                                      PRECEDENTIAL

      UNITED STATES COURT OF APPEALS
           FOR THE THIRD CIRCUIT
                _____________

                    No. 14-3147
                   _____________

NATIONAL PARKS CONSERVATION ASSOCIATION;
     SIERRA CLUB; CLEAN AIR COUNCIL,
                             Petitioners

                          v.

UNITED STATES ENVIRONMENTAL PROTECTION
                AGENCY,
                          Respondent

* Pennsylvania Department of Environmental Protection;
               Homer City Generation LP,
                                     Intervenors


       *(Pursuant to Clerk Order dated 08/21/14)
                    _____________

     On Petition for Review of Final Agency Action
 of the United States Environmental Protection Agency
              (EPA-R03-OAR-2012-0002)
                    ______________

                Argued April 14, 2015
                    ______________

         Before: AMBRO, VANASKIE, and SHWARTZ,
                    Circuit Judges

           (Opinion Filed: September 29, 2015)

David S. Baron, Esq.
Earthjustice Legal Defense Fund
1625 Massachusetts Avenue, N.W.
Suite 702
Washington, DC 20036

Charles McPhedran, Esq. [ARGUED]
Earthjustice
1617 John F. Kennedy Boulevard
Suite 1675
Philadelphia, PA 19103
       Counsel for Petitioners

Kate R. Bowers, Esq. [ARGUED]
United States Department of Justice
Environment and Natural Resources Division
P.O. Box 7611
Ben Franklin Station
Washington, DC 20044

Regina McCarthy, Esq.
Environmental Protection Agency
Ariel Rios Building
1200 Pennsylvania Avenue, N.W.
Washington, DC 20460
       Counsel for Respondent




                            2
Robert A. Reiley, Esq. [ARGUED]
Kristen M. Furlan, Esq.
Pennsylvania Department of Environmental Resources
Office of Chief Counsel
9th Floor
400 Market Street
Harrisburg, PA 17101
       Counsel for Intervenor Pennsylvania Department of
Environmental Protection

Chet M. Thompson, Esq. [ARGUED]
Kirsten L. Nathanson, Esq.
Crowell & Moring
1001 Pennsylvania Avenue, N.W.
Washington, DC 20004
       Counsel for Intervenor Homer City Generation LP
                        ___________

                OPINION OF THE COURT
                     ___________

VANASKIE, Circuit Judge.

        Section 169A of the Clean Air Act, 42 U.S.C. § 7491,
and implementing regulations promulgated by the United
States Environmental Protection Agency (“EPA”) require
states to evaluate the impact that emissions from certain
sources of pollution within their borders have on atmospheric
visibility in national parks and wilderness areas. After
conducting this evaluation, the Commonwealth of
Pennsylvania declined to require its sources to implement
additional pollution controls because it concluded that the




                             3
costs associated with the controls outweighed the limited
visibility improvements they would produce.            The
Commonwealth’s conclusions were set forth in its 2010 State
Implementation Plan (“SIP”), which was approved by the
EPA in 2014.

       Alleging that the EPA’s approval of Pennsylvania’s
SIP was arbitrary and capricious, the National Parks
Conservation Association, Sierra Club, and Clean Air Council
(collectively, “Conservation Groups”) filed the petition for
review presently before the Court. For the reasons that
follow, we will grant the petition in part and deny it in part,
and remand the matter to the EPA for further consideration.

                              I.

           A. Statutory and Regulatory Framework

        In 1970, Congress enacted the Clean Air Act, 42
U.S.C. §§ 7401–7671q, to address the increasing amount of
air pollution created by the industrialization of the United
States and the resulting threat to public health and welfare.
Employing “cooperative federalism,” the Clean Air Act gives
both the federal government and the states responsibility for
maintaining and improving air quality: “the federal
government develops baseline standards that the states
individually implement and enforce.” Bell v. Cheswick
Generating Station, 734 F.3d 188, 190 (3d Cir. 2013)
(citation and quotation marks omitted).

       As originally enacted, the Clean Air Act “did not
elaborate on the protection of visibility as an air-quality
related value.” Chevron U.S.A., Inc. v. EPA, 658 F.2d 271,
272 (5th Cir. 1981) (emphasis added). In 1977, however,




                              4
Congress added § 169A to the Clean Air Act “[i]n response to
a growing awareness that visibility was rapidly deteriorating
in many places, such as wilderness areas and national parks . .
. .” Id. With § 169A, Congress “established as a national
goal the ‘prevention of any future, and the remedying of any
existing, impairment in visibility in mandatory class I areas
which impairment results from man-made air pollution.’”
Am. Corn Growers Ass’n v. EPA, 291 F.3d 1, 3 (D.C. Cir.
2002) (per curiam) (quoting 42 U.S.C. § 7491(a)(1)). The
protected “Class I areas” include certain national parks and
wilderness areas under 42 U.S.C. § 7472(a).1 “Visibility
impairment” means both “reduction in visual range and
atmospheric discoloration.” Id. § 7491(g)(6).

        In connection with § 169A, Congress directed the EPA
to issue regulations to ensure “reasonable progress” toward
the national goal of restoring visibility conditions to their
natural state in Class I areas. Id. § 7491(a)(4). Congress
dictated that the EPA’s regulations require adoption of a State
Implementation Plan (“SIP”) by each state that has a Class I
area within its borders or whose emissions “may reasonably
be anticipated to cause or contribute to any impairment of
visibility” in any Class I area. Id. § 7491(b)(2). Each SIP
must include, inter alia, emission limits, compliance

      1
          There are 156 Class I areas in the United States,
including 47 national parks, 108 wilderness areas, and one
international park. No Class I area is located within
Pennsylvania’s borders. 40 C.F.R. pt. 51, app. Y.; EPA, List
of     156    Mandatory      Class     I   Federal     Areas,
http://www.epa.gov/visibility/class1.html (last visited Aug.
26, 2015).




