State of Wisconsin v. Environmental Protection Agency

          Argued October 3, 2018

          On Petitions for Review of Final Action of the United States Environmental Protection Agency

          Neil Gormley argued the cause for petitioners Conservation Groups. Valerie S. Edge, Deputy Attorney General, Office of the Attorney General for the State of Delaware, argued the cause for petitioner State of Delaware. With them on the briefs were David Baron, Charles McPhedran, Joshua R. Stebbins, and Zachary M. Fabish. Seth L. Johnson entered an appearance.

          Misha Tseytlin, Solicitor General, Office of the Attorney General for the State of Wisconsin, and Harvey M. Sheldon argued the causes for State Petitioners, Cedar Falls Utilities, and City of Ames, Iowa. With them on the briefs were Brad D. Schimel, Attorney General, Luke N. Berg, Deputy Solicitor General, Peter Michael, Attorney General, Office of the Attorney General for the State of Wyoming, James Kaste, Deputy Attorney General, Erik Petersen, Senior Assistant Attorney General, Leslie Sue Ritts, Steve Marshall, Attorney General, Office of the Attorney General for the State of Alabama, Robert D. Tambling, Assistant Attorney General, Leslie Rutledge, Attorney General, Office of the Attorney General for the State of Arkansas, Nicholas J. Bronni, Deputy Solicitor General, Michael DeWine, Attorney General, Office of the Attorney General for the State of Ohio, Eric E. Murphy, State Solicitor, Ken Paxton, Attorney General, Office of the Attorney General for the State of Texas, Priscilla M. Hubenak, and Craig J. Pritzlaff and Linda B. Secord, Assistant Attorneys General. Andrew L. Brasher, Deputy Solicitor, Office of the Attorney General for the State of Alabama, Michael J. McGrady, Senior Assistant Attorney General, Office of the Attorney General for the State of Wyoming, Lee P. Rudofsky, Solicitor, Office of the Attorney General for the State of Arkansas, and Ryan Walsh entered appearances.

          Norman W. Fichthorn, Aaron M. Streett, and C. Grady Moore, III argued the causes for Industry Petitioners. With them on the briefs were E. Carter Chandler Clements, Peter S. Glaser, Margaret Claiborne Campbell, M. Buck Dixon, Scott C. Oostdyk, E. Duncan Getchell, Jr., Michael H. Brady, Jane E. Montgomery, J. Michael Showalter, Amy Antoniolli, P. Stephen Gidiere, III, Julia B. Barber, David W. Mitchell, Daniel J. Kelly, David M. Flannery, Kathy G. Beckett, Edward L. Kropp, Megan H. Berge, Charles T. Wehland, Todd E. Palmer, John A. Sheehan, Valerie L. Green, Ben H. Stone, Terese T. Wyly, M. Brant Pettis, Louis E. Tosi, Cheri A. Budzynski, and Michael A. Born. Alina Fortson and Jordan Hemaidan entered appearances.

          Amy J. Dona and Chloe H. Kolman, Attorneys, U.S. Department of Justice, argued the causes for respondents. With them on the brief were Jonathan Brightbill, Deputy Assistant Attorney General, and Stephanie L. Hogan, Attorney, U.S. Environmental Protection Agency.

          E. Carter Chandler Clements argued the cause for Industry Respondent-Intervenors. With her on the brief were Norman W. Fichthorn, Peter S. Glaser, Margaret Claiborne Campbell, M. Buck Dixon, Scott C. Oostdyk, E. Duncan Getchell, Jr., Michael H. Brady, Robert A. Manning, and Joseph A. Brown.

          Andrew G. Frank, Assistant Attorney General, argued the cause for State Intervenors. With him on the brief were Eric T. Schneiderman, Attorney General at the time the brief was filed, Office of the Attorney General for the State of New York, Barbara D. Underwood, Solicitor General, Steven C. Wu, Deputy Solicitor General, Michael J. Myers, Senior Counsel, Maura Healey, Attorney General, Office of the Attorney General for the Commonwealth of Massachusetts, Jillian M. Riley, Assistant Attorney General, Environmental Protection Division, Peter F. Kilmartin, Attorney General, Rhode Island Department of Attorney General, Gregory S. Schultz, Special Assistant Attorney General, Brian E. Frosh, Attorney General, Office of the Attorney General for the State of Maryland, Michael F. Strande, Assistant Attorney General, Gordon J. MacDonald, Attorney General, K. Allen Brooks, Assistant Attorney General, New Hampshire Office of the Attorney General, Thomas J. Donovan, Jr., Attorney General, Office of the Attorney General for the State of Vermont, and Nicholas F. Persampieri, Assistant Attorney General. Morgan A. Costello, Assistant Attorney General, Office of the Attorney General for the State of New York, entered an appearance.

