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,AMODERN

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 surfacebound, 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 “capand-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-byriver, 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.