 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued October 24, 2013              Decided April 18, 2014

                       No. 10-1371

         NATURAL RESOURCES DEFENSE COUNCIL,
                     PETITIONER

                             v.

    ENVIRONMENTAL PROTECTION AGENCY AND GINA
 MCCARTHY, IN HER OFFICIAL CAPACITY AS ADMINISTRATOR,
      U.S. ENVIRONMENTAL PROTECTION AGENCY,
                    RESPONDENTS

                   CEMEX, INC., ET AL.,
                     INTERVENORS


            Consolidated with 10-1378, 13-1112


      On Petitions for Review of Final Actions of the
      United States Environmental Protection Agency


     James S. Pew and Seth L. Johnson argued the causes for
petitioners. With them on the briefs were John Walke,
Meleah Geertsma, and Avinash Kar.

    Matthew R. Oakes, Attorney, U.S. Department of Justice,
argued the cause for respondents. With him on the brief were
                              2
Robert G. Dreher, Acting Assistant Attorney General, and
Steven E. Silverman, Attorney, U.S. Environmental Protection
Agency.     Daniel R. Dertke and T. Monique Peoples,
Attorneys, U.S. Department of Justice, entered appearances.

    Carter G. Phillips argued the cause for intervenors. With
him on the briefs were Roger R. Martella Jr., Timothy K.
Webster, Ryan C. Morris, William M. Bumpers, Debra J.
Jezouit, Michael B. Schon, Deborah E. Jennings, Chet M.
Thompson, Beth S. Ginsberg, Jason T. Morgan, Ashley C.
Parrish, Cynthia A.M. Stroman, and Richard G. Stoll.

    Russell S. Frye and Richard G. Stoll were on the brief for
amici curiae SSM Coalition, et al. in support of respondents.

    Before: KAVANAUGH and SRINIVASAN, Circuit Judges,
and EDWARDS, Senior Circuit Judge.

   Opinion for      the   Court   filed   by   Circuit   Judge
KAVANAUGH.

     KAVANAUGH, Circuit Judge: Portland cement is the key
ingredient in concrete. The basic process for making Portland
cement is much the same today as it was when the material
was first developed nearly 200 years ago.              Cement
manufacturers pulverize limestone and minerals, and then
heat those raw materials to several thousand degrees. The
resulting substance, called clinker, is then cooled and ground
into a fine gray powder. This powder – called Portland
cement – is later combined with sand, rocks, and water to
make concrete.

    The grinding and heating involved in cement
manufacturing has an unfortunate side effect: It releases into
the air a number of hazardous air pollutants, most notably
                              3
mercury, hydrochloric acid, hydrocarbons, and particulate
matter. This case concerns EPA’s efforts to develop rules
under the Clean Air Act to limit emissions of those pollutants
from cement plants.

     In a previous decision, we considered EPA’s first attempt
to create emission standards for the cement industry, and we
found the agency’s action arbitrary and capricious. See
Portland Cement Association v. EPA, 665 F.3d 177 (D.C. Cir.
2011). Following our ruling, EPA went back to the drawing
board and developed the emission standards at issue here, the
2013 Rule.

     Several environmental organizations, including the
Natural Resources Defense Council and the Sierra Club, have
petitioned for review of the 2013 Rule, arguing primarily that
certain aspects of the Rule contravene the Clean Air Act.
They also challenge EPA’s decision to create an affirmative
defense for private civil suits in which plaintiffs sue sources
of pollution and seek penalties for violations of emission
standards. EPA’s affirmative defense would be available to
defendants in cases where an “unavoidable” malfunction had
resulted in impermissible levels of emissions.

    We conclude that the emissions-related provisions of
EPA’s 2013 Rule are permissible but that the affirmative
defense for private civil suits exceeds EPA’s statutory
authority. We therefore grant the petitions in part and vacate
the portion of the Rule pertaining to the affirmative defense.
We deny the petitions in all other respects.

