FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

NATIONAL PARKS CONSERVATION

ASSOCIATION; MONTANA

ENVIRONMENTAL INFORMATION

CENTER; SIERRA CLUB,

Petitioners,

v.

U.S. ENVIRONMENTAL PROTECTION

AGENCY and LISA P. JACKSON,

Administrator, United States

Environmental Protection Agency,

Respondents,

PPL MONTANA, LLC,

Respondent-Intervenor.

No. 12-73710
2 NAT’L PARKS CONSERVATION ASS’N V. EPA

PPL MONTANA LLC,

Petitioner,

NATIONAL PARKS CONSERVATION

ASSOCIATION, MONTANA

ENVIRONMENTAL INFORMATION

CENTER, AND SIERRA CLUB,

Intervenors,

v.

U.S. ENVIRONMENTAL PROTECTION

AGENCY and LISA P. JACKSON,

Respondents.

No. 12-73757

EPA No.

EPA-R08-OAR2011-0851

OPINION

On Petition for Review of an Order of the

Environmental Protection Agency

Argued and Submitted

May 16, 2014—Seattle, Washington

Filed June 9, 2015

Before: Diarmuid F. O’Scannlain, Marsha S. Berzon,

and Richard C. Tallman, Circuit Judges.

Opinion by Judge O’Scannlain;

Concurrence by Judge Berzon
NAT’L PARKS CONSERVATION ASS’N V. EPA 3

SUMMARY*

Environmental Law

The panel granted in part and denied in part petitions for

review brought by environmental organizations and PPL

Montana, LLC, the operator and partial owner of

hydroelectric power plants in Montana, challenging the

Environmental ProtectionAgency’s regional haze regulations

for the State of Montana, which prescribe emission limits at

certain power plants.

The Clean Air Act requires that all implementation plans

require installation of the “best available retrofit technology”

(BART) to reduce emissions from certain emission sources

that were operational between 1962 and 1977. PPL Montana

operates and partially owns coal-fired and hydroelectric

power plants in Montana, including the Colstrip Steam

Electric Generating Station and the J.E. Corette Steam

Electric Station.

Concerning the Colstrip station, the panel held that EPA’s

BART determination for Nitrogen Oxide emissions at

Colstrip Units 1 and 2 was arbitrary and capricious. The panel

also held that EPA’s determination of BART to control

sulphur dioxide emissions at Colstrip Units 1 and 2 was

arbitrary and capricious. The panel held that the seeming

inconsistency in EPA’s BART determinations at Colstrip

Units 1 and 2 and Corette, absent explanation, was arbitrary

and capricious. The panel further held that EPA did not

* This summary constitutes no part of the opinion of the court. It has

been prepared by court staff for the convenience of the reader.
4 NAT’L PARKS CONSERVATION ASS’N V. EPA

meaningfully address PPL Montana’s comment concerning

the EPA’s use of the CALPUFF visibility model in

determining BART at Colstrip Units 1 and 2.

Concerning the Corette station, the panel held that EPA’s

determination – that installation of additional technology to

control emissions from the Corette station was not cost

effective – suffered the same failure of explanation as its

BART determinations at Colstrip. The panel noted that the

Final Rule tightened the emissions limits identified for

Corette in the Proposed Rule. The panel held that after EPA

found Corette already had BART technology in place, it was

authorized by the Regional Haze Rule to skip the remaining

analyses in the section. The panel further held that PPL

Montana’s contention that EPA was nevertheless required to

proceed with the remaining BART analysis was a challenge

to the provision of the Regional Haze Rule itself, and was not

properly asserted in the challenge to the Montana Final

Implementation Plan. The panel also held that EPA properly

set emissions limits for Corette on a 30-day rolling average.

The panel rejected the environmental groups’ contention that

EPA’s decision not to require any additional

emission-reducing technology at Colstrip Units 3 and 4 was

arbitrary and capricious because it failed to satisfy the Clean

Air Act’s reasonable progress requirements.

The panel vacated the portions of the Final Rule setting

emissions limits at Colstrip Units 1 and 2 and Corette, and

remanded to EPA for further proceedings.

Concurring, Judge Berzon wrote separately to emphasize

her understanding that the lead opinion is not impugning the

EPA’s use of the CALPUFF model generally, but only

requiring a sufficiently reasoned response to a particular
NAT’L PARKS CONSERVATION ASS’N V. EPA 5

comment regarding CALPUFF’s usefulness in these specific

circumstances.

COUNSEL

Lisa S. Blatt, Arnold & Porter LLP, Washington, DC, argued

the cause and filed the briefs for petitioner PPL Montana. 

With her on the briefs were Jonathan S. Martel and

Christopher A. Jaros, Arnold & Porter, LLP, Washington,

DC.

Jenny K. Harbine, EARTHJUSTICE, Bozeman, MT, argued

the cause and filed the briefs for NPCA. Wither her on the

briefs was Janette K. Brimmer, EARTHJUSTICE, Seattle,

WA.

Daniel Pinkston, Environmental Defense Section,

Environment and Natural Resources Division, United States

Department of Justice, Denver, CO, argued the cause and

filed the brief for respondents. With him on the brief were

Ignacia S. Moreno, Assistant Attorney General, Environment

and Natural Resources Division, United States Department of

Justice, Denver, CO, and Elizabeth B. Dawson,

Environmental Defense Section, Environment and Natural

Resources Division, United States Department of Justice,

Denver, CO.

Michael G. Jenkins, Assistant General Counsel, PacifiCorp

Energy, Salt Lake City, UT and E. Blaine Rawson, Ray

Quinney & Nebeker, P.C., Salt Lake City, UT, filed the brief

for Amicus Curiae PacifiCorp.
6 NAT’L PARKS CONSERVATION ASS’N V. EPA

OPINION

O’SCANNLAIN, Circuit Judge:

We must decide whether the Environmental Protection

Agency’s regional haze regulations for the State of Montana

lawfully prescribe emission limits at certain power plants.

I

Petitioner PPL Montana operates and partially owns coalfired and hydroelectric power plants in Montana, including

the Colstrip Steam Electric Generating Station (“Colstrip”)

and the J.E. Corette Steam Electric Station (“Corette”). 

Petitioners National Parks Conservation Association,

Montana Environmental Information Center, and Sierra Club

(collectively, “NPCA”) are nonprofit conservation

organizations whose members enjoy wilderness areas

impacted by EPA’s regional haze regulations for the State of

Montana. Both petitioners are dissatisfied with such

regulations. PPL Montana argues, in essence, that they are

too stringent; NPCA argues, to the contrary, that they do not

do enough to remedyvisibility impairment caused by regional

haze in various relevant wilderness areas.

