Court Opinion

ID: 9946852
Source: CourtListenerOpinion
Date Created: 2024-03-01 17:01:24.510669+00
Date Added: 2024-06-11T14:25:41.827352
License: Public Domain

United States Court of Appeals
        FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued March 25, 2022                 Decided March 1, 2024

                        No. 15-1239

  ENVIRONMENTAL COMMITTEE OF THE FLORIDA ELECTRIC
          POWER COORDINATING GROUP, INC.,
                   PETITIONER

                            v.

  ENVIRONMENTAL PROTECTION AGENCY AND MICHAEL S.
                     REGAN,
                  RESPONDENTS

      CITIZENS FOR ENVIRONMENTAL JUSTICE, ET AL.,
                     INTERVENORS

  Consolidated with 15-1256, 15-1265, 15-1267, 15-1268,
               15-1270, 15-1271, 15-1272

         On Petitions for Review of a Final Action
         of the Environmental Protection Agency
     Evan M. Ezray, Deputy Solicitor General, Office of the
Attorney General for the State of Florida, argued the cause for
state petitioners. On the briefs were Ashley Moody, Attorney
General, Henry C. Whitaker, Solicitor General, Daniel W. Bell,
Chief Deputy Solicitor General, Jason H. Hilborn, Deputy
Solicitor General at the time the brief was filed, Steve Marshall,
Attorney General, Office of the Attorney General for the State
of Alabama, Edmund LaCour, Solicitor General, Tim Griffin,
Attorney General, Office of the Attorney General for the State
of Arkansas, Nicholas Bronni, Solicitor General, Vincent M.
Wagner, Deputy Solicitor General at the time the brief was
filed, Kris Mayes, Attorney General, Office of the Attorney
General for the State of Arizona, Drew C. Ensign, Deputy
Solicitor General at the time the brief was filed, Kathy
Jennings, Attorney General, Office of the Attorney General for
the State of Delaware, Valerie Satterfield Edge, Deputy
Attorney General, Kris Kobach, Attorney General, Office of
the Attorney General for the State of Kansas, Jeffrey A.
Chanay, Chief Deputy Attorney General, Christopher M. Carr,
Attorney General, Office of the Attorney General for the State
of Georgia, Stephen J. Petrany, Solicitor General, Russell
Coleman, Attorney General, Office of the Attorney General for
the Commonwealth of Kentucky, Matthew F. Kuhn, Solicitor
General, Brett R. Nolan, Principal Deputy Solicitor General at
the time the brief was filed, Jeff Landry, Attorney General,
Office of the Attorney General for the State of Louisiana,
Elizabeth B. Murrill, Solicitor General, Lynn Fitch, Attorney
General, Office of the Attorney General for the State of
Mississippi, Mary Jo Woods, Special Assistant Attorney
General, Andrew Bailey, Attorney General, Office of the
Attorney General for the State of Missouri, D. John Sauer,
Deputy Attorney General, Sam M. Hayes, David Yost, Attorney
General, Office of the Attorney General for the State of Ohio,
Benjamin M. Flowers, Solicitor General at the time the brief
was filed, Gentner Drummond, Attorney General, Office of the
Attorney General for the State of Oklahoma, P. Clayton
Eubanks, Assistant Attorney General, Marty Jackley, Attorney
                              3
General, Office of the Attorney General for the State of South
Dakota, Steven R. Blair, Deputy Attorney General, Alan
Wilson, Attorney General, Office of the Attorney General for
the State of South Carolina, J. Emory Smith, Jr., Deputy
Solicitor General, Jonathan Skrmetti, Attorney General, Office
of the Attorney General for the State of Tennessee, Wilson S.
Buntin, Senior Assistant Attorney General, Ken Paxton,
Attorney General, Office of the Attorney General for the State
of Texas, Priscilla M. Hubenak, Chief, Environmental
Protection Division, Kellie E. Billings-Ray, Assistant Attorney
General, Patrick Morrisey, Attorney General, Office of the
Attorney General for the State of West Virginia, Lindsay S.
See, Solicitor General. Christopher J. Baum, Deputy Solicitor,
Office of the Attorney General for the State of Florida, Aaron
S. Farmer, Principal Assistant Attorney General, Office of the
Attorney General for the State of Ohio, Phillip R. Hilliard,
Assistant Attorney General, Office of the Attorney General for
the State of Tennessee, Thomas T. Lampman, Assistant
Attorney General, Office of the Attorney General for the State
of West Virginia, Justin L. Matheny, Deputy Solicitor, Office
of the Attorney General for the State of Mississippi, Joseph A.
Newberg, II, Assistant Attorney General, Office of the
Attorney General for the Commonwealth of Kentucky, James
H. Percival, II, Chief Deputy Solicitor General, Office of the
Attorney General for the State of Florida, Andrew A. Pinson,
Solicitor General, Office of the Attorney General for the State
of Georgia, Lee P. Rudofsky, Solicitor, Office of the Attorney
General for the State of Arkansas, and Megan K. Terrell,
Assistant Attorney General, Office of the Attorney General for
the State of Louisiana, entered appearances.

    Russell S. Frye argued the cause for industry petitioners.
With him on the briefs were Lauren E. Freeman, C. Max
Zygmont, C. Grady Moore, III, Randy E. Brogdon, Robert A.
Manning, Joseph A. Brown, Margaret C. Campbell, Carroll W.
                              4
McGuffey, III, M. Brant Pettis, Melissa Horne, Terese T. Wyly,
P. Stephen Gidiere, III, Gary V. Perko, Leslie Sue Ritts, J.
Michael Showalter, Patrick F. Veasy, Matthew Kuryla, Devi
Chandrasekaran, and Samara L. Kline. Amy C. Antoniolli and
Hahnah Williams entered appearances.

    David J. Kaplan and Sarah A. Buckley, Attorneys, U.S.
Department of Justice, argued the causes for respondents. With
them on the briefs were Todd Kim, Assistant Attorney General,
Andrew S. Coghlan, Attorney, Seth Buchsbaum, Attorney, U.S.
Environmental Protection Agency, and Paul Bangser, Sheila
Igoe, and Jan Tierney, Attorneys.

     Andrea Issod argued the cause for environmental
intervenors. With her on the briefs were Joshua D. Smith, Seth
L. Johnson, James S. Pew, Patton Dycus, John Walke, Emily
Davis, and Paul Cort. Eric Schaeffer entered an appearance.

    Before: SRINIVASAN, Chief Judge, PILLARD and WALKER,
Circuit Judges.

    Opinion of the Court filed PER CURIAM.

    Opinion concurring in part and dissenting in part filed by
Circuit Judge PILLARD.

     PER CURIAM: The Clean Air Act requires the federal
government and the states to work together to protect the
nation’s air. First, the Environmental Protection Agency
identifies pollutants that endanger public health and welfare
and sets air-quality standards that the states must meet. Then,
the states develop state implementation plans to meet and
enforce those standards. Those plans are called SIPs.
                                 5
     EPA’s role goes beyond simply making sure that SIPs will
enable states to meet the air-quality standards. Before a SIP
can go into effect, EPA also makes sure that it complies with
specific requirements that the Clean Air Act imposes for SIPs.
Then, after a SIP is approved, EPA must call for the state to
revise it if the SIP is substantially inadequate to comply with a
requirement of the Act.

     In this case, EPA called for 35 states and the District of
Columbia to revise their SIPs, though it has since withdrawn
its calls to three of those states. Two sets of petitioners, a group
of about half the states whose SIPs EPA called and a set of
companies that are subject to those SIPs, level an array of
challenges against EPA’s SIP Calls.

    We grant their petitions in part and deny them in part.

                        BACKGROUND

     We first explain the relevant parts of the Clean Air Act.
Then, we provide background on the types of SIP provisions at
issue in this case. Last, we describe the underlying EPA action
and this case’s winding path to our decision today.

                                 I.

     Congress passed the Clean Air Act in 1963. Pub L. No.
88-206, 77 Stat. 392 (1963). But until 1970, states “generally
retained wide latitude to determine both the air quality
standards which they would meet and the period of time in
which they would do so.” See Train v. NRDC, 421 U.S. 60, 64
(1975). That year, frustrated with the states’ lack of progress
toward cleaner air, Congress enacted the 1970 Amendments to
the Act. Id.
                               6
     The 1970 Amendments changed the regulatory paradigm,
replacing the old, state-centered model with “an exercise in
cooperative federalism.” Dominion Transmission, Inc. v.
Summers, 723 F.3d 238, 240 (D.C. Cir. 2013). The
Amendments empowered the newly created EPA to set air-
quality standards to protect public health and welfare. Train,
421 U.S. at 64-65. And they required the states to implement,
maintain, and enforce those standards in a timely manner. Id.

     Among other things, SIPs set rules to limit the emissions
that a source can release. Those rules are often source-specific
and sometimes take the form of a formula. See, e.g., GA. COMP.
R. & REGS. 391-3-1.02(2)(c)-(z). Emission-control rules may
also include, for example, technological control requirements
or work practice requirements.

    In addition to enabling states to meet the air-quality
standards that EPA sets, SIPs have to comply with a number of
the Clean Air Act’s specific legal requirements. Among the
most important is a requirement that SIPs:

    include enforceable emission limitations and other
    control measures, means, or techniques (including
    economic incentives such as fees, marketable permits,
    and auctions of emissions rights), as well as schedules
    and timetables for compliance, as may be necessary or
    appropriate to meet the applicable requirements of this
    chapter.

42 U.S.C. § 7410(a)(2)(A). SIPs also must, for example,
provide for monitoring systems, set up a permitting scheme,
and “include a program to provide for the enforcement of the”
emission-control measures. Id. § 7410(a)(2)(C); see also id.
§ 7410(a)(2)(B), (a)(2)(L).
                               7
     Congress tasked EPA with ensuring that SIPs comply with
the Act’s requirements. If a SIP submission meets “all of the
applicable requirements” of the Act, EPA must approve it. Id.
§ 7410(k)(3). If a state wants to revise its SIP, EPA reviews
the proposed revision and may only approve it if it does not
interfere with attainment of the national ambient air quality
standards (NAAQS) “or any other applicable requirement” of
the Act. Id. §§ 7410(k)(2)-(3), 7410(l); see also id. § 7515.

     Then, when EPA signs off on a new or revised SIP, it
incorporates the SIP into the Code of Federal Regulations,
which makes it a federally enforceable regulation. Dominion
Transmission, Inc., 723 F.3d at 244. Once the SIP is approved
and incorporated, EPA, state and local governments, and
citizens can all sue to enforce it. Nat’l Mining Ass’n v. EPA,
59 F.3d 1351, 1363 (D.C. Cir. 1995); 42 U.S.C. §§ 7413(b),
7602(e), 7604(a). In those suits, courts can award injunctive
relief or monetary damages. 42 U.S.C. §§ 7413(b), 7602(e),
7604(a).

     If that system works, that can sometimes be the end of the
story. But if problems arise, EPA must address them.
Specifically, the Act mandates that:

    Whenever the Administrator finds that the applicable
    implementation plan for any area is substantially
    inadequate to attain or maintain the relevant national
    ambient air quality standard, to mitigate adequately
    the interstate pollutant transport described in section
    7506a of this title or section 7511c of this title, or to
    otherwise comply with any requirement of this
    chapter, the Administrator shall require the State to
    revise the plan as necessary to correct such
    inadequacies.
                               8
Id. § 7410(k)(5).

     When EPA uses that authority to require states to revise
their SIPs, it is referred to as a SIP Call. If the EPA issues a
SIP Call, it must identify the SIP’s substantial inadequacies and
set a deadline that affords the state no more than 18 months to
“revise the [SIP] as necessary to correct such inadequacies.”
Id. The state need not undergo “a wholesale revision of its
entire plan,” but must make revisions necessary to correct the
substantial inadequacies EPA identified. Virginia v. EPA, 108
F.3d 1397, 1410 (D.C. Cir. 1997), modified on reh’g on other
grounds, 116 F.3d 499. Should a state fail to timely revise its
SIP, or should EPA disapprove the state’s submission, EPA
must timely promulgate a federal implementation plan for that
state. See 42 U.S.C. § 7410(c)(1).

                               II.

     During periods when a source starts up, shuts down, or
malfunctions—SSM periods—Petitioners say the source may
not be able to comply with the emission rules that apply during
regular operation. See Restatement and Update of EPA’s SSM
Policy Applicable to SIPs, 80 Fed. Reg. 33,840, 33,843 (June
12, 2015) (defining “SSM”). Some SIPs thus include a variety
of SSM provisions that can insulate sources from liability for
emissions during SSM periods that exceed the emission levels
permitted under the regular rule.

    Four types of SSM provisions are at issue in this case.

     First, some SIPs include “automatic exemptions,” which
exclude SSM periods from otherwise applicable emission
rules.
                              9
    Second, other SIPs include “director’s discretion”
provisions, which allow state officials to independently and
conclusively decide that excess emissions are not violations
during SSM periods.

     Third, at least one SIP includes provisions that EPA
believes could be read to allow state officials to excuse
emission violations during SSM periods in a way that
forecloses EPA or citizen-suit enforcement. EPA called those
“overbroad enforcement discretion” provisions.       Unlike
director’s discretion provisions—which let state officials
determine that there is no violation—overbroad enforcement
discretion provisions let state officials recognize that a
violation happened but bar enforcement.

    Fourth, many SIPs include affirmative defenses for excess
emissions that occur during SSM periods. Some affirmative
defenses protect sources against all liability, while others
protect only against certain forms of relief. E.g., 118-01-19
ARK. CODE § 602 (all liability); ARIZ. ADMIN. CODE § 18-2-
310(B)-(C) (providing an affirmative defense except in a
“judicial action seeking injunctive relief”).

                             III.

     This case’s origins date back over a decade. In June 2011,
Sierra Club filed a petition for rulemaking. Among other
requests, it identified 39 SIPs that it believed included SSM
provisions that made the SIPs unlawful, and it asked EPA to
call them.

     Twenty months later, EPA published a proposed rule in
response to Sierra Club’s petition. It initially indicated its
intent to grant Sierra Club’s petition as to all types of SSM
provisions except affirmative defenses to monetary damages
                              10
for excess emissions during malfunctions. Then, in NRDC v.
EPA, we vacated a similar affirmative defense in a federal
emission standard. 749 F.3d 1055, 1058, 1064 (D.C. Cir.
2014). So EPA published a supplemental notice in which it
proposed to grant Sierra Club’s petition as to those affirmative
defenses as well.

     Comments flowed in. Sixty-nine thousand different
commenters offered their thoughts on the original proposed
rule, and twenty thousand commenters responded to the
supplemental notice.

     In 2015, after considering those comments, EPA published
its Final Action. It called SIPs from 35 states and the District
of Columbia.

     EPA began by explaining why it was now calling SIPs it
had previously approved. When Congress enacted the 1970
Clean Air Act amendments, it set an ambitious timetable for
implementing the cooperative-federalism framework. It gave
EPA 30 days (plus a 90-day notice-and-comment period) to
develop the first air-quality standards. Train, 421 U.S. at 65.
Then, the states had nine months from EPA’s promulgation of
air-quality standards to submit SIPs. Id. From there, EPA had
four months to verify that each SIP complied with the Clean
Air Act and, if so, to approve it. Id. That timetable was a tall
order for an EPA that had been created less than a month
earlier. Reorganization Plan No. 3 of 1970, reprinted in 5
U.S.C. app. 1 (creating EPA effective December 2, 1970); Pub.
L. No. 91-604, 84 Stat. 1676 (Dec. 31, 1970) (1970
Amendments).

    In the SIP Calls here, EPA explained that the timetable
proved too tall an order: “[B]ecause the EPA was inundated
with proposed SIPs and had limited experience in processing
                                11
them, not enough attention was given to the adequacy,
enforceability and consistency of” SSM provisions. 80 Fed.
Reg. 33,840, 33,843/3 (June 12, 2015).

     On the merits of its call, EPA first explained its view that
it can call any SIP that “contains a provision that is inconsistent
with fundamental requirements of the” Clean Air Act. Id. at
33,926/3. As EPA reads the Act, it does not need to
demonstrate that the substantial legal infirmity has had any
demonstrated ill effects; its existence is enough.

     EPA then identified the aspects of the Act that it believes
the SSM provisions are inconsistent with.

     It started with automatic exemptions. Because EPA reads
the Act to “require that SIPs contain ‘emissions limitations’ to
meet CAA requirements” and “those emissions limitations
must be continuous,” 80 Fed. Reg. 33,927/2, it argues that
automatic exemptions bring SIPs out of compliance with the
Act. The Act defines an emission limitation as a requirement
that limits emissions “on a continuous basis.” 42 U.S.C.
§ 7602(k). According to EPA, an exemption would make the
limitation discontinuous and thus violate the Act’s definition of
an emission limitation.

     Next, EPA applied the same logic to director’s discretion
provisions. It determined that allowing state officials to decide
that excess emissions during SSM periods were not a violation
makes an emission limitation as discontinuous as simply
having a categorical exemption. In addition, EPA thought that
if a state official can decide to waive an emission limitation’s
application, such waivers would amount to “de facto revisions
of the approved emission limitations” without EPA approval.
80 Fed. Reg. at 33,928/1.
                               12
     After that, EPA explained that overbroad enforcement
discretion provisions unlawfully blocked the Act’s
enforcement scheme by allowing a state to unilaterally bar an
EPA or citizen suit.

     Finally, EPA concluded that affirmative defense
provisions similarly interfere with the Act’s enforcement
scheme. EPA’s reasoning tracked the logic of the NRDC
decision that had persuaded EPA to revise its interpretation and
issue the supplemental notice of proposed rulemaking. See
NRDC, 749 F.3d at 1063-64.

     Three sets of petitioners sought our review: Texas and a
coalition of Texas companies and trade groups; a group of other
states that petitioned together; and a group of industrial
companies and organizations. The Sierra Club and other
environmental groups intervened to support EPA. The many
parties completed extensive briefing, and we scheduled an oral
argument for May 8, 2017. Then, less than a month before
argument, EPA asked us to postpone the argument as the new
administration reconsidered the SIP Calls. We agreed, and the
case remained in abeyance for more than four years.

     During those four years, EPA withdrew its calls to Iowa,
North Carolina, and Texas—leaving 32 states and the District
of Columbia subject to the SIP Calls. That mooted the Texans’
petitions for review and meant that the SIP Calls issued to
North Carolina and Iowa are not before this court, nor are the
SIP Calls directed at states that never joined the petition. That
leaves sixteen State Petitioners and the Industry Petitioners.

     In November 2021, EPA reaffirmed the original SIP Calls
as to the remaining states and moved to reopen this case. We
granted the motion and now resolve the case as follows.
                               13
     In part I of the analysis, we evaluate EPA’s SIP-call
authority. In part II, we evaluate EPA’s specific objections to
the four types of SSM provisions.

                          ANALYSIS

     We set aside an EPA SIP Call if it is “arbitrary, capricious,
an abuse of discretion, or otherwise not in accordance with
law” or if it is “in excess of [EPA’s] statutory jurisdiction,
authority, or limitations.” 42 U.S.C. § 7607(d)(9)(A), (C); see
also Maryland v. EPA, 958 F.3d 1185, 1196 (D.C. Cir. 2020).
We employ the traditional tools of statutory interpretation to
determine what the Clean Air Act requires. Sierra Club v.
EPA, 551 F.3d 1019, 1026-27 (D.C. Cir. 2008). Courts, like
EPA, “must give effect to the unambiguously expressed intent
of Congress.” Id. at 1026 (quoting Chevron U.S.A., Inc. v.
NRDC, 467 U.S. 837, 844 (1984)). If, however, the Clean Air
Act is ambiguous on a particular issue, we defer to the agency’s
reasonable interpretation of the Act. Id.

