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Federal Register / Vol. 86, No. 53 / Monday, March 22, 2021 / Rules and Regulations but does not compare the complexities involved in analyzing interactions between multiple pollutants. We disagree with this point in relation to the alternative analysis here. First, particularly for purposes of a BART
alternative analysis for a single facility with two BART units, EPAs regulations recognize CALPUFF to be an acceptable model, explaining that CALPUFF is particularly suited for BART and BART alternative applications at a single source.71
Further, Method 2, incorporated by the State in its SIP submittal, is a full assessment method where all sources and pollutants are combined into a single CALPUFF modeling run per year for the baseline and each control scenario. Method 2 allows for interaction of the pollutants from both boilers, as emitted pollutants from each unit disperse and compete for the same reactants in the atmosphere, providing modeled overall impacts due to emissions from both units. It is because of this that method 2 analysis results are a more reliable assessment of the anticipated overall visibility improvement of controls under each scenario. Thus, this is an entirely suitable application of the CALPUFF
model, and the commenter is incorrect to state that the CALPUFF modeling did not account for the interactive chemistry of visibility pollutants.
EPA recognizes that the CALPUFF
model includes simplified chemistry to account for interactions between pollutants. The simplified chemistry tends to magnify the actual visibility effects of a single source; thus, it is appropriate to use the 98th percentile to avoid overprediction and not give undue weight to the extreme tail of the distribution. This approach will effectively capture the sources that contribute to visibility impairment in a Class I area, while minimizing the likelihood that the highest modeled visibility impacts might be caused by unusual meteorology or conservative assumptions in the model.
The EPA has previously recognized this approach of using CALPUFF as an acceptable approach in the past when analyzing BART alternatives that only include emission reductions at a single or small group of BART sources.
Specifically, we approved this approach for the State of Arizona which established a BART alternative for Steam Units 2 and 3 at Arizona Electric Power Cooperatives Apache Generating 71 See
71 FR 60616.

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recognizing CALPUFF as particularly appropriate for single-source applications.
The commenter states that the FIP
considered each pollutant separately, whereas the alternative attempts to analyze and take credit for combined emission reductions from three pollutants, which allegedly fails to assess the effect of the alternative on visibility as compared to BART. The commenter is incorrect in their premise.
The CALPUFF modeling in the FIP
evaluated each unit separately, but modeled the visibility impacts from all pollutants from that unit. For example, in evaluating the visibility benefit from NOX controls on Power Boiler No. 1, the NOX emissions varied between each control scenario modeled, while the SO2
and PM emissions were included but held constant in these NOX control scenarios. In evaluating the BART
alternative, the State provided EPA with two separate methods of using the CALPUFF modeling to evaluate visibility impacts of the BART
alternative as compared to BART, including Method 2 described above that modeled all pollutants from both BART units to assess the total visibility impact from these two units.
For these reasons, we disagree that the modeling data was of insufficient quality and failed to meet the requirements of the regulations.
Comment A.6: EPA lacks authority to give one Class I area more weight than others. EPA suggests that it is reasonable to give one of the Class I areas greater weight when considering visibility benefits and cherry-picks the Class I
area with the greatest visibility improvement, which is closest to Domtar. Focusing on that Class I area serves to support a sources preferred control outcome. Showing that one Class I area will have greater visibility benefits does nothing to tip the weight of evidence scale in favor of the BART
alternative. It merely shows one area will see more benefits. In addition, EPA
fails to provide a basis for applying the 0.5 deciview threshold used by the State to determine if a source contributes to visibility impairment at a Class I area with the BART alternative analysis.
Response: We disagree with the commenters assertion that EPA cherry picks the Class I areas with the greatest visibility improvement. We considered many metrics in analyzing the weight of evidence approach by the State, including the overall visibility 72 See Arizonas September 19, 2014 proposed approval 79 FR 56322 which was finalized on April 10, 2015 80 FR 19220.

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improvement on average across the four impacted Class I areas. As a whole, these factors supported a conclusion that the BART alternative achieves greater reasonable progress than BART
at the subject facility. One metric that we analyzed was the breakdown of pollutant speciation impacts across each Class I area due to modeled emissions from each power boiler. We highlighted impacts at Caney Creek specifically in this analysis because Domtars Ashdown facility impacts this Class I
area the greatest, and this is due to NOX
emissions from Power Boiler No. 2. We also found that NOX emissions contributed more to visibility impairment across all four Class I areas for Power Boiler No. 1. The greater impact due to NOX emissions is relevant because it demonstrates that the higher SO2 emissions allowed under the BART
alternative is offset by the larger reduction in NOX emissions. This is just one factor among many that we considered in analyzing the States weight of evidence approach as explained in the proposed approval and in preceding responses in this final approval. We took into account the visibility impacts at all impacted Class I areas individually and on average and did not solely focus on the benefits at the most impacted area.
We disagree with the assertion that we are supporting the sources preferred control outcome instead of addressing emissions cumulatively across all Class I areas. The commenter points out that the court in Natl Parks Conservation Assn v. EPA held that EPAs analysis in reviewing SIP submittals must take into account the visibility impacts at all impacted Class I areas rather than focusing solely on the benefits at the most impacted areas, 803 F.3d 151, 165
3d Cir. 2015. However, the facts of Natl Parks Conservation Assn v. EPA, are not analogous to the facts surrounding our proposed approval. In Natl Parks Conservation Assn v. EPA, the court was reviewing EPAs approval of the states assessment of the visibility-improvement factor within the five-factor BART analysis. The state calculated visibility improvement that could be achieved at Class I areas by implementing additional controls at BART-eligible sources.73 The states calculations for each source, however, took into account only the potential impact such controls would have on the visibility in the Class I area most severely impacted by the source. The state did not consider cumulative visibility impact, which the EPA itself had conceded was improper under the 73 Id.

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