A

Regional haze is “visibility impairment caused by

geographically dispersed sources emitting fine particles and

their precursors into the air.” Am. Corn Growers Ass’n v.

EPA, 291 F.3d 1, 3 (D.C. Cir. 2002) (per curiam) (citing

Regional Haze Regulations, 64 Fed. Reg. 35,714 (July 1,

1999) (codified at 40 C.F.R. Pt. 51)). Congress enacted

§§ 169A and 169B of the Clean Air Act (the “CAA” or the
NAT’L PARKS CONSERVATION ASS’N V. EPA 7

“Act”) to address the problem of regional haze. Id. at 3–4;

see Clean Air Act Amendments of 1977, Pub. L. No. 95–95,

§ 128, 91 Stat. 685, 742 (current version at 42 U.S.C.

§ 7491); Clean Air Act Amendments, Pub L. No. 101–549,

§ 816, 104 Stat. 2695 (1990) (current version at 42 U.S.C.

§ 7492). These provisions establish as a national goal the

“prevention of any future, and the remedying of any existing,

impairment of visibility in mandatory Class I areas which

impairment results from manmade air pollution.”1

42 U.S.C.

§ 7491(a)(1).

The Act imposes several requirements on States and on

EPA relevant to this case. First, the Act requires EPA to

promulgate regulations to “assure . . . reasonable progress

toward meeting the national goal” of regional haze reduction. 

42 U.S.C. § 7491(a)(4). Second, the Act invites each State to

submit to EPA a “State Implementation Plan” (“SIP”) setting

forth emission limits and other measures necessary to make

reasonable progress toward the national visibility goal. See

42 U.S.C. §§ 7410(a), 7491(b)(2). If, like Montana, a State

chooses not to submit such a plan, the Act requires EPA to

produce a “Federal Implementation Plan” (“FIP”) for that

State. See 42 U.S.C. § 7410(c)(1)(A).

The Act further provides that all implementation plans

must require installation of the “best available retrofit

1 Class I areas include, inter alia, national wilderness areas exceeding

5,000 acres in size and national parks in existence on August 7, 1977

exceeding 6,000 acres in size. See 42 U.S.C. § 7472(a). The term

“mandatory class I federal areas” describes those that “may not be

designated as other than class I.” Id. § 7491(g)(5). Relevant here,

Yellowstone National Park, Glacier National Park, UL Bend National

Wildlife Refuge, and Medicine Lake Wilderness Area have been

designated mandatory Class I areas. See 40 C.F.R. §§ 81.400, 81.417.
8 NAT’L PARKS CONSERVATION ASS’N V. EPA

technology” (“BART”) to reduce emissions from certain

emission sources that were operational between 1962 and

1977 (“BART-eligible sources”). See 42 U.S.C.

§ 7491(b)(2), (g). Five statutory factors determine which

type of emissions-reducing technology constitutes BART for

such sources:

(a) the costs of compliance;

(b) the energy and non-air quality environmental impacts

of compliance;

(c) any existing pollution control technology at a source;

(d) the remaining useful life of the emission source; and

(e) the degree of visibility improvement anticipated[.]

42 U.S.C. § 7491(g)(2).

Pursuant to the Act, EPA promulgated its Regional Haze

Regulations (the “Regulations”), which asked certain States,

including Montana, to analyze sources of emissions within

the State and to develop a plan to eliminate all man-made

visibility impacts by 2064. See 64 Fed. Reg. at 35,714;

40 C.F.R. § 51.308. The Regulations require any

implementation plan to include (1) “reasonable progress

goals”; (2) a calculation of baseline and natural visibility

conditions; (3) a long-term strategy for achieving “reasonable

progress goals”; and (4) additional monitoring of emission

sources in Class I federal areas. See 40 C.F.R.

§ 51.308(d)(1)–(4). After the D.C. Circuit vacated the

provisions of the Regulations relating to BART

determinations, see Am. Corn Growers, 291 F.3d at 6, EPA
NAT’L PARKS CONSERVATION ASS’N V. EPA 9

promulgated new BART regulations in its Regional Haze

Regulations and Guidelines for Best Available Retrofit

Technology (BART) Determinations (the “2005

Regulations”), which revised the text of the earlier

Regulations. See 70 Fed. Reg. 39,104 (July 6, 2005).

EPA also published its Guidelines for BART

Determinations Under the Regional Haze Rule (the

“Guidelines”), 40 C.F.R. Pt. 51, App. Y (Sept. 6, 2005),

prescribing five steps for application of the five statutory

BART factors:

(Step 1) Identify all available retrofit control

technologies;

(Step 2) Eliminate technically infeasible options;

(Step 3) Evaluate the control effectiveness of

remaining control technologies;

(Step 4) Evaluate impacts (identified in § 7491(g)(2),

see 40 C.F.R. Pt. 51, App. Y, § IV.D.4,

70 Fed. Reg. at 39,166) and document the

results;

(Step 5) Evaluate visibility impacts (measured in

“deciviews,” see 40 C.F.R. § 51.301).2

Id. App. Y § IV.D.

2

“Deciview means a measurement of visibility impairment.” 40 C.F.R.

§ 51.301. “Each deciview change is an equal incremental change in

visibility perceived by the human eye. Most people can detect a change

in visibility at one deciview.” Proposed Rule, 77 Fed. Reg. at 23,992.
10 NAT’L PARKS CONSERVATION ASS’N V. EPA

B

In 2006, the Montana Department of Environmental

Quality notified EPA that it did not intend to produce a SIP

triggering EPA’s obligation to produce a FIP for the State of

Montana. See 42 U.S.C. § 7410(c)(1)(A). EPA published a

proposed FIP for Montana on April 20, 2012 (the “Proposed

Rule”). See Approval and Promulgation of Implementation

Plans; State of Montana, 77 Fed. Reg. 23,988 (Apr. 20, 2012).

The Proposed Rule required petitioner PPL Montana to

take various actions to reduce emissions of two

compounds—nitrogen oxide (“NOX”) and sulfur dioxide

(“SO2”)—at two power plants it partially owns and operates,

Colstrip and Corette. First, PPL Montana was required to

implement several new technologies at Units 1 and 2 of the

four-unit Colstrip station. To reduce NOX emissions to a 30-

day rolling average of 0.15 lb/mmBtu, the Proposed Rule

required PPL Montana to install two new

technologies—separated overfire air (“SOFA”) and selective

non-catalytic reduction (“SNCR”) at Colstrip Units 1 and 2. 

