 United States Court of Appeals
           FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued October 10, 2013               Decided May 27, 2014

                        No. 12-1238

        CENTER FOR BIOLOGICAL DIVERSITY, ET AL.,
                     PETITIONERS

                             v.

     ENVIRONMENTAL PROTECTION AGENCY AND GINA
                    MCCARTHY,
                   RESPONDENTS

    AMERICAN PETROLEUM INSTITUTE AND UTILITY AIR
                REGULATORY GROUP,
                   INTERVENORS


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


    Kevin P. Bundy argued the cause for petitioners. With him
on the briefs were Kassia R. Siegel, Charles McPhedran, and
David S. Baron.

    Daniel R. Dertke, Attorney, U.S. Department of Justice,
argued the cause and filed the brief for respondents.

     Andrea Bear Field, Lucinda Minton Langworthy, and Aaron
M. Flynn were on the brief for intervenors American Petroleum
Institute, et al. in support of respondents.
                               2

   Before: KAVANAUGH, Circuit Judge, and SENTELLE and
RANDOLPH, Senior Circuit Judges.

   Opinion for the Court filed by Senior Circuit Judge
RANDOLPH.

     RANDOLPH, Senior Circuit Judge: This petition for judicial
review deals mainly with what is popularly known as “acid
rain.”

     The Environmental Protection Agency decided in 2012,
after an exhaustive rulemaking proceeding, that it needed further
studies before it could set a new, joint, “secondary” national
ambient air quality standard for oxides of nitrogen and oxides of
sulphur, and other related compounds found in the ambient air
and considered precursors of acid deposits on the land and in the
waters of the continental United States. Secondary National
Ambient Air Quality Standards for Oxides of Nitrogen and
Sulphur, 77 Fed. Reg. 20,218, 20,226 (Apr. 3, 2012) [Final
Rule]. EPA’s failure to issue a new multi-pollutant rule at that
time, petitioners claim, violated the Clean Air Act.

                               I

    We begin with a brief description of the subjects of EPA’s
rulemaking.

                               A

    The ambient air—the air we breathe—is made up of
approximately 80 percent non-reactive nitrogen (N2) and 20
percent oxygen (O2). Like oxygen, nitrogen is essential to what
we think of as life. See Ag 101: Nitrogen, U.S. ENVTL. PROT.
AGENCY, http://www.epa.gov/oecaagct/ag101/impactnitrogen.
                                3

html (last updated June 27, 2012). This is so both as a matter of
biochemistry—nitrogen is “an essential nutrient required by all
living organisms,” id.—and as a matter of global economics.
The mass agriculture that feeds the world’s population is reliant
on nitrogen, which is “normally supplied in the form of organic
or inorganic fertilizers.” Margaret Rosso Grossman, Nitrates
from Agriculture in Europe: The EC Nitrates Directive and Its
Implementation in England, 27 B.C. ENVTL. AFF. L. REV. 567,
567 (2000).

     But nitrogen takes many forms, some of which are harmful
to the environment. Oxides of nitrogen (NOy), ammonia (NH3),
and ammonium (NH4),1 together with oxides of sulphur (SOx) in
the ambient air, “undergo a complex mix of reactions in
gaseous, liquid, and solid phases to form various acidic
compounds.” Final Rule, 77 Fed. Reg. at 20,224-25. Those
compounds are, in turn, “removed from the atmosphere through
deposition.” Id. at 20,225. “Wet” deposition occurs when the
compounds return to earth through rain, snow, sleet, hail, fog,
and dew. “Dry” deposition occurs when gases and particles of
these compounds drop onto Earth without mixing with water in
the atmosphere. Id.

     Wet deposition has attracted most popular attention, but
EPA has estimated that “[d]ry deposition now accounts [for] 20-
60%” of total acid deposition. Causes of Acid Rain, U.S. ENVTL.
PROT. AGENCY, http://www.epa.gov/region1/eco/acidrain/causes.
html (last visited May 2014). EPA’s number is a very rough
estimate because dry deposition is “not easily measured” and
because “[v]ery little falls at one time or at one location.” Id.;


    1
      Ammonia and ammonium are considered “reduced” forms of
nitrogen. Reduced nitrogen is abbreviated NHx. U.S. ENVTL. PROT.
AGENCY, REACTIVE NITROGEN IN THE UNITED STATES, at ES-4 n.5
(2011) [REACTIVE NITROGEN].
                                  4

see Final Rule, 77 Fed. Reg. at 20,249 (attributing the lack of
data about dry deposition to “the lack of efficient measurement
technologies”). The shorthand “acid rain,” coined in the 1800s,2
refers to wet and dry deposition collectively.

    The effects of acid rain vary depending upon where it lands.
Deposition in water bodies—aquatic acidification—can affect
the pH3 of the water and affect its habitability for aquatic
organisms. EPA’s rulemaking focused on these effects, rather
than those of terrestrial acidification (deposits on land), because
more and better data were available for aquatic ecosystems.4
Final Rule, 77 Fed. Reg. at 20,242. Even so, the data, from
many studies, indicated that the effects of acid rain on surface
waters vary widely throughout the United States. See id. at
20,227.

