Redoil v. Epa ( 2013 )


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  •                 FOR PUBLICATION
    UNITED STATES COURT OF APPEALS
    FOR THE NINTH CIRCUIT
    RESISTING ENVIRONMENTAL                  No. 12-70518
    DESTRUCTION ON INDIGENOUS
    LANDS, REDOIL; ALASKA
    WILDERNESS LEAGUE; CENTER FOR            AMENDED
    BIOLOGICAL DIVERSITY ; NATURAL            OPINION
    RESOURCES DEFENSE COUNCIL;
    NORTHERN ALASKA
    ENVIRONMENTAL CENTER ; OCEANA ;
    PACIFIC ENVIRONMENT ; SIERRA
    CLUB; THE WILDERNESS SOCIETY ,
    Petitioners,
    v.
    UNITED STATES ENVIRONMENTAL
    PROTECTION AGENCY ,
    Respondent,
    SHELL GULF OF MEXICO INC.; SHELL
    OFFSHORE INC.,
    Respondents-Intervenors.
    On Petition for Review of an Order of the
    Environmental Protection Agency
    Environmental Appeals Board
    Argued and Submitted
    August 28, 2012—Anchorage, Alaska
    2                        REDOIL V . EPA
    Filed December 26, 2012
    Amended April 23, 2013
    Before: Michael Daly Hawkins, M. Margaret McKeown,
    and Carlos T. Bea, Circuit Judges.
    Opinion by Judge McKeown
    SUMMARY*
    Environmental Law
    The panel denied a petition for review, and upheld a
    decision of the Environmental Protection Agency granting
    two air permits authorizing exploratory drilling operations in
    the Arctic Ocean by a drillship and its associated fleet of
    support vessels.
    The panel upheld the EPA’s statutory and regulatory
    interpretations. Specifically, the panel held that the Clean Air
    Act is ambiguous as to the applicability of the best available
    control emissions to support vessels not attached to an Outer
    Continental Shelf source, and concluded under Chevron
    U.S.A., Inc. v. NRDC, Inc., 
    467 U.S. 837
     (1984), deference
    that the EPA’s construction of the statute was permissible and
    reasonable. The panel also held that the EPA’s grant of a 500
    meter ambient air exemption was not plainly erroneous or
    inconsistent with the EPA’s regulations.
    *
    This summary constitutes no part of the opinion of the court. It has
    been prepared by court staff for the convenience of the reader.
    REDOIL V . EPA                      3
    COUNSEL
    Colin C. O’Brien (argued), Earthjustice, Anchorage, Alaska;
    Eric P. Jorgensen, Earthjustice, Juneau, Alaska, for
    Petitioners.
    Ignacia S. Moreno, Daniel Pinkston (argued), Alan D.
    Greenberg, United States Department of Justice, Denver,
    Colorado, for Respondent.
    Kathleen M. Sullivan (argued), William B. Adams, David S.
    Mader, Quinn Emanuel Urquhart & Sullivan LLP, New York,
    New York; Duane A. Siler, Sarah C. Bordelon, Tony G.
    Mendoza, Crowell & Moring LLP, Washington, D.C., for
    Intervenors-Respondents.
    Lisa E. Jones, Samuel B. Boxerman, and James R. Wedeking,
    Sidley Austin, LLP, Washington, D.C., Mara E. Zimmerman,
    Washington, D.C., for Amicus Curiae American Petroleum
    Institute.
    Cameron M. Leonard, Office of the Attorney General,
    Fairbanks, Alaska, for Amicus Curiae State of Alaska.
    OPINION
    McKEOWN, Circuit Judge:
    Since 1990, the Environmental Protection Agency
    (“EPA”) has been responsible for regulating air pollution
    from offshore sources on the Outer Continental Shelf
    (“OCS”) under the Clean Air Act (“the Act”). 
    42 U.S.C. § 7627
    . We consider here whether the EPA’s Environmental
    4                         REDOIL V . EPA
    Appeals Board (“EAB”) properly upheld two air permits
    authorizing exploratory drilling operations in the Arctic
    Ocean by a drillship and its associated fleet of support
    vessels. The petition for review challenges two aspects of the
    permits: (1) the determination that support vessels, unlike the
    drillship itself, do not require the best available control
    technology (“BACT”) to control emissions; and (2) the
    exemption of the area within a 500-meter radius of the
    drillship from ambient air quality standards.
