                     FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

SAFE AIR FOR EVERYONE; AMERICAN             
LUNG ASSOCIATION OF IDAHO; NOËL
STURGEON,
                       Petitioners,
                v.                                  No. 05-75269
UNITED STATES ENVIRONMENTAL                           EPA No.
PROTECTION AGENCY; STEPHEN L.                        ID-03-003
JOHNSON, Administrator of the
United States Environmental                          ORDER
Protection Agency; ELIN D.                          AMENDING
MILLER,* Regional Administrator                    OPINION AND
of the United States                                AMENDED
Environmental Protection Agency,                     OPINION
Region X,
                     Respondents,
STATE OF IDAHO,
                       Intervenor.
                                            
           On Petition for Review of an Order of the
              Environmental Protection Agency

                  Argued and Submitted
           November 15, 2006—Seattle, Washington

                      Filed January 30, 2007
                      Amended May 29, 2007

   *Elin D. Miller is substituted for her predecessor as Regional Adminis-
trator of the United States Environmental Protection Agency. Fed. R. App.
P. 43(c)(2).

                                  6265
6266      SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
       Before: Arthur L. Alarcón, Pamela Ann Rymer, and
               Marsha S. Berzon, Circuit Judges.

                     Opinion by Judge Berzon
6268    SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA


                           COUNSEL

David S. Baron, Earthjustice, Washington, D.C., for the peti-
tioners.

Paul Cirino, Environmental Defense Section, U.S. Depart-
ment of Justice, Washington, D.C., for the respondents.

Lisa J. Kronberg, Deputy Attorney General, Idaho Depart-
ment of Environmental Quality, Boise, Idaho, for the interve-
nor.
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA   6269
                              ORDER

  Petitioners’ Petition for Clarification is GRANTED. The
opinion filed on January 30, 2007 and reported at 475 F.3d
1096 is hereby amended as follows:

   1) On 475 F.3d at 1099, the sentence beginning “Because
our review . . .” is replaced with: “Because our review of an
administrative agency’s decision begins and ends with the
reasoning that the agency relied upon in making that decision,
see SEC v. Chenery Corp., 318 U.S. 80, 87, 63 S.Ct. 454, 87
L.Ed. 626 (1943); Ctr. for Biological Diversity v. Kemp-
thorne, 466 F.3d 1098, 1103-04 (9th Cir. 2006), we grant the
petition for review, vacate EPA’s approval of the amendment,
and remand for EPA’s reconsideration of SAFE’s objections
under a correct understanding of the preexisting Idaho SIP.”

  2) On 475 F.3d at 1099, the sentence beginning “Finally,
we explain why our interpretation . . .” is replaced with: “Fi-
nally, we explain why our interpretation of the SIP as it
existed at the time of the 2005 amendment resolves this case
and requires that we grant the petition for review, vacate
EPA’s approval of the amendment, and remand for further
proceedings.”

   3) On 475 F.3d at 1109-10, the sentence beginning “We
therefore grant . . .” is replaced with: “We therefore grant
SAFE’s petition, vacate EPA’s approval of the amendment,
and remand to EPA for its consideration of Idaho’s proposed
amendment as a change in the preexisting SIP, rather than as
simply a “clarification” of it.”

  4) On 475 F.3d at 1110, the final line of the opinion is
changed to: “Petition for Review GRANTED; EPA’s
approval VACATED; REMANDED to EPA.”

  Petitioners’ Petition for Panel Rehearing is DENIED as
moot. No further petitions for rehearing or rehearing en banc
will be accepted.
6270     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
                             OPINION

BERZON, Circuit Judge:

   The Clean Air Act (“CAA” or “the Act”), 42 U.S.C.
§§ 7401-7671q, authorizes the creation of air quality stan-
dards for a number of pollutants, including particulate matter
produced as a byproduct of burning. To implement these stan-
dards, the Act establishes a system of State Implementation
Plans (“SIPs”), whereby states submit, subject to the United
States Environmental Protection Agency’s (“EPA”) review
and approval, proposed methods for maintaining air quality.
Once approved by EPA these plans “[h]av[e] ‘the force and
effect of federal law.’ ” Trs. for Alaska v. Fink, 17 F.3d 1209,
1210 n.3 (9th Cir. 1994) (quoting Union Elec. Co. v. EPA,
515 F.2d 206, 211 (8th Cir. 1975), aff’d, 427 U.S. 246
(1976)).

   In this case, we are presented with a preexisting SIP con-
taining language that prohibits open burning generally and
contains no exception allowing farmers to burn the residue
left in their fields after harvesting their crops. Petitioner, Safe
Air for Everyone (“SAFE”), challenges EPA’s decision to
approve an amendment to that SIP authorizing such burning.
SAFE argues that certain CAA provisions which prohibit
amending SIPs so that they interfere with meeting air quality
standards forbid EPA’s action, at least absent further analysis
of field burning’s impact on Idaho’s air quality; EPA main-
tains that its approval of the amendment does not contravene
any CAA provisions.