                              5
schedules, and a long-term strategy for meeting the national
visibility goal. Id. In response to this statutory directive, the
EPA promulgated the Regional Haze Rule in 1999. Regional
Haze Regulations, 64 Fed. Reg. 35,714 (July 1, 1999).2



       2
        The EPA has explained the visibility impairment
known as “regional haze” as follows:

              Regional haze is visibility
              impairment that is produced by a
              multitude of sources and activities
              which are located across a broad
              geographic area and emit fine
              particles (PM2.5) (e.g., sulfates,
              nitrates, organic carbon, elemental
              carbon, and soil dust) and their
              precursors (e.g., sulfur dioxide
              (SO2), nitrogen oxides (NOX), and
              in some cases, ammonia (NH3)
              and volatile organic compounds
              (VOC)). Fine particle precursors
              react in the atmosphere to form
              fine particulate matter, which
              impairs visibility by scattering
              and absorbing light. Visibility
              impairment reduces the clarity,
              color, and visible distance that
              one can see. PM2.5 can also cause
              serious health effects and
              mortality     in    humans     and
              contributes to environmental




                               6
        Section 169A and the Regional Haze Rule also require
each SIP to include a determination of the best available
retrofit technology (“BART”) for certain major stationary
sources of pollution that are reasonably anticipated to cause
or contribute to visibility impairment in any Class I area.
North Dakota v. EPA, 730 F.3d 750, 756 (8th Cir. 2013)
(citing 42 U.S.C. § 7491(b)(2)(A); 40 C.F.R. §§ 51.301,
51.308(e)). BART is defined as “an emission limitation
based on the degree of reduction achievable through the
application of the best system of continuous emission
reduction for each pollutant which is emitted by an existing
stationary facility.” 40 C.F.R. § 51.301.

        To satisfy the BART requirements, a state’s SIP must
first identify all “BART-eligible” sources within its borders.
Under the regulations, a stationary source of air pollution is
BART-eligible if it: (1) was in existence on August 7, 1977,
but not in operation prior to August 7, 1962; (2) fits within
one of 26 identified categories; and (3) has the potential to
emit annually at least 250 tons of any air pollutant. Id.

       Next, a state’s SIP must determine which of these
BART-eligible sources are “subject to BART.” A source is
subject to BART if it “emits any air pollutant which may

             effects such as acid deposition
             and eutrophication.

Approval and Promulgation of Air Quality Implementation
Plans; Commonwealth of Pennsylvania; Regional Haze State
Implementation Plan, 77 Fed. Reg. 3,984, 3,985 (Jan. 26,
2012).




                              7
reasonably be anticipated to cause or contribute to any
impairment of visibility in any mandatory Class I Federal
area.” Id. § 51.308(e)(1)(ii) (emphasis added). The EPA
recommends that a state consider a source to “cause”
visibility impairment if it is responsible for a change in
visibility in a Class I area of at least 1.0 deciview.3 Regional
Haze Regulations and Guidelines for Best Available Retrofit
Technology (BART) Determinations, 70 Fed. Reg. 39,104,
39,118 (July 6, 2005).           The suggested threshold for
determining whether a source “contributes” to visibility
impairment at a level no higher than 0.5 deciviews. Id.

       For each BART-eligible source that is subject to
BART, the state must conduct a source-specific analysis to
determine appropriate emission limitations. In so doing,
states “weigh[] the following five factors: (1) ‘the costs of
compliance’; (2) ‘the energy and non[-]air quality
environmental impacts of compliance’; (3) ‘any existing
pollution control technology in use at the source’; (4) ‘the
remaining useful life of the source’; and (5) ‘the degree of
improvement in visibility which may reasonably be
anticipated to result from the use of such technology.’”

       3
          Changes in visibility are expressed in a standard unit
of measurement known as the deciview. See 40 C.F.R. §
51.301 (stating that the deciview is “a measurement of
visibility impairment” that is “derived from calculated light
extinction, such that uniform changes in haziness correspond
to uniform incremental changes in perception across the
entire range of conditions, from pristine to highly impaired”).
A higher deciview value corresponds with a greater level of
visibility impairment.




                               8
WildEarth Guardians v. EPA, 759 F.3d 1064, 1068 (9th Cir.
2014) (quoting 42 U.S.C. § 7491(g)(2); 40 C.F.R. pt. 51, app.
Y).

        To aid states in identifying BART-eligible sources and
determining appropriate emission limitations, the EPA issued
the BART Guidelines, 70 Fed. Reg. 39,156. WildEarth
Guardians, 759 F.3d at 1068. The Guidelines, issued in
2005, provide states with a five-step process for making their
source-specific BART determinations, and these five steps
subsume the statutory considerations listed above. Id. at
1068–69 (citing 70 Fed. Reg. 39,127). Under the Guidelines,
a state is to first identify all available retrofit control
technologies. Second, technically infeasible options are
eliminated. Third, the effectiveness of the remaining control
techniques is assessed. Fourth, the impacts, including the
cost of compliance, energy impacts, non-air quality impacts,
and the remaining useful life of the facility, are evaluated.
Finally, a state must estimate the visibility impacts at Class I
areas. Id. at 1069 (citing 70 Fed. Reg. 39,164, 39,166).
While states are required to use the Guidelines when making
BART determinations for any fossil fuel-fired power plant
with a total electricity generating capacity of 750 megawatts
or more, the Guidelines are advisory for smaller BART-
eligible sources. Id. (citing 42 U.S.C. § 7491(b)(2)(B); 40
C.F.R. § 51.308(e)(1)(ii)(B)).

       As an alternative to conducting this source-specific
analysis, states may instead implement another program if
they can demonstrate it is “better-than-BART” at reducing
emissions. Specifically, the regional haze regulations permit
a state to “opt to implement or require participation in an
emissions trading program or other alternative measure” if it
can show that the program would result in “greater reasonable



                               9
progress” toward the national goal of restoring natural
visibility “than would be achieved through the installation
and operation of BART.” 40 C.F.R. § 51.308(e)(2). States
participating in such programs do not have to conduct a
source-specific BART analysis or compel pollution sources
within their borders to install, operate, and maintain BART at
their facilities. Id.

       Regardless of whether a state conducts the source-
specific BART analysis or follows the better-than-BART
approach, it must ultimately submit its SIP to the EPA. The
EPA, in turn, must review the SIP and determine whether it
meets the requirements of the Clean Air Act. 42 U.S.C. §
7410(a)(1). The EPA is required to approve a SIP as a whole
if it meets all the statutory requirements, and it may approve
any portion of a SIP that meets the requirements. Id. at §
7410(k)(3). If a state fails to submit a SIP, submits an
incomplete SIP, or submits a SIP that does not meet the
statutory requirements, the EPA must enact its own Federal
Implementation Plan (“FIP”), unless the state can provide a
SIP that the EPA can approve within two years. North
Dakota, 730 F.3d at 757 (citing 42 U.S.C. § 7410(c)).

