 381 F.3d 826, 833
(9th Cir. 2004). Under the APA, we must uphold an agency
action unless it is “arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with law.”
5 U.S.C. § 706(2)(A). This standard is “highly deferential,
presuming the agency action to be valid and affirming the
agency action if a reasonable basis exists for its decision.”
Ranchers Cattlemen Action Legal Fund United Stockgrowers
of Am. v. U.S. Dep’t of Agric., 499 F.3d 1108, 1115 (9th Cir.
2007). Generally, “[a]n agency decision will be upheld as
long as there is a rational connection between the facts found
and the conclusions made.” Barnes v. U.S. Dep’t of Transp.,
655 F.3d 1124, 1132 (9th Cir. 2011). We will deem an
                     BAHR V. U.S. EPA                       21

agency action to be arbitrary and capricious only “if the
agency has relied on factors which Congress has not intended
it to consider, entirely failed to consider an important aspect
of the problem, offered an explanation for its decision that
runs counter to the evidence before the agency, or is so
implausible that it could not be ascribed to a difference in
view or the product of agency expertise.” Motor Vehicle
Mfrs. Ass’n of the U.S. v. State Farm Mut. Auto. Ins. Co.,
463 U.S. 29, 43 (1983). Where the question presented for
review is a factual dispute which implicates “a high level of
technical expertise” we defer to “the informed discretion of
the responsible federal agencies.” Kleppe v. Sierra Club,
427 U.S. 390, 412 (1976). “Even when an agency explains its
decision with ‘less than ideal clarity,’ a reviewing court will
not upset the decision on that account ‘if the agency's path
may reasonably be discerned.’” Alaska Dep’t of Envtl.
Conservation v. EPA, 540 U.S. 461, 497 (2004) (quoting
Bowman Transp., Inc. v. Ark.-Best Freight Sys., Inc.,
419 U.S. 281, 286 (1974)).

     Where the petitioner challenges the agency’s action as
inconsistent with the agency’s own policies, we examine
whether the agency has actually departed from its policy and,
if so, whether the agency has offered a reasoned explanation
for such departure. See Encino Motorcars, LLC v. Navarro,
136 S. Ct. 2117, 2125–26 (2016). Generally, “[a]gencies are
free to change their existing policies as long as they provide
a reasoned explanation for the change.” Id. at 2125. In
contrast, where the agency is not offering a policy
explanation but is instead interpreting a binding regulation,
the agency’s interpretation is “controlling” unless “plainly
erroneous or inconsistent with the regulation.” Auer v.
Robbins, 519 U.S. 452, 461 (1997).
22                   BAHR V. U.S. EPA

    Where the agency’s action is an interpretation of a statute
that the agency administers, “we follow the two-step
approach set out in Chevron U.S.A. Inc. v. Natural Resources
Defense Council, Inc., 467 U.S. 837 (1984).” Latino Issues
Forum, 558 F.3d at 941. First, “if Congress has ‘directly
spoken to the precise question at issue,’ then the matter is
capable of but one interpretation by which the court and the
agency must abide.” Vigil, 381 F.3d at 834 (quoting
Chevron, 467 U.S. at 842).

    At the second step of Chevron, if we determine that
Congress was silent on the issue, or the statute is subject to
multiple interpretations, we must determine the degree of
deference to give the agency’s interpretation of a statute.
United States v. Mead Corp., 533 U.S. 218, 227–29 (2001).
“Not all agency statutory interpretations are entitled to
Chevron deference.” Sierra Club v. EPA, 671 F.3d 955, 962
(9th Cir. 2012). “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.’” Id. (quoting Wilderness Soc’y v. U.S. Fish &
Wildlife Serv., 353 F.3d 1051, 1067 (9th Cir. 2003) (en
banc)). We generally deem Congress to have delegated such
authority when it authorizes the agency to engage in notice-
and-comment rulemaking. Mead, 533 U.S. at 229–30. In
such circumstances, a court should accept the agency’s
interpretation “if Congress has not previously spoken to the
point at issue and the agency’s interpretation is reasonable.”
Id. at 229.

