          United States Court of Appeals
                     For the First Circuit

No. 19-1794
  TOWN OF WEYMOUTH, MASSACHUSETTS; ROBERT HEDLUND, Mayor of Town
 of Weymouth; PATRICK M. O'CONNOR, State Senator; MICHAEL SMART,
 Vice President District Six; KENNETH J. DIFAZIO, District Three
    Councilor; JANE HACKETT, Councilor at Large; ED HARRINGTON,
 District Five Councilor; REBECCA HAUGH, District One Councilor;
      ARTHUR MATHEWS, District Four Councilor; MICHAEL MOLISSE,
     Councilor at Large; SCOTT DOWD, Conservation Commissioner;
       GEORGE LORING, Conservation Commissioner; THOMAS TANNER,
       Conservation Commissioner; FRANK SINGLETON, Conservation
   Commissioner; JOHN REILLY, Conservation Commissioner; CITY OF
  BRAINTREE, MASSACHUSETTS; TOWN OF HINGHAM, MASSACHUSETTS; CITY
                       OF QUINCY, MASSACHUSETTS,

                          Petitioners,

                               v.

      MASSACHUSETTS DEPARTMENT OF ENVIRONMENTAL PROTECTION,

                           Respondent,

                ALGONQUIN GAS TRANSMISSION, LLC,

                           Intervenor.


No. 19-1797

 ELIZABETH MOULDS; JENNIFER MATHIAN; OLIVIA LANNA; PRIYA HOWELL;
KATHERINE ROGERS; MICHAEL MULLALEY; HEATHER KAAS; KATIE MCBRINE;
       JANICE DEYOUNG; A. SILVIA FABRIZIO; KATHLEEN CRONIN,

                          Petitioners,

                               v.

      MASSACHUSETTS DEPARTMENT OF ENVIRONMENTAL PROTECTION,

                           Respondent,
                ALGONQUIN GAS TRANSMISSION, LLC,

                           Intervenor.


No. 19-1803

    DOROTHY ANDERSON; ALICE ARENA; MARGARET BELLAFIORE; WENDY
  CULLIVAN; SUSAN GREENE; REBECCA HAUGH; ANDREA HONORE; MICHAEL
 LANG; CURTIS NORDGAARD, M.D.; THOMAS PENDERGAST; JUDY ROBERTS;
        FRANK SINGLETON; BETSY SOWERS; BERNADETTE WILSON,

                          Petitioners,

                               v.

      MASSACHUSETTS DEPARTMENT OF ENVIRONMENTAL PROTECTION,

                           Respondent,

                ALGONQUIN GAS TRANSMISSION, LLC,

                           Intervenor.


               PETITIONS FOR REVIEW OF AN ORDER OF
    THE MASSACHUSETTS DEPARTMENT OF ENVIRONMENTAL PROTECTION


                             Before

                  Thompson, Lipez, and Kayatta,
                         Circuit Judges.


     Brian F. Bertram, J. Raymond Miyares, Katherine E. Stock,
Miyares and Harrington, LLP, Joseph Callanan, Town Solicitor, Town
of Weymouth, Nicole I. Taub, Town Solicitor, Town of Braintree,
Kerry T. Ryan, Special Counsel, Town of Hingham, Bogle, DeAscentis
& Coughlin, P.C., and Janet Petkun, Assistant City Solicitor, City
of Quincy, on brief for petitioners Town of Weymouth, et. al.
     Lawrence K. Kolodney, Adam J. Kessel, Natalie Galley, Eda
Stark, Kayleigh E. McGlynn, and Fish & Richardson P.C. on brief
for petitioners Moulds, et. al.
     Michael H. Hayden and Morrison Mahoney LLP on brief for
petitioners Anderson, et. al.
     Seth Schofield, Senior Appellate Counsel, Office of the
Attorney General of Massachusetts, Maura Healey, Attorney General
for the Commonwealth of Massachusetts, Julie E. Green, Assistant
Attorney General, Office of the Attorney General of Massachusetts,
and Joshua Olszewski-Jubelirer, Assistant Attorney General, Office
of the Attorney General of Massachusetts, on brief for respondent.
     Jeremy C. Marwell, Joshua S. Johnson, Vinson & Elkins LLP,
James T. Finnigan, and Rich May, P.C. on brief for intervenor.


                          June 3, 2020
            KAYATTA,   Circuit   Judge.     These    consolidated      cases

involve a proposed natural gas compressor station set to be built

in Weymouth, Massachusetts, as part of Algonquin Gas Transmission,

LLC's "Atlantic Bridge Project," a natural gas pipeline connecting

the Northeastern United States and Canada.                The Massachusetts

Department of Environmental Protection (DEP) approved Algonquin's

non-major   comprehensive   plan    application   for      the   station   and

granted the station's air permit, certifying its compliance with

the Massachusetts Clean Air Act (CAA), Mass. Gen. Laws ch. 111,

§§ 142A–142F.     Nearby municipalities and two citizen-petition

groups challenge DEP's decision in this court, invoking original

jurisdiction     pursuant   to   the   Natural      Gas     Act,   15 U.S.C.

§ 717r(d)(1).    The petitioners raise a slew of arguments that DEP

violated the Massachusetts CAA and related laws and regulations.

Because we find that DEP did not follow its own established

procedures for assessing whether an electric motor was the Best

Available Control Technology (BACT), we vacate the air permit and

remand to the agency to redo that analysis.                 We resolve the

remaining issues in favor of DEP.          See, e.g., Swajian v. Gen.

Motors Corp., 916 F.2d 31, 35 (1st Cir. 1990) ("The remainder of

this opinion will discuss other issues raised by the parties which

are likely to recur [on remand] and should therefore be passed

upon by us.").




                                   - 4 -
                                           I.

            Algonquin, a natural gas transmission company based in

Houston, Texas, proposed its Atlantic Bridge Project in response

to rising demand for natural gas in the Northeastern United States

and Canada.    See Algonquin Gas Transmission, LLC v. Weymouth, 919

F.3d 54, 59 (1st Cir. 2019).          Algonquin submitted its proposal for

the project to the Federal Energy Regulatory Commission (FERC) in

October 2015, id., and at the same time filed its air-permit

application with DEP seeking the agency's approval of the project,

see 310 Mass. Code Regs. § 7.02.

            As with all natural gas pipelines, the Atlantic Bridge

Project needs "[c]ompressor stations" to be "strategically placed

along the pipeline to boost the system pressure to maintain

required flow rates."        FERC, An Interstate Natural Gas Facility on

My   Land?:           What    Do      I        Need        to     Know?     28      (2015),

http://bit.ly/2PBe0Tz.          One       of    the    compressor          stations    that

Algonquin     plans    to     build       will        be    located        in    Weymouth,

Massachusetts, near the Fore River Energy Center (an unrelated

power plant) and King's Cove recreation area (a public park).

Algonquin   proposed     to    operate         the    Weymouth        station     using    a

"SoLoNOx"     Solar    Taurus 60      combustion                turbine,    which     is   a

proprietary model of a Dry Low Nitrogen Oxide (NOx) combustion




                                          - 5 -
turbine owned by Solar.1             Dry Low NOx turbines burn natural gas and

reduce      emissions    of    NOx    by    operating      at    a     lower    combustion

temperature.      In layman's terms, the Weymouth station will burn a

small amount of natural gas in order to generate pressure that

will allow the bulk of the gas to flow through the pipeline.

              FERC, for its part, approved Algonquin's plans for the

Atlantic      Bridge    Project,       including     the    Weymouth         Station,    and

issued a "certificate of public convenience and necessity" under

15 U.S.C. § 717f(c) in January 2017.                  Algonquin Gas Transmission,

919 F.3d at 57, 59 (citing Algonquin Gas Transmission, LLC Mars.

& Ne. Pipeline, LLC, 158 FERC ¶ 61061, 2017 WL 383829, at *1

(Jan. 25, 2017)); see also Town of Weymouth v. FERC, No. 17-1135,

2018 WL 6921213, at *1 (D.C. Cir. Dec. 27, 2018) (per curiam)

(denying      Weymouth's      petition      challenging         FERC's      certificate).

This       approval    came    after       FERC    completed         its    environmental

assessment      pursuant      to     the   National      Environmental         Policy    Act

(NEPA), 42 U.S.C. § 4332(2)(C), finding that, "with appropriate

mitigating measures," the project "would not constitute a major

federal action significantly affecting the quality of the human

environment."           FERC's     approval,       however,       is       conditioned    on

Algonquin's      compliance        with    the     CAA   (state      and     federal2)    as


       1"Solar" is a turbine manufacturer owned by Caterpillar,
Inc. "Taurus" is a family of turbines that Solar manufactures.
     2  The Environmental Protection Agency (EPA) has approved the
Massachusetts CAA and accompanying regulations as a State


                                           - 6 -
determined by DEP.      Algonquin Gas Transmission, 2017 WL 383829, at

*45; see also 15 U.S.C. § 717b(d)(2) ("[N]othing in this chapter

affects   the    rights      of   States     under . . .     the   Clean    Air

Act . . . ."); cf. Algonquin Gas Transmission, 919 F.3d at 57–60,

63–66   (dealing      with   DEP's   approval    under     the   Coastal   Zone

Management Act).

             DEP staff reviewed Algonquin's air-permit application

and issued a "draft permit" in March 2017 stating that the proposed

Weymouth station was "in conformance with the Air Pollution Control

regulations     and    current    air   pollution    control       engineering

practice."      DEP provided a public comment period on the draft

permit, and in response to public concerns, Massachusetts Governor

Charles Baker directed DEP and the Massachusetts Department of

Public Health to prepare a health impact assessment (HIA) of the

Weymouth station.