    State and Industry Petitioners challenge EPA’s SIP Calls
on two fronts. First, they argue that EPA misinterpreted its
SIP-call authority under section 7410(k)(5) of the Clean Air
Act. Second, they claim that EPA incorrectly interpreted the
Act, as well as the SIPs in question, when it called the SIPs for
containing at least one of four types of SSM provisions that
EPA deemed impermissible: (1) automatic exemption
provisions, (2) director’s discretion provisions, (3) overbroad
enforcement discretion provisions, and (4) affirmative defense
provisions. We consider each challenge in turn.

                                I.

    We begin by addressing Petitioners’ crosscutting
challenges to EPA’s statutory authority to issue any of these
disputed SIP Calls. Section 7410(k)(5) of the Clean Air Act
                              14
sets out EPA’s SIP-call authority. It states, as relevant here,
that whenever EPA “finds” that a SIP is “substantially
inadequate” to “comply with any requirement of this chapter,”
then EPA “shall” require the state to “revise the [SIP] as
necessary to correct such inadequacies.”             42 U.S.C.
§ 7410(k)(5). State and Industry Petitioners advance four
arguments why EPA misinterpreted its SIP-call authority.
First, they assert that EPA lacked authority to call the SIPs
without making factual findings about the real-world effects of
the inadequate SIPs, even when calling SIPs as substantially
legally inadequate to “comply with any requirement” of the
Act. Second, Petitioners insist that, when issuing a SIP Call,
EPA must consider each SIP “as a whole,” rather than object
to individual provisions in isolation. Third, they maintain that
ambiguity in a SIP provision—that is, where it is unclear
whether a SIP provision conflicts with the Act—can never be
a sufficient basis for issuing a SIP Call. Fourth, and finally,
they argue that EPA must engage in a cost-benefit analysis
whenever it calls a SIP and that failure to do so renders the
challenged action arbitrary and capricious. We consider each
of those arguments and conclude that, in its Final Action, EPA
abided by the strictures of its SIP-call authority under section
7410(k)(5).

                              A.

     Petitioners contend that, before EPA issues any SIP Call
under section 7410(k)(5), it must make factual findings about
adverse effects resulting from the SIP’s deficiencies—for
example, by identifying instances in which the SSM provisions
at issue prevented or will prevent attainment of the national
ambient air quality standards (NAAQS). That argument is at
odds with the statutory text and structure. While factual
findings about the effects of SIP inadequacies may be needed
to support some types of SIP Calls, section 7410(k)(5) does not
                               15
categorically require them. The Act obligates EPA to issue a
SIP Call whenever it determines that a SIP does not comply
with the Act’s legal requirements, so long as the legal
deficiencies are “substantial[].” 42 U.S.C. § 7410(k)(5). A SIP
Call based on failure to comply with the Act’s legal
requirements need not be withheld until the agency is able to
document actual or anticipated damage to the environment, to
human health or welfare, or to any other objective of the Act.
Requiring such a factual showing would impermissibly
“place[] an information submission obligation on EPA [that]
Congress did not impose,” EPA v. EME Homer City
Generation, L.P., 572 U.S. 489, 510 (2014).

     We start with the statutory source of EPA’s authority to
call a SIP. Section 7410(k)(5) of the Act compels the agency
to call a SIP whenever it “finds” that the SIP is “substantially
inadequate” for one of three reasons: (1) “to attain or maintain
the relevant national ambient air quality standard,” (2) “to
mitigate adequately the interstate pollutant transport,” or (3)
“to otherwise comply with any requirement” of the Act. 42
U.S.C. § 7410(k)(5). Those three distinct grounds for issuing
a SIP Call are stated as alternatives, separated by the word “or.”
In using that conjunction, Congress made clear that a
substantial inadequacy to “otherwise comply with any
requirement” of the Act (per the third stated ground) is by itself
sufficient to require EPA to call the SIP. Id. EPA need not
also show the SIP meets the standard to be called under the first
ground for failure to attain or maintain the NAAQS, or under
the second ground for failure to mitigate interstate pollutant
transport.

      EPA might call a SIP for failure to comply with the Act
(i.e., under the third ground) in a variety of circumstances. To
provide a few examples, a SIP might be challenged as
inadequate under the third ground if the state fails to provide
                               16
for the operation of air quality monitoring devices, id.
§ 7410(a)(2)(B), to ensure personnel to carry out the plan, id.
§ 7410(a)(2)(E), to require periodic reporting of emissions-
related data from stationary sources, id. § 7410(a)(2)(F)(ii), to
provide for consultation with local governments, id.
§§ 7410(a)(2)(J), 7421, to submit air quality modeling data to
EPA, id. § 7410(a)(2)(K)(ii), or to address non-NAAQS
pollutants in new source review permit programs, id. §§ 7470-
79, 7503.

     In the action under review, EPA relied on that third ground
for issuing a SIP Call. 80 Fed. Reg. 33,840, 33,925/3 (June 12,
2015). EPA concluded that the SIPs were substantially
inadequate to comply with the Act’s requirements for emission
limitations, its remedial and enforcement provisions, and its
procedural requirements for revising a SIP. See, e.g., id. at
33,874/2-75/2, 33,957/2-74/2.        The inadequacies EPA
identified were legal in nature—that is, the SIPs on their face
conflicted with the Act’s legal requirements vis-à-vis SIPs.

     We first consider what findings, if any, the plain text of
section 7410(k)(5) requires when EPA calls a SIP for legal
inadequacies. Under the third ground, EPA plainly must find
that a SIP is substantially inadequate to “comply with any
requirement” of the Act. 42 U.S.C. § 7410(k)(5). That is, EPA
must determine that a SIP provision has not conformed to
“those statutory and regulatory requirements that are germane
to the SIP provision at issue.” 80 Fed. Reg. at 33,925/3.

    The shortcoming EPA identifies must also be one that
renders the SIP “substantially inadequate” to comply with the
Act. 42 U.S.C. § 7410(k)(5). In the SIP Calls, EPA noted that
“section 110(k)(5) [i.e., 42 U.S.C. § 7410(k)(5)] does not
specify a particular form of analysis or methodology that the
EPA must use to evaluate SIP provisions for substantial
                               17
inadequacy.” 80 Fed. Reg. at 33,926/1. EPA noted that, for
some SIP Calls, a “technical evaluation” of the inadequacy
might be warranted. Id. at 33,937/1. But calling a SIP that on
its face violates key legal requirements of the Act “does not
require that type of technical analysis and does not require a
‘quantification’ of the extent of the deficiency.” Id. To
determine that a SIP is “substantially inadequate,” EPA need
conclude only that it is materially deficient, or falls short in a
meaningful, nontrivial way. See, e.g., WEBSTER’S NINTH NEW
COLLEGIATE DICTIONARY 1176 (1990) (“substantial” includes
“being largely but not wholly that which is specified”);
WEBSTER’S NEW WORLD DICTIONARY 1336 (3d. coll. ed.
1988) (“substantial” includes “with regard to essential
elements; in substance”); id. at 680 (“inadequate” means “not
adequate; not sufficient; not equal to what is required”). The
Final Action abides by that ordinary understanding of
“substantially inadequate”: Where SIPs threaten to undermine
the “fundamental integrity of the [Clean Air Act]’s SIP process
and structure” and allow the Act’s emission rules to be
“violated without potential recourse,” 80 Fed. Reg. at 33,926/3,
they readily clear the threshold of substantial inadequacy.

     Beyond determining that the SIP is deficient and that such
deficiency is material, EPA need not invariably make factual
findings about the effects of that asserted legal deficiency—for
example, on air quality. Section 7410(k)(5)’s third ground
imposes no such limitation. Under that clause, EPA is
obligated to call a SIP that is substantially inadequate for
purposes of “any” Clean Air Act requirement, 42 U.S.C.
§ 7410(k)(5), no matter the degree to which that requirement is
shown to impair the SIP’s ability to protect the environment or
human health and welfare. In some cases, as here, the
requirement will be legal in nature—for example, a
requirement about what SIPs must contain, not what emission
reductions the state must achieve. Under those circumstances,
                                18
noncompliance may be readily apparent from the statutory text
and the SIP itself. As EPA recognized, “it is not necessary to
establish that these deficiencies literally caused a specific
violation of the NAAQS on a particular day or undermined a
specific enforcement case.” 80 Fed. Reg. at 33,937/1. Failure
to comply with the Clean Air Act’s legal requirements “does
not become legally permissible merely because there is not
definitive evidence” of adverse environmental or other effects
caused by that noncompliance. Id. at 33,926/2.

      Our conclusion that EPA need not, as a blanket rule, make
factual findings about the effects of a legal inadequacy accords
with section 7410(k)(5)’s requirement that EPA “find” a
substantial inadequacy. Because Congress has not defined the
term “find” as used in section 7410(k)(5), we “construe [that]
statutory term in accordance with its ordinary or natural
meaning.” FDIC v. Meyer, 510 U.S. 471, 476 (1994); see also
Food Mktg. Inst. v. Argus Leader Media, 139 S. Ct. 2356, 2363
(2019). The ordinary meaning of “find” is “to settle upon and
make a statement about (as a conclusion).” WEBSTER’S NINTH
NEW COLLEGIATE DICTIONARY 464 (1990); see also Find,
BLACK’S LAW DICTIONARY (6th ed. 1990) (“to determine”).
To be sure, in legal usage, “find” sometimes connotes
factfinding, as distinct from reaching a legal conclusion. See,
e.g., FED. R. CIV. P. 52 (requiring courts in a bench trial to make
findings of fact and state conclusions of law separately); Find,
BLACK’S LAW DICTIONARY (6th ed. 1990) (defining “find” to
include “[t]o announce a conclusion upon a disputed fact or
state of facts”). But the word “find” “easily admits of multiple
meanings,” Schiller v. Tower Semiconductor Ltd., 449 F.3d
286, 300 (2d Cir. 2006), and is not limited to that particular
legal usage. “And it is normal usage that, in the absence of
contrary indication, governs our interpretation of texts.”
Freeman v. Quicken Loans, Inc., 566 U.S. 624, 634 (2012).
EPA’s reading—that it may “find” a SIP provision inadequate
                               19
where the SIP “does not meet applicable legal requirements,”
even “without definitive proof that this legal deficiency caused
a specific outcome,” 80 Fed. Reg. at 33,934/2—is faithful to
the ordinary, popular meaning of the word “find.”

     In short, the text of section 7410(k)(5) instructs that EPA
shall issue a SIP Call whenever it concludes that a SIP is
materially deficient to comply with any requirement of the Act.
In the Final Action, EPA did exactly that, explaining why in its
view the SIPs were deficient to comply with the Act’s
requirements for emission limitations, its remedial and
enforcement provisions, and its procedural requirements for
revising a SIP. See, e.g., id. at 33,874/2-75/2, 33,957/2-74/2.
EPA further explained why those asserted deficiencies were
“substantial.” See, e.g., id. at 33,926/3, 33,927/1-29/3. At least
where EPA calls a SIP for substantial legal inadequacies,
section 7410(k)(5) requires nothing more.

     Statutory structure and context confirm that EPA need not
in all cases make factual findings about the practical effects of
asserted inadequacies before calling a SIP for substantial legal
inadequacies.

     First, our reading comports with section 7410(a)(2)(H)(ii),
which requires SIPs to “provide for revision” of their own
terms “whenever [EPA] finds on the basis of information
available to [EPA] that the plan is substantially inadequate . . .
to otherwise comply with any additional requirements
established under this chapter.” 42 U.S.C. § 7410(a)(2)(H).
Sections 7410(a)(2)(H)(ii) and 7410(k)(5) “complement[]” one
another. Virginia, 108 F.3d at 1410. The former describes
what the states must include in their SIPs—that is, they must
reserve the legal authority to revise their SIPs in the event EPA
deems them substantially inadequate, see 42 U.S.C.
                              20
§ 7410(a)(2)(H)(ii)—while the latter codifies EPA’s SIP-call
authority, see id. § 7410(k)(5).

     Petitioners home in on the phrase “on the basis of
information available,” which appears only in section
7410(a)(2)(H)(ii), to assert that EPA must make factual
findings. They read EPA’s SIP-call authority in section
7410(k)(5) to incorporate language from section
7410(a)(2)(H)(ii) requiring EPA to find substantial inadequacy
“on the basis of information available.” That claim fails. For
one, we agree with EPA that, unlike section 7410(k)(5),
“section 110(a)(2)(H)(ii) [42 U.S.C. § 7410(A)(2)(H)(ii)] does
not on its face directly address the scope” of EPA’s authority
to call SIPs. 80 Fed. Reg. at 33,934/2. We thus do not interpret
section 7410(a)(2)(H)(ii)’s “listing of specific structural or
program requirements” for SIPs “in a way that contradicts or
curtails the broad [SIP-call] authority” that Congress codified
in section 7410(k)(5). Id. at 33,934/2-3.

     Even if one were to consider section 7410(a)(2)(H)(ii) in
isolation, Petitioners overread the phrase “on the basis of
information available.”       By its own terms, section
7410(a)(2)(H)(ii) does not specify any particular type of
supportive material for an EPA determination of substantial
inadequacy. It demands only that a SIP Call be based on
available “information,” which ordinarily refers to “data,”
“facts,” or “knowledge acquired in any manner.” WEBSTER’S
NEW WORLD DICTIONARY 222 (2d. coll. ed. 1974). It is well
within EPA’s authority under section 7410 to base a finding of
substantial inadequacy on the agency’s knowledge acquired
through comparing a SIP’s provisions to the relevant Clean Air
Act requirements. By allowing EPA to decide to call a SIP
based on “available” information, Congress made clear that
EPA does not need to gather new information or set forth
factual findings. Put otherwise, EPA has the authority to
                                21
request additional information from the states as needed, see 42
U.S.C. § 7410(p), but is under no obligation to do so.

     Second, our reading respects that section 7410(k)(5),
governing EPA’s SIP-call authority, suggests the agency
should evaluate states’ earlier-approved SIPs with a degree of
leeway not afforded when EPA reviews states’ initial SIP
submissions or their proposed revisions. When a state submits
a SIP or SIP revision for approval, EPA may not approve it
unless “it meets all of the applicable requirements” of the Act.
Id. § 7410(k)(3). Moreover, EPA may not approve a SIP
revision if it “would interfere with any applicable requirement”
of the Act, id. § 7410(l)—that is, if it would cause the state to
backslide or come out of compliance with the Clean Air Act’s
requirements, see 80 Fed. Reg at 33,941/3. Those standards
governing EPA’s approval of SIPs and their revisions appear
to require EPA to ensure absolute or near-absolute compliance
with the Act.

     In contrast, Congress directed EPA, when calling an
earlier-approved SIP, to apply a more forgiving compliance
standard. Under section 7410(k)(5), a SIP’s failure to comply
with the Act’s requirements must be “substantial[],” 42 U.S.C.
§ 7410(k)(5), thereby preventing EPA from calling an
approved SIP based on slight variations from the Act’s
requirements. The higher bar on SIP Calls than on initial
approvals makes sense: While Congress sought to ensure strict
compliance with the Act in the first instance, it did not want
EPA calling long-approved implementation plans for minimal
forms of noncompliance too insubstantial to justify a re-do.

      There may well be provisions of the Act, then, with which
a SIP’s failure to ensure compliance should prevent the SIP’s
initial approval but, if only identified later, would not justify a
SIP Call. To the extent that those provisions exist, Petitioners
                                 22
do not argue that the main provision at issue in the Final
Action—section 7410(a)(2)(A), specifying some of the key
required contents of SIPs—is among them. And for good
reason. That section, which directs that each SIP “shall”
include “emission limitations” and “other control measures,
means, or techniques” as “necessary or appropriate” to meet
the Act’s requirements, goes to the heart of SIPs’ role in EPA’s
and states’ implementation and enforcement of the Act. See id.
§ 7410(a)(2)(A).

     We now turn to a third and final reason why the statutory
context and structure demand only an EPA determination of
substantial legal inadequacy and not necessarily factual
findings about the adverse effects resulting from the SIP’s
deficiencies: Namely, an across-the-board obligation for EPA
to make such factual findings would perturb the federal-state
balance Congress struck in the Clean Air Act.

     The Clean Air Act is an exercise in cooperative federalism.
One of the core obligations it imposes on EPA is to identify air
pollutants that endanger public health and welfare, and to set
standards for permissible ambient concentrations of those
pollutants. See id. §§ 7408-09. Congress then obligates states
to determine how they will meet those air-quality standards,
affording each state leeway to select means consistent with its
particular circumstances and priorities, and to accordingly
develop its own implementation plan for EPA’s approval. See
id. § 7410(a)(2). EPA cannot require states to adopt a
particular emission-control measure, see Virginia, 108 F.3d at
1408, and did not do so in the challenged SIP Calls, giving
states a range of options to correct their deficient SIPs, see, e.g.,
80 Fed. Reg. at 33,947/1; see also id. at 33,976/2-82/2. But,
while states generally have “the power to determine which
sources w[ill] be burdened by regulation and to what extent,”
Union Elec. Co. v. EPA, 427 U.S. 246, 269 (1976), the Act
                                23
“‘subject[s] the states to strict minimum compliance
requirements’ and gives EPA the authority to determine a
state’s compliance with the requirements,” Michigan v. EPA,
213 F.3d 663, 687 (D.C. Cir. 2000) (quoting Union Elec. Co.,
427 U.S. at 256-57).

      By requiring a determination of “substantial[]
inadequa[cy]” before EPA must issue a SIP Call, Congress
ensured that EPA cannot call states’ already-approved SIPs for
slight or immaterial noncompliance with the Act. 42 U.S.C.
§ 7410(k)(5). At the same time, Congress recognized EPA’s
need to act prophylactically to protect air quality and entrusted
EPA to ensure states achieve the Act’s objectives. See
generally id. §§ 7401(a)(3)-(4), (c), 7410(k)-(l). Mandating
that EPA factually demonstrate observed adverse effects on air
quality or enforcement, as State Petitioners urge, would
contravene the statutory text and undercut the role that
Congress reserved for EPA in the Clean Air Act. Just as EPA
cannot force the states to adopt a particular control measure,
the states cannot force EPA to wait for air quality to deteriorate,
or for human health and welfare to suffer, before seeking
corrections to SIPs with substantial legal deficiencies. After
all, “[a]n agency need not suffer the flood before building the
levee.” See Stilwell v. Off. of Thrift Supervision, 569 F.3d 514,
519 (D.C. Cir. 2009).

     Finally, we disagree with Industry Petitioners that the
Act’s legislative history shows Congress intended EPA to
make factual findings before calling a SIP, even when the SIP’s
inadequacy is a legal shortcoming discernible from the text of
the statute and the SIP itself. Industry Petitioners rely on a
Senate report from 1970 that describes EPA’s job to “find[]
from new information developed after a plan is approved that
the plan is not or will not be adequate to achieve promulgated
ambient air quality standards” and “notify the appropriate
                                24
States and give them an opportunity to respond to the new
information.” Ind. Pet. Br. at 31-32 (emphasis omitted)
(quoting S. REP. NO. 91-1196, at 55-56 (1970)). That report
has little relevance here and, in any case, does not support
Petitioners’ argument. It speaks to the first ground for issuing
a SIP Call, i.e., where a SIP is substantially inadequate “to
attain or maintain the relevant national ambient air quality
standard.” 42 U.S.C. § 7410(k)(5). The report does not
address EPA SIP Calls under the third ground for failure to
“comply with any requirement” of the Act, id.; indeed, in 1970
when the report was written, the Act did not yet include the
third ground for a SIP Call. Only in 1977 did Congress expand
section 7410(a)(2)(H)(ii) to recognize that a SIP may be
substantially inadequate to otherwise comply with the Act’s
legal requirements. See Clean Air Act Amendments of 1977,
Pub. L. No. 95-95, § 108(a)(6)(A), 91 Stat. 685, 693-94. And
only in 1990 did Congress codify the section 7410(k)(5) SIP-
call authority. See Clean Air Act Amendments of 1990, Pub.
L. No. 101-549, § 101(c), 104 Stat. 2399, 2407. Even if that
legislative history were relevant, it is not nearly as illuminating
as Petitioners make it out to be. As EPA recognized, the
legislative history does not require Industry Petitioners’
reading of “find” or “information” any more than does the
statute itself. See 80 Fed. Reg. at 33,935/1.