    The application of BACT to support vessels requires us to
    reconcile conflicting provisions of the Act. In doing so,
    under Chevron U.S.A., Inc. v. NRDC, Inc., we defer to the
    EAB’s reasonable interpretation of those provisions and
    related regulations. 
    467 U.S. 837
     (1984). Likewise, we
    evaluate whether the EAB’s decision on the ambient air
    boundary is a permissible application of the EPA’s
    regulations. In both cases, we uphold the EPA’s statutory and
    regulatory interpretations, and we deny the petition.
    I. BACKGROUND
    Shell Gulf of Mexico, Inc. and Shell Offshore, Inc.
    (collectively “Shell”) purchased lease blocks1 in the Chukchi
    and Beaufort Seas off the North Slope of Alaska for oil and
    gas exploration. Shell plans to conduct this exploration via
    its drillship, the Discoverer, along with an associated fleet of
    support vessels, including icebreakers, oil spill response
    vessels, and a supply ship. As required by the Act, Shell
    applied for permits to emit pollutants in connection with its
    1
    Outer Continental Shelf lease blocks are defined geographic areas over
    the outer continental shelf that identify federal land ownership and
    facilitate the management of offshore resources.
    REDOIL V . EPA                          5
    exploration activities. The EPA granted the permits, which
    were upheld in two administrative appeals to the EAB.
    Petitioners, Resisting Environmental Destruction on
    Indigenous Lands, an environmental organization, and other
    environmental groups (collectively “REDOIL”), challenge
    the permits on the basis that they do not satisfy the Act’s air
    permit requirements. Shell intervened to oppose REDOIL’s
    petitions.
    Two permits are at issue, one for operation in the Chukchi
    Sea and the other for the Beaufort Sea. The permits allow
    Shell, subject to conditions, to construct and operate its
    Discoverer drillship and use its associated fleet for
    exploratory drilling activities between July 1 and November
    30 each year. The Chukchi permit underwent two rounds of
    notice-and-comment before it was issued in March 2010. The
    Beaufort permit underwent one round of notice-and-comment
    before it was issued in April 2010. The EAB addressed the
    two permits together in the administrative proceedings that
    followed.
    Under the permits, Shell must apply BACT—consisting
    of specific technologies selected by the EPA, such as good
    combustion practices, a particular ventilation system, or a
    type of fuel—to limit the emissions of specific pollutants
    subject to regulation under the Act. Central to this appeal, the
    EPA determined that BACT applies to the Discoverer when
    it is attached to the seabed at a drill site by at least one
    anchor, and to any vessel that is tied to the Discoverer under
    that condition. In short, the permits require Shell to comply
    with technological requirements for the Discoverer and the
    supply vessel whenever it is tied to the Discoverer. However,
    the permits do not prescribe technological requirements for
    6                     REDOIL V . EPA
    the remaining vessels in the associated fleet because they will
    not be physically attached to the Discoverer.
    REDOIL appealed the approval of the permits to the
    EAB, seeking to have BACT applied to the entire associated
    fleet whenever it is operating within 25 miles of the
    Discoverer, regardless of whether the vessels are tied to the
    drillship. REDOIL argued that § 7627 establishes an
    “unambiguous mandate” requiring the EPA to do so. Citing
    ambiguity in § 7627’s requirements, the EAB denied review
    in December 2010, but remanded for reasons not at issue
    here. In re: Shell Gulf of Mexico, Inc. & Shell Offshore, Inc.,
    
    2010 WL 5478647
     (E.P.A. Dec. 30, 2010).
    On remand, the EPA issued revised air permits in
    September 2011. The revised permits allow the Discoverer
    an area with a 500-meter radius, measured from the center of
    the drillship, that is exempt from “ambient air” standards.
    The exemption is conditioned on the United States Coast
    Guard’s establishment of an effective safety zone that
    prohibits members of the public from entering the area. Shell
    must also develop and implement a public access control
    program to (1) notify the general public of the prohibition on
    entering the safety zone, and (2) communicate to North Slope
    communities relevant information about exploration
    operations, such as timing and location.