    Though we have previously applied Chevron deference to
the EPA’s interpretation of the CAA issued in connection
with a SIP approval, see Association of Irritated Residents v.
                     BAHR V. U.S. EPA                      23

EPA, 686 F.3d 668, 679–81 (9th Cir. 2011), we have never
expressly held that such deference is appropriate, but rather
have held the question open, see Vigil, 381 F.3d at 835
(declining to address “whether the EPA’s interpretation of the
Act in the course of approving Arizona’s SIP is entitled to
Chevron deference”). But because “[a] very good indicator
of delegation meriting Chevron treatment [is] express
congressional authorizations to engage in the rulemaking or
adjudication process that produces the regulations or rulings
for which deference is claimed,” Mead Corp., 533 U.S. at
229, and because the EPA engages in such a rulemaking
process in approving a SIP, we agree with the Fifth Circuit
that “EPA’s final rules approving the [Arizona] SIP, to the
extent they involve the reasonable resolution of ambiguities
in the CAA, will be afforded Chevron deference,” BCCA
Appeal Grp. v. EPA, 355 F.3d 817, 825 (5th Cir. 2003).

                             IV

    On appeal, petitioners argue that the EPA acted contrary
to law by failing to require that Arizona include an updated
analysis of best available control measures and most stringent
measures in the Five Percent Plan. They also argue that the
EPA abused its discretion by excluding 135 exceedances
from the monitoring data as “exceptional events.” Finally,
they argue that the EPA violated the CAA by allowing
Arizona to satisfy the “contingency measures” requirement
with previously implemented control measures. We consider
each argument in turn.

                              A

   We first consider petitioners’ argument that the EPA’s
approval of the Five Percent Plan constituted an abuse of
24                   BAHR V. U.S. EPA

discretion because the EPA did not require Arizona to
demonstrate that the plan included best available control
measures (BACM) or most stringent measures (MSM) for the
control of PM-10. According to petitioners, the Five Percent
Plan must demonstrate that Arizona’s control measures meet
the BACM and MSM standards because Arizona has a
continuing obligation to comply with § 7513a(b)(1)(B) and
§ 7513(e), which set the control requirements for serious
nonattainment areas. Petitioners argue that had a BACM
demonstration been required, the Five Percent Plan would
have failed because the agricultural control measures in
Arizona’s 2000 SIP are no longer BACM, as stated in EPA’s
2010 Proposed Rule. Further, petitioners argue, the EPA’s
statement in the 2010 Proposed Rule regarding Arizona’s
failure to meet the BACM standard is evidence that the EPA
generally requires an updated BACM demonstration in each
SIP, meaning that the EPA acted inconsistently by not
requiring such a demonstration in the Five Percent Plan.

    We disagree with these arguments. The EPA’s decision
not to require an updated demonstration of BACM and MSM
in the Five Percent Plan was not an abuse of discretion
because it was not contrary to any language in the CAA.
Section 7513a(d), which governs five percent plans such as
the one before us here, provides only that if a state fails to
achieve attainment after receiving the five-year extension, it
must submit plan revisions providing for “attainment of the
PM-10 air quality standard,” and an annual five percent
reduction in PM-10 within the Serious Area. It does not
mention BACM or MSM. The CAA sections that do require
BACM and MSM demonstrations do not expressly apply to
a five percent plan. Section § 7513a(b) provides that when
the EPA reclassifies a Moderate Area as a Serious Area, the
state must then submit a SIP that “assure[s] that the best
                     BAHR V. U.S. EPA                       25

available control measures for the control of PM-10 shall be
implemented.” It does not require the state to update that
assurance when submitting a five percent plan, nor does it
require the EPA to review its previous BACM determination.
Similarly, § 7513(e) provides that when the EPA grants a
state a five-year extension of the deadline to meet the
NAAQS for PM-10, the state must submit a SIP that
“includes the most stringent measures” that are included in
any SIP or achieved in any state. Again, this language does
not require the state to include an updated demonstration of
MSM when submitting a five percent plan.