     In sum, we hold that when EPA calls a SIP for a substantial
legal inadequacy, it need only identify the deficiency and
explain why it is substantial. Whether a SIP is “substantially
inadequate” to comply with the Act may depend on the
particular circumstances of the SIP Call at issue, including the
nature of the Clean Air Act provisions the SIP violates, as well
as the extent of its noncompliance. We further hold that the
Act does not categorically require EPA, when calling a SIP for
a substantial legal inadequacy, to make specific factual
findings of actual or projected harm to the Act’s objectives as
                               25
a result of that deficiency. In so holding, we note that the Tenth
Circuit came to a similar conclusion in U.S. Magnesium, LLC
v. EPA, 690 F.3d 1157 (10th Cir. 2012). There, the court
explained that because section 7410(k)(5) “says nothing about
whether the agency is required to make a specific factual
finding” before calling a SIP, EPA reasonably interpreted its
authority not to require factual findings when calling a SIP for
failure to comply with the Act’s legal requirements. Id. at
1167; see id. at 1167-68. In reaching a similar bottom line, we
see no need to resort to deference. The statute itself is clear:
Factual findings about the knock-on effects of a SIP’s
deficiencies are not categorically required when EPA calls a
SIP for substantial inadequacies to comply with the Act.

                               B.

    We next consider Industry Petitioners’ assertion that EPA
cannot base a SIP Call on legal deficiencies in specific SIP
provisions so long as the SIP “as a whole” is adequate to
comply with the Act. We conclude that we need not decide
whether section 7410(k)(5) requires EPA to review SIPs “as a
whole,” because even if EPA were subject to such a
requirement, the agency clearly satisfied it here.

     Industry Petitioners seem to argue that EPA cannot object
to particular SIP provisions, even if inconsistent with the Act’s
explicit requirements, so long as the SIP as a whole ultimately
complies with the NAAQS. To the extent this argument
reprises Petitioners’ contention that EPA must make factual
findings about a SIP’s overall effectiveness in attaining the
NAAQS before calling it, we have already explained why no
such factual findings are necessary when EPA calls a SIP for
substantial legal inadequacies. See pp. 14-25, supra. In any
event, Petitioners’ argument is belied by the text of section
7410(k)(5). Recall that section 7410(k)(5) identifies three
                               26
distinct grounds for calling a SIP. Congress expressly
recognized that some SIPs might be deficient in ways that
interfere with the NAAQS and thus could be called under the
first ground, or that fail to curtail interstate pollution and so
could be called under the second. Acknowledging, however,
that SIPs can be substantially inadequate in other ways,
Congress conferred on EPA the third type of SIP-call authority,
the power to call SIPs that are “otherwise” legally deficient. 42
U.S.C. § 7410(k)(5). Requiring EPA to demonstrate adverse
effects on the NAAQS for all SIP Calls—even those under the
third ground—would contravene the plain text of section
7410(k)(5) and would render the third ground surplusage.

     Industry Petitioners further claim that EPA overlooked
other provisions in the called SIPs that bring the SIPs “as a
whole” into compliance with the Act’s enforcement scheme
and its specification of continuous emission limitations. In
particular, commenters pointed to SIP provisions that confer a
“general duty” on sources to limit emissions at all times,
including during SSM periods. Those general-duty provisions,
Petitioners argue, resolve any perceived violations of the Act’s
requirements vis-à-vis emission limitations. Setting aside
whether those general-duty provisions satisfy the Act’s
definition of emission limitation, see Sierra Club, 551 F.3d at
1027-28, the record demonstrates that EPA did in fact consider
those general-duty provisions, see 80 Fed. Reg. at 33,889/1-
90/1, 33,903/1-04/2, 33,979/3-80/1; see also EPA Br. at 68-84,
129-31. The agency explained in detail why, in its view, the
general-duty provisions failed to bring the SIPs into
compliance. See 80 Fed. Reg. at 33,903/1-04/2. On appeal,
Petitioners do not cite any other SIP provisions they claim EPA
overlooked and that they assert obviate the need for a SIP Call.
Thus, whether or not EPA was required to consider the SIPs
“as a whole,” it plainly did so.
                               27
                               C.

     State Petitioners argue that EPA impermissibly called SIPs
to address ambiguous SIP provisions—that is, provisions that
could be read as incompatible with the requirements of the
Clean Air Act. According to State Petitioners, ambiguous
provisions have only the “mere potential for” inadequacy.
State Pet. Br. at 19. Thus, say the states, EPA has arbitrarily
lowered the standard for calling SIPs from “substantial[]
inadequa[cy]” to potential substantial inadequacy. For its part,
EPA explained that ambiguous SIPs undermine the Act’s
effectiveness: Because ambiguous provisions leave the
regulated community, public, regulators, and courts uncertain
as to what the SIP does—and does not—require, such
provisions dilute the Clean Air Act’s compliance requirements
and enforcement scheme. See 80 Fed. Reg. at 33,885/1-2,
33,886/2, 33,926/3-27/1. In other words, the inadequacy is not
merely “potential” because lack of clarity on its own hinders
enforcement.

     We reject Petitioners’ claims that ambiguity cannot
support a SIP Call. Nothing in section 7410(k)(5) requires
substantial inadequacies to be unambiguous.               Rather,
consistent with the Tenth Circuit’s decision in U.S.
Magnesium, we conclude that section 7410(k)(5) empowers
EPA to call SIPs to clarify language that may be read to violate
the Act. See 690 F.3d at 1169-70. By calling SIPs for
ambiguous provisions, EPA fulfills its role within the statutory
scheme. Congress charged EPA with ensuring that SIPs meet
the requisites of the Clean Air Act and fulfill the Act’s purpose
of improving air quality. See 42 U.S.C. § 7410(k)(5); id.
§ 7410(k)(3); id. § 7410(l); see generally id. §§ 7401,
7410(a)(2). Ambiguous SIP provisions leave unclear what
regulated parties must do to conform to the SIP’s requirements
and hamstring EPA in its efforts to police compliance with the
                                28
Act. As the agency observed, if EPA is “unable to ascertain”
what a SIP provision requires, then courts, regulated entities,
and the public “will have the same problem.” 80 Fed. Reg. at
33,943/3. By seeking clarification or revision of ambiguous
SIP provisions, EPA dispels any uncertainties as to the SIP’s
demands, enabling SIP provisions to fulfill their role in
achieving the mandates of the Clean Air Act.

     State Petitioners advance a distinct argument against
EPA’s stance on ambiguous provisions. EPA’s position is
especially vexing, they say, because EPA will not consider
states’ post hoc interpretive letters—i.e., letters submitted after
EPA’s approval of the SIP—to clarify ambiguous provisions.
See id. at 33,888/1. Instead, if states wish to rely on interpretive
letters to cure material ambiguities in their SIPs, EPA requires
that the letters be provided during notice and comment. That
ensures that EPA approval rests on a shared EPA and state
understanding, appropriately memorialized in the public
rulemaking docket. Id. at 33,885/2-88/3. Whatever its merit,
EPA’s decision not to rely on after-the-fact interpretive letters
to resolve any ambiguity in SIPs is beside the point: Petitioners
did not challenge in the period for public comment EPA’s
policy against reliance on post hoc interpretive letters. See id.
at 33,887/1-9/1.

     Thus, under section 7410(k)(5), if EPA reasonably
determines that a SIP provision could reasonably be understood
to conflict with the Act, that can suffice to warrant a finding of
substantial inadequacy.

                                D.

     Last, Industry Petitioners maintain that EPA did not fulfill
its duty to consider the economic costs and benefits of the SIP
Calls.     They assert that a finding of “substantial[]
inadequa[cy]” requires consideration of costs and benefits, and
                               29
that EPA’s decision not to carry out such an analysis
necessarily renders the Final Action arbitrary and capricious.
According to Petitioners, EPA should have evaluated not just
the “direct costs” to states of editing the violative SIP
provisions, see 80 Fed. Reg. at 33,883/3-84/2, but also the costs
to revise other portions of their SIPs that refer to the violative
provisions and/or to create new, compliant emission rules, as
well as the cost to industry to meet revised SIP provisions.

     Contrary to EPA’s contention, Industry Petitioners timely
objected during the period for public comment to EPA’s lack
of cost-benefit analysis. Several commenters critiqued EPA
for failing to conduct a cost-benefit analysis and cautioned that
the SIP Calls would impose outsized costs on states and
industry without commensurate benefits. See SSM Coalition
Comments, J.A. 598 (“EPA failed to perform such an
assessment of the costs and benefits.”), 595 (“EPA Unlawfully
Failed To Assess the Undoubtedly High Costs the Proposed
SIP Calls Would Impose on States and Regulated Sources . . .
The SIP Call Notice is totally lacking in any analysis of what
this EPA action would cost the states, stationary sources, and
the public.”), 579 (“The SIP Call Notice would impose
tremendous resource demands . . . and costly new constraints
on [source] operations.”), 580 (“The proposed SIP Calls would
impose huge administrative burdens on state agencies, as well
as significant costs for regulated facilities, without any clear
environmental benefit.”); Southern Company Comments, J.A.
506-12 (section titled “EPA Has Arbitrarily and Capriciously
Failed To Consider the Significant Cost, Technical, and
Operational Burden of This Rule”). Because Petitioners
objected “with reasonable specificity during the period for
public comment,” we consider their objections. 42 U.S.C.
§ 7607(d)(7)(B).
                              30
     Contrary to Industry Petitioners’ contention, however,
section 7410(k)(5) does not impose an across-the-board
obligation on EPA to quantify and weigh costs and benefits
before calling a SIP. In calling for cost-benefit analysis,
Industry Petitioners principally rely on Michigan v. EPA, 576
U.S. 743 (2015). Michigan and other states challenged EPA’s
disregard of costs when the agency first considered whether to
regulate power plants’ emissions of hazardous air pollutants
under Clean Air Act section 112. Id. at 750. Interpreting
section 112’s instruction that the agency regulate power plants
only if it found such regulation “appropriate and necessary,” 42
U.S.C. § 7412(n)(1)(A), the Supreme Court held that EPA had
unreasonably ignored costs in its threshold determination
whether to regulate. Michigan, 576 U.S. at 753. The Court
reasoned that the word “appropriate” was the “classic broad
and all-encompassing term that naturally and traditionally
includes consideration of all the relevant factors,” including
cost. Id. at 752. Because cost is a “centrally relevant factor
when deciding whether to regulate,” it was unreasonable under
the circumstances to read the phrase “‘appropriate and
necessary’ as an invitation to ignore cost.” Id. at 752-53.
Furthermore, in section 112, there was an express statutory
directive for EPA to study “the costs of [available]
technologies” to control hazardous mercury emissions from
power plants. 42 U.S.C. § 7412(n)(1)(B). That statutorily
mandated consideration of cost bolstered the Michigan Court’s
decision that EPA was required to conduct a cost-benefit
analysis under the circumstances presented there. 576 U.S. at
753. Even as it recognized a cost-benefit analysis requirement
in that context, however, the Court acknowledged that “[t]here
are undoubtedly settings in which the phrase ‘appropriate and
necessary’ does not encompass cost.” Id. at 752.

    Seeking support from Michigan, Petitioners insist the
phrase “substantially inadequate” signals the need for cost-
                               31
benefit analysis, just as the phrase “appropriate and necessary”
did with respect to EPA’s decision whether to regulate certain
hazardous air pollutants. That analogy proves too much.
Unlike the provision at issue in Michigan, section 7410(k)(5)
does not prompt EPA to consider all relevant factors (including
cost). To the contrary, section 7410(k)(5) puts significant
bounds on EPA’s duty to call a SIP. For starters, EPA is neither
required nor authorized to call a SIP whenever the agency
decides it would be “appropriate and necessary” to do so.
Instead, to call a SIP, EPA must determine it to be substantially
inadequate—that is, materially deficient. See pp. 16-18, supra.
And not every substantial inadequacy suffices:               The
inadequacy must also be on one of three statutorily defined
grounds. See 42 U.S.C. § 7410(k)(5).

     None of the other considerations in Michigan has any
bearing on this challenge to EPA’s SIP-call authority. Unlike
section 112, the provisions governing EPA’s SIP Call authority
contain no “express reference to cost.” Michigan, 576 U.S. at
753. Nor is EPA’s duty to call for revision of a substantially
inadequate SIP akin to the agency action at issue in Michigan
of “first deciding whether to regulate” an industry’s hazardous
emissions. Id. at 756. Far from it. By the time a SIP Call
occurs, EPA has already identified air pollutants that endanger
public health and welfare and has promulgated the NAAQS, 42
U.S.C. §§ 7408-09; states have already submitted SIPs as
required to meet the NAAQS, id. § 7410(a)(1), and other
obligations of the Act; EPA has already approved those plans,
id. § 7410(k)(3)-(4); and states may have even revised those
plans with further EPA approval, id. § 7410(l).

     The cooperative federalism required by the Clean Air Act
makes it especially anomalous in the SIP-call context to read
an implied cost-benefit mandate into the statutory authorization
for EPA to call a substantially inadequate SIP. When calling a
                              32
SIP, EPA cannot dictate, or even predict, the particular
measures states will adopt to rectify the SIP’s deficiencies. 80
Fed. Reg. at 33,883/3; see Train, 421 U.S. at 78-79; Virginia,
108 F.3d at 1409-10, 1414-15. In response to the SIP Calls
under review, for instance, states might decide to remove
legally deficient provisions. Or they could adopt alternative
numerical emission limits. Or perhaps they would seek to put
in place technological controls or work practices. Or they
might refashion their applicable emission rules altogether. See
80 Fed. Reg. at 33,974/2-82/2. Requiring consideration of
costs in every SIP Call would thus upset the federal-state
balance Congress established in the Clean Air Act. Because
the states retain broad discretion in crafting SIP provisions so
long as they comply with the Act, balancing the benefits and
burdens of a particular SIP revision is best left to the state
agency tasked with revising the implementation plan. After all,
“[p]erhaps the most important forum for consideration of
claims of economic and technological infeasibility is before the
state agency formulating the implementation plan.” Union
Elec. Co., 427 U.S. at 266.

     That said, nothing in our decision prevents EPA from
choosing to consider costs to the extent feasible and not
prohibited by the Act. See Entergy Corp. v. Riverkeeper, Inc.,
556 U.S. 208, 222-26 (2009); see also Michigan, 213 F.3d at
674-79. We hold only that section 7410(k)(5) does not
invariably require EPA to assess costs and benefits when
calling SIPs for failure to comply with the Act’s legal
requirements. EPA’s decision not to do so here is no ground
for invalidating the challenged action.

                              II.

     In addition to Petitioners’ overarching challenges to EPA’s
statutory authority to call the SIPs at issue, Petitioners also
                               33
more particularly challenge EPA’s decision to call four specific
categories of SIP provisions: (1) automatic exemptions, (2)
director’s discretion provisions, (3) overbroad enforcement
discretion provisions, and (4) affirmative defense provisions.
We now take up those specific challenges.

     With respect to automatic exemptions and director’s
discretion provisions, we agree with Petitioners and set aside
the SIP Calls insofar as they rest on those provisions. With
respect to overbroad enforcement discretion provisions, we
reject that challenge and uphold EPA’s Final Action. And with
respect to affirmative defense provisions, we agree with
Petitioners as to certain types of affirmative defense provisions
but reject Petitioners’ challenge as to other types.

                               A.

     We first consider automatic exemptions. We cannot
sustain EPA’s rationale for concluding that automatic
exemptions are substantially inadequate to comply with the
CAA. See 42 U.S.C. § 7410(k)(5). We thus vacate the SIP
Calls insofar as they are predicated on that conclusion.

                               1.

      Automatic exemptions exclude SSM periods from
otherwise applicable emission restrictions. In calling SIPs on
the basis that they contain automatic exemptions, EPA relied
on its authority to call SIPs that are “substantially inadequate
to . . . comply with any requirement of [the CAA].” Id.

     The centerpiece of EPA’s belief that automatic exemptions
violate a requirement of the CAA is that, by excluding SSM
periods from an emission restriction, an automatic exemption
impermissibly renders the limitation discontinuous. The
agency’s analysis proceeds in three steps.
                               34
      First, EPA starts with the premise that it “interprets CAA
sections 110(a)(2)(A) and 110(a)(2)(C) [i.e., 42 U.S.C.
§§ 7410(a)(2)(A) and (C)] to require that SIPs contain
‘emission limitations.’” State Implementation Plans, 80 Fed.
Reg. at 33,927/2. Second, EPA reasons that the “emission
limitations” that SIPs ostensibly must contain are defined by
the CAA as restrictions that apply on a “continuous” basis. Id.
(citing 42 U.S.C. § 7602(k), which defines “emission
limitation”). Third, EPA concludes that, because automatic
exemptions from “otherwise applicable emission limitations”
render limitations “less than continuous” in that they need not
apply during SSM periods, automatic exemptions are
“inconsistent with a fundamental requirement of the CAA”—
i.e., the statute’s definition of an “emission limitation” as a
“continuous” measure. Id.

     That rationale begins with—and rests on—an essential
premise at the first step: that SIPs must contain “emission
limitations.” From that premise, EPA builds its argument that:
(a) The CAA defines an “emission limitation” as a measure that
operates “on a continuous basis,” 42 U.S.C. § 7602(k); and
(b) automatic exemptions conflict with the CAA because they
render an emission limitation discontinuous, contravening the
statutory definition of “emission limitation.” Even if we
assume the correctness of the last step of the analysis—i.e., that
an automatic exemption renders a given measure incompatible
with the statutory definition of “emission limitation”—EPA’s
rationale breaks down if the measure need not qualify as an
“emission limitation” in the first place. In that event, the fact
that the statute defines an “emission limitation” as operating
“on a continuous basis,” id., would not matter: If the measure
in question need not qualify as an “emission limitation,” then
it need not satisfy the CAA’s definition of an “emission
limitation” (including the requirement that it apply “on a
continuous basis”).
                               35
     A great deal turns, then, on EPA’s starting premise that
SIPs invariably must contain “emission limitations.” Indeed,
because EPA views automatic exemptions to be inconsistent
with the CAA as a categorical matter, and because the agency
thus called every automatic exemption in any SIP, the essential
premise of EPA’s Final Action is still broader: The premise is
not just that SIPs must contain “emission limitations,” but that
every emission restriction in a SIP that is subject to an
automatic exemption (and hence was called by EPA) must
qualify as an “emission limitation.”

     What is the basis for that essential premise of EPA’s
rationale for calling automatic exemptions as a blanket matter?
In its brief, EPA at one point states, without elaboration, that
“[i]t is indisputably a ‘requirement[] of this chapter’ that SIPs
include ‘enforceable emission limitations.’” EPA Br. 48-49
(quoting 42 U.S.C. § 7410(a)(2)(A)). That assertion, it bears
noting, does not even purport to support the full breadth of the
essential premise of EPA’s categorical call of automatic
exemptions—i.e., that every emission restriction in a SIP that
is subject to an automatic exemption must qualify as an
“emission limitation.” At any rate, even on the narrower
question whether SIPs must include at least some “emission
limitations,” that assertion in EPA’s brief still falls short.

     In stating that it “is indisputably a ‘requirement[] of this
chapter’ that SIPs include ‘enforceable emission limitations,’”
id., EPA quotes from and relies upon 42 U.S.C.
§ 7410(a)(2)(A). As Petitioners emphasize, though, that
provision does not state that any SIP, no matter the
circumstances, must include “emission limitations.” Rather, it
states that a SIP “shall—

    (A) include enforceable emission limitations and
    other control measures, means, or techniques . . . , as
                                36
    well as schedules and timetables for compliance, as
    may be necessary or appropriate to meet the
    applicable requirements of this chapter.”