          Charles McPhedran argued the cause for Public Health and Environmental Intervenors. With him on the brief were Sean H. Donahue, Susannah L. Weaver, Graham G. McCahan, Vickie L. Patton, Ann Brewster Weeks, Neil Gormley, David Baron, Howard Fox, Joshua R. Stebbins, and Zachary M. Fabish.

          Hope M. Babcock was on the brief for amicus curiae American Thoracic Society in support of respondent-intervenors American Lung Association, Appalachian Mountain Club, Environmental Defense Fund, and Sierra Club.

          Before: Srinivasan, Millett and Wilkins, Circuit Judges.

          PER CURIAM.

         When upwind States pollute, downwind States can suffer the consequences. Congress addressed that problem in the Clean Air Act by enacting a "Good Neighbor Provision." The Provision requires upwind States to eliminate their significant contributions to air quality problems in downwind States.

         In 2016, the Environmental Protection Agency implemented that requirement by promulgating a regulation addressing the interstate transport of ozone, or smog. A number of parties brought challenges to the Rule, some contending that the Rule is too strict and others contending that it is too lenient.

         We conclude that, in one respect, the Rule is inconsistent with the Act: it allows upwind States to continue their significant contributions to downwind air quality problems beyond the statutory deadlines by which downwind States must demonstrate their attainment of air quality standards. In all other respects, though, we determine that EPA acted lawfully and rationally.

         I

         The Clean Air Act tasks EPA with setting national ambient air quality standards, or NAAQS. See 42 U.S.C. § 7409(a). Individual States must ensure that their ambient air quality complies with the national standard. To that end, the Clean Air Act requires States to adopt State implementation plans, or SIPs, that provide for implementation, maintenance and enforcement of the national standard. Id. § 7410(a)(1). If a State fails to submit a SIP, or if EPA disapproves it, EPA must issue a federal implementation plan, or FIP, to correct any deficiency. Id. § 7410(c)(1).

         State-level regulation of air quality faces a confounding variable. Air pollution, once emitted, drifts with the wind. Upwind pollutants affect air quality in downwind States via various chemical processes. Ozone, for example, forms from the interaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. For downwind States, upwind emissions of these ozone precursors can pose a significant problem. According to a study referenced by EPA, on average, over three-quarters of the ground-level ozone in downwind States comes from upwind emissions. 81 Fed. Reg. at 74, 514.

         Congress included a Good Neighbor Provision in the Clean Air Act to address the problem of upwind States' pollution impairing downwind States' air quality. The Provision prohibits States from "emitting any air pollutant in amounts" that will "contribute significantly to nonattainment" or "interfere with maintenance" of air quality in other States. 42 U.S.C. § 7410(a)(2)(D)(i).

         EPA has addressed the Good Neighbor Provision's requirements in a series of rulemakings. In 2011, EPA promulgated the Cross-State Air Pollution Rule (CSAPR), which applied to States whose upwind pollution violated good neighbor obligations under the 1997 ozone NAAQS and the 1997 and 2006 fine particulate matter NAAQS. See 76 Fed. Reg. 48, 208 (Aug. 8, 2011).

         In 2008, EPA reduced the ozone NAAQS from 80 parts per billion (ppb) to 75 ppb. As a result, EPA promulgated the rule at issue in this case: an update to the CSAPR for eastern States that accounts for the stricter 2008 ozone NAAQS. See Cross-State Air Pollution Rule Update for the 2008 Ozone NAAQS, 81 Fed. Reg. 74, 504 (Oct. 26, 2016) (Update Rule). Under the Update Rule, EPA proceeded in four steps.

         At the first step, EPA identified downwind States expected to have problems attaining or maintaining air quality in compliance with the 2008 ozone NAAQS. To identify those States, EPA had to estimate the future air quality in each State. Id. at 74, 516-17. EPA devised a measure to turn 2011 ozone measurements into 2017 projections.