    REDOIL appealed the revised permits to the EAB, this
    time targeting the ambient air exemption. REDOIL urged
    that the exemption contravened both the definition of
    “ambient air” in the EPA’s regulations and the agency’s
    longstanding interpretation of those regulations. The EAB
    dismissed this second round of appeals in a January 2012
    order, noting that ambient air exemptions are determined on
    REDOIL V . EPA                         7
    a case-by-case basis. In re: Shell Gulf of Mexico, Inc. & Shell
    Offshore, Inc., 
    2012 WL 119962
    , at *27 (E.P.A. Jan. 12,
    2012). The permits became effective on January 27, 2012.
    Notice of Approval of Clean Air Act Outer Continental Shelf
    Permits Issued to Shell Gulf of Mexico, Inc., and Shell
    Offshore, Inc. for the Discoverer Drillship, 
    77 Fed. Reg. 7148
    , 7148 (Feb. 10, 2012).
    REDOIL now seeks review of both the December 2010
    EAB ruling declining to apply BACT to the entire associated
    fleet and the January 2012 EAB ruling on the 500-meter
    ambient air exemption.
    II. APPLICATION OF BACT TO THE ASSOCIATED FLEET
    A. STATUTORY FRAMEWORK
    Enacted in 1970, the Clean Air Act established a
    comprehensive program to protect and enhance air quality by
    limiting emissions from both stationary industrial sources and
    mobile sources. 
    42 U.S.C. § 7401
     et seq. Central to this
    legislation are national air standards, known as “National
    Ambient Air Quality Standards” (“NAAQS”), set by the EPA
    for pollutants considered harmful to public health and the
    environment. 
    42 U.S.C. §§ 7408
    –10. The Act has been
    significantly amended twice, in 1977 and 1990. Clean Air
    Act Amendments of 1977, Pub. L. No. 95-95, 
    91 Stat. 685
    (1977); Clean Air Act Amendments of 1990, Pub. L. No.
    101-549, 
    104 Stat. 2399
     (1990). The Prevention of
    Significant Deterioration (“PSD”) program and its related
    BACT requirement were first included in the 1977
    amendment, which dealt with onshore sources of air
    pollution. 
    42 U.S.C. §§ 7470
    –7492. The PSD program was
    later applied to offshore sources on the OCS through the 1990
    8                      REDOIL V . EPA
    amendment. 
    42 U.S.C. § 7627
    . The extent to which BACT
    applies to an associated fleet turns on the interaction between
    the two amendments.
    The 1977 amendment directed that major new stationary
    sources of pollution and major existing stationary sources that
    are being significantly modified must obtain preconstruction
    permits through a process called New Source Review. PSD
    is the New Source Review program for areas with relatively
    clean air—those areas that the EPA designates as in
    “attainment” with NAAQS or as “unclassifiable” due to
    insufficient data. 
    42 U.S.C. § 7471
    . The program’s purpose
    is to protect the public from any adverse health or welfare
    effects of air pollution that may occur despite achievement of
    NAAQS, and to require careful evaluation of all
    consequences of new industrial development. 
    42 U.S.C. § 7470
    (1), (5).
    Under the PSD program, “[n]o major emitting facility”
    “may be constructed” without a conforming permit.
    
    42 U.S.C. § 7475
    (a)(1). “Major emitting facility” is defined,
    in part, as a “stationary source[] of air pollutants” with the
    potential to emit certain threshold levels of specified air
    pollutants subject to regulation. 
    42 U.S.C. § 7479
    (1). To
    obtain a PSD permit, such a facility must, among other
    things, satisfy two independent requirements. The first
    requirement concerns emissions: the owner or operator of the
    major emitting facility must conduct air quality analyses
    demonstrating that emissions from the facility will not cause
    or contribute to air pollution in violation of various air quality
    standards. 
    42 U.S.C. § 7475
    (a)(3). The second requirement,
    and the one at issue here, concerns technology: “the proposed
    facility is subject to the best available control technology for
    each pollutant subject to regulation” emitted from such
    REDOIL V . EPA                         9
    facility. 