Id. at 24,027, 24,035. To reduce SO2 emissions to a 30-day

rolling average of 0.08 lb/mmBtu, the Proposed Rule required

PPL Montana to implement two additional new technologies

at Colstrip Units 1 and 2—lime injection and a fourth

“scrubber.” Id. at 24,028, 24,035. The Proposed Rule did not

require PPL Montana to implement new technologies at

Colstrip Units 3 and 4.

Second, the Proposed Rule required PPLMontana to limit

NOX and SO2 emissions at the Corette station. The Proposed

Rule imposed 30-day average rolling emission limits of 0.40

lb/mmBtu for NOX and 0.70 lb/mmBtu for SO2. See id. at

24,042, 24,046. It required PPL Montana to achieve such
NAT’L PARKS CONSERVATION ASS’N V. EPA 11

emissions using current technology; unlike at Colstrip Units

1 and 2, the Proposed Rule does not require installation of

new technology at Corette. Id. at 24,043, 24,047.

Both PPL Montana and NPCA commented on the

Proposed Rule. At the conclusion of the notice and comment

period, EPA issued its final FIP for Montana on September

18, 2012. See Approval and Promulgation of Implementation

Plans; State of Montana (the “Final Rule” or the “Rule”),

77 Fed. Reg. 57,864 (Sept. 18, 2012) (codified at 40 C.F.R.

§ 52.1396). The Final Rule implemented the Proposed Rule

in almost all respects relevant to this appeal.3

C

PPL Montana and NPCA both filed petitions for review

of EPA’s BART determinations at Colstrip and Corette,

which petitions were consolidated for purposes of this appeal. 

PPL Montana contends that the emissions limits set forth in

the Rule for NOX and SO2 at Colstrip Units 1 and 2—as well

as the BART determinations underlying such limits—are

essentially too stringent. It argues that EPA failed reasonably

to explain the Rule’s BART selections and that EPA’s

visibility model does not reasonably anticipate visibility

improvement as a result of the Rule’s requirements. PPL

Montana also challenges the emissions set forth in the Rule

for Corette.

3 Although the Final Rule did not alter EPA’s BART determinations for

either Colstrip or Corette, it did lower the Proposed Rule’s emission limits

for NOX and SO2 at Corette from 0.40 lb/mmBtu to 0.35 lb/mmBtu and

from 0.70 lb/mmBtu to 0.57 lb/mmBtu, respectively. See Final Rule,

77 Fed. Reg. at 57,911.
12 NAT’L PARKS CONSERVATION ASS’N V. EPA

NPCA also challenges the Rule’s emissions limits and

BART determinations at Colstrip Units 1 and 2 and Corette,

contending essentially that such limits are not stringent

enough. According to NPCA, EPA’s decision not to require

installation of more advanced technology at these locations

was unexplained, arbitrary, and capricious. Moreover,

according to NPCA, EPA’s decision not to require

installation of any new technologies at Colstrip Units 3 and

4 fails to satisfy the requirement that the Rule make

reasonable progress toward visibility improvement.

D

EPA’s implementation of the regional haze plan is

governed by Section 307(d) of the Act, 42 U.S.C. § 7607(d). 

The statute requires that EPA explain the basis for its

decisions, including underlying factual bases, methods of

analysis, and legal and policy considerations. Id.

§ 7607(d)(6)(A). EPA must also respond to the comments,

criticism, and new data submitted during the comment period. 

Id.

When we review an agency action “involv[ing] primarily

issues of fact,” and where “analysis ofthe relevant documents

‘requires a high level of technical expertise,’ we must defer

to ‘the informed discretion of the responsible federal

agencies.’” Marsh v. Oregon Natural Res. Council, 490 U.S.

360, 377 (1989) (quoting Kleppe v. Sierra Club, 427 U.S.

390. 412 (1976)). However, we do not defer to EPA actions

that are “arbitrary, capricious, an abuse of discretion, or

otherwise not in accordance with law,” or that exceed EPA’s

statutory jurisdiction. 42 U.S.C. §§ 7607(D)(9)(A), (C). 

EPA’s actions must be reasoned; EPA acts in an arbitrary and

capricious manner if it fails to consider an important aspect
NAT’L PARKS CONSERVATION ASS’N V. EPA 13

of a decision or if its explanation contradicts the evidence

before it. Motor Vehicle Mfrs. Ass’n of U.S., Inc. v. State

Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983). EPA’s

actions must also be consistent; an internally inconsistent

analysis is arbitrary and capricious. Gen. Chem. Corp. v.

United States, 817 F.2d 844, 857 (D.C. Cir. 1987) (per

curiam).

II

PPL Montana and NPCA each contest the Rule’s BART

determinations and prescribed limits for NOX and SO2

emissions at Colstrip Units 1 and 2—and, for similar reasons,

Corette.

A

The heart of the dispute about EPA’s BART selection at

Colstrip Units 1 and 2 is its determination that use of SNCR

in addition to SOFA is cost-effective, and that use of selective

catalytic reduction (“SCR”)—a more aggressive

technology—in addition to SOFA is not. Both parties urge

that EPA’s cost-effectiveness analysis fails appropriately to

consider the costs of compliance and degree of visibility

impairment, as required by the Act, see 42 U.S.C.

§ 7491(g)(2), at Step Three of EPA’s BART analysis under

the Guidelines, see 40 C.F.R. Pt. 51, App. Y § IV.D.

The Rule requires PPL Montana to reduce NOX emissions

at Colstrip Units 1 and 2 to 0.15 lb/mmBtu over a 30-day

rolling average. See 40 C.F.R. § 52.1396(c)(1). EPA

concluded that such reduction could be achieved by installing

both SOFA and SNCR technologies. See Final Rule, 77 Fed.

Reg. at 57,866. NPCA contends that EPA offered insufficient
14 NAT’L PARKS CONSERVATION ASS’N V. EPA

justification for its rejection of SCR as BART in favor of

SOFA and SNCR together, and argues that PPL Montana

should be required to install SCR instead of SNCR, and in

addition to SOFA, at all four Colstrip units. PPL Montana

makes a related point, but on the other side of the argument,

contending that EPA offered insufficient justification for

requiring SOFA and SNCR as BART instead of SOFA

alone—a remedy PPL Montana does not oppose. EPA

responds that the BART determination is discretionary and

that it considered all of the relevant factors in reaching its

conclusions.

The parties also challenge several more minor aspects of

EPA’s cost-effectiveness analysis. PPL Montana argues that

EPA’s use of the dollar-per-ton metric for balancing cost and

visibility benefit was improper. NPCA, for its part, maintains

that EPA underestimated the emissions baseline at Colstrip

Units 1 and 2, and that it miscalculated the cost of

implementing SCR at these units.