                  B. Procedural Background

       Pennsylvania submitted its regional haze SIP to the
EPA in December 2010, identifying 34 BART-eligible
sources of pollution within its borders. App. 43–171. These
pollution sources—various power plants, mills, refineries, and
other facilities around the state—emit visibility-impairing
particulate matter (“PM”) into the atmosphere, as well as the
chemical precursors to PM, which include sulfur dioxide
(“SO2”) and oxides of nitrogen (“NOx”). Pennsylvania
elected to treat each of these 34 BART-eligible sources as




                             10
subject to BART,4 and it opted to follow the five-step process
outlined in the Guidelines for making source-specific BART
determinations.5 Pennsylvania, however, chose to follow the
better-than-BART approach with respect to the eight fossil
fuel electric generating stations with a capacity of 750
megawatts or more.

Thus, Pennsylvania conducted a source-specific BART
analysis regarding the SO2 and NOx emissions of each source
with an electricity generating capacity below 750 megawatts,
but did not do so for the fossil fuel electric generating stations
having a capacity of 750 megawatts or more. Pennsylvania
noted that these sources participated in the “cap and trade”
program6 for SO2 and NOx emissions established by EPA


       4
           This practice ensures that a BART analysis is
conducted for every BART-eligible source, even if the
deciview impact from the source is not high enough that the
source would be considered to “cause” or “contribute” to
visibility impairment in any Class I area under 40 C.F.R. §
51.308(e)(1)(ii).
       5
        Pennsylvania was obligated to follow the Guidelines
for each of the eight fossil fuel-fired power plants in the state
that have electricity generating capacity of at least 750
megawatts, but the Guidelines were advisory for the
remaining BART-eligible sources.           See 42 U.S.C. §
7491(b)(2)(B); 40 C.F.R. § 51.308(e)(1)(ii)(B).
       6
         A cap and trade program is an environmental policy
tool that involves setting a mandatory cap on emissions while
providing pollution sources with flexibility as to how they




                               11
Clean Air Interstate Rule (“CAIR”), 70 Fed. Reg. 25,162
(May 12, 2005), and concluded that the sources’ participation
in the cap and trade program was better than BART at
reducing such emissions.

       Ultimately, Pennsylvania’s SIP found that requiring
additional emission controls at any of the 34 BART-eligible
sources would result in only minimal visibility improvement
in affected Class I areas.         Weighing this minimal
improvement against the cost of implementing the controls,
Pennsylvania concluded that additional controls were not
warranted.

       In January 2012, the EPA issued a proposed rule
providing for a limited approval of Pennsylvania’s SIP
(“2012 Proposed Rule”). Approval and Promulgation of Air
Quality Implementation Plans;             Commonwealth       of
Pennsylvania; Regional Haze State Implementation Plan, 77
Fed. Reg. 3,984 (Jan. 26, 2012). The EPA concluded that
Pennsylvania’s BART analysis complied with the statutory
requirements of the Clean Air Act and the regional haze
regulations.    However, the EPA declined to address
Pennsylvania’s reliance on the better-than-BART CAIR
program regarding SO2 and NOx emissions for certain
pollution sources, noting that particular issue was the subject
of a separate rulemaking proceeding.           The EPA also
announced a one-month period for interested parties to
comment on the 2012 Proposed Rule.


comply with the cap.         See EPA, Cap and Trade,
http://www.epa.gov/captrade (last visited Aug. 26, 2015).




                              12
        On June 7, 2012, the EPA issued its final rule (the
“National Rule”) in the separate proceeding referenced by the
2012 Proposed Rule, disapproving the SIPs submitted by
Pennsylvania and 14 other states to the extent they relied on
the CAIR program to limit SO2 and NOx emissions. Regional
Haze: Revisions to Provisions Governing Alternatives to
Source-Specific Best Available Retrofit Technology (BART)
Determinations, Limited SIP Disapprovals, and Federal
Implementation Plans, 77 Fed. Reg. 33,642 (June 7, 2012).
With this disapproval, the EPA also promulgated FIPs for 13
of the states (including Pennsylvania), effectively replacing
the states’ reliance on the CAIR program with reliance on the
newly promulgated Cross-State Air Pollution Rule, better
known as the Transport Rule. By issuing the National Rule,
the EPA also finalized its conclusion that the Transport Rule
was better-than-BART at reducing SO2 and NOx emissions,
and that it addressed the shortcomings of the CAIR program
previously identified by the United States Court of Appeals
for the District of Columbia Circuit.7


      7
         The EPA initially promulgated CAIR in 2005, but the
D.C. Circuit vacated the rule in 2008, noting multiple fatal
flaws not pertinent to the present case. North Carolina v.
EPA, 531 F.3d 896, 921 (D.C. Cir. 2008) (per curiam). On
rehearing, the D.C. Circuit elected to leave CAIR in place
while the EPA crafted a new program to address CAIR’s
deficiencies. North Carolina v. EPA, 550 F.3d 1176, 1178
(D.C. Cir. 2008) (per curiam).          EPA responded by
promulgating the Transfer Rule. The D.C. Circuit vacated
this rule in 2012, EME Homer City Generation, L.P. v. EPA,
696 F.3d 7, 37 (D.C. Cir. 2012), but the Supreme Court later




                             13
       Shortly thereafter, on July 13, 2012, the EPA finalized
its limited approval of Pennsylvania’s SIP. Approval and
Promulgation of Air Quality Implementation Plans;
Pennsylvania; Regional Haze State Implementation Plan, 77
Fed. Reg. 41,279 (July 13, 2012). With this “2012 Final
Rule,” the EPA responded to comments regarding the 2012
Proposed Rule and reaffirmed its conclusion that
Pennsylvania’s BART analysis was proper.

       In response to the 2012 Final Rule, the Conservation
Groups filed a petition for review with this Court, challenging
the rule on a number of fronts. Nat’l Parks Conservation
Assoc. v. EPA, No. 12-3534. We did not reach the merits of
the petition, though, since the EPA filed a motion for
voluntary remand without vacatur in order to consider and
respond in greater detail to the Conservation Groups’
concerns. We granted the motion on October 22, 2013, and
remanded the matter to the EPA.

       Following remand, the EPA entered a final rule on
April 30, 2014 (“2014 Final Rule”), reissuing its limited
approval of Pennsylvania’s SIP. Approval and Promulgation
of Air Quality Implementation Plans; Pennsylvania; Regional
Haze State Implementation Plan, 79 Fed. Reg. 24,340 (Apr.
30, 2014). With this rule, the EPA expanded its responses to
certain comments and acknowledged numerous deficiencies
in Pennsylvania’s source-specific BART analysis. In the end,
however, the EPA approved the SIP, finding that

overturned the decision, upheld the Transport Rule, and
remanded for further proceedings. EPA v. EME Homer City
Generation, L.P., 134 S. Ct. 1584, 1609–10 (2014).




