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Federal Register / Vol. 86, No. 232 / Tuesday, December 7, 2021 / Proposed Rules
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measures. We also proposed to find that the State had demonstrated that its current nonattainment new source review NNSR program covered the 2010 one-hour SO2 NAAQS; therefore, no revision to the SIP was required for the NNSR element.
After the close of the public comment period to the April 19, 2018 proposal, the LDEQ submitted additional information to EPA on August 24, 2018.8 The additional information was submitted to EPA partly in response to a public comment that expressed concern that Rain would need to modify the February 2018 AOC entered between Rain and LDEQ as Rain did not believe that it could meet the limits set forth in the AOC without an additional extension to the compliance dates.9 In response to the comment, and in order to determine feasible emission limits for operations during transitions from exhaust flow through the hot stack to the cold stack, LDEQ granted an extension of the deadline of the February 2018 AOC on April 27, 2018.10
On August 2, 2018, Rain and LDEQ
revised their existing AOC. On August 24, 2018, LDEQ supplemented their SIP
submittal with the revised AOC and additional modeling analysis. On October 9, 2018, LDEQ again supplemented their SIP with an updated modeling analysis. The revised AOC 11
and the October 9, 2018 modeling files served as a supplement to the November 9, 2017 and February 8, 2018 SIP
submittals and incorporated certain additional AOC revisions dated August 2, 2018 and supporting modeling into the 2010 SO2 NAAQS Nonattainment Area SIP revision for St. Bernard Parish.
On February 8, 2019, EPA proposed to approve LDEQs August 24, 2018 and October 9, 2018 submittals as a supplement to the prior SIP submittals 84 FR 2801. Please refer to EPAs April 19, 2018 proposed approval and February 8, 2019 supplemental notice of proposed rulemaking.
In a May 29, 2019 final action, EPA
approved the nonattainment SIP for the St. Bernard area 84 FR 24712. For additional information concerning the St. Bernard Parish, Louisiana 8 See letter from Secretary Chuck Carr Brown to Anne Idsal, August 24, 2018, St. Bernard 2008
Sulfur Dioxide State Implementation Plan Supplemental Information and Executed Administrative Order on Consent AOC included in the docket for this action.
9 See the April 24, 2018 letter in the docket to this action from Senator Cassidy to EPA that referred to Rains need to modify the February 2, 2018 AOC.
10 See April 27, 2018 Letter from Secretary Chuck Carr Brown to Rain in the docket for this action.
11 AOC signed by LDEQ and Rain on August 2, 2018, and submitted to EPA on August 24, 2018.

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nonattainment SIP revision see docket ID No. EPAR06OAR20170558
available at https
www.regulations.gov.
II. Proposed Determination A. Applicable Statutory and Regulatory Provisions Section 179c1 of the Act requires the EPA to determine whether a nonattainment area has achieved an applicable attainment date based on the areas air quality as of the attainment date. A determination of whether an areas air quality meets applicable standards is generally based upon the most recent three years of complete, quality-assured data gathered at established state and local air monitoring stations SLAMS in a nonattainment area and entered into the EPAs Air Quality System AQS
database.12 Data from ambient air monitors operated by state and local agencies in compliance with the EPA
monitoring requirements must be submitted to AQS.13 Monitoring agencies annually certify that these data are accurate to the best of their knowledge.14 All SO2 data are reviewed to determine the areas air quality status in accordance with 40 CFR part 50, Appendix T.
Under EPA regulations in 40 CFR
50.17 and in accordance with 40 CFR
part 50 Appendix T, the 2010 one-hour annual SO2 standard is met when the design value is less than or equal to 75
ppb. Design values are calculated by computing the three-year average of the annual 99th percentile daily maximum one-hour average concentrations.15 An SO2 one-hour primary standard design value is valid if it encompasses three consecutive calendar years of complete data. A year is considered complete when all four quarters are complete, and a quarter is complete when at least 75
percent of the sampling days are complete. A sampling day is considered complete if 75 percent of the hourly concentration values are reported; this includes data affected by exceptional events that have been approved for exclusion by the Administrator.16
We note that when determining the attainment status of SO2 nonattainment areas, including when making 12 AQS is the EPAs repository of ambient air quality data.
13 See 40 CFR 58.16.
14 See 40 CFR 58.15.
15 As defined in 40 CFR part 50, Appendix T
section 1c, daily maximum 1-hour values refer to the maximum one-hour SO2 concentration values measured from midnight to midnight that are used in the NAAQS computations.
16 See 40 CFR part 50, Appendix T sections 1c, 3b, 4c, and 5a.

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determinations of attainment by the attainment date, in addition to ambient monitoring data, the EPA may also consider air quality dispersion modeling and/or a demonstration that the control strategy in the SIP has been fully implemented.17 With regard to the use of monitoring data for such determinations, the EPAs SO2
Nonattainment Area Guidance specifically notes that if the EPA
determines that the air quality monitors located in the affected area are located in the area of maximum concentration, the EPA may be able to use the data from these monitors to make the determination of attainment without the use of air quality modeling data. If there are no air quality monitors located in the affected area, or there are air quality monitors located in the area, but analyses show that none of the monitors are located in the area of maximum concentration,18 then air quality dispersion modeling will generally be needed to estimate SO2 concentrations in the area. In this case, as discussed in our proposed actions on the St. Bernard nonattainment plan and Technical Support Documents TSDs,19 the monitors are not located in the area of expected maximum concentration, meaning we-must also consider the available modeling data in determining whether the area attained by the attainment date. When relying on a modeling demonstration based on allowable emissions for purposes of determining attainment by the attainment date, the EPA looks to whether the emission limit or limits were adopted and whether the relevant source or sources were complying with those modeled limits prior to the attainment date. That is, when determining attainment by the attainment date using air quality modeling of allowable emissions, EPA
looks to whether the state has demonstrated that the control strategy in the SIP has been fully implemented compliance records demonstrating that the control measures have been implemented as required by the approved SIP. This is necessary because a modeling demonstration based on allowable emissions is not itself sufficient since, without the supporting emissions information 17 EPA, April 23, 2014 Guidance for 1-Hour SO
2 Nonattainment Area SIP Submissions SO2
Nonattainment Area Guidance, page 49.
18 See section VIII.A of the SO Nonattainment 2
Area Guidance 19 See EPAs April 19, 2018 proposed approval 83 FR 17349, February 8, 2019 supplemental notice of proposed rulemaking 84 FR 2801 and EPAs Technical Support Documents, available in the docket for this action.

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