 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued March 22, 2018                  Decided July 20, 2018

                         No. 16-1413

NATURAL RESOURCES DEFENSE COUNCIL AND SIERRA CLUB,
                   PETITIONERS

                              v.

    ENVIRONMENTAL PROTECTION AGENCY AND ANDREW
                     WHEELER,
                   RESPONDENTS

             AMERICAN PETROLEUM INSTITUTE,
                     INTERVENOR


      On Petition for Review of a Final Agency Action
    of the United States Environmental Protection Agency


    Margaret T. Hsieh argued the cause for petitioners. With
her on the briefs were Sanjay Narayan, John Walke, and Emily
K. Davis. Nancy S. Marks entered an appearance.

     Sue Chen, Attorney, U.S. Department of Justice, argued
the cause for respondents. With her on the brief was Jeffrey H.
Wood, Acting Assistant Attorney General.

    Aaron M. Flynn and Lucinda Minton Langworthy were on
the brief for intervenor-respondent.
                               2

   Caroline Lobdell was on the brief for amici curiae National
Cattlemen’s Beef Association, et al. in support of respondents.

  Before: GRIFFITH and KATSAS, Circuit Judges, and
EDWARDS, Senior Circuit Judge.

   Opinion for the Court filed by Circuit Judge GRIFFITH.

    GRIFFITH, Circuit Judge: The Clean Air Act calls upon the
Environmental Protection Agency to protect air quality by
enforcing state and local limits on the amount of pollution. The
agency need not count against those limits pollution caused by
“exceptional events.” In this case, Natural Resources Defense
Council and Sierra Club challenge a rule the agency uses to
determine whether an event caused by recurring activity is
“natural,” and thus “exceptional,” or “caused by human
activity,” and thus not. 42 U.S.C. § 7619(b)(1)(A)(iii). We
think the agency’s rule is permitted by the Clean Air Act.

                               I

     To “protect the public health,” 42 U.S.C. § 7409(b)(1), the
Clean Air Act (the “Act”) established a nationwide policy for
limiting air pollution on the state and local level, id. § 7410.
The Act directs the Environmental Protection Agency (EPA)
to set uniform levels of concentrations of various pollutants,
National Ambient Air Quality Standards (NAAQS), that local
areas must not exceed. Id. § 7409. Each state must earn EPA’s
approval of a state implementation plan (SIP), which commits
the state to recording levels of specified pollutants using a
network of air-quality monitors. Id. § 7410(a). By recording the
concentration levels of these pollutants, the monitors identify
areas that exceed the NAAQS. States report pollutant levels to
EPA quarterly and receive from the agency “attainment”
designations when the levels are below the NAAQS and
                               3

“nonattainment” designations, accompanied by additional air-
quality regulations, when the levels exceed the NAAQS. Id.
§ 7407; see also, e.g., 40 C.F.R. § 50.6 (establishing the
NAAQS for large particulate matter, setting attainment to be
exceeding a 24-hour average concentration of 150 µg/m3 no
more than one day within a calendar year).

    Since 1977, EPA has recognized that “[f]ederal, [s]tate,
and local air pollution control officials have expressed a great
deal of concern” that counting emissions caused by
“exceptional events” inflates reported levels of pollutants,
which sometimes pushes an area otherwise in attainment to be
designated as nonattainment. EPA, EPA-450/4-86-007,
Guideline on the Identification and Use of Air Quality Data
Affected by Exceptional Events 1 (1986). To avoid this, EPA
suggested in a series of informal guidelines that state and
federal agencies need not include in their pollution reports
those pollutants emitted from exceptional events. See, e.g.,
EPA, OAPQS No. 1.2-008, Guideline for the Interpretation of
Air Quality Standards (1977). The agency considered events to
be exceptional if “they are not expected to recur routinely at a
given location, or they are possibly uncontrollable or
unrealistic to control through the [SIP] process.” EPA-450/4-
86-007 at 1. In 2005, Congress added this practice to the Act.
Act of Aug. 10, 2005, Pub. L. No. 109-59, sec. 6013(a), § 319,
119 Stat 1144, 1882-884 (codified as amended at 42 U.S.C.
§ 7619(b)) (“Air quality monitoring data influenced by
exceptional events.”). Since then, EPA has had statutory
authority to exclude from a state’s reported pollutant levels
emissions that result from exceptional events. Id.

