St. Clair Power Plant 

St. Clair, Michigan 

Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

Conducted by: 

Steven Klafka, P.E., BCEE 

Wingra Engineering, S.C. 

Madison, Wisconsin
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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1. Introduction 

Wingra Engineering, S.C. was hired by Sierra Club to conduct an air modeling impact analysis to 

help the U.S. Environmental Protection Agency (USEPA), state and local air agencies identify 

facilities that are likely causing exceedances of the 1-hour sulfur dioxide (SO2) national ambient air 

quality standard (NAAQS). This document describes the results and procedures for an evaluation 

conducted for the St. Clair Power Plant located in St. Clair, Michigan. 

To ensure the modeling analysis reflected the cumulative concentration of SO2 emissions, it included 

emissions from the Belle River Power Plant in St. Clair, Michigan, which is an additional source of 

SO2 emissions located within 50 kilometers of the St. Clair Power Plant. 

The dispersion modeling analysis predicted ambient air concentrations for comparison with the 1-

hour SO2 NAAQS. The modeling was performed using the most recent version of AERMOD, 

AERMET, and AERMINUTE, with data provided to Sierra Club by regulatory air agencies and 

through other publicly-available sources as documented below. The analysis was conducted in 

adherence to all available USEPA guidance for evaluating source impacts on attainment of the 1-

hour SO2 NAAQS via aerial dispersion modeling, including the AERMOD Implementation Guide; 

USEPA's Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient 

Air Quality Standard, August 23, 2010; modeling guidance promulgated by USEPA in Appendix W 

to 40 CFR Part 51; USEPA’s March 2011 Modeling Guidance for SO2 NAAQS Designations; 1 and 

USEPA’s December 2013 SO2 NAAQS Designations Technical Assistance Document.2 

2. Compliance with the 1-hour SO2 NAAQS 

2.1 1-hour SO2 NAAQS 

The 1-hour SO2 NAAQS takes the form of a three-year average of the 99th-percentile of the annual 

distribution of daily maximum 1-hour concentrations, which cannot exceed 75 ppb.3

 Compliance 

with this standard was verified using USEPA’s AERMOD air dispersion model, which produces air 

concentrations in units of μg/m3

. The 1-hour SO2 NAAQS of 75 parts per billion (ppb) equals 196.2 

μg/m3

, and this is the value used for determining whether modeled impacts exceed the NAAQS.4 

The 99th-percentile of the annual distribution of daily maximum 1-hour concentrations corresponds 

to the fourth-highest value at each receptor for a given year. 

 

1

 http://www.epa.gov/scram001/so2_modeling_guidance.htm 2

 http://www.epa.gov/oaqps001/sulfurdioxide/pdfs/SO2ModelingTAD.pdf 

3 USEPA, Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard, 

August 23, 2010.

4 The ppb to μg/m3

 conversion is found in the source code to AERMOD v. 14134, subroutine Modules. The conversion 

calculation is 75/0.3823 = 196.2 μg/m3

. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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2.2 Modeling Results 

Model results for both sources included in the analysis of SO2 are summarized in Table 1. Results 

are provided for each source alone, and for both sources combined. 

Modeling results for St. Clair Power Plant are summarized in Table 1. It was determined that based 

on either current allowable emissions or measured actual emissions, the St. Clair Power Plant is 

estimated to create downwind SO2 concentrations which exceed the 1-hour NAAQS. 

More specifically, the modeling results presented in Table 1, show exceedances of the NAAQS by 

the plant’s allowable and actual emissions. “Allowable” is the peak emission rate from each unit as 

approved by the current air quality operation permit for the facility. “Actual” are the measured 

emissions for each hour between January 1, 2012 and December 31, 2014 as taken from USEPA Air 

Markets Program Data.

5

In addition, the emissions from the Belle River significantly contribute to the ambient SO2

concentration in the area impacted by St. Clair power plant. 

Air quality impacts in Michigan are based on a background concentration of 23.5 μg/m3. This is the 

2011-13 design value for Kent County, Michigan – the lowest measured background concentration 

in the state. This is the most recently available design value. See Section 5 for further discussion of 

the background concentrations used for this analysis. 