                              14
Pennsylvania reasonably concluded that no additional
pollution controls were required at the 34 BART-eligible
sources given the low visibility impact of the sources in Class
I areas and the high cost of implementing the controls.

       This petition for review followed, with the
Conservation Groups alleging that the EPA arbitrarily and
capriciously approved Pennsylvania’s SIP. We subsequently
granted motions to intervene filed by the Pennsylvania
Department of Environmental Protection (the state agency
responsible for drafting Pennsylvania’s SIP) and Homer City
Generation, L.P., a coal-fired power plant in Indiana County,
Pennsylvania.

                              II.

        Under § 307(b)(1) of the Clean Air Act, we have
jurisdiction to review a final EPA action that is “locally or
regionally applicable” within our Circuit. 42 U.S.C. §
7607(b)(1); GenOn REMA, LLC v. EPA, 722 F.3d 513, 519
(3d Cir. 2013). However, a petition for review regarding any
“nationally applicable regulations promulgated, or final action
taken, by the Administrator [of the EPA] . . . may be filed
only in the United States Court of Appeals for the District of
Columbia.” 42 U.S.C. § 7607(b)(1) (emphasis added).

       When reviewing a final EPA action, we must
“determine whether it is ‘arbitrary, capricious, an abuse of
discretion, or otherwise not in accordance with the law.’”
GenOn REMA, 722 F.3d at 525 (quoting 42 U.S.C. §
7607(d)(9)(A)). While this is a narrow and deferential
standard of review, Motor Vehicle Mfrs. Ass’n v. State Farm
Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983), we must
nevertheless ensure that the EPA “examined the relevant data




                              15
and articulated a satisfactory explanation for its action,
including a rational connection between the facts found and
the choice made.” Prometheus Radio Project v. FCC, 373
F.3d 372, 389–90 (3d Cir. 2004) (citation and quotation
marks omitted).

                             III.

                      A. Transport Rule

        The Conservation Groups challenge the EPA’s
decision to allow Pennsylvania to rely on the Transport Rule
in lieu of conducting a source-specific BART analysis
regarding SO2 and NOx emissions from each source with an
electricity generating capacity of at least 750 megawatts. In
particular, they argue that the Transport Rule is not better-
than-BART at reducing SO2 and NOx emissions, has not been
implemented as the EPA assumed it would be when it
permitted Pennsylvania to rely on the rule, and is subject to
further delays and legal challenges.

       The EPA counters that this appeal is not the
appropriate vehicle to challenge its finding that the Transport
Rule is better-than-BART or its decision to approve states’
reliance on this rule, as both these determinations stem from a
final rule and separate rulemaking proceeding not presently
before this Court. Moreover, the EPA argues that under 42
U.S.C. § 7607(b)(1), the Conservation Groups must pursue
any such challenge in the D.C. Circuit. We agree with the
EPA on both points.

      Following extensive administrative proceedings, the
EPA issued its National Rule on June 7, 2012. 77 Fed. Reg.
33,642. With it, the EPA finalized the emissions-limiting




                              16
Transport Rule, a replacement to the CAIR program that had
been invalidated by the D.C. Circuit in North Carolina v.
EPA, 531 F.3d 896, 921 (D.C. Cir. 2008) (per curiam). The
National Rule included the finding that the emission trading
programs established by the Transport Rule are better-than-
BART. 77 Fed. Reg. 33,643 (“In this action, the EPA is
finalizing our finding that the trading programs in the
Transport Rule . . . achieve greater reasonable progress
towards the national goal of achieving natural visibility
conditions in Class I areas than source-specific . . . (BART) in
those states covered by the Transport Rule.”). The EPA also
finalized its disapproval of the SIPs submitted by
Pennsylvania and 14 other states to the extent they relied on
the CAIR program to limit SO2 and NOx emissions, and
promulgated FIPs for 13 states (including Pennsylvania),
effectively replacing the states’ reliance on the CAIR
program with reliance on the newly promulgated Transfer
Rule. Id.

       By contrast, the 2014 Final Rule, which the
Conservation Groups challenge here, does not address the
merits of the Transport Rule or Pennsylvania’s reliance on it.
Instead, it notes those issues were addressed in a “separate but
related action,” referring to the National Rule. See 79 Fed.
Reg. 24,340–41. Prior to issuing the 2014 Final Rule, the
EPA repeatedly explained that the propriety of the Transport
Rule, the CAIR program, and Pennsylvania’s reliance on the
Transport Rule or the CAIR program were beyond the scope
of these rulemaking proceedings. See, e.g., 2012 Final Rule,
77 Fed. Reg. 41,282 (“Comments related to [the Transport
Rule] as an alternative to BART for [electricity generating
units] are beyond the scope of this rulemaking. The EPA
addressed similar comments concerning the Transport Rule as




                              17
a BART alternative in [the National Rule].”); 2012 Proposed
Rule, 77 Fed. Reg. 3,984 (“[W]e are not taking action in this
notice to address the Commonwealth’s reliance on CAIR to
meet certain regional haze requirements.”).

        In short, the Conservation Groups seek to use this
appeal from the administrative proceedings that culminated in
the 2014 Final Rule to challenge decisions the EPA reached
in separate proceedings. We find no support for this approach
in the text of the Clean Air Act provision authorizing judicial
review of EPA actions. See 42 U.S.C. § 7607(b)(1).
Additionally, as the administrative record upon which these
decisions were made is not before us, we lack the information
necessary to evaluate the EPA’s action regarding the
Transport Rule. See Fed. Power Comm’n v. Transcontinental
Gas Pipe Line Corp., 423 U.S. 326, 331 (1976) (stating that
“we have consistently expressed the view that ordinarily
review of administrative decisions is to be confined to
consideration of the decision of the agency . . . and of the
evidence on which it is based”) (citation and quotation marks
omitted). Accordingly, we cannot entertain the Conservation
Groups’ challenge to the Transport Rule.