   We hold that as it presently stands, EPA’s approval is
legally unsustainable. EPA grounded its approval of this
amendment on the premise that the preexisting Idaho SIP did
not ban field burning, so that the amendment only clarified
what was already the case. This view of the preexisting SIP
is one with which we cannot agree. Because our review of an
administrative agency’s decision begins and ends with the
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA    6271
reasoning that the agency relied upon in making that decision,
see SEC v. Chenery Corp., 318 U.S. 80, 87, 63 S.Ct. 454, 87
L.Ed. 626 (1943); Ctr. for Biological Diversity v. Kemp-
thorne, 466 F.3d 1098, 1103-04 (9th Cir. 2006), we grant the
petition for review, vacate EPA’s approval of the amendment,
and remand for EPA’s reconsideration of SAFE’s objections
under a correct understanding of the preexisting Idaho SIP.

   The current treatment of field burning in the Idaho SIP
came about as the result of a thirty-five-year regulatory evolu-
tion. After reviewing the factual administrative record, we
first explain the regulatory process established by the CAA
and then trace the development of the current SIP provisions
related to field burning in Idaho. We then closely examine
how the preexisting Idaho SIP treated field burning prior to
2005, when EPA approved an amendment to the SIP that
explicitly authorized the practice. Finally, we explain why our
interpretation of the SIP as it existed at the time of the 2005
amendment resolves this case and requires that we grant the
petition for review, vacate EPA’s approval of the amendment,
and remand for further proceedings.

                                 I.

                                 A.

   Open burning of agricultural fields is a common practice in
Idaho, particularly among bluegrass farmers in the northern
portion of the state. Those farmers maintain that burning the
grass residue improves the productivity of their fields and has
certain environmental benefits, views with which the Idaho
legislature has expressed agreement. See IDAHO CODE ANN.
§ 22-4801 (2006) (“The legislature finds that the current
knowledge and technology support the practice of burning
crop residue to control disease, weeds, pests, and to enhance
crop rotations. . . . The legislature finds that due to the cli-
mate, soils, and crop rotations unique to north Idaho counties,
crop residue burning is a prevalent agricultural practice and
6272      SAFE AIR    FOR   EVERYONE v. UNITED STATES EPA
that there is an environmental benefit to protecting water
quality from the growing of certain crops in environmentally
sensitive areas.”); Safe Air for Everyone v. Meyer, 373 F.3d
1035, 1043-47 (9th Cir. 2004) (recognizing that “the Growers
realize farming benefits from reusing grass residue in the pro-
cess of open burning”).

   Despite these attested benefits, the administrative record
establishes that such field burning is also a source of particu-
late matter that contributes to air pollution. SAFE submitted
evidence indicating that the burning of agricultural fields in
Idaho creates significant air quality problems. That evidence
documents: (1) that clouds of smoke cover large portions of
rural Idaho and surrounding states during burning season; (2)
that area doctors believe that this smoke has had particularly
severe health consequences for individuals with respiratory
ailments; (3) that some individuals with such ailments have
fled their homes during burning season to avoid the smoke;
and (4) that a coroner’s report linked at least one fatality to
field burning. EPA has recognized that substantial pollution
and health problems are created by the practice. See EPA,
AGRICULTURAL BURNING: EPA MAKES NORTHWEST FIELD BURN-
ING A TOP PRIORITY 2 (2000) (“[F]ield burning can cause seri-
ous environmental and health effects. . . . Scientific studies —
along with thousands of complaints — indicate that smoke is
unhealthy. . . . Exposure to fine particles, either alone or com-
bined with other air pollutants, has been linked to difficulty
in breathing, aggravated asthma, increased emergency room
visits and hospital admissions, and, in some cases, premature
deaths.”).

                                    B.

   Under the CAA, EPA has the authority to issue national air
quality standards setting the maximum allowable concentra-
tion of a given pollutant. 42 U.S.C. § 7409(a).1 Using this
  1
   All citations to the United States Code refer to the 2000 edition, unless
noted otherwise.
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA    6273
authority, EPA has issued limits for particulate matter. 40
C.F.R. §§ 50.6, 50.7.

   To assure that such air quality standards are met, the CAA
establishes a system heavily dependant upon state participa-
tion. See 42 U.S.C. § 7407(a) (“Each State shall have the pri-
mary responsibility for assuring air quality within the entire
geographic area comprising such State . . . .”); see generally
Train v. Natural Res. Def. Council, 421 U.S. 60, 64-70
(1975). As a central aspect of this system, states promulgate
SIPs that “provide[ ] for implementation, maintenance, and
enforcement” of the CAA’s air quality standards within the
state. 42 U.S.C. § 7410(a)(1). Although states retain signifi-
cant flexibility in establishing the details of these plans, the
CAA, and EPA regulations, outline many required features.
Id. § 7410(a)(2) (6); 40 C.F.R. pt. 51. Among them is the
mandate that state plans provide for regular revisions to
reflect evolving air quality conditions and standards. 42
U.S.C. § 7410(a)(2)(H). These revisions need not be whole-
sale recastings of SIPs; instead, the CAA allows the states to
submit, and EPA to review, piecemeal amendments dealing
with discrete SIP provisions, leaving most of the plan
untouched. See Hall v. U.S. EPA, 273 F.3d 1146, 1159-60
(9th Cir. 2001).