    We also disagree with petitioners’ argument that the EPA
acted inconsistently in failing to require an updated
demonstration of BACM and MSM in the Five Percent Plan.
Even though the EPA had previously reviewed and proposed
to disapprove the agricultural control measure in Arizona’s
2007 SIP as not meeting the BACM standard, the EPA
provided a reasonable interpretation of its approach in the
2014 Final Rule. According to the EPA, the CAA lays out a
series of escalating control measures that are triggered by a
finding of noncompliance with a series of statutory
requirements. The EPA will assess compliance with the
control measures that were triggered at each step, but will not
reassess compliance with those measures at subsequent steps
unless a state proposes changes to control measures that were
previously approved. 79 Fed. Reg. at 33108–09. This
approach is consistent with the CAA, which does not require
the EPA to reassess a state’s controls in each SIP submission.
Here, the EPA’s 2010 Proposed Rule reassessed Arizona’s
agricultural control measures under the BACM standard
because Arizona proposed to revise agricultural control
measures that were previously approved as BACM in the
2002 Final Rule. Arizona did not propose any revisions to its
26                   BAHR V. U.S. EPA

agricultural controls in the Five Percent Plan, however, so
EPA did not have any occasion to reevaluate those measures.
Given the EPA’s reasonable explanation for its approach, and
the lack of any contrary statutory command in the CAA, we
conclude that the EPA did not abuse its discretion or act
contrary to law by declining to require an updated
demonstration of BACM or MSM in the Five Percent Plan.

    Petitioners also argue that the EPA acted in an arbitrary
and capricious manner in reviewing the Five Percent Plan
because it evaluated Arizona’s compliance with CAA
requirements regarding emission inventories, reasonable
further progress, and contingency measures, but ignored
BACM and MSM. We also reject this argument. The EPA
reasonably explained that those particular CAA requirements
are procedural or otherwise applicable to all SIP submissions,
see 42 U.S.C. §§ 7410(a), 7502(c), 7506(c), 7513a(c)(1), and
the EPA reviews such measures whenever it reviews a
proposed SIP.       By contrast, the BACM and MSM
requirements are not applicable to all SIP submissions, and so
the EPA reviews them only when the state is required to
demonstrate compliance with these requirements.

                              B

    We next turn to the petitioners’ argument that the EPA
acted contrary to law by excluding 135 exceedances in the
Maricopa Area from Arizona’s air quality monitoring data.

    Petitioners raise several arguments as to why the EPA
erred in concluding that the dust sources causing the 135
exceedances were from anthropogenic sources that were
“reasonably well-controlled,” and therefore were excludable
as exceptional events. See 40 C.F.R. § 50.14(b)(1). The
                     BAHR V. U.S. EPA                       27

petitioners begin by pointing to the statement in the Interim
Guidance that “[g]enerally, the EPA will consider windblown
dust BACM to constitute reasonable controls if these
measures have been reviewed and approved in the context of
a SIP revision for the emission source area within the past
three years.” Interim Guidance at 15. The EPA’s decision
was inconsistent with this guidance, petitioners argue, for two
reasons. First, Arizona’s dust control measures for the
Maricopa Area had not been approved since 2002, well over
three years before the 2014 Final Rule. Second, agricultural
emissions are among the sources of windblown dust in the
Maricopa Area, and the EPA’s 2010 Proposed Rule had
proposed to disapprove of Arizona’s agricultural control
measures because they were not BACM. Moreover,
petitioners argue, the EPA failed to offer a reasonable
explanation for its departure from its guidance.

    We disagree; the EPA’s 2014 Final Rule did not conflict
with the Interim Guidance. First, nothing in the Interim
Guidance indicates that EPA must find that control measures
for windblown dust have been reviewed and approved as
BACM within the past three years in order for the dust to be
deemed reasonably well-controlled. Rather, the Interim
Guidance gives the EPA flexibility to consider a wide range
of issues, and emphasizes that a prior BACM determination
“may be a reference point, but not the sole means, by which
the EPA assess the reasonableness of controls.” Interim
Guidance at 15. The EPA’s interpretation is therefore
consistent with the Interim Guidance and is a reasonable
interpretation of the Exceptional Events Rule, to which we
owe deference. Auer, 519 U.S. at 461.