      Factors such as “biota, climate, geochemistry, and
hydrology” have an impact. Id. at 20,229; see also id. at 20,225
(listing additional factors such as geology, topography, land use,
and hydrologic flowpath). This short sentence, accurate as it is,
masks an enormity of scientific complications because every


     2
      See Joseph Mac D. Schwartz, Comment, On Doubting Thomas:
Judicial Compulsion and Other Controls of Transboundary Acid Rain,
2 AM. U. J. INT’L L. & POL’Y 361, 361 n.1 (1987).
     3
       “The pH . . . of a solution is a measure of its acidity or
alkalinity. A pH of 7.0 is neutral; a pH below 7.0 is acidic; and a pH
above 7.0 is alkaline.” Warner-Jenkinson Co. v. Hilton Davis Chem.
Co., 520 U.S. 17, 22 n.1 (1997). The scale is logarithmic: each whole
number decrease signifies a ten-fold increase in acidity. Id.
     4
       Although it is difficult to measure, terrestrial acidification
affects the acidity of soil, which is correlated with “decreased growth
and increased susceptibility to disease and injury” among red spruce
and sugar maple trees. Final Rule, 77 Fed. Reg. at 20,226.
                                 5

body of water is unique. How large and how deep is it? Is it a
still lake or a flowing stream? And if it is a stream, is it a
freestone stream slowly winding down from a mountain
meadow, or does it move as rapidly as Niagara? What is the
water body’s mineral content, its vegetative content, its altitude,
its temperature, its location?

     “Parts of the West are naturally less sensitive to
acidification,” while other areas—for instance, “lakes in the
Adirondacks and streams in Shenandoah National Park”—are
considered “acid sensitive aquatic ecosytems.” Id. at 20,236. In
such “acid sensitive” waters, acid rain’s effect on the water’s pH
can make the water uninhabitable for some fish and aquatic
organisms. The disappearance of species can disrupt delicate
food chains. Id. at 20,233. Less aquatic life may also mean less
recreational fishing. Id.

     In other areas, or in water bodies within the same area, acid
rain may have no measurable effect.5 Id. at 20,235. The
limestone streams6 of the Cumberland Valley of Pennsylvania


    5
       Because nitrogen fosters the growth of organisms, deposition
can also cause “nutrient enrichment”—an alteration of the balance of
available nutrients in a given ecosystem. Final Rule, 77 Fed. Reg. at
20,226-28. Despite its positive-sounding name, nutrient enrichment
can lead to species death, foul odors, and outbreaks of harmful
organisms. See MARK D. MUNN & PIXIE A. HAMILTON, U.S.
GEOLOGICAL SURVEY, FS-118-03, NEW STUDIES INITIATED BY THE
U.S. GEOLOGICAL SURVEY—EFFECTS OF NUTRIENT ENRICHMENT ON
STREAM ECOSYSTEMS 2 (2003), available at http://pubs.usgs.gov/fs/
fs11803/pdf/fs-118-03.pdf.
    6
       “Limestone streams are different from most streams because of
their unique composition and structure.” Andrew H. Shaw, Comment,
The Public Trust Doctrine: Protector of Pennsylvania’s Natural
Resources?, 9 DICK. J. ENVTL. L. & POL’Y 383, 384 (2000). For an in-
                               6

(Letort Spring Run, Falling Springs Run, Big Spring Creek)—
the birthplace of modern American dry fly fishing—have
produced stream-bred, trophy-size brown and rainbow trout for
generations. See generally VINCENT C. MARINARO, A MODERN
DRY-FLY CODE (1950). Because of their mineral content, these
spring creeks—rich in aquatic vegetation (watercress, elodea
grass) and insect life (mayflies, stoneflies, caddis, cress
bugs)—are nature’s antacid, quickly neutralizing whatever
acidic compounds the rain may bring. See, Joe Kendall Neel,
Interrelations of Certain Physical and Chemical Features in a
Headwater Limestone Stream, 32 ECOLOGY 368, 386 (1951); cf.
Final Rule, 77 Fed. Reg. at 20,235 (“[T]he same levels of
deposition falling on limestone dominated soils have a very
different effect from those falling on shallow glaciated soils
underlain with granite.”). The Firehole River in Yellowstone
National Park provides another example, but a rather different
one. The Firehole is a geothermal freestone stream rising out of
the Park’s geyser basins. Caldrons and mud pots and other
thermal features along its banks emit vast quantities of sulphur
gases as the stream winds its way to join the Gibbon River to
form the Madison River. Along the Firehole the odor of rotten
eggs (hydrogen sulfide) hangs in the air. Yet despite constant
doses of these sulphur compounds, the Firehole River is one of
the most productive trout streams in the world—and one of the
most beautiful. See generally Aquatic Ecology of Yellowstone,
NAT’L PARK SERV., http://www.nps.gov/yell/naturescience/
fishing_ecology.htm (last visited May 2014).




