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Federal Register / Vol. 86, No. 33 / Monday, February 22, 2021 / Proposed Rules
ambient data, Alaska assessed the contribution of SO2, NOX, and ammonia for all four monitor sites between 2011
and 2015 on the highest concentration days. Alaska did not perform a concentration-based analysis using ambient data for VOCs. Through these analyses, Alaska identified that ammonia was a significant precursor in the Fairbanks PM2.5 Nonattainment Area.
For the concentration-based analysis using air quality modeling, Alaska utilized version 4.7.1 of the Community Multiscale Air Quality CMAQ
photochemical model. The modeling relied on many elements from the precursor analysis provided by the State in the Fairbanks Moderate Plan e.g., meteorological inputs, emissions processing methods, nested modeling grids.
Alaska performed modeling analyses using both the base year emissions inventory 2013 and the future year emissions inventory 2019 for VOCs and NOX. First, the State evaluated precursor significance using a zero-out approach that compared a baseline model run with a model run where a precursors emissions were set to zero in order to determine the influence of that precursor on PM2.5 formation. For VOCs, Alaska performed a single analysis where it zeroed out all anthropogenic VOC emissions. For NOX, Alaska performed two zero-out analyses:
One where all anthropogenic NOX
emissions were zeroed out and one where only major stationary source NOX
emissions were zeroed out. Next, Alaska further evaluated NOX precursor significance through a 75% sensitivity analysis. In this analysis, the State compared a baseline model run with a model run where all anthropogenic NOX
emissions were reduced by 75%. Alaska concluded that these analyses showed that VOCs and NOX were not significant precursors in the Fairbanks PM2.5
Nonattainment Area.
Alaska performed two SO2 precursor analyses using modeling elements from the Moderate Area Plan and updated baseline and future year emission inventories, as described previously in this preamble for the VOC and NOX
modeling. Alaska first performed a zeroout analysis where it zeroed out major stationary source SO2 emissions. To address concerns about model underprediction of secondary sulfate, Alaska next performed an analysis that incorporated the base case model performance evaluation to estimate the impact of removing all major stationary source SO2 emissions. Based on these analyses, Alaska concluded that SO2

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was a significant precursor in the Fairbanks PM2.5 Nonattainment Area.
Additionally, on March 18, 2020, Alaska provided clarifications on the precursor analyses, included in the docket for this action. Included in these clarifications were further calculations projecting NOX significance at a 50%
sensitivity level i.e., the comparison of a baseline model run with a model run including a 50% reduction of all anthropogenic NOX emissions.

Regarding the States analytical approach, EPA proposes to find that the State used the appropriate methods and data to evaluate PM2.5 formation in the Fairbanks PM2.5 Nonattainment Area from precursor emissions. Alaska began with concentration-based analyses for the precursors and proceeded with sensitivity-based analyses if necessary, which is an acceptable progression of analyses under the PM2.5 SIP
Requirements Rule. The State utilized the appropriate threshold recommended 3. EPAs Evaluation and Proposed in the EPAs guidance 1.5 mg/m3 in Action evaluating the significance of precursor emissions to the formation of 24-hour EPA has evaluated the States precursor demonstration for the Serious PM2.5 and utilized data from all four area nonattainment plan consistent with monitors in the Fairbanks PM2.5
Nonattainment Area.
the PM2.5 SIP Requirements Rule and Regarding the results of the States the recommendations in the PM2.5
analysis, the concentration-based Precursor Guidance. Additional details modeling Alaskas second tier of EPAs evaluation of Alaskas precursor PM2.5 analyses are included in precursor analysis of VOC emissions demonstrates that anthropogenic VOCs a Technical Support Document included in the docket for this action.31 have impacts on PM2.5 concentrations in the Fairbanks PM2.5 Nonattainment Area Based on this evaluation, EPA agrees that are well below the 1.5 mg/m3
that SO2 and ammonia emissions significance threshold. Therefore, we contribute significantly to ambient propose to concur with the States PM2.5 levels that exceed the 2006 PM2.5
conclusion that VOCs are not significant NAAQS in the Fairbanks PM2.5
for PM2.5 formation in the Fairbanks Nonattainment Area and that SO2 and PM2.5 Nonattainment Area.
ammonia emission sources, therefore, Further, we propose to find that the remain subject to control requirements weight of evidence presented in the under subparts 1 and 4 of part D, title Fairbanks Serious Plan and Alaskas I of the Act.
March 18, 2020, clarification document For the reasons provided in this suggests that NOX emitted from all preamble and further detailed in the sources is an insignificant contributor to Technical Support Document, EPA
local PM2.5 concentrations, based on the proposes to approve the States following evidence. First, the NOX
demonstration that NOX and VOC
100% major stationary source reduction emissions do not contribute significantly to ambient PM2.5 levels that analysis demonstrated that NOX
emissions are insignificant contributors exceed the 2006 PM2.5 NAAQS in the to PM2.5 concentrations at the four Fairbanks PM2.5 Nonattainment Area.
monitor locations. Second, the NOX
Our proposed approval of Alaskas 75% all source reduction sensitivity precursor demonstration does not analysis demonstrated that only 10% of extend to nonattainment NSR
requirements for the area. The State did the modeled days showed significant contributions of NOX to PM2.5
not address the issue of precursors for concentrations at the Fairbanks purposes of nonattainment NSR
monitors, and no days with significant requirements in the Fairbanks Serious contributions at the North Pole Plan because Alaska previously monitors. Third, the 75% all source determined that it was appropriate to regulate NOX, SO2, VOCs, and ammonia reduction sensitivity analysis was conservative given that EPA guidance as precursors to PM2.5 with respect to recommends evaluation of 3070%
nonattainment NSR and submitted rule reductions of the pollutant for analytical changes to that effect on October 25, purposes.32 Lastly, Alaskas projected 2018. The EPA approved the submitted 50% reduction of NOX from all sources revised program as meeting sensitivity analysis suggested there nonattainment NSR requirements would be insignificant contributions triggered upon reclassification of the Fairbanks PM2.5 Nonattainment Area to 32 PM
2.5 Precursor Demonstration Guidance, Serious 84 FR 45419, August 29, 2019.
31 Review of Fairbanks Nonattainment Area Precursor Demonstrations for Volatile Organic Compounds and Nitrogen Oxides in the 2019 State Implementation Plan Submission. Nicole Briggs and Robert Kotchenruther, November 4, 2020.

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EPA454/R19004, May 2019, including Memo dated May 30, 2019 from Scott Mathias, Acting Director, Air Quality Policy Division and Richard Wayland, Director, Air Quality Assessment Division, Office of Air Quality Planning and Standards OAQPS, EPA to Regional Air Division Directors, Regions 110, EPA. Page 29.

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