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Federal Register / Vol. 86, No. 232 / Tuesday, December 7, 2021 / Rules and Regulations monitoring experts found that the thirdparty monitoring network and the data quality at the St. Clair area monitors are consistent with EPA requirements and are acceptable to rely upon to characterize air quality in the St. Clair area. The NPRM inadvertently omitted specific reference to a letter EGLE
submitted to EPA on October 28, 2020, which provides EGLEs confirmation that the two industrial SO2 monitoring sites operated by DTE meet the quality assurance and siting requirements in 40
CFR part 58, appendix A and D, respectively. This letter has been added to Docket ID No. EPAR05OAR2020
0385. Additionally, the SO2 monitoring methods used at these two monitoring sites are reference or equivalent methods as defined in 40 CFR part 50.
Comment E: The commenter expressed concern that the two DTE
monitors could be missing maximum concentrations of the SO2 plume. The commenter cited diagrams from modeling results shown in the 2019
draft. The commenter stated that diagrams in this document appear to indicate an additional area of high modeled concentrations in the St. Clair area which does not currently contain a monitor. The commenter asked EPA to consider how to obtain monitoring results from that third location.
Response E: As previously stated, EPA relied on the modeling analysis in EGLEs July 24, 2020 CDD submittal, which used actual facility SO2
emissions and an updated meteorological data set from Pontiac, Michigan, 20172019. This meteorology was determined to be more complete and more representative of the St. Clair area than other available meteorological datasets which EGLE had considered or used earlier in its other work for the St.
Clair area. The CDD modeling of 2017
2019 actual emissions which EGLE
submitted indicated that the highest modeled concentrations tended to occur most frequently near the Remer monitor location. EPAs SO2 NAAQS
Designations Source-Oriented Monitoring Technical Assistance Document SO2 Monitoring TAD
considers both high relative magnitude of modeled results, and the frequency of a location experiencing maximum values, in helping to choose appropriate monitoring sites. The third location in the St. Clair area northwest of the plants, which the commenter appears to refer to, does not appear as a location of higher concentrations than the monitored locations in EGLEs CDD
modeling analysis. The CDDs modeled values in the northwest location are similar to but lower than the CDDs modeled values in the area of maximum
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concentration near the Remer monitors location. EPA is satisfied that the two DTE monitors provide a reasonable representation of the maximum impacts from the two St. Clair sources and that the imposition of a third monitor is not justified by current information.
Comment F: The commenter noted that the Belle River plant had a 7-month outage in 2019 and stated that EPA does not address how this outage affects its assessment that the 20172019
monitoring data represents three full years, particularly in the warmer months, or whether the outage skewed the results of the modeling so that it is not representative of maximum SO2
emissions observed during typical operations.
Response F: The Belle River plant did have outages at Unit 1 from February 2019 to June 2019; from November 2019
to December 2019, and from January 2020 to February 2020, which led to an overall emission reduction of over 6,000
tons of SO2. These outages would not affect most of the warmer months in the St. Clair area, so presumably the ambient air concentrations measured at the DTE monitors during the summer and early fall of 2019 would represent normal expected conditions for that year.
The monitoring data used to support the CDD represents actual ambient air quality during 20172019. Air quality monitoring data can reflect fluctuations in source operating conditions, meteorology, and other factors. The Belle River plant Unit 1 outage does not invalidate the monitoring data. The use of three years of data to calculate a monitors design value also helps balance variations in emissions and other factors. In addition, the CDD is supported by modeling of actual current facility emissions in this case, 2017
2019, in order to demonstrate that the NAAQS are attained. The analysis is not intended to evaluate only maximum typical emissions. EPA believes it is appropriate to model the true actual emissions for the modeling period, which encompassed the most recent three years of data available when the CDD was requested.
Comment G: The commenter noted that EGLE had used a single background value in its modeling for the initial nonattainment designation recommendation for the St. Clair area, but later revised the background concentration to a set of lower values for the 2019 draft and another set of background values in the CDD
submittal. The commenter questioned EGLEs claim that approximately 90
hours of data were considered in each season and asked that EPA explain the
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appropriateness of the final background values EGLE used. The commenter asked that EGLEs background spreadsheet be added to the CDD
actions docket record and inquired whether EPA limits the number of hours or wind sectors that can be excluded from a background data set.
Response G: Dispersion modeling analysis can be an iterative process, in which initial conservative input data is later evaluated to better reflect actual ambient air conditions within the modeling domain, or more accurate emissions and facility configuration data at the modeled sources. Such adjustments can provide for more appropriate and accurate results. In its initial nonattainment recommendation analysis of the St. Clair areas 2012
2014 SO2 emissions submitted on September 18, 2015, Michigan chose a conservative Tier I background value.
Based in part on the results of the modeling analysis, the State recommended to EPA that the St. Clair area be designated nonattainment.
These modeling results were also used to help suggest boundaries for the St.
Clair nonattainment area. Having made its nonattainment recommendation, Michigan did not decide to further refine its 2015 modeling or the background value it used.
However, EPA concurs with EGLE
that additional refinement of input data such as background concentrations can be part of an acceptable approach to support future planning, or to characterize an areas air quality. The background analysis EGLE submitted with its July 24, 2020, CDD submittal used monitored ambient air quality data from 20172019 at the Port Huron monitor, selected by season and hour of day with wind direction exclusions to avoid double-counting of the St. Clair plants impacts and to avoid overestimating SO2 impacts from facilities closer to the background monitor which would not be expected to actually impact the St. Clair area when winds came from their locations.
EPA accepted this approach, which is a commonly used method of addressing background in SO2 modeling analyses, fully supported by EPAs modeling guidance. The background values used in the CDD submittal work come from a newer set of air quality data than the background values in the 2019 draft, which may help explain the difference between the data sets cited by the commenter. The actual number of acceptable background exclusions depends on the wind patterns experienced at the Port Huron monitor, and is not specifically limited by EPA
guidance as long as the monitor meets
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