                               I

    Section 112 of the Clean Air Act, 42 U.S.C. § 7412,
requires EPA to establish technology-based emission
standards for major sources of certain hazardous air
                              4
pollutants. Emission standards must reflect “the maximum
degree of reduction in emissions” that EPA determines is
“achievable,” taking into consideration “the cost of achieving
such emission reduction, and any non-air quality health and
environmental impacts and energy requirements.” 42 U.S.C.
§ 7412(d)(2). The resulting standards are commonly known
as the “maximum achievable control technology,” or
“MACT” standards. See National Lime Association v. EPA,
233 F.3d 625, 630 (D.C. Cir. 2000).

     EPA uses a two-step process for establishing MACT
standards. The agency begins by setting a minimum
stringency level, or “floor,” based on the results achieved by
the best-performing similar sources.          See 42 U.S.C.
§ 7412(d)(3). Once EPA sets the statutory floor, it then
determines, considering cost and the other factors listed in
Section 112(d)(2), whether a more restrictive standard is
“achievable,” and if so then adopts that standard. EPA calls
these stricter requirements “beyond-the-floor” standards.
Sierra Club v. EPA, 479 F.3d 875, 877 (D.C. Cir. 2007).

     When EPA sets an emission standard, it also determines a
schedule for compliance with that standard. For existing
sources, EPA must “provide for compliance as expeditiously
as practicable, but in no event later than 3 years after the
effective date” of the emission standard.         42 U.S.C.
§ 7412(i)(3)(A).

     In 2010, pursuant to its Section 112 authority, EPA
promulgated National Emission Standards for Hazardous Air
Pollutants from the Portland Cement Manufacturing Industry
and Standards of Performance for Portland Cement Plants, 75
Fed. Reg. 54,970 (Sept. 9, 2010). That 2010 Rule set or
revised emissions limits for mercury, hydrogen chloride, total
hydrocarbons (a surrogate for organic hazardous air pollutants
                              5
such as benzene and formaldehyde), and particulate matter (a
surrogate for certain non-mercury metals). Cement plants
would be required to comply with the new standards
beginning in September 2013. 75 Fed. Reg. at 54,993.

     In addition to establishing emission standards, the 2010
Rule created an affirmative defense in private civil suits when
violations of the standards occurred because of “unavoidable”
malfunctions. See id. at 54,993, 55,053. The affirmative
defense replaced a previous EPA policy creating an
exemption from emissions limitations during malfunction
events. In a prior decision, this Court struck down that
exemption because it was inconsistent with the requirement
that emission standards apply continuously. See Sierra Club
v. EPA, 551 F.3d 1019, 1027-28 (D.C. Cir. 2008).

     Industry groups petitioned this Court for review and we
found the 2010 Rule arbitrary and capricious and remanded to
EPA. We ruled specifically that, in calculating the floor for
MACT purposes, EPA had arbitrarily included in its dataset
information from cement kilns properly classified as
commercial incinerators, which are regulated under a separate
provision of the Act. See Portland Cement Association v.
EPA, 665 F.3d 177, 186-89 (D.C. Cir. 2011).

     In our 2011 decision, however, we did not vacate the
emission standards set forth in the 2010 Rule or stay its
implementation pending EPA’s reconsideration process,
stating that “it is unlikely that significant changes will be
made to the standards upon reconsideration.” Id. at 189.

     On remand, however, EPA made several relevant
changes to the Portland cement emission standards. See
National Emission Standards for Hazardous Air Pollutants for
the Portland Cement Manufacturing Industry and Standards of
                              6
Performance for Portland Cement Plants, 78 Fed. Reg. 10,006
(Feb. 12, 2013) (the 2013 Rule).

