Scherer Steam-Electric Generating Plant 

Juliette, Georgia 

Evaluation of Compliance with the 1-hour NAAQS for SO2

July 30, 2015 

Conducted by: 

Steven Klafka, P.E., BCEE 

Wingra Engineering, S.C. 

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

July 30, 2015 

Page 2

1. Introduction 

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

help 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 Scherer Steam-Electric 

Generating Plant located in Juliette, Georgia. 

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 the 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 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. 

2.2 Modeling Results 

Modeling results for Scherer Steam-Electric Generating Plant are summarized in Table 1. It was 

 

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

July 30, 2015 

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determined that based on either current allowable emissions or measured actual emissions, the 

Scherer Steam-Electric Generating 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

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

2011-13 design value for Muscogee, Georgia - 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 Scherer Steam-Electric Generating Plant Modeling Analysis 

Emission Rates Averaging 

Period 

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

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 1-hour 1,001.2 23.5 1,024.7 196.2 No 

Actual 1-hour 210.7 23.5 234.2 196.2 No 

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

Table 2 - Modeled SO2 Emissions from Scherer Steam-Electric Generating Plant 6

Stack 

ID 

Unit 

ID 

Allowable Emissions 

24-hour Average 

 (lbs/hr) 

S01 Unit 1 9,834 

S02 Unit 2 9,834 

S03 Unit 3 9,834 

S04 Unit 4 9,834 

Total All Units 39,336.0 

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

allowable rates necessary to achieve compliance with the 1-hour NAAQS. 

 

5

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

6

 Georgia Department of Natural Resources, Part 70 Operating Permit No. 4911-207-0008-V-03-0, May 8, 2012. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

July 30, 2015 

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Table 3 - Required Emission Reductions 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 83% 6,785.2 0.21 

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

throughout the region to a maximum distance of 44 kilometers. 

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

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

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 or GEP height. If the stack height exceeds GEP, the predicted concentrations will 

increase. 

 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

July 30, 2015 

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Figure 1 - Regional View of Impacts Based on Allowable Emissions
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Figure 2 - Regional View of Impacts Based on Actual Emissions for the 2012-14 Period 

 
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

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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 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. Based on 

the output from the AERSURFACE, approximately 0.2% of surrounding land use around the 

modeled facility was of urban land use types including Type 21 – Low Intensity Residential, Type 

22 – High Intensity Residential and Type 23 – Commercial / Industrial / Transportation. 

This is less than the 50% value considered appropriate for the use of urban dispersion coefficients. 

Based on the AERSURFACE analysis, it was concluded that the rural option would be used for the 

modeling summarized in this report. Please refer to Section 4.5.3 for a discussion of the 

AERSURFACE analysis.

 

 

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

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4.2 Emission Rates and Source Parameters 

The modeling analyses only considered SO2 emissions from the facility. Off-site sources were not 

considered. 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, 2011 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

Stack S01 S02 S03 S04 

Description Unit 1 Unit 2 Unit 3 Unit 4 

X Coord. [m] 237957 237957 237971.26 237982.8 

Y Coord. [m] 3661152 3661162.86 3661822.09 3661827.52 

Base Elevation [m] 141.94 141.95 141.95 141.96 

Release Height [m] 258.17 258.17 258.17 258.17 

Gas Exit Temperature [°K] 329.261 329.261 329.261 329.261 

Gas Exit Velocity [m/s] 17.345 17.345 17.345 17.345 

Inside Diameter [m] 10.363 10.363 10.363 10.363 

Allowable Emission Rate [g/s] 1,239 1,239 1,239 1,239 

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 the U.S. Energy Information Agency, http://www.eia.gov/electricity/data/eia860/, 2013 Form

EIA-860 Data - Schedule 6, 'Stack & Flue Data' , 6_2_EnviroEquip_Y2013.xlsx
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4.3 Building Dimensions and GEP 

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. 

4.4 Receptors 

For Scherer Steam-Electric Generating Plant, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on Scherer Steam-Electric Generating Plant 

and extending out 5 kilometers. 

2. A 500-meter Cartesian receptor grid centered on Scherer Steam-Electric Generating Plant 

and extending out 10 kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on Scherer Steam-Electric Generating 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 2012-2014 

period 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. 

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

 

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

July 30, 2015 

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

4.5.1 Surface Meteorology 

Surface meteorology was obtained for Middle Georgia Regional Airport located near the Scherer 

Steam-Electric Generating 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 Scherer Steam-Electric Generating Plant, the concurrent 2012-2014 upper air data from twicedaily radiosonde measurements obtained at the most representative location were used. This 

location was the Peachtree City / Falcon Field measurement station. These data are in Forecast 

Systems Laboratory (FSL) format and were downloaded in ASCII text format from NOAA’s FSL 

website.14 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 

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. Seasonal moisture conditions were considered average with winter 

 

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

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months having no continuous snow cover. 

4.5.4 Data Review 

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

requirement.15 The AERMOD output file shows there were 0.46% 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 Middle 

Georgia Regional Airport is located close to Scherer Steam-Electric Generating Plant, this 

meteorological data set was considered appropriate for this modeling analysis. 16 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. Lastly, the weather stations selected for 

the modeling analysis were those recommended for use of the AERMOD dispersion model by the 

Georgia Department of Natural Resources.17

5. Background SO2 Concentrations 

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

NAAQS Designations.18,19 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.20 Background 

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

Georgia.21 

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. 

 

15 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. 

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

17 http://www.georgiaair.org/airpermit/html/sspp/modeling/aermetdata.htm 

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

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

19 USEPA, SO2 NAAQS Designations Modeling Technical Assistance Document, Dec. 2013, section 8.1, pp 27-28. 

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

August 23, 2010, p. 3. 

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