        Moreover, even if the Conservation Groups could use
this appeal to challenge the Transport Rule, we are not the
proper court to hear the challenge. Under 42 U.S.C. §
7607(b)(1), petitions for review of “nationally applicable
regulations promulgated, or final action taken, by the
Administrator [of the EPA] . . . may be filed only in the [D.C.
Circuit].” Id. (emphasis added). We conclude that the EPA’s
National Rule, which finalized the Transport Rule (applicable
to 28 states and the District of Columbia) and resulted in 13
FIPs permitting various states to rely on the Transport Rule,
falls into this category. See Texas v. EPA, No. 10-60961,



                              18
2011 WL 710598, at *5 (5th Cir. Feb. 24, 2011)
(unpublished) (“Our conclusion today—that an EPA action
involving the SIPs of numerous far-flung states is ‘nationally
applicable’ and thus reviewable only in the D.C. Circuit—is
consistent with the holdings of our sister circuits to have
considered the question.”); W. Va. Chamber of Commerce v.
Browner, No. 98-1013, 1998 WL 827315, at *4 (4th Cir. Dec.
1, 1998) (unpublished) (“An EPA rule need not span ‘from
sea to shining sea’ to be nationally applicable.”) (footnote
omitted); Puerto Rican Cement Co. v. EPA, 889 F.2d 292,
299–300 (1st Cir. 1989) (finding EPA regulations to be
“nationally applicable” where they applied to any SIP “that
ha[d] been disapproved with respect to prevention of
significant deterioration of air quality in any portion of any
State where the existing air quality is better than the national
ambient air quality standards,” and the list of states governed
by the regulations changed as SIPs were approved and
disapproved by the EPA).8

       Accordingly, we will deny the Conservation Groups’
petition for review to the extent it challenges the Transport
Rule or Pennsylvania’s reliance on it.



       8
         What’s more, even the Conservation Groups appear
to recognize that their challenge to the Transport Rule should
be heard by the D.C. Circuit: the National Parks Conservation
Association and Sierra Club are participants in consolidated
appeals challenging the Transport Rule that are currently
pending before the D.C. Circuit. See Util. Air Regulatory
Grp. v. EPA, No. 12-1342 (D.C. Cir.).




                              19
              B. Source-Specific BART Analysis

       The Conservation Groups also contend that
Pennsylvania’s source-specific BART analysis failed to
comply with the Guidelines in many respects, and that the
EPA violated the Clean Air Act by arbitrarily approving
Pennsylvania’s SIP despite these fatal flaws. The EPA
counters that Pennsylvania’s analysis was largely proper, and
that the errors it committed did not affect the reasonableness
of the state’s decision not to require its BART-eligible
sources to implement additional pollution controls. In what
resembles a harmless-error argument, the EPA asserts that,
despite Pennsylvania’s flawed analysis, the resulting overall
picture supported its ultimate decision. As discussed below,
while we reject some of the arguments advanced by the
Conservation Groups, we are nevertheless compelled to
conclude that the EPA arbitrarily approved Pennsylvania’s
SIP given the multiple flaws in Pennsylvania’s BART
analysis and the EPA’s insufficient explanation as to why it
could overlook them.

         1.   Identification of All Available Retrofit Control
              Technologies

       The Conservation Groups contend that Pennsylvania
failed to satisfy the BART requirement of identifying all
available pollution control technologies. In particular, they
argue that the state did not consider upgrades to existing
electrostatic precipitator (“ESP”) control technologies for
BART-eligible power plants within the state, or other
available combinations of controls.

     The EPA counters that Pennsylvania’s SIP notes that
ESP upgrades were considered for all but two power plants,




                             20
and that Pennsylvania had declined to consider upgrades at
those two facilities because they had recently installed “state-
of-the-art” ESP controls.        The EPA also argues that
Pennsylvania did consider combinations of controls,
including fabric filters on sources where technically feasible.

        While we agree with the EPA that Pennsylvania’s SIP
states that upgrades and combinations were considered, we
cannot discern from the administrative record the specifics of
Pennsylvania’s analysis or why it rejected certain upgrades or
combinations. As the Conservation Groups noted in their
comments to the 2012 Final Rule, App. 487, Pennsylvania’s
SIP states in conclusory fashion that ESP upgrades,
enhancements, or replacements were considered for certain
sources. See, e.g., App. 221 (stating that “[t]he retrofit
technologies reviewed” during the course of the BART
analysis for the Mitchell Power Station “included fuel-related
modifications, ESP upgrades, enhancements or replacement,
replacement of the ESPs with fabric filters or compact hybrid
particulate collectors”). What the SIP fails to do, however, is
identify or describe the upgrades considered or explain why
these controls were rejected. Similarly, the EPA has failed to
explain—either in the 2014 Final Rule or now on appeal—
how it could meaningfully evaluate Pennsylvania’s analysis
described in such conclusory fashion. We acknowledge that
EPA and BART regulations do not require exhaustive
analysis of every conceivable emissions control. See 40
C.F.R. pt. 51, app. Y § IV.D. n.12 (explaining that “[i]t is not
necessary to list all permutations of available control levels
that exist for a given technology”). Nonetheless, the EPA has
failed to satisfactorily explain why the SIP’s conclusory
listings are acceptable.

         2.   Baseline Level for PM Emissions



                              21
        The     Conservation    Groups     next     challenge
Pennsylvania’s source-specific BART analysis regarding PM
emissions from 13 power plants. Specifically, they contend
the state improperly concluded that the filterable emission
limit of 0.1 pound of particulate matter per million British
thermal units (“0.1 lb/MMBtu”) represents BART for those
facilities.9 The Conservation Groups argue the limit is not
sufficiently stringent, and note that lower limits (between
0.07 lb/MMBtu and 0.012 lb/MMBtu) have qualified as
BART at other facilities.       In short, they assert that
Pennsylvania had no reasoned basis for selecting the emission
limit that it did, and that the EPA arbitrarily approved
Pennsylvania’s BART analysis regarding PM emissions
predicated on this threshold.

      In the 2014 Final Rule, the EPA concedes that
Pennsylvania failed to determine whether the 0.1 lb/MMBtu
emission limit actually represents BART for those facilities.
See 79 Fed. Reg. 24,344 (“Here, Pennsylvania determined
that PM BART for most of the subject-to-BART [electricity
generating units] was their existing permitted emission limits

       9
         After a state has identified the best available control
technology for reducing emissions at a particular source, it
must then set an “emission limit.” This limit represents the
emission-reduction capabilities of the identified control
technology. See 2014 Final Rule, 79 Fed. Reg. 24,344
(stating that “once a state has selected a control technology
that represents BART, the state must then complete the
BART analysis by selecting an emission limit that represents
the emission-reduction capabilities of that control
technology”).




                              22
of 0.1 lb/MMBtu, which can be achieved by the existing
[control technology]. While the EPA agrees with the
commenter that Pennsylvania ideally should have examined
whether 0.1 lb/MMBtu actually reflects the ‘degree of
reduction achievable’ for the particular [control technology]
at each facility, EPA thinks that Pennsylvania’s failure to do
so was not fatal in this instance . . . .”) (footnote omitted).
The EPA excuses this failure for two reasons. First, it argues
that Pennsylvania’s error was essentially harmless, as
imposing a stricter PM emission limit on these sources would
have minimal visibility impact in Class I areas since the PM
emissions from these sources were responsible for only a
minimal portion of the visibility impairment in these areas.
Second, the EPA claims that the issue is “largely moot[].” Id.
at 24,345. Specifically, the agency notes that many of these
13 power plants have retired or put in motion plans to retire
or convert to cleaner burning fuels since Pennsylvania
conducted its BART determinations. The EPA also notes that
the remaining sources will have to comply with a more
stringent PM emission limit of 0.03 lb/MMBtu by 2015 due
to the implementation of the Mercury and Air Toxics
Standards (“MATS”) Rule. Id. at 24,344.