42 U.S.C. § 7410(a)(2)(A) (emphasis added). The only
“enforceable emission limitations” that must be included in a
SIP, then, are those that “may be necessary or appropriate” to
enable the state “to meet the applicable requirements of this
chapter [i.e., the CAA].” Id. Or, as Petitioners articulate the
point: “[R]egardless of what ‘emission limitation’ means,
Congress explicitly provided states discretion to impose them
only as ‘necessary or appropriate’ to meet some other
applicable requirement of the CAA.” Ind. Pet. Br. 49
(emphasis added) (citing 42 U.S.C. § 7410(a)(2)(A)).

     Put in the alternative, if it is not “necessary or appropriate”
that a given measure qualify as an “emission limitation” to
enable a state “to meet the [CAA’s] applicable requirements,”
the measure can be included in a SIP even if it does not meet
the CAA’s definition of an “emission limitation.” Indeed, the
plain terms of section 7410(a)(2)(A) specifically allow for
inclusion in a SIP of measures other than “enforceable
emission limitations”: The provision states that, in addition to
“enforceable emission limitations,” a SIP can also
“include . . . other control measures, means, or techniques” as
“may be necessary or appropriate to meet [the CAA’s]
requirements.” Id. (emphasis added). So, even if a given
emission restriction does not qualify as an “emission
limitation” under the CAA—including, for instance, because it
does not operate on a “continuous basis,” id. § 7602(k)—it
could still be part of a SIP. And EPA would lack authority to
call such a measure solely on the ground that it fails to meet the
statutory definition of an “emission limitation”—a definition it
did not need to satisfy.
                               37
     To illustrate the point, consider a hypothetical example of
a state that submits a SIP containing various measures, some of
which restrict emissions but none of which satisfies the CAA’s
definition of “emission limitation.”            Under section
7410(a)(2)(A), as noted, the SIP must “include enforceable
emission limitations and other control measures, means, or
techniques . . . as may be necessary or appropriate to meet the
[CAA’s] applicable requirements.” Id. § 7410(a)(2)(A). The
most prominent “applicable requirements” are the NAAQS.
See id. § 7410(a)(1). Suppose that the state concludes it can
satisfy the NAAQS for a given pollutant through its particular
combination of other types of control measures (and without
any “emission limitations,” as defined by 42 U.S.C. § 7602(k)).
The state might then consider that mix of “other control
measures” to be “necessary or appropriate” to meet the
NAAQS. Id. § 7410(a)(2)(A).

     In that situation, EPA could not call the SIP solely on the
ground that the SIP’s “other control measures” fail to satisfy
the statutory definition of an “emission limitation.” Even if so,
the SIP may still “include . . . other control measures . . . as
[are] necessary or appropriate to meet the [CAA’s] applicable
requirements.” Id. Indeed, EPA acknowledged at oral
argument that the agency could as a conceptual matter approve
“a SIP that doesn’t include an emission limitation at all,” so
long as that SIP still “would then meet [the] requirements of
the Clean Air Act.” Oral Arg. 1:34:40-1:34:53.

     To be sure, EPA could determine that the hypothetical
state is wrong in concluding that its chosen mix of “other
control measures” is “necessary or appropriate” to meet the
NAAQS. If so, EPA might decide that, for the state to meet the
NAAQS, at least one of the “other control measures” must be
adjusted such that it satisfies the definition of an “emission
limitation”—including, for instance, by converting it from a
                                38
discontinuous to a continuous measure. And EPA could call
the SIP on that basis.

     But EPA, in that event, would not rest its SIP Call solely
on the ground that the SIP’s “other control measures” do not
satisfy the statutory definition of an “emission limitation.”
Rather, EPA would rely on its determination that, for the state
to meet the NAAQS, it is “necessary or appropriate” that one of
the SIP’s measures be converted from a non-“emission
limitation” to an “emission limitation.” EPA’s SIP-call
authority, that is, would be predicated on that kind of
“necessary or appropriate” determination. Here, though, EPA
made no such finding that, for a state to meet the NAAQS (or
satisfy some other pertinent requirement of the Act), it is
“necessary or appropriate” for the SIP measures subject to
automatic exemptions instead to operate during SSM periods.

     Our dissenting colleague protests that the hypothetical
example of a state whose SIP contains no provisions satisfying
the statutory definition of “emission limitation” has never in
fact happened in the real world. Dissenting Op. 22. That may
well be so; no one contends otherwise. The purpose of the
hypothetical example is simply to illustrate that, insofar as such
a SIP runs afoul of the statute, it would be because including
measures satisfying the statutory definition of “emission
limitation” is “necessary or appropriate” to enable the state to
meet the NAAQS or some other CAA requirement. 42 U.S.C.
§ 7410(a)(2)(A). It would not be merely because the SIP’s
emission restrictions fail to fit within the statutory definition of
“emission limitation,” without regard to the implications for
complying with the Act’s requirements.

    Our colleague posits two reasons, apart from the statutory
definition, that EPA might believe a SIP’s emission restrictions
must apply during SSM periods (and thus cannot be subject to
                               39
automatic exemption provisions). Neither reason she puts
forward, however, can possibly justify EPA’s blanket call of
automatic exemptions in this case. EPA does not (and could
not) claim otherwise.

     Our colleague first surmises that automatic exemptions
from SSM periods “undercut states’ ability to meet Clean Air
Act requirements, such as attaining and maintaining the
NAAQS.” Dissenting Op. 5. If that were so, and if EPA
reasonably so concluded, we agree that EPA could call a SIP
on that basis.       EPA, though, never made any such
determination and does not purport to have done so—indeed,
that is exactly the problem with EPA’s blanket call of
automatic exemption provisions. EPA instead relied on a
supposed obligation to satisfy the statutory definition of an
“emission limitation”—which, for the reasons explained, is not
itself a requirement of the Act for the emission restrictions in a
SIP. Rather, those measures must operate during SSM periods
only if it is “necessary or appropriate” for them to do so to
enable the state to meet the NAAQS (or comply with the Act’s
other pertinent requirements). 42 U.S.C. § 7410(a)(2)(A).

     Second, according to our colleague, automatic exemptions
tend to frustrate accurate tracking of emissions because
monitoring generally assumes a source’s compliance at all
times. Dissenting Op. 5-6. The resulting “inaccuracies in
emission inventories,” our colleague submits, “distort[]
strategies for attaining the NAAQS and downstream modeling
of NAAQS attainment.” Id. at 6. Again, though, EPA did not
rely on that explanation to justify its blanket call of automatic
exemptions. Quite the opposite: EPA reviewed that precise
theory, deemed it an “oversimplification,” and expressly
declined to rest the SIP Calls on “how SSM exemptions may
or may not negatively impact things like emissions
inventories.” 80 Fed. Reg. at 33,950/1.
                              40
     Where does this leave us with respect to EPA’s across-the-
board call of automatic exemption provisions in this case?
EPA’s authority to call a SIP is conditioned on the agency’s
finding that the SIP is “substantially inadequate to attain or
maintain the relevant [NAAQS] . . . or to otherwise comply
with any requirement of” the CAA. 42 U.S.C. § 7410(k)(5).
And here, EPA premised its call of automatic exemption
provisions on section 7410(a)(2)(A)’s reference to “emission
limitations” when setting out what a SIP must include. See 80
Fed. Reg. at 33,927/2. For EPA to justify its call of every
automatic exemption based on that provision, the agency
would need to find that, to enable a state to meet the NAAQS
or some other “applicable requirement[],” it is “necessary or
appropriate” that emission restrictions subject to automatic
exemptions satisfy the statutory definition of an “emission
limitation.” 42 U.S.C. § 7410(a)(2)(A). As a result, EPA
cannot ground its blanket call of automatic exemptions on
section 7410(a)(2)(A)’s specification that SIPs must “include
enforceable emission limitations” in certain situations (when
“necessary or appropriate to meet the [CAA’s] applicable
requirements”). 42 U.S.C. § 7410(a)(2)(A). The agency did
not make—and does not purport to have made—the predicate
“necessary or appropriate” determination with respect to any
automatic exemption, much less to all automatic exemptions as
a class.

     To be sure, EPA grounds its authority to call automatic
exemption provisions not just in section 7410(a)(2)(A) but also
in section 7602(k), the CAA’s definition of “emission
limitation.” The latter provision, though, does not say anything
about when a SIP must include an “emission limitation.”
Rather, it defines an “emission limitation” as a measure “which
limits the quantity, rate, or concentration of emissions of air
pollutants on a continuous basis,” 42 U.S.C. § 7602(k), without
speaking to the antecedent question of when such a measure
                               41
must be part of a SIP. That question instead is addressed in
section 7410(a)(2)(A), which, again, requires a SIP to include
an “emission limitation[]” only when it is “necessary or
appropriate to meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A).

     For that reason, EPA’s reliance on Sierra Club v. EPA, 551
F.3d 1019, is misplaced. In that case, we considered 42 U.S.C.
§ 7412(d)(1), which requires EPA to “establish[] emission
standards” for certain sources and pollutants. The CAA gives
“emission standard” the same definition as “emission
limitation.” 42 U.S.C. § 7602(k) (“The terms ‘emission
limitation’        and     ‘emission     standard’     mean     a
requirement . . . which limits the quantity, rate, or
concentration of emissions of air pollutants on a continuous
basis . . . .”). In Sierra Club, we held that EPA’s establishment
of an SSM exemption for an “emission standard” was
inconsistent with the definition’s requirement that an emission
standard “be continuous.” 551 F.3d at 1027-28. EPA submits
that, in this case, automatic exemptions for SSM periods are
likewise inconsistent with the definition of “emission
limitation[]” and so should also be deemed invalid.

     The relevant provision in Sierra Club, however, simply
required EPA to “establish[] emission standards,” 42 U.S.C.
§ 7412(d)(1), without any proviso conditioning that obligation
on a predicate determination that it is “necessary or
appropriate” for a measure to qualify as an “emission
standard.” As a result, every measure established by EPA
under that provision needed to qualify as an “emission
standard,” including by satisfying the requirement that the
measure operate on a “continuous basis.” There are also other
provisions that likewise require the use of “emission
limitations” without condition.      See, e.g., 42 U.S.C.
§ 7429(a)(1)(A) (requiring EPA to establish performance
                               42
standards for solid waste incineration units and stating that
“[s]uch standards shall include emission limitations”
(emphasis added)).

     Here, by contrast, a SIP must include “emission
limitations” only when “necessary or appropriate to meet the
[CAA’s]       applicable    requirements.”         42  U.S.C.
§ 7410(a)(2)(A). And EPA has not purported to find that it is
“necessary or appropriate” for every (or indeed any) emission
restriction subject to an automatic exemption to qualify as an
“emission limitation” under the statutory definition.

     Our dissenting colleague stresses that the emission
restrictions at issue meet at least part of the definition of
“emission limitation” in that they limit “the quantity, rate, or
concentration of emissions.” Dissenting Op. 10, 20 (quoting
42 U.S.C. § 7602(k)). And because they meet part of the
definition, she suggests, they must satisfy the rest of the
definition too, by limiting emissions “on a continuous basis.”
Id. (quoting 42 U.S.C. § 7602(k)). That view could in theory
have some purchase if the two relevant types of emission
restrictions referenced in section 7410(a)(2)(A)—“emission
limitations” and “control measures”—were mutually
exclusive, such that a limit on “the quantity, rate, or
concentration of emissions” could fit only in the former
category. If so, such a limit perhaps would need to meet every
part of the definition of “emission limitation,” or else it would
be neither an “emission limitation” nor a “control measure.”
But the statute makes plain that “emission limitations” are a
subset of “control measures,” not an entirely distinct category.
See 42 U.S.C. § 7410(a)(2)(A) (referring to “emission
limitations and other control measures”) (emphasis added).
The statute thus contemplates restrictions satisfying only part
of the definition of “emission limitation”: Such measures, even
if not “emission limitations,” can be “control measures.”
                               43
      In the SIP Calls, but not in its brief in our court, EPA
sought to ground its authority to call automatic exemptions in
one additional provision: 42 U.S.C. § 7410(a)(2)(C). See State
Implementation Plans, 80 Fed. Reg. at 33,927/2 (“Automatic
exemptions from otherwise applicable emission limitations
thus render those limits less than continuous as required by
CAA sections 302(k), 110(a)(2)(A) and 110(a)(2)(C) . . . .”
(emphasis added)). That provision does not help the agency.
It requires SIPs to “include a program to provide for the
enforcement of the measures described in subparagraph (A),”
i.e., section 7410(a)(2)(A). 42 U.S.C. § 7410(a)(2)(C). While
section 7410(a)(2)(C) thus calls for SIPs to provide for the
“enforcement of the measures” that must be included in a SIP
under section 7410(a)(2)(A), it does not address which
measures must be contained in a SIP in the first place. That
instead is the office of section 7410(a)(2)(A), which requires a
SIP to include only those measures that are “necessary or
appropriate to meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A).

     In the end, then, EPA’s authority to issue a blanket call of
automatic exemptions must be supported by the terms of
section 7410(a)(2)(A). And because reliance on that provision,
under the provision’s plain terms, is conditioned on a
“necessary or appropriate” determination that EPA has not
made, the agency’s call of automatic exemptions must be set
aside.

                               2.

    EPA in varying measure floats two alternative reasons for
why it ostensibly could call automatic exemptions without
having to make any “necessary or appropriate” determination
under section 7410(a)(2)(A):         (i) The “necessary or
appropriate” condition in that provision in fact does not apply
                              44
to “emission limitations” at all; or (ii) EPA had already made
any required “necessary or appropriate” finding when initially
approving the SIPs. Neither of those theories withstands
scrutiny.

                              a.

     In its briefing in this case, EPA nowhere disputes that,
under the text of section 7410(a)(2)(A), SIPs must “include
enforceable emission limitations” only “as may be necessary or
appropriate to meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A). That is, EPA does not contest that, per
the language of the provision, the “necessary or appropriate”
condition applies to “enforceable emission limitations.” True,
EPA, as explained, issued its blanket call of automatic
exemptions without making the requisite “necessary or
appropriate” determination. But with respect to construing the
language of section 7410(a)(2)(A), EPA’s briefing does not
dispute that the words “as may be necessary or appropriate”
modify “enforceable emission limitations.”

     In fact, EPA explains in its brief that, “under section
7410(a)(2)(A), each SIP must contain enforceable emission
limitations as necessary to meet all applicable CAA
requirements.” EPA Br. 36-37 (emphasis added). And in the
Final Action itself, EPA expressly acknowledged that section
7410(a)(2)(A) “means that a SIP must . . . contain legitimate,
enforceable emission limitations to the extent they are
necessary or appropriate ‘to meet the applicable requirements’
of the Act.” State Implementation Plans, 80 Fed. Reg. at
33,879/2     (emphasis    added)     (quoting     42    U.S.C.
§ 7410(a)(2)(A)).

     In another part of the SIP Calls, moreover, EPA responded
to comments supposing that the agency espoused a contrary
interpretation of section 7410(a)(2)(A) under which “the
                               45
statutory phrase ‘as may be necessary’ only qualifies what
‘other control[s]’ are required rather than also qualifying what
emission limitations are required.” Id. at 33,902/1-2. Under
such an interpretation, in other words, the “necessary or
appropriate” clause would modify “other control measures,
means, or techniques,” but would not trace back to also modify
“enforceable     emission      limitations.”       42     U.S.C.
§ 7410(a)(2)(A).

     EPA, though, did not accept the commenters’
understanding of the agency’s interpretation, instead referring
to it only as “EPA’s purported interpretation.” State
Implementation Plans, 80 Fed. Reg. at 33,902/1 (emphasis
added). And in EPA’s response to those comments, the agency
acted on an understanding that “necessary or appropriate” does
modify “enforceable emission limitations,” and sought to
explain why its call of automatic exemptions should be upheld
anyway (an explanation we consider and reject below, see Part
II.A.2.b, infra). Id. at 33,902/2-3. For the most part, then, EPA
accepts that under a straightforward reading of the terms of
section 7410(a)(2)(A), the “necessary or appropriate” clause
applies to “enforceable emission limitations.”

     In a response to a comment elsewhere in the SIP Calls,
however, the agency put forward an explanation that appears
to rest implicitly on a contrary reading of section 7410(a)(2)(A)
under which the “necessary or appropriate” clause modifies
only “other control measures, means, or techniques,” not
“enforceable emission limitations.” The comment contended
that when an emission restriction in a SIP is subject to an SSM
exemption, section 7410(a)(2)(A) still allows the measure to be
included in a SIP as an “other control measure[],” even if it
does not qualify as an “emission limitation[]” due to its
discontinuity. See State Implementation Plans, 80 Fed. Reg. at
33,896/1. EPA rejected that possibility, reasoning that its logic
                              46
would “theoretically allow a SIP to contain no emission
limitations whatsoever.” Id. at 33,896/3. EPA then observed:
“This result is contrary to judicially approved EPA
interpretations of prior versions of the CAA as requiring all
SIPs to include continuously applicable emission limitations
and only requiring ‘other’ additional controls ‘as may be
necessary’ to satisfy the NAAQS.” Id. at 33,896/3-33,897/1.

     That observation references an interpretation of a “prior
version[],” id., of section 7410(a)(2)(A) under which the words
“as may be necessary” were construed to modify only “other
control measures” and not to modify “emission limitations.”
Consistent with such an understanding, EPA supported its
observation by citing an opinion that had adopted such an
interpretation of section 7410(a)(2)(A)’s predecessor. Id. at
33,897/1 n.175 (citing Kennecott Copper Corp. v. Train, 526
F.2d 1149, 1153 (9th Cir. 1975)).

      Even assuming that was the correct interpretation of the
previous language, however, the language is materially
different now. The prior version called for approval of a SIP if
it “includes emission limitations, schedules, and timetables for
compliance with such limitations, and such other measures as
may be necessary to insure attainment and maintenance of [the
NAAQS].” Kennecott Copper, 526 F.2d at 1153 (quoting 42
U.S.C. § 1857c-5(a)(2)(B) (1970)). The 1990 Clean Air Act
Amendments changed the language in several ways, including
by expanding the qualifying phrase “as may be necessary to”
to “as may be necessary or appropriate to,” and also by
expanding the ensuing words from “insure attainment and
maintenance of [the NAAQS]” to “meet the [CAA’s]
applicable requirements” (including but not limited to the
NAAQS). 42 U.S.C. § 7410(a)(2)(A). And crucially for our
purposes, the previous version linked its “necessary” clause
more closely to “such other measures” by placing them in the
                                47
same clause while putting “emission limitations” in a separate
clause, whereas the current version, exhibiting no such
imbalance, links its “necessary or appropriate” clause with both
“other control measures” and “emission limitations” to the
same degree.

     In particular, the previous version, as just noted, called for
a SIP to “include[] emission limitations, . . . and such other
measures as may be necessary to insure attainment” of the
NAAQS. 42 U.S.C. § 1857c-5(a)(2)(B) (1970). Because the
qualifying phrase “as may be necessary” appeared immediately
after (and in the same clause as) “such other measures,”
whereas “emission limitations” appeared in a distinct, previous
clause set off by a comma, the most natural reading was that
“as may be necessary” modified only “such other measures,”
not “emission limitations.” The result of that reading was that
a SIP always needed to include “emission limitations”
regardless of their necessity in achieving the NAAQS—i.e.,
regardless of whether “other measures” could render an
“emission limitation” unnecessary to achieve the NAAQS. See
Train, 421 U.S. at 67. And a SIP therefore could contain “other
measures” only if “emission limitations” could not attain the
NAAQS on their own. See id.