   Before a SIP becomes effective, EPA must determine that
it meets the CAA’s requirements. 42 U.S.C. § 7410(k)(3).
EPA must also approve plan amendments and “shall not
approve a revision of a plan if the revision would interfere
with any applicable requirement concerning attainment and
reasonable further progress . . . or any other applicable
requirement of [the CAA].” Id. § 7410(l).

                                 C.

  Idaho, like every other state, was first required to submit a
SIP to EPA within thirteen months of the Act’s 1970 passage.
See Train, 421 U.S. at 65. The original Idaho SIP was
6274     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
approved by EPA in May 1972. Approval and Promulgation
of Implementation Plans, 37 Fed. Reg. 10,842, 10,861 (May
31, 1972). A provision on open burning was among the Idaho
state regulations incorporated into that SIP: “No person shall
allow, suffer, cause or permit any open burning operation
which does not fall into at least one of the categories of Sec-
tion 3.” Field burning was included in the types of burning
allowed by Section 3, but with significant limitations:

    The open burning of plant life grown on the prem-
    ises in the course of any agricultural, forestry, or
    land clearing operation may be permitted when it can
    be shown that such burning is necessary and that no
    fire or traffic hazard will occur. Convenience of dis-
    posal is not of itself a valid necessity for burning.

    1.   It shall be the responsibility of any person con-
         ducting such burning to make every reasonable
         effort to burn only when weather conditions are
         conducive to a good smoke dissipation and only
         when an economical and reasonable alternate
         method of disposal is not available.

    2.   When such alternate method is made available,
         it shall be put into use within a reasonable time.

    3.   Any person conducting an agricultural, forestry,
         or land clearing burning operation similar to an
         operation carried out by a governmental agency
         shall follow the rules and procedures of the
         agency with regard to minimizing air pollution.

    4.   When such burning creates air pollution or a
         public nuisance, additional restrictions may be
         imposed to minimize the effect upon the envi-
         ronment.
             SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA          6275
Section 3 also allowed eight other categories of open burning:
food preparation and recreational fires; weed control fires;
fires for firefighting training; industrial flares; residential
solid waste disposal fires in rural areas; disposal site fires;
junked motor vehicle fires; and orchard fires.2

   In 1993, EPA approved amendments to the Idaho SIP that
substantially changed the open burning provisions. See
Approval and Promulgation of Implementation Plans, 58 Fed.
Reg. 39,445, 39,446 (July 23, 1993) (noting that in Idaho’s
submission, “the existing Rules for Control of Open Burning
and Categories of Allowable Burning were revised extensive-
ly” and that “[t]hese new and revised provisions for open
burning comply with EPA’s general requirements for SIP
control strategies” (citations omitted)). These SIP provisions,
incorporating section 01.01151.04(a) of Idaho air pollution
regulations in effect on December 31, 1991, contained a gen-
eral prohibition on open air burning:

      No person shall allow, suffer, cause or permit any
      open burning operation unless it is a category of
      open burning set forth in Section 01.01153 and does
      not include any of the following materials:

      i.     Garbage;

      ii.    Dead animals or parts thereof;

      iii.   Junked motor vehicles or any materials result-
             ing from a salvage operation;
  2
    In 1982, EPA approved amendments to Idaho’s SIP that incorporated
Idaho’s recodified air pollution regulations. See Approval and Promulga-
tion of Implementation Plans, 47 Fed. Reg. 32,530, 32,531 (July 28,
1982). Those recodified regulations maintained the identical substantive
language of the open burning regulations incorporated into the 1972 SIP,
including the limited permission for field burning and permission for eight
other categories of fires.
6276      SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
    iv.   Tires or other rubber materials or products;

    v.    Plastics;

    vi.   Asphalt or composition roofing or any other
          asphaltic material or product;

    vii. Tar, tar paper, waste or heavy petroleum prod-
         ucts, or paints;

    viii. Lumber or timbers treated with preservatives;

    ix.   Trade wastes except as allowed in Section
          01.01153;

    x.    Insulated wire;

    xi.   Pathogenic wastes; or

    xii. Hazardous wastes.

(Emphases added). The revised regulation incorporated into
the SIP listed categories of allowable burning that no longer
included field burning. Instead, that list retained seven of the
nine categories of permitted fires included in the 1972 SIP
regulations — food preparation and recreational fires, weed
control fires, fires for firefighting training, industrial flares,
residential solid waste disposal fires in rural areas, disposal
site fires, and orchard fires; omitted two categories of permit-
ted fires from the 1972 list — junked motor vehicle fires and
agricultural fires; and added three new categories of permitted
fires — prescribed burning, dangerous material fires, and
infectious waste burning.

   In 2003, EPA approved another set of Idaho SIP amend-
ments. Those amendments incorporated updated versions of
Idaho regulations. This round of revisions, however, updated
the open burning regulations only to reflect a recodification.
           SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA               6277
See Approval and Promulgation of Implementation Plans, 68
Fed. Reg. 2217, 2218 (Jan. 16, 2003) (“[S]ince EPA last
approved the Idaho SIP in 1993, Idaho has revised nearly
every section of its air quality rules to some degree. Many of
these amendments have been editorial and are renumberings,
changes to citations for cross-referenced rules or statutes,
changes in terminology, or grammatical corrections.”). The
substantive language of the incorporated provisions on open
burning was identical to the language approved in the 1993
SIP.