       We find the EPA’s arguments unconvincing. As
discussed in greater detail infra, Part III.B.7, the EPA’s claim
of harmless error is unpersuasive since the agency has offered
scant justification for this position, apart from its own
assurances that the multiple flaws in Pennsylvania’s analysis
did not impact the reasonableness of its conclusions.
Similarly, the EPA has not identified, nor have we located,
any legal support for the EPA’s contention that it may excuse
errors in a state’s BART analysis as moot based on events
that are yet to transpire. To the contrary, the EPA has a




                              23
statutory obligation to disapprove a SIP that does not comply
with the Clean Air Act and to promulgate a FIP if the
deficiencies are not timely cured. See 42 U.S.C. § 7410(k)
(requiring the EPA to review SIPs to ensure compliance); id.
§ 7410(l) (prohibiting the EPA from approving a revision to a
SIP if it would interfere with any applicable requirement of
the Clean Air Act).

            3.   Alternative Pollution Control Limits: BACT,
                 LAER, and MACT

       The Conservation Groups also contend Pennsylvania’s
BART analysis regarding PM emissions did not comply with
the Guidelines because the state did not consider more
stringent emission limits developed as part of separate air
quality permitting processes under the Clean Air Act. In
particular, they argue that limits imposed by other
programs—known as best available control technology
(“BACT”), lowest achievable emission rate (“LAER”), and
maximum achievable control technology (“MACT”)—are
relevant to the BART analysis because they demonstrate
achievable emission reductions.10


       10
           BACT is “an emission limitation based on the
maximum degree of reduction of each pollutant . . . which the
permitting authority, on a case-by-case-basis, taking into
account energy, environmental, and economic impacts and
other costs, determines is achievable for [the] facility . . . .”
42 U.S.C. § 7479(3). Under the Clean Air Act’s Prevention
of Significant Deterioration program, no new major air
pollutant emitting facility may be constructed unless the
facility is equipped with BACT. Alaska Dep’t of Envtl.




                               24
       In response, the EPA notes that the BART Guidelines
do not require states to consider the exact emission limits
determined to be BACT and LAER. Instead, they must
consider the technologies used to achieve BACT and LAER
when conducting the first step of the BART analysis:
identifying all available control technologies for their
pollution sources. See BART Guidelines, 40 C.F.R. pt. 51,
app. Y (“Technologies required as BACT or LAER are
available for BART purposes and must be included as control
alternatives.”) (emphasis added). Moreover, the EPA notes
that the stringent emission levels determined to be BACT or
LAER are not necessarily achievable by BART-eligible
sources because those programs apply to new and newly



Conservation v. EPA, 540 U.S. 461, 468 (2004). In
“nonattainment areas”—areas that are not in attainment with
the Clean Air Act’s National Ambient Air Quality
Standards—new and modified pollution sources are required
to install LAER, which is more stringent than BACT. See
Citizens Against Ruining the Env’t v. EPA, 535 F.3d 670, 673
n.3 (7th Cir. 2008). Under the Clean Air Act’s National
Emission Standards for Hazardous Air Pollutants program,
the EPA imposes MACT on major sources of certain
hazardous air pollutants. MACT “must reflect ‘the maximum
degree of reduction in emissions’ that the EPA determines is
‘achievable,’ taking into consideration ‘the cost of achieving
such emission reduction, and any non-air quality health and
environmental impacts and energy requirements.’” Nat’l Res.
Def. Council v. EPA, 749 F.3d 1055, 1057 (D.C. Cir. 2014)
(quoting 42 U.S.C. § 7412(d)(2)).




                             25
modified sources, while BART governs pollution sources
constructed before 1977.

       The EPA also notes that, for sources of PM emissions
that are subject to MACT standards, the BART Guidelines
permit—but do not require—states to rely on the stringent
MACT standards for purposes of BART. In other words, the
Guidelines create a presumption that a state’s reliance on the
MACT standards satisfies BART, but they do not require the
state to rely on the MACT standard to satisfy BART. See
BART Guidelines, 40 C.F.R. pt. 51, app. Y (“We believe that,
in many cases, it will be unlikely that States will identify
emission controls more stringent than the MACT standards
without identifying control options that would cost many
thousands of dollars per ton.        Unless there are new
technologies subsequent to [issuance of] the MACT standards
which would lead to cost-effective increases in the level of
control, you may rely on the MACT standards for purposes of
BART.”).

      We agree with the EPA’s reading of the BART
Guidelines on these points. As a result, we reject the
Conservation Groups’ contention that Pennsylvania
improperly failed to consider BACT, LAER, and MACT
emission limitations.

         4.   Cost-Effectiveness Threshold

       The Conservation Groups argue that Pennsylvania
failed to properly evaluate the cost-effectiveness of the
pollution controls available for each BART-eligible source.
In particular, they note that Pennsylvania did not set a
“threshold” for cost-effectiveness—that is, an amount of
money at which it would reject any available control option




                             26
as too expensive. Absent such a threshold, the Conservation
Groups contend, Pennsylvania had no principled way of
determining when a pollution control was a cost-effective
method of improving visibility in affected Class I areas.

        The EPA asserts that nothing in the Clean Air Act
requires Pennsylvania to set a fixed threshold of cost-
effectiveness, and that the Guidelines make no mention of
such a threshold in their instructions on how to evaluate cost-
effectiveness. See BART Guidelines, 40 C.F.R. pt. 51, app.
Y; Nat’l Parks Conservation Ass’n v. EPA, 788 F.3d 1134,
1142 (9th Cir. 2015) (“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.”). Instead of drawing a line in the sand on cost-
effectiveness, the EPA notes that Pennsylvania’s SIP
appropriately determined that pollution “sources with a higher
degree of potential visibility improvement from control would
justify higher cost controls,” and that “only low cost controls
would be justified for sources with a lower degree of potential
visibility improvement.” App. 100.

      Because we agree that Pennsylvania was not
compelled to set a threshold for cost-effectiveness, we
conclude that the EPA did not act arbitrarily by approving
Pennsylvania’s SIP absent such a threshold.