             Frustrated by the protracted approval process, Algonquin

sued DEP in the D.C. Circuit under 15 U.S.C. § 717r(d)(2) seeking

to hurry along DEP's final decision.             See Petition for Review,

Algonquin Gas Transmission, LLC v. Mass. Dep't of Envtl. Prot.,

No. 18-1045 (D.C. Cir. Feb. 16, 2018).          The result of that lawsuit




Implementation Plan (SIP) under the federal CAA. See 40 C.F.R.
§§ 52.1119–52.1169; see also 42 U.S.C. § 7410(a). As such, "[t]he
EPA has delegated authority to the Massachusetts DEP to administer
the [federal] CAA in Massachusetts." Algonquin Gas Transmission,
2017 WL 383829, at *45.


                                     - 7 -
was an out-of-court agreement between DEP and Algonquin to the

following timeline:     DEP promised to complete the HIA by January

2019 and to limit any subsequent adjudicatory hearings on the air

permit to six months.

             In keeping with the established timeline, DEP completed

the HIA on January 4, 2019, "predict[ing] no substantial changes

in health from direct exposures from the station itself with the

exception of estimated sound levels during construction."       The

following week, DEP's regional Air Quality Section Chief issued a

"Non-Major Comprehensive Air Quality Plan Approval" (i.e., air-

permit approval) for the Weymouth station.

             Petitioners in this case (described below) filed an

administrative appeal to DEP's Office of Appeals and Dispute

Resolution, raising a number of claims that the permit violated

Massachusetts law.    The Presiding Officer of that appeal dismissed

some of these claims without opinion on April 11, 2019, and then,

after a hearing, issued a full "Recommended Final Decision" on

June 27, 2019.     That decision affirmed the air permit with some

revisions.    On July 12, 2019, the DEP Commissioner issued a "Final

Decision" adopting in full the Presiding Officer's Recommended

Final Decision.      In response to the Commissioner's request for

clarification (on issues not relevant to this case), the Presiding

Officer issued a "Recommended Final Decision on Reconsideration"




                                 - 8 -
on August 5, 2019, and the Commissioner adopted this decision in

full on August 7, 2019.

           The three groups of petitioners all challenge DEP's

approval of the Weymouth station air permit.              In case 19-1794

(docketed Aug. 8, 2019), petitioners include the Town of Weymouth,

several nearby municipalities, and state and local officials.               In

cases    19-1797    and   19-1803     (both   docketed    Aug. 9,        2019),

petitioners are eleven and fourteen (respectively) residents of

these nearby municipalities.        See Mass. Gen. Laws ch. 30A, § 10A

(authorizing "not less than ten persons [to] intervene in any

adjudicatory       proceeding . . .     in    which      damage     to     the

environment . . . is or might be at issue").             We have original

jurisdiction3 under the Natural Gas Act, which states that:

           The United States Court of Appeals for the
           circuit in which a facility subject to [15
           U.S.C. § 717f] is proposed to be constructed,
           expanded, or operated shall have original and
           exclusive jurisdiction over any civil action
           for the review of an order or action of a
           Federal agency (other than [FERC]) or State
           administrative agency acting pursuant to
           Federal law to issue, condition, or deny any
           permit,      license,     concurrence,     or
           approval . . . required under Federal law,
           other than the Coastal Zone Management Act of
           1972 . . . .


     3  No party has argued that any justiciability barrier
(mootness, ripeness, finality, standing, etc.) precludes our
review of these cases. See generally Berkshire Envtl. Action Team,
Inc. v. Tenn. Gas Pipeline Co., 851 F.3d 105 (1st Cir. 2017)
(addressing    the   finality   requirement   under    15   U.S.C.
§ 717r(d)(1)). We are satisfied that there are no such barriers.


                                    - 9 -
15 U.S.C. § 717r(d)(1).      Here, we review "an order" of a "State

administrative agency" (DEP) "acting pursuant to Federal law" (the

CAA4) "to issue" a "permit" required by that federal law.               We

consolidated    these   cases,   and      Algonquin   intervened   as   a

respondent.    As required by the Natural Gas Act, we expedited our

review.   See 15 U.S.C. § 717r(d)(5).

                                  II.

                        A.   Standard of Review

           The Natural Gas Act does not provide a standard of

review, so we are left to look elsewhere.         The parties dispute,

however, where we should look.      According to petitioners, we can

consult the federal Administrative Procedure Act (APA), 5 U.S.C.

§ 706(2), and "borrow" its standards of review.        And several other

circuits have done so in reviewing state approval of natural-gas

facilities.    See Del. Riverkeeper Network v. Sec'y Pa. Dep't of

Envtl. Prot., 833 F.3d 360, 377 (3d Cir. 2016); AES Sparrows Point




     4  As indicated, see supra note 2, the CAA adopts a
"cooperative federalism" approach, see Berkshire Envtl. Action
Team, 851 F.3d at 113, such that DEP, in enforcing the
Massachusetts CAA, is in fact acting pursuant to the federal CAA.
This also provides the federal "ingredient" for purposes of
Article III jurisdiction. See Osborn v. Bank of U.S., 22 U.S. (9
Wheat.) 738, 823 (1824) ("[W]hen a question to which the judicial
power of the Union is extended by the constitution, forms an
ingredient of the original cause, it is in the power of Congress
to give the Circuit Courts jurisdiction of that cause, although
other questions of fact or of law may be involved in it."); see
also Verlinden B.V. v. Cent. Bank of Nigeria, 461 U.S. 480, 494–
95 (1983) (describing Article III "arising under" jurisdiction).


                                 - 10 -
LNG, LLC v. Wilson, 589 F.3d 721, 727 (4th Cir. 2009); Islander E.

Pipeline Co. v. Conn. Dep't of Envtl. Prot., 482 F.3d 79, 94 (2d

Cir. 2006).     But DEP says the Massachusetts APA, Mass. Gen. Laws

ch. 30A, § 10A, applies.       This makes some sense, as we will be

mostly looking to Massachusetts substantive law to resolve the

issues in this case, and because the federal APA by its terms does

not apply to states.      See 5 U.S.C. § 551(1).

          Fortunately, the issue appears to be strictly academic

in this case, for no party has demonstrated how it would make any

difference whether the Massachusetts APA or federal APA applies.

As relevant here, the standards do not vary materially, so we will

apply those standards without worrying about their source.            Cf.

Friends of Buckingham v. State Air Pollution Control Bd., 947 F.3d

68, 80–82 (4th Cir. 2020) (likewise avoiding the issue where the

Virginia standards matched the federal standards).           As in most

cases involving a decision of an administrative agency, we review

formally adjudicated findings of fact for "substantial evidence,"

and   reverse    agency   decisions   if   they    are   "arbitrary   and

capricious."     Cent. Me. Power Co. v. FERC, 252 F.3d 34, 40 n.3

(1st Cir. 2001); accord Friends & Fishers of Edgartown Great Pond,

Inc. v. Dep't of Envtl. Prot., 848 N.E.2d 393, 399 (Mass. 2006).

We will defer to an agency's otherwise lawful interpretation of

its own regulation unless the regulation unambiguously forecloses

that interpretation.      See Kisor v. Wilkie, 139 S. Ct. 2400, 2414–


                                 - 11 -
23 (2019) (reaffirming and clarifying Auer v. Robbins, 519 U.S.

452 (1997)); Town of Brookline v. Comm'r of Dep't of Envtl. Quality

Eng'g ("Brookline II"), 497 N.E.2d 9, 15 (Mass. 1986).

                             B.   Discussion

                 1.   Best Available Control Technology

          All parties agree that, under Massachusetts regulations,

the Weymouth station, a non-major source of air pollution, must

employ the Best Available Control Technology (BACT) for reducing

NOx emissions.    See 310 Mass. Code Regs. § 7.02(8)(a)(2).   BACT is

defined as:

          [A]n emission limitation based on the maximum
          degree of reduction of any regulated air
          contaminant emitted from or which results
          from any regulated facility which the
          Department, on a case-by-case basis taking
          into account energy, environmental, and
          economic impacts and other costs, determines
          is achievable for such facility through
          application of production processes and
          available methods, systems and techniques for
          control of each such contaminant. The best
          available         control          technology
          determination . . . may include a design
          feature,   equipment    specification,   work
          practice, operating standard, or combination
          thereof.

Id. § 7.00; see also 42 U.S.C. § 7479(3).      BACT is distinct from

more stringent standards, like Lowest Achievable Emission Rate

(LAER), in that BACT takes into consideration economic factors

(i.e., costs).    See 310 Mass. Code Regs. § 7.00.




                                  - 12 -
             The     Environmental       Protection       Agency     (EPA)      has

established a five-step "top-down" approach for determining BACT,

see   EPA,   New    Source    Review    Workshop      Manual:      Prevention    of

Significant Deterioration and Nonattainment Area Permitting B.5–

B.6 (1990)5 [hereinafter NSR Workshop Manual], and DEP has adopted

this approach, see DEP, Best Available Control Technology (BACT)

Guidance:        Air Pollution Control Requirements for Construction,

Substantial Reconstruction or Alteration of Facilities that Emit

Air Contaminants 3 (2011) [hereinafter BACT Guidance].                  The five

steps are:

             •    STEP 1:    IDENTIFY           ALL         CONTROL
                             TECHNOLOGIES.

             •    STEP 2:    ELIMINATE TECHNICALLY INFEASIBLE
                             OPTIONS.

             •    STEP 3:    RANK      REMAINING            CONTROL
                             TECHNOLOGIES     BY            CONTROL
                             EFFECTIVENESS.

             •    STEP 4:    EVALUATE MOST EFFECTIVE CONTROLS
                             AND DOCUMENT RESULTS.

             •    STEP 5:    SELECT BACT.

NSR Workshop Manual, supra, at B.6.