   That 2005 rulemaking approved amendments to the SIP
that added field burning as an eleventh category of allowed
burning. The relevant provision, incorporating section
58.01.01.617 of the Idaho Administrative Code in effect on
March 21, 2003, states: “The open burning of crop residue on
fields where the crops were grown is an allowable form of
open burning if conducted in accordance with the Smoke
Management and Crop Residue Disposal Act and the rules
promulgated pursuant thereto.” (Citations omitted). Although
Idaho first enacted statutes dealing with field burning in 1985
and amended them in 1986, 1999, and 2003, see Act of March
12, 1985, ch. 248, 1985 Idaho Sess. Laws 580 (codified as
amended at IDAHO CODE ANN. §§ 22-4801 to -4804 (2006)),
the SIP amendment approved by EPA in 2005 was the first
explicit reference to those statutes in the SIP, see Approval
and Promulgation of Air Quality Implementation Plan (“Final
SIP”), 70 Fed. Reg. 39,658, 39,659 (July 11, 2005) (noting
that the field burning legislation “was not [previously] specifi-
cally submitted to EPA as a SIP revision”).3

   SAFE submitted comments to EPA during the 2005 rule-
  3
    Although Idaho’s field burning statute was referenced in a 1993 report
submitted to EPA detailing Idaho’s strategy for bringing one portion of the
state into attainment with particulate matter pollution standards, that strat-
egy did not purport to alter the provisions of Idaho’s statewide SIP, which
contains the language banning field burning.
6278     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
making process and now challenges EPA’s approval of the
amendment permitting field burning in this court. We have
jurisdiction over SAFE’s challenge to this 2005 rulemaking
under 42 U.S.C. § 7607(b)(1). This court reviews EPA’s deci-
sion to approve SIP amendments under the “arbitrary, capri-
cious, or otherwise not in accordance with law” standard of
the Administrative Procedure Act (“APA”). Hall, 273 F.3d at
1155; see 5 U.S.C. § 706(2)(A).

                                 II.

                                 A.

   As detailed above, Idaho’s 2003 SIP mandated that “[n]o
person shall allow, suffer, cause or permit any open burning
operation unless it is a category of open burning set forth” in
ten specified sections that “establish categories of open burn-
ing that are allowed when done according to the prescribed
conditions.” (Emphases added). Those ten sections cover:
“Recreational and Warming Fires”; “Weed Control Fires” for
“abatement along fence lines, canal banks, and ditch banks”;
“Training Fires” for firefighting training; “Industrial Flares”;
“Residential Solid Waste Disposal Fires”; “Landfill Disposal
Site Fires”; “Orchard Fires”; “Prescribed Burning” for fire
management purposes; “Dangerous Material Fires”; and “In-
fectious Waste Burning.” Field burning does not fit into any
of these categories. EPA so acknowledged during the 2005
rulemaking proceedings. See Final SIP, 70 Fed. Reg. at
39,659 (“EPA recognizes the rule language . . . does not, on
its face, appear to identify crop residue as a category of
allowed burning”); id. at 39,660 n.1 (noting EPA’s agreement
with SAFE that field burning does not come within the “pre-
scribed burning” exception). Nor is it debatable that field
burning is an “open burning operation” covered by the SIP’s
expansive mandatory terms.

  In short, given the SIP’s broad prohibition and the absence
of any pertinent exception, the plain meaning of the SIP, in
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA      6279
the clearest of terms, prohibits field burning. Cf. Craft v. Nat’l
Park Serv., 34 F.3d 918, 922 (9th Cir. 1994) (“[T]he regula-
tion by its terms clearly prohibits appellants’ activities. With
two exceptions, the regulation prohibits ‘dredg[ing] or other-
wise alter[ing] the seabed in any way.’ . . . There can be no
question but that this language prohibits the excavation activi-
ties in which appellants were engaged.” (second and third
alterations in original) (quoting 15 C.F.R. § 935.7(a)(2)(iii)
(1994))).

   [1] In interpreting a SIP, we begin with a look toward the
plain meaning of the plan and stop there if the language is
clear. This much is clear from Bayview Hunters Point Com-
munity Advocates v. Metropolitan Transportation Commis-
sion (BHPCA), 366 F.3d 692 (9th Cir. 2004), a leading case
in this court in which the meaning of a SIP was at stake. In
considering the SIP for the San Francisco Bay Area related to
transit ridership, BHPCA began by observing that “[w]e start
with the plain language of [the SIP]. ‘A regulation should be
construed to give effect to the natural and plain meaning of
its words,’ ” and then noted that “[t]he expected ridership
increase was never described as anything more than a ‘tar-
get.’ ” Id. at 698 (quoting Crown Pac. v. Occupational Safety
& Health Review Comm’n, 197 F.3d 1036, 1038 (9th Cir.
1999)). Because the plan did “not, on its face, require a rider-
ship increase of 15%,” we held “[t]hat by its plain language
[the SIP] does not establish a mandatory requirement to
increase transit ridership by a specified percentage weighs
heavily against the conclusion that such an obligation can be
imposed based upon [the SIP].” Id. (internal quotation mark
omitted); see also Idaho Conservation League v. Boer, 362 F.
Supp. 2d 1211, 1216 (D. Idaho 2004) (refusing to defer to a
state environmental agency’s interpretation that “cannot be
reconciled with the plain language of the regulations”
included in the SIP); United States v. Gen. Dynamics Corp.,
755 F. Supp. 720, 723 (N.D. Tex. 1991) (“The [Texas state
agency’s] interpretation of the Texas SIP to allow plantwide
averaging is unreasonable, because it contradicts specific lan-
6280       SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
guage of the SIP . . . .”); Citizens for a Better Env’t v. Deuk-
mejian, 731 F. Supp. 1448, 1454-55 (N.D. Cal. 1990)
(refusing to credit evidence that the provisions of a SIP did
not make a binding commitment when the SIP included “un-
equivocal[ ]” phrasing). Applying that same methodology
here, we would quite readily conclude that the pre-2005 Idaho
SIP did not permit field burning.