         EPA started with 2011 modeled data from "receptors," devices in each State that measure air quality. EPA modeled ozone concentrations in a three-by-three grid around each receptor. EPA chose the ten days with the highest projected ozone concentration, noted which of the nine 12-km2 grid cells had the highest ozone concentration on that day, and averaged the ten observations. See id. at 74, 526-27. EPA then ran the model for 2017, inputting 2011 environmental conditions (like rainfall and fire emissions) but projected 2017 NOx emissions rates. The percentage change from 2011 to 2017 was deemed a receptor's "relative response factor," which measures the sensitivity of an area to ozone formation. Multiplying a 2011 observation by the relevant response factor yielded a projection for 2017 for the receptor.

         EPA projected 2017 ozone levels for each receptor by applying the relative response factor to three "design values" from a receptor across a five-year period. A "design value" is a three-year historical average of monitored ozone data. The selected design values represented 2009-2011, 2010-2012, and 2011-2013. See id. at 74, 532. Multiplying the three design values by the applicable relative response factor resulted in three different ozone projections for 2017.

         In light of the 2008 ozone NAAQS of 75 ppb, EPA considered any projected value of up to 75.9 ppb to constitute attainment. Id. EPA defined a receptor to be in "nonattainment" status if the average of its three projected design values and its most recent monitored design value (2013-2015) exceeded the NAAQS. Id. EPA also identified a third category (beyond attainment and nonattainment): a receptor would be deemed in "maintenance" status if the highest of the three projected design values exceeded the NAAQS but the other two values did not. Id.

         At the second step, EPA identified those upwind States whose pollution was linked to nonattainment or nonmaintenance at downwind receptors. EPA quantified the impact of each State's pollution on downwind receptors using a model that apportioned responsibility for ozone formation at a given receptor to various categories of emitters. See id. at 74.536. EPA then multiplied a given State's contribution factor by the projected average ozone concentration at each receptor (calculated in Step 1) to yield each State's contribution to ozone formation at each downwind receptor.

         Next, EPA screened out States whose contributions to ozone formation in a downwind State comprised less than 1% of the NAAQS (0.75 ppb) in that downwind State. See id. at 74.537. In other words, EPA construed "contribute significantly" and "interfere with" in the Good Neighbor Provision to require an upwind State's pollution to cause at least 0.75 ppb of ozone formation in a downwind State. EPA's analysis concluded that twenty-three States and the District of Columbia were linked to nonattainment or nonmaintenance in at least one downwind State. Fourteen States' contributions fell below the 1% threshold and thus were screened out at this step. See id.

         At the third step, EPA quantified the amount of emissions reductions that the twenty-three upwind States and the District of Columbia would be required to make under the Good Neighbor Provision. To do so, EPA applied a multifactor test that balanced costs against benefits.

         EPA first measured the emissions reductions that would occur at various levels of cost controls. It began with a baseline case (i.e., a cost-control level of $0). It then ran the model for a cost-control level of $800 per ton-in other words, measuring the emissions reductions that would occur if EPA required all States to apply all possible emissions controls at its EGUs (electric generating units, or power plants) up to the marginal price of $800 per ton of NOx. See id. at 74, 540-41. EPA then ran the model for higher cost-control levels of $1, 400/ton, $3, 400/ton, $5, 000/ton, and $6, 400/ton. For each of these cost-control levels, EPA also estimated the air quality improvements that would occur at each receptor in downwind States. Comparing the cost-control levels against the resulting emissions reductions and air quality improvements, EPA concluded that the cost-control level of $1, 400 per ton represented the point at which upwind "NOx reduction potential and corresponding downwind ozone air quality improvements are maximized with respect to marginal cost" - that is, the point at which EPA would get the biggest bang for its buck. Id. at 74, 550.

         As part of that analysis, EPA also performed an "overcontrol" analysis to ensure that no upwind State would be required to reduce its emissions more than called for by the Good Neighbor Provision. Overcontrol would occur if either (1) a State's downwind receptors all reduced their ozone concentrations below the NAAQS, or (2) a State's contributions to all downwind receptors was reduced below the 1% contribution threshold of 0.75 ppb. See EME Homer City Gen., L.P. v. EPA, 572 U.S. 489, 521 (2014) ("EME Homer II"). EPA concluded that neither of those indicia of overcontrol existed at the $800/ton and $1, 400/ton cost-control levels. See 81 Fed. Reg. at 74, 551-52.

         At the fourth step, EPA quantified State emissions "budgets" by calculating the emissions amount that would occur under $1, 400/ton cost controls. Under the CSAPR, States may emit more NOx than permitted by their budgets by acquiring allowances from other States, and States may sell allowances to other States if they reduce their emissions more than required by their budgets. Id. at 74, 554. But to ensure that a State does not entirely skirt its good neighbor obligations by buying a large number of allowances, each State is limited to emitting no more than 121% of the emissions budget (the "assurance level"), irrespective of allowances. See id.