             Algonquin's air permit application applied the five-step

approach and concluded that the SoLoNOx turbine was the BACT for

NOx for the Weymouth station.          DEP agreed.      Petitioners argue that


      5 Despite being marked as a "draft," this document is
frequently cited by courts as authoritative on BACT issues. See,
e.g., Sierra Club v. EPA, 499 F.3d 653, 654 (7th Cir. 2007).


                                       - 13 -
that conclusion (or at least the analysis getting there) was flawed

in two respects:         (1) it excluded consideration of using an

electric motor instead of the SoLoNOx turbine, and (2) it wrongly

determined that a Dry Low NOx turbine plus a selective catalytic

reduction (SCR) was not cost feasible.       We address each argument

in turn.

                           a.   Electric Motor

           Algonquin's air-permit application initially made no

mention of an electric-motor option.       But during the adjudication

process, petitioners asserted that an electric motor would be a

more   effective   and   economically   feasible   alternative   to   the

SoLoNOx turbine.    Essentially, the petitioners proposed that the

compressor station could be hooked up to the existing electrical

grid and create the necessary pressure without burning any natural

gas.   This would eliminate all emissions of NOx from the Weymouth

station.   And at least some compressor stations in other parts of

the country operate with such an electric motor.

           Algonquin revised its application in May 2018 to address

the feasibility of an electric motor.        Algonquin explained that

this option was excluded for several reasons, including the high

cost of upgrading the existing power infrastructure.         Algonquin

also cited the fact that FERC's environmental assessment concluded

that an electric motor would not offer a significant environmental

advantage over the proposed gas-fired turbine.      DEP staff accepted


                                  - 14 -
Algonquin's exclusion, relying on FERC's assessment and concededly

not making an independent determination for purposes of BACT.

            Algonquin and DEP refocused their position before the

Presiding Officer, arguing that the electric-motor option was

properly excluded from Step 1 of the BACT analysis as a project

"redesign."      Per the NSR Workshop Manual, Step 1 is a wide-ranging

process,    in    which   applicants   "should   initially   identify   all

control options with potential application to the emissions unit

under review."        NSR Workshop Manual, supra, at B.7 (emphasis

added).     However, a technology may be excluded from Step 1 if it

would "redefine the source."       Helping Hand Tools v. EPA, 848 F.3d

1185, 1194 (9th Cir. 2016).       "In a classic and simple example, a

coal-burning power plant need not consider a nuclear fuel option

as a 'cleaner' fuel because it would require a complete redesign

of the coal-burning power-plant."         Id. (citing Sierra Club v. EPA,

499 F.3d 653, 655 (7th Cir. 2007)); see also Friends of Buckingham,

947 F.3d at 74, 82–85 (analyzing whether an electric motor would

redefine the source of a proposed gas-fired compressor station

turbine).

            The     Presiding   Officer    was   not   persuaded   by   the

"redesign" argument.        She stated that DEP erred in relying on

FERC's environmental assessment and that DEP should have included

"all control technologies in the BACT analysis" (emphasis in

original).       She left unresolved whether the electric motor would


                                  - 15 -
in fact constitute a redesign if properly analyzed as such by DEP

staff.6       Instead, she determined that, "even assuming use of an

electric driven compressor would not redefine the source," the

electric motor would properly be excluded at Step 4 of the BACT

analysis as not cost feasible.

               The support for the Presiding Officer's cost-feasibility

conclusion came largely from the testimony of Algonquin witness

William Welch.           Welch testified, with respect to the redesign

issue, that an electric motor at the Weymouth station would require

substantial infrastructure investment, including construction of

a new substation and the laying of half a mile of underground

electric transmission line.            In total, Welch estimated that these

upgrades could cost between $9 million and $12 million.                          The

Presiding Officer acknowledged that "there is no corroboration of

these       numbers,    and   [that]   they   seem   to   be     based   on   several

conversations or meetings at which no notes apparently were taken."

However, she stated, "there is no evidence disputing them," since

petitioners'           witness   did    not     take      into     account      these




        Neither DEP nor Algonquin argue on this appeal that we can
        6

affirm on the ground that an electric motor would constitute a
redesign.   Nor could they, since DEP's final decision does not
rest on that ground. See Motor Vehicle Mfrs. Ass'n of U.S., Inc.
v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 50 (1983) (citing
SEC v. Chenery Corp., 332 U.S. 194, 196 (1947)); NSTAR Elec. Co.
v. Dep't of Pub. Utils., 968 N.E.2d 895, 900–01 (Mass. 2012).
Algonquin reserves the right to reassert its redesign argument on
remand to DEP.


                                       - 16 -
infrastructure       costs   in   his   own   cost   estimates.     She   thus

"infer[red] that the total cost for this infrastructure would be

substantial."    So, she concluded, Algonquin's evidence, "though

scant and uncorroborated by any documentation, at least provides

some basis to infer that" the electric motor would not be cost

feasible.

            Petitioners challenge that conclusion on three grounds:

(1) they    assert    that   "scant     and   uncorroborated"     evidence   of

infrastructure costs cannot be considered "substantial evidence,"

as would be needed for us to affirm an agency's finding of fact;

(2) they argue that neither the Presiding Officer nor anyone else

at DEP ever provided a full Step 4 analysis as required by DEP's

BACT Guidance; and (3) they contend that the Presiding Officer

raised the Step 4 issue sua sponte after the hearing without

providing an opportunity for the parties to weigh in, thus denying

them their Due Process rights and violating Massachusetts law.

            We easily dispatch with the first of these arguments.

Welch offered an estimate that does not seem irrational on its

face, and petitioners offered no contrary estimate of what must be

a real cost.     So while the Presiding Officer fairly noted the

unimpressive support for the estimate, we cannot say that the

evidence was insubstantial as a matter of law. See Bath Iron Works

Corp. v. U.S. Dep't of Labor, 336 F.3d 51, 56 (1st Cir. 2003)

(recognizing that, under the "substantial evidence" standard, "we


                                    - 17 -
will accept the findings and inferences drawn" by an agency so

long as they are not "irrational," meaning that "the record

contains 'such relevant evidence as a reasonable mind might accept

as adequate to support a conclusion'" (quoting Barker v. U.S. Dep't

of Labor, 138 F.3d 431, 434 (1st Cir. 1998), and Sprague v. Dir.,

Office of Workers' Comp. Programs, U.S. Dep't of Labor, 688 F.2d

862, 865 (1st Cir. 1982))).

               Petitioners' second argument fares better.7            According

to the NSR Workshop Manual, "[c]ost effectiveness is the economic

criterion used to assess the potential for achieving an objective

at least cost.         Effectiveness is measured in terms of tons of

pollutant emissions removed." NSR Workshop Manual, supra, at B.36.

So at Step 4 of the BACT analysis, the agency (or the applicant)

must       calculate   the   cost   effectiveness   of   the   most   effective

technology remaining after Step 3 and eliminate that technology if

it falls above a predetermined cost-feasibility threshold.                 For

NOx, DEP has established that technologies falling in (or below)

the range of $11,000 to $13,000 per ton of NOx removed per year

will be considered cost feasible.          BACT Guidance, supra, at 5.

               DEP never calculated cost effectiveness for an electric

motor, nor did it compare that figure to the range established in


       7Because we vacate DEP's decision on this ground, see infra
subpart II(C), we need not address petitioners' third argument or
Algonquin's response that petitioners waived that particular
argument by not moving for reconsideration.


                                      - 18 -
its BACT Guidance.       And even in their briefs before us, DEP and

Algonquin do not attempt to perform the required mathematical

calculations.      Instead, DEP states that "the full calculation was

unnecessary because the infrastructure costs were so obviously

substantial."        Effectively,    DEP    argues   that   a   $9–12 million

infrastructure cost is so high that the cost effectiveness, if

calculated, would necessarily exceed the $13,000-per-ton cutoff.

              Without a more detailed explanation by DEP, we cannot be

so    sure.       According   to   the     NSR   Workshop   Manual,   "[c]ost

effectiveness calculations can be conducted on an average, or

incremental basis." NSR Workshop Manual, supra, at B.36. Starting

with average cost effectiveness, the manual provides us with the

following formula:

              Average cost Effectiveness (in dollars per
              ton removed) =
              (Control option annualized cost) /
              (Baseline emissions rate – Control option
              emissions rate)

Id.   at   B.37    (mathematical    notations     reformatted).       And,   to

annualize costs for capital investments, the manual tells us to

multiply up-front costs by:

              (real interest rate) *
              (1 + real interest rate)^(economic life of
              equipment in years) /
              ((1 + real interest rate)^(economic life of
              equipment in years) - 1)

Id. at b.10 (mathematical notations reformatted).




                                    - 19 -
           When we attempt to solve for average cost effectiveness,

it becomes apparent that the record before us does not contain

enough information.    As to the numerator, we are assuming that the

infrastructure     costs   of   the    electric   motor    would     be   between

$9 million and $12 million based on Welch's testimony.                     But we

cannot annualize that figure because we do not know the lifespan

of the equipment.      The NSR Workshop Manual tells us that "[t]he

economic life of a control system typically varies between 10 to

20 years and longer," id., but that hardly narrows things.                       We

also do not know what interest rate DEP would use.                   The manual

says   that   "[t]he   value     used     in   most     control    analyses      is

10 percent," id. at b.11, but again this is not a categorical

pronouncement.     So we cannot tell what the annualized cost of the

electric   motor    infrastructure       would    be.      We     also    have   no

information on the annual operating expenses for the electric

motor, although anything above zero would be helpful to DEP in

this exercise.

           Even more difficult is the denominator.                 We know that

the emissions rate for the electric motor is zero, but the record

is incomplete as to what the baseline emissions rate would be.