     The current provision, by contrast, sets off “as may be
necessary or appropriate” in a separate clause from both
“emission limitations” and “other control measures,” while
pairing the latter two in the same clause: Section 7410(a)(2)(A)
now calls for SIPs to “include enforceable emission limitations
and other control measures, . . . , as may be necessary or
appropriate to meet the applicable requirements of [the CAA].”
42 U.S.C. § 7410(a)(2)(A). And when a qualifying phrase like
“as may be necessary or appropriate” is set off by a comma in
that manner from both “emission limitations” and “other
control measures,” the most natural reading is that the
                              48
qualifying phrase modifies both “emission limitations” and
“other control measures,” not just the latter. As the Supreme
Court recently explained: A “qualifying phrase separated from
antecedents by a comma is evidence that the qualifier is
supposed to apply to all the antecedents instead of only to the
immediately preceding one.” Facebook, Inc. v. Duguid, 141
S. Ct. 1163, 1170 (2021) (quoting W. ESKRIDGE,
INTERPRETING LAW: A PRIMER ON HOW TO READ STATUTES
AND THE CONSTITUTION 67-68 (2016)).

     In accordance with that reading, a SIP must include
“emission limitations” only to the extent they are “necessary or
appropriate to meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A).        EPA does not dispute that
interpretation to be the most natural reading of section
7410(a)(2)(A). Rather, EPA, in the referenced part of the SIP
Calls, invokes only the legislative history, positing that the
change from the prior to current versions of the statute sought
merely to “combine and streamline” it rather than alter its
meaning. 80 Fed. Reg. 33,897/1. But as explained, the
amendments changed the relationship between the qualifying
phrase and its potential objects, such that the “necessary or
appropriate” clause in section 7410(a)(2)(A) is naturally (and
best) read to modify “emission limitations.” Indeed, as noted,
EPA itself recognizes exactly that elsewhere in the SIP Calls.
80 Fed. Reg. 33,879/2 (“With respect to section 110(a)(2)(A),
this means that a SIP must . . . contain legitimate, enforceable
emission limitations to the extent they are necessary or
appropriate ‘to meet the applicable requirements’ of the Act.”
(quoting 42 U.S.C. § 7410(a)(2)(A)) (emphasis added)). And
EPA’s brief in this case, we reiterate, says the same about the
statutory language: “[U]nder section 7410(a)(2)(A), each SIP
must contain enforceable emission limitations as necessary to
meet all applicable CAA requirements.” EPA Br. 36-37
                               49
(emphasis added). We agree with that straightforward reading
of section 7410(a)(2)(A)’s terms.

     Notably, our dissenting colleague nowhere advances any
alternate interpretation of the statute’s plain terms. That is
unsurprising, as there is no sound way to read section
7410(a)(2)(A) other than to call for a SIP to include “emission
limitations” only to the extent they are “necessary or
appropriate to meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A). While our colleague offers no
competing interpretation of the provision, she characterizes our
straightforward reading of it (which is also EPA’s own reading,
as just noted) as “novel.” Dissenting Op. 3, 19. Insofar as our
colleague considers that reading to be “novel” in that it differs
from the provision’s previous understanding, that is because
the statute has changed, as we have explained. Insofar as she
deems that reading “novel” because, in her view, Petitioners
have “never argued” for it “before us,” Dissenting Op. 17, she
is mistaken: As we set out below, Petitioners plainly argue that
a SIP needs to include emission limitations only to the extent
necessary or appropriate to meet the CAA’s applicable
requirements. See Part II.C, infra.

     In fact, our colleague herself ultimately understands the
changes to section 7410(a)(2)(A) to mean that the provision’s
“necessary or appropriate” clause now applies to “emission
limitations,” even if that was not previously the case. She
emphasizes that, under the provision, states are charged with
initially determining whether an “emission limitation” is
“necessary or appropriate” to meet the CAA’s applicable
requirements. See Dissenting Op. 26. That of course is true,
as the language of section 7410(a)(2)(A) sets out in
enumerating what a state must include in its SIP. See 42 U.S.C.
§ 7410(a)(2)(A). But if states initially determine whether
“emission limitations” are “necessary or appropriate” pursuant
                                50
to that provision, then the provision’s “necessary or
appropriate” condition necessarily applies to the words
“emission limitations.” In short, there is no plausible reading
of the words “enforceable emission limitations and other
control measures, means, or techniques . . . , as may be
necessary or appropriate to meet the applicable requirements of
this chapter” under which “as may be necessary or appropriate”
modifies only “other control measures, means or techniques”
without also modifying “emission limitations.” Id.

                                b.

     In the SIP Calls (but not in its brief in our court), EPA also
maintained that it acted consistently with section 7410(a)(2)(A)
even if the “necessary or appropriate” clause does modify
“emission limitations.” That argument is grounded in the idea
that, when EPA initially approved the called SIPs, the agency
considered measures it regarded as “emission limitations” to be
“necessary” to meet the CAA’s requirements. See 42 U.S.C.
§§ 7410(a)(2)(A), (k)(3). EPA explains its reasoning this way:
“In every state subject to this SIP call, the EPA has previously
concluded in approving the existing SIP provisions that the
emission limitations are necessary to comply with the legal
requirements of the CAA.” 80 Fed. Reg. 33,902/2.

     But whatever may be the potential implications of EPA’s
initial approval of those so-called “limitations” in the SIPs, the
SIPs also contained—and EPA thus also approved—the
exemptions to those limitations. For instance, one of the states
whose SIP was called for containing automatic exemptions is
Delaware. And Delaware places its automatic exemptions
directly alongside the corresponding emission restrictions. See
7-1100-1104 DEL. ADMIN. CODE § 1.5; 7-1100-1105 DEL.
ADMIN. CODE § 1.7; 7-1100-1109 DEL. ADMIN. CODE § 1.4; 7-
1100-1114 DEL. ADMIN. CODE § 1.3. As an example,
                              51
Delaware’s emission restriction for “visible emissions” says
that it “shall not apply to the start-up and shutdown of
equipment” in specified circumstances, and then immediately
sets forth the restriction to which the exemption applies—i.e.,
“[n]o person shall cause or allow the emission of visible air
contaminants or smoke from a stationary or mobile source, the
shade or appearance of which is greater than 20% opacity for
an aggregate of more than three minutes in any one hour or
more than 15 minutes in any 24 hour period.” 7-1100-1114
DEL. ADMIN. CODE §§ 1.3, 2.0.

     Anyone reading those neighboring provisions (including,
of course, EPA) would see that Delaware deems the emission
restriction as a whole—the restriction as tempered by the
immediately adjacent exemption—to be necessary to comply
with the CAA. See 7-1100-1101 DEL. ADMIN. CODE § 3.2
(noting that “emission requirements are selected as minimum
controls necessary to ensure a reasonable quality of air
throughout the State”). And EPA thus approved those
exemptions—no less than the restrictions to which they are
attached—when it initially approved the SIPs. As a result,
EPA’s initial approval of the SIPs cannot now somehow justify
leaving the so-called “limitations” in place but excising the
interconnected exemptions. The agency approved both as an
integrated unit.

     That is true of all the automatic exemptions now called by
EPA: EPA approved all those exemptions when it initially
approved the associated restrictions. In that light, EPA cannot
get very far by observing that it “previously concluded in
approving the existing SIP provisions that the emission
limitations are necessary to comply with [the] legal
requirements of the CAA.” 80 Fed. Reg. at 33,902/2. It
necessarily also approved the exemptions to those “emission
limitations.”
                                52
     Our dissenting colleague, though, says that we should just
ignore the agency’s approval of the exemptions because “EPA
was rushing to approve the SIPs.” Dissenting Op. 26. But to
the extent that EPA rashly approved the exemptions in the SIPs
because it was in too much of a hurry, that indictment of EPA’s
work would also apply to its approval of the restrictions in the
same SIPs. Once again, there is no sound basis for leaving the
restrictions in place on the theory that they were initially
approved by the agency but nonetheless jettisoning the
interconnected—and also-approved—exemptions.

     At times in the SIP Calls, EPA additionally appears to nod
at—but never ultimately advances—a somewhat related
argument also rooted in the states’ submission of the SIPs for
EPA’s approval. The idea is that the states’ use of the term
“emission limitations” to describe the emission restrictions in
their SIPs had the effect of locking the states into satisfying the
statutory definition of that term (including that an “emission
limitation” operate “on a continuous basis,” 42 U.S.C.
§ 7602(k)). Cf. 80 Fed. Reg. 33,879/3 (“Among [the CAA’s]
requirements are that an emission limitation in a SIP must be
an ‘emission limitation’ as defined in section 302(k).”).

     Even if EPA never wholeheartedly embraces that sort of
argument, our dissenting colleague subscribes to it. She
stresses that “the states have repeatedly told us that their SIPs
do contain emission limitations as contemplated by the Clean
Air Act.” Dissenting Op. 20. The apparent upshot of that view
is that, whenever a state calls an emission restriction in its SIP
an “emission limitation,” it necessarily buys into the notion that
the restriction must satisfy the statutory definition of that term.
That rationale, in other words, essentially tells the states: “You
at times have referred to the emission restrictions in your SIPs
as ‘emission limitations.’ Because you did so, you are now
stuck with those restrictions’ having to meet the statutory
                              53
definition of that term, regardless of whether meeting the
definition is in fact necessary or appropriate for you to comply
with the CAA’s requirements.”

     EPA never adopts that kind of argument. The agency
understandably may not wish to treat the statute as creating
what amounts to a semantic “gotcha” game. When a state
develops and submits a SIP for approval, the SIP includes a
suite of substantive restrictions that the state—per section
7410(a)(2)(A)—considers “necessary or appropriate” to
comply with the CAA. Even if a state uses the words “emission
limitation” to describe a given emission restriction in its SIP,
there is no indication that the state thereby means it wants,
above all else, for the restriction to qualify as an “emission
limitation” as defined by the CAA, however that term may be
construed by a court—let alone that the state wants the
restriction to be an “emission limitation” that operates during
SSM periods. To the contrary, when the state includes an
automatic exemption for SSM periods in a measure it might
describe as an “emission limitation,” that necessarily negates
any notion that, by using the label “emission limitation,” the
state somehow accepts that its so-called “emission limitation”
would have to apply during SSM periods if a court were
ultimately to decide that the “limitation” would otherwise fail
to meet the statutory definition.

     A state, in other words, presumably wants the substance of
the emission restriction it puts in a SIP, including any
exemption for SSM periods. The state does not simply want to
satisfy the statutory label “emission limitation,” even if that
means letting go of the SSM exemption it adopted. Why
should any state be understood to prize the label over the
substance—such that the state is somehow deemed to have
accepted that, even though it made the considered decision to
adopt an SSM exemption, it would cast aside the exemption
                                54
just to enable the associated emission restriction to be
accurately called an “emission limitation” per the statutory
definition? Why should any state be thought to have accepted
(let alone desired) that label-over-substance outcome?

     Imagine, for example, a hypothetical state whose emission
levels comply with the NAAQS and all other applicable CAA
requirements, but whose emission restrictions are subject to
automatic exemptions. Why would such a state care (at least
for purposes of the issue in this case) if its emission restrictions
amount to “emission limitations” per the statutory definition?
The state is already complying with the CAA regardless of the
answer to that question. Our dissenting colleague believes that
EPA nonetheless could call such a state’s SIP solely on the
ground that its emission restrictions fail to satisfy the definition
of an “emission limitation”—apparently because the state at
one point might have described its restrictions as “emission
limitations.” Calling a SIP in that situation would serve no
purpose other than a semantic one: advising the state that it
cannot accurately refer to its emission restrictions by using the
statutory term of art “emission limitations.” Nothing in the
statute supports EPA’s authority to call a state’s SIP for that
sort of language-policing reason.

     It is irrelevant for these purposes that states first developed
their SIPs under an older version of section 7410(a)(2)(A) that
may have required SIPs to contain emission limitations. See
Dissenting Op. 23. The statute’s text has changed, as
explained, and EPA must justify its actions under the revised
version of the statute in effect when it called the SIPs. EPA
does not suggest otherwise—and in fact, EPA in its briefing in
our court never once mentions the previous statutory language
or that the language has changed.
                                55
      Nor does it matter that Petitioners may believe “the SIPs
at issue here do include emission limitations.” Id. at 10-11.
True, Petitioners argue that the emission restrictions in the SIPs
at issue meet the definition of “emission limitations” even
though they are subject to automatic exemptions. See Ind. Pet.
Br. 40-51; State Pet. Br. 24-28. But by making that argument,
Petitioners in no way abandon their position that those
restrictions need not qualify as “emission limitations” in the
first place unless their doing so is “necessary or appropriate” to
enable a state’s compliance with the CAA’s requirements.
That compliance is ultimately what matters to a state, not
whether its emission restrictions happen to meet the statutory
definition of an “emission limitation.” Again, why would a
state prioritize a restriction’s fitting within the statutory label
“emission limitation” over retaining the substance of the
restrictions it adopted (including any SSM exemption)? It
would not.

                            *   *    *

     Although we set aside EPA’s call of automatic
exemptions, our decision is necessarily confined to the
particular “grounds on which the agency acted.” Michigan,
576 U.S. at 760. The central deficiency in EPA’s rationale is
that, in relying chiefly on section 7410(a)(2)(A) to support its
action, the agency did not make the kind of predicate
“necessary or appropriate” determination required by the
straightforward language of that provision. We thus do not
reach the question whether the called SIPs’ relevant emission
restrictions in fact amount to (or must amount to) “emission
limitations” per the statutory definition.

    If EPA in the future were to determine that, for states to
meet the CAA’s applicable requirements, it is “necessary or
appropriate” for their emission reduction measures to meet the
                               56
statutory definition of “emission limitations” and operate
during SSM periods, the agency could explain and implement
that rationale and its action would be subject to judicial review.
Here, however, the agency merely reasoned that every
emission restriction in a SIP needs to be continuous to qualify
as an “emission limitation” per the statutory definition, without
explaining why that continuity is “necessary or appropriate” to
meet any of the CAA’s requirements (beyond the definition
itself). That rationale cannot be sustained.

                               B.

     We next consider EPA’s call of SIPs containing director’s
discretion provisions. For largely the same reasons that we set
aside EPA’s call of automatic exemptions, we also set aside
EPA’s call of director’s discretion provisions.

     Director’s discretion provisions are essentially exemptions
but with an added step. See State Implementation Plans, 80
Fed. Reg. at 33,927/3. Whereas automatic exemptions go into
effect automatically whenever there is an SSM event, director’s
discretion provisions give state officials discretion to grant
SSM-related exemptions from otherwise applicable emission
limitations. According to EPA, the “director’s discretion SIP
provisions at issue present the same problems and
inconsistencies with CAA requirements as those that create
automatic exemptions.” EPA Br. 51. Specifically, director’s
discretion provisions render “emission limitations less-than-
continuous and preclud[e] enforcement for what would be
violations absent the discretionary exemptions.” Id. at 68.

     That continuity-based rationale fails as to director’s
discretion provisions for the same reasons it fails as to
automatic exemptions: Before concluding that emission
restrictions in a SIP must apply continuously (including during
SSM periods), EPA needed to determine that it is “necessary
                               57
or appropriate” that the restrictions be continuous to enable the
state to “meet the [CAA’s] applicable requirements.” 42
U.S.C. § 7410(a)(2)(A). But EPA did not make the requisite
“necessary or appropriate” finding. And insofar as EPA
relatedly believes that director’s discretion provisions unduly
impede enforcement of the CAA, that rationale falls short for
the same reasons as with automatic exemptions: While SIPs
must provide for enforcement of their measures, see id.
§ 7410(a)(2)(C), that requirement does not speak to the content
of the measures to begin with, including whether they must
operate continuously. See p. 43, supra.

     EPA also offers one additional rationale in support of its
call of director’s discretion provisions, this time one that does
not apply to automatic exemptions. In the agency’s view, a
director’s discretion provision, by enabling a state official to
exempt an otherwise applicable emission restriction from SSM
periods, impermissibly allows the official to modify a SIP
unilaterally, without going through the CAA’s prescribed
procedures for revising a SIP. See 42 U.S.C. § 7410(l). We
find that rationale no more persuasive than the others.

      A SIP with a director’s discretion provision has built
within it the possibility that a state official will grant an
exemption from an otherwise applicable emission restriction.
And EPA itself, when it initially approved the SIP, authorized
inclusion of that provision in the SIP. 42 U.S.C. § 7410(k)(3).
So when a state official exercises the discretion conferred by
such a provision, that is merely an application of the EPA-
approved SIP to a particular situation, not a revision of the SIP.
To be sure, in granting an exemption, the official alters the
applicability of an emission restriction. But the official does
not alter the underlying SIP. Rather, the SIP is what conferred
the power to grant the exemption in the first place.
                               58
     For those reasons, we conclude that EPA arbitrarily acted
in excess of its authority in calling director’s discretion
provisions. In reaching that conclusion, we do not foreclose
the possibility that EPA in the SIP Calls touched on reasons
that specific director’s discretion provisions, depending on
their particulars, might interfere with the CAA. For instance,
EPA broached whether a provision might be so unbounded as
to interfere with the agency’s ability to predict the impact on
compliance with the CAA’s requirements.                See State
Implementation Plans, 80 Fed. Reg. 33,927/3-33,928/1. But
EPA seeks to sustain its call of director’s discretion provisions
as a group based either on the same continuity-based rationale
it offered for calling automatic exemptions or on its distinct
rationale about SIP revisions. The agency does not argue that
its call of specific director’s discretion provisions can be
sustained absent either of those rationales that apply to the full
group of called provisions. We thus have no occasion to
address any other reasons that could be offered in support of
calling specific director’s discretion provisions.

                               C.

     As to both automatic exemptions and director’s discretion
provisions, in short, EPA called the SIPs at issue without
making the “necessary or appropriate” determination called for
by the terms of section 7410(a)(2)(A). To the extent our
dissenting colleague doubts the substantive merits of that
conclusion, we have explained why we believe any such view
is groundless. We now address an equally misconceived
procedural objection she repeatedly presses.

    Our colleague claims that we set aside EPA’s call of
automatic exemptions and director’s discretion provisions
based on an argument Petitioners nowhere make. She says that
“Petitioners did not argue” that “emission limitations are
                               59
required only as ‘necessary or appropriate.’” Dissenting Op.
17-18. Or, equivalently: It is a “new reading” of the statute,
nowhere urged by Petitioners, “that a SIP must include
‘emission limitations’ only to the extent ‘necessary or
appropriate to meet the applicable requirements’ of the Clean
Air Act.” Id. at 18 (quoting 42 U.S.C. § 7410(a)(2)(A)).

     That is a proposition “Petitioners did not argue”? Id. at 17.
Hardly so. Here is what Petitioners contend in their brief, in
their own words: “[R]egardless of what ‘emission limitation’
means, Congress explicitly provided states discretion to
impose them only as ‘necessary or appropriate’ to meet some
other applicable requirement of the CAA.” Ind. Pet. Br. 49
(emphasis added) (quoting 42 U.S.C. § 7410(a)(2)(A)). That
is exactly the proposition our colleague insists Petitioners never
argued. There is no daylight between arguing “that a SIP must
include ‘emission limitations’ only to the extent ‘necessary or
appropriate to meet the applicable requirements’ of the Clean
Air Act,” Dissenting Op. at 19 (quoting 42 U.S.C.
§ 7410(a)(2)(A)), and arguing that Congress gave “discretion
to impose them [i.e., emission limitations] only as ‘necessary
or appropriate’ to meet some other applicable requirement of
the CAA,” Ind. Pet. Br. 49 (quoting 42 U.S.C.
§ 7410(a)(2)(A)). Those contentions are self-evidently one and
the same.