Table 1 - SO2 Modeling Results for St. Clair Power Plant Modeling Analysis 

Emission Rates Averaging 

Period 

99th Percentile 1-hour Daily Maximum (μg/m3

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 

Belle River 

246.5 23.5 270.0 196.2 No 

Actual 101.0 23.5 124.5 196.2 Yes 

Allowable 

St. Clair 

513.1 23.5 536.6 196.2 No 

Actual 238.9 23.5 262.4 196.2 No 

Allowable 

Both 

622.3 23.5 645.8 196.2 No 

Actual 246.7 23.5 270.2 196.2 No 

The emissions used for the modeling analysis are summarized in Table 2. 

 

5

 http://ampd.epa.gov/ampd/ 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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Table 2 - Modeled SO2 Emissions 6

Stack 

ID 

Unit 

ID 

Allowable Emissions 

Monthly Average 

 (lbs/hr) 

B01 Unit 1 8,176.8 

B02 Unit 2 8,176.8 

Belle River Power Plant Subtotal 16,353.6 

S01 Unit 1 2,355.0 

S02 Unit 2 2,355.0 

S03 Unit 3 2,354.7 

S04 Unit 4 2,357.7 

S06 Unit 6 5,187.0 

S07 Unit 7 7,840.7 

St. Clair Power Plant Subtotal 22,450.1 

Both Stations Total 38,803.7 

Based on the modeling results, Table 3 provides the emission reductions from current allowable 

rates necessary to achieve compliance with the 1-hour NAAQS. This assumes a one-hour averaging 

period for the emission rate and that the emission rate is binding at all times. However, given the 

conservative aspects of this modeling protocol, it is extremely likely that this limit is too high to 

protect the NAAQS. For example, startup or shutdown periods were not evaluated. During these 

periods, decreased gas velocities and temperatures may lead to greater ambient impacts at ground 

level. Further, the hypothetical emission limitation in Table 3 would allow St. Clair Power Plant to 

consume the entire NAAQS, leaving little to no room for any other source of SO2 in the area. No 

margin of safety has been included in the hypothetical emission limitation. 

Table 3 - Required Emission Reductions from St. Clair Power Plant for Compliance with the 1-

hour NAAQS for SO2 

Acceptable Impact 

(NAAQS - Background) 

99th Percentile 

1-hour Daily Max 

(μg/m3

) 

Required 

Total Facility 

Reduction Based on 

Allowable Emissions 

(%) 

Required 

Total Facility 

Emission Rate 

(lbs/hr) 

Required 

Total Facility 

1-hour Average 

Emission Rate 

(lbs/mmbtu) 

172.7 72% 6,230.3 0.46 

Predicted exceedances of the 1-hour NAAQS for SO2 based on allowable emissions extend 

throughout the region to a maximum distance of 46 kilometers. 

 

6

 Michigan Department of Environmental Quality, Renewal Operating Permit MI-ROP-B2796-2009, June 1, 2009. The 

emission limit for each boiler at the Belle River Power Plant is 1.2 lbs/mbtu heat input (3-hour average) and for each 

boiler at the St. Clair Power Plant is 1.67 lbs/mmbtu (monthly average). 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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Figure 1 shows the extent of NAAQS violations based on allowable emissions from both sources. 

Figure 2 shows the extent of NAAQS violations based on actual hourly emissions from both sources. 

2.3 Conservative Modeling Assumptions 

A dispersion modeling analysis requires the selection of numerous parameters which affect the 

predicted concentrations. For the enclosed analysis, several parameters were selected which underpredict facility impacts. 

Assumptions used in this modeling analysis which likely under-estimate concentrations include the 

following: 

 Allowable emissions are based on a limitation with an averaging period which is greater than 

the 1-hour average used for the SO2 air quality standard. Emissions and impacts during any 

1-hour period may be higher than assumed for the modeling analysis. 

 No consideration of facility operation at less than 100% load. Stack parameters such as exit 

flow rate and temperature are typically lower at less than full load, reducing pollutant 

dispersion and increasing predicted air quality impacts. 

 No consideration of building or structure downwash. These downwash effects typically 

increase predicted concentrations near the facility. 

 No evaluation has been conducted to determine if the stack height exceeds Good Engineering 

Practice (GEP) height. If the stack height exceeds GEP, the predicted concentrations will 

increase. 