According to Algonquin's air-permit application, the "Base Case"

is "Good Combustion Practices" (presumably a gas-fired turbine




                                      - 20 -
that, unlike SoLoNOx, employs no control of NOx emissions).8             But

the application does not give a value for "Potential NOx Emissions"

for   this     option.    The   control   technology   just   above   "Good

Combustion Practices" is "Water Injection," which the application

tells us has an emissions rate of "20 to 42 ppm (water)."             So, it

is probably safe to assume that the baseline emissions rate is at

least that high, and probably higher.         We are also not provided

with a formula for converting ppm (parts per million) to tons per

year.       We know that the SoLoNOx turbine will result in 10.03 tons

of NOx per year and that it has an emissions rate of 9 ppm, so for

ballparking purposes a one-to-one conversion would seem to be good

enough (although we must accept a wide margin of error, especially

since we do not know what "(water)" means).

               So, if we assume, reasonably, that the interest rate is

10% and that the lifespan of the electric motor infrastructure is

twenty years, then the average cost effectiveness of a $12 million

electric motor would be below $13,000 per ton per year if the "Good

Combustion Practices" emissions exceed 108 tons per year.9            For a



        The electric motor, unlike the SCR discussed below, is a
        8

process-control technology, rather than an add-on technology
(i.e., the compressor station needs either an electric motor or a
SoLoNOx turbine, but not both). As such, the baseline emissions
rate is not the emissions rate of the SoLoNOx turbine.
      9 $13,000    per    ton ≥ ($12,000,000 *  0.1 *    (1.120) /
(1.120 - 1)) / (Baseline emissions rate - 0).
         Baseline emissions rate ≥ ($12,000,000 * 0.1 * (1.120) /
(1.1 - 1)) / ($13,000 per ton).
    20

         Baseline emissions rate ≥ 108.42 tons.


                                   - 21 -
$9 million motor, that value would drop to 81 tons per year.10

These values are higher than the 42 ppm for "Water Injection" (as

we expected they would be), but not so high as to be unthinkable,

given what we know from this incomplete record.11

          Turning to incremental cost effectiveness, we run into

similar, though different, problems. The NSR Workshop Manual gives

us this formula:

          Incremental Cost (in dollars per incremental
          ton removed) =
          (Total costs (annualized) of control option –
          Total costs (annualized) of next control
          option) /
          (Next control option emission rate – Control
          option emissions rate)

Id. at B.41 (mathematical notations reformatted).

          Here the "control option" is the electric motor, and the

"next control option" is the SoLoNOx turbine.       And we know the

denominator will be 10.03 tons (10.03 minus zero).      But we run


     10    $13,000    per    ton ≥ ($9,000,000 *   0.1 *   (1.120) /
(1.120   - 1)) / (Baseline emissions rate - 0).
           Baseline emissions rate ≥ ($9,000,000 * 0.1 * (1.120) /
(1.1 - 1)) / ($13,000 per ton).
    20

           Baseline emissions rate ≥ 81.32 tons.
      11   To illustrate how much wiggle room there is in these
numbers, we can adjust our assumptions to a 1% interest rate and
a fifty-year equipment lifespan.        At that point, a $9 million
electric motor would be cost feasible if the uncontrolled emissions
rate is above 17.7 tons per year (which, based on the "Water
Injection" figures, it almost certainly is).
           $13,000   per   ton ≥ ($9,000,000 *   0.01 *   (1.0150) /
(1.0150 - 1)) / (Baseline emissions rate - 0).
           Baseline emissions rate ≥ ($9,000,000 * 0.01 * (1.0150) /
(1.01 - 1)) / ($13,000 per ton).
      50

           Baseline emissions rate ≥ 17.66 tons.


                              - 22 -
into the same problems as before with annualizing the costs of the

electric motor, and more importantly, we have no information from

the record of what the costs -- annual or capital -- are for the

SoLoNOx   turbine.    Indeed,   Algonquin's     application   includes   a

line-item cost breakdown of the SCR (discussed below), but in the

column for SoLoNOx, the fields are all blank.         Sticking with our

ballparking approach and assuming a 10% interest rate and twenty-

year lifespan on the electric motor (and zero costs for the

electric motor beyond capital expenses), a $9 million electric

motor would be cost feasible if the annualized SoLoNOx costs

(factoring   in   capital   investments   and    operating    costs)   are

$926,747 or higher.12   The actual costs for SoLoNOx may in fact be

far less than that, but not so "obviously" that we can shrug off

the lack of data in the record.      And, in any event, it is DEP's

job, not ours, to do these calculations properly.              See Motor

Vehicle Mfrs. Ass'n of U.S., Inc. v. State Farm Mut. Auto. Ins.




     12    $13,000    per   ton ≥ (($9,000,000 *    0.1 *   (1.120) /
(1.120   - 1)) - annualized SoLoNOx costs) / (10.03 tons - 0).
           Annualized SoLoNOx costs ≥ ($9,000,000 * 0.1 * (1.120) /
(1.1 - 1)) - ($13,000 per ton * 10.03 tons).
    20

           Annualized SoLoNOx costs ≥ $926,746.62.
           With a 1% interest rate and a fifty-year lifespan, see
supra note 11, the $9 million motor would be cost feasible if the
annualized SoLoNOx costs exceed $99,225.
           $13,000   per   ton ≥ (($9,000,000 *   0.01 *   (1.0150) /
(1.0150 - 1)) - annualized SoLoNOx costs) / (10.03 tons - 0).
           Annualized SoLoNOx costs ≥ ($9,000,000 * 0.01 * (1.0150) /
(1.01 - 1)) - ($13,000 per ton * 10.03 tons).
      50

           Annualized SoLoNOx costs ≥ $99,224.58.


                                - 23 -
Co., 463 U.S. 29, 50 (1983) ("It is well-established that an

agency's action must be upheld, if at all, on the basis articulated

by the agency itself." (citing SEC v. Chenery Corp., 332 U.S. 194,

196 (1947))); NSTAR Elec. Co. v. Dep't of Pub. Utils., 968 N.E.2d

895, 900–01 (Mass. 2012).

            Algonquin tries to paper over the gaps in the record by

pointing to something for which there is ample evidence: the costs

for the SCR.      As will be discussed in the next section, the

Presiding Officer found that the SCR/turbine combination was not

cost feasible.        And, based on Algonquin's line-item analysis in

its application, the total capital costs for SCR were $1,432,058,

which     Algonquin    translated    into     $135,176     annualized.      So,

Algonquin reasons, a technology with a $9–12 million capital cost

must be even more infeasible.

            We consider the comparison to the SCR unhelpful for two

reasons.     First, SCR is an add-on technology, and, as will be

discussed shortly, the calculations for cost effectiveness for

add-on     technologies    differ     from    those      for   process-control

technologies like the electric motor.              See also supra note 8.

Second,     Algonquin     compares     only     one   variable --        capital

costs -- where the formulae encompass multiple variables.                  Even

assuming the lifespan and annual operating costs of each technology

are identical, we know that the electric motor is more effective

at reducing NOx emissions than the SCR.         So the denominator of each


                                     - 24 -
formula (average and incremental cost effectiveness) would be

higher for the electric motor, thus offsetting (at least in part)

the higher numerator.

             We concede that our own calculations are not obviously

correct.     The problem for the DEP and Algonquin is that no one has

provided a basis for concluding that our calculations are so

obviously incorrect as to obviate the need for any calculation at

all by Algonquin or DEP.        The record does not even contain a Fermi

estimate13       fixing   the   magnitude    of   the    quotient   above      the

regulatory cost-effectiveness cut-off.

             The bottom line is this: DEP's established BACT protocol

requires     a    cost-effectiveness    analysis        before   eliminating    a

technology at Step 4, and the results of such an analysis do not

strike us as so obvious as to overlook as harmless DEP's failure

either to follow that protocol or at least do enough to make it

clear that following the protocol would eliminate the electric

motor as a cost-effective option.           "An agency may not . . . depart

from a prior policy sub silentio or simply disregard rules that

are still on the books."          FCC v. Fox Television Stations, Inc.,

556 U.S. 502, 515 (2009); see also Nat'l Envtl. Dev. Ass'n's Clean

Air Project v. EPA, 752 F.3d 999, 1009 (D.C. Cir. 2014) ("[A]n




     13 See Robert N. Ronau, Number Sense, 81 Mathematics Tchr.
437, 439–40 (1988). See generally Hans Christian von Baeyer, The
Fermi Solution: Essays on Science (1993).


                                    - 25 -
agency action may be set aside as arbitrary and capricious if the

agency fails to 'comply with its own regulations.'"               (quoting

Environmentel, LLC v. FCC, 661 F.3d 80, 85 (D.C. Cir. 2011)));

Tofias v. Energy Facilities Siting Bd., 757 N.E.2d 1104, 1111

(Mass. 2001); Town of Northbridge v. Town of Natick, 474 N.E.2d

551, 556 (Mass. 1985).         Thus, we find that DEP's final decision

excluding the electric motor on this ground was arbitrary and

capricious.

                    b.     Selective Catalytic Reduction

             Unlike the electric motor, the SCR was analyzed in

Algonquin's application.        According to the application, "SCR is a

post-combustion gas treatment process in which NH3 [ammonia] is

injected into the exhaust gas upstream of a catalyst bed."             SCR

can reduce NOx emissions from a gas-fired turbine like SoLoNOx by

up to 90%.

             In Step 3 of the BACT analysis, Algonquin's application

ranked the SCR as the most effective technology at reducing NOx

emissions,    and    the    only   technology   included   that   outranked

SoLoNOx.     However, Algonquin excluded the SCR at Step 4 as not

cost feasible.       Using the 9-ppm SoLoNOx emissions rate as a

baseline, Algonquin calculated the SCR's cost effectiveness at

$41,541 per ton of NOx removed, which well exceeded DEP's $11,000–

$13,000 range.      DEP agreed, leaving SoLoNOx as the BACT.