depth discussion of the chemistry of limestone streams, see Joe
Kendall Neel, Interrelations of Certain Physical and Chemical
Features in a Headwater Limestone Stream, 32 ECOLOGY 368 (1951).
                                 7

                                 B

     The sources of atmospheric NOy and SOx are fairly well
known. Small amounts of both types of oxides occur naturally.
Volcanic eruptions and sea spray produce SOx. Causes of Acid
Rain, U.S. ENVTL. PROT. AGENCY, http://www.epa.gov/region1/
eco/acidrain/causes.html (last visited May 2014). Lightning
strikes and rotting vegetation produce NOy. Id. The largest
sources of SOx are “fossil fuel combustion at power plants
(73%) and other industrial facilities (20%).” Sulfur Dioxide,
U.S. ENVTL. PROT. AGENCY, http://www.epa.gov/airquality/
sulfurdioxide (last visited May 2014). The bulk of NOy in the
atmosphere also comes from the combustion of fossil fuels,
mostly by motor vehicles and, to a lesser extent, from industrial
operations. Nitrogen Oxides, U.S. ENVTL. PROT. AGENCY,
http://www.epa.gov/cgi-bin/broker?_service=data&_debug=
0&_program=dataprog.national_1.sas&polchoice=NOX (last
visited May 2014). With respect to ammonia, agriculture—
particularly livestock operations—represents by far the largest
source of emissions to the atmosphere, by one estimate 85
percent, although this estimate and others “are characterized by
a high degree of uncertainty.” COMM. ON ENV’T & NAT.
RESOURCES AIR QUALITY RESEARCH SUBCOMM., NAT’L
OCEANOGRAPHIC & ATMOSPHERIC ADMIN., ATMOSPHERIC
AMMONIA: SOURCES & FATE 1 (2000), available at http://www.
esrl.noaa.gov/csd/AQRS/reports/ammonia.pdf.

     This explanation, too, overlooks many complexities. As
explained, emitted oxides of nitrogen and sulfur cause the
formation of acid rain which, in turn, causes both aquatic and
terrestrial acidification. But acid rain is not the only or even the
largest source of nitrogen’s total environmental impact. Today,
                                   8

the largest source of so-called “reactive nitrogen”7 is
agriculture—the manufacture and use of nitrogen-based
fertilizers. ENVTL. PROT. AGENCY SCIENCE ADVISORY BD.,
REACTIVE NITROGEN IN THE UNITED STATES, at ES-4 (2011),
available at http://yosemite.epa.gov/sab/sabproduct.nsf/
WebBOARD/INCSupplemental. Agriculture is responsible for
some NOy emissions, see id. at 12 tbl.1, but, to a much greater
extent, agriculture-related nitrogen compounds are surface-
bound, affecting nearby land and water via leaching and runoff,
id. at 15, 34; see also id. at 46 box 2 (estimating the varied
impacts of Nr—from atmospheric, terrestrial, and water-based
nitrogen—on the Chesapeake Bay watershed). Scientific
uncertainty regarding all of the above chemical processes
remains high. Id. at 33 finding 8.

                                   II

                                   A

    EPA has been regulating NOy and SOx emissions, not
together but separately, in a variety of ways since 1971, as
required by the 1970 amendments to the Clean Air Act. See
Pub. L. No. 91-604, 84 Stat. 1676. The Act established the
regulatory structure still in force today.

    Under the Act, EPA is required to regulate any airborne
pollutant which, in the Administrator’s judgment, “may
reasonably be anticipated to endanger public health or welfare.”
42 U.S.C. § 7408(a)(1)(A). For pollutants within that



     7
       Reactive nitrogen (Nr) is a broad term encompassing “all
biologically active, chemically reactive, and radiatively active nitrogen
compounds in the atmosphere and biosphere of the Earth.” REACTIVE
NITROGEN, supra note 1, at ES-1.
                                  9

category—so-called “criteria air pollutants”8—EPA must
promulgate national ambient air quality standards. Id.
§ 7409(a)(1)(A). Air quality standards are of two sorts.
“Primary” national ambient air quality standards must be set at
a level the attainment of which the EPA Administrator judges to
be “requisite” to protect “the public health,” id. § 7409(b)(1);
“secondary” national ambient air quality standards must be set
at a level the attainment of which is “requisite” to protect “the
public welfare,” id. § 7409(b)(2). The term “public welfare”
“includes, but is not limited to, effects on soils, water, crops,
vegetation, manmade materials, animals, wildlife, weather,
visibility, and climate.” Id. § 7602(h).