         After EPA promulgated the original CSAPR, various parties brought a challenge to that rule. We initially vacated the rule, see EME Homer City Generation, L.P. v. EPA, 696 F.3d 7 (D.C. Cir. 2012) ("EME Homer I"), but the Supreme Court reversed our decision and upheld the rule in its entirety, although it left open the possibility of as-applied challenges, see EME Homer II, 572 U.S. at 495. On remand, we found that emissions budgets for thirteen States were invalid due to overcontrol. EME Homer City Generation, L.P. v. EPA, 795 F.3d 118, 124 (D.C. Cir. 2015) ("EME Homer III").

         In 2016, EPA promulgated the Update Rule at issue here. A number of parties have petitioned for review of the Rule in this court. See 42 U.S.C. § 7607(d)(9). One group of petitioners, including environmental groups and the State of Delaware (collectively, Environmental Petitioners), contends that EPA was required to adopt a more stringent rule. Another group of petitioners, including various States and industry groups (collectively, State and Industry Petitioners), argues that EPA was required to issue a more lenient rule.

         II

         We begin by addressing Environmental Petitioners' claims. Environmental Petitioners challenge the Update Rule on the grounds that it: (i) fails to square with the statutory attainment deadlines, (ii) makes impermissible modeling and implementation choices, and (iii) incorrectly classifies Delaware as an attaining downwind State.

         The Rule must be set aside if it is "arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law" or "in excess of statutory jurisdiction, authority, or limitations." 42 U.S.C. § 7607(d)(9). That standard is "essentially the same" as the familiar standard of review contained in § 706(2) of the Administrative Procedure Act. Ethyl Corp. v. EPA, 51 F.3d 1053, 1064 (D.C. Cir. 1995).

         Applying that standard, we agree with Environmental Petitioners that the Rule is inconsistent with the Act's attainment deadlines. We reject Environmental Petitioners' remaining claims.

         A

         We first consider Environmental Petitioners' argument that the Update Rule infringes the Good Neighbor Provision by permitting upwind States to continue their significant contributions to downwind air quality problems for too long- i.e., past the statutory deadlines for nonattaining downwind areas to meet the NAAQS for ozone. In fact, EPA set no concrete deadline at all for upwind States to eliminate their contributions to downwind States' nonattainment.

         The Good Neighbor Provision requires States to submit SIPs that "prohibit[], consistent with the provisions of this subchapter [i.e., Title I of the Act], any source . . . from emitting any air pollutant in amounts which will . . . contribute significantly to nonattainment in . . . any other State with respect to any" NAAQS. 42 U.S.C. § 7410(a)(2)(D)(i). If a State fails to submit a SIP or submits one deemed inadequate, such that EPA must then prepare a FIP, EPA must likewise satisfy the Good Neighbor Provision in the FIP. See EME Homer II, 572 U.S. at 512-14 & n.15.

         The question we face is one of timing: the Good Neighbor Provision calls for upwind States to eliminate their significant contributions to air pollution in downwind States, but by when must upwind States do so? In particular, does the Provision call for upwind States to eliminate their significant contributions to downwind pollution by the deadlines for downwind areas to comply with the relevant NAAQS-here, the 2008 NAAQS for ozone?

         Those deadlines are prescribed by the Act. The Act first tasks EPA with designating as "nonattainment" any area that does not meet a NAAQS. See 42 U.S.C. § 7407(d)(1)(A). For areas designated as nonattainment for ozone, the Act specifies that each State must secure compliance "as expeditiously as practicable but not later than" a date certain. Id. § 7511(a)(1) (emphasis added). That date is July 20, 2018, for areas in "moderate" nonattainment with respect to the 2008 NAAQS (and was July 20, 2015 for areas in "marginal" nonattainment). See Nat. Res. Def. Council v. EPA, 777 F.3d. 456, 465-66 (D.C. Cir. 2014); Implementation of the 2008 National Ambient Air Quality Standards for Ozone: State Implementation Plan Requirements, 80 Fed. Reg. 12, 264, 12, 268 (Mar. 6, 2015).