1

EPA identified the costs of the various technologies for

NOX reduction at Colstrip Units 1 and 2 as follows:
NAT’L PARKS CONSERVATION ASS’N V. EPA 15

Technology Capital

Cost

Annualized

Cost

Tons

per

Year

Cost

per

Ton

COLSTRIP UNIT 1

SOFA $4.508

mm

$1.090mm 1,432 $761

SOFA +

SNCR

$13.381

mm

$3.279mm 2,097 $1,564

SOFA +

SCR

$82.772

mm

$10.942mm 3,425 $3,195

COLSTRIP UNIT 2

SOFA $4.508

mm

$1.090mm 1,420 $768

SOFA +

SNCR

$13.381

mm

$3.256mm 2,072 $1,571

SOFA + SCR $82.771

mm

$10.920mm 3,376 $3,235

See Proposed Rule, Tables 66–74, 89–97, 77 Fed. Reg. at

24,024–27, 24,032–34.

The Rule offers essentially no reasoning behind EPA’s

selection of SOFA and SNCR together, as opposed either to

SOFA alone or SOFA and SCR together, as BART to reduce

NOX emissions at Colstrip Units 1 and 2. With respect to the

decision to require SNCR in addition to SOFA, EPA asserted

in the Proposed Rule only that the cost of SOFA and SNCR

together “is justified when the visibility improvement is

considered.” See Proposed Rule, 77 Fed. Reg. at 24,027. In
16 NAT’L PARKS CONSERVATION ASS’N V. EPA

response to PPL Montana’s comment requesting that EPA

explain this assertion, EPA noted that the Proposed Rule

discloses the cost for both SOFA alone and SOFA combined

with SNCR, and that it “selected SNCR as BART in

consideration of these costs, all of which were presented to

the public in our proposed rule.” Final Rule, 77 Fed. Reg. at

57,886. But PPL Montana’s objection is not that EPA failed

to disclose the cost differential between implementing SOFA

alone and SOFA along with SNCR. It contends instead that

EPA failed to explain the reasons why the improvement

achieved through SOFA alone was insufficient, but the

improvement achieved through SOFA and SNCR together

was deemed both sufficient and cost-effective.

With respect to SCR, EPA simply asserted that the cost of

SOFA and SCR together ($3,195/ton at Colstrip Unit 1) was

“not justified by the visibility improvement of .404

deciviews,” but that “[t]he lower cost of SOFA + SNCR

($1,564/ton) is justified when the visibility improvement [of

.264 deciviews] is considered.” Proposed Rule, 77 Fed. Reg.

at 24,027. EPA maintains that it did not use a set costeffectiveness or improvement threshold to disqualify SCR,

and we do not suggest that it must do so. But absent any

explanation at all of how EPA determines cost-effectiveness,

it is impossible for the Rule’s reader to determine why EPA

ruled SOFA and SNCR in and ruled SCR out.

EPA acknowledged that the Regional Haze Rule does not

prevent it from implementing what it called a “bright line”

rule for cost-effectiveness, but contends that its regulations do

not require it to do so. See Final Rule, 77 Fed. Reg. at

57,872. To be sure, the Act and the Regulations do not

specifically require that EPA explain its cost-effectiveness

decisions through use of a “bright line” rule. But the law
NAT’L PARKS CONSERVATION ASS’N V. EPA 17

does require EPA to “cogently explain why it has exercised

its discretion in a given manner.” See State Farm, 463 U.S.

at 48; Greater Yellowstone Coalition, Inc. v. Servheen,

665 F.3d 1015, 1030 (9th Cir. 2011) (requiring “a rational

connection between the data before [the agency] and its

conclusion”); Nw. Envtl. Def. Ctr. v. Bonneville Power

Admin., 477 F.3d 668, 691 (9th Cir. 2007). EPA’s

unexplained assertions that the combination of SOFA and

SNCR is cost-effective, but that SOFA alone is insufficient,

and that the combination of SOFA and SCR is not costeffective, are unsupported by any explained reasoning. These

assertions leave the Rule’s reader wondering what metric, if

any, EPA used to determine BART, or if EPA employed no

metric, why not. Therefore, we conclude that EPA’s BART

determination for NOX emissions at Colstrip Units 1 and 2 is

arbitrary and capricious.

2

EPA’s responses to petitioners’ more minor challenges to

its cost-effectiveness analysis make clear that it is capable of

the required rational explanation.

First, NPCA challenges EPA’s selection of the years

2008–2010 as the emissions baseline period for calculating

BART at Colstrip Units 1 and 2. It contends that EPA

underestimated the potential benefit of using SCR by first

underestimating the existing emissions baseline. EPA’s

BART Guidelines require it to choose a representative

baseline period that supplies “a realistic depiction of

anticipated annual emissions for the source.” BART

Guidelines, 70 Fed. Reg. at 39,167. EPA chose the period

2008–2010 on the reasoning that Colstrip had installed

additional combustion controls in 2007, reducing its
18 NAT’L PARKS CONSERVATION ASS’N V. EPA

emissions. See id. NPCA argues that EPA should have

chosen earlier periods because PPL Montana is not required

to maintain the rate of emissions achieved between

2008–2010, describing the changes at Colstrip as

“unenforceable.” But EPA offered a reasoned response to

NPCA’s comment about its choice—that Colstrip had

achieved reduced emissions using technology it has no plans

to deactivate—and NPCA has suggested no reason to believe

that PPL Montana would change course and remove the

additional combustion controls it had already installed. EPA

gave a reasoned, rational response to this argument, a

response to which a reviewing court defers. See Latino Issues

Forum v. EPA, 558 F.3d 936, 941 (9th Cir. 2009).

Second, PPL Montana objects that EPA’s reliance on a

dollars-per-ton metric to calculate cost effectiveness is

inappropriate because it does not actually measure the

improvement achieved in visibility. But EPA responded that

it has previously defined “cost effectiveness” as “tons of

pollutant emissions removed” compared with “annualized

control costs.” 2005 Regulations, 70 Fed. Reg. at 39,167. 

Indeed, PPL Montana essentially abandoned this argument in

reply after EPA pointed to its regulation in its response brief.