     The Act sets out several requirements that events must
satisfy to be exceptional. Id. § 7619(b)(1)(A). However, one of
those requirements applies only to events “caused by human
activity” and not “natural event[s].” Id. § 7619(b)(1)(A)(iii)
                                  4

(“[A]n event [must be] caused by human activity that is
unlikely to recur at a particular location or a natural event.”).
Through notice-and-comment rulemaking, EPA proposed that
“natural events” include events that are caused by both natural
and human activity, so long as such human activity complies
with relevant environmental regulations. Treatment of Data
Influenced by Exceptional Events, 80 Fed. Reg. 72,840, 72,854
(Nov. 20, 2015). Natural Resources Defense Council and
Sierra Club (together, the “environmental groups”) objected to
the definition, arguing that an event caused by human activity
cannot be a natural event. EPA replied that “there is not always
a bright line” between natural and human-caused events, J.A.
135, and adopted the definition as a final rule, Treatment of
Data Influenced by Exceptional Events, 81 Fed. Reg. 68,216
(Oct. 3, 2016) (“2016 Rule”).

     The environmental groups filed a timely petition for
review in our court, 1 and we have jurisdiction to review the
2016 Rule for compliance with the Act. See 42 U.S.C.
§ 7607(b)(1). The American Petroleum Institute (API) moved
to intervene on behalf of EPA but failed to show the required
Article III standing. See Deutsche Bank National Trust Co. v.
FDIC, 717 F.3d 189, 193 (D.C. Cir. 2013). API claims
representational standing, 2 API Mot. to Intervene 6 n.2, but that

     1
        Although an EPA rule previously defined “natural event” in
2007, the 2016 Rule reopened the issue. See Sierra Club v. EPA, 551
F.3d 1019, 1024 (D.C. Cir. 2008).
      2
        An association has standing on behalf of its members when:
“(1) ‘its members would otherwise have standing to sue in their own
right’; (2) ‘the interests it seeks to protect are germane to the
organization’s purpose’; and (3) ‘neither the claim asserted nor the
relief requested requires the participation of individual members in
the lawsuit.’” Ctr. for Sustainable Econ. v. Jewell, 779 F.3d 588, 596
(D.C. Cir. 2015) (quoting Hunt v. Wash. State Apple Adver.
Comm’n, 432 U.S. 333, 343 (1977)).
                                5

requires “specifically identify[ing] members who have
suffered the requisite harm,” Chamber of Commerce of the U.S.
v. EPA, 642 F.3d 192, 199 (D.C. Cir. 2011) (citations and
internal quotation marks omitted). Nowhere in its motion or
brief does API identify a single member of its organization or
support with evidence its vague assertion that an adverse result
in this case will injure any member. Because API failed to
establish the constitutional standing required to participate as
an intervenor, we instead grant it the status of amicus curiae.
See Fed. R. App. P. 29(a); see also Old Dominion Elec. Coop.
v. FERC, 892 F.3d 1223, 1232-34 (D.C. Cir. 2018); Rio
Grande Pipeline Co. v. FERC, 178 F.3d 533, 539 (D.C. Cir.
1999).

    We now consider whether the Act’s exceptional-event
provision permits EPA to attribute emissions to natural causes
when they were also caused by regulated human activity.

                                II

    The Act allows areas to keep their attainment designations
when their pollutant levels exceed the NAAQS so long as those
emissions resulted from an exceptional event. An exceptional
event is one that “affects air quality,” is “not reasonably
controllable or preventable,” and is “caused by . . . activity that
is unlikely to recur at a particular location.” 42 U.S.C.
§ 7619(b)(1)(A). But even a recurring event can be
“exceptional” if it is a “natural event.” Id. § 7619(b)(1)(A)(iii).

   The Act does not give a definition for “natural event,” so
EPA defined it in the 2016 Rule:

    [A natural event is] an event and its resulting emissions,
    which may recur at the same location, in which human
    activity plays little or no direct causal role. For purposes
                                6

    of the definition of a natural event, anthropogenic sources
    that are reasonably controlled shall be considered to not
    play a direct role in causing emissions.

81 Fed. Reg. at 68,277 (codified at 40 C.F.R. § 50.1(k)). In
other words, to determine whether a recurring event is natural,
and thus exceptional, EPA looks at the activities that caused the
emissions. See id. at 68,232 (explaining that EPA classifies
events based on the underlying sources of their emissions).
When making this decision, EPA will disregard contributions
to the emissions made by human activities, or “anthropogenic
sources,” that “are reasonably controlled” by complying with
emissions regulations. 3 As a consequence, an event is natural
if it resulted from at least some natural activity and any amount,
no matter how significant, of reasonably controlled human
activity. See id. at 68,231 (explaining that an event cannot be
“natural if all of the event-related emissions originated from
anthropogenic sources”).