                                    - 26 -
           Petitioners argued before the Presiding Officer, and now

before us, that Algonquin incorrectly calculated the SCR's cost

effectiveness.      They claim that Algonquin should have used a

baseline emissions rate of 25 ppm, which represents the emissions

rate of older models of Dry Low NOx turbines.           Put differently,

petitioners fault Algonquin for considering only the SCR added to

its   preferred     technology,   SoLoNOx,    without   considering     the

combination of SCR plus a cheaper, less effective turbine.

           According    to   petitioners,    the   SCR-plus-older-turbine

combination would be more effective at reducing emissions than

SoLoNOx alone and would have a cost effectiveness of $14,483. That

value is still over the $13,000 threshold, but petitioners go on

to argue that DEP set that threshold in 1990 and that it should be

updated for inflation at a minimum.14         So adjusting, petitioners

claim, would make the SCR/turbine combination cost feasible.

           We need not reach the inflation issue because we agree

with DEP that 9 ppm was the correct baseline under the applicable

BACT guidance.15    The NSR Workshop Manual addresses this situation

under the instructions for average cost effectiveness:                "When

calculating   the    cost    effectiveness    of   adding   post   process




      14 The Presiding Officer rejected petitioners' argument in
this case but recommended that DEP consider updating its thresholds
for inflation going forward.
     15  Petitioners do not argue that an inflation adjustment
alone would make $41,541 per ton cost feasible.


                                  - 27 -
emissions     controls   to   certain     inherently   lower   polluting

processes, baseline emissions may be assumed to be the emissions

from the lower polluting process itself.       In other words, emission

reduction credit can be taken for use of inherently lower polluting

processes."    NSR Workshop Manual, supra, at B.37.       The SCR is a

post-process emissions control, and SoLoNOx is an inherently lower

polluting process (as compared to other Dry Low NOx turbines), so

SoLoNOx's 9-ppm emissions rate is the proper baseline.

            If there was any doubt as to whether the above rule

applies here, DEP's 2011 BACT Guidance offers an even more on-

point case study.    It says:

            In the recent past, boiler manufacturers have
            developed "ultra-low NOx burners" (UNLBs)
            which can achieve an oxides of nitrogen
            emission rate of 9 parts per million (ppm).
            Before the advent of UNLBs, BACT for NOx for
            boilers with capacity above approximately
            50 million British thermal units per hour was
            achieved by the use of Selective Catalytic
            Reduction (SCR) to reduce NOx emissions to
            5 ppm, accompanied by a 5 ppm ammonia (NH3)
            slip. When analyzing the incremental cost of
            using SCR to reduce the 9 ppm NOx emission
            rate attained by UNLB to reach a 5 ppm NOx
            emission limit, it became readily apparent
            that requiring SCR with added NH3 emissions
            would be economically infeasible, on a
            dollar-per-ton-of-pollutant-removed    basis.
            Therefore, NOx BACT for this category of
            emission units is now 9 ppm, with no NH3
            emissions.




                                 - 28 -
BACT Guidance, supra, at 5 (footnote omitted).    Algonquin and DEP

followed this guidance to a T, so we can hardly call DEP's decision

arbitrary and capricious.16

           Petitioners argue that DEP's approach yields undesirable

results.    "Algonquin's 9-ppm turbine may be state-of-the-art,"

they say, "but the BACT process is focused not on technological

progress for its own sake. . . .       If pairing two older or less

effective technologies will achieve a better result than cutting

edge, standalone technology, BACT favors the former."

           Perhaps. But on the other hand, there may be good policy

reasons for DEP's current approach.    Requiring applicants to fully

analyze every combination of add-on technology and process-control

technology, including different models of the same technology,

would make an already drawn out and expensive process even more

so.   And, as the case study shows, exclusion of the SCR in this

situation may result in slightly higher NOx emissions, but it also

results in lower emissions of ammonia, another air pollutant.


      16Petitioners attempt to distinguish the NSR Workshop Manual
rule by arguing that the phrase "inherently lower polluting
processes" does not apply to newer models of an existing technology
like SoLoNOx, which provide "incremental refinement." But as the
BACT Guidance case study suggests, that is not necessarily the
case. See also 310 Mass. Code Regs. § 7.00 ("The best available
control technology determination . . . may include a[n] . . .
equipment specification . . . .").    The better understanding of
the phrase "inherently lower polluting processes," as DEP has
interpreted it, is any control technology yielding a lower
emissions rate that is not an add-on technology (i.e., a process-
control technology).


                              - 29 -
Lastly,      promoting          technological          development        of        cleaner

technologies may indeed be a goal of the BACT framework.                            Even if

the application of that technology in the instant case does not

reduce emissions, the fact that there is an economic incentive for

manufacturers to develop cleaner technologies may benefit the

state   as   a     whole     by,   for   example,      improving    the     New      Source

Performance Standards (NSPS) applicable to other facilities.                           See

42 U.S.C. § 7411; 310 Mass. Code Regs. § 7.00 (citing 40 C.F.R.

pt. 60).

             In any event, our task is not to pick the better policy.

As DEP points out, nothing in its rules or regulations requires

applicants       to   consider      every      possible   combination          of   older,

dirtier technologies in order to achieve the lowest possible

emissions outcome.

             Finally, we note that petitioners argue that Algonquin

used the wrong formula in calculating the SCR's cost effectiveness,

saying that Algonquin "focused only on incremental costs when the

analysis required demands a focus on average cost."                        Petitioners

miss the point.             The NSR Workshop Manual says that, for add-on

technologies,         the    baseline    for    the    average-cost-effectiveness

calculation is the emissions rate for the technology to which it

is being added (here, SoLoNOx).                  Incremental cost effectiveness

compares     one      control      technology     to   the   next    most      effective

technology (again, SoLoNOx).                So for the SCR, the two formulae


                                         - 30 -
would yield the same result of $41,541 per ton.       It matters not

whether Algonquin called this "average" or "incremental" cost

effectiveness.

                          2.   Toxic Emissions

          Petitioners raise two arguments concerning the Weymouth

station's emissions of toxic air pollutants.       According to DEP,

the Weymouth station's SoLoNOx turbine may emit up to 0.91 tons of

toxic pollutants (combined) per year, with up to 0.41 tons of that

being attributable to formaldehyde emissions.      Formaldehyde is a

genotoxic carcinogenic chemical that can form from incomplete

combustion of natural gas.       Incomplete combustion can occur at

lower temperatures, so formaldehyde can be a particular problem

for Dry Low NOx turbines.

          Since the 1980s, DEP has handled air toxics through

Allowable Ambient Limits (AAL) and Threshold Effects Exposure

Limits (TEL).17 See DEP Office of Research & Standards, Methodology

for Updating Air Guidelines:     Allowable Ambient Limits (AALS) and

Threshold Effects Exposure Limits (TELS) 1–2 (2011); see also DEP,

Ambient   Air    Toxics    Guidelines,    https://www.mass.gov/files/

documents/2017/11/07/Ambient%20Air%20Toxics%20Guidelines.pdf. TEL




     17  For context, EPA regulates air toxics at the federal level
through the National Emission Standards for Hazardous Air
Pollutants (NESHAP). See 42 U.S.C. § 7412. Petitioners do not
argue that the Weymouth station will exceed or in any way implicate
the NESHAP for formaldehyde or any other pollutant.


                                 - 31 -
is a 24-hour-based measurement reflecting toxic concentrations at

a   low-enough      level         that    no     health     effects       at    all,     even

noncarcinogenic effects like eye irritation, "are expected in the

population, including sensitive populations, over a lifetime of

continuous exposure."             In common parlance, the record labels the

risk at these levels "de minimis; i.e., the added risk is so small

that   it   makes      no   meaningful         difference."         AAL    is   an     annual

measurement focused on risks of cancer in humans and is determined

by the lower of the TEL and the Non-Threshold Effects Exposure

Limits (NTEL).         For certain air toxics, scientists consider any

non-zero concentration in the air to produce some risk of cancer

in humans, see Brookline II, 497 N.E.2d at 11, so NTEL represents

a de minimis cancer risk rather than no risk at all.

            We   turn       now    to     each   of   the     petitioners'       arguments

concerning AAL and TEL.

                            a.    Background Toxic Levels

            Petitioners claim that DEP violated the Massachusetts

CAA and associated regulations because it failed to account for

background levels of air toxics near the Weymouth station when

considering      AAL    and       TEL.         They   point    to    three      chemicals:

formaldehyde, benzene, and acrolein.                  They accept, for purposes of

this   argument,       but       see     infra   section II(B)(2)(b),            that    the

marginal emissions from the Weymouth station for each of these air

toxics fall below the AAL and TEL, but they contend that the


                                           - 32 -
Weymouth     emissions   plus    the      already-existing      background

concentrations exceed those values.         In fact, even without the

Weymouth emissions, background levels from other sources in that

area already exceed the AAL and TEL for these three air toxics,

such that any additional emissions would, as petitioners argue,

contribute to an exceedance.     Since Algonquin and DEP ignored this

cumulative effect and only compared the marginal increase to the

AAL   and   TEL,   petitioners    claim    the   air   permit     violates

Massachusetts law.

            In support of their position, petitioners point us to

two Massachusetts regulations.         First, they point us to the

definition of "air pollution," which says in full:

            AIR POLLUTION means the presence in the
            ambient air space of one or more air
            contaminants or combinations thereof in such
            concentrations and of such duration as to:
            (a) cause a nuisance;
            (b) be injurious, or be on the basis of
            current information, potentially injurious to
            human or animal life, to vegetation, or to
            property; or
            (c) unreasonably    interfere     with    the
            comfortable enjoyment of life and property or
            the conduct of business.