         5.   Cost-Effectiveness Metric

       The Conservation Groups also assert that Pennsylvania
used an improper metric when calculating the cost-
effectiveness of additional pollution controls. Specifically,
they argue that Pennsylvania evaluated the cost of controls
based on the dollars-per-deciview metric rather than the




                              27
dollars-per-ton metric required by the Guidelines.11 The
Conservation Groups contend that Pennsylvania’s use of the
dollars-per-deciview metric distorted the true cost of pollution
controls and led to the state’s conclusion that additional
pollution controls were not warranted at any of the BART-
eligible sources.

       In responding to this argument during the notice-and-
comment period and now on appeal, the EPA has taken
seemingly inconsistent positions. In the text of the 2014 Final
Rule, the EPA states, without elaboration, that Pennsylvania’s
use of the dollars-per-deciview metric was “flawed.” 2014
Final Rule, 79 Fed. Reg. 24,342 (stating that “EPA agrees
with the commenters that Pennsylvania’s reliance on the
[dollars-per-deciview] metric was flawed for multiple
reasons”).     On appeal, however, the EPA responds that
the Guidelines specify that cost-effectiveness calculations be
expressed in terms of dollars-per-ton, but they do not forbid
the consideration of the dollars-per-deciview metric as well.12

       11
          As its name implies, the dollars-per-ton metric is a
measurement of the costs associated with removing a ton of a
particular pollutant from a source’s emission. The dollars-
per-deciview metric, by contrast, considers the costs
associated with pollution reduction that would result in a 1.0
deciview visibility improvement. The dollars-per-ton metric
is frequently abbreviated as “$/ton,” while the dollars-per-
deciview metric is abbreviated as “$/dv.”
       12
          As the Tenth Circuit has noted, the Guidelines
“permit the BART-determining authority to use dollar per
deciview as an optional method of evaluating cost
effectiveness.” Oklahoma v. EPA, 723 F.3d 1201, 1221 (10th




                              28
Cir. 2013) (citing 40 C.F.R. pt. 51, app. Y(IV)(E)(1)). As to
the issue of whether states are required to use the dollars-per-
ton metric in evaluating cost-effectiveness, however, “[t]he
guidelines themselves are a bit unclear.” Id. at 1221 n.13.
The Tenth Circuit explains:

              In     the   section     on    cost
              effectiveness,    the    guidelines
              mention only the dollar-per-ton
              metric. 40 C.F.R. pt. 51 app.
              Y(IV)(D)(4)(c).      However, the
              guidelines later state that, in
              evaluating     alternatives,   “we
              recommend you develop a chart
              (or charts) displaying for each of
              the alternatives” that includes,
              among other factors, the cost of
              compliance         defined       as
              “compliance—total        annualized
              costs ($), cost effectiveness
              ($/ton), and incremental cost
              effectiveness ($/ton), and/or any
              other cost-effectiveness measures
              (such as $/deciview).” Id. app.
              Y(IV)(E)(1) (emphasis added).

Id.




                              29
The EPA also notes that Pennsylvania considered both
metrics with respect to 33 of its 34 BART-eligible sources.
Resp. Br. 46.

        Our review of the EPA’s decision is limited to the
reasoning supplied in its final rule, not the justifications
subsequently crafted and proffered by the agency’s appellate
counsel. See Motor Vehicle Mfrs. Ass’n, 463 U.S. at 50 (“It is
well-established that an agency’s action must be upheld, if at
all, on the basis articulated by the agency itself.”) (citations
omitted); Safe Air for Everyone v. EPA, 488 F.3d 1088, 1091
(9th Cir. 2007) (stating that “our review of an administrative
agency’s decision begins and ends with the reasoning that the
agency relied upon in making that decision”). As a result, we
are left with the EPA’s conclusion that Pennsylvania’s use of
the dollars-per-deciview metric is “flawed” in multiple
unidentified respects and no meaningful explanation as to
why the EPA ignored these flaws. This rationale is
insufficient to justify the EPA’s approval of Pennsylvania’s
analysis of cost-effectiveness.

         6.   Cumulative Visibility Impact

        As part of its source-specific BART analysis,
Pennsylvania was required to calculate the visibility
improvement that could be achieved in Class I areas by
implementing additional pollution controls at its BART-
eligible sources. The state’s calculations for each source,
however, took into account only the potential impact such
controls would have on the visibility in the Class I area most
severely impacted by the source. Pennsylvania did not
consider the “cumulative visibility impact”—that is, it did not
calculate the total visibility improvement for all affected
Class I areas that would result from installing additional




                              30
controls at each source. As a result, the Conservation Groups
argue, Pennsylvania underestimated the visibility impact of
each source and, correspondingly, underestimated the cost-
effectiveness of additional control technologies.

       In the 2014 Final Rule, the EPA admits that
Pennsylvania should have calculated the cumulative visibility
impact from its sources. 79 Fed. Reg. 24,342 (“EPA also
agrees with the commenters that, in considering the visibility
improvement expected from the use of controls, Pennsylvania
should have taken into account the visibility impacts at all
impacted Class I areas rather than focusing solely on the
benefits at the most impacted area.”). The EPA contends this
error, among others, was harmless, a contention we address
below.

         7.   Harmless Error

        To justify its approval of Pennsylvania’s admittedly
flawed BART analysis, the EPA advances a harmless error
argument. In particular, the EPA contends it reasonably
approved Pennsylvania’s conclusion that pollution controls
were not warranted as the overall picture that emerged from
the state’s analysis demonstrated that the improvement in
visibility at affected Class I areas as a result of the controls
would be minimal. Based on the administrative record before
us, however, that conclusion is a bridge too far.

       In the 2014 Final Rule, the EPA concedes that
Pennsylvania’s BART determinations contained “systemic
deficiencies” and a “large number” of errors. 79 Fed. Reg.
24,341, 24,343 (quotation marks omitted). On a broad scale,
the EPA acknowledges that Pennsylvania’s SIP lacked
necessary     technical    information   and    supporting




                               31
documentation, and that it was insufficiently thorough. Id. at
24,342 (noting that “many of the comments criticizing
Pennsylvania’s BART determinations are correct,” and that
“the Pennsylvania regional haze SIP contains very limited
information describing Pennsylvania’s analyses and
consideration of the BART factors”); id. (stating
“Pennsylvania should have provided a more thorough and
detailed analysis of costs and visibility impacts in its regional
haze SIP”). More specifically, the EPA concedes that
Pennsylvania erred at multiple steps of the BART analysis.
For example, by failing to consider the cumulative visibility
impact of each source, Pennsylvania understated the impact
that pollution originating within its borders had on Class I
areas beyond those borders. Id. (“EPA also agrees . . . that . .
. Pennsylvania should have taken into account the visibility
impacts at all impacted Class I areas rather than focusing
solely on the benefits at the most impacted area.”). The EPA
also admits that Pennsylvania’s cost-effectiveness
calculations were flawed. Id. (“Similarly, EPA agrees with
the commenters that Pennsylvania’s reliance on the $/dv
metric was flawed for multiple reasons.”); id. (agreeing with
the commenters “that many of the [pollution] controls under
consideration [by Pennsylvania] were likely cost-effective
measures,” even though the state rejected them as too
expensive).

