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Federal Register / Vol. 86, No. 185 / Tuesday, September 28, 2021 / Proposed Rules
nonattainment or maintenance receptor, that site is excluded from further analysis under the EPAs four-step interstate transport framework. For monitoring sites that are identified as nonattainment or maintenance receptors in 2021, we proceed to the next step of our four-step framework by identifying the upwind states contribution to those receptors.
The EPAs approach to identifying ozone nonattainment and maintenance receptors in this proposed action is consistent with the approach used in the CSAPR, the CSAPR Update, and the Revised CSAPR Update. The EPAs approach gives independent consideration to both the contribute significantly to nonattainment and the interfere with maintenance prongs of section 110a2DiI, consistent with the D.C. Circuits direction in North Carolina.27 Further, in its decision on the remand of CSAPR from the Supreme Court in the EME Homer City case, the D.C. Circuit confirmed that the EPAs approach to identifying maintenance receptors in CSAPR comported with the courts prior instruction to give independent meaning to the interfere with maintenance prong in the good neighbor provision.28
For purposes of this proposed action, the EPA identifies nonattainment receptors as those monitoring sites that are projected to have average design values that exceed the NAAQS and that are also measuring nonattainment based on the most recent monitored design values.29 This approach is consistent with prior transport rulemakings, such as the CSAPR Update, where the EPA
defined nonattainment receptors as those areas that both currently monitor nonattainment and that the EPA projects will be in nonattainment in the future analytic year.30 In addition, in this 27 531 F.3d at 910911 holding that the EPA
must give independent significance to each prong of CAA section 110a2DiI.
28 EME Homer City Generation, L.P. v. EPA, 795
F.3d 118, 136 D.C. Cir. 2015 EME Homer City II.
29 Average projected design values are based on the average design value during the five-year base monitoring period i.e., 20142016, 20152017 and 20162018, as discussed in the Final Revised Cross-State Air Pollution Rule Update for the 2008
Ozone NAAQS 86 FR 23054, April 30, 2021 and further clarified in the Air Quality Modeling Technical Support Document for the Final Revised Cross State Air Pollution Rule Update, which is available in the docket for that rulemaking EPA
HQOAR20200272.
30 See 81 FR 74504 October 26, 2016. The Revised CSAPR Update also used this approach.
See 86 FR 23054 April 30, 2021. This same concept, relying on both current monitoring data and modeling to define nonattainment receptors, was also applied in CAIR. See 70 FR 25241 January 14, 2005; see also North Carolina, 531 F.3d at 913
14 affirming as reasonable EPAs approach to defining nonattainment in CAIR.

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proposed action, the EPA identifies a receptor to be a maintenance receptor for purposes of defining interference with maintenance, consistent with the method used in CSAPR and upheld by the D.C. Circuit in EME Homer City II.31 32
Recognizing that nonattainment receptors are also, by definition, maintenance receptors, the EPA often uses the term maintenance-only to refer to receptors that are not also nonattainment receptors. Consistent with the methodology described above, monitoring sites with a projected maximum design value that exceeds the NAAQS, but with a projected average design value that is below the NAAQS, are identified as maintenance-only receptors. In addition, those sites that are currently measuring ozone concentrations below the level of the applicable NAAQS but are projected to be nonattainment based on the average design value and that, by definition, are projected to have a maximum design value above the standard are also identified as maintenance-only receptors.
To evaluate future air quality in steps 1 and 2 of the interstate transport framework, the EPA is using the 2016
and 2023 base case emissions developed under the EPA/MJO/state collaborative emissions modeling platform project as the primary source for base year and 2023 future year emissions data for this proposed rule. Because this platform does not include emissions for 2021, the EPA developed an interpolation technique based on modeling for 2023
and measured ozone data to determine ozone concentrations for 2021. To estimate average and maximum design values for 2021, the EPA first performed air quality modeling for 2016 and 2023
to obtain design values in 2023. The 2023 design values were then coupled with the corresponding 2016 measured design values to estimate design values in 2021. Details on the modeling, including the interpolation methodology, can be found in the Air Quality Modeling TSD, in the docket for this proposed action.
To quantify the contribution of emissions from specific upwind states on 2021 8-hour design values for the identified downwind nonattainment and maintenance receptors, the EPA
first performed nationwide, state-level ozone source apportionment modeling for 2023. The source apportionment modeling provided contributions to 31 See
795 F.3d at 136.
projected design values are based on the maximum design value during the five-year base monitoring period from 2014 to 2018.
32 Maximum
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ozone from precursor emissions of anthropogenic NOX and VOCs in each individual state. The modeled contributions were then applied in a relative sense to the 2021 average design value to estimate the contributions in 2021 from each state to each receptor.
Details on the source apportionment modeling and the methods for determining contributions in 2021 are in the Air Quality Modeling TSD in the docket.
The EPA generally does not consider modeling to be necessary for isolated states like Hawaii for the purposes of evaluating interstate transport.
Therefore, Hawaii was not included in the modeling domain, and the apportionment modeling analysis described above does not calculate emissions contributions from Hawaii to the downwind nonattainment and maintenance areas identified in step 1
in the contiguous United States. In lieu of apportionment modeling, at step 2 of the interstate transport framework, a proper and well-supported weight of evidence approach can provide sufficient information for purposes of addressing Hawaiis interstate transport for the 2015 ozone NAAQS. In a weight of evidence analysis, no single piece of information is by itself dispositive of the issue. Instead, the total weight of all the evidence taken together is used to evaluate significant contribution to nonattainment or interference with maintenance of the 2015 ozone NAAQS
in another state. In the weight of evidence analysis detailed below, we consider 1 the distance between sources in Hawaii and the nonattainment and maintenance receptors identified in step 1; 2 the relative magnitude of state-wide emissions of ozone precursors; 3 an evaluation of prevailing wind direction that may impact of transport of emissions from Hawaii during the summer ozone season; and 4 a comparison of Hawaiis impact on California to Californias impact on Connecticut.
The state with the nearest nonattainment receptors to Hawaii is California, based on the modeling supporting the Revised CSAPR
Update.33 The nearest California 33 Projected ozone 2021 receptor concentrations and interstate contributions are contained in spreadsheet titled, ozone_design_values_
contributions_proposed_revised_csapr_update.xlsx.
The spreadsheet and accompanying TSD, Air Quality Modeling TSD for the Proposed Revised Cross-State Air Pollution Rule Update, are contained in the docket for the Proposed Revised CSAPR Update, Docket Number EPAHQOAR
20200272, and have also been included in the docket for this action. In total, in California 22

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