    “At least every five years, EPA must reevaluate the
standards and, if appropriate, revise them.” Cmties. for Better
Env’t v. EPA, No. 11-1423, 2014 WL 1394655, at *1 (D.C. Cir.
Apr. 11, 2014). The reevaluation requires EPA to undertake a
“thorough review” of all primary and secondary standards and to
“make such revisions in such criteria and standards and
promulgate such new standards as may be appropriate in
accordance with” § 7409(b). 42 U.S.C. § 7409(d)(1). Section
7409(b) is the provision requiring air quality standards to be set
to protect the public health (primary standards) or welfare
(secondary standards).

     EPA’s secondary national ambient air quality standards for
sulfur and nitrogen oxides were initially set to protect against the
harm to vegetation from direct exposure to those gases. The
SOx standard initially imposed both short term (three-hour



     8
      There are currently six criteria air pollutants: carbon monoxide,
lead, nitrogen oxides, ozone, particulate matter, and sulfur oxides.
Cmties. for Better Env’t v. EPA, No. 11-1423, 2014 WL 1394655, at
*1 (D.C. Cir. Apr. 11, 2014).
                                 10

average) and longer term (annual arithmetic mean) requirements.9
The three-hour standard has remained in effect, but EPA
revoked the annual standard in 1973 after this court remanded
it to EPA for further explanation of its basis. See Kennecott
Copper Corp. v. EPA, 462 F.2d 846 (D.C. Cir. 1972). The
secondary standard for nitrogen oxides has remained essentially
unchanged since its promulgation in 1971, and no area of the
country has been found not to comply with it. Nitrogen Dioxide,
U.S. ENVTL. PROT. AGENCY, http://www.epa.gov/air/
nitrogenoxides (last updated April 4, 2013).

     The existing secondary standards do not account for the
public welfare harms associated with acid rain. The issue,
however, has not gone unaddressed. In 1990 Congress enacted,
as Title IV of the Clean Air Act Amendments, an Acid Rain
Program. See Pub. L. No. 101-549, tit. IV, 104 Stat. 2399
(codified at 42 U.S.C. §§ 7651 et seq.). Title IV created a “cap-
and-trade program for sulfur dioxide (‘SO2’) emitted by fossil
fuel–fired combustion devices. Congress capped SO2 emissions
for affected units, . . . distributed ‘allowances’ among those
units,” and permitted the transfer of allowances among those
units so as to keep total emissions beneath the cap. North
Carolina v. EPA, 531 F.3d 896, 902 (D.C. Cir. 2008). The
program also imposed “more traditional”—that is, non–market
based—emissions limitations on NOy emitters. U.S. ENVTL.


    9
        Every national ambient air quality standard includes an
“averaging time,” which specifies the duration over which compliance
with the standard will be measured. Am. Farm. Bureau Fed’n v. EPA,
559 F.3d 512, 516 (D.C. Cir. 2009) (per curiam). A standard with a
three-hour averaging time will be exceeded each time the average
level of the pollutant over a three-hour span exceeds the standard. A
standard employing an annual arithmetic mean allows more seasonal
volatility: it measures compliance based on the pollutant’s average
level over the course of the year.
                                 11

PROT. AGENCY, CLEARING THE AIR: THE FACTS ABOUT CAPPING
AND TRADING EMISSIONS (2002) [CLEARING THE AIR], available
at http://www.epa.gov/airmarkt/progsregs/arp/docs/clearingtheair.
pdf; see 42 U.S.C. § 7651f.

     Since 1980, the national average SO2 concentration has
dropped 78 percent. Air Trends, U.S. ENVTL. PROT. AGENCY,
http://www.epa.gov/air/airtrends/sulfur.html (last updated Sept.
3, 2013). This is the result, at least in part, of the cap-and-trade
program established by the Acid Rain Program. See CLEARING
THE AIR. The level of NOy has also declined, but these
reductions have been “offset by increases in emissions from
automobiles.” David B. Spence, Coal-Fired Power in a
Restructured Electricity Market, 15 DUKE ENVTL. L. & POL’Y F.
187, 194 (2005).

                                 B

     EPA’s current rulemaking proceeding, conducted pursuant
to a consent decree,10 began as a review of the existing
secondary national ambient air quality standards for NOy and
SOx pursuant to 42 U.S.C. § 7409(d). As mentioned earlier, the
existing standards, issued in 1971, were not “directed toward
depositional effects” on surface waters; both standards were


    10
       The decree required EPA to issue a “decision” “concerning its
review of the secondary NAAQS for NO2 and SO2” and to make “such
revisions in the secondary NO2 and SOx NAAQS and/or promulgat[e]
such new secondary standards for NOx and SOx as may be appropriate
pursuant to 42 U.S.C. §§ 7408 and 7409(b)(2).” Second Stipulation to
Amend Consent Decree at 2, Ctr. for Biol. Diversity v. EPA, No. 05-
1814 (D.D.C. Oct. 22, 2009). As a result of the rulemaking now under
review, the parties have jointly terminated the decree as having been
complied with. Notice of Termination of Consent Decree, Ctr. for
Biol. Diversity v. EPA, No. 05-1814 (D.D.C. Aug. 30, 2012).
                                12

designed “to protect against direct effects of gaseous oxides of
nitrogen and sulfur” on plants—what biologists call
“phytotoxic” effects. Final Rule, 77 Fed. Reg. at 20,239-40. In
less scientific terms, the existing standards dealt with the vapors
of these two compounds descending on plants and causing the
plants to suffer stunted growth, or wilting, or death. Given the
purpose of the existing secondary standards, there is general
agreement they have done their job. Id. at 20,239.