    42 U.S.C. § 7475
    (a)(4). The obligation to apply
    BACT requires the EPA to select emission control
    technologies that result in the maximum reduction of
    specified pollutants in view of “energy, environmental, and
    economic impacts and other costs.” 
    42 U.S.C. § 7479
    (3).
    The 1990 amendment extended the applicability of the
    PSD requirements beyond major onshore stationary sources
    of air pollution. As a result of the 1990 amendment, the EPA,
    for the first time, was given jurisdiction to regulate OCS
    sources “located offshore of the States along the Pacific,
    Arctic and Atlantic Coasts” and certain areas of the Gulf
    Coast. 
    42 U.S.C. § 7627
    (a)(1). To achieve the goals of the
    PSD program, Congress directed the EPA to “establish
    requirements” so that OCS sources would attain and maintain
    ambient air quality standards and comply with the PSD
    program. 
    Id.
    A key provision of § 7627 is the definition of the term
    “Outer Continental Shelf source,” which means “any
    equipment, activity, or facility” that “(i) emits or has the
    potential to emit any air pollutant, (ii) is regulated or
    authorized under the Outer Continental Shelf Lands Act
    [(“OCSLA”), 
    43 U.S.C. § 1331
     et seq.], and (iii) is located on
    the Outer Continental Shelf or in or on waters above the
    Outer Continental Shelf.” 
    42 U.S.C. § 7627
    (a)(4)(C).
    Significantly, jurisdiction under OCSLA extends only to “the
    subsoil and seabed of the outer Continental Shelf and . . . all
    installations and other devices permanently or temporarily
    attached to the seabed.” 
    43 U.S.C. § 1333
    (a)(1). The
    statutory definition of “OCS source” additionally directs that,
    “[f]or purposes of this subsection, emissions from any vessel
    servicing or associated with an OCS source, including
    emissions while at the OCS source or en route to or from the
    10                    REDOIL V . EPA
    OCS source within 25 miles of the OCS source, shall be
    considered direct emissions from the OCS source.”
    
    42 U.S.C. § 7627
    (a)(4)(C)(iii).
    In follow-on regulations adopted in 1992, the EPA
    incorporated sections (i), (ii) and (iii) of the statutory
    definition of “OCS source” and added that it would include
    vessels only when “(1) [p]ermanently or temporarily attached
    to the seabed,” or “(2) [p]hysically attached to an OCS
    facility, in which case only the stationary sources aspects of
    the vessels will be regulated.” 
    40 C.F.R. § 55.2
    .
    B. THE CLEAN AIR ACT IS AMBIGUOUS AS TO BACT’S
    APPLICATION TO ASSOCIATED VESSELS NOT
    ATTACHED TO AN OCS SOURCE
    We next consider whether the Act clearly and
    unambiguously requires the application of all aspects of the
    PSD program, including BACT, to associated vessels
    operating within 25 miles of the OCS source, regardless of
    whether they are tied to the OCS source, as urged by
    REDOIL. In contrast, the EPA and Shell argue that the
    statute is ambiguous and that we owe deference to the EPA’s
    permissible construction that BACT applies to the
    Discoverer, which is a stationary OCS source, but not to
    mobile support vessels unattached to the drillship.
    To interpret the statute, we look first to the statute’s
    “language itself [and] the specific context in which that
    language is used.” McNeill v. United States, 
    131 S. Ct. 2218
    ,
    2221 (2011) (quoting Robinson v. Shell Oil Co., 
    519 U.S. 337
    , 341 (1997) (internal quotation marks omitted)). If “the
    [expressed] intent of Congress is clear,” then the court and
    the agency “must give effect to [that] unambiguously
    REDOIL V . EPA                         11
    expressed intent.” Chevron, 
    467 U.S. at
    842–43. If,
    however, “Congress has not directly addressed the precise
    question at issue,” then we must not “simply impose [our]
    construction on the statute, as would be necessary in the
    absence of an administrative interpretation,” but rather ask
    “whether the agency’s answer is based on a permissible
    construction of the statute.” 
    Id. at 843
    .
    An additional question is whether the EAB’s
    interpretation deserves Chevron deference.             Congress
    explicitly granted to the EPA the authority to promulgate
    regulations and grant air permits for activities on the OCS.
