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Federal Register / Vol. 86, No. 156 / Tuesday, August 17, 2021 / Proposed Rules maximum SO2 concentrations have been predicted at or near this location. The other monitor, known as the Mills monitor, is sited 3 km west of the Belle River plant, so that it can capture the worst-case combined impacts when winds are blowing from the St. Clair plant toward the Belle River plant.
EPA reviewed the ambient air monitoring data for the 20172019
period, which were the three most recent full calendar years of data
available. Ambient and quality assurance data for these two monitoring sites are recorded in EPAs AQS
database. EGLE and EPA have reviewed the data and have determined that this data meets completeness and data quality indicators confirm that the data is suitable to be used in support of a clean data determination for the St.
Clair area.
The data cited by EGLE in its request show attainment of the 2010 SO2

NAAQS at both monitors for the 2017
2019 time period, with three-year average 99th percentile daily maximum 1-hour concentrations design values of 54 and 45 parts per billion ppb, which are below the 2010 SO2 NAAQS of 75
ppb. Data for 2020 indicate that the monitors have continued to show attainment. Table 1 shows the 2017
2020 SO2 monitoring results for the St.
Clair area monitors.

TABLE 120172020 MONITORED SO2 VALUES IN THE ST. CLAIR AREA
Annual 99th percentile ppb
20172019
design value ppb
Monitor 2017

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Mills Monitor
Remer Monitor

46
51

EPA also reviewed the dispersion modeling analysis for the St. Clair area which EGLE submitted on July 24, 2020.
The SO2 Modeling TAD outlines modeling approaches for SO2 NAAQS
attainment status designations and states that, for the purposes of modeling to characterize air quality for use in SO2
designations, EPA recommends using a minimum of the most recent three years of actual emissions data and concurrent meteorological data to allow the modeling to simulate what a monitor would observe.
EGLEs analysis followed the Modeling TAD and modeled the impacts of the Belle River and St. Clair plants in the St. Clair nonattainment area. EGLE used the actual 20172019
hourly SO2 emissions for the Belle River and St. Clair plants as measured by continuous emissions monitor CEM
data. EGLE also characterized the buildings at the two plants using the AERMOD component BPIPPRM, to address building downwash. There were no additional nearby sources that were expected to produce a significant SO2
concentration gradient in the nonattainment area.
To model the St. Clair nonattainment area, EGLE used EPAs AERMOD model, version 19191, with meteorological data for 20172019 from the Oakland County International Airport Pontiac, located 75 km to the west of the St. Clair plants.
This meteorological data set is considered to be representative of the St. Clair area. The area was modeled as rural, based on local land use characteristics. Terrain information was included in the modeling analysis. The nonattainment area is flat and mostly residential or agricultural. The river valley is not deep, although some wind channeling could occur. The
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2018

2019
50
65

2020
40
45

geographical and topographical features of the area are not considered to significantly impact air pollution transport. The St. Clair modeling analysis used a nested receptor grid with resolution from 50 meters near the facilities to 100 meters in the central portion, and then 250 meters to the edge of the modeling domain, 10 km from the power plants.
For a background concentration for the modeling analysis, EGLE used monitored SO2 data from Michigans SO2 monitor in Port Huron, located 21
km to the north of the St. Clair plants.
The Port Huron monitor has an SO2
design value of 67 ppb for 20172019.
EGLE determined its background concentration using a temporally varying approach to characterize background SO2 emissions, based on the 99th percentile monitored concentrations by season and hour of day. In this analysis, EGLE used data measured when winds were blowing from wind direction sectors which were chosen to avoid double-counting emissions from the St. Clair and Belle River plants and to avoid overestimating impacts from sources which are located in Canada, 35 km east of Port Huron but 1520 km from the St. Clair area.
The Modeling TAD provides for this approach. At such distances, the Canadian sources are not expected to provide a significant concentration gradient in the St. Clair area. The modeling analysis results match well with the monitored values near the St.
Clair plants, which suggests that the modeling analysis is not missing significant additional ambient contributions at those locations.
Therefore, EPA concurs with the background values EGLE used in its analysis. The background
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25

45
54

20182020
design value ppb 40
45

concentrations for the St. Clair modeling analysis were determined to vary from 1.3 to 6.5 ppb, with an average value of 2.4 ppb.
The states modeling resulted in a three-year maximum predicted 99th percentile daily maximum 1-hour concentration of 64.4 ppb, including background. This design value was predicted at a receptor located very near the St. Clair plant. As the predicted design value is below the 2010 SO2
NAAQS of 75 ppb, the states modeling demonstrates attainment of the 2010
SO2 NAAQS.
EGLEs modeling results for receptors placed at the two SO2 monitors locations matched well with the actual monitored design values. The models predicted design value at the Remer monitor location was 47.7 ppb, compared to the monitored design value of 45 ppb, and the models predicted design value at the Mills monitor location was 52.7 ppb, compared to the monitored design value of 54 ppb. The location of the maximum modeled 99th percentile concentration was less than half a kilometer from the Remer monitor, which lends support to EPAs expectation that the Remer monitor is located in the area of expected maximum concentrations. Other areas of predicted high concentrations were at approximately the same distance to the northwest and west of the power plants as the Mills monitor, again lending support to EPAs expectation that the Mills monitor location is also representative of areas of high expected concentrations.
After reviewing EGLEs July 24, 2020
submittal, EPA proposes to find that the St. Clair area has attained the 2010 SO2
NAAQS and satisfies the requirements of the Clean Data Policy.

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