     As we have been explaining, however, direct exposure is
not the only way in which these chemicals can cause harm.
With respect to acid rain, EPA concluded—and all parties
agree—that the existing secondary standards for NOy and SOx
were “not adequate to protect against the adverse impacts of
aquatic acidification on sensitive ecosystems.” Id. at 20,236.
Part of the problem is that the current NOy and SOx secondary
standards did not, and were not designed to, “capture all relevant
chemical species of oxides of nitrogen and sulphur that
contribute to deposition-related effects.” Id. at 20,234-35. The
nitrogen standard is confined to nitrogen dioxide (NO2) and the
sulphur standard is confined to sulphur dioxide (SO2). In
addition, the current NO2 and SO2 standards measure exposure
in terms of hours, a period that is not “relevant for ecosystem
impacts . . . that occur over periods of months to years.” Id. at
20,234. The current standards also did not take into account
“variability in ecosystem sensitivity” to acid deposition. Id. at
20,235. “Ecosystems are not uniformly distributed either
spatially or temporally in their sensitivity to oxides of nitrogen
and sulphur.” Id.

    Once EPA found that the two current standards were
inadequate with respect to acid rain, it sought to determine
“what new multi-pollutant standard would be appropriate.” Id.
at 20,242. EPA recognized that such a national ambient air
quality standard “would necessarily be more complex than the
                                13

NAAQS that have been set historically to address effects
associated with ambient concentrations of a single pollutant.”
Id. And while the standard would be national in scope, EPA
knew that the effects from acid rain vary throughout the country
and that any new standard would have to reflect that variability.
Id. To that end, EPA developed a “Policy Assessment” that
divided the United States into 84 “ecoregions.” See Office of
Air Quality Planning & Standards, EPA, Policy Assessment for
the Review of the Secondary National Ambient Air Quality
Standards for Oxides of Nitrogen and Oxides of Sulphur (2011).
Although acid sensitivity varies water-body-by-water-body, the
ecoregions attempted to categorize certain areas, based on “a
variety of vegetation, geological, and hydrological attributes that
are directly relevant to aquatic acidification,” as more or less
sensitive. Id. at ES-7. The ecoregions, the Policy Assessment
posited, would “allow for a practical application of an aquatic
acidification standard on a national scale.” Id.

     To make this region-sensitive standard work, EPA
developed what it called an “Aquatic Acidification Index.” A
relatively simple explanation of the Index is that it attempts to
quantify the connection between the NOy and SOx in the air—
the compounds EPA is authorized to regulate—and the expected
harm from acid rain in any particular region. In a sensitive
region, the Index would require more regulation of NOy and
SOx; in a resilient region, it would require less. EPA expressed
its Aquatic Acidification Index in the form of an equation,
unnecessary to describe in this opinion.

     “But, like any model, the Index may be scientifically sound
in theory, or general concept yet, without the appropriate inputs,
too uncertain to apply in practice.” Br. for Resp’ts 3. The
validity of the Index depends on its ability “to generate values
that are representative throughout an entire ecosystem.” Id.
EPA ultimately concluded that those practical hurdles were too
                                  14

high.11 Citing doubts about the accuracy of the Aquatic
Acidification Index, EPA decided not to promulgate a new,
combined NOy–SOx secondary standard. It reasoned that limits
on the accuracy of the Index’s inputs prevented EPA from
identifying with sufficient certainty a standard that would be, as
the Clean Air Act requires, “requisite to protect the public
welfare.” 42 U.S.C. § 7409(b)(2). Instead EPA decided to
“undertake a field pilot program to gather additional data.”
Final Rule, 77 Fed. Reg. at 20,263.12 EPA also retained the
individual NO2 and SO2 standards for the protection of
vegetation, as its Scientific Advisory Committee recommended.13


     11
        The independent Clean Air Scientific Advisory Committee
made a similar point: “It is difficult to judge the adequacy of the
uncertainty analysis performed by EPA because of lack of details on
data inputs and the methodology used, and lack of clarity in its
presentation. . . . The parameters of the [Aquatic Acidification Index]
are derived using air quality and aquatic models. Given that these
models are being used in the standard setting process, a more rigorous
model evaluation should have been conducted to provide more
confidence in the use of the models.” Letter from Drs. Armistead
Russell & Jonathan M. Samet, Chairs, Clean Air Scientific Advisory
Comm., to Lisa P. Jackson, Administrator, EPA (May 17, 2011), at
encl. B, p. 11 [CASAC Report].
     12
       The chief objective of the pilot program is to “enhance [EPA’s]
understanding of the degree of protectiveness that would likely be
afforded by a standard based on the” Aquatic Acidification Index.
Final Rule, 77 Fed. Reg. at 20,264. To further that objective, the
program aims to evaluate the current methods for measuring NOy and
SOx in the ambient air; better understand the “concentration and
deposition patterns” of the compounds; collect further data for all the
elements of the Aquatic Acidification Index formula; and further
develop EPA’s air monitoring network. Id.
     13
        “The current public-welfare-based (secondary) NAAQS
standards for oxides of nitrogen (NOx) and sulphur oxides (SOx) were
                                 15