    Our dissenting colleague asserts that Petitioners, in
arguing that a SIP needs to include emission limitations “only
as ‘necessary or appropriate’ to meet some other applicable
requirements of the CAA,” id., assumed the need to satisfy the
definition of “emission limitations” and disputed only whether
the definition’s “continuity requirement” is met. Dissenting
Op. 18. That is incorrect. Far from assuming the definition’s
applicability, Petitioners argued that, regardless of the
definition, a SIP must include emission limitations only if
                               60
necessary or appropriate: “[R]egardless of what ‘emission
limitation’ means, Congress explicitly provided states
discretion to impose them only as ‘necessary or appropriate’ to
meet some other applicable requirement of the CAA.” Ind. Pet.
Br. 49 (emphasis added) (quoting 42 U.S.C. § 7410(a)(2)(A)).

     What is more, after so setting out exactly the interpretation
of section 7410(a)(2)(A) on which our rejection of the SIP
Calls rests, Petitioners go on to make several integrally
associated, follow-on points that we have also found
persuasive. Petitioners explain that, while “EPA attempts to
dismiss the ‘necessary or appropriate’ language by interpreting
the subsequent phrase ‘requirement of this chapter’ to include
the definition of ‘emission limitation,’” that definition,
“[w]hatever [it] means,” is “not a standalone ‘requirement’ of
the Act.” Id. at 50. Precisely so. See pp. 34-38, 40-41, supra.
That definition, Petitioners elaborate, “says nothing about the
applicability of emission limitations.” Ind. Pet. Br. 42. True.
See pp. 40-41, supra. Those points solely make sense in the
context of Petitioners’ argument that a SIP needs to contain
“emission limitations” only as necessary or appropriate.
Petitioners add that “EPA’s interpretation” of section
7410(a)(2)(A) would “depriv[e] states[] the choice of not
applying emission limitations to some or all sources during
SSM (or other) periods.” Ind. Pet. Br. 51. Yes again. See pp.
52-55, supra. And “[i]f Congress did not want states to have
discretion to determine how ‘emission limitations’ were
applied in SIPs, it could have adopted more prescriptive
measures,” as “it did in CAA § 112,” Ind. Pet. Br. 51, the
materially different provision we considered in Sierra Club.
Right once more. See pp. 41-42, supra.

     Tellingly, moreover, EPA fully understands the argument
that Petitioners are making (and that our colleague nonetheless
asserts Petitioners have not made). In describing “Petitioners’
                              61
arguments” against it, EPA includes the following as a “key
point” pressed by Petitioners: “EPA misinterprets section
7410(a)(2)(A).” EPA Br. 43. In particular, EPA explains,
“Petitioners contend that EPA is trampling the States’
discretion under section 7410(a)(2)(A) to determine what
‘enforceable emission limitations and other control
measures . . . [are] necessary or appropriate’ to meet CAA
requirements.” Id. at 48 (alterations in original) (quoting Ind.
Pet. Br. 50-52; and 42 U.S.C. § 7410(a)(2)(A)). That is indeed
what Petitioners argue is the upshot of EPA’s mistaken (in
Petitioners’ view) understanding of the terms of section
7410(a)(2)(A), particularly the provision’s “necessary or
appropriate” clause. And as we have explained, we find
Petitioners’ argument on that score to be persuasive.

     EPA, in attempting to respond to Petitioners’ argument in
that regard, further confirms that it understands Petitioners to
be making the argument our colleague professes Petitioners do
not advance. EPA states: “States do have the discretion not to
regulate a source or source category entirely if doing so is not
necessary or appropriate to meet CAA requirements.” Id. at
50. Why would EPA make it a point to concede that there is
some discretion to refrain from applying an emission limitation
when it is not “necessary or appropriate” to meet the CAA’s
requirements, unless EPA understands Petitioners to be
arguing that emission limitations need only be included in a
SIP if they are “necessary or appropriate” to meet the CAA’s
requirements? To be sure, EPA ultimately has no persuasive
response to Petitioners’ argument on that score; but as to
whether Petitioners in fact make the argument, EPA plainly
believes they do. Indeed, EPA, in attempting to respond to
Petitioners’ argument, cites the exact four pages of the 146-
page Final Action containing the agency’s response to the
argument that the Action mistakenly overlooks the significance
of section 7410(a)(2)(A)’s “necessary or appropriate”
                                62
language—the argument our colleague believes Petitioners do
not make. See id. at 50 (citing 80 Fed. Reg. at 33,902-03); id.
at 51 (citing 80 Fed. Reg. at 33,896-97).

     All of which is to say: Petitioners absolutely do argue that
a SIP needs to “include ‘emission limitations’ only to the extent
‘necessary or appropriate to meet the applicable requirements’
of the Clean Air Act.” Dissenting Op. 19 (quoting 42 U.S.C.
§ 7410(a)(2)(A)). And EPA understands Petitioners to so
contend. Our colleague might wish that EPA responded more
fulsomely or in a different way to that argument. EPA, for
instance, does not rely in its brief on our colleague’s notion that
the SIPs must contain “emission limitations” as defined by the
CAA because the states sometimes used those words when
referring to the emission restrictions in the challenged SIPs.
Perhaps EPA opted against relying on that notion upon
recognizing its shortcomings. See pp. 52-55, supra.

     We note, lastly, that even if Petitioners had not made the
“necessary or appropriate” argument that we ultimately find
persuasive, we are “not limited to the particular legal theories
advanced by the parties, but rather retain[] the independent
power to identify and apply the proper construction of
governing law.” Kamen v. Kemper Fin. Servs., Inc., 500 U.S.
90, 99 (1991). After all, because “[o]ur task is to construe what
Congress has enacted,” we need not “accept an interpretation
of a statute simply because it is agreed to by the parties.”
Rumsfeld v. F. for Acad. & Inst. Rts., 547 U.S. 47, 56 (2006)
(quoting Duncan v. Walker, 533 U.S. 167, 172 (2001)).
Petitioners of course pointed us to the relevant “governing
law,” Kamen, 500 U.S. at 99—i.e., the terms of 42 U.S.C.
§ 7410(a)(2)(A). See EPA Br. 43 (understanding Petitioners to
argue that “EPA misinterprets section 7410(a)(2)(A)”). And
once we look at that provision, the significance of its
“necessary or appropriate” clause to the validity of EPA’s SIP
                              63
Calls is hardly hidden beneath the surface: It essentially jumps
off the page upon a scan of the provision’s terms.

     Here, in any event, Petitioners not only directed us to
section 7410(a)(2)(A), but they also explained precisely how
the provision’s language undermines EPA’s basic rationale for
calling the automatic exemption and director’s discretion
provisions: “[R]egardless of what ‘emission limitation’ means,
Congress explicitly provided states discretion to impose them
only as ‘necessary or appropriate’ to meet some other
applicable requirement of the CAA.” Ind. Pet. Br. 49 (quoting
42 U.S.C. § 7410(a)(2)(A)). EPA has no persuasive answer to
that contention. Nor, in our respectful view, does our
dissenting colleague.

                              D.

     EPA (by its own account) called only one SIP,
Tennessee’s, for containing overbroad enforcement discretion
provisions. See TENN. COMP. R. & REGS. 1200-3-20-.07-(1),
.07(3). It found those provisions ambiguous: The provisions
could be read to limit only Tennessee’s enforcement discretion,
but EPA thought that they could also be read to allow
Tennessee officials to foreclose EPA enforcement actions and
citizen suits.

     The Petitioners do not question that the provision would
be unlawful if the second reading is correct. Instead, they say
that EPA cannot call a SIP for being ambiguous, and that EPA
should have deferred to Tennessee’s construction of any
potentially ambiguous terms.

    We have already rejected those arguments. We therefore
deny the petitions for review as to Tennessee’s overbroad
enforcement discretion provisions.
                              64

                              E.

    We finally turn to EPA’s calls based on affirmative
defense provisions.

     States have included two kinds of affirmative defenses in
their SIPs, and each requires a different analysis.

     One kind provides “a complete affirmative defense to an
action brought for non-compliance” with an emission rule,
provided the source complies with certain conditions. 118-01-
19 ARK. CODE R. § 602. Those affirmative defenses create an
exemption from the normal emission rule. So our automatic-
exemption analysis applies equally to them.

     The other kind precludes certain remedies after a source
has violated an emission rule. In Arizona, for example, a
source generally has an affirmative defense for excess
emissions during SSM period, except in a “judicial action
seeking injunctive relief.” ARIZ. ADMIN. CODE § 18-2-310(B)-
(C). Those raise a different legal question: whether states can
limit the relief that Congress empowered federal courts to grant
for violations of emission rules. We hold that they cannot.

     As part of its enforcement regime, the Clean Air Act
authorizes citizens and EPA to seek injunctive relief and
monetary penalties against sources that violate SIPs’ emission
rules. 42 U.S.C. §§ 7604(a), 7413(b). Affirmative defenses
against certain remedies block that aspect of the Act’s
enforcement regime.

    That is why, in NRDC v. EPA, we determined that EPA
could not provide an affirmative defense against monetary
damages as part of a federal emission rule. 749 F.3d at 1063-
                                 65
64. We explained that the Act’s citizen-suit provision “clearly
vests authority over private suits in the courts, not EPA.” Id.
at 1063. Thus, “the Judiciary, not any executive agency,
determines the scope—including the available remedies—of
judicial power vested by statutes establishing private rights of
action.” Id. (internal quotation marks omitted) (quoting City of
Arlington v. FCC, 569 U.S. 290, 302 n.3 (2013)). We therefore
held that once a court finds a violation in a private suit, it is for
the court alone to determine the appropriate civil penalties. Id.

     Of course, NRDC did not address SIPs. 749 F.3d at 1064
n.2. But “statutes are not chameleons, acquiring different
meanings when presented in different contexts.” Maryland,
958 F.3d at 1202. The enforcement provisions that we
analyzed in NRDC—sections 113 and 304 of the Act—apply
to state-created emission rules just as they do to EPA-created
rules. See 749 F.3d 1055.

    The Petitioners offer three reasons that they believe the
SIP context is different. Each is unpersuasive.

     First, the Petitioners argue the Act grants states the power
to alter its enforcement regime through the Act’s requirement
that SIPs “include a program to provide for the enforcement of
the” SIPs’ substantive rules. 42 U.S.C. § 7410(a)(2)(C).

    They are incorrect. That provision, § 110(a)(2)(C) of the
Act, instructs states to “provide for the enforcement” of
substantive emission rules. To provide is to “furnish” or
“supply” something—here, enforcement.         Provide, THE
AMERICAN HERITAGE DICTIONARY OF THE ENGLISH
                               66
LANGUAGE (1976).1 A duty to supply enforcement does not
carry with it the power to limit other enforcement efforts that
the Clean Air Act authorizes.

     That understanding is confirmed when the words in
section 7410(a)(2)(C) are read “in their context and with a view
to their place in the overall statutory scheme.” Roberts v. Sea-
Land Servs., Inc., 566 U.S. 93, 101 (2012) (quoting Davis v.
Mich. Dep’t of Treasury, 489 U.S. 803, 809 (1989)). That
section is part of a long list of duties that Congress imposed on
states.    E.g., 42 U.S.C. § 7410(a)(2)(B) (“provide for
establishment and operation of appropriate devices, methods,
systems, and procedures necessary to monitor, compile, and
analyze data on ambient air quality”). None of those other
duties empowers states to alter other aspects of Congress’s
scheme. And neither does section 7410(a)(2)(C).

     Second, the Petitioners observe that Congress left it to the
states “to determine which sources would be burdened by
regulation and to what extent.” Union Elec. Co., 427 U.S. at
269. From that premise, they conclude that states’ general
discretion to determine what types of emission rules are
“necessary or appropriate” to meet the Act’s requirements
allows them to decide how those rules should be enforced. 42
U.S.C. § 7410(a)(2)(A).

    But states’ power to decide whom and how to regulate
does not carry with it the power to alter the consequences
Congress chose for violating those regulations. Rather,
Congress specifically determined how EPA and citizens could

1
  See also Provide, WEBSTER’S NEW WORLD DICTIONARY OF THE
AMERICAN LANGUAGE (2d ed. 1972) (same); Clean Air Act
Amendments of 1977, Pub. L. No. 95-95, § 108, 91 Stat. 685, 693
(adding the provision that is now section 110(a)(2)(C)).
                                67
enforce the regulations that the states choose to impose. And
it left it to the courts to determine the appropriate penalties for
violations. See 42 U.S.C. §§ 7413, 7604(a); NRDC v. EPA, 749
F.3d at 1063.

     Third, the Petitioners say that if we agree with EPA, we
will create a split with the Fifth Circuit’s decision in Luminant
Generation Co. v. EPA, 714 F.3d 841 (5th Cir. 2013). There,
the Fifth Circuit’s textual analysis consisted of the following
sentence: “[S]ection 7413 does not discuss whether a state may
include in its SIP the availability of an affirmative defense
against civil penalties for unplanned SSM activity.” Id. at 852.
It then deferred to EPA’s pre-NRDC view that affirmative
defenses against monetary damages for unavoidable excess
emissions were lawful. Id. at 853.

     We “avoid creating circuit splits when possible.” United
States v. Philip Morris, USA, Inc., 396 F.3d 1190, 1201 (D.C.
Cir. 2005). But NRDC already rejected the Fifth Circuit’s
silence-as-delegation logic, and we must follow NRDC. 749
F.3d at 1064; Ali v. Rumsfeld, 649 F.3d 762, 775 n.20 (D.C.
Cir. 2011) (“We are of course bound by circuit precedent.”).
So any disagreement with the Fifth Circuit about the Act’s
meaning has existed since 2014; we are just applying it now in
the same context that the Fifth Circuit did. Just as NRDC’s rule
foreclosed EPA’s affirmative defense, it likewise forecloses
affirmative defenses in SIPs. NRDC v. EPA, 749 F.3d at 1064.

     In short, states cannot limit courts’ discretion to determine
and apply appropriate civil penalties for violations of SIPs. We
therefore deny the petitions for review as to affirmative
defenses against monetary damages.

                           *    *    *
                             68
     As to the calls based on automatic exemptions, director’s
discretion provisions, and affirmative defenses that are
functionally exemptions, we grant the petitions and vacate the
SIP-call order. We deny the petitions as to the calls based on
the enforcement-discretion provision and affirmative defenses
against specific relief.
                                                  So ordered.
     PILLARD, Circuit Judge, concurring in part and dissenting
in part:

     The majority and I agree on quite a bit. We agree that
EPA correctly interpreted its SIP-call authority under the
Clean Air Act and that Petitioners’ crosscutting challenges to
that authority fail. I also join the majority in denying the
petitions for review with respect to Tennessee’s overbroad
enforcement-discretion provisions and those affirmative
defenses that operate to preclude certain remedies after a
source has violated an emission rule. I thus join Analysis
Sections I and II.C in full and Section II.D in part.

     But the majority missteps in vacating EPA’s decision to
call state implementation plans that are substantially
inadequate under the Clean Air Act because they include
automatic exemptions, director’s discretion provisions, or
affirmative defenses that operate as wholesale exemptions for
SSM events. As for those provisions, EPA got it right: The
Clean Air Act requires “emission limitations” (also referred to
as “emission standards”) to apply continuously, and the
emission limitations in the SIPs subject to this call violate that
requirement.

     Reading the majority, one might think this case is about
whether the SIP provisions challenged as discontinuous are
emission limitations at all. But make no mistake: No
challenger pressed before this court that the SIPs at issue did
not contain emission limitations. No challenger defended on
the ground that pollution-restricting measures that limit the
quantity, rate, or concentration of air pollutants were not
“emission limitations,” or that EPA fell short of establishing
before issuing its SIP Calls that the measures the states
included as “emissions limitations” were “necessary or
appropriate” to their SIPs. Quite the opposite. The states
have long and repeatedly used the statutorily defined term
“emission limitations” to refer to their disputed pollution
                               2
restrictions. And they have defended them as “emissions
limitations” that they view as appropriate to meet their Clean
Air Act obligations. The Petitioners’ briefs object only that
EPA misunderstood what it means for an emission limitation
to be “continuous” and that, even under EPA’s reading of the
continuity requirement, the SIPs contain continuous emission
limitations. See States Br. 7, 22-34; States Reply Br. 1, 8, 11-
12; Indus. Br. 36-52.

     My colleagues nevertheless reject EPA’s SIP Call by
hypothesizing that the plans at issue may be entirely devoid of
“emission limitations” as the Act defines them. Positing that
a discontinuous emission limitation is no emission limitation
at all, they say that the challenged SIPs cannot be
substantially legally deficient for failing to meet the
requirement that “emission limitations” apply on a continuous
basis. See 42 U.S.C. § 7602(k); Maj. Op. 37. The majority
also sees the states’ submission and EPA’s approval of these
SIPs as reason to treat the emission limitations in the SIPs as
not “enforceable emission limitations” under the Act: Given
the statutory definition of “emission limitations” as
continuous, say my colleagues, the states cannot have
proffered, nor EPA approved, those overtly discontinuous
measures as “enforceable emission limitations.” 42 U.S.C.
§ 7410(a)(2)(A); Maj. Op. 50-52.

    But that is wrong for at least three reasons.

     First, EPA has been candid that its initial approval was in
error to the extent it overlooked the discontinuity of the
emission limitations when it initially approved them. And
Petitioners do not argue that EPA’s initial approval of their
SIPs estops the agency from calling the SIPs to fix its errors.

   Second, Petitioners themselves refer to the disputed
measures as emission limitations. In legally consequential
                               3
filings—including their briefs in this very case, comments to
EPA, and the SIPS themselves—the petitioning states have
repeatedly referred to their chosen control measures as
containing “emission limitations.” The majority brushes
those statements aside as semantic missteps: Because it was
against the petitioning states’ interests to do so, they must not
have meant to acknowledge that their discontinuous rules are
“emission limitations” when, on the majority’s logic, they
could have avoided the Act’s continuous-applicability
requirement simply by calling them something else. But that
cuts the other way. In both law and common sense,
statements made against a speaker’s own interest are taken as
more likely to be accurate, not less.

     Third, the statute appears to treat “emission limitations”
and “other control measures” as distinct categories of
restrictions. Without briefing on the issue, I would hesitate to
reduce that distinction to whether a restriction is continuous.
The crux of my colleagues’ approach is that the discontinuity
of an emission limitation just redefines it as an “other control
measure.” In the face of textual and structural reasons to
doubt Congress intended that workaround, we should not rely
on it here to excuse the deficiencies EPA identifies.

     I would have answered the questions presented to us—
namely, how to interpret Congress’s direction that emission
limitations operate on a continuous basis, and whether the
emission limitations in the SIPs at issue violate that continuity
requirement. On the issues as to which today’s decision both
departs from the briefing before us and arrives at a novel
interpretation of the strictures of the Clean Air Act, I
respectfully dissent.
                              4
                              I.

     This case concerns the difficult task of curbing harmful
air pollution released by industrial facilities during their
startup, shutdown, or malfunction. Those so-called SSM
events may involve, for example, leaks, flares, and even
explosions. Releases during SSM events often far exceed
emissions from normal operations because facilities may
operate less efficiently than during steady-state operation and
because facilities often bypass controls when they are starting
up, shutting down, or malfunctioning. See Intervenors’ Br. 1;
Indus. Br. 7 n.4. Excess emissions from SSM events can be
regular occurrences: One Georgia facility, for example,
exceeded applicable emission limits on “thousands of
occasions” over a four-year period. Sierra Club v. Ga. Power
Co., 443 F.3d 1346, 1347 (11th Cir. 2006); see id. at 1350.
The exceedances can be dramatic: Another plant released
“three times its daily limit” of a pollutant “over a nine-hour
period.” US Magnesium, LLC v. EPA, 690 F.3d 1157, 1163
(10th Cir. 2012).