Third, NPCA objects that EPA improperly calculated the

cost of installing SCR at Colstrip Units 1 and 2, leading it to

reject SCR as BART. It challenges EPA’s calculation of the

capital cost, its choice of interest rate, and the lifespan and

maintenance costs assumed in EPA’s calculation. EPA

provided a reasoned response to NPCA’s comments on these

questions. See Final Rule, 77 Fed. Reg. at 57,888–89. NPCA

maintains that EPA should have looked to different data

sources to determine these costs. But it offers a reason that

EPA should look to different underlying cost data in only one
NAT’L PARKS CONSERVATION ASS’N V. EPA 19

example case. It contends EPA should have chosen a 5%

interest rate instead of a 7% interest rate in calculating these

costs to comport with the requirements set forth in the EPA

Air Pollution Cost Manual. But that manual instructs

industry actors that they may wish to use a different interest

rate for their own calculations. As EPA explained, see Final

Rule, 77 Fed. Reg. at 57,888–89, EPA’s Manual clearly

contemplates that EPA will use the interest rate set by the

OMB, as it properly did here, in pursuing its own

calculations. See id.

3

Both petitioners dispute the rationality of EPA’s selection

of a fourth scrubber as BART for SO2 emissions control at

Colstrip Units 1 and 2 for essentially the same reasons they

dispute EPA’s NOX BART determinations. EPA determined

the costs of the various technologies for reducing SO2

emissions at Colstrip as follows:
20 NAT’L PARKS CONSERVATION ASS’N V. EPA

Technology Capital

Cost

Annualized

Cost

Tons

per

Year

Cost

per

Ton

COLSTRIP I

Lime

injection

$3.00mm $1.883mm 3,557 $529

Lime

injection +

New

scrubber

$28.00

mm

$4.100mm 4,486 $912

COLSTRIP II

Lime

injection

$3.00mm $1.883mm 3,212 $586

Lime

injection +

New

scrubber

$28.00

mm

$4.093mm 4,129 $991

See Proposed Rule, Tables 78–87, 102–111, 77 Fed. Reg. at

24,028–29, 24,036–37.

PPL Montana contends that EPA underestimated the cost

of installing a fourth scrubber at Colstrip Units 1 and 2 and

failed sufficiently to explain its cost-effectiveness

determination for this requirement. NPCA, to the contrary,

contends that EPA should have required installation of

replacement advanced scrubbers at Colstrip, not just the

introduction of an additional scrubber. EPA responds that it

appropriately explained its calculation of the costs involved

in adding an additional scrubber and that it appropriately
NAT’L PARKS CONSERVATION ASS’N V. EPA 21

decided not to require an entirely new system of scrubbers, as

opposed only to adding one additional scrubber.

EPA’s cost-effectiveness analysis with respect to SO2

emissions at Colstrip Units 1 and 2 suffers the same defect as

its cost-effectiveness with respect to NOX emissions. EPA’s

SO2 analysis is an improvement in one respect: EPA

acknowledged the incremental cost of adding an additional

scrubber vessel ($2,410/ton at Colstrip Unit 2, see Final Rule,

77 Fed. Reg. at 57,892), and noted that adding a scrubber

vessel would incrementally improve visibility from 0.225

deciview (using lime injection alone) to 0.280 deciview

(using lime injection and an additional scrubber vessel). See

id. But EPA then concluded that it “continue[s] to find that

the cost is reasonable given the visibility benefits[.]” Id.

Although in this case EPA acknowledged the comment

concerning incremental cost-effectiveness, it neverthelessstill

failed to explain what makes a cost reasonable in light of

potential visibility benefits.

The Rule’s SO2 emission limits for Colstrip Units 1 and

2 force PPL Montana to spend an additional $25,000,000 in

capital costs and an additional $2,210,000 annuallyto achieve

a visibility improvement of 0.055 deciviews at the sampled

location. See id. This improvement very well may be

necessary and ultimately cost-effective. But EPA has

supplied no reasons justifying that determination. Based on

its explanation, the Rule’s reader is left to wonder what

rationale EPA used to determine cost-effectiveness. Again,

the law requires a reasoned answer to that question. See State

Farm, 463 U.S. at 48; Servheen, 665 F.3d at 1028; Nw. Envtl.

Def. Ctr., 477 F.3d at 691.
22 NAT’L PARKS CONSERVATION ASS’N V. EPA

By contrast, EPA thoroughly and rationally explained its

response to NPCA’s objection on this point. NPCA contends

that EPA should have considered requiring complete

replacement of the existing control systems for SO2 emissions

at Colstrip Units 1 and 2. EPA explained its decision not to

do so: its BART Guidelines recommend constructing a new

system when a current control system achieves “less than 50

percent removal efficiencies.” Final Rule, 77 Fed. Reg. at

57,892 (quoting 2005 Regulations, 70 Fed. Reg. at 39,171)

(internal quotation marks omitted). The current SO2

emissions control system at Colstrip Units 1 and 2 achieves

removal efficiencies exceeding 50 percent, see Proposed

Rule, 77 Fed. Reg. at 24,028, and therefore EPA is not

required to consider replacement technology. See Final Rule,

77 Fed. Reg. at 57,892.

This kind of explanation—not elaborate, but rational, and

thus adequate—is missing from EPA’s conclusion that PPL

Montana must install a fourth scrubber at Colstrip Units 1 and

2. EPA’s determination of BART to control SO2 emissions

suffers the same failure of rational explanation as its BART

determination for NOX emissions. Because the rule offers no

reasoned explanation to support its requirement of a fourth

scrubber at Colstrip Units 1 and 2, we conclude that such

requirement is arbitrary and capricious.

B

PPL Montana also, relatedly, contends that EPA’s BART

determinations at Colstrip Units 1 and 2 are arbitrary and

capricious because they are inconsistent with EPA’s Corette

analysis, which does not require additional controls at that

station, see Final Rule, 77 Fed. Reg. at 57,893. PPL Montana

points out that EPA rejected implementation of SOFA at
NAT’L PARKS CONSERVATION ASS’N V. EPA 23

Corette, but found similar costs for SOFA installation

justified at Colstrip Units 1 and 2. See supra § II.A.1. 

NPCA, on the other hand, argues that EPA’s conclusion that

additional controls at Corette were not cost-effective was

arbitrary and capricious.

The entirety of EPA’s cost-effectiveness reasoning with

respect to Corette follows: “We have weighed costs against

the anticipated visibility impacts for Corette. Any of the

control options would have a positive impact on visibility;

however, the cost of controls is not justified by the visibility

improvement.” Proposed Rule, 77 Fed. Reg. at 24,043. 

Again, this reasoning fails to reveal to a reader how EPA

determined that the cost of controls were not justified. 

Moreover, PPLMontana is correct that this reasoning appears

inconsistent with EPA’s analysis for Colstrip Units 1 and 2. 