     First, consistent with our Portland Cement opinion, EPA
revised its dataset to exclude commercial incinerators. When
recalculated using the revised data, the maximum emissions
level for particulate matter – the floor – was ultimately
revised from 0.04 lb/ton to 0.07 lb/ton of clinker for existing
kilns. See id. at 10,017-19. And EPA declined to re-adopt the
more stringent, 0.04 lb/ton limit of the 2010 Rule as a
beyond-the-floor standard.       The agency reasoned that
achieving that additional increment of particulate reduction
would not be cost effective on a cost-per-ton basis. See id. at
10,020-21.

     Second, citing additional compliance strategies afforded
cement manufacturers by the revised particulate standard,
EPA established a new compliance date of September 2015
for that standard. See id. at 10,014. EPA further concluded
that although the emissions limits for mercury, hydrochloric
acid, and hydrocarbons remained the same as in the 2010
Rule, the new September 2015 compliance date would also
apply to those emission standards. According to EPA,
coordinating the compliance date for particulate matter,
mercury, hydrochloric acid, and hydrocarbons was essential
because the latter standards “all typically involve some
element of [particulate matter] generation and capture and so
the controls must be integrated with [particulate matter]
control strategies.” Id. at 10,022.

     The 2013 Rule also retained the affirmative defense for
private civil suits when the defendant violated emission
standards due to an unavoidable malfunction. EPA explained
that in its view, the affirmative defense was necessary to
resolve a “tension” between the Clean Air Act’s requirement
                               7
that emission standards apply at all times and the fact that
emission limits may sometimes be exceeded for reasons
beyond the control of the source. See id. at 10,014.

     NRDC, the Sierra Club, and other environmental
organizations have petitioned for review of various aspects of
the 2013 Rule. In Part II of this opinion, we address
petitioners’ arguments regarding the emission standards for
particulate matter. In Part III, we address petitioners’
challenge to the compliance schedule implementing some of
the 2013 Rule’s emission standards. In Part IV, we consider
whether EPA’s decision to create the affirmative defense to
civil penalties for certain malfunction-related events exceeds
the agency’s statutory authority.

                               II

    We first consider petitioners’ challenges to the emission
standards for particulate matter.

                               A

     The 2013 Rule ultimately set the emissions level for
particulate matter at 0.07 lb/ton of clinker for existing kilns.
The 2010 Rule had set the level at 0.04 lb/ton of clinker.
Petitioners argue that the 2013 Rule weakens the particulate
matter standard in violation of Section 112(d)(7) of the Clean
Air Act, 42 U.S.C. § 7412(d)(7). That provision, titled “Other
requirements preserved,” states:

       No emission standard or other requirement
       promulgated under this section shall be interpreted,
       construed or applied to diminish or replace the
       requirements of a more stringent emission limitation
       or other applicable requirement established pursuant to
                              8
       section 7411 of this title, part C or D of this
       subchapter, or other authority of this chapter or a
       standard issued under State authority.

(emphases added). Petitioners maintain that EPA violated
Section 112(d)(7) because the 2013 Rule’s particulate matter
standards “diminish or replace” the more stringent standards
in the 2010 Rule.

    EPA responds that such a reduction does not violate
Section 112(d)(7). In EPA’s view, the most natural reading of
Section 112(d)(7) is that “other authority” refers to authority
other than Section 112 and other than the parts of the Clean
Air Act specifically enumerated in Section 112(d)(7). Stated
another way, EPA suggests that Section 112(d)(7) is simply a
savings clause that makes clear that Section 112 does not
supersede the requirements of other, more restrictive
provisions of the Act.

     By contrast, petitioners say that “other authority” of the
Act includes Section 112 itself, as well as other provisions in
the Act. Petitioners read the statute as an anti-backsliding
restriction on EPA’s ability to voluntarily reduce the
stringency of any emission standard issued under Section 112.