310 Mass. Code Regs. § 7.00 (second emphasis added).         Second, the

"General Regulations to Prevent Air Pollution" state:

            No person owning, leasing, or controlling the
            operation of any air contamination source
            shall willfully, negligently, or through
            failure to provide necessary equipment or to
            take   necessary   precautions,  permit   any
            emission for said air contamination source or


                                 - 33 -
           sources   of    such   quantities    of  air
           contaminants which will cause, by themselves
           or   in    conjunction    with    other  air
           contaminants, a condition of air pollution.

Id.   § 7.01(1)    (emphasis   added).       Because   these    regulations

contemplate "combinations" of air contaminants, or contaminants

"in conjunction" with one another, petitioners say that DEP is

mandated to consider background levels of air toxics.

           DEP    responds   that    its   "longstanding    policy"   is   to

compare only emissions from the new source to the applicable AAL

and TEL, without regard to background levels.              According to its

1989 policy statement, DEP requires new or modified sources of air

contaminants to "assess, through computer modeling, the ambient

concentrations caused solely by that source's emissions," and

"[t]hese modelled concentrations are then compared to the AALs to

determine whether there may be potentially unacceptable risks

associated with that particular source."         DEP Div. of Air Quality

Control, Air Toxics Implementation Update 2 (1989) [hereinafter

1989 Air Toxics Update] (emphasis added).          DEP calls the AAL and

TEL "screening guidelines," whereby new sources that exceed these

values are subject to "further evaluation" and new sources below

these values receive no further scrutiny.          And, DEP points out,

most states have a similar two-step approach to air toxics, whereby




                                    - 34 -
step one (here AAL and TEL comparison) is for screening purposes

only.18

             Petitioners argue, in substance, that to interpret the

regulation's "in conjunction with" language as not requiring an

assessment of the cumulative level of background and proposed new

emissions would be to adopt an irrational or absurd interpretation

of the regulation.     We disagree, finding it perfectly rational to

use   a    low   threshold    to   identify   those   instances   in   which

additional, cumulative impacts need be examined.            Consider, for

the sake of analogy, a baking hobbyist who plans on making a pie

for a family reunion.        The baker knows he has sugar, but he is not

certain how much, and he may not even have the full cup needed for

the recipe.      Before he can start baking, his neighbor knocks on

his door and asks to borrow some sugar.         And, to make this analogy

more like this case, imagine that measuring the baker's current

supply of sugar would be "resource intensive" for the baker and

his neighbor at that particular moment when the neighbor needs the

sugar.

             It would be a perfectly reasonable response in this

scenario for the baker to ask his neighbor how much sugar he needs.

If the neighbor wants only a teaspoon, the baker might simply give


      18 In practice, it appears, that applicants regard surviving
step-one screening as essential, given that DEP Air Quality Section
Chief Thomas Cushing could recall no instance in which DEP received
an application for a project that exceeded an AAL or TEL.


                                    - 35 -
it to the neighbor without first measuring his own supply.                    A

teaspoon is likely too little to make a difference between having

and not having a cup, and even if it would, the practical effect

will not be noticeable.        But if the neighbor wants a quarter cup,

then the baker might decide to spend the resources to measure his

supply before agreeing to the neighbor's request.

           This is in essence the purpose of AAL and TEL.                Before

deciding whether to require that the resources be spent to measure

the current cumulative amount of air toxics, DEP asks whether the

amount to be produced by the new source is like the teaspoon or

like the quarter cup.      If it is like the teaspoon, DEP decides

that it is unlikely to make a practical difference. And here DEP's

case is perhaps even stronger than the pie example, because

petitioners have not pointed us to any other cap on how much

pollutant is too much cumulatively (other than NESHAP, see supra

note 17, and petitioners do not allege that the Weymouth station

is even close to violating that standard).              In other words, it

would be as if the pie recipe said "roughly one cup, depending on

how sweet you want it."

           Petitioners,    for     their    own     analogy,   point     us   to

California's rules for automobile tailpipe emissions.               See Motor

Vehicle   Mfrs.   Ass'n   of    U.S.,   Inc.   v.    N.Y.   Dep't   of   Envtl.

Conservation, 17 F.3d 521, 524–25 (2d Cir. 1994) (explaining

California's unique exemption from federal preemption over mobile-


                                   - 36 -
source regulations).         "[P]rior to the creation of California's

stringent air pollution regulations," petitioners tell us, "daily

emissions from millions of . . . vehicles resulted in a chronic

condition    of     air   pollution --   smog --     in    the   City   of   Los

Angeles . . . even though the incremental emissions from each of

those   individual        vehicles   undoubtedly      represented       a    tiny

contribution to the overall problem, and a de minimis risk to human

health."

             The tailpipe example would be like our hypothetical

example if the baker had twelve neighbors at his door all asking

for sugar.    In that case, he might want to measure his supply even

if each neighbor wanted only a teaspoon.           But surely there are not

"millions"     of    proposed   stationary       sources   of    formaldehyde,

benzene, and acrolein around Weymouth.            It is completely rational

for DEP to treat this limited number of sources differently for

screening purposes than California treats personal automobiles.

Cf. 310 Mass. Code Regs. § 7.02(2)(b)(7) (excluding stationary

sources not capable of emitting one ton or more of any pollutant

from the air-permit requirement).             Moreover, there are plenty of

other examples of air-pollution regulatory schemes that similarly

screen out de minimis sources.        In addition to other states' rules

on air toxics, DEP points us to the Significant Impact Limits

(SILs) used by EPA when assessing compliance with the National

Ambient Air Quality Standards (NAAQS).             See Sierra Club v. EPA,


                                     - 37 -
705 F.3d 458, 461 (D.C. Cir. 2013) (explaining SILs); see also

Sierra Club v. EPA, 955 F.3d 56, 58–60 (D.C. Cir. 2020); 40 C.F.R.

§ 51.166(b)(23)(i) (setting net-emissions-increase levels deemed

"[s]ignficant"    for     purposes    of    Prevention     of    Significant

Deterioration (PSD)).     The fact that some regulatory programs take

a different approach does not make these programs irrational.

           Of course, the fact that DEP's two-step approach is

rational does not dispense with petitioners' argument that the

Massachusetts    regulations    compel     consideration    of   background

levels.    Nevertheless, we defer to the agency's interpretation.

The regulations to which petitioners point us, 310 Mass. Code Regs.

§§ 7.00,   7.01(1),     are   broad   regulations    concerning    all   air

pollution generally, not just air toxics.           See Town of Brookline

v. Comm'r of Dep't of Envtl. Quality Eng'g ("Brookline I"), 439

N.E.2d 792, 799 (Mass. 1982) (giving DEP discretion to interpret

310 Mass. Code Regs. § 7.01); see also Brookline II, 497 N.E.2d at

13 ("The Legislature has granted [DEP] broad authority.").            And we

do not think that the language from those general regulations

unambiguously forecloses DEP's approach to air toxics. The phrases

"in conjunction with other air contaminants," 310 Mass. Code Regs.

§§ 7.01(1), and "combinations thereof," id. § 7.00, might mean, as

petitioners argue, that DEP should consider background levels of

a given pollutant.      Or they might reasonably be read as referring

to situations where two different air pollutants produce a chemical


                                  - 38 -
reaction in the air, as with ozone precursors, see Ill. State

Chamber of Commerce v. EPA, 775 F.2d 1141, 1143 & n.3 (7th Cir.

1985), in which case the regulations do not compel petitioners'

approach.     Since the text is ambiguous, and traditional tools of

construction do not resolve that ambiguity, we defer to DEP's

reasonable interpretation.         See Kisor, 139 S. Ct. at 2414–18;

Brookline II, 497 N.E.2d at 15.

                           b.   Startup Emissions

            Petitioners also argue that the formaldehyde emissions

solely    from   the   Weymouth   station   will,   in   fact,   exceed   the

applicable AAL and TEL. They argue that Algonquin's air-dispersion

model, which showed that formaldehyde emissions would be no more

than 70% of the AAL, underestimated the actual emission rate

because it did not include emissions during intermittent startup

events.

            The proposed SoLoNOx turbine will not run continuously.

Instead, it will be shut down and restarted up to 416 times per

year.    Normally, the turbine will employ an oxidation catalyst to

reduce formaldehyde emissions, but during the time it takes the

turbine to start up, this catalyst will not be fully operational,

meaning     formaldehyde    emissions   will   be    higher.      Algonquin

estimates that startups will usually last only nine minutes, though

the time may be longer if a startup fails initially.




                                   - 39 -
           The air permit as initially drafted by DEP staff allowed

for startup times up to thirty minutes.         Petitioners opposed the

allowed   startup   period   before   the   Presiding   Officer.    Their

witness, John Hinckley, performed his own dispersion modeling

that, unlike Algonquin's initial modeling,19 accounted for startup

emissions -- one model representing thirty-minute startups, and

another representing nine-minute startups.          Hinckley's thirty-

minute model showed that formaldehyde emissions from the Weymouth

station would exceed the AAL and TEL when measured at the property

line as required by DEP guidance.      See 1 DEP, The Chemical Health

Effects Assessment Methodology and the Method to Derive Allowable

Ambient Limits 21 (1990).     Hinckley's nine-minute model showed an

exceedance of the AAL, but not the TEL, at the property line.           In

response, Algonquin's witness, Justin Fickas, put together a model

representing   eighteen-minute    startups,     which   also   showed   an

exceedance of the AAL.

           The Presiding Officer acknowledged that EPA's guidance

does not require modeling of intermittent startups at all, but

nevertheless expressed concern over the modeled exceedances.            As




     19 DEP policy requires an applicant to model emissions of air
toxics only for sources subject to PSD, plus a few other types of
facilities. See 1989 Air Toxics Update, supra, at 3. The Weymouth
Station does not meet any of these criteria, but DEP exercised its
discretion to request that Algonquin perform this modeling. See
310 Mass. Code Regs. § 7.02(5)(c)(6) (requiring applicants to
furnish air-dispersion modeling "upon request by" DEP).