   Second, the EPA provided a reasonable explanation as to
why the Maricopa Area had reasonable controls for
28                    BAHR V. U.S. EPA

windblown dust even though the 2010 Proposed Rule had
proposed to disapprove of Arizona’s agricultural control
measures. The EPA’s 2002 Final Rule determined that the
2000 SIP contained the best available control measures for
the highest emitters of PM-10 (including unpaved roads and
alleys, construction, paved road dust, and non-agricultural
windblown dust). 79 Fed. Reg. at 33111–13. The 2014 Final
Rule then explained that it was still appropriate to rely on that
determination because neither the highest emitters of PM-10
nor the techniques for controlling fugitive dust had changed
significantly since 2002. Id. at 33112. Moreover, Arizona
made its dust control rules even more stringent in the years
following its 2000 SIP, further bolstering the conclusion that
the controls remained reasonable. Id. Although the EPA had
proposed to disapprove of the agricultural controls as not
meeting the BACM standard in 2010, data showed that
agricultural sources were only a minimal contributor to the
Maricopa Area’s overall level of PM-10 emissions. Id.
Accordingly, the EPA’s judgment was that the control
measures in the 2000 SIP ensured that anthropogenic
windblown dust was reasonably controlled for purposes of the
Exceptional Events Rule.

    “[W]e generally must be at [our] most deferential when
reviewing scientific judgments and technical analyses within
the agency’s expertise.” Lands Council v. McNair, 629 F.3d
1070, 1074 (9th Cir. 2010) (second alteration in original)
(internal quotation marks omitted). Here, the EPA considered
the relevant factors and articulated a rational connection
between the facts found and the choice made. As a general
rule, a determination that particular control measures are
reasonable relies on technical considerations that are
                        BAHR V. U.S. EPA                            29

“properly left to the informed discretion of” the EPA.
Kleppe, 427 U.S. at 412. We defer to its conclusion here.10

                                  C

    Petitioners also claim that the EPA’s approval of
Arizona’s 135 exceedances as exceptional events violated the
Interim Guidance because the EPA failed to adequately
address the controls in upwind areas outside the Maricopa
Area. To qualify as an exceptional event under the
Exceptional Events Rule, an exceedance must be “caused by
human activity that is unlikely to recur at a particular location
or a natural event.” 40 C.F.R. §§ 50.1(j), 50.14. The
Treatment of Data Guidance states that high wind events will
be considered “natural” when “(1) the dust originated from
nonanthropogenic sources, or (2) the dust originated from
anthropogenic sources within the State, that are determined
to have been reasonably well-controlled at the time that the
event occurred, or from anthropogenic sources outside the
State.” 72 Fed. Reg. at 13576. To assist the EPA in
determining compliance with this requirement, the Interim
Guidance requires a state to provide “a brief description” of
“all contributing emission sources in upwind areas and
provide evidence that those sources were reasonably
controlled, whether anthropogenic or natural.” Interim
Guidance at 42.

    Petitioners argue that Arizona’s submissions were
inadequate because Arizona did not identify all contributing
emission sources outside the Maricopa Area, failed to


    10
       Because we conclude that the EPA did not depart from its Interim
Guidance, we do not reach the petitioners’ argument that the EPA failed
to provide a reasoned explanation for departing from its guidance.
30                   BAHR V. U.S. EPA

distinguish between natural and anthropogenic sources, and
failed to submit evidence that Pinal County had reasonable
controls in place.

    We again disagree. Under the Treatment of Data
Guidance, a high wind event may meet the criteria of the
Exceptional Events Rule when the dust originated from
nonanthropogenic sources, or from anthropogenic sources
that are reasonably well-controlled at the time that the event
occurred. 72 Fed. Reg. at 13576–77. Arizona explained that
the high wind events causing the 135 exceedances stemmed
from monsoonal dust storms. Because “outflow from
thunderstorms can carry dust over vast distances
encompassing many source areas,” Arizona could not clearly
distinguish between nonanthropogenic and anthropogenic
sources of dust. Nevertheless, Arizona adequately provided
a “brief description” of contributing dust sources outside the
Maricopa Area and demonstrated that reasonable controls
were in place for any anthropogenic sources of dust. For
instance, in describing exceedances that occurred during the
August 11, 2012, event, Arizona provided a “conceptual
model” identifying a nonanthropogenic source, the
“undeveloped lands south of Maricopa County,” as being the
primary contributing source areas in Pima and Pinal Counties.
Arizona also provided evidence that any anthropogenic
sources of dust in those areas were reasonably controlled,
pointing to two Pinal County rules applicable to fugitive dust
and construction sites. Arizona compiled similar submissions
for each of its exceptional event submittals. Arizona’s
submissions provided enough detail for the EPA to
reasonably conclude that the dust originated either from
“nonanthropogenic sources” or “anthropogenic sources” that
were “reasonably well-controlled.” Interim Guidance at 42.
Accordingly, the EPA’s conclusion that Arizona’s description
                     BAHR V. U.S. EPA                       31

of the upwind sources was adequate was not an abuse of
discretion.