     As EPA points out, however, when Congress has sought
to include that sort of anti-backsliding provision in the Clean
Air Act, it has done so directly and explicitly. Cf., e.g., 42
U.S.C. § 7410(l) (“The Administrator shall not approve a
revision of a [State Implementation Plan] if the revision
would interfere with any applicable requirement concerning
attainment and reasonable further progress”); id. § 7502(e)
(specifying pollution control requirements if “the
Administrator relaxes a national primary ambient air quality
standard”). Section 112(d)(7) contains no such language.
Furthermore, EPA argues that petitioners’ interpretation of
                              9
Section 112(d)(7) would mean that any change to a rule
issued under Section 112 – even a necessary change made just
for technical reasons or because there was a calculation error
– would be impermissible if the change made the standard
less stringent. EPA says that it would be extraordinary if the
statute precluded that kind of necessary change.

     In wading through this back-and-forth, we ultimately
need not decide whether EPA’s reading is the better or only
reading of this statutory provision, but simply whether it is a
permissible reading. EPA administers the Clean Air Act, and
we must defer to its reasonable interpretation of any
ambiguities in the statute. See Chevron U.S.A. Inc. v. NRDC,
467 U.S. 837, 843-44 (1984). Here, even if the statute does
not compel EPA’s reading, and indeed even if EPA’s reading
is not the better reading, the statute at a minimum is
sufficiently ambiguous on this point to permit EPA’s reading
of “other authority.” Because EPA’s reading is at least
reasonable, we reject petitioners’ argument and rule for EPA
at Chevron step two.

                              B

     Petitioners also contend that EPA should have set a more
restrictive particulate matter standard when considering
whether to set “beyond-the-floor” standards. Petitioners
argue in particular that EPA misinterpreted the statute to
allow it to consider cost-effectiveness when setting beyond-
the-floor standards.

     Under Section 112(d), EPA must require “the maximum
degree of reduction in emissions” that EPA determines is
“achievable.” 42 U.S.C. § 7412(d)(2). To determine that
emission level, EPA first establishes a minimum stringency
level, or “floor,” based on the emission results achieved by
                                10
the best-performing sources in the category at issue. See id.
§ 7412(d)(3). Once EPA sets the statutory floor, it then
determines, based on cost and the other factors listed in
Section 112(d)(2), whether a more restrictive, beyond-the-
floor standard is achievable.1

     When it promulgated the 2013 Rule, EPA rejected
petitioners’ argument to set a 0.04 lb/ton limit as a beyond-
the-floor standard. 78 Fed. Reg. at 10,020. EPA estimated
that a beyond-the-floor standard set at the 0.04 lb/ton level
would result in a reduction of 138 tons of particulate matter
per year, at a cost of $37 million. Id. Based on those
estimates, EPA noted that the cost-effectiveness of the
potential beyond-the-floor standard – $268,000 per ton of
particulate matter removed – was substantially lower than the
cost-effectiveness of other emission standards previously
rejected by EPA. See 78 Fed. Reg. at 10,021.

     Petitioners take issue with EPA’s consideration of cost-
effectiveness as a component of the Section 112(d)(2) cost
analysis. Petitioners contend that “cost” for purposes of the
statute only concerns whether “the standard is too expensive
for industry to achieve,” in essence, whether the standards
would bankrupt the industry. Pet’rs Br. 34.


    1
       In relevant part, the statute reads: “Emissions standards
promulgated under this subsection and applicable to new or existing
sources of hazardous air pollutants shall require the maximum
degree of reduction in emissions of the hazardous air pollutants
subject to this section (including a prohibition on such emissions,
where achievable) that the Administrator, taking into consideration
the cost of achieving such emission reduction, and any non-air
quality health and environmental impacts and energy requirements,
determines is achievable for new or existing sources in the category
or subcategory . . . .” 42 U.S.C. § 7412(d)(2) (emphasis added).
                               11
     EPA says that Congress afforded it wide latitude in
deciding how to balance cost and other considerations when
determining that maximum achievable reduction in emissions.
According to EPA, Section 112 does not command EPA to
use a particular form of cost analysis. In taking cost into
account, EPA contends that it may determine whether the
proposed emission levels would be cost-effective. Indeed,
EPA notes that this Court has previously recognized EPA’s
authority to consider cost-effectiveness in setting standards
under nearly identical provisions of the Clean Air Act. See,
e.g., Husqvarna AB v. EPA, 254 F.3d 195, 200 (D.C. Cir.
2001) (“Because section 213 does not mandate a specific
method of cost analysis, we find reasonable the EPA’s choice
to consider costs on the per ton of emissions removed
basis.”); National Association of Clean Water Agencies v.
EPA, 734 F.3d 1115, 1156-57 (D.C. Cir. 2013) (citing
Husqvarna; EPA could consider cost-effectiveness in setting
a beyond-the-floor standard under Section 129(a)(2) of the
Act).