 Except for the Belle River Power Plant, no consideration of off-site sources. These other 

sources of SO2 will increase the predicted impacts.
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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Figure 1 - Regional View of Impacts Due to Allowable Emissions from Both Sources
Evaluation of Compliance with the 1-hour NAAQS for SO2

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Figure 2 - Regional View of Impacts Due to Actual Emissions for the 2012-14 Period from Both Sources 

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

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3. Modeling Methodology 

3.1 Air Dispersion Model 

The modeling analysis used USEPA’s AERMOD program, v. 14134. AERMOD, as available from 

the Support Center for Regulatory Atmospheric Modeling (SCRAM) website, was used in 

conjunction with a third-party modeling software program, AERMOD View, sold by Lakes 

Environmental Software. 

3.2 Control Options 

 

The AERMOD model was run with the following control options: 

 1-hour average air concentrations 

 Regulatory defaults 

 Flagpole receptors 

To reflect a representative inhalation level, a flagpole height of 1.5 meters was used for all modeled 

receptors. This parameter was added to the receptor file when running AERMAP, as described in 

Section 4.4. 

An evaluation was conducted to determine if the modeled facility was located in a rural or urban 

setting using USEPA’s methodology outlined in Section 7.2.3 of the Guideline on Air Quality 

Models.7

 For urban sources, the URBANOPT option is used in conjunction with the urban 

population from an appropriate nearby city and a default surface roughness of 1.0 meter. Methods 

described in Section 4.1 were used to determine whether rural or urban dispersion coefficients were 

appropriate for the modeling analysis. 

 

3.3 Output Options 

The AERMOD analysis was based on three years of recent meteorological data. The modeling 

analyses used one run with three years of sequential meteorological data from 2012-2014. Consistent 

with USEPA’s Modeling Guidance for SO2 NAAQS Designations, AERMOD provided a table of 

fourth-high 1-hour SO2 impacts concentrations consistent with the form of the 1-hour SO2 NAAQS.8 

Please refer to Table 1 for the modeling results. 

 

7

 USEPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex 

Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005.

8 USEPA, Area Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, 

Attachment 3, March 24, 2011, pp. 24-26. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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4. Model Inputs 

4.1 Geographical Inputs 

The “ground floor” of all air dispersion modeling analyses is establishing a coordinate system for 

identifying the geographical location of emission sources and receptors. These geographical 

locations are used to determine local characteristics (such as land use and elevation), and also to 

ascertain source to receptor distances and relationships. 

The Universal Transverse Mercator (UTM) NAD83 coordinate system was used for identifying the 

easting (x) and northing (y) coordinates of the modeled sources and receptors. Stack locations were 

obtained from facility permits and prior modeling files provided by the state regulatory agency. The 

stack locations were then verified using aerial photographs. 

The facility was evaluated to determine if it should be modeled using the rural or urban dispersion 

coefficient option in AERMOD. A Geographic Information System (GIS) was used to determine 

whether rural or urban dispersion coefficients apply to a site. Land use within a three-kilometer 

radius circle surrounding the facility was considered. USEPA guidance states that urban dispersion 

coefficients are used if more than 50% of the area within 3 kilometers has urban land uses. 

Otherwise, rural dispersion coefficients are appropriate.9

 

USEPA’s AERSURFACE v. 13016 was used to develop the meteorological data for the modeling 

analysis. This model was also used to evaluate surrounding land use within 3 kilometers. The 

evaluated power plants are located on the Canadian border with the U.S. The 1992 National Land 

Cover Data or NLCD data required by AERSURFACE does not extend into Canada. For this reason, 

AERSURFACE was not used to evaluate surrounding land use. Instead, based on a review of aerial 

photographs, it was clear the land use surrounding both power plants is rural. It was concluded that 

the rural option would be used for the modeling summarized in this report. 

 

 

9

 USEPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex 

Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005, Section 7.2.3. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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4.2 Emission Rates and Source Parameters 

Concentrations were predicted for the scenarios shown in Tables 1 and 2: 

1) allowable emissions based on the current permit issued by the regulatory agency, and 

2) actual hourly emissions measured each hour between January 1, 2012 and December 31, 

2014 as taken from USEPA Air Markets Program Data.

10

Stack parameters and emissions used for the modeling analysis are summarized in Table 4. 