    Petitioners also argue that the EPA ignored the Interim
Guidance and thus abused its discretion in concluding that the
anthropogenic dust sources in the areas of Pinal County
outside of the Maricopa Area were reasonably controlled.
The Interim Guidance states that “[f]or the anthropogenic
sources to be considered to be reasonably controlled, the EPA
anticipates that it is reasonable for an air agency to have the
controls required for an area’s attainment status.” Interim
Guidance at 15. “[T]he EPA does not expect areas classified
as attainment, unclassifiable, or maintenance for a NAAQS
to have the same level of controls as areas that are
nonattainment for the same NAAQS.” Id. In other words, an
area that is in attainment should have the control measures
appropriate for an attainment area, while an area that has been
designated a serious nonattainment area should have the
control measures appropriate for that level of classification.
Id. Where “an area has been recently designated to
nonattainment but has not yet been required to implement
controls, the EPA will expect the level of controls that is
appropriate for the planning stage.” Id.

    Under the Interim Guidance, the EPA did not abuse its
discretion in concluding that the anthropogenic dust sources
in the areas of Pinal County outside of the Maricopa Area
were reasonably well-controlled. The EPA provided a
reasoned explanation as to why it deemed the Pinal County
area to have the controls required for that area’s attainment
status. The 135 exceedances approved by the EPA all
occurred between 2010 and 2012. 79 Fed. Reg. at 33111.
From January 2011 to June 2012, Pinal County (excluding the
portion within the Maricopa Area) was an attainment area,
32                   BAHR V. U.S. EPA

and had adopted two rules addressing fugitive dust emissions
that were appropriate for this status. Although Pinal County
was redesignated a nonattainment area in July 2012, it was
not required to submit a SIP until 18 months after
redesignation, leaving it in the planning stage for the
remainder of 2012. See Designation of Areas for Air Quality
Planning Purposes; State of Arizona; Pinal County; PM10,
77 Fed. Reg. 32024, 32030 (May 31, 2012). Because Pinal
County had not yet been required to implement controls, the
EPA reasonably concluded that Pinal County’s fugitive dust
rules were appropriate for the planning stage.

                             V

    Finally, we turn to petitioners’ argument that the EPA
erred in approving the contingency measures in the Five
Percent Plan because those measures had already been
implemented. As noted above, four of five contingency
measures in the Five Percent Plan were permanent changes to
infrastructure that had been completed by 2012 (paving and
stabilizing existing public dirt roads and alleys, paving and
stabilizing unpaved shoulders, repaving or overlaying paved
roads with rubberized asphalt, and lowering speed limits on
dirt roads and alleys). The fifth contingency measure
involved the purchase of PM-10 certified sweepers and
sweeping of freeways, though the purchase occurred in 2009
and Arizona had procured contracts for sweeping services by
2010. The CAA provides that a nonattainment SIP:

       [S]hall provide for the implementation of
       specific measures to be undertaken if the area
       fails to make reasonable further progress, or
       to attain the national primary ambient air
       quality standard by the attainment date
                      BAHR V. U.S. EPA                         33

        applicable under this part. Such measures
        shall be included in the plan revision as
        contingency measures to take effect in any
        such case without further action by the State
        or the Administrator.

42 U.S.C. § 7502(c)(9). In its 2014 Final Rule, the EPA
explained its interpretation of this requirement. While
“[c]ontingency measures must provide for additional
emission reductions that are not relied on for RFP [reasonable
further progress] or attainment and that are not included in
the attainment demonstration,” the EPA concluded that
“[n]othing in the statute precludes a state from implementing
such measures before they are triggered.” 79 Fed. Reg. at
33114.