    The EPA exercised that authority through a formal process
    that included multiple rounds of public notice and comment,
    various petitions for administrative review, and two reasoned
    EAB decisions upholding the air permits at issue. We join
    our sister circuits in concluding that the EAB proceeding is a
    formal adjudication that warrants Chevron deference. See In
    re Lyon Cnty. Landfill, 
    406 F.3d 981
    , 984 (8th Cir. 2005)
    (“EAB decisions . . . are formal adjudications consistent with
    the Administrative Procedure Act . . . , and due Chevron
    deference.”) (citing United States v. Mead Corp., 
    533 U.S. 218
    , 228 (2001)); Sultan Chemists, Inc. v. EPA, 
    281 F.3d 73
    ,
    79 (3d Cir. 2002) (holding that an EAB proceeding is a
    formal adjudication to which courts should defer under Mead
    if the statutory language is ambiguous); Piney Run Pres.
    Ass’n v. Cnty. Com’rs of Carroll Cnty., 
    268 F.3d 255
    , 267–68
    (4th Cir. 2001) (noting that an earlier EAB decision
    articulating a reasonable statutory interpretation is entitled to
    Chevron deference). Chevron deference also extends to the
    EAB’s interpretation of the Clean Air Act, which is
    administered by the EPA.
    12                    REDOIL V . EPA
    It is useful to begin with what is clear and undisputed,
    both in the statute and by the parties. Under a plain reading
    of the statute, the PSD program and the BACT requirement
    apply to an “Outer Continental Shelf source.” 
    42 U.S.C. § 7627
    (a)(1) (The EPA is required to “control air pollution
    from Outer Continental Shelf sources” by establishing
    requirements to ensure that such sources “comply with the
    provisions of part C of subchapter I of this chapter [the PSD
    program].” ). The Discoverer, when it is attached to the
    seabed by an anchor, is an OCS source. Additionally, an
    associated vessel that is tied to the Discoverer while it is
    anchored to the seabed also becomes an OCS source because
    it is “[p]hysically attached to an OCS facility,” an alternative
    way to qualify as an OCS source under the EPA’s regulatory
    definition. 
    40 C.F.R. § 55.2
    . However, support vessels that
    are not “[p]ermanently or temporarily attached to the seabed,”
    or “[p]hysically attached to an OCS facility,” are not
    “regulated or authorized under the Outer Continental Shelf
    Lands Act” and thus cannot be an OCS source under the
    statute or under the EPA’s regulatory definition. See 
    40 C.F.R. § 55.2
    ; 
    43 U.S.C. § 1333
    (a)(1); 
    42 U.S.C. § 7627
    (a)(4)(C)(ii). Congress therefore did not express an
    intention to regulate associated vessels as OCS sources, or to
    apply BACT to associated vessels on that basis.
    Here is the rub. Section 7627 also provides that “[f]or
    purposes of this subsection, emissions from any vessel
    servicing or associated with an OCS source, including
    emissions while at the OCS source or en route to or from the
    OCS source within 25 miles of the OCS source, shall be
    considered direct emissions from the OCS source.”
    
    42 U.S.C. § 7627
    (a)(4)(C)(iii). REDOIL elliptically argues,
    based on this “direct emissions” clause, that Congress
    intended to apply all PSD requirements, including BACT, to
    REDOIL V . EPA                        13
    support vessels operating within 25 miles of the OCS source,
    thereby effectively turning such support vessels into “OCS
    sources.” However, that conclusion does not follow
    unambiguously from the statutory language because the direct
    emissions clause does not redefine “OCS source.”
    The statute does not instruct that “any vessel servicing or
    associated with an OCS source” “while at the OCS source or
    en route to or from the OCS source within 25 miles of the
    OCS source” shall be considered an OCS source. Rather, the
    direct emissions clause maintains a distinction between an
    OCS source, to which all PSD requirements apply, and
    vessels servicing an OCS source, to which unspecified
    requirements apply because their emissions must be
    considered direct emissions from the OCS source. As noted
    in the EAB decision, § 7627 provides no further explanation
    as to why emissions from associated vessels should be
    considered direct emissions from the OCS source or to what
    effect. See In re: Shell Gulf of Mexico, 
    2010 WL 5478647
    , at
    *12. Thus, Congress gave conflicting signals by clearly and
    unambiguously excluding the associated fleet from the
    definition of “OCS source” and yet including the associated
    fleet’s emissions as direct emissions of the OCS source.