                                     B.

   [2] EPA, however, assumed during the 2005 rulemaking
proceedings that this clear-as-day prohibition of field burning
does not resolve the meaning of the SIP as it existed as of the
2005 proceedings. Instead, the agency considered Idaho’s “in-
tent” in drafting the SIP, conducting “an examination of the
State’s overall approach to field burning” and “consider[ing]
such things as the legislative history of Idaho’s provisions
related to agricultural burning and smoke management,” vari-
ous reports and plans prepared by the State, and various
agreements signed by the State. Final SIP, 70 Fed. Reg. at
39,659. EPA also noted that its own past actions “indicate[ ]
that EPA understood agricultural burning to be allowed in
Idaho and that the SIP does not prohibit it.”4 Id. at 39,660.
EPA did not, however, justify or explain this approach to
interpreting a SIP, and the approach cannot be reconciled with
the role of SIPs in the federal regulatory scheme.5

  [3] Interpreting a similar state implementation plan scheme
under the Clean Water Act,6 the Supreme Court held that the
  4
     None of these past actions, however, purported to interpret the relevant
SIP provisions. Thus, there is no agency interpretation to which courts
must afford deference on the determinative question in this case.
   5
     Nor is there any indication that EPA has previously taken a position on
the proper way to interpret a SIP. Given this vacuum, the case also does
not implicate agency deference considerations on this key conceptual
issue.
   6
     Under the Clean Water Act,
          SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA             6281
Clean Water Act “effectively incorporates into federal law
those state-law standards the Agency reasonably determines
to be ‘applicable.’ In such a situation, then, state water quality
standards — promulgated by the States with substantial guid-
ance from the EPA and approved by the Agency — are part
of the federal law of water pollution control.” Arkansas v.
Oklahoma, 503 U.S. 91, 110 (1992) (footnote omitted). Simi-
larly, a SIP, once approved by EPA, has “the force and effect
of federal law.” Trs. for Alaska, 17 F.3d at 1210 n.3 (quoting
Union Elec., 515 F.2d at 211) (internal quotation marks omit-
ted). In accord with this general proposition, a state may not
unilaterally alter the legal commitments of its SIP once EPA
approves the plan. See 42 U.S.C. § 7416 (“[I]f an emission
standard or limitation is in effect under an applicable imple-
mentation plan . . . such State or political subdivision may not
adopt or enforce any emission standard or limitation which is
less stringent than the standard or limitation under such plan
. . . .”); Gen. Motors Corp. v. United States, 496 U.S. 530, 540
(1990) (“There can be little or no doubt that the existing SIP
remains the ‘applicable implementation plan’ even after the
State has submitted a proposed revision.”).

   [4] Thus, the SIP became federal law, not state law, once
EPA approved it, and could not be changed unless and until
EPA approved any change. Consequently, the state’s interpre-
tation of the regulations incorporated into the SIP, even if
binding as a matter of state law, is not directly dispositive of
the meaning of the SIP.

    “water quality standards” are, in general, promulgated by the
    States and establish the desired condition of a waterway. . . .
    [T]he Act requires, inter alia, that state authorities periodically
    review water quality standards and secure the EPA’s approval of
    any revisions in the standards. If the EPA recommends changes
    to the standards and the State fails to comply with that recom-
    mendation, the Act authorizes the EPA to promulgate water qual-
    ity standards for the State.
Arkansas v. Oklahoma, 503 U.S. 91, 101 (1992) (citing 33 U.S.C. § 1313).
6282       SAFE AIR    FOR   EVERYONE v. UNITED STATES EPA
   [5] Accordingly, we look to the standards governing the
interpretation of federal regulations. As a general interpreta-
tive principle, “the plain meaning of a regulation governs.”
Wards Cove Packing Corp. v. Nat’l Marine Fisheries Serv.,
307 F.3d 1214, 1219 (9th Cir. 2002). Other interpretative
materials, such as the agency’s own interpretation of the regu-
lation, should not be considered when the regulation has a
plain meaning. See id. (citing Christensen v. Harris County,
529 U.S. 576, 588 (2000)); see also Roberto v. Dep’t of the
Navy, 440 F.3d 1341, 1350 (Fed. Cir. 2006) (“If the regula-
tory language is clear and unambiguous, the inquiry ends with
the plain meaning.”).