    The issue in this petition for judicial review is whether
EPA’s decision to defer adopting a new standard at this time,
pending further scientific study, violated § 7409(d)(1).14

                                 III

     When EPA undertakes a review of a secondary national
ambient air quality standard, § 7409(d)(1) requires EPA to
revise the standard or issue a new one “as may be appropriate in
accordance with” § 7409(b). Section 7409(b)(2) requires such
a secondary standard to be at a level of air quality that, “in the
judgment of the Administrator,” “is requisite to protect the
public welfare from any known or anticipated adverse effects”
from the pollutant in the ambient air. The phrase “requisite to
protect” means that a secondary standard must be neither higher
nor lower than necessary. Whitman v. Am. Trucking Ass’ns, 531
U.S. 457, 473, 475-76 (2001).

     As we have said, all parties agree that the two secondary
standards for NO2 and SO2 are not adequate to protect against
adverse effects on water bodies from acid rain. According to
petitioners, it follows that the Clean Air Act required EPA to
issue a new secondary standard that would provide sufficient


designed to protect vegetation from exposures to injurious
concentrations of gaseous NOx and SOx. This protection is a desirable
goal and for that reason the [Scientific Advisory] Panel recommends
that the current secondary NOx and SOx NAAQS standards should be
retained.” CASAC Report at 1.
     14
       We do not consider the intervenors’ argument that EPA lacked
the statutory authority to set a national ambient air quality standard
using the Aquatic Acidification Index. EPA ultimately decided not to
set a new standard. Thus, the question whether an Aquatic
Acidification Index–based standard would have been lawful is
hypothetical. See Golden v. Zwickler, 394 U.S. 103, 108 (1969).
                                16

protection.   EPA interprets the Act differently: if the
Administrator cannot make a reasoned judgment that a proposed
new standard would comply with § 7409(b)(2), then—in the
language of § 7409(d)(1)—it cannot be “appropriate” for EPA
to issue it.

     Even if we had no obligation to defer to EPA’s
interpretation of the Clean Air Act—but of course we do, see
Chevron, U.S.A., Inc. v. Natural Res. Def. Council, 467 U.S.
837, 842-43 (1984)—EPA has by far the better of the argument.
When EPA selects a standard during a rulemaking, it must
exercise “reasoned” decisionmaking. Am. Farm Bureau Fed’n
v. EPA, 559 F.3d 512, 530 (D.C. Cir. 2009) (per curiam). If, as
EPA found, the available information was insufficient to permit
a reasoned judgment about whether any proposed standard
would be “requisite to protect the public welfare,” promulgating
that standard would have been arbitrary and capricious. See 42
U.S.C. § 7607(d)(9); Motor Vehicle Mfrs. Ass’n v. State Farm
Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983).15 It is ridiculous to
suppose that the Clean Air Act required EPA to promulgate a
secondary standard that would immediately violate the Act. Yet
that is where petitioners’ arguments lead.

     Petitioners dispute EPA’s conclusion that it lacked
sufficient scientific information to make a reasoned judgment,
and they argue that, even if EPA was correct, it failed to provide
an adequate explanation on that score. Decades of decisions in
this court stand in the way of these arguments.



    15
        Just like the Administrative Procedure Act, § 7607(d)(9)
authorizes a reviewing court to reverse “arbitrary” and “capricious”
agency action. Compare 42 U.S.C. § 7607(d)(9), with 5 U.S.C.
§ 706(2)(A). We have held the two standards to be equivalent. See
West Virginia v. EPA, 362 F.3d 861, 867 (D.C. Cir. 2004).
                               17

    Consider this sample of circuit law:

    “We must look at the decision not as the chemist,
    biologist or statistician that we are qualified neither by
    training nor experience to be, but as a reviewing court
    exercising our narrowly defined duty of holding
    agencies to certain minimal standards of rationality.”

Ethyl Corp. v. EPA, 541 F.2d 1, 24, 36 (D.C. Cir. 1976) (en
banc) (citations and footnotes omitted);

    “[I]n an area characterized by scientific and
    technological uncertainty . . . this court must proceed
    with particular caution, avoiding all temptation to
    direct the agency in a choice between rational
    alternatives.”