     According to the environmental groups, this approach
stretches the meaning of “natural event” beyond what the text
of the Act can bear. They concede that, in some circumstances,
the Act permits EPA to classify an event and its resulting
emissions as natural even though human activity played a small
role. However, they contend that EPA must count, for the
purposes of characterizing an event as natural, the role played
by both types of human activity—that which complies with
environmental regulations and that which does not.

     We review EPA’s definition of natural event under
Chevron U.S.A. Inc. v. Natural Resources Defense Council,
Inc., 467 U.S. 837 (1984): “If the Act unambiguously
    3
       In general, “reasonably controlled” means that the human
activity satisfied the relevant SIP standards. 40 C.F.R.
§ 50.14(b)(8)(v).
                               7

authorizes or forecloses EPA’s . . . rule, step one of the
Chevron analysis requires that we follow Congress’s express
policy choice. If the Act is unclear on the matter, step two of
Chevron requires that we defer to EPA’s reasonable
interpretation.” Sierra Club v. EPA, 536 F.3d 673, 677 (D.C.
Cir. 2008) (citing Chevron, 467 U.S. at 842-43).

     At step one, we consider whether the Act issued
unambiguous instructions for distinguishing natural events
from events caused by human activity. Outside its statutory
context, “natural” ordinarily means something unaffected by
human activity. See, e.g., Merriam-Webster’s Collegiate
Dictionary 774 (10th ed. 1997) (defining natural as “growing
without human care . . . existing in or produced by nature: not
artificial”);   Oxford        English     Dictionary     Online,
http://www.oed.com/view/Entry/125333 (3d ed. 2003)
(defining natural as “[f]ormed by nature; not subject to human
intervention, not artificial”); Black’s Law Dictionary 1048 (7th
ed. 1999) (defining natural as “[b]rought about by nature as
opposed to artificial means”). And an ordinary reading of
“natural event” summons images of natural disasters such as
tornados and volcanic eruptions; cosmic episodes, such as
comets and harvest moons; and organic processes, such as viral
epidemics and seasonal changes. These examples leave little
room for human causation.

     But what “natural event” means in the Act does not depend
entirely on its ordinary reading because “the words of a statute
must be read in their context and with a view to their place in
the overall statutory scheme.” Nat’l Ass’n of Home Builders v.
Defenders of Wildlife, 551 U.S. 644, 666 (2007) (internal
quotation marks omitted); see also Pereira v. Sessions, 138 S.
Ct. 2105, 2110 (2018) (interpreting the statute in light of its
statutory context). By pairing “natural event” alongside “an
event caused by human activity,” the Act uses the phrase as a
                                8

tool to separate events into the two categories, requiring it to
carry a special meaning. Perhaps if EPA had to separate only
those events caused by either solely natural or solely human
activity, the ordinary understanding of “natural event” might
do the trick. But many events are caused by a combination of
the two. For example, consider a windstorm that sweeps dust
into the air so that it is emitted as small particulate matter,
which is subject to the NAAQS. At first blush, the emissions
appear to be the result of the windstorm and, therefore, a natural
event. But this is less obvious if the storm swept up the dust
only because the ground’s surface had been loosened by recent
construction. See 80 Fed. Reg. at 72,854 n.34. In that case, the
natural event caused the emissions only because human activity
changed the landscape. The point at which human
contributions convert a natural event into one caused by human
activity is blurry at best.

     But EPA must draw that line, and the Act provides little
guidance beyond establishing that the distinction exists.4 See
42 U.S.C. § 7619(b)(1)(A)(iii) (allowing an event to be
exceptional only if it is “natural” or “caused by human activity
that is unlikely to recur at a particular location”). Many
possible rules for sorting events may be permissible under the
statute. Some may be easier to administer than others, but the
Act leaves the choice to EPA. The statutory language is far
from unambiguous and is, instead, a classic example of
Congress leaving a gap for EPA to fill with reasonable
regulations. See Nat’l Cable & Telecomm. Ass’n v. Brand X
Internet Servs., 545 U.S. 967, 980 (2005) (“[A]mbiguities in
statutes within an agency’s jurisdiction to administer are
delegations of authority to the agency to fill the statutory gap
in reasonable fashion.”); Chevron, 467 U.S. at 843-44.
    4
      The Act also provides broad governing principles, such as the
supremacy of public health, that could place some limits on EPA’s
choice of rules. See 42 U.S.C. § 7619(b)(3).
                                9


      It is at Chevron step two that we determine if the 2016 Rule
“fill[s] the statutory gap in reasonable fashion.” Brand X, 545
U.S. at 980. The environmental groups think the rule upends
the statutory distinction between natural events and those
caused by human activity. They contend it is unreasonable for
EPA to assume that human activity did not cause an event
simply because that activity complied with environmental
regulations. As a consequence, they worry, EPA will treat
emissions from recurring human activity as emissions from an
exceptional event.