       Tellingly, the EPA concedes that these various failures
impaired its ability to independently assess Pennsylvania’s
analysis. In the agency’s own words, it has a duty under the
Clean Air Act “to exercise independent technical judgment in
evaluating the adequacy of a state’s regional haze SIP,
including its BART determinations.” Approval, Disapproval
and Promulgation of Implementation Plans; State of




                               32
Wyoming; Regional Haze State Implementation Plan;
Federal Implementation Plan for Regional Haze, 79 Fed.
Reg. 5,032, 5,064 (Jan. 30, 2014). Here, however, with
respect to the control technologies considered by
Pennsylvania and the costs associated with those controls, the
EPA concedes that “the cursory information available in the
record does not allow for an assessment of how these
numbers were derived or whether Pennsylvania’s analyses
were reasonably done.” 2014 Final Rule, 79 Fed. Reg.
24,342. Regarding Pennsylvania’s determination of potential
visibility improvements in Class I areas, the EPA similarly
notes that “it is difficult to assess the estimates of the
improvements in visibility associated with various controls
given the limited information in the SIP as to the assumptions
relied on in the modeling and the summary nature of the
results provided.” Id. Likewise, regarding Pennsylvania’s
estimates of the costs of implementing certain pollution
controls, the EPA laments: “Unfortunately, where controls
were estimated to be more cost-effective, EPA cannot assess
the extent to which Pennsylvania’s analyses are reasonable
estimates for purposes of making a BART determination.”
Id.

        Despite the multitude of problems with Pennsylvania’s
SIP, and the EPA’s admitted inability to adequately assess the
state’s analysis, the EPA asserts that “the information that
Pennsylvania did provide” is sufficient to conclude “that
Pennsylvania’s ultimate BART determinations were
nevertheless reasonable.” Id. Without citation to supporting
authorities or further explanation, the EPA broadly claims
that, “based on the cost estimates for other BART sources in
other states” it has reviewed, “Pennsylvania’s cost numbers
appear to be generally consistent for such controls . . . .” Id.




                              33
The EPA further concludes that “[w]here Pennsylvania
estimated the costs of controls to be in the tens of thousands
or hundreds of thousands of dollars per ton of pollutant
removed, Pennsylvania’s conclusions that such controls are
not cost-effective seem reasonable, even assuming that the
true cost[s] of controls are likely less than what Pennsylvania
estimated.” Id.

        As a reviewing court, we must ensure that the EPA
“articulate[s] a satisfactory explanation” for its decision to
approve Pennsylvania’s SIP, “including a rational connection
between the facts found and the choice made.” Prometheus
Radio Project, 373 F.3d at 389–90 (citation and quotation
marks omitted). The EPA’s conclusory assertions on the
issue of control costs and its invocation of its own experience
addressing cost estimates do not suffice. See Natural Res.
Def. Council, Inc. v. Hodel, 865 F.2d 288, 298 (D.C. Cir.
1988) (per curiam) (“[C]onclusory remarks . . . do not equip a
decisionmaker to make an informed decision about alternative
courses of action or a court to review the [agency’s]
reasoning.”); see also Ass’n of Private Colleges & Univs. v.
Duncan, 870 F. Supp. 2d 133, 154 (D.D.C. 2012) (“That this
explanation could be used to justify any [determination] at all
demonstrates its arbitrariness.”); Nat’l Parks Conservation
Ass’n, 788 F.3d at 1145 (remanding where the “reasoning
fails to reveal to a reader how EPA determined that the cost
of controls were not justified”).

       The EPA also asserts that “[w]hen the other key BART
factor—visibility—is taken into account, . . . an overall
picture emerges that supports Pennsylvania’s BART
determinations.” 2014 Final Rule, 79 Fed. Reg. 24,342. In
essence, the EPA contends that, given Pennsylvania’s
calculations showing that its BART-eligible sources had



                              34
minimal visibility impact at Class I areas, it was reasonable to
conclude that additional pollution controls were unwarranted.

        We are unpersuaded by this reasoning. As noted
above, the 2014 Final Rule repeatedly criticizes
Pennsylvania’s      SIP      calculations     and supporting
documentation, noting that the SIP is so lacking that it is
difficult to assess the visibility impact calculations
Pennsylvania did conduct. What the EPA could determine,
however, was that Pennsylvania underestimated the impact of
pollution from its sources because it failed to calculate the
cumulative visibility impact from each source. The EPA now
urges us to rely on these very same visibility impact
calculations to conclude that the “overall picture” supports
Pennsylvania’s BART analysis. The EPA unconvincingly
insists we rely on what it has said is flawed.13


       13
           The EPA also argues that because 26 of
Pennsylvania’s 34 BART-eligible sources had less than a 0.5
deciview impact on any Class I area, the state could have
exempted these 26 sources from its BART analysis. Under
the agency’s own regulations and the BART Guidelines,
however, a state need not exempt these sources. See, e.g.,
Regional Haze Regulations, 70 Fed. Reg. 39,104, 39,107
(“States certainly have the discretion to consider that all
BART-eligible sources within the State are ‘reasonably
anticipated to cause or contribute’ to some degree of visibility
impairment in a Class I area.”); BART Guidelines, 40 C.F.R.
pt. 51, app. Y (“Once you have compiled your list of BART-
eligible sources, you need to determine whether . . . to make
BART determinations for all of them . . . .”).




                              35
        In the end, the EPA has identified a host of problems
with Pennsylvania’s BART analysis. What it has not done,
however, is provide a sufficient explanation as to why it
overlooked these problems and approved Pennsylvania’s SIP.
Because we, as a reviewing court, need an agency to show its
work before we can accept its conclusions, we will remand
this case to the EPA for further consideration.

                            IV.

       For the aforementioned reasons, we will vacate the
2014 Final Rule to the extent it approved Pennsylvania’s
source-specific BART analysis and remand to the EPA for
further proceedings consistent with this Opinion.




                             36