Envtl. Def. Fund v. Costle, 578 F.2d 337, 339 (D.C. Cir. 1978);

    “[When EPA decisons] turn on . . . predictions dealing
    with matters on the frontiers of scientific knowledge,
    we will demand adequate reasons and explanations, but
    not ‘findings’ of the sort familiar from the world of
    adjudication.”

Amoco Oil Co. v. EPA, 501 F.2d 722, 741 (D.C. Cir. 1974);

    “Happily, it is not for the judicial branch to undertake
    comparative evaluations of conflicting scientific
    evidence. Our review aims only to discern whether the
    agency’s evaluation was rational.”

Natural Res. Def. Council v. EPA, 824 F.2d 1211, 1216 (D.C.
Cir. 1987);
                               18

    “[P]articular deference is given by the court to an
    agency with regard to scientific matters in its area of
    technical expertise . . ..”

Nat’l Wildlife Fed’n v. EPA, 286 F.3d 554, 560 (D.C. Cir.
2002);

    “[C]ourts give a high level of deference to an agency’s
    evaluations of scientific data within its area of
    expertise.”

A.L. Pharma, Inc. v. Shalala, 62 F.3d 1484, 1490 (D.C. Cir.
1995);

    “In conducting this review, we show considerable
    deference, especially where the agency’s decision rests
    on an evaluation of complex scientific data within the
    agency’s technical expertise . . ..”

Troy Corp. v. Browner, 120 F.3d 277, 283 (D.C. Cir. 1997);

    “[W]e will give an extreme degree of deference to the
    agency when it ‘is evaluating scientific data within its
    technical expertise.’”

Hüls Am. Inc. v. Browner, 83 F.3d 445, 452 (D.C. Cir. 1996)
(quoting Int’l Fabricare Inst. v. EPA, 972 F.3d 384, 389 (D.C.
Cir. 1992)).

    In its explanation in the Federal Register, and in its Policy
Assessment, EPA explained in great detail the uncertainties it
faced in seeking to set a secondary standard to protect against
aquatic acidification. The problems, EPA thought, were not so
much in the “conceptual formulation” of the theory embodied in
the Aquatic Acidification Index. Final Rule, 77 Fed. Reg. at
                                 19

20,249. But as EPA knew, the true test of a theory is whether it
will work in practice. And EPA thoroughly explained why it
believed that the “much higher uncertainties” surrounding “the
specific elements within the structure of an [Aquatic
Acidification Index–based] standard” were prohibitive. Id. at
20,249.

     For example, as we mentioned earlier, although dry
deposition may contribute as much as 60 percent of acidic
deposits, it has not been adequately measured, due in large part
to the “lack of efficient measurement technologies.” Id. As to
nitrogen, “although ambient measurements of NOy are made as
part of a national monitoring network, the monitors are not
located in locations that have been determined to be
representative of sensitive aquatic ecosystems or individual
ecoregions.” Id. at 20,261. Field measurements of NHx
(ammonia and ammonium), released from agriculture
operations, were also “extremely limited.” Id. at 20,249 n.12.
In addition, EPA recognized “the inherent complexity of
characterizing NHx with respect to source emissions and dry
deposition.” Id. at 20,249. These and other “gaps in field
measurement data increase uncertainties in modeled processes
and in the specific application of such models,” such as the
Aquatic Acidification Index, to the 84 ecoregions EPA
identified. Id. at 20,250.

    Another large, unsolved problem relating to the Aquatic
Acidification Index deals with what EPA designated as the
“Acid Neutralizing Capacity” level, an “ecological indicator”
used in its modeling. Id. at 20,255.16 We have mentioned the


     16
       The Index-based standard would have calculated the Acid
Neutralizing Capacity for a given water body using what EPA termed
that water body’s “critical load.” Final Rule, 77 Fed. Reg. at 20,230,
20,240, 20,245-46. In this context, a water body’s “critical load” is
                               20

example of the limestone streams of the Cumberland Valley of
Pennsylvania, which have the ability to neutralize acidic
deposits. That example, and several others, demonstrate what
EPA knew to be true: both within and among EPA’s 84
ecoregions, the water bodies in the lower 48 states vary widely
in their Acid Neutralizing Capacities. EPA did not attempt the
monumental task of a stream-by-stream, lake-by-lake, river-by-
river, estuary-by-estuary analysis for the United States. Instead
it sought to estimate an acid neutralizing figure for each of the
84 ecoregions and to plug that figure into the Aquatic
Acidification Index formula. But, as EPA explained, those
region-wide estimates were based on very little data. “[T]here
is,” for example, “relatively sparse coverage in mountainous
western areas where a number of sensitive aquatic ecosystems
are located.” Id. at 20,261. And even “in areas where relevant
data are available, small sample sizes in some areas impede
efforts to characterize the representativeness of the available
data at an ecoregion scale.” Id.