At those units, EPA concluded that a cost-per-ton rate of

approximately $1,500 for NOX emissions controls was

justified. See supra § II.A.1. Yet at Corette, EPA concluded

that a cost-per-ton rate of $1,487 did not justify the potential

emissions reductions. See Proposed Rule, 77 Fed. Reg. at

24,040–43. Corette, moreover, is closer to Class I areas than

Colstrip, and emissions controls there could have improved

visibility at least as much as they were set to control visibility

at Colstrip. See id. The seeming inconsistency in EPA’s

BART determinations at Colstrip Units 1 and 2 and Corette

is, absent explanation, “the hallmark of arbitrary action.” 

Sierra Club v. EPA, 719 F.2d 436, 459 (D.C. Cir. 1983); see

also Gen. Chem. Corp. v. United States, 817 F.2d 844, 846

(D.C. Cir. 1987) (per curiam) (finding analysis arbitrary and

capricious because it was “internally inconsistent and

inadequately explained”).
24 NAT’L PARKS CONSERVATION ASS’N V. EPA

C

Finally, PPL Montana objects, on two grounds, to EPA’s

use of the CALPUFF

4

visibility model in determining BART

at Colstrip Units 1 and 2. PPL Montana first objects that the

maximum potential incremental visibility benefit of SNCR is

below the range of perceptibility and falls within the model’s

margin of error, meaning such improvement cannot be

“reasonably . . . anticipated” as required by the Act. See

42 U.S.C. § 7491(g)(2). It also challenges the application of

CALPUFF to the emissions sources at issue because of their

distance from Class I areas, contending the model is

inaccurate at such great distances.

EPA responds that, because the CALPUFF model was

approved in the Guidelines, EPA need not defend its every

application. See Final Rule, 77 Fed. Reg. at 57,867. EPA

also responds that the Regional Haze Rule itself anticipated

and rejected PPL Montana’s argument, reasoning that

“[f]ailing to consider less-than-perceptible contributions to

visibility impairment would ignore the CAA’s intent to have

BART requirements apply to sources that contribute to, as

well as cause, such impairment.” Final Rule, 77 Fed. Reg. at

57,883 (quoting 2005 Regulations, 70 Fed. Reg. at 39,129). 

EPA further responds that it “has acknowledged that there is

uncertainty in the CALPUFF model predicted visibility

impacts,” and “the CALPUFF model can both underpredict

and overpredict visibility impacts,” but that “CALPUFF . . .

is a reasonable application to determine whether such a

facility [with an impact exceeding 2 or 3 deciviews] can

reasonably be anticipated to cause or contribute to any

4 CALPUFF is a model used to estimate an emissions source’s impact

on visibility. See 40 C.F.R. Pt. 51 App. Y § III.A.3.
NAT’L PARKS CONSERVATION ASS’N V. EPA 25

impairment of visibility.” Final Rule, 77 Fed. Reg. at 57,868

(quoting 2005 Regulations, 77 Fed. Reg. 39,123).

EPA fails to understand PPL Montana’s first argument. 

PPL Montana does not object wholesale to EPA’s use of the

CALPUFF model for visibility measurement. Instead, it

argues that the results the model predicts in this case are too

insignificant for the model to measure, and therefore cannot

be reasonably anticipated as the Act requires. During the

notice and comment period, PPL Montana argued that the

maximum incremental visibility benefit to be gained by

installing SNCR at Colstrip Units 1 and 2 would be 0.085

deciviews, an incremental improvement PPL Montana

contends falls within the CALPUFF model’s margin of error. 

EPA responded by explaining that its regulation permits

visibility improvements to be required even when visibility

impacts fall below the threshold of perceptibility. See Final

Rule, 77 Fed. Reg. at 57,867. EPA further responded that its

2005 Regulations direct use of the CALPUFF model to

estimate the 98th percentile of visibility impairment, rather

than the highest data collected, to minimize uncertainty in its

calculations. See id. at 57,868. But PPL Montana does not

ask EPA to discontinue application of CALPUFF below the

one-deciview perceptibility threshold, or to run the model

using different data points than the ones selected; it asks how

CALPUFF explains EPA’s conclusion that additional

measures will lead to reasonable anticipation of visibility

improvement in this case, when, PPLasserts, an improvement

of 0.085 deciview is “beyond the CALPUFF model’s ability

to predict with any confidence.”

EPA’s response does not meaningfully address PPL

Montana’s comment, as it must. See Columbia Falls

Aluminum Co. v. EPA, 139 F.3d 914, 923 (D.C. Cir. 1998);
26 NAT’L PARKS CONSERVATION ASS’N V. EPA

Eagle-Picher Indus., Inc. v. EPA, 759 F.2d 905, 922 (D.C.

Cir. 1985) (“[I]f . . . the model is challenged, the agency must

provide a full analytical defense.”).5It is no answer to

respond, as EPA did, that low levels of visibility impairment

must be addressed even though they are not perceptible to the

human eye, or that measures have been taken to minimize the

margin of error. The issue is not the perceptibility of the

proposed improvements, but the model’s ability to anticipate

improvements at a level allegedly within its margin of error,

whether perceptible or not to the human eye. EPA simply

offered no response to this objection.

EPA’s only detailed defense of the model addressed PPL

Montana’s concern about distance—not the margin of error

issue. See Final Rule, 77 Fed. Reg. at 57,867–68. With

regard to distance, EPA noted that guidance issued by the

Interagency Workgroup on Air Quality Modeling “provides

for the use of the CALPUFF model at receptor distances of

up to 200 to 300 km.” EPA then explained why it believed

CALPUFF could be used “cautiously” for distances in that

range, even though the puffs would be more dispersed at

5 Requiring such an explanation does not, as EPA argues, improperly

require de novo review of EPA’s use of the CALPUFF model itself. EPA

“need not justify the model on an ad hoc basis for every chemical to which

the model is applied, even when faced with data indicating that it is not a

perfect fit.” Chem. Mfrs. Ass’n v. EPA, 28 F.3d 1259, 1265 (D.C. Cir.

1994). But PPL Montana does not ask EPA to justify its model. 

Accepting CALPUFF as the applicable model, PPL Montana asks how

CALPUFF explains EPA’s selection of an additional technology for

BART, a question EPA has not answered. In such a situation, deference

even on methodological questions is inappropriate. See Brower v. Evans,

257 F.3d 1058, 1067 (9th Cir. 2001) (holding deference overcome where

agency “completely failed to address some factor consideration of which

was essential to [making an] informed decision” (internal quotation marks

and citation omitted)).
NAT’L PARKS CONSERVATION ASS’N V. EPA 27

greater distances, and stated that the model was not suitable

for “very long-range transport (300 km and beyond).” Id. at

57,868. EPA thus offered a reasoned response to PPL

Montana’s challenge to the use of CALPUFF at the distances

in question. That explanation did not, however, also suffice

as a reasoned response regarding how CALPUFF could be

relied upon to predict an improvement of as little as 0.085

deciviews when PPL offered reasons to think that doing so

was outside the model’s capabilities.