         The Update Rule does not require upwind States to eliminate their significant contributions to downwind ozone pollution by that date-or by any date, for that matter. EPA acknowledges that, except for one State (Tennessee), it "is only quantifying a subset of each State's emission reduction obligation pursuant to the good neighbor provision." 81 Fed. Reg. at 74, 520 (emphasis added); see id. at 74, 508 n.19. And the Rule states that it represents only a "first, partial step to addressing a given upwind State's significant contribution to downwind air quality impacts for the 2008 ozone NAAQS." Id. at 74, 522. That is in large part because the Update Rule confines itself to addressing upwind contributions from EGUs due to an ostensible lack of information about non-EGUs. EPA "expects that a full resolution of upwind transport obligations would require emission reductions from sectors besides EGUs," along with "further EGU reductions that are achievable after 2017." Id. The upshot is that, while the Rule calls for a certain level of reductions in upwind contributions by the 2017 ozone year-"in time to assist downwind states to meet the July 2018 attainment deadlines"-the Rule does not purport to require upwind States to fully meet their good neighbor obligations by that time. Id.

         Under the Update Rule, then, downwind States face a dilemma. On one hand, they operate under a statutory obligation to secure compliance with the ozone NAAQS by July 20, 2018. But on the other hand, the Rule does not require upwind States to eliminate their significant contributions to downwind pollution by that deadline. Environmental Petitioners argue that the Rule is inconsistent with the Act in failing to require upwind States to eliminate their significant contributions in accordance with the deadline by which downwind States must come into compliance with the NAAQS. We agree.

         1

         That conclusion follows from our decision in North Carolina v. EPA, 531 F.3d 896 (D.C. Cir. 2008). There, we considered essentially the same question we now face here: whether EPA can allow upwind States to continue their significant contributions to downwind pollution beyond the statutory deadlines for downwind States to meet the NAAQS. North Carolina involved the Clean Air Interstate Rule (CAIR), a prior EPA effort to implement the Good Neighbor Provision with regard to the then-applicable NAAQS for two pollutants, fine particulate matter (PM_2.5) and ozone. See id. at 903-06. Although the statutory deadline for nonattaining areas to comply with those NAAQS was 2010, CAIR gave upwind States until 2015 to eliminate their significant contributions to downwind nonattainment. See id. at 911.

         We held that CAIR's "deadline of 2015 [was] unlawful." Id. at 913. We explained that, under the terms of the Good Neighbor Provision, upwind States must eliminate their significant contributions "consistent with the provisions" of Title I of the Act. See id. at 911-12; 42 U.S.C. § 7410(a)(2)(D)(i). And the incorporated provisions of Title I, we further explained, include ones setting the attainment deadlines for downwind areas. See 42 U.S.C. § 7502(a)(2)(A) (PM_2.5); id. § 7511 (ozone). But under CAIR, "downwind nonattainment areas [were required to] attain NAAQS for ozone and PM_2.5" by 2010, "without the elimination" by then "of upwind states' significant contribution to downwind nonattainment, forcing downwind areas to make greater reductions than [the Good Neighbor Provision] requires." North Carolina, 531 F.3d at 912. As a result, we concluded, "EPA ignored its statutory mandate to promulgate CAIR consistent with the provisions in Title I mandating compliance deadlines for downwind states in 2010." Id.

         All of that is equally true here. Just as with CAIR, the CSAPR Update Rule we consider in this case fails to eliminate upwind States' significant contributions to downwind pollution by the statutory deadline for downwind States to meet the NAAQS for ozone. That in turn "forc[es] downwind areas to make greater reductions than [the Good Neighbor Provision] requires." Id. Indeed, CAIR at least imposed some deadline for upwind States to fully satisfy their good neighbor obligations, albeit a deadline we held was too late. Here, by contrast, EPA established no deadline at all for upwind States to eliminate their significant contributions. And while EPA concluded that requiring upwind States to meet their good neighbor obligations by the 2018 attainment deadline "simply [was] not feasible in the existing timeframe," 81 Fed. Reg. 74, 523, in North Carolina, EPA had likewise sought (unsuccessfully) to rely "on reasons of feasibility." 531 F.3d at 911.

         EPA contends that North Carolina required it only to "consider" the attainment deadline in some fashion when establishing upwind States' good neighbor responsibilities, not to align the attainment deadline with the deadline for satisfying good neighbor obligations. And EPA argues that the Update Rule gives the requisite consideration to the July 2018 attainment deadline by requiring at least some level of good neighbor reductions by that date.

