                FOR PUBLICATION

  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT


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

PPL MONTANA LLC,                             No. 12-73757
                             Petitioner,
                                               EPA No.
NATIONAL PARKS CONSERVATION                 EPA-R08-OAR-
ASSOCIATION, MONTANA                          2011-0851
ENVIRONMENTAL INFORMATION
CENTER, AND SIERRA CLUB,
                     Intervenors,             OPINION

                  v.

U.S. ENVIRONMENTAL PROTECTION
AGENCY and LISA P. JACKSON,
                     Respondents.


         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 Protection Agency’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 coal-
fired 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 remedy visibility 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 PPL Montana 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 of the 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    Annualized      Tons      Cost
                 Cost        Cost           per      per
                                           Year      Ton
                   COLSTRIP UNIT 1
 SOFA           $4.508     $1.090mm       1,432     $761
                mm
 SOFA +         $13.381    $3.279mm       2,097     $1,564
 SNCR           mm
 SOFA +         $82.772    $10.942mm      3,425     $3,195
 SCR            mm
                    COLSTRIP UNIT 2
 SOFA           $4.508     $1.090mm       1,420     $768
                mm
 SOFA +         $13.381    $3.256mm       2,072     $1,571
 SNCR           mm
 SOFA + SCR     $82.771    $10.920mm      3,376     $3,235
                mm

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 cost-
effectiveness 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 cost-
effective, 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     Annualized      Tons     Cost
                 Cost         Cost           per     per
                                            Year     Ton
                         COLSTRIP I
 Lime           $3.00mm     $1.883mm       3,557     $529
 injection
 Lime           $28.00      $4.100mm       4,486     $912
 injection +    mm
 New
 scrubber
                         COLSTRIP II
 Lime           $3.00mm     $1.883mm       3,212     $586
 injection
 Lime           $28.00      $4.093mm       4,129     $991
 injection +    mm
 New
 scrubber

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 nevertheless still
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 annually to 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, PPL Montana 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 CALPUFF4 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, PPL asserts, 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.”).5 It 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 any Class 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 rulemaking must 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 thirty-
day 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
Valley Station—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
AND DENIED IN PART; VACATED AND REMANDED.



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 Steam Electric 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 “reasonably be 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 data-
based 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-and-
capricious 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.
