GRDA Chouteau Coal-Fired Complex 

Chouteau, Oklahoma 

Evaluation of Compliance with the 1-hour NAAQS for SO2

May 29, 2014 

Conducted by: 

Steven Klafka, P.E., BCEE 

Wingra Engineering, S.C. 

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

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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 violations 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 GRDA Chouteau Coal-Fired Complex 

located in Chouteau, Oklahoma. 

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

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; and, USEPA’s March 2011 Modeling Guidance for SO2 NAAQS Designations.1 

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.2

 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.3

 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 GRDA Chouteau Coal-Fired Complex are summarized in Table 1. It was 

determined that based on currently approved emissions, the GRDA Chouteau Coal-Fired Complex is 

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

 

1

 http://www.epa.gov/scram001/so2_modeling_guidance.htm 2 USEPA, Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard, 

August 23, 2010.

3 The ppb to μg/m3

 conversion is found in the source code to AERMOD v. 13350, subroutine Modules. The conversion 

calculation is 75/0.3823 = 196.2 μg/m3

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

May 29, 2014 

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Table 1 presents the modeled impacts based on the plant’s allowable emissions rate. “Allowable” is 

the peak emission rate from each unit as approved by the current air quality operation permit or 

applicable discharge regulation for the facility. 

Air quality impacts in Oklahoma are based on a background concentration of 13.1 μg/m3

. This is the 

2010-12 design value for Oklahoma County, Oklahoma - the lowest measured background 

concentration in the state. This is the most recently available design value. 

Table 1 - SO2 Modeling Results for GRDA Chouteau Coal-Fired Complex Modeling Analysis 

Emission Rates Averaging 

Period 

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

NAAQS? Impact Background Total NAAQS 

Allowable 1-hour 318.4 13.1 331.5 196.2 No 

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

Table 2 - Modeled SO2 Emissions from GRDA Chouteau Coal-Fired Complex 4,5

Stack 

ID 

Unit 

ID 

Allowable Emissions 

3-hour Average 

(lbs/hr) 

Maximum Emissions

1-hour Average 

(lbs/hr) 

S01 Unit 1 6,157.2 3,318.8 

S02 Unit 2 6,355.2 3,185.9 

Stack Total All Units 12,512.4 6,504.7 

Based on the modeling results, emission reductions from current allowable rates considered 

necessary to achieve compliance with the 1-hour NAAQS were calculated and presented in Table 3. 

 

 

4

 Allowable emissions used for the modeling analysis are based on the 3-hour limitation of 1.2 lbs/mmbtu under OAC 

252:100-31-25. Long-term annual-average limits of 1.2 and 0.6 lbs/mmbtu for Units 1 and 2, respectively are contained 

in OKDEQ, Permit No. 2009-179-RVR2 (M-1), December 1, 2011. Applicable emission limitations were confirmed 

during a telephone conversation between H. Neumann – OKDEQ and S. Klafka – Wingra Engineering, S.C., May 28, 

2014. 

5

 Maximum emissions are based on the measured hourly rates reported for the 2011 to 2013 period in USEPA Air 

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

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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) 

183.1 43% 7,195.4 0.69 

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

throughout the region to a maximum distance of 9 kilometers. 

Figure 1 provides a regional view of NAAQS violations based on allowable emissions. 

Figure 2 provides a local view of NAAQS violations based on allowable 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 off-site sources. These other sources of SO2 will increase the predicted 

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

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Figure 1 - Regional View of SO2 Concentrations Due to the Chouteau Coal-Fired Complex Based on Allowable Emissions
Evaluation of Compliance with the 1-hour NAAQS for SO2

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Figure 2 - Local View of SO2 Concentrations Due to the Chouteau Coal-Fired Complex Based on Allowable Emissions 

 
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. 13350. 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.6

 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 2011-2013. 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.7 

Please refer to Table 1 for the modeling results. 

 

6

 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. 7 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.8

 

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 9.3% 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.

 

 

8

 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. Table 1 presents the concentrations which were predicted using the approved or 

allowable emissions based on applicable regulations or permits issued by the regulatory agency. 

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

Table 4 – Facility Stack Parameters and Emissions 9

Stack S01 S02 

Description Unit 1 Unit 2 

X Coord. [m] 294133 294203 

Y Coord. [m] 4007351 4007271 

Base Elevation [m] 189.6 189.13 

Release Height [m] 153.92 153.92 

Gas Exit Temperature [°K] 422.594 363.15 

Gas Exit Velocity [m/s] 29.745 30.643 

Inside Diameter [m] 6.096 6.096 

Allowable Emission Rate [g/s] 775.8 800.7 

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

databases identified in Section 2.3. 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. 

 

9 Stack parameters are taken from Evaluation of Permit Application No. 2009-179-C (M-2) PSD, Page 16, August 1, 

2012. 
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4.3 Building Dimensions and GEP 

Building dimensions from a prior downwash evaluation were available so this modeling analysis did 

address the effects of downwash.10 The BPIP model was used to re-evaluate downwash. 

4.4 Receptors 

For GRDA Chouteau Coal-Fired Complex, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on GRDA Chouteau Coal-Fired Complex and 

extending out 5 kilometers. 

2. A 500-meter Cartesian receptor grid centered on GRDA Chouteau Coal-Fired Complex and 

extending out 10 kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on GRDA Chouteau Coal-Fired Complex and 

extending out 50 kilometers. 50 kilometers is the maximum distance accepted by USEPA for 

the use of the AERMOD dispersion model.11

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 2011-2013 

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.12 The USEPA 

software program AERMINUTE v. 11325 is used for these tasks. 

 

10 Email, E. Mulligan – OKDEQ to L. Williams – Sierra Club, May 28, 2014. Provided a BPIP output file from a prior 

downwash evaluation, PRIME.PIP. 

11 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. 

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

Attachment 3, March 24, 2011, p. 19. 
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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. 13350 is used for these tasks. 

4.5.1 Surface Meteorology 

Surface meteorology was obtained for Tulsa International Airport located near the GRDA Chouteau 

Coal-Fired Complex. Integrated Surface Hourly (ISH) data for the 2011-2013 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 GRDA Chouteau Coal-Fired Complex, the concurrent 2011-2013 upper air data from twicedaily radiosonde measurements obtained at the most representative location were used. This 

location was the Lamont, Oklahoma measurement station. These data are in Forecast Systems 

Laboratory (FSL) format and were downloaded in ASCII text format from NOAA’s FSL website.13 

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 no 

 

13 Available at: http://esrl.noaa.gov/raobs/ 
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months with 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.14 The AERMOD output file shows there were 6.2% 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 Tulsa 

International Airport is located close to GRDA Chouteau Coal-Fired Complex, this meteorological 

data set was considered appropriate for this modeling analysis. 15 Additionally, this weather station 

provided high quality surface measurements for the most recent 5-year time, and had similar land 

use, surface characteristics, terrain features and climate. 

5. Background SO2 Concentrations 

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

NAAQS Designations.16 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.17 

Background concentrations were based on the 2010-12 design value measured by the ambient 

monitors located in Oklahoma.18 

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. 

 

14 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. 

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

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

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

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

August 23, 2010, p. 3. 

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