     The pollutants that SIPs must control include particulate
matter, sulfur dioxide, lead, and others that damage human
respiratory, cardiac, and neurological health. See 40 C.F.R.
pt. 50. SSM events can involve bursts of such pollutants in
high concentrations. Mounting scientific evidence links
concentrated bursts of pollutants to severe harm to public
health and welfare. Those harms fall disproportionately on
industrial facilities’ neighboring communities, many of which
are socially and economically disadvantaged. For those
communities, frequent industrial flaring events, for example,
cause myriad health and welfare problems—from burning
sensations in the nose and throat to respiratory illness to
cancer. See generally Intervenors’ Br. 2-3, 7-13.
                               5
     The harms caused by SSM emissions are often avoidable.
Industrial sources are technologically capable of minimizing
emissions during startup and shutdown and reducing the
frequency of, and damage caused by, malfunctions. Id. at 16;
see also SIP Calls, 80 Fed. Reg. 33,840, 33,874/3 (June 12,
2015). But they may lack the incentive to do so. After all,
many state implementation plans simply exempt excess
emissions during SSM events from the otherwise-applicable
emission limitations that are designed to curb air pollution.
As a result, emissions during SSM events that exceed
otherwise-applicable emission limitations are not treated as
enforcement risks that should drive compliance. EPA Br. 17-
19. No wonder that many industrial sources have not made
the investments necessary to protect our air from the harmful
emissions released during startup, shutdown, and malfunction.

     SSM exemptions are regulatory loopholes. In the years
since EPA initially approved such exemptions, the agency has
come to recognize that they conflict with the Clean Air Act’s
requirement that emission restrictions apply continuously.
SSM exemptions allow sources to emit pollutants causing
“unacceptable air pollution in nearby communities,” without
any legal mechanism for state air agencies, EPA, the public,
or courts to require greater efforts to reduce emissions. 80
Fed. Reg. at 33,843/3. The exemptions also undercut states’
ability to meet Clean Air Act requirements, such as attaining
and maintaining the NAAQS, preventing significant
deterioration of air quality in attainment areas, and protecting
visibility.   EPA Br. 18; Memorandum to Docket for
Rulemaking: Statutory, Regulatory, and Policy Context for
this Rulemaking, Docket No. EPA-HQ-OAR-2012-0322-
0029, at 23 (Feb. 4, 2013) (J.A. 430).

    EPA has identified these substantial regulatory loopholes,
but it remains difficult to measure just how damaging
                              6
uncontrolled SSM emissions are. Because emissions during
SSM events are exempt from emission limitations, they are
often not tracked with any precision or regularity. Indeed,
emission inventories typically “presume compliance by
sources at all times throughout the year” and are “not adjusted
to include excess emissions that occur as a result [of] SSM
events.” Memorandum to Docket for Rulemaking at 23 (J.A.
430). Those inaccuracies in emission inventories ripple
throughout the SIP planning process, distorting strategies for
attaining the NAAQS and downstream modeling of NAAQS
attainment. See id. at 23-24 (J.A. 430-31). EPA did not “rest
the SIP Call” on these distortions, Maj. Op. 39, but it did not
dismiss them either, cf. id. According to EPA, discontinuous
source compliance with emission limitations has a “negative
impact” on the accuracy of emissions inventories and SIP
planning and could “have a larger negative effect” if not
disallowed. 80 Fed. Reg. at 33,950/1-51/1.

     Most of the SSM exemptions are, as EPA put it, “artifacts
of the early phases of the SIP program, approved before state
and EPA regulators recognized the implications of such
exemptions.” 80 Fed. Reg. at 33,957/3. After hurriedly
approving such SSM exemptions in the wake of the 1970
Amendments to the Clean Air Act, EPA soon realized that
they were “not consistent” with the Act. Id. In a series of
guidance documents dating back to 1982, EPA communicated
to states that the Clean Air Act bars exemptions from
emission limitations for SSM events. Id.; see Memorandum
to Docket for Rulemaking at 8-16 (J.A. 415-23) (describing
EPA’s longstanding SSM policy). But, while EPA has long
interpreted the Act to prohibit SSM exemptions, many states
did not update their SIPs to remove those loopholes.

    Recall that the Clean Air Act sets out requirements for
SIPs, including that:
                             7
Each SIP shall—

    (A) include enforceable emission limitations and
    other     control      measures,       means,      or
    techniques (including economic incentives such as
    fees, marketable permits, and auction of emissions
    right), as well as schedules and timetables for
    compliance, as may be necessary or appropriate to
    meet the applicable requirements of this chapter.”

42 U.S.C. § 7410(a)(2)(A). Congress defined an “emission
limitation” and “emission standard” as:

    a requirement established by the State or the
    Administrator which limits the quantity, rate, or
    concentration of emissions of air pollutants on a
    continuous basis, including any requirement relating
    to the operation or maintenance of a source to assure
    continuous emission reduction, and any design,
    equipment, work practice or operational standard
    promulgated under [the Clean Air Act].

42 U.S.C. § 7602(k) (emphasis added).

     That brings us to the SIP Calls at issue here. After
decades of inaction to correct those deficient SIPs, EPA in
2015 called SIPs from 35 states and the District of Columbia
as substantially inadequate to the extent they exempted SSM
events from emission limitations. EPA explained that, by
carving out SSM events from otherwise-applicable emission
limitations, those SIPs violated the Clean Air Act’s
requirement that emission limitations operate “on a
continuous basis,” 42 U.S.C. § 7602(k). See, e.g., 80 Fed.
Reg. at 33,852/1-2, 33,927/1-3, 33,928/3. A subset of the
states—roughly half of those originally subject to the SIP
                               8
Calls—and certain industry groups and companies seek
review in this court.

                               II.

     Petitioners’ challenges to EPA’s SIP Calls present two
questions of relevance here: (1) whether emission limitations
in SIPs must apply at all times to satisfy the Clean Air Act’s
requirement that emission limitations operate “on a
continuous basis,” 42 U.S.C. § 7602(k), and (2) whether the
state plans subject to this SIP Call violated that requirement
when they exempted SSM emissions from otherwise-
applicable emission limitations. The answer to both questions
is yes. The Clean Air Act requires that emission limitations
apply continuously. The called SIPs violated that requirement
when they included automatic exemptions, director’s
discretion provisions, and certain affirmative defenses that
exempt emissions during SSM events from the SIPs’
otherwise-applicable emission limitations.

     Perplexingly, the majority answers neither question.
Rather, my colleagues devote their attention to what they say
is an “essential premise” of EPA’s SIP Calls—namely, that
the Clean Air Act invariably requires all SIPs to contain
emission limitations as the Act defines them. Maj. Op. 34.
The majority then questions the legitimacy of that premise by
deciding that the Act does not require every SIP to include
such a limitation. But that so-called premise is not before us.
Petitioners do not controvert it, nor does EPA see the need to
defend it in this case. The position adopted by the majority is
nowhere to be found in the briefing. That is because the straw
“premise” it targets is not at all “essential” to the SIP Call
before us. Even if the majority is correct that the Act does not
invariably require every SIP to include emission limitations, it
is enough that as a practical matter the called SIPs in this case
                              9
all purport to include them. Indeed, States submitted and
EPA approved them under an earlier version of the Act, when
“a SIP always needed to include ‘emission limitations.’” Maj.
Op. 47. Accordingly, the parties all understood the SIPs at
issue to contain “emission limitations” to which the
challenged SSM carveouts apply. See infra 20-21. That is
reason enough not to go down the path taken by the majority.

                              A.

     Start with the arguments the parties did bring before this
court. The Petitioners claim that exemptions for SSM events
do not violate the Clean Air Act’s requirement that emission
limitations be continuous. The thrust of their argument
proceeds in two steps. First, Petitioners argue that an
emission limitation meets the Act’s continuity requirement so
long as it is “‘continuous’ over some period of time or
condition,” even if “not necessarily all periods of time.”
Indus Br. 42. See Indus. Br. 40-47, 50-52. Under that
reading, the Act’s continuity requirement merely prohibits the
use of intermittent emission control technologies, which “vary
their level of emission control based on air quality.” Indus.
Br. 46 (emphasis omitted); see also States Br. 25. Second,
Petitioners claim in the alternative that, even if emission
limitations must apply at all times, the SIPs comply because
they “include ‘general duty’ requirements” that obligate
sources “to minimize emissions either at all times, or as a
prerequisite to an exception to application of a numeric
‘emission limitation.’” Indus. Br. 38; see id. at 38-40; States
Br. 10, 22-28. Neither of those arguments treats the called
SIPs as devoid of “emission limitations” within the meaning
of the Act. Rather, they defend as “continuous” the emission
limitations the states chose to include. Because the emission
limitations in the called SIPs are not continuous, Petitioners’
challenges fail.
                              10
     Start with the text. In the Clean Air Act, Congress
specified what constitutes an emission limitation.          An
emission limitation is “a requirement” that “limits the
quantity, rate, or concentration of emissions of air pollutants
on a continuous basis.” 42 U.S.C. § 7602(k). Recognizing
the myriad methods by which a state might limit emissions—
from numerical caps to work practice standards to
technological     controls—Congress      defined     “emission
limitations” expansively. The statutory definition “includ[es]
any requirement relating to the operation or maintenance of a
source” as well as “any design, equipment, work practice or
operational standard….” Id.

     Importantly for our purposes, the Clean Air Act demands
that, whatever of those various forms it takes, a limitation on
“the quantity, rate, or concentration of emissions of air
pollutants” must operate “on a continuous basis,” id.; that is,
an emission limitation must apply at all times. As EPA
explained, “the word ‘continuous’ is not separately defined in
the Act,” but “its plain and unambiguous meaning is
‘uninterrupted.’” 80 Fed. Reg. at 33,901/1; WEBSTER’S
THIRD NEW INTERNATIONAL DICTIONARY 493-94 (Phillip
Babcock ed. 1976) (“operated without interruption”). An
emission limitation is not continuous if it “provides for any
period of time when a source is not subject to any requirement
that limits emissions.” 80 Fed. Reg. at 33,901/1-2. That
continuity requirement helps protect our nation’s air quality.
As Congress emphasized when amending the Clean Air Act,
“[w]ithout an enforceable emission limitation which will be
complied with at all times, there can be no assurance that
ambient standards will be attained and maintained.” H.R.
REP. NO. 95-294, at 92 (1977) (emphasis added).

    Against that backdrop, we have interpreted the Clean Air
Act’s requirement of “continuous” emission limitations to
                              11
mean that some emission limitation or standard must apply at
all times. In Sierra Club v. EPA, 551 F.3d 1019 (D.C. Cir.
2008), we considered a decision by EPA to exempt SSM
events from compliance with hazardous air pollutant
standards under section 112 of the Clean Air Act. Id. at 1026.
We there read section 112, requiring EPA to establish
emission standards for hazardous air pollutants, alongside
section 302(k), requiring emission limitations and standards to
apply “on a continuous basis,” 42 U.S.C. § 7602(k). Sierra
Club, 551 F.3d at 1027. Interpreting those provisions
together, we concluded that “Congress has required that there
must be continuous section 112-compliant standards” and that
“the SSM exemption violate[d]” that requirement. Id. at
1027-28.

     Sierra Club’s logic applies with equal force here. In the
Clean Air Act, Congress required that emission limitations
included in SIPs be continuous. SSM exemptions violate that
requirement. The majority attempts to distinguish Sierra
Club, claiming it does not answer the question the majority
injects into this case: that is, whether the Clean Air Act
requires any emission limitations whatsoever in SIPs. See
Maj. Op. 41-42. That is simply beside the point, because all
parties before us agreed that the SIPs at issue here do include
emission limitations. See infra at 20-21. And Sierra Club
instructs that when SIPs do include emission limitations those
limitations must be continuous. 551 F.3d at 1027-28. In
other words, when emission limitations are used, there is no
option to exempt SSM periods altogether.

     Other circuits have likewise accepted EPA’s
interpretation that emission limitations must apply
continuously, including during startup, shutdown, and
malfunction. In Mont. Sulphur & Chem. Co. v. EPA, 666
F.3d 1174 (9th Cir. 2012), the Ninth Circuit held that EPA,
                             12
when promulgating a federal implementation plan to fill gaps
in a state SIP, “reasonably interpreted the Clean Air Act to
require continuous limits on emissions,” including during
SSM events. Id. at 1193; see also Kamp v. Hernandez, 752
F.2d 1444, 1452-53 (9th Cir. 1985). The Sixth and Tenth
Circuits, too, have upheld as reasonable EPA’s longstanding
interpretation of the Clean Air Act to bar SSM exemptions
from emission limitations. See Mich. Dep’t of Env’t Quality
v. Browner, 230 F.3d 181, 183-86 (6th Cir. 2000); Ariz. Pub.
Serv. Co. v. EPA, 562 F.3d 1116, 1129 (10th Cir. 2009).

     In short, there can be little doubt that emission
limitations, wherever used, must apply continuously. And
Petitioners do not argue that EPA’s initial approval of a SIP,
in the earliest days of its statutory role in doing so, should
estop the agency from calling the SIPs to fix any legal
deficiencies in such a plan. That then leaves the question
whether the SSM provisions at issue in these SIP calls—
namely, automatic exemptions, director’s discretion
provisions, and affirmative defenses—disrupt that continuity.
EPA correctly found that they do.

     1. Automatic exemptions violate the Act’s requirement
that emission limitations be continuous. Such provisions
exempt sources from compliance with emission limitations
during SSM events or other modes of operation, such that any
excess emissions are not considered violations of the emission
limitation. Take West Virginia’s SIP as an example. West
Virginia provides that limitations on visible emissions “shall
apply at all times except in periods of start-ups, shutdowns
and malfunctions.” W. VA. CODE R. § 45-2-9.1. West
Virginia’s emission limitations thus apply continuously except
during SSM periods. Such carve-outs run afoul of Congress’s
requirement for continuous emission limitations under section
7602(k). As in Sierra Club, those automatic exemptions
                              13
mean that no Clean Air Act-compliant standard governs
periods of startup, shutdown, and malfunction—i.e., there is
no continuously applicable emission limitation. See 551 F.3d
at 1027-28.

     Petitioners claim that, even if the Act requires
continuously applicable emission limitations, “general duty”
provisions in SIPs remedy any discontinuity because a general
duty applies to all periods of source operation. See Indus. Br.
38-40; State Br. 22-24. General duty provisions require, for
example, sources to “minimize emissions,” to “use good
engineering judgment at all times,” to avoid “improperly
operating or maintaining facilities,” or to avoid “caus[ing] a
violation of the NAAQS at any time.” 80 Fed. Reg. at
33,889/3-90/1, 33,903/1. We have held that such general duty
provisions fail to satisfy the Clean Air Act’s requirement for
continuous emission standards. See Sierra Club, 551 F.3d at
1027-28.

     Much as in Sierra Club, the generic general duty
provisions in the SIPs called by EPA do not meet the Act’s
requirement for continuous emission limitations. For one,
where general duty provisions “are not clearly part of or
explicitly cross-referenced in a SIP emission limitation,” they
“cannot be viewed as a component of a continuous emission
limitation.” 80 Fed. Reg. at 33,890/1. And, absent any clear
indication that general duty provisions are part of a
continuous emission limitation, they may not be enforceable
by EPA or citizens. Cf. McEvoy v. IEI Barge Servs., Inc., 622
F.3d 671, 678 (7th Cir. 2010) (concluding that an Illinois SIP
provision amounting to a “commandment ‘thou shall not
pollute’” could not be enforced through the CAA’s citizen-
suit provision); 80 Fed. Reg. at 33,903-904.
                              14
     Moreover, some of the states’ general duty provisions fail
to meet the Act’s “applicable stringency requirements for
th[e] type of SIP provision” at issue. 80 Fed. Reg. at
33,890/1; see id. at 33,904/1; cf. Sierra Club, 551 F.3d at
1027-28. For example, a generic general duty provision may
not satisfy the Act’s requirement that stationary sources in
nonattainment areas use “reasonably available control
technology,” 42 U.S.C. § 7502(c)(1), that certain sources in
attainment areas use “best available control technology,” id.
§ 7475(a)(4), or that certain older sources use “best available
retrofit technology” as part of the regional haze program, id.
§ 7491(b)(2)(A).

     For those reasons, EPA rightly determined that the
generic general duty provisions in the called SIPs “do not
meet applicable stringency requirements, are not clearly part
of the emission limitations in the SIP-called provisions, and
are likely not legally and/or practically enforceable.” EPA
Br. 69; see 80 Fed. Reg. at 33,903/2-04/2. As such, those
general duty provisions do not “legitimize exemptions for
emissions during SSM events” from otherwise-applicable
emission limitations. 80 Fed. Reg. at 33,890/1.

     2. Automatic exemptions are not the only SIP provisions
that violate the continuity requirement. So do director’s
discretion provisions. They allow state air agency personnel
to make “unilateral decisions on an ad hoc basis” to excuse
compliance with emission limitations, “up to and including
the granting of complete exemptions for emissions during
SSM events.” Id. at 33,917/2. The exercise of a director’s
discretion thereby has the effect of removing the otherwise-
applicable emission limitation during the SSM event. Just as
a state cannot automatically exempt SSM emissions from
emission limitations, it cannot authorize a director’s
                              15
insufficiently bounded, ad hoc exercise of discretion to do so.
See id.

     Director’s discretion provisions have another defect:
They interfere with EPA and citizen enforcement. “[B]y
granting exemptions for emissions that should be treated as
violations of the applicable SIP emission limitations,”
director’s discretion “provisions functionally allow the air
agency to impose its own enforcement discretion decisions on
the EPA and other parties.” Id. at 33,917/3. Such provisions
thus interfere with federal and citizen enforcement of the Act.
See 42 U.S.C. §§ 7413 (federal enforcement), 7604 (citizen
enforcement). In that way, director’s discretion provisions
cannot be squared with the Clean Air Act’s various
enforcement requirements, including that SIPs generally
provide for enforcement, see id. § 7410(a)(1), that emission
limitations be “enforceable,” id. § 7410(a)(2)(A), and that
SIPs “include a program to provide for the enforcement” of
their emission limitations, id. § 7410(a)(2)(C).

     3. Last are the SIPs’ affirmative defense provisions. As
the majority recognizes, some affirmative defenses
functionally “create an exemption from the normal emission
rule.” Maj. Op. 64. Therefore, like any other SSM
exemption, they disrupt an emission limitation’s continuity.
That discontinuity suffices to justify the SIP Calls for the
handful of affirmative defense provisions as to which my
colleagues grant the petitions.

     But there is another reason why EPA correctly called all
SIPs containing affirmative defenses for SSM events.
Affirmative defenses interfere with the Act’s enforcement
structure no matter “what forms of remedy they purport to
limit or eliminate,” 80 Fed. Reg. at 33,851/3—not just, as the
majority holds, when they “preclude[] certain remedies” such
                              16
as civil penalties, Maj. Op. 64. Here is why. The Clean Air
Act gives federal district courts jurisdiction over EPA
enforcement actions, 42 U.S.C. § 7413(b), and citizen suits,
id. § 7604(a), that claim violations of a SIP’s emission
limitations or standards. Congress vested in the courts the
authority both “to determine liability” and “to impose
remedies of various kinds,” including but not limited to civil
penalties.    80 Fed. Reg. at 33,851/3; see 42 U.S.C.
§§ 7413(b), (e), 7604(a). That “grant of jurisdiction comes
directly from Congress,” and EPA is not, nor are the states,
“authorized to alter or eliminate” it. 80 Fed. Reg. at 33,851/3.
By including affirmative defense provisions for SSM
exemptions in their SIPs, states attempt to restrict federal
courts’ jurisdiction to hold violators liable and fashion
remedies for noncompliance with emission limitations. Id. at
33,851/3-52/1. Such defenses usurp the role that Congress
reserved for courts in the Clean Air Act and so cannot stand.
See NRDC v. EPA, 749 F.3d 1055, 1062-64 (D.C. Cir. 2014).

     In short, Petitioners’ challenges to EPA’s SIP Calls of
automatic exemptions, director’s discretion provisions, and
affirmative defenses lack merit. Because our review is
generally confined to the issues presented by the parties, that
should suffice for this court to deny the petitions in full.