      Although we defer to the EPA’s interpretation of the
CAA contained in a final rule approving a SIP if that
interpretation involves the reasonable interpretation of
ambiguous statutory terms, see supra at 22, we cannot defer
to the EPA’s interpretation of § 7502(c)(9) here. Where
Congress has “directly spoken to the precise question at issue
. . . 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.” Chevron, 467 U.S. at 842–43. The
statutory language in § 7502(c)(9) is clear: it requires the SIP
to provide for the implementation of measures “to be
undertaken” in the future, triggered by the state’s failure “to
make reasonable further progress” or to attain the NAAQS.
These measures are included in the SIP as “contingency
measures” and are “to take effect” automatically in the future.
Although the statute does not define the word “contingency,”
the meaning of the term is not ambiguous. According to the
dictionary definition, it means “a possible future event or
34                    BAHR V. U.S. EPA

condition or an unforeseen occurrence that may necessitate
special measures.” Webster’s Third New International
Dictionary (2002). Because Congress was clear that
“contingency measures” are control measures that will be
implemented in the future, and the statutory language is not
susceptible to multiple interpretations, we must give effect to
its plain meaning. Chevron, 467 U.S. at 842–43.

    In arguing that its interpretation of § 7502(c) is entitled to
deference despite the clear language of the statute, the EPA
relies on the Fifth Circuit’s decision in Louisiana Envtl.
Action Network v. EPA, 382 F.3d 575, 580 (5th Cir. 2004).
In Louisiana Envtl. Action Network, the petitioners
challenged the EPA’s 2002 approval of Louisiana’s SIP
because the contingency measure in the SIP (a compressor
station’s permanent reduction of its emissions) had been
implemented in 1998, and therefore was not a measure “to be
undertaken” or “to take effect” in the future, as § 7502(c)(9)
requires. Id. at 582. The Fifth Circuit first acknowledged
that “a plain reading of the terms ‘to take effect’ and ‘to be
undertaken’ imply a prospective, forward looking
orientation” that would “preclude the use of past reductions
which have already failed to achieve attainment.” Id. at 583.
Nevertheless, the Fifth Circuit held that § 7502(c)(9) was
ambiguous because it “neither affirms nor prohibits
continuing emissions reductions—measures which originate
prior to the SIP failing, but whose effects continue to
manifest an effect after the plan fails—from being utilized as
a contingency measure.” Id. (emphasis omitted). Having
found this “ambiguity,” it deferred to the EPA’s interpretation
that “contingency measures” could include measures that had
already been implemented by the state.
                      BAHR V. U.S. EPA                        35

    We cannot agree with the Fifth Circuit’s interpretative
approach. Having determined that the “plain reading of the
terms” indicates a forward looking approach, the Fifth Circuit
was bound by Chevron to give effect to the plain meaning of
the statute. We disagree that the lack of any discussion in
§ 7502(c)(9) regarding treatment of continuing emissions
reductions makes the statute ambiguous. Rather, unless such
continuing emissions reductions are “to be undertaken” in the
event of a contingency, they do not fit the definition of
“contingency measures” provided in § 7502(c)(9). We also
disagree with the dissent’s contention that previously
implemented control measures that provide continuing
emissions reductions “take effect” and are “undertaken” both
“at the time they are first implemented but also thereafter.”
Dissent at 38. This is a misreading of the statute, which
defines contingency measures as measures “to be undertaken”
or “to take effect” if a future event occurs, namely “if the area
fails to make reasonable further progress, or to attain the
[NAAQS].” 42 U.S.C. § 7502(c)(9) (emphases added).
Control measures that have already been implemented are not
measures “to be undertaken” or “to take effect” in the future,
and the statute cannot reasonably be so interpreted.

    The EPA argues that its interpretation is consistent with
the CAA’s policy goals, because permitting early
implementation of contingency measures is consistent with
the overall policy of the CAA to reduce particulate emissions
and protect public health. La. Envtl. Action Network,
382 F.3d at 583. The Fifth Circuit likewise relied on these
policy considerations, stating that allowing states to
implement measures before the contingency occurs was
consistent with the CAA’s purpose of creating incentives for
states to reach NAAQS compliance earlier and more
efficiently. Id. at 583–84. The dissent agrees, adopting the
36                          BAHR V. U.S. EPA

Fifth Circuit’s policy analysis.11 Dissent at 39–40. Even if
we agreed that the EPA’s policy considerations are
compelling, such considerations cannot override the plain
language of the statute. We therefore cannot give them
controlling weight here.