    What we have described thus far is but a fraction of the
analysis and explanation EPA provided in support of its decision
not to establish a new multi-pollutant national ambient air
quality standard without further studies. Citing these and many
other problems, EPA concluded that “given the current high
degree of uncertainties and the large complexities inherent in
quantifying the elements of” the Aquatic Acidification Index, it
had “no reasoned way to choose” a nationwide Acid
Neutralizing Capacity or to apply the Index to each of the
ecoregions throughout the country. Id. In light of the deference
due EPA’s scientific judgment, it is clear that its judgment must
be sustained here.



the highest load of acidic compounds it can absorb before chemical
changes leading to long-term adverse effects are caused.
                               21

                               IV

     We will end with a few words about Massachusetts v. EPA,
549 U.S. 497 (2007). There the Supreme Court held that an
agency’s decision not to promulgate a new rule “must conform
to the authorizing statute.” Id. at 533. We think it important to
explain why, despite that decision, the Clean Air Act authorizes
EPA to take the course it chose here.

     As part of its review of the existing standards, EPA was
obligated to promulgate a revision “as appropriate” under
§ 7409(b). EPA, exercising its authority to interpret ambiguous
provisions of the Clean Air Act, determined that a revision was
not “appropriate” when scientific uncertainty deprived the
agency of a “reasoned way to choose” an appropriate standard.
Final Rule, 77 Fed. Reg. at 20,252. Petitioners contend, citing
a string of circuit precedent, that EPA’s rationale violated the
Clean Air Act because uncertainty is an illegitimate reason not
to regulate under the Act. We have held that the Clean Air Act
“demand[s] regulatory action to prevent harm, even if the
regulator is less than certain that harm is otherwise inevitable”
because “[a]waiting certainty will often allow for only reactive,
and not preventive, regulation.” Ethyl Corp., 541 F.2d at 25; see
also Coal. for Responsible Regulation, Inc. v. EPA, 684 F.3d
102, 122 (D.C. Cir. 2012); Lead Indus. Ass’n v. EPA, 647 F.2d
1130, 1155 (D.C. Cir. 1980). Petitioners therefore argue that
despite the uncertainty, EPA violated the statute “by simply
leaving in place” a standard it knew to be inadequate. Br. for
Pet’r 29-30. We disagree.

     First, EPA did not “simply leav[e] in place” the old
standard. Although it did not promulgate a new standard, it
identified the data gaps that prevented it from doing so and
initiated a data-collection program designed precisely to fill
those gaps and facilitate future regulation. See Final Rule, 77
                                  22

Fed. Reg. at 20,264-67. EPA has not argued, and we do not
hold, that after finding its existing standards inadequate, EPA
was at liberty simply to leave them in place and take no action
at all. And as we recently explained in WildEarth Guardians v.
EPA, Massachusetts subjects “the manner, timing, content, and
coordination” of EPA’s rulemaking docket to very limited
review. No. 13-1212, slip op. at 9 (D.C. Cir. May 13, 2014)
(quoting Massachusetts, 549 U.S. at 533). By statute, EPA must
address the “known or anticipated” harms associated with
criteria air pollutants. 42 U.S.C. § 7409(b)(2). We cannot say,
under deferential Massachusetts review, that targeted data
collection is not a valid step toward that end.17

     Second, the Clean Air Act’s preference for preventive—and
thus potentially speculative—regulation is not absolute. We
explained that the Act proscribes “arbitrary” and “capricious”
agency action, 42 U.S.C. § 7607(d)(9)(A), and that new rules
must be based on reasoned judgment. See State Farm, 462 U.S.
at 43. Reasoned judgment may at times permit—and the Act
may at times require—action in the face of uncertainty, lest “the
precautionary purpose of the statute” be undermined. Ethyl
Corp., 541 F.2d at 28. But, at some point, action infected by
enough uncertainty cannot be called reasoned.18 Distinguishing
among these degrees is emphatically the province of EPA. See
cases cited supra.


     17
       Even in Massachusetts itself, the Court, despite rejecting
EPA’s rationale for its decision not to regulate, “did not say that EPA
was obliged to pursue rulemaking” on remand. WildEarth Guardians,
No. 13-1212, slip. op. at 10; see Massachusetts, 549 U.S. at 533.
     18
      In other words, the fact that we have rejected certainty as an
appropriate goal, see generally Ethyl Corp., 541 F.2d at 24-29, does
not mean that regulation is required (or permitted) no matter how
much uncertainty the agency faces.
                               23

     Here, EPA explained at length that the uncertainty it faced
was unusually profound. Across the Aquatic Acidification
Index model, data gaps were “of such a significant nature and
degree” that any rule promulgated would not have been based on
“reasoned judgment.” Final Rule, 77 Fed. Reg. at 20,256.
Petitioners question that conclusion, but as between petitioners’
critique and EPA’s scientific analysis, EPA’s judgment prevails.
Because the Act requires a reasoned judgment, and because EPA
found it could not form one, EPA’s explanation “conform[ed] to
the authorizing statute.” Massachusetts, 549 U.S. at 533.

    For all of these reasons, the petition for judicial review is
denied.

                                                    So ordered.