The predictability concern is important because the Act

requires that any visibility improvement be “reasonably . . .

anticipated” as a result of BART installation. 42 U.S.C.

§ 7491(g)(2). As the D.C. Circuit explained in vacating a

portion of the Regional Haze Rule itself, it is arbitrary and

capricious for EPA to force an emissions source “to spend

millions of dollars for new technology that will have no

appreciable effect on the haze in anyClass I area.” Am. Corn

Growers Ass’n, 291 F.3d at 7. In response to PPL Montana’s

contentions that the Final Rule would do just that, by

requiring PPL Montana to install SNCR at Colstrip Units 1

and 2 without sufficient assurance of any improvement at all,

EPA has offered no reasoned explanation.

III

PPL Montana also challenges the emissions limitations

EPA imposed at Corette—namely, the requirement that

Corette lower its emissions levels even without installing

additional technology. As explained, EPA’s determination

that installation of additional technology to control emissions

from Corette was not cost-effective suffers the same failure

of explanation as its BART determinations at Colstrip. See

supra § II.B. But the Rule also tightened the emissions limits
28 NAT’L PARKS CONSERVATION ASS’N V. EPA

identified for Corette in the Proposed Rule. Specifically, in

response to comments (including comments made by PPL

Montana), EPA altered its rule to establish an emission rate

of 0.57 lb/MMbtu for SO2 emissions and an emission rate of

0.35 lb/MMBtu for NoX emissions, both monitored on a 30-

day rolling average. See Final Rule, 77 Fed. Reg. at

57,893–94. PPL Montana argues that the CAA does not

authorize EPA to impose emissions limits without

determining BART, but EPA insists that its analysis

comported with the BART Guidelines.6

EPA correctly argues that, after it found Corette already

had BART technology in place, it was authorized by the

Regional Haze Rule to “skip the remaining analyses in this

section, including the visibility analysis in step 5.” See 2005

Regulations, 70 Fed. Reg. at 39,165. PPL Montana’s

contention that EPA was nevertheless required to proceed

with the remaining BART analysis is a challenge to this

provision of the Regional Haze Rule itself, not properly

asserted in this challenge to the Montana FIP. See 42 U.S.C.

§ 7607(b)(1) (challenge to rulemakingmust be brought within

sixty days).

6 EPA contends first that PPL Montana waived its objection on this point

by failing to make a relevant comment to the Proposed Rule. Contrary to

EPA’s argument, PPL Montana preserved its objection to the Corette

emissions limits when it made its comments, which urged that, with

Corette’s current technology, converting annualized emissions limits to

30-day rolling average limits would require an increase in emissions

limits. Moreover, EPA specifically addressed this comment in the Rule,

noting that a commenter had suggested an increase in emissions limits to

0.81 lb/MMBtu for SO2 emissions and to 0.46 lb/MMBtu for NOX

emissions. See Final Rule, 77 Fed. Reg. at 57,893.
NAT’L PARKS CONSERVATION ASS’N V. EPA 29

EPA also properly set emissions limits for Corette on a

30-day rolling average. The Regional Haze Rule prescribes

such limits. See 2005 Regulations, 70 Fed. Reg. 39,172

(requiring emissions limits to “specify an averaging time of

a 30-day rolling average”). EPA noted and addressed PPL

Montana’s comment about the 30-day rolling average

requirement. See Final Rule, 77 Fed. Reg. at 57,893–94. 

PPL Montana contended that converting current emissions

limits to 30-day rolling averages would require EPA to raise

the limits set forth in the Proposed Rule. EPA concluded to

the contrary that, by choosing 99th percentile monthly

emission rates and adding an additional margin for

compliance, the 30-day rolling averages provided in the rule

comported with Corette’s current technological capabilities. 

See id. EPA’s reasoned disagreement on this topic with PPL

Montana’s comment reflects its conclusion on a highly

scientific question—the variance in emissions calculations

that occurs when annualized rates are translated into thirtyday rolling averages—precisely the kind of question

justifying deference to EPA’s discretion. See Nat’l Wildlife

Fed’n v. U.S. Army Corps of Eng’rs, 384 F.3d 1163, 1177–78

(9th Cir. 2004).

IV

NPCA contends that EPA’s decision not to require any

additional emission-reducing technology, let alone

installation of SCR, at Colstrip Units 3 and 4 was arbitrary

and capricious because it fails to satisfy the Act’s reasonable

progress requirements. Colstrip Units 3 and 4 are not subject

to BART requirements because they were constructed after

1977. See 42 U.S.C. § 7491(b)(2), (g). Nevertheless, the Act

still directs EPA to issue regulations assuring “reasonable

progress” toward improving visibility in Class I areas. See
30 NAT’L PARKS CONSERVATION ASS’N V. EPA

42 U.S.C. § 7491(a)(4). The statute requires EPA to consider

several factors in determining reasonable progress: (a) costs

of compliance; (b) the time necessary for compliance; (c) the

energy and other environmental impacts of compliance; and

(d) the remaining useful life of a source subject to reasonable

progress requirements. See 42 U.S.C. § 7491(g)(1). EPA’s

Regulations prescribe reasonable progress toward attaining

natural visibility conditions by 2064. See 40 C.F.R.

§ 51.308(f).

EPA responded to comments contending that the visibility

benefits to be gained from SCR at Colstrip Units 3 and 4

justify the requirement for reasonable progress purposes. See

Final Rule, 77 Fed. Reg. at 57,902–03. It explained that the

cost of compliance is only one of the four statutory

requirements for reasonable progress analysis. See id. EPA

also offered a better explanation for its cost-based decision on

this point than it did for its cost-effectiveness determination

of BART at Colstrip and Corette. It reasoned that the

visibility benefits to be gained from requiring SCR (ranging

from 0.273 deciviews to 0.260 deciviews) were not sufficient

considering their cost, between $4,574 and $4,607 per ton. 

See id. EPA contrasted this effectiveness with the reasonable

progress goals it implemented in North Dakota at Antelope

ValleyStation—a location to which NPCA specifically urged

comparison. See id. EPA explained that it had found

improvements of 0.2 deciviews at each unit of that location

to be cost-justified for reasonable progress purposes when the

costs of such improvement ranged from $586/ton to $661/ton. 