                                 - 40 -
such, she recommended reducing the allowable startup time from

thirty minutes to eighteen minutes, and the Commissioner adopted

this recommendation.    As to the fact that even the eighteen-minute

model showed a concentration above the AAL, the Presiding Officer

stated that "[t]he slight exceedance shown in the revised modeling

beyond the fence line . . . does not, in my opinion, justify

denying the permit."

          Petitioners' main argument before us is straightforward:

the Weymouth station will, even under the revised air permit,

create an exceedance of the AAL for formaldehyde, and so the permit

should have been denied, and we should vacate DEP's contrary

decision as arbitrary and capricious.       As its name suggests, AAL

is meant to be a "limit," petitioners argue, so an exceedance

should not be allowed, no matter how "slight."

          DEP counters that under the applicable policy, there was

no exceedance here. EPA's guidance explains that "the intermittent

nature   of   the      actual   emissions     associated   with . . .

startup/shutdown [operations] in many cases, when coupled with the

probabilistic form of the standard, could result in modeled impacts

being significantly higher than actual impacts would realistically

be expected to be for these emission scenarios."      Memorandum from

Tyler Fox, Leader, Air Quality Modeling Grp., EPA, to Reg'l Air

Div. Dirs., Additional Clarification Regarding Application of

Appendix W Modeling Guidance for the 1-Hour NO2 National Ambient


                                - 41 -
Air Quality Standard 8 (Mar. 1, 2011), https://www.epa.gov/sites/

production/files/2015-07/documents/appwno2_2.pdf.                         EPA        thus

advises against using startup modeling.                 See id. at 9–10; see also

DEP, Modeling Guidance for Significant Stationary Sources of Air

Pollution    1    (2011)    (noting    that     DEP     generally    follows     EPA's

guidance on air modeling); cf. 40 C.F.R. § 60.8(c) ("Operations

during periods of startup, shutdown, and malfunction shall not

constitute       representative       conditions        for   the    purpose     of     a

performance test . . . .").            DEP relied on this guidance in not

requiring Algonquin to account for startups in its application,

and   petitioners       have   not   pointed       to   anything     in   DEP   policy

requiring a contrary rule.           Although Algonquin provided additional

(non-mandatory) modeling to explore the effects of startup events,

such models have not been adopted into official department policy

or regulations.         Thus, DEP acted within its discretion when it

relied on a model excluding startup emissions to find, without

further evaluation, that there was no exceedance of AAL or TEL.

                     3.    Environmental Justice Policy

            Petitioners claim that DEP failed to comply with the

Massachusetts Environmental Justice (EJ) Policy.

            The    EJ     Policy,    first    implemented       in    2002      by    the

Massachusetts       Executive       Office    of    Energy     and    Environmental

Affairs, states that "all people have a right to be protected from

environmental pollution and to live in and enjoy a clean and


                                       - 42 -
healthful environment," regardless of "race, ethnicity, class,

gender,    or   handicap."    Environmental             Justice   Policy   of   the

Executive       Office   of   Environmental             Affairs     2–3    (2002),

https://www.mass.gov/files/documents/2017/11/29/ej%20policy%2020

02.pdf; see City of Brockton v. Energy Facilities Siting Bd., 14

N.E.3d 167, 171 n.9 (Mass. 2014) (describing the various iterations

of the EJ Policy prior to 2014).                The EJ Policy requires that

agencies subject to it, including DEP, engage in "enhanced public

participation" for projects that meet two criteria:                        (1) the

project site is located within five miles (for air pollutants) of

an   "EJ    population,"20    and     (2) emissions          will     exceed    the

Environmental      Notification     Form        (ENF)    threshold     under    the

Massachusetts Environmental Protection Act (MEPA), Mass. Gen. Laws

ch. 30, §§ 61–62I. City of Brockton, 14 N.E.3d at 172. The policy

requires    "enhanced     analysis        and     review     of     'impacts    and

mitigation'" for projects that meet the first of these criteria

and where emissions will exceed the Environmental Impact Report

(EIR) threshold under MEPA.         Id.

            DEP and Algonquin acknowledge that the Weymouth station

is located within five miles of EJ populations.                     However, they


     20  An EJ population is "a neighborhood where 25 percent of
the households have an annual median household income that is equal
to or less than 65 percent of the statewide median or 25% of its
population is Minority or identifies as a household that has
English Isolation." Environmental Justice Policy of the Executive
Office of Energy and Environmental Affairs 3 (2017).


                                    - 43 -
argue, the Weymouth station's emissions exceed neither the ENF nor

the EIR thresholds, so the EJ Policy is not implicated.               Seeing no

rejoinder from petitioners on this point, we agree.

           Petitioners nevertheless argue that DEP was required to

do something more.        They cite City of Brockton, which stated in

dicta that "[t]he EJ policy does impose a general, but affirmative,

requirement   on    all    agencies   covered       by   it . . .   to    develop

strategies designed 'to proactively promote environmental justice

in all neighborhoods' in a manner tailored to and consistent with

that agency's 'specific mission.'"           Id. at 174 n.17.       The City of

Brockton court said there "may be an argument that under this

general requirement," agencies must incorporate EJ principles into

certain agency decisions for projects not implicating the enhanced

public participation or enhanced analysis criteria, but the court

ultimately left the question unresolved.                 Id. (emphasis added).

Since DEP has not, in petitioners' view, developed any special

"strategies," petitioners say we should invalidate the air permit

for noncompliance with the EJ Policy.

           We decline to do so.       City of Brockton does not mandate

that agencies go beyond the two requirements set out in the EJ

Policy, only that there "may" be such a requirement. In this case,

there is no real need to resolve this issue of Massachusetts law.

Even   assuming    DEP    is   required   to   go    beyond   the   two    stated

requirements, here DEP allowed for enhanced public participation


                                    - 44 -
even though there was no exceedance of the ENF threshold.                     City of

Brockton also recognized that agencies would need time to implement

any special strategies, id., and the 2017 updated EJ Policy (issued

after City of Brockton) says that all agency strategies "will be

consolidated into one Secretariat EJ Strategy and will be finalized

by   a    date    established   by     the    Secretary       [for     Energy       and

Environmental      Affairs],"   Environmental          Justice      Policy    of   the

Executive Office of Energy and Environmental Affairs 9 (2017).                      So

we can hardly blame DEP for the fact that this future date has not

arrived    yet.     Petitioners      also    do    not    explain    what    special

procedures they have in mind, only that DEP should have implemented

something more than it did.           We are unwilling to disturb DEP's

decision in this case with only the vague admonition that it needed

to do more, without saying what more is needed.21

            Petitioners     point      us         to     Brockton     Power        Co.,

Nos. 2011-025,      2011-026,   2016 WL 8542559           (Mass.     DEP     July 29,

2016), in which DEP "performed an enhanced substantive review" of

a power plant even though the relevant MEPA thresholds were not




     21  Petitioners also argue, in one sentence, that the
Presiding Officer's decision to dismiss the EJ Policy claim prior
to the hearing was "arbitrary, capricious, an abuse of discretion,
otherwise not in accordance with law, contrary to constitutional
right or short of statutory right."     To the extent petitioners
attempt to raise a separate challenge -- constitutional or
otherwise -- to the Presiding Officer's procedure, we deem this
argument waived for lack of development.     See United States v.
Zannino, 895 F.2d 1, 17 (1st Cir. 1990).


                                     - 45 -
triggered.     Id. at *57.     So, petitioners say, the EJ Policy does

not prevent DEP from voluntarily doing more, and the unexplained

departure from what DEP did in Brockton Power was arbitrary and

capricious.    We disagree.     As Brockton Power recognizes, DEP can,

in its discretion, engage in further review "on a case-by-case

basis," id., and in this case it chose not to do so (except for

the enhanced public participation).

             Finally, petitioners draw our attention to Friends of

Buckingham,    in     which   the   Fourth     Circuit   vacated   Virginia's

approval of a compressor station because the agency failed to

comply with Virginia's EJ requirements.                  947 F.3d at 87–92.

Friends of Buckingham is easily distinguishable, though, because

Virginia's      EJ     requirements      are     not     Massachusetts's   EJ

requirements.        A violation of the former, even on similar facts,

would not necessarily be a violation of the latter, and as we have

determined, there was no violation of Massachusetts's EJ policy

here.

                                    4.   Noise

             Petitioners argue that noise from the Weymouth station

will cause a nuisance, and hence an impermissible "condition of

air pollution," in violation of 310 Mass. Code Regs. §§ 7.00,

7.01(1) (quoted supra section II(B)(2)(a)).22



        Petitioners also mention "unpleasant odors" from the
        22

Weymouth station, but do not explain how such odors would create


                                     - 46 -
             According to the HIA, the Weymouth station will produce,

under   normal    nighttime    operating       conditions,    up   to   47 dB(A)

(A-weighted      decibels)    of    noise     as   measured   at   King's    Cove

recreation    area.     When       combined    with   background    noise,    the

nighttime noise at one nearby residence will be 46 dB(A).                   While

the station is under construction, noise is estimated to be up to

113 dB(A).     The HIA also noted that "EPA recommends an average

24-hr exposure limit of 45 dB(A) indoors and 55 dB(A) outdoors,"

and that the World Health Organization (WHO) recommends nighttime

exposure of "45 dB(A) or less."         Petitioners argue in effect that,

because the estimated noise levels will exceed EPA's and WHO's

recommendations, the station will create a nuisance.               They present

no other argument for how the noise would be considered a nuisance,

nor do they cite to any case or statute pertaining to Massachusetts

nuisance law.