   [6] The plain language of a regulation, however, will not
control if “clearly expressed [administrative] intent is to the
contrary or [if] such plain meaning would lead to absurd
results.” Dyer v. United States, 832 F.2d 1062, 1066 (9th Cir.
1987).7 Although “clearly expressed . . . intent” of regulators
therefore could overcome the plain meaning of a regulation,
see id., we have never considered how definitely and in what
form such intent must be expressed. Doing so now, we con-
clude that the notice requirements of the APA, 5 U.S.C.
§§ 552(a)(1), 553(b),8 requires that some indication of the reg-
   7
     Dyer uses the term “legislative intent,” but then inquires into the intent
of the promulgating executive agency, not that of Congress. 832 F.2d at
1066. That focus makes sense, so, for clarity, we use “administrative
intent.”
   8
     As pertinent here, these requirements provide:
    Each agency shall separately state and currently publish in the
    Federal Register for the guidance of the public —
         ...
         (D) substantive rules of general applicability adopted as
         authorized by law, and statements of general policy or inter-
         pretations of general applicability formulated and adopted by
         the agency; and
         (E) each amendment, revision, or repeal of the foregoing.
          SAFE AIR      FOR   EVERYONE v. UNITED STATES EPA           6283
ulatory intent that overcomes plain language must be refer-
enced in the published notices that accompanied the
rulemaking process. Otherwise, interested parties would not
have the meaningful opportunity to comment on proposed
regulations that the APA contemplates, id. § 553(c),9 because

    Except to the extent that a person has actual and timely notice of
    the terms thereof, a person may not in any manner be required to
    resort to, or be adversely affected by, a matter required to be pub-
    lished in the Federal Register and not so published. For the pur-
    pose of this paragraph, matter reasonably available to the class of
    persons affected thereby is deemed published in the Federal Reg-
    ister when incorporated by reference therein with the approval of
    the Director of the Federal Register.
5 U.S.C. § 552(a)(1).
    General notice of proposed rule making shall be published in the
    Federal Register, unless persons subject thereto are named and
    either personally served or otherwise have actual notice thereof
    in accordance with law. The notice shall include —
        (1) a statement of the time, place, and nature of public rule
        making proceedings;
        (2) reference to the legal authority under which the rule is
        proposed; and
        (3) either the terms or substance of the proposed rule or a
        description of the subjects and issues involved. . . .
Id. § 553(b).
   9
     That provision mandates:
    After notice required by this section, the agency shall give inter-
    ested persons an opportunity to participate in the rule making
    through submission of written data, views, or arguments with or
    without opportunity for oral presentation. After consideration of
    the relevant matter presented, the agency shall incorporate in the
    rules adopted a concise general statement of their basis and pur-
    pose. When rules are required by statute to be made on the record
    after opportunity for an agency hearing, sections 556 and 557 of
    this title apply instead of this subsection.
5 U.S.C. § 553(c). This opportunity for comment applies to SIP revisions.
Ober v. U.S. EPA, 84 F.3d 304, 312 (9th Cir. 1996).
6284     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
they would have had no way of knowing what was actually
proposed. For, as the D.C. Circuit has observed:

       Courts’ reliance on the “plain meaning” rule in
    this setting [of interpreting administrative regula-
    tions] is not a product of some fetishistic attraction
    to legal “formalism.” In order to infuse a measure of
    public accountability into administrative practices,
    the APA mandates that agencies provide interested
    parties notice and an opportunity for comment
    before promulgating rules of general applicability.
    This right to participate in the rulemaking process
    can be meaningfully exercised, however, only if the
    public can understand proposed rules as meaning
    what they appear to say. Moreover, if permitted to
    adopt unforeseen interpretations, agencies could con-
    structively amend their regulations while evading
    their duty to engage in notice and comment proce-
    dures. As applied to agency regulations, then, the
    plain meaning doctrine is an interpretive norm essen-
    tial to perfecting the scheme of administrative gover-
    nance established by the APA.

       ....

       . . . To protect the integrity of [the APA’s
    required] procedures, we cannot permit an agency to
    rely on its unexpressed intentions to trump the ordi-
    nary import of its regulatory language.

Exportal Ltda. v. United States, 902 F.2d 45, 50-51 (D.C. Cir.
1990) (citations and emphases omitted).

   Such a mode of interpretation is particularly sensible under
the CAA, which requires that judicial challenges be filed
within sixty days of a SIP’s approval. 42 U.S.C. § 7607(b)(1).
If an agency can promulgate a regulation with plain language
that dictates one meaning but later interpret it according to an
          SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA             6285
intent indicated neither in the regulatory language nor in the
promulgation documents, parties may depend on the plain
meaning of the regulation in deciding not to launch a chal-
lenge within the prescribed time limit. If, later, the agency
relies on an undisclosed intended meaning, interested parties
might be foreclosed from challenging the regulation, contrary
to the statutory permission to launch such challenges.10

   [7] Here, following the plain language prohibiting field
burning in Idaho’s 2003 SIP does not produce “absurd
results” or contravene the pertinent administrative history. See
Dyer, 832 F.2d at 1066. As the administrative record of air
quality and health problems created by field burning demon-
strates, it is far from patently inconceivable that the federal air
pollution law covering Idaho would ban a significant source
of the state’s particulate pollution. Indeed, one of Idaho’s
neighbors has enacted a broad ban, except in limited circum-
stances, on the open burning on farms that produce grass seed.
See WASH. ADMIN. CODE 173-430-045 (2006). So the interpre-
tation of the 2003 SIP mandated by its plain language is not
absurd at all, much less sufficiently absurd to justify departure
from a plain words interpretation. See Crooks v. Harrelson,
282 U.S. 55, 60 (1930) (“[T]o justify a departure from the let-
ter of the law upon that ground, the absurdity must be so gross
as to shock the general moral or common sense.”).