    The Act’s structure provides a further wrinkle suggesting
    that associated vessels may be treated differently from an
    OCS source. The PSD program is found in Title I of the Act,
    which governs major stationary sources such as industrial
    plants, while mobile sources such as motor vehicles are
    regulated separately by Title II. 
    42 U.S.C. §§ 7401
    –7515; 
    id.
    at §§ 7521–7590. By definition, a “stationary source”
    excludes any source of “emissions resulting directly from an
    internal combustion engine for transportation purposes or
    from a nonroad engine or nonroad vehicle.” 42 U.S.C.
    14                    REDOIL V . EPA
    § 7602(z). The EPA’s regulations in turn define “marine
    engine” as “a nonroad engine that is installed or intended to
    be installed on a marine vessel.” 
    40 C.F.R. § 89.2
    . Reading
    § 7627 within the context of the PSD provisions points to the
    conclusion that PSD requirements should apply to stationary
    sources on the OCS, but not mobile marine vessels. We
    agree with the EAB ruling that this distinction between
    stationary and mobile sources is consistent with application
    of BACT to installations attached to the seabed but not to
    vessels, such as those in the associated fleet, moving freely in
    the waters above the OCS. In re: Shell Gulf of Mexico, 
    2010 WL 5478647
    , at *14. Whether Congress expressed an
    intention that BACT were to apply to associated vessels that
    are not attached to an OCS source is, at the very least,
    ambiguous.
    REDOIL ironically resorts to legislative history in an
    effort to avoid ambiguity. However, were the statutory
    language clear, as REDOIL posits, reference to the legislative
    history would be both unnecessary and inappropriate to
    illuminate unambiguous text. Ratzlaf v. United States,
    
    510 U.S. 135
    , 147–48 (1994) (“[W]e do not resort to
    legislative history to cloud a statutory text that is clear.”);
    Barnhill v. Johnson, 
    503 U.S. 393
    , 401 (1992) (“[A]ppeals to
    statutory history are well taken only to resolve statutory
    ambiguity.”) (internal quotation marks omitted).
    Of course, in the case of an ambiguous statute, we may
    consider legislative history. See, e.g., N. Cal. River Watch v.
    Wilcox, 
    633 F.3d 766
    , 773 (9th Cir. 2011) (“If the proper
    interpretation is not clear from this textual analysis, the
    legislative history offers valuable guidance and insight into
    Congressional intent.”) (internal quotation marks and citation
    omitted). But in this case, the legislative history of § 7627
    REDOIL V . EPA                         15
    offers no useful guidance as to congressional intent. A report
    submitted during the floor debate in the House of
    Representatives stated that “[m]arine vessels emissions . . .
    which are associated with an OCS activity, will be included
    as part of the OCS facility emissions for the purposes of
    regulation,” so as to “ensure that the cruising emissions from
    marine vessels are controlled and offset as if they were part
    of the OCS facility’s emissions.” 136 Cong. Rec. H12,848
    (daily ed. Oct. 26, 1990) (report submitted by Rep. Robert
    Lagomarsino); see also 136 Cong. Rec. S16,895 (daily ed.
    Oct. 27, 1990) (conference report submitted by Senator Max
    Baucus). Without referencing BACT or explaining what it
    means to be regulated “as part of the OCS facility emissions,”
    it is impossible to discern if Congress specifically considered
    the question of whether BACT applies to associated vessels
    in the same manner and to the same degree as an OCS source
    itself. We agree with the EAB that the legislative history
    merely “shows that Congress intended vessel emissions to be
    ‘controlled,’ ‘offset,’ ‘mitigated,’ or subject to ‘regulation,’
    all of which are accomplished to some degree” by the EPA’s
    interpretation of § 7627. In re: Shell Gulf of Mexico, 
    2010 WL 5478647
    , at *15.
    Neither does the fact that Congress enacted § 7627 after
    the D.C. Circuit’s decision in NRDC, Inc. v. EPA resolve the
    provision’s ambiguity, as REDOIL posits. 