      Again on this issue as with the first, we need not decide
whether EPA’s reading is the only reading of this provision.
Even if the statute does not compel EPA’s approach, and even
if EPA’s reading is not the better reading, we conclude that it
is still at least a reasonable reading given the various potential
meanings of “cost” in this context. Therefore, we reject
petitioners’ argument that EPA was required to exclude
consideration of cost-effectiveness and to set a beyond-the-
floor standard of 0.04 lb/ton of clinker.

                               III

    Next, we consider petitioners’ claim that EPA acted
unreasonably in setting a compliance date of September 2015
                              12
for the emission standards for particulate matter, mercury,
hydrochloric acid, and hydrocarbons.

     Under Section 112(i)(3)(A) of the Clean Air Act, EPA
must require compliance with emission standards for existing
sources “as expeditiously as practicable, but in no event later
than 3 years after the effective date of such standard.” 42
U.S.C. § 7412(i)(3)(A). Petitioners here contend that EPA
violated this timing provision by “extending” the compliance
deadline for the 2010 Rule from September 2013 to
September 2015.

     As applied to the particulate matter standard, there is a
basic flaw in petitioners’ argument: The 2013 Rule did not
simply “extend” the deadline for complying with the 2010
Rule. Rather, the 2013 Rule established new particulate
matter standards with a new effective date of February 2013,
and a new compliance date of September 2015. EPA
concluded that any earlier date of compliance would not be
practicable because of the multi-year timeline for upgrading
the technology necessary to ensure compliance. See 78 Fed.
Reg. at 10,024. As we have ruled before, EPA may reset the
compliance date for an emission standard when it introduces a
new standard with a new effective date, as was the case for
particulate matter in the 2013 Rule. See NRDC v. EPA, 489
F.3d 1364 (D.C. Cir. 2007).

     According to petitioners, even if EPA could change the
compliance date for the particulate matter standard, the
agency could not do the same for the mercury, hydrochloric
acid, and hydrocarbon standards. The 2010 Rule set the
emission levels for those pollutants, and the 2013 Rule did not
alter the emission levels for those pollutants. This situation
does indeed present a bit of a conundrum. On the one hand,
we know under the terms of the statute that EPA has set a
                               13
compliance date of 2015 as the date that is as expeditious as
practicable for particulate matter. On the other hand, our
prior decision did not vacate the prior 2010 Rule, and the
compliance date for the other pollutants as set forth in that
Rule would otherwise be 2013.

     This conundrum is resolved when one realizes that it
would be irrational and even absurd to have different
compliance dates for the different pollutants. EPA explained
that the technology is such that it would be senseless to have
different compliance dates. See 78 Fed. Reg. at 10,024. Even
petitioners do not deny that the compliance date for all the
pollutants should be the same. To be sure, they want 2013 not
2015 as the compliance date. But they recognize the general
undesirability of a compliance date of 2013 for some of the
pollutants and of 2015 for other pollutants. See Tr. of Oral
Argument at 10-11. Finally and perhaps most importantly,
our prior decision in this case also recognized, at least
implicitly, that there must be a single compliance date for all
of the pollutants. See Portland Cement Association, 665 F.3d
at 189. Our decision necessarily relied on an assumption that
if EPA did not alter the level for any of the pollutants, the date
would be 2013. But if EPA changed the level for one of the
pollutants, the compliance date for all the pollutants would
move together. Petitioners’ argument for a 2013 compliance
date would be inconsistent with our prior decision.