See id. EPA’s comparison of the two sources provided

NPCA with at least some broad metric for understanding

which cost-per-ton ratios EPA will approve and which it will

not—a rational explanation why the reasonable progress

options available at Colstrip Units 3 and 4 were not feasible.
NAT’L PARKS CONSERVATION ASS’N V. EPA 31

V

For the reasons explained, we GRANT IN PART and

DENY IN PART the petitions for review, VACATE the

portions of the Rule setting emissions limits at Colstrip Units

1 and 2 and Corette and REMAND to EPA for further

proceedings consistent with this opinion. Each party shall

bear its own costs.7

PETITIONS FOR REVIEW GRANTED IN PART

ANDDENIEDINPART;VACATEDANDREMANDED.

BERZON, Circuit Judge, concurring.

I concur. I write separately to underline my

understanding that, in Part II.C of the lead opinion, we are not

impugning EPA’s use of the CALPUFF model generally. 

Instead, we are requiring a sufficiently reasoned response to

a particular comment regarding CALPUFF’s usefulness in

these specific circumstances. Had EPA given any reasonable

explanation as to why CALPUFF was not just “sufficiently

7 Petitioners National Parks Conservation Association, Montana

Environmental Information Center, and Sierra Club’s Motion for Judicial

Notice of Standing Declarations filed March 14, 2013 is DENIED as

moot.

Petitioner PPL Montana, LLC’s Unopposed Motion to Take Judicial

Notice Regarding J.E. Corette SteamElectric Station filed March 17, 2015

is DENIED. The facts contained therein are neither generally known

within the court’s territorial jurisdiction nor accurately and readily

determinable from sources whose accuracy cannot reasonably be

questioned. See Fed. R. Evid. 201(b).
32 NAT’L PARKS CONSERVATION ASS’N V. EPA

reliable to inform the decision making process” generally,

77 Fed. Reg. 57,864, 57,868 (Sept. 18, 2012), but also

specifically suitable for predicting visibility improvement in

the pertinent factual context at increments as small as 0.085

deciviews, then we could require no more.

The arbitrary-and-capricious standard of review

authorizes remand where, inter alia, the agency has “entirely

failed to consider an important aspect of the problem,” by

“g[iving] no consideration whatever” to it. Motor Vehicle

Mfrs. Ass’n of the United States, Inc. v. State Farm Mut.

Auto. Ins. Co., 463 U.S. 29, 43, 46 (1983) (emphasis added). 

The “standard of review . . . [requires] only reasonableness,

not perfection.” Kennecott Greens Creek Mining Co. v. Mine

Safety & Health Admin., 476 F.3d 946, 954 (D.C. Cir. 2007);

see also J &G Sales Ltd. v. Truscott, 473 F.3d 1043, 1051–52

(9th Cir. 2007). “[W]here the agency ‘considered the

relevant factors and articulated a rational connection between

the facts found and the choice made,’” we must defer to the

“reasonable basis” of that decision. Arrington v. Daniels,

516 F.3d 1106, 1112 (9th Cir. 2008) (quoting Ranchers

Cattlemen Action Legal Fund v. U.S. Dep’t of Agric., 415

F.3d 1078, 1099 (9th Cir. 2005)). As the Supreme Court has

frequently reiterated, courts “should ‘uphold a decision of

less than ideal clarity if the agency’s path may reasonably be

discerned.’” FCC v. Fox Television Stations, Inc., 556 U.S.

502, 513–14 (2009) (quoting Bowman Transp., Inc. v.

Arkansas-Best Freight Sys., Inc., 419 U.S. 281, 286 (1974)).

Here, as the majority opinion concludes, in light of the

comments received, EPA did not give any adequate, reasoned

explanation as to why CALPUFF supported installation of

SNCR at Colstrip Units 1 and 2. PPL Montana argued in its

comment that the predicted improvement of 0.085 deciviews
NAT’L PARKS CONSERVATION ASS’N V. EPA 33

from SCNR was “within the error range of the model,” and

therefore such improvement could not “reasonably be

anticipated” to result from SNCR. 42 U.S.C. § 7491(g)(2). 

EPA’s generic response, that “the CALPUFF model can both

underpredict and overpredict visibility impacts,” but that it

viewed the model as nonetheless “sufficiently reliable to

inform the decisionmaking process,” does not meaningfully

respond to PPL’s argument. 77 Fed. Reg. at 57,868.

Several kinds of responsive answers to PPL Montana’s

comment about CALPUFF’s margin of error, if supportable,

could have sufficed to enable “the agency’s path . . .

reasonably [to] be discerned.” Fox Television Stations,

556 U.S. at 513–14 (quoting Bowman Transp., 419 U.S. at

286) (internal quotation marks omitted). For example,

perhaps there is a basis to dispute the assertion that a

visibility improvement of 0.085 deciviews fell within the

model’s margin of error. Or perhaps it is significant that the

fact that a predicted improvement falls within a model’s

margin of error does not prove the predicted improvement

will not occur, just that it is less likely to occur. Or perhaps

there is another reason why EPA thought a 0.085 deciview

improvement could “reasonablybe anticipated” to result from

SNCR even if such increment fell within the model’s margin

of error. 42 U.S.C. § 7491(g)(2).

My examples are not meant to put words in EPA’s mouth

(if an agency can be said to have a mouth). But EPA said

nothing of either sort, or anything else responsive to the PPL

CALPUFF comment. Instead, it just professed general

confidence in the CALPUFF model—which may well be

warranted—but was not responsive to the particular concerns

expressed. Courts are just not in a position to provide databased or statistically based explanations, which is one reason
34 NAT’L PARKS CONSERVATION ASS’N V. EPA

why this Court “may not supply a reasoned basis for the

agency’s action that the [expert] agency itself has not given.” 

Motor Vehicle Mfrs. Ass’n, 463 U.S. at 43 (quoting SEC v.

Chenery Corp., 332 U.S. 194, 196 (1947)) (internal quotation

mark omitted).

In short, even under the deferential arbitrary-andcapricious standard of review, it is impossible to say whether

EPA has “‘considered the relevant factors and articulated a

rational connection between the facts found and the choice

made’” with regard to CALPUFF’s ability to predict

improvements as small as 0.085 deciview at Colstrip Units 1

and 2. Arrington, 516 F.3d at 1112 (quoting Ranchers

Cattlemen Action Legal Fund, 415 F.3d at 1093). Perhaps it

did. But it has not, even minimally, provided any indication

that it did in the comments and analysis accompanying the

Rule. For that reason alone, and not because I have any

reason to question whether the CALPUFF model is generally

fully adequate to the purposes for which EPA uses it, I

concur.