    Because the “contingency measures” in Arizona’s SIP
were not “specific measures to be undertaken if the area fails
to make reasonable further progress, or to attain the national
primary ambient air quality standard by the attainment date
applicable under this part,” the EPA’s approval of this part of
the Five Percent Plan was contrary to the CAA. Accordingly,
we remand to the EPA for further consideration of this
portion of the SIP but otherwise deny the petition.12

  PETITION GRANTED IN PART AND DENIED IN
PART.




     11
       In addition to relying on the Fifth Circuit’s policy arguments, the
dissent argues that precluding the use of previously implemented controls
as contingency measures “imposes an additional and unnecessary burden
upon states where the failure to attain the NAAQS also triggers a bump up
of an area’s classification under the Act” because states will not be able
to “focus their efforts on implementing the newly imposed requirements.”
Dissent at 40. This is incorrect: because contingency measures
automatically take effect when the contingency occurs, “without further
action by the State,” 42 U.S.C. § 7502(c)(9), the implementation of
contingency measures cannot distract a state from meeting the other CAA
requirements.
     12
          Each party is to bear its own costs on appeal.
                      BAHR V. U.S. EPA                       37

CLIFTON, Circuit Judge, concurring in part and dissenting
in part:

    I fully concur in sections I–IV of the majority opinion. I
disagree, however, with the majority’s conclusion in section
V that EPA’s approval of the contingency measures in
Arizona’s SIP is contrary to the clear language of the CAA.
In my view, the scope of the CAA’s contingency measures
requirement is ambiguous and EPA’s reasonable
interpretation of that requirement is entitled to deference.

    Like the majority, I begin by analyzing the text of the
relevant statutory provision, 42 U.S.C. § 7502(c)(9). That
section of the CAA requires state nonattainment plans to
include contingency measures “to be undertaken” and “to
take effect” in the event that an area “fails to make reasonable
further progress, or to attain” the NAAQS by the applicable
attainment date. Id. Although I agree with the majority that
this language most often refers to measures that are to be
implemented in the future, in the event that the other
measures included in a state’s SIP are not sufficient to meet
CAA requirements, I am not persuaded that the provision’s
text forecloses the interpretation advanced by EPA and
applied in this case.

    The language of the statute prohibits states from labeling
as “contingency measures” the same proposed reductions
relied upon to achieve NAAQS compliance. See La. Envtl.
Action Network v. EPA, 382 F.3d 575, 583 (5th Cir. 2004)
(“Such a prospective reading of the text would seemingly
preclude the use of past reductions which have already failed
to achieve attainment.”). It requires states to identify
additional measures that must be put into effect without
further action by the state or EPA if reasonable progress is
38                    BAHR V. U.S. EPA

not made or the air quality standard is not met by the
attainment date. Those additional measures are what the
statute describes as “contingency measures.”

    Arizona’s SIP identified additional measures that were
not relied upon to obtain the anticipated compliance. The
practical issue before us in this case is whether Arizona was
prohibited from putting those additional measures into effect
in advance. The majority opinion concludes that it was, that
the state’s contingency measures must be left undone, sitting
on the sidelines in reserve. I do not believe that the language
or intent of the statute requires that conclusion.

    The early implementation of infrastructure improvements
that are expected to result in additional and continuing
emissions reductions is consistent with the language of
§ 7502(c)(9). These early-implemented contingency measures
result in a net reduction in emissions following their
implementation. They “take effect” and are “undertaken” not
only at the time they are first implemented but also thereafter,
including at the time they might formally be required due to
nonattainment. So long as these reductions are not relied upon
to meet other CAA requirements, they function as a backup
plan, reducing the likelihood that the state will fail to attain
the NAAQS even in the “contingency” that the measures
explicitly included in the SIP for that purpose are not enough.

    The majority responds by asserting that the statute
“defines contingency measures as measures ‘to be
undertaken’ or ‘to take effect’ if a future event occurs,
namely ‘if the area fails to make reasonable further progress,
or to attain the [NAAQS].’” Majority opinion at 35 (emphasis
in original). But this language fits just as well with EPA’s
interpretation as it does with the view of the majority. In both
                      BAHR V. U.S. EPA                       39

scenarios, the contingency measures must be in effect at the
time an area fails to achieve the goals outlined in the SIP.
What is at issue here is whether states are prohibited from
also implementing the measures before that “future event
occurs.” The language quoted by the majority contains no
such prohibition and it is not our role to read one into the
statute.