   In short, we reject petitioners’ argument about the 2015
compliance date.

                               IV

    We next consider petitioners’ challenge to the affirmative
defense that EPA created for cases of “unavoidable”
malfunctions.
                               14
     Section 304(a) of the Clean Air Act, 42 U.S.C. § 7604(a),
allows individuals to file citizen suits in federal district court
against sources that violate emission standards. Under the
law as originally enacted, a court could order only injunctive
relief as a remedy for a violation. But as part of the 1990
amendments to the Act, Congress expanded the citizen suit
provision to give district courts authority to impose “any
appropriate civil penalties,” which may include monetary
penalties. 42 U.S.C. § 7604(a).

     In the 2010 Rule, EPA created an affirmative defense to
Section 304(a) for certain emissions violations caused by
“unavoidable” malfunctions. Under the affirmative defense,
the district court may assess penalties only if violators “fail to
meet [their] burden of proving all of the requirements in the
affirmative defense.” 78 Fed. Reg. at 10,039. EPA retained
the affirmative defense when it promulgated the 2013 Rule.
See 40 C.F.R. § 63.1344.

    Petitioners now argue that the affirmative defense
exceeds EPA’s statutory authority and that it is for the courts
to decide whether to create an affirmative defense in these
private civil suits, not EPA. We agree.

     The threshold question is whether petitioners have
standing to challenge EPA’s adoption of the affirmative
defense. Petitioners are environmental associations with
individual members across the country. EPA’s affirmative
defense would immunize certain emissions that petitioners
contend should be penalized. Some of petitioners’ members
will suffer from those higher emissions, according to their
affidavits. A ruling in their favor would prevent those
emissions and help alleviate that harm. That’s good enough.
Petitioners have shown injury-in-fact, causation, and
redressability, and they thus have standing under Article III.
                               15
     We turn, then, to the substance of petitioners’ challenge
to the affirmative defense.

     Section 304(a) grants “any person” the right to
“commence a civil action” against any person “who is alleged
to have violated (if there is evidence that the alleged violation
has been repeated) or to be in violation of” an emission
standard or limitation under the Clean Air Act. 42 U.S.C.
§ 7604(a). The statute further provides that the federal district
courts “shall have jurisdiction, without regard to the amount
in controversy or the citizenship of the parties, to enforce such
an emission standard or limitation . . . and to apply any
appropriate civil penalties.” Id.

     When determining whether civil penalties are
appropriate, district courts look to Section 113(e)(1) of the
Act, which directs courts to “take into consideration (in
addition to such other factors as justice may require) the size
of the business, the economic impact of the penalty on the
business, the violator’s full compliance history and good faith
efforts to comply, the duration of the violation as established
by any credible evidence . . . , payment by the violator of
penalties previously assessed for the same violation, the
economic benefit of noncompliance, and the seriousness of
the violation.” 42 U.S.C. § 7413(e)(1).

     Section 304(a) creates a private right of action, and as the
Supreme Court has explained, “the Judiciary, not any
executive agency, determines ‘the scope’ – including the
available remedies – ‘of judicial power vested by’ statutes
establishing private rights of action.” City of Arlington v.
FCC, 133 S. Ct. 1863, 1871 n.3 (2013) (emphasis added)
(quoting Adams Fruit Co. v. Barrett, 494 U.S. 638, 650
(1990)). Section 304(a) is in keeping with that principle. By
its terms, Section 304(a) clearly vests authority over private
                               16
suits in the courts, not EPA. As the language of the statute
makes clear, the courts determine, on a case-by-case basis,
whether civil penalties are “appropriate.” By contrast, EPA’s
ability to determine whether penalties should be assessed for
Clean Air Act violations extends only to administrative
penalties, not to civil penalties imposed by a court. See 42
U.S.C. § 7413(d)(2)(B) (Administrator may “compromise,
modify, or remit, with or without conditions, any
administrative penalty”). To the extent that the Clean Air Act
contemplates a role for EPA in private civil suits, it is only as
an intervenor. See id. § 7604(c)(2). EPA also of course could
seek to participate as an amicus curiae.