    725 F.2d 761
    (D.C. Cir. 1984). In that case, the D.C. Circuit upheld the
    EPA’s revocation of regulations treating emissions produced
    by ships moving to or from a marine terminal as “secondary
    emissions” of the terminal itself, meaning that such emissions
    would be counted toward the marine terminal for various
    purposes, including the PSD program’s air quality impact
    analysis. 
    Id. at 766
    , 772–73. However, the court’s
    acceptance of the EPA’s revocation of the “secondary
    16                    REDOIL V . EPA
    emissions” regulation, which the agency lacked authority to
    promulgate in the first place, sheds little light on Congress’s
    intent in later employing the term “direct emissions” in the
    OCS provisions. It hardly follows that Congress’s use of
    “direct emissions” as opposed to “secondary emissions”
    manifested a clear intent to subject “direct emissions” to the
    full suite of PSD requirements.
    C. EPA’S INTERPRETATION        OF   
    42 U.S.C. § 7627
        IS
    REASONABLE
    Because the Clean Air Act is ambiguous as to the
    applicability of BACT to support vessels not attached to an
    OCS source, our task is to assess whether the agency’s
    construction of the statute is “permissible” or “reasonable.”
    Chevron, 
    467 U.S. at
    843–44. In its December 2010 ruling,
    the EAB concluded that it is permissible to apply BACT to
    support vessels only insofar as they are attached to the
    Discoverer because § 7627 “simply does not contain any
    words expressly, or by implication, explaining why the statute
    distinguishes between the OCS source and vessels servicing
    the OCS source when directing that such vessels’ emissions
    shall be considered direct emissions from the OCS source.”
    In re: Shell Gulf of Mexico, 
    2010 WL 5478647
    , at *2. We
    agree with the EAB’s characterization of the ambiguity.
    The EAB reconciled this ambiguity in two ways. First,
    the associated fleet’s emissions provide a baseline for
    triggering PSD requirements. Emissions from the entire
    associated fleet and the Discoverer were used to determine
    whether the drillship met the threshold of having the potential
    to emit two hundred and fifty tons or more of regulated air
    pollutants, thereby subjecting the drillship to PSD
    requirements. This approach followed the EPA’s rationale in
    REDOIL V . EPA                        17
    its OCS rulemaking. Having determined that, under OCSLA,
    it could not regulate vessels other than drillships and vessels
    attached to the drillship as “OCS sources,” the EPA explained
    that emissions from the associated fleet are “accounted for by
    including vessel emissions in the ‘potential to emit.’” Outer
    Continental Shelf Air Regulations, 
    56 Fed. Reg. 63,774
    ,
    63,777 (Dec. 5, 1991). This interpretation gives meaning to
    the “direct emissions” language—by including the associated
    fleet’s emissions “while at the OCS source or en route to or
    from the OCS source within 25 miles of the OCS source”
    toward the drillship itself—while respecting the limitations
    imposed by the definition of OCS source.
    The emissions from the entire fleet were also attributed to
    the OCS source in a second way—in assessing whether the
    Discoverer would cause a violation of various air quality
    standards, one of the PSD requirements. In granting the PSD
    permits, the EAB thus imposed conditions that incorporated
    the emissions of the associated fleet but found that “the
    Associated Fleet [except for a supply vessel] will not be
    physically attached to the Discoverer and therefore will not
    be part of the OSC source and not subject to the BACT
    requirements.”
    The EAB’s approach is consistent with the D.C. Circuit’s
    decision in Santa Barbara Cnty. Air Pollution Control Dist.
    v. EPA, which upheld EPA regulations refusing to regulate
    in-transit maritime vessels as OCS sources. 
    31 F.3d 1179
    (D.C. Cir. 1994). The court held that “it was reasonable for
    the EPA to conclude that OCS sources did not include vessels
    that were merely traveling over the OCS.” 
    Id. at 1181
    .
    In sum, we conclude that while the BACT requirement
    clearly applies to an OCS source, the statute is ambiguous as
    18                    REDOIL V . EPA
    to application of BACT to associated vessels within 25 miles
    of an OCS source. We defer to the EPA’s reasonable
    construction of the statute, as adopted by the EAB, that
    BACT does not apply to mobile support vessels unattached to
    the drillship.