                              B.

     My colleagues never reach the questions presented by
Petitioners because, in their view, EPA failed to make a
“predicate ‘necessary or appropriate’ finding” that they deem
“required” by the Clean Air Act. Maj. Op. 55. In so holding,
the majority disposes of the case on an unbriefed theory that
depends on a blinkered reading of the record.

    My colleagues vacate the SIP Calls because EPA “merely
reasoned” that emission limitations must be continuous,
                              17
“without explaining why that continuity is ‘necessary or
appropriate’ to meet any of the CAA’s requirements.” Maj.
Op. 56. Their holding hangs on a distinctive reading of the
Clean Air Act’s section 7410(a)(2)(A). Recall the terms of
that provision: It instructs that each implementation plan
shall “include enforceable emission limitations and other
control measures, means, or techniques (including economic
incentives such as fees, marketable permits, and auctions of
emissions rights), as well as schedules and timetables for
compliance, as may be necessary or appropriate to meet the
applicable requirements” of the Act.           42 U.S.C. §
7410(a)(2)(A).     The majority says the “necessary or
appropriate” proviso prevents EPA from faulting the
discontinuity of a SIP’s emission limitations until it deems it
at least appropriate to treat them as “enforceable emission
limitations”—rather than, say, “other control measures,
means, or techniques.” See Maj. Op. 36, 42.

     But the Petitioners never argued before us that EPA
needed to make—let alone that it did not make—a predicate
“necessary or appropriate” determination before accepting
Petitioners’ own labeling of the subject emission controls as
“emission limitations.” See generally Indus. Br. 36-60; States
Br. 22-38. In fact, the Petitioners took issue with the SIP
Calls in part on the ground that it is up to states when they
fashion the SIPs, not EPA, to decide what is “necessary or
appropriate.” See States Br. 4-5; Indus. Br. 21, 36. Likewise,
the Petitioners do not dispute that the assertedly discontinuous
control measures subject to these SIP Calls were included in
SIPs because states believed them “necessary or appropriate”
for Clean Air Act compliance, and EPA approved them as
such (albeit in approvals it now says overlooked unlawful
discontinuities). To the contrary, Petitioners fault EPA for
“overruling state decisions” about the measures necessary or
appropriate to comply with the Act. Indus. Br. 20; see States
                               18
Br. 4-5. The majority’s reliance on the fact that EPA
“nowhere disputes” that emission limitations are required
only as “necessary or appropriate,” Maj. Op. 44, is startling
given that the Petitioners did not argue as much before us.

     The sentence my colleagues quote from the Industry
Petitioners’ brief as “exactly” making their argument, Maj.
Op. 59 (quoting Ind. Pet. Br. 49), in fact disputes only EPA’s
understanding of the continuity requirement. It appears under
the header “EPA’s Prohibition on So-Called ‘Exemptions’
from Emission Limitations Is Not Supported,” Ind. Pet. Br.
36, where Industry Petitioners argue against EPA’s view of
the “so-called exemptions,” Ind. Pet. Br. 36-52. Only the
majority doubts the characterization of the subject SIP
provisions as (so called) emission limitations per section
7602(k).

     To be sure, the Industry Petitioners (but not the states) at
times argued that SSM exemptions were permissible because
a state could determine that emission limitations were only
“necessary or appropriate” for particular periods of source
operation. Indus. Br. 36, 51. They assert that the “necessary
or appropriate” language gives states discretion to decide that
an emission limitation will apply only to steady-state
operation and not to SSM periods. Indus. Br. 49, 51. Under
that reading, an emission limitation meets the Clean Air Act’s
requirements if it is “‘continuous’ over some period of time or
condition,” even if “not necessarily [over] all periods of
time.” Indus. Br. 42. In short, Industry Petitioners assume
that the SIPs at issue include emission limitations within the
meaning of the Act but argue that the challenged SSM
exemptions permissibly apply because the Act empowers the
states to decide that fully continuous application is not
“necessary or appropriate.” Indus. Br. 49.
                              19
     All Industry Petitioners argued, then, is that the
“necessary or appropriate” clause justifies exemptions to
emission limitations—not that the SIPs might not even,
technically speaking, include emission limitations at all, see
Maj. Op. 35-38, nor that EPA needed to determine that
continuous emission limitations were necessary or appropriate
before calling the SIPs, see Maj. Op. 48-49. At core, Industry
Petitioners fought the definition of an emission limitation as
“continuous” and our construction of that definition in Sierra
Club, 551 F.3d at 1026-28, not the reality that the SIPs at
issue do include “emission limitations” that must comply with
the Act’s definition.

     For us to rule for Industry Petitioners on that theory, we
would therefore have needed to conclude first that Sierra
Club was wrong or somehow did not apply. The majority
instead eschews interpreting the continuity requirement and
pivots to even more fundamental (and unbriefed) questions:
must SIPs contain “emission limitations” at all, and must we
presume they do not unless EPA has made a predicate
determination that it is “necessary or appropriate” for a
measure to qualify as an “emission standard”? See Maj. Op.
34.

     To answer that question the majority deploys a novel
reading of the Clean Air Act, concluding that a SIP must
include “emission limitations” only to the extent “necessary
or appropriate to meet the applicable requirements” of the
Clean Air Act.        Maj. Op. 7-8 (quoting 42 U.S.C.
§ 7410(a)(2)(A)). Relying on this new reading of section
7410, my colleagues conclude that, in the absence of an
explicit EPA categorization of challenged provisions as
“emission limitations,” we should presume the SIPs at issue
here do not contain emission limitations at all. Their reading
seems to require EPA to make a “predicate finding” as to any
                               20
SIP provision that it is “necessary or appropriate” to treat it as
what a state says it is—as opposed to some other measure or
means—before a court may so treat it. Maj. Op. 36. Consider
a SIP that transparently announces what appears to be a
schedule or timetable for compliance as contemplated by
section 7410. 42 U.S.C. § 7410(a)(2)(A). On the majority’s
logic, may we no longer take a SIP at its word and treat its
compliance deadlines and timeframes as “enforceable”
“schedule[s] and timetable[s] of compliance” under the Act,
42 U.S.C. § 7602(p), unless EPA finds it “appropriate” or
“necessary” that the timing requirements be so treated?

    More to the point, the Petitioners do not dispute that the
SIPs at issue needed to, and appropriately do, include
“emission limitations.” Indeed, the states have repeatedly told
us that their SIPs do contain emission limitations as
contemplated by the Clean Air Act:

      In their briefing the states refer to the disputed
       emission limitations as “emission limitations.” See,
       e.g., States Br. 24-29; States Reply Br. 1, 8, 11-12.
       Industry Petitioners even describe the provisions
       subject to the disputed SSM carveouts as “numerical”
       or “numeric emission limitation[s].” Ind. Pet. Br. 37.
       That quantitative descriptor highlights that the
       referenced “emission limitations” are indeed limits on
       the “quantity, rate, or concentration of emissions”
       within the meaning of section 7602(k), and so subject
       to its continuity requirement.
      In their comments on the SIP Calls the states refer to
       the disputed emission limitations as “emission
       limitations.” See, e.g., Comment of Ohio (J.A. 529-
       30) (explaining that Ohio has never taken the position
       that excess emissions are not violations of emission
       limitations); Comment of Georgia (J.A. 575-76)
                              21
       (“EPA’s proposal if finalized would likely require that
       Georgia evaluate all of the emission limitations in
       Georgia Rule 391-3-1-.02 . . . .”); Comment of
       Delaware (J.A. 600) (“As such, Delaware’s SIP
       provides for continuous limits; at no time does
       Delaware’s SIP provide for any unit to be ‘exempted’
       from being covered by an emission limitation.”);
       Comment of West Virginia (J.A. 630) (“Emissions
       that occur during SSM must be subject to a continuous
       limitation, and emissions that occur during regular
       operations may be subject to a different, but also
       continuously applied, limitation.”); Comment of
       Florida (J.A. 746) (“Florida’s rule establishes
       enforceable, alternative emissions limitations and
       measures such that an emission limitation applies
       continuously . . . .”).
      Even in the enacted SIPs themselves the states refer to
       the disputed emission limitations as “emission
       limitations.” See e.g., S.C. CODE ANN. REGS. 61-62.1
       §§ I(29), V(A) (2016) (explaining that the state
       developed its implementation plan “to provide
       enforceable emission limitations” and that an emission
       limitation “limits the quantity, rate, or concentration of
       emissions of air pollutants on a continuous basis”);
       118-01-19 ARK. CODE. R. § 602 (2022) (providing an
       affirmative defense for exceedances of “emission
       limitation[s]” during emergency conditions); LA.
       ADMIN. CODE tit. 33, pt. III, § 103(A) (2023)
       (directing that the SIP’s “emission limitations apply to
       any source of emissions existing partially or wholly
       within the state of Louisiana”).

There is thus every indication that the states, in submitting
SIPs to fulfill their legal obligations under the Clean Air Act,
intended to include “‘emission limitation[s]’ as defined by the
                              22
CAA.” Maj. Op. 53 (emphasis added). And there is every
indication the Petitioners are defending those emission
limitations’ compliance with the Act’s requirement that they
be continuous. Why not take them at their word?

    The majority sidesteps those admissions to posit that the
Clean Air Act could allow for a SIP that contains various
emission control measures, “none of which satisf[ies] the
CAA’s definition of ‘emission limitation.’” Maj. Op. 37.
Assuming the text may be so read, the permissibility of that
hypothetical is beside the point. As EPA explained at
argument, “if [a] state[] were to bring a plan to EPA that
contained just control measures and not emission limitations
and said that [it] me[t] the requirements of the Clean Air Act,
then EPA could assess that plan.” Oral Arg. Tr. 68:14-17.
But here, states “picked emission limitation[s] as one element
of their plan[s].” Oral Arg. Tr. 68:18-19. That choice is
reason enough to treat the SIPs under review as containing
“emission limitations” within the meaning of the Act.

     Moreover, the majority provides no indication (nor is
there any, as far as I can tell) that any state has ever—in the
more than fifty years of the Clean Air Act’s existence—
submitted and received approval for a SIP without provisions
that appear to be (and have been treated as) “emission
limitations.” That is no accident. Emission limitations are at
the heart of a state’s responsibilities to reduce emissions
pursuant to the Clean Air Act. In assigning to the states
certain responsibilities under the Act, Congress sought to
ensure that “[e]ach source’s prescribed emission limitation”
would be “the fundamental tool for assuring that ambient
standards are attained and maintained.” H.R. REP. NO. 95-
294, at 92.
                              23
      Whatever the case for statutory permissibility of a
hypothetical SIP devoid of emission limitations, there is
certainly no basis for concluding that the challenged SIP
provisions in this case were simply not emission limitations at
all, nor have the Petitioners so argued. Yet that conclusion is
the linchpin of the majority’s call for a re-do.

     It makes sense that these state plans, first formulated in
the early 1970s, all do include “emissions limitations” within
the meaning of the Clean Air Act. Before the 1990
amendment of the Act, an earlier version of section 7410
(then codified at 42 U.S.C. § 1857c-5) instructed that state
plans shall “include[] emission limitations, schedules, and
timetables for compliance with such limitations, and such
other measures as may be necessary . . . .”—a formulation
that courts read to require “emission limitations.” See
Kennecott Copper Corp. v. Train, 526 F.2d 1149, 1153-54
(9th Cir. 1975) (quoting 42 U.S.C. § 1857c-5(a)(2)(B)
(1970)); see id. at 1153-56 (collecting cases). It was not until
Congress amended that provision in 1990 that it added “or
appropriate” and changed the syntax to the current version, on
which the majority depends for its holding that “emission
limitations” are optional unless EPA deems them “necessary
or appropriate.” Given the pre-1990 origins of the relevant
emission-control measures in the SIPs (and the challenged
loopholes), it is no wonder that every one of the petitioning
states treats them as “emission limitations.”

    The states proffered and EPA accepted SIPs that
purported to include “emission limitations” as defined by the
Clean Air Act. See 80 Fed. Reg. at 33,902/2. Petitioners seek
to defend them as “emission limitations” that meet the
requirement of “continuous emission reduction.” It is entirely
unsurprising that EPA “never wholeheartedly embraces” my
emphasis on the states’ own consistent description of the
                              24
relevant SIP provisions as “emissions limitations.” Maj. Op.
52; see id. 25. The Petitions never said they were anything
else.

     Our judicial role is as “neutral arbiter[s] of matters the
parties present.” United States v. Sineneng-Smith, 140 S. Ct.
1575, 1579 (2020) (quoting Greenlaw v. United States, 554
U.S. 237, 243 (2008)). In our adversarial system, “we rely on
the parties to frame the issues for decision.” Id. (quoting
Greenlaw, 554 U.S. at 243). So, our “well-established” rule
is that “we do not consider arguments not presented to us,”
Diamond Walnut Growers, Inc. v. NLRB, 113 F.3d 1259,
1263 (D.C. Cir. 1997) (en banc), even if those arguments can
be supported by citations to the administrative record, see
NRDC v. EPA, 25 F.3d 1063, 1071 n.4 (D.C. Cir. 1994). And
we are especially reluctant to consider unbriefed or
inadequately briefed arguments where they raise “important
questions of far-reaching significance,” Carducci v. Regan,
714 F.2d 171, 177 (D.C. Cir. 1983) (quoting Ala. Power Co.
v. Gorsuch, 672 F.2d 1, 7 (D.C. Cir. 1982)), or implicate a
complex regulatory scheme, see Time Warner Ent. Co., L.P.
v. FCC, 56 F.3d 151, 202 (D.C. Cir. 1995). Every reason
calls for restraint.

     Today’s decision well illustrates why we ordinarily stick
to what the parties have argued. Without the benefit of the
parties’ presentation, it is difficult to say just how wrong or
damaging the majority opinion might be. What I can say is
that my colleagues unearth—and, in some cases, answer—a
host of questions that were not raised in the briefing and that
we had never resolved before today.

     Here is the strange upshot of the majority’s interpretation
of section 7410(a)(2)(A): Provisions that otherwise look like
and are treated as emission limitations are reviewed as “other
                               25
control measures,” undefined under the Act, precisely
because they are discontinuous. See Maj. Op. 37. Under that
logic, a flawed emission limitation is simply no emission
limitation at all and so the continuity question that was briefed
and argued in this case just goes away. For the majority, it
does not matter whether the called SIPs lack continuous
protection against harmful emissions, because if the emission
limitations in the SIPs were actually just “other control
measures,” the statute does not require them to apply
continuously.

     But it is not evident that emission limitations and “other
control measures” are interchangeable but for the requisite
continuity of the former, and we lack the benefit of briefing
on the matter to determine the boundaries of either. In the
Clean Air Act, Congress gave examples of “other control
measures, means, or techniques”—namely, “economic
incentives such as fees, marketable permits, and auctions of
emissions rights.” 42 U.S.C. § 7410(a)(2)(A). In so doing,
Congress illustrated the kinds of “other control measures” it
had in mind. They stand in contrast to an emission limitation,
“which limits the quantity, rate, or concentration of emissions
of air pollutants on a continuous basis, including . . . any
design, equipment, work practice or operational standard
promulgated under this chapter.” Id. § 7602(k). Indeed, to
the extent “other” measures are ways of harnessing market
power to make underlying emission limitations more efficient,
they seem to presuppose “emission limitations” as a distinct
but complementary counterpart.

                               C.

     Cabining the reach of its holding, the majority notes that,
going forward, EPA may ensure that states’ pollution
restrictions are “enforceable emission limitations” for
                              26
purposes of a SIP by making a predicate “necessary or
appropriate” finding to that effect. Maj. Op. 37-38, 40, 55.
Never mind that Petitioners did not argue that the Clean Air
Act imposes any such predicate, nor did they object that
provisions in the SIPs failed to meet it.

     In so doing, the majority surmises both who must make a
necessary-or-appropriate determination (EPA, say my
colleagues) and when (before calling the SIPs, they say).
Remember, though, that section 7410(a)(2) describes what
“[e]ach implementation plan submitted by a State” shall
include. 42 U.S.C. § 7410(a)(2) (emphasis added). It thus
appears—as the Petitioners themselves argued—that
Congress called on the states to determine in the first instance
what measures they believe are “necessary or appropriate,”
subject to EPA approval. See Indus. Br. 53; States Br. 4-5.
Perplexingly, though, it is not enough for the majority that the
states that proffered the SIPs, and EPA when initially
approving them, concluded that emission limitations are at the
very least appropriate to comply with the Act. See 80 Fed.
Reg. at 33,902/2-3. The fact that EPA simultaneously
approved SSM exemptions, see Maj. Op. 50–51, shows only
that a newly created EPA was rushing to approve the SIPs on
too tight a timeline—not that the challenged SIP provisions
were simply not “emission limitations” at all.

                              D.

     In short, if anyone has manufactured a “semantic ‘gotcha’
game,” Maj. Op. 53, it is the majority. According to my
colleagues, an emission reduction measure that is a “design,
equipment, work practice or operational standard” that “limits
the quantity, rate, or concentration of emissions of air
pollutants,” 42 U.S.C. § 7602(k), that a state labeled an
“emission limitation” and submitted to EPA to show how it
                              27
planned to meet the Act’s requirements, that EPA then
approved as an appropriate way for the state to do what the
Act necessitates, 80 Fed. Reg. at 33,902/2-3, and that states,
industry, and EPA all have for decades treated as an emission
limitation, might not be an emission limitation after all. That
cannot be right. If it looks like a duck, swims like a duck, and
quacks like a duck, then it probably is a duck.

     While I believe the majority erred, it helps that its core
holding is limited. Presumably EPA can re-issue the vacated
SIP Calls. My colleagues tell the agency simply to show that
continuous emission limitations are “necessary”—or even just
“appropriate”—to meet Clean Air Act requirements. That is a
low bar. No factual showing would necessarily be required.
See Maj. Op. 14-25. And EPA’s insistence that emission
limitations operate continuously would simply reiterate its
longstanding policy, beginning in 1977, that SSM exemptions
are inconsistent with the Clean Air Act. See J.A. 125. It
might even be enough for EPA to point out that the called
SIPs are ambiguous as to whether they do contain “emission
limitations.” See Maj. Op. 27-28. What is more, nothing in
today’s decision forecloses EPA from calling SIPs because
they contain emission limitations that are discontinuous due to
SSM exemptions, see Maj. Op. 34, or fail to include measures
that meet the statutory definition of emission limitations, see
Maj. Op. 37, or interfere with the Clean Air Act’s
enforcement scheme, see Maj. Op. 39, or interfere with EPA’s
ability to predict compliance with the Act, see id. The court’s
opinion also does not categorically preclude EPA, going
forward, from pointing to its initial approval of SIP provisions
to establish that those provisions were “necessary or
appropriate” to meet Clean Air Act requirements. See Maj.
Op. 55-56.
                             28
    So, while the majority goes down a path that it never
should have considered, resulting in more years of delay in
controlling harmful emissions, the fallout may be more
contained than it appears at first glance.

                            ***

     In brief, EPA correctly called SIPs for containing SSM
provisions that violated the Clean Air Act’s requirement of
continuous emission limitations. This court should have
acknowledged that the states appropriately included emission
limitations in their SIPs, the Act says an emission limitation
must be continuous, and the SIP provisions rendering them
discontinuous necessitated EPA’s SIP Calls. My colleagues
provide no practical reason to doubt that the continuous
applicability of the SIPs’ emission rules is indeed “necessary
or appropriate.” Yet the majority assigns to EPA a procedural
step it claims was missed: What everyone refers to as
emission limitations should not be treated as such under the
Act unless EPA first finds that it was “necessary or
appropriate” that they were included as “emission limitations”
in the SIPs. In vacating EPA’s SIP Calls on that ground, the
majority strays from the briefing, from our circumscribed
judicial role, and from the mandates of the Clean Air Act. On
these issues only, I respectfully dissent.