Table 4 – Facility Stack Parameters and Emissions 11

Facility Belle River St. Clair 

Stack B01 B02 S01 S02 S03 S04 S06 S07 

Description Unit 1 Unit 2 Unit 1 Unit 2 Unit 3 Unit 4 Unit 6 Unit 7 

X Coord. [m] 377804 377713 379538 379538 379538 379538 379597 379629 

Y Coord. [m] 4736933 4736972 4735415 4735415 4735415 4735415 4735659 4735718 

Base Elevation [m] 179.91 179.81 177.63 177.63 177.63 177.63 177.4 177.35 

Release Height [m] 200.04 200.04 182.58 182.58 182.58 182.58 129.54 182.88 

Gas Exit Temperature [°K] 416.48 416.48 422.04 424.82 422.04 424.82 422.04 408.15 

Gas Exit Velocity [m/s] 27.432 27.432 27.889 32.034 30.267 27.706 49.225 40.234 

Inside Diameter [m] 7.772 7.772 4.06 4.06 4.06 4.06 4.054 4.877

Allowable Emission Rate [g/s] 1,030 1,030 296.7 296.7 296.7 296.7 653.6 987.9 

Actual Emission Rate [g/s] - - - - - - - - 

The above stack parameters and emissions were obtained from regulatory agency documents and 

databases identified in Section 2.2. The analysis was conducted based on 100% operating load using 

maximum exhaust flow rates and temperatures. Operation at less than full capacity loads was not 

considered. This assumption tends to under-predict impacts since stack parameters such as exit flow 

rate and temperature are typically lower at less than full load, reducing pollutant dispersion and 

increasing predicted air quality impacts. Stack location, height and diameter were verified using 

aerial photographs, and flue gas flow rate and temperature were verified using combustion 

calculations. 

 

 

10 http://ampd.epa.gov/ampd/ 

11 Stack parameters were obtained from an AERMOD input file from a previous modeling anlaysis provided by the 

Michigan Department of Environmental Quality, SCPP BRPP 1Hr SO2 SIA Run 50km 05_09 Aux DG 

15PPM_5yrs_SO2.DTA, April 24, 2012 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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4.3 Building Dimensions and Good Engineering Practice Height 

No building dimensions or prior downwash evaluations were available. Therefore this modeling 

analysis did not address the effects of downwash and this may under-predict impacts. 

In addition, no evaluation has been conducted to determine if the stack height exceeds GEP height. If 

the stack height exceeds GEP, the predicted concentrations may under-predict impacts. 

4.4 Receptors 

For St. Clair Power Plant, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on St. Clair Power Plant and extending out 5 

kilometers. 

2. A 500-meter Cartesian receptor grid centered on St. Clair Power Plant and extending out 10 

kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on St. Clair Power Plant and extending out 50 

kilometers. 50 kilometers is the maximum distance accepted by USEPA for the use of the 

AERMOD dispersion model.12

A flagpole height of 1.5 meters was used for all these receptors. 

Elevations from stacks and receptors were obtained from National Elevation Dataset (NED) GeoTiff 

data. GeoTiff is a binary file that includes data descriptors and geo-referencing information 

necessary for extracting terrain elevations. These elevations were extracted from 1 arc-second (30 

meter) resolution NED files. The USEPA software program AERMAP v. 11103 is used for these 

tasks. 

4.5 Meteorological Data 

To improve the accuracy of the modeling analysis, recent meteorological data for the 2012-

2014period were prepared using the USEPA’s program AERMET which creates the model-ready 

surface and profile data files required by AERMOD. Required data inputs to AERMET included 

surface meteorological measurements, twice-daily soundings of upper air measurements, and the 

micrometeorological parameters surface roughness, albedo, and Bowen ratio. One-minute ASOS 

data were available so USEPA methods were used to reduce calm and missing hours.13 The USEPA 

software program AERMINUTE v. 14237 is used for these tasks. 

 

12 USEPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and 

Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, Section A.1.(1), November 9, 

2005. 

13 USEPA, Area Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, 

Attachment 3, March 24, 2011, p. 19. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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For this modeling project, pre-processed meteorological data were provided by the Michigan 

Department of Environmental Quality (DEQ).14, 15 Data for the 2012 to 2013 period were 

downloaded from the DEQ web site and data for the 2014 period were provided directly by DEQ. 

The agency prepared its meteorological data using the procedures described in this report. 

This section discusses how the meteorological data was prepared for use in the 1-hour SO2 NAAQS 

modeling analyses. The USEPA software program AERMET v. 14134 is used for these tasks. 

4.5.1 Surface Meteorology 

Surface meteorology was obtained for St. Clair International Airport located near the St. Clair Power 

Plant. Integrated Surface Hourly (ISH) data for the 2012-2014 period were obtained from the 

National Climatic Data Center (NCDC). The ISH surface data was processed through AERMET 

Stage 1, which performs data extraction and quality control checks. 