   [8] Likewise, no administrative intent expressed in an
appropriate way contradicts the plain meaning of the SIP:
None of the published notices that accompanied the consider-
ation or adoption of Idaho’s previous SIPs established any
intent concerning field burning. Instead, EPA’s purported
intent to allow field burning in Idaho is demonstrated, if at all,
  10
     When review is sought “based solely on grounds arising after such
sixtieth day,” the CAA also allows for the challenge to be “filed within
sixty days after such grounds arise.” 42 U.S.C. § 7607(b). We take no
position on whether EPA’s reliance on its previously undisclosed intent in
approving a SIP could constitute “grounds arising after such sixtieth day.”
6286      SAFE AIR    FOR   EVERYONE v. UNITED STATES EPA
only through informal materials such as letters and presenta-
tions and its silent acquiescence when approving certain anti-
pollution strategies submitted by Idaho.11 Although Idaho law-
makers and regulators made their intentions toward field
burning known through more formal actions, such as enacting
legislation and regulations allowing field burning, none of
these measures were referenced in the published materials that
accompanied adoption of the earlier SIPs.

                                    C.

   For the first time on appeal, EPA proffers two additional
reasons we should not rely on the plain meaning of the 2003
and earlier SIPs. We owe no deference to these post hoc liti-
gating positions, adopted by counsel for EPA. See Bowen v.
Georgetown Univ. Hosp., 488 U.S. 204, 212 (1988) (“[W]e
have declined to give deference to an agency counsel’s inter-
pretation of a statute where the agency itself has articulated no
position on the question . . . .”). We do not find EPA’s argu-
ments persuasive.

   EPA argues, first, that giving effect to the plain meaning of
a SIP contrary to the true intent of state policymakers would
violate case law prohibiting EPA from enacting more strin-
gent SIP provisions than those proposed by the state. See Riv-
erside Cement Co. v. Thomas, 843 F.2d 1246, 1247-48 (9th
Cir. 1988) (holding that EPA’s approval of a SIP after remov-
ing a proviso submitted by the state was arbitrary and capri-
cious); Bethlehem Steel Corp. v. Gorsuch, 742 F.2d 1028,
1035-36 (7th Cir. 1984) (holding the CAA’s partial approval
provision did not allow EPA to make a SIP stricter); cf. Train,
  11
     EPA cites its approval of an area-specific plan that referenced Idaho’s
field burning statute, see supra note 3, to demonstrate that it understood
prior to 2005 that field burning was not banned in Idaho. Final SIP, 70
Fed. Reg. at 39,660. EPA, however, did not refer to the provisions on field
burning when explaining its decision to approve that strategy. See
Approval and Promulgation of Sandpoint, Idaho, Air Quality Implementa-
tion Plan, 67 Fed. Reg. 43,006 (June 26, 2002).
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA     6287
421 U.S. at 79 (“[S]o long as the ultimate effect of a State’s
choice of emission limitations is compliance with the national
standards for ambient air, the State is at liberty to adopt what-
ever mix of emission limitations it deems best suited to its
particular situation.”); Hall, 273 F.3d at 1153 (“By virtue of
the States’ roles in devising a strategy and adopting an imple-
mentation plan, . . . ‘[i]t is to the States that the Act assigns
initial and primary responsibility for deciding what emissions
reductions will be required from which sources.’ ” (alteration
in original) (quoting Whitman v. Am. Trucking Ass’ns, Inc.,
531 U.S. 457, 470 (2001))). Those decisions, however, inter-
preted the CAA’s provisions concerning the authority of EPA
to approve or deny SIPs. They are not relevant to the task
presently before the court — interpreting SIP language that
was originally proposed by the state. As to that endeavor,
requiring states to express their intent understandably when
submitting proposed SIPs in no way detracts from states’ crit-
ical role in devising the strategy to be used in achieving the
requisite air quality standards.

   Second, EPA argues that crediting the SIP’s plain meaning
would contradict case law prohibiting EPA from approving
SIPs based on “an elusive and illusory measure.” Riverside
Cement, 843 F.2d at 1248. In Riverside Cement, EPA
approved a SIP that contained a provision that explicitly
stated its operation was “contingent upon the results of ongo-
ing factfinding.” Id. at 1247 (internal quotation mark omit-
ted). Because that provision, by its own terms, might never
have become effective, the court held EPA could not rely on
that provision in determining whether the SIP met the CAA’s
pollution reduction requirements. Id. at 1248. In this case, by
contrast, nothing on the face of Idaho’s SIP suggests that the
field burning prohibition is in any way contingent or indefi-
nite. The plan is therefore not illusory under our case law.
Moreover, by relying on a SIP’s explicit language to find it
illusory, Riverside Cement supports our broader conclusion
that the plain meaning of a SIP controls.
6288     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
                               ***

   [10] In sum, we hold that SIPs are interpreted based on
their plain meaning when such a meaning is apparent, not
absurd, and not contradicted by the manifest intent of EPA, as
expressed in the promulgating documents available to the
public. Because the prohibitory language of the preexisting
Idaho SIP plainly applies to field burning, federal law banned
field burning in Idaho prior to EPA’s 2005 approval of the
SIP amendment.