    EPA’s interpretation also comports well with the purpose
of the CAA. “In determining whether Congress has
specifically addressed the question at issue, a reviewing court
should not confine itself to examining a particular statutory
provision in isolation. . . . It is a ‘fundamental canon of
statutory construction that the words of a statute must be read
in their context and with a view to their place in the overall
statutory scheme.’” FDA v. Brown & Williamson Tobacco
Corp., 529 U.S. 120, 132–33 (2000) (quoting Davis v. Mich.
Dept. of Treasury, 489 U.S. 803, 809 (1989)).

    Allowing states to implement contingency measures
before they are triggered makes sense in light of that same
provision’s requirement that such measures “take effect . . .
without further action by the State or the Administrator.” That
requirement, read in context with the Act’s mandate that
states implement emission-control measures “as expeditiously
as practicable,” id. § 7502(c)(1), evinces a clear congressional
preference that contingency measures operate to reduce the
emission of harmful pollutants in as efficient and timely a
manner as possible. Taking this statutory purpose into
account, “it seems illogical to penalize nonattainment areas
that are taking extra steps, such as implementing contingency
measures prior to a deadline” as a cushion to ensure NAAQS
compliance and prevent the need for the contingency
measures requirement to be triggered in the first place. La.
40                        BAHR V. U.S. EPA

Envtl. Action Network, 382 F.3d at 584. It does not benefit
ordinary citizens to read the CAA to incentivize states to hold
off from purchasing new street sweepers or repaving their
roads until the contingency measures requirement is
triggered.

    The majority’s interpretation also imposes an additional
and unnecessary burden upon states in circumstances where
the failure to attain the NAAQS also triggers a bump up of an
area’s classification under the Act. For example, the failure
of a moderate nonattainment area to achieve NAAQS
compliance by the applicable deadline triggers both
§ 7502(c)(9)’s contingency measures requirement and the
additional requirements imposed upon serious nonattainment
areas. 42 U.S.C. § 7513(b)(2). The early implementation of
contingency measures allows states in this situation to focus
their efforts on implementing the newly imposed
requirements while the contingency measures continue to
operate in the interim. See La. Envtl. Action Network,
382 F.3d at 583.1



     1
       The majority argues that because contingency measures must take
effect “without further action by the State” or EPA, 42 U.S.C.
§ 7502(c)(9), they “cannot distract a state from meeting the other CAA
requirements.” Majority opinion at 36 n. 11. However, EPA has
interpreted this language to require only “that no further rulemaking
activities by the State or EPA would be needed to implement the
contingency measures.” Greenbaum v. EPA, 370 F.3d 527, 541 (6th Cir.
2004) (quoting State Implementation Plans; General Preamble for the
Implementation of Title I of the Clean Air Act Amendments of 1990,
57 Fed. Reg. 13498, 13512 (Apr. 16, 1992)). Thus, under the majority’s
interpretation, states would still be required to perform the actual business
of implementing the identified contingency measures at the same time
they address other requirements imposed by a failure to attain the
NAAQS.
                      BAHR V. U.S. EPA                       41

    I recognize that what is lost by EPA’s interpretation of the
statute is the advance identification of “still more” measures
aimed at improving air quality that can be newly and
additionally implemented in the event of nonattainment. If
Arizona’s SIP does not reach its goal, then reliance upon
contingency measures that have already been put into effect
will not further reduce airborne particulate matter, and more
will have to be done at that point. But EPA applied its
expertise and exercised its judgment in concluding that the
goal was likely to be reached by the measures proposed in the
SIP, without regard to the additional contingency measures.
By letting Arizona identify actions that have already been
implemented as contingency measures, EPA has obtained a
cushion. It is not an unreasonable judgment for EPA to
conclude that implementing the cushion right away is more
valuable than advance identification of what else might be
done, if necessary.

   For these reasons, I would give Chevron deference to the
EPA’s interpretation of § 7502(c)(9). I respectfully dissent
from the majority’s conclusion to the contrary.