4.5.2 Upper Air Data 

Upper-air data are collected by a “weather balloon” that is released twice per day at selected 

locations. As the balloon is released, it rises through the atmosphere, and radios the data back to the 

surface. The measuring and transmitting device is known as either a radiosonde, or rawindsonde. 

Data collected and radioed back include: air pressure, height, temperature, dew point, wind speed, 

and wind direction. The upper air data were processed through AERMET Stage 1, which performs 

data extraction and quality control checks. 

For St. Clair Power Plant, the concurrent 2012-2014 upper air data from twice-daily radiosonde 

measurements obtained at the most representative location were used. This location was the White 

Lake, Michigan measurement station. These data are in Forecast Systems Laboratory (FSL) format 

and were downloaded in ASCII text format from NOAA’s FSL website.16 All reporting levels were 

downloaded and processed with AERMET. 

4.5.3 AERSURFACE 

AERSURFACE is a program that extracts surface roughness, albedo, and daytime Bowen ratio for 

an area surrounding a given location. AERSURFACE uses land use and land cover (LULC) data in 

the U.S. Geological Survey’s 1992 National Land Cover Dataset to extract the necessary 

 

14 Michigan Department of Environmental Quality, Map of Available Meteorological Stations, 

http://www.michigan.gov/documents/deq/deq-aqd-mm-met_support_256121_7.pdf 

15 Email from J. Haywood – MDEQ to S. Klafka – Wingra Engineering, S.C., Request for AERMOD Modeling 

Meteorology for Michigan Power Plants, May 18, 2015. 

16 Available at: http://esrl.noaa.gov/raobs/ 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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micrometeorological data. LULC data was used for processing meteorological data sets used as 

input to AERMOD. 

AERSURFACE v. 13016 was used to develop surface roughness, albedo, and daytime Bowen ratio 

values in a region surrounding the meteorological data collection site. AERSURFACE was used to 

develop surface roughness in a one kilometer radius surrounding the data collection site. Bowen 

ratio and albedo was developed for a 10 kilometer by 10 kilometer area centered on the 

meteorological data collection site. These micrometeorological data were processed for seasonal 

periods using 30-degree sectors. 

4.5.4 Data Review 

Missing meteorological data were not filled as the data file met USEPA’s 90% data completeness 

requirement.17 The AERMOD output file shows there were 0.12% missing data. 

To confirm the representativeness of the airport meteorological data, the surface characteristics of 

the airport data collection site and the modeled source location were compared. Since the St. Clair 

International Airport is located close to St. Clair Power Plant, this meteorological data set was 

considered appropriate for this modeling analysis. 18 This weather station provided high quality 

surface measurements for the most recent 3-year time, and had similar land use, surface 

characteristics, terrain features and climate. Finally, DEQ provided pre-processed meteorological 

data for this project, whichhad concluded the weather stations were representative of the project 

location. 

5. Background SO2 Concentrations 

Background concentrations were determined consistent with USEPA’s Modeling Guidance for SO2

NAAQS Designations.19, 20 To preserve the form of the 1-hour SO2 standard, based on the 99th

percentile of the annual distribution of daily maximum 1-hour concentrations averaged across the 

number of years modeled, the background fourth-highest daily maximum 1-hour SO2 concentration 

was added to the modeled fourth-highest daily maximum 1-hour SO2 concentration.21 Background 

concentrations were based on the 2011-13 design value measured by the ambient monitors located in 

 

17 USEPA, Meteorological Monitoring Guidance for Regulatory Modeling Applications, EPA-454/R-99-05, February 

2000, Section 5.3.2, pp. 5-4 to 5-5. 

18 USEPA, AERMOD Implementation Guide, March 19, 2009, pp. 3-4. 

19 USEPA, Area Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, 

Attachment 3, March 24, 2011, pp. 20-23. 

20 USEPA, SO2 NAAQS Designations Modeling Technical Assistance Document, Dec. 2013, section 8.1, pp 27-28. 21 USEPA, Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard, 

August 23, 2010, p. 3. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

Page 14 

Michigan.22 

6. Reporting 

All files from the programs used for this modeling analysis are available to regulatory agencies. 

These include analyses prepared with AERSURFACE, AERMET, AERMAP, and AERMOD. 

 

22 http://www.epa.gov/airtrends/values.html