                                III.

   In commenting to EPA about Idaho’s proposed amendment
to the SIP, SAFE maintained that its approval would weaken
the prior SIP and thereby violate sections 110(l) and 193 of
the CAA. Section 110(l) provides that EPA “shall not approve
a revision of a [SIP] if the revision would interfere with any
applicable requirement concerning attainment and reasonable
further progress . . . or any other applicable requirement of
this chapter.” 42 U.S.C. § 7410(l). Section 193 provides that
“[n]o control requirement in effect, or required to be adopted
by an order, settlement agreement, or plan in effect before
November 15, 1990, in any area which is a nonattainment
area for any air pollutant may be modified after November 15,
1990, in any manner unless the modification insures equiva-
lent or greater emission reductions of such air pollutant.” Id.
§ 7515. In its 2005 approval of the amendment, EPA denied
the amendment contravened either of these statutes. Final SIP,
70 Fed. Reg. at 39,659-60. SAFE now challenges those deter-
minations. We do not reach those broad statutory challenges,
except to hold that EPA’s reasoning in rejecting them cannot
be squared with our interpretation of Idaho’s pre-2005 SIPs.

  As we have explained, EPA’s decision to approve the 2005
amendment to Idaho’s SIP rested on the fundamental premise
that “EPA does not believe that Idaho’s existing SIP when
viewed in its entirety prohibits the burning of crop residue.”
         SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA     6289
Id. at 39,659 (emphasis added). EPA relied on this premise in
rejecting SAFE’s claims under sections 110(l) and 193. See
id. at 39,660 (“The proposed SIP revision is merely a clarifi-
cation of the existing SIP and does not change or otherwise
relax an existing control measure and therefore will not inter-
fere with any applicable requirements concerning attainment
and reasonable further progress or other applicable require-
ment of the Act. EPA believes that the requirement of section
110(l) is satisfied.” (emphasis added)); id. (“In sum, EPA
believes that approving the proposed SIP revision does not
change or alter the existing SIP in Idaho which does not pro-
hibit burning of crop residue. . . . Therefore, the requirements
of section 193 of the Act are satisfied.” (emphasis added)).
Moreover, EPA has continued to rely on the same logic in its
brief to this court: “In approving [the 2005] amendment to
Idaho’s SIP, EPA understood it to be a clarification of exist-
ing state law and the SIP, governing open burning of crop res-
idue. EPA’s action to approve Idaho’s SIP revision request
therefore did not relax Idaho’s pre-existing SIP with respect
to open burning of crop residue, or any control requirements
in the SIP that had been in effect before November 15, 1990.”
(Emphases added).

    We must review the EPA’s actions based on the “grounds
. . . upon which the record discloses that its action was based.”
Chenery, 318 U.S. at 87; see also Ctr. for Biological Diver-
sity, 466 F.3d at 1103-04. On one hand, that principle means
that we can only uphold EPA’s action “on the basis articu-
lated by the agency itself.” Motor Vehicle Mfrs. Ass’n of the
U.S., Inc. v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 50
(1983). On the other hand, it also means we must “remand to
the agency for additional investigation or explanation” when
the agency’s analysis is incomplete after its flawed basis is
removed. INS v. Ventura, 537 U.S. 12, 16 (2002) (per curiam)
(quoting Fla. Power & Light Co. v. Lorion, 470 U.S. 729, 744
(1985) (internal quotation mark omitted)). The record demon-
strates that because EPA based the action under review on its
belief that the preexisting SIP did not ban agricultural burn-
6290     SAFE AIR   FOR   EVERYONE v. UNITED STATES EPA
ing, the agency did not address the question whether the 2005
amendment, if indeed a change, contravened the statutory
requirements. We therefore cannot reach that question either.

   [11] We have held EPA’s conclusion that the preexisting
SIP did not ban field burning legally erroneous. Because that
flawed premise is fundamental to EPA’s determination that it
did not contravene sections 110(l) or 193 of the CAA by
approving the 2005 SIP, EPA’s outcome on those statutory
interpretation questions is “arbitrary, capricious, or otherwise
not in accordance with law” for the purposes of our review.
Hall, 273 F.3d at 1155. We therefore grant SAFE’s petition,
vacate EPA’s approval of the amendment, and remand to EPA
for its consideration of Idaho’s proposed amendment as a
change in the preexisting SIP, rather than as simply a “clarifi-
cation” of it. Final SIP, 70 Fed. Reg. at 39,660. Accordingly,
we have no reason to interpret the meaning of either CAA
provision relied upon by SAFE but will instead allow EPA the
first opportunity to apply those provisions, this time in accord
with the understanding that the preexisting SIP bans field
burning while the proposed amendment clearly allows, and
regulates, the practice.

 Petition for Review GRANTED;                 EPA’s       approval
VACATED; REMANDED to EPA.