         North Carolina, though, requires more than merely "considering" attainment deadlines in that manner. In fact, CAIR provided for a first phase of reductions in upwind contributions to take place before the attainment deadlines. See id. at 903. But that was not enough to satisfy the statute. The problem was that the eventual elimination of significant upwind contributions in the second phase of reductions would occur only long after the attainment deadlines had passed.

         We explained that EPA needed to "harmonize" the "Phase Two deadline for upwind contributors to eliminate their significant contribution with the attainment deadlines for downwind areas." Id. at 912 (emphasis added). Otherwise, downwind areas would need to attain the NAAQS "without the elimination of upwind states' significant contribution." Id. The Rule here creates the same situation. (And we note it does so with respect to both the 2018 and 2015 deadlines. Although EPA contends that the claim as to the 2015 deadlines was forfeited, we disagree. See Comment of Sierra Club et al., at 8-9, EPA Docket No. 2015-0500-0287 (Feb. 1, 2016), J.A. 1015-16.)

         EPA notes that, when we addressed the issue of the appropriate remedy at the conclusion of our opinion in North Carolina, we observed that EPA would need to "decide what date, whether 2015 or earlier, is as expeditious as practicable for states to eliminate their significant contributions to downwind nonattainment." 561 F.3d at 930 (emphasis added). But our reference to 2015 did not suggest that EPA could delay the deadline for upwind States to eliminate their significant contributions until 5 years after the 2010 attainment deadline. The entire object of our analysis was to reject the notion that the Phase Two deadline of 2015 could be squared with the Good Neighbor Provision. See id. at 913 (EPA operated "under the assumption that 2015 was an appropriate deadline for CAIR compliance. It is not."). Rather, we presumably referred to 2015 because, as we had earlier specifically noted, EPA has separate statutory authority to extend the deadline for attaining the NAAQS for PM_2.5-and thereby correspondingly also extend the good neighbor deadline-for up to five years, or until 2015. See id. at 911 (citing 42 U.S.C. § 7502(a)(2)(A)).

         In sum, under our decision in North Carolina, the Good Neighbor Provision calls for elimination of upwind States' significant contributions on par with the relevant downwind attainment deadlines. The Update Rule fails to do so.

         2

         North Carolina's understanding of the Good Neighbor Provision is confirmed by examining the Update Rule under the framework set out in Chevron U.S.A. Inc. v. Nat. Res. Def. Council, Inc., 467 U.S. 837 (1984). See EME Homer II, 572 U.S. at 512-14 (evaluating previous EPA implementation of the Good Neighbor Provision under Chevron); see also Util. Air Regulatory Grp. v. EPA, 573 U.S. 302, 315 (2014) ("We review EPA's interpretations of the Clean Air Act using the standard set forth in Chevron.").

         "Under Chevron, we presume that when an agency-administered statute is ambiguous with respect to what it prescribes, Congress has empowered the agency to resolve the ambiguity." Util. Air. Regulatory Grp., 573 U.S. at 315. The question then "is whether in doing so the agency has acted reasonably and thus has 'stayed within the bounds of its statutory authority.'" Id. (quoting Arlington v. FCC, 569 U.S. 290, 296 (2013)). Here, the Update Rule's open-ended compliance timeframe exceeds the bounds of EPA's statutory authority by allowing upwind States to continue their significant contributions to downwind nonattainment well past the deadline for downwind areas to comply with the NAAQS.

         The threshold question under Chevron ordinarily would be whether the statute is ambiguous on that issue, such that the agency then would have discretion to choose among reasonable interpretations. E.g., id. But there is no need to resolve that threshold issue in this case, because, regardless of ambiguity, the Update Rule amounts to an unreasonable-and hence impermissible-interpretation of the statute in any event. See Michigan v. EPA, 135 S.Ct. 2699, 2707 (2015); Massachusetts v. U.S. Dep't of Transp., 93 F.3d 890, 892 (D.C. Cir. 1996) (declining to resolve how Chevron might apply because "the agency's determination here cannot be upheld with or without deference").

         The Good Neighbor Provision, as North Carolina emphasized, requires upwind States to eliminate their significant contributions to downwind pollution "consistent with the provisions of this subchapter," i.e., Title I of the Clean Air Act. 42 U.S.C. § 7410(a)(2). One of the "provisions of this subchapter" is § 7511(a)(1), which in turn requires downwind areas in moderate nonattainment to attain the NAAQS by July 20, 2018. See id. § 7511(a)(1); 81 Fed. Reg. at 74, 507. The statute cannot reasonably be understood to enable upwind States to continue their significant contributions ...