     EPA argues that its proposed affirmative defense simply
fleshes out the statutory requirement that penalties be applied
only when “appropriate.” But under this statute, deciding
whether penalties are “appropriate” in a given private civil
suit is a job for the courts, not for EPA. When a private suit is
filed, the defendant can argue that penalties should not be
assessed, based on the factors in Section 113(e)(1) such as the
defendant’s “full compliance history and good faith efforts to
comply.” Id. § 7413(e)(1). EPA can support that argument as
intervenor or amicus, to the extent such status is deemed
appropriate by the relevant court. But under the statutory
scheme, the decision whether to accept the defendant’s
argument is for the court in the first instance, not for EPA.

     EPA alternatively contends that it is permitted to create
the affirmative defense because of Section 301(a)(1) of the
Clean Air Act. See 78 Fed. Reg. at 10,014. That provision
authorizes EPA’s Administrator to “prescribe such regulations
as are necessary to carry out his functions under” the Act. 42
U.S.C. § 7601(a)(1). But we have consistently held that
EPA’s authority to issue ancillary regulations is not open-
ended, particularly when there is statutory language on point.
                              17
See, e.g., American Petroleum Institute v. EPA, 52 F.3d 1113,
1119 (D.C. Cir. 1995) (“the general grant of rulemaking
power to EPA cannot trump specific portions of the CAA”);
NRDC v. Reilly, 976 F.2d 36, 41 (D.C. Cir. 1992) (EPA
cannot use its general rulemaking authority as justification for
adding to a statutorily specified list); Sierra Club v. EPA, 719
F.2d 436, 453 (D.C. Cir. 1983) (same); see also Gonzales v.
Oregon, 546 U.S. 243, 264-65 (2006) (“It would
go . . . against the plain language of the text to treat a
delegation for the ‘execution’ of [the Attorney General’s]
functions as a further delegation to define other functions well
beyond the statute’s specific grants of authority.”). Those
precedents establish a simple and sensible rule: EPA cannot
rely on its gap-filling authority to supplement the Clean Air
Act’s provisions when Congress has not left the agency a gap
to fill. So it is here.

     On a different tack, EPA notes that Section 304(a)(1)
does not expressly deny EPA the ability to create an
affirmative defense, and EPA emphasizes that this Court has
frequently recognized the need for flexibility in the
administrative process. EPA Br. 46. That’s true. But the
suggestion implicit in EPA’s argument – that we should
“presume a delegation of power absent an express
withholding of such power” – is “plainly out of keeping with
Chevron . . . .” Railway Labor Executives’ Association v.
National Mediation Board, 29 F.3d 655, 671 (D.C. Cir. 1994)
(en banc).

    Finally, EPA suggests that an affirmative defense for
malfunctions is necessary to account for the tension between
requirements that emissions limitations be “continuous” and
the practical reality that control technology can fail
unavoidably. See 78 Fed. Reg. at 10,014. That is a good
argument for EPA to make to the courts – and for the courts
                               18
to then consider – in future civil cases when this issue arises.
But it does not suffice to give EPA authority to create an
affirmative defense.2

                              ***

     We grant the petitions for review with regard to EPA’s
affirmative defense and vacate those portions of the 2013
Rule pertaining to the defense. We deny the petitions in all
other respects.

                                                     So ordered.




    2
       The Fifth Circuit recently upheld EPA’s partial approval of
an affirmative defense provision in a State Implementation Plan.
See Luminant Generation Co. v. EPA, 714 F.3d 841 (5th Cir. 2013).
We do not here confront the question whether an affirmative
defense may be appropriate in a State Implementation Plan.
