Clifty Creek Station 

Madison, Indiana 

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 

Page 2

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 Clifty Creek Station located in Madison, Indiana. 

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

emissions from the following additional sources of SO2, which are located within 50 kilometers of 

the Clifty Creek Station: 

 Trimble County Generating Station – Bedford, Kentucky 

 Ghent Generating Station – Ghent, Kentucky 

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 or 

obtained 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 parts per billion 

(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 

 

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

September 17, 2015 

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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 Clifty Creek, Trimble County, and Ghent Generating Stations are summarized 

in Table 1. It was determined that based on either current allowable emissions or measured actual 

emissions, the Clifty Creek Station 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 Trimble County and Ghent Generating Stations, both 

individually and cumulatively, significantly contribute to the ambient SO2 concentration in the area 

impacted by Clifty Creek Generating Station. 

Air quality impacts in Indiana are based on a background concentration of 41.7 μg/m3. This is the 

2011-13 design value for Vanderburgh County, Indiana - 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 Clifty Creek Station Modeling Analysis 

Emission Rates Averaging 

Period 

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

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 

Clifty Creek 

1,681.8 47.1 1,728.9 196.2 No 

Actual 234.3 47.1 281.4 196.2 No 

Actual Trimble 33.5 47.1 80.6 196.2 Yes 

Actual Ghent 76.0 47.1 123.1 196.2 Yes 

Actual All Plants 236.5 47.1 283.6 196.2 No 

 

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. 5

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

September 17, 2015 

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

Table 2 - Modeled SO2 Emissions from Clifty Creek Station 6

Stack 

ID 

Unit 

ID 

Allowable Emissions 

30-day Average 

 (lbs/hr) 

S13 

Unit 1 14,054.9 

Unit 2 14,054.9 

Unit 3 14,054.9 

Subtotal 42,164.7 

S46 

Unit 4 14,054.9 

Unit 5 14,054.9 

Unit 6 14,054.9 

Subtotal 42,164.7 

Facility Total 84,329.4 

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 Clifty Creek Station 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 the Clifty Creek Generating Station 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) 

149.1 91% 7,476.2 0.67 

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

throughout the region to a maximum distance of 50 kilometers. 

 

6

 Indiana Department of Environmental Management,Part 70 Operating Permit Renewal, Operation Permit No. T077-

29920-00001, July 7, 2001. Allowable emissions for Units 1 to 6 are 7.52 lbs per million BTU. 
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 Clifty Creek 

Generating Station. 

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

 Except for Trimble County and Ghent Generating Stations, no consideration of off-site 

sources. Any 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 Clifty Creek Station
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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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. Based on 

the output from the AERSURFACE, approximately 11.6% 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

September 17, 2015 

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4.2 Emission Rates and Source Parameters 

The modeling analyses only considered SO2 emissions from the Gibson, Trimble County, and Ghent 

facilities. Other 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, 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 Clifty Creek Ghent Trimble 

Stack S13 S46 G01 G02 G04 T01 T02 

Description Units 

1, 2 and 3 

Units 

4, 5 and 6 Unit 1 Units 

2 and 3 Unit 4 Unit 1 Unit 2 

X Coord. [m] 637424 637433.05 670779 670554 670479 638151 638155 

Y Coord. [m] 4289051 4289043.51 4290835 4290716 4290646 4271869 4271865 

Base Elevation [m] 143.21 143.06 147.94 147.82 148.8 146.38 146.34 

Release Height [m] 299.31 299.31 201.78 176.78 201.78 231.65 231.65 

Gas Exit Temperature [°K] 329.261 328.706 324.817 324.817 324.817 324.817 329.261 

Gas Exit Velocity [m/s] 15.084 15.453 15.342 14.976 14.62 27.077 27.836 

Inside Diameter [m] 9.504 9.504 8.077 15.941 8.077 5.486 6.279 

Allowable Emission Rate [g/s] 5,313 5,313 - - - - - 

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 annual survey compiled by the U.S. Energy Information Administration. 

http://www.eia.gov/electricity/data/eia860/ 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 17, 2015 

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4.3 Building Dimensions 

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 Clifty Creek Station, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on Clifty Creek Station and extending out 5 

kilometers. 

2. A 500-meter Cartesian receptor grid centered on Clifty Creek Station and extending out 10 

kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on Clifty Creek Station 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

September 17, 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 Louisville International Airport located near the Clifty Creek 

Station. 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 Clifty Creek Station, the concurrent 2012-2014 upper air data from twice-daily radiosonde 

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

Wilmington, Ohio 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

September 17, 2015 

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months having 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 0.12% 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 Louisville 

International Airport is located close to Clifty Creek Station, 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. Finally, the meteorological data from the selected 

surface and upper air stations were recommended by the Indiana Department of Environmental 

Management for modeling facilities located in Jefferson County.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 

Indiana.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 Indiana DEM, Air Dispersion Meteorological Data, http://www.in.gov/idem/airquality/2376.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