     To illustrate their concern, they describe how the 2016
Rule would apply to a windstorm that blew pollutants emitted
from a reasonably controlled power plant to another
jurisdiction’s air-quality monitor, which then registered a much
higher pollutant concentration than would be typical for that
area. The environmental groups argue that EPA would consider
the emissions to be the result of a natural event. Although
human activity produced the emissions, they believe EPA
would not consider the role played by the power plant because
it complied with the relevant air-quality regulations. Instead,
EPA would look to only the windstorm and conclude that the
event was natural.

     But such an outcome wouldn’t be possible under the
statutory and regulatory safeguards in place. The
environmental groups do not challenge EPA’s understanding
that an “event” must be an occurrence that “deviat[es] from
normal or expected conditions.” 81 Fed. Reg. at 68,228.
Emissions that result from only “routine” activity cannot be
treated as exceptional-event emissions. Id.

    The environmental groups seem to suggest that emissions
from “routine” activities could become emissions from an
                                 10

“event” if they are later affected by unexpected activity.
However, this misreads the 2016 Rule. “Natural event means
an event and its resulting emissions,” 40 C.F.R. § 50.1(k)
(second emphasis added), and EPA has made clear that
emissions only result from the events that generate them, 81
Fed. Reg. at 68,226. 5 Activities that cause emissions to behave
in a certain way, such as migrating to new areas or
concentrating in dangerous amounts, are not events that cause
the emissions. Id. In the windstorm example, it was the power
plant and not the windstorm that generated the emissions.
Although the pollutants only reached the monitor because they
were carried by the windstorm, they were emitted into the air
before the wind arrived at the scene. Under the 2016 Rule,
those emissions would be attributed to the power plant, not the
natural activity of the windstorm. 6

     The agency would give a different answer for emissions
generated by a windstorm that swept up particulate matter from
a dirt road. In that case, no pollutants would be emitted until
the wind struck the road and swept particles into the air. If the
road were reasonably controlled, EPA would discount the
     5
       To the extent the environmental groups are challenging EPA’s
interpretation of its own regulations, we defer to the agency’s
understanding. See Auer v. Robbins, 519 U.S. 452, 461 (1997)
(holding that an agency’s interpretation of its own regulation, unless
“plainly erroneous or inconsistent with the regulation,” controls
(internal quotation marks omitted)). Even if the regulation could be
read to include emissions that have already been generated, EPA’s
reading is certainly consistent with the 2016 Rule.
     6
       This does not mean that emissions produced by human activity
and transported by wind to a new area can never be the result of an
exceptional event. It simply means that EPA would treat them as a
result of human activity and consequently subject to the recurrence
condition for exceptional events. See, e.g., 81 Fed. Reg. at 68,280
(discussing “emissions-generating activity that occurs outside of the
State’s jurisdictional boundaries”).
                              11

road’s role and look only to the windstorm. But if the road had
been improperly maintained, EPA would consider both the
road’s and the windstorm’s contributions to the emissions.

     We think the 2016 Rule preserves the Act’s distinct
treatment of natural events. Although we recognize the
possibility raised, but not demonstrated, by the environmental
groups that extreme and unforeseen applications of the rule
might have problematic results, the 2016 Rule still passes
muster under Chevron step two. The “possibility that the rule,
in uncommon particular applications, might exceed EPA’s
statutory authority does not warrant judicial condemnation of
the rule in its entirety.” EPA v. EME Homer City Generation,
L.P., 134 S. Ct. 1584, 1609 (2014); see also Babbitt v. Sweet
Home Chapter of Cmtys. for a Great Or., 515 U.S. 687, 699-
700 (1995) (upholding a rule as reasonable despite possible
applications that would be impermissible under the statute). If
EPA applies the rule in a way that the Act would not permit, an
injured party can petition us to review the agency’s action at
that time. See EME Homer City, 134 S. Ct. at 1609 (explaining
that as-applied challenges remain available if EPA were to
apply a rule, which the Court had upheld under Chevron, in a
way that was impermissible under the statute); Util. Air
Regulatory Grp. v. EPA, 885 F.3d 714, 723 (D.C. Cir. 2018).
For now, we uphold the definition of natural event against the
environmental group’s facial challenge to the 2016 Rule.

                              III

    We deny the petition for review because the 2016 Rule’s
definition of natural event is permissible under the Act.

                                                   So ordered.