             DEP regulations specifically address noise.            See Morales

v. Trans World Airlines, Inc., 504 U.S. 374, 384 (1992) ("[I]t is

a commonplace of statutory construction that the specific governs

the general . . . .").         310 Mass. Code Regs. § 7.10(1) states:

"No person owning, leasing, or controlling a source of sound shall

willfully, negligently, or through failure to provide necessary



a nuisance.    We consider this argument waived for lack of
development.   See Zannino, 895 F.2d at 17.        To the extent
petitioners claim that the Weymouth station will in any other way
cause a nuisance, those arguments are likewise waived.


                                      - 47 -
equipment,    service,   or   maintenance    or   to   take   necessary

precautions cause, suffer, allow, or permit unnecessary emissions

from said source of sound that may cause noise."         Id. (emphasis

added).23    DEP has established a "Noise Policy" interpreting when

emissions of sound are unnecessary.         See Mass. Exec. Office of

Envtl. Affairs, Dep't of Envtl. Quality Eng'g, Division of Air

Quality            Control            Policy 90-001            (1990),

https://www.mass.gov/doc/massdep-noise-policy/download.          Under

that policy, a source of sound will violate the noise regulation

if the source "[i]ncreases the broadband sound level by more than

10 dB(A) above ambient" or "[p]roduces a 'pure tone' condition."

Id.

            Petitioners do not argue that DEP's Noise Policy is an

unreasonable interpretation of 310 Mass. Code Regs. § 7.10(1), so

we give deference to that policy.     See Kisor, 139 S. Ct. at 2414–

18; Brookline II, 497 N.E.2d at 15.      Petitioners also do not argue

that the Weymouth station will create a "pure tone" condition or,

except for the construction period and "emergency blowdowns,"24

increase broadband sound by more than 10 dB(A) above ambient.

Thus, there is no violation of the Noise Policy.         The fact that


      23"Noise" is defined as "sound of sufficient intensity
and/or duration as to cause or contribute to a condition of air
pollution." 310 Mass. Code Regs. § 7.00.
     24 A "blowdown" is a venting of gas.     Emergency blowdowns
"will be extremely rare after initial commissioning" of the
Weymouth station and would last no more than five minutes.


                                - 48 -
EPA or WHO might recommend a lower level of noise would presumably

be of interest to DEP.             But that fact provides no basis for saying

that DEP has violated any relevant law.

                   As to the construction period, the HIA estimated that

sound from construction equipment will increase ambient sound

levels by 12 dB(A), which is over the Noise Policy's 10-dB(A)

limit.25            However,      DEP    says   that    it     has    a   "longstanding

practice" --             to    which      its   witnesses       testified        at     the

hearing -- "not to apply the Noise Policy to temporary construction

sound" for purposes of air permitting, and "instead to require

appropriate noise mitigation measures."                       And DEP required just

such mitigating measures for construction of the Weymouth station,

including           limited     construction      hours,       mufflers        for     heavy

equipment, quieter backup alarms, portable noise barriers, and a

noise complaint hotline.                 Similarly, DEP says that it does not

apply        the    Noise     Policy    to   "unplanned      emergency       events"   like

blowdowns, which could increase ambient sound by up to 17 dB(A),

and it instead required a "blowdown silencer" to muffle the noise

from     such       an   event.         Petitioners    do    not     argue    that     DEP's

"longstanding practice[s]" regarding construction and emergency

events are unreasonable, nonexistent, or in any other way invalid,

so we will again defer to those practices.                         See Doe v. Leavitt,



        DEP contends that the actual increase will be closer to
        25

8 dB(A), below the limit.


                                             - 49 -
552 F.3d 75, 80–81 (1st Cir. 2009) (describing deference based on

Skidmore v. Swift & Co., 323 U.S. 134 (1944)).

                        5.     Manufacturer Guarantee

            Petitioners fault the Presiding Officer's finding that

"[t]he Solar turbine specified for the proposed Project has a

guaranteed    emission       rate   for    NOx   of    9 ppm,"      given   that   the

technical proposal from Solar Turbines contains no guarantee of

emissions rate.        Therefore, petitioners reason, the Presiding

Officer's finding was not supported by substantial evidence, or

DEP's decision based on that finding was otherwise arbitrary and

capricious.

            Algonquin    contends         that   the       record    does   otherwise

contain evidence of a guaranteed emissions rate from Solar.                        But

even   assuming   it    does    not,      we   see    no   merit    in   petitioners'

argument.     Petitioners point us to nothing in Massachusetts law

suggesting that a guarantee from the equipment manufacturer is

required for approval of Algonquin's air permit.                            And, more

importantly, if the SoLoNOx turbine does not work as claimed in

the permit, that will be Algonquin's problem, not petitioners' or

even DEP's.    Simply put, Algonquin could be fined and ordered to

reduce operations to eliminate any violation of its permitted

limits. See Mass. Gen. Laws ch. 111, § 142B. Whether or not Solar

could be liable to Algonquin for such a violation affects only the

allocation of costs as between those parties.                      See Catlin v. Bd.


                                       - 50 -
of Registration of Architects, 604 N.E.2d 1301, 1305 (Mass. 1992)

("The   appealing   party   has   the   burden     of   showing   that    his

'substantial   rights . . .    may   have   been    prejudiced'    by     the

agency's error."    (quoting Mass. Gen. Laws ch. 30A, § 14(7))); see

also Sasen v. Spencer, 879 F.3d 354, 366 (1st Cir. 2018).

                      6.    Insurance Requirement

           Petitioners claim that DEP violated Mass. Gen. Laws

ch. 21C, § 4 by granting Algonquin's air permit without requiring

Algonquin to obtain liability insurance or a surety bond.                That

statute states:

           The department shall require that a licensee
           obtain and maintain in effect a contract of
           liability insurance, a surety bond or other
           evidence of financial responsibility in favor
           of the commonwealth sufficient to assure
           financial responsibility in the event of
           damages resulting from accidents, negligence,
           misconduct,   or    malfunctioning   in   the
           construction, maintenance and operation of a
           facility, or from any other circumstances
           reasonably foreseeable occurring during or
           after construction or in the course of the
           maintenance and operation of hazardous waste
           facilities.

Mass. Gen. Laws ch. 21C, § 4.

           Petitioners' argument lacks merit because Mass. Gen.

Laws ch. 21C, the Massachusetts Hazardous Waste Management Act,

has nothing to do with air permitting.      Indeed, petitioners do not

even attempt to argue that the Weymouth station is a "hazardous

waste facilit[y]" subject to the insurance requirement.            See id.



                                  - 51 -
§ 2 (defining "facility" and "hazardous waste"); see also 310 Mass.

Code Regs. §§ 30.131–30.136 (listing hazardous wastes).                 We thus

reject petitioners' argument.

                                  C.    Remedy

            Having determined that DEP erred in one regard, see supra

section II(B)(1)(a), we finally must determine the appropriate

remedy. Algonquin urges us to remand to DEP to address the defects

in the permitting process without vacating the permit approval.

Petitioners say we should vacate and remand.

            Whether to vacate an agency's flawed decision or remand

without vacatur is within our discretion as the reviewing court,

and   "depends   inter   alia    on    the   severity   of   the   errors,   the

likelihood that they can be mended without altering the order, and

on the balance of equities and public interest considerations."

Cent. Me. Power, 252 F.3d at 48.                Algonquin argues that these

factors favor remand without vacatur.            It says that DEP's failure

to fully explain the BACT result could be easily remedied by DEP

providing the missing explanation.                And Congress has already

expressed   a    preference     for    the   speedy   resolution   of   matters

concerning natural-gas facilities, see 15 U.S.C. § 717r(d)(5),

particularly where FERC has already declared the facility to be

required by considerations of "public convenience and necessity."

So, Algonquin argues, the balance of equities favor a narrow

remedy.


                                       - 52 -
             Petitioners say that, under the federal APA, vacatur is

the default remedy, see 5 U.S.C. § 706(2) (requiring reviewing

courts to "hold unlawful and set aside" defective agency actions),

and that remand without vacatur is a limited exception that should

apply mainly to agency rulemaking, see generally Note, Kristina

Daugirdas, Evaluating Remand Without Vacatur:            A New Judicial

Remedy for Defective Agency Rulemakings, 80 N.Y.U. L. Rev. 278

(2005).   And they argue that the Central Maine Power factors favor

vacatur because the BACT error was severe, correcting the error

may potentially alter DEP's final decision (i.e., DEP might decide

that the electric motor is the BACT), and the interests of the

public in being protected against harmful air pollution tip the

balance of equities.

             Both sides' arguments are persuasive, but we decide to

vacate based on three additional considerations. First, we believe

the administrative record as it exists now is insufficient for DEP

to complete the BACT analysis. As shown by our attempts to perform

the missing calculations, see supra section II(B)(1)(a), it is

impossible     even   to   calculate   the   magnitude    of   the   cost

effectiveness of an electric motor without more information about

either SoLoNOx's costs or the base-case emissions rate.               By

vacating, we allow DEP to reopen the administrative record for the

purpose of filling these evidentiary gaps.       DEP may also wish to

elicit more evidence on the actual costs of the electric motor.


                                 - 53 -
Second, Algonquin has expressed a desire to pursue its redesign

argument, see supra note 6, and vacating will allow DEP to fully

consider that issue as well.   Third, we expect and anticipate that

any further proceeding before the DEP will be limited to these

purposes and will be expedited.

                                III.

          For the foregoing reasons, we vacate the air permit and

remand to DEP for it to conduct further proceedings, limited to

the purposes we have identified.        We further order that if and

when DEP determines that it cannot reasonably conclude those

proceedings and issue a decision within seventy-five days of the

date of this opinion, then DEP will consult with the other parties

and make a filing with this court in this action showing cause why

such additional time is reasonably required.       Any opposition to

the show-cause filing must be filed within seven calendar days of

DEP's filing.   We retain jurisdiction for the limited purpose of

receiving and responding to such a filing.




                               - 54 -
