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Federal Register / Vol. 86, No. 165 / Monday, August 30, 2021 / Proposed Rules
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four-step interstate transport framework.
The March 2018 memo also included the then newly available contribution modeling results to assist states in evaluating their impact on potential downwind air quality problems for the 2015 ozone NAAQS under step 2 of the interstate transport framework. EPA
subsequently issued two more memoranda in August and October 2018, providing additional information to states developing good neighbor SIP
submissions for the 2015 ozone NAAQS
concerning, respectively, potential contribution thresholds that may be appropriate to apply in step 2 of the framework, and considerations for identifying downwind areas that may have problems maintaining the standard at step 1 of the framework.11
On October 30, 2020, in the Notice of Proposed Rulemaking for the Revised CSAPR Update, EPA released and accepted public comment on updated 2023 modeling that used a 2016
emissions platform developed under the EPA/Multi-Jurisdictional Organization MJO/state collaborative project as the primary source for the base year and future year emissions data.12 On March 15, 2021, EPA signed the final Revised CSAPR Update using the same modeling released at proposal.13 Although Connecticut relied on the modeling included in the March 2018 memo to develop its SIP submission as EPA had suggested, EPA now proposes to primarily rely on the updated and newly available 2016 base year modeling in evaluating these submissions. By using the updated modeling results, EPA is using the most current and technically appropriate information as the primary basis for this proposed rulemaking. EPAs independent analysis, which also evaluated historical monitoring data, recent DVs, and emissions trends, found that such information provides additional support and further 11 See Analysis of Contribution Thresholds for Use in Clean Air Act Section 110a2DiI
Interstate Transport State Implementation Plan Submissions for the 2015 Ozone National Ambient Air Quality Standards, August 31, 2018 August 2018 memo, and Considerations for Identifying Maintenance Receptors for Use in Clean Air Act Section 110a2DiI Interstate Transport State Implementation Plan Submissions for the 2015
Ozone National Ambient Air Quality Standards, October 19, 2018, available in the docket for this action or at https www.epa.gov/airmarkets/memoand-supplemental-information-regarding-interstatetransport-sips-2015-ozone-naaqs.
12 See 85 FR 68964, 68981. The results of this modeling are included in a spreadsheet in the docket for this action. The underlying modeling files are available for public review in the docket for the Revised CSAPR Update EPAHQOAR
20200272.
13 See 86 FR 23054 at 23075, 23164 April 30, 2021.

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substantiates the results of the 2016 base year modeling as the basis for this proposed rulemaking. Section III of this document and the Air Quality Modeling technical support document TSD
included in the docket for this proposal contain additional detail on this modeling.14
In the CSAPR, CSAPR Update, and the Revised CSAPR Update, EPA used a threshold of one percent of the NAAQS
to determine whether a given upwind state was linked at step 2 of the interstate transport framework and would, therefore, contribute to downwind nonattainment and maintenance sites identified in step 1. If a states impact did not equal or exceed the one percent threshold, the upwind state was not linked to a downwind air quality problem, and EPA, therefore, concluded the state would not significantly contribute to nonattainment or interfere with maintenance of the NAAQS in the downwind states. However, if a states impact equaled or exceeded the one percent threshold, the states emissions were further evaluated in step 3, considering both air quality and cost considerations, to determine what, if any, emissions might be deemed significant and, thus, must be eliminated under the good neighbor provision. EPA is proposing to rely on the one percent threshold which is 0.70
ppb for the purpose of evaluating Connecticuts contribution to nonattainment or maintenance of the 2015 ozone NAAQS in downwind areas.
Several D.C. Circuit court decisions address the issue of the relevant analytic year for the purposes of evaluating ozone transport air-quality problems.
On September 13, 2019, the D.C. Circuit issued a decision in Wisconsin v. EPA, remanding the CSAPR Update to the extent that it failed to require upwind states to eliminate their significant contribution by the next applicable attainment date by which downwind states must come into compliance with the NAAQS, as established under CAA
section 181a. 938 F.3d 303, 313.
On May 19, 2020, the D.C. Circuit issued a decision in Maryland v. EPA
that cited the Wisconsin decision in holding that EPA must assess the impact 14 See Air Quality Modeling Technical Support Document for the Revised Cross-State Air Pollution Rule Update, 86 FR 23054 April 30, 2021, available in the docket for this action. This TSD was originally developed to support EPAs action in the Revised CSAPR Update, as relating to outstanding good neighbor obligations under the 2008 ozone NAAQS. While developed in this separate context, the data and modeling outputs, including interpolated design values for 2021, may be evaluated with respect to the 2015 ozone NAAQS
and used in support of this proposal.

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of interstate transport on air quality at the next downwind attainment date, including Marginal area attainment dates, in evaluating the basis for EPAs denial of a petition under CAA section 126b. Maryland v. EPA, 958 F.3d 1185, 120304 D.C. Cir. 2020. The court noted that section 126b incorporates the Good Neighbor Provision, and, therefore, EPA must find a violation of section 126 if an upwind source will significantly contribute to downwind nonattainment at the next downwind attainment deadline. Therefore, the agency must evaluate downwind air quality at that deadline, not at some later date. Id. at 1204 emphasis added. EPA interprets the courts holding in Maryland as requiring the Agency, under the good neighbor provision, to assess downwind air quality by the next applicable attainment date, including a Marginal area attainment date under CAA section 181 for ozone nonattainment.15 The Marginal area attainment date for the 2015 ozone NAAQS is August 3, 2021.16
Historically, EPA has considered the full ozone season prior to the attainment date as supplying an appropriate analytic year for assessing good neighbor obligations. While this would be 2020 for an August 2021 attainment date which falls within the 2021 ozone season running from May 1 to September 30, in this circumstance, when the 2020 ozone season is wholly in the past, it is appropriate to focus on 2021 to address good neighbor obligations to the extent possible by the 2021 attainment date. EPA does not believe it would be appropriate to select an analytical year that is wholly in the past, because the agency interprets the good neighbor provision as forward looking. See 86 FR 23054 at 23074; see also Wisconsin, 938 F.3d at 322.
Consequently, in this proposal EPA will use the analytical year of 2021 to evaluate Connecticuts good neighbor 15 We note that the court in Maryland did not have occasion to evaluate circumstances in which EPA may determine that an upwind linkage to a downwind air quality problem exists at steps 1 and 2 of the interstate transport framework by a particular attainment date, but for reasons of impossibility or profound uncertainty the Agency is unable to mandate upwind pollution controls by that date. See Wisconsin, 938 F.3d at 320. The D.C.
Circuit noted in Wisconsin that upon a sufficient showing, these circumstances may warrant flexibility in effectuating the purpose of the good neighbor provision. Such circumstances are not at issue in the present proposal.
16 CAA section 181a; 40 CFR 51.1303;
Additional Air Quality Designations for the 2015
Ozone National Ambient Air Quality Standards, 83
FR 25776 June 4, 2018, effective Aug. 3, 2018.

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