    III.   EPA’S GRANT       OF A   500   METER    AMBIENT AIR
    EXEMPTION IS       NOT PLAINLY       ERRONEOUS OR
    INCONSISTENT WITH THE AGENCY ’S REGULATIONS
    In the revised air permits issued in September 2011, the
    EPA granted Shell’s request for a 500-meter radius “ambient
    air” exemption. The EAB, in its January 2012 decision,
    upheld this exemption, which allows Shell to assess
    compliance with air quality standards at a distance of 500
    meters from the center of the drillship. In re: Shell Gulf of
    Mexico, 
    2012 WL 119962
    . Air quality impacts within the
    500-meter radius are not subject to analysis or regulation.
    The exemption is contingent on the Coast Guard’s
    establishment of an effective safety zone precluding public
    access to the area, and Shell’s development and
    implementation of a public access program.
    According to the Supreme Court, “‘ambient air’ [] is the
    statute’s term for the outdoor air used by the general public.”
    Train v. NRDC, Inc., 
    421 U.S. 60
    , 65 (1975). Curiously, the
    Clean Air Act does not define “ambient air,” but the EPA’s
    regulations define the term as “that portion of the atmosphere,
    external to buildings, to which the general public has access.”
    
    40 C.F.R. § 50.1
    (e). The agency has occasionally exempted
    certain areas from being labeled ambient air. The parties
    agree that the agency’s “longstanding interpretation” of this
    exemption is described in a 1980 letter from former EPA
    Administrator Douglas Costle. That letter states that an
    REDOIL V . EPA                       19
    “exemption from ambient air is available only for the
    atmosphere over land owned or controlled by the source and
    to which public access is precluded by a fence or other
    physical barriers.”
    The Supreme Court instructs that an agency’s
    interpretation of its own regulations is “controlling unless
    ‘plainly erroneous or inconsistent with the regulation.’” Auer
    v. Robbins, 
    519 U.S. 452
    , 461 (1997) (quoting Robertson v.
    Methow Valley Citizens Council, 
    490 U.S. 332
    , 359 (1989)).
    The relevant inquiry is whether the EPA’s grant of the
    ambient air exemption to Shell is plainly erroneous or
    inconsistent with its regulatory definition of ambient air.
    It is obvious that the exemption here is not “for the
    atmosphere over land owned or controlled by the source and
    to which public access is precluded by a fence or other
    physical barriers” (emphasis added). However, neither is the
    exemption inconsistent with the EPA’s regulatory definition
    of ambient air or clearly erroneous. The essence of the EPA’s
    regulatory definition links ambient air to public access.
    Because the EPA conditioned Shell’s permit and ambient air
    exemption on the establishment of an effective safety zone
    that precludes public access, the grant is consistent with the
    regulation.
    Further, the EPA has not impermissibly departed from its
    longstanding regulatory interpretation of “ambient air”
    without explanation or justification, as REDOIL claims. We
    are persuaded by the EAB’s reasonable explanation that
    Costle’s 1980 letter prescribing a fence or other physical
    barrier to preclude public access was “clearly written with
    overland situations in mind.” In re: Shell Gulf of Mexico,
    
    2012 WL 119962
    , at *29. The agency did not have the
    20                    REDOIL V . EPA
    occasion to consider how it might apply the exemption in the
    context of open waters until ten years later, when Congress
    first gave the EPA jurisdiction to regulate air pollution from
    OCS sources. The EAB’s assessment that the agency
    “requires some leeway” in determining how to apply “the
    regulation and the interpretive letter to an ‘overwater’
    situation” is just common sense. 
    Id.
     Constructing a fence in
    the Arctic Ocean would make little sense, and the EPA has
    previously recognized a safety zone established by the Coast
    Guard as evidence of sufficient ownership or control over
    open water areas to qualify as a boundary for defining
    ambient air. Here, as in that precedent, the EPA imposed
    conditions that approximated the criteria in the Costle
    letter—control of property and limited public access—for a
    marine environment. We conclude that the EPA’s grant of an
    ambient air exemption to Shell conditioned on an effective
    safety zone excluding the public is a permissible
    interpretation of its ambient air regulation and earlier letter
    ruling.
    PETITION DENIED.