Gibson Generating Station 

Owensville, Indiana 

Evaluation of Compliance with the 1-hour NAAQS for SO2

September 3, 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 3, 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 Gibson Generating Station located in Owensville, Indiana. 

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

emissions from the Petersburg Generating Station in Petersburg, Indiana, which is an additional 

source of SO2 emissions located within 50 kilometers of the Gibson Generating Station. 

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 

NAAQS.4

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

 

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 3, 2015 

Page 3 

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

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 Gibson and Petersburg Generating Stations are summarized in Table 1. It was 

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

Gibson Generating 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 Petersburg Generating Station significantly contribute to the 

ambient SO2 concentration in the area impacted by Gibson Generating Station. 

Air quality impacts in Indiana are based on a background concentration of 47.1 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 Gibson Generating Station Modeling Analysis 

Emission Rates Averaging 

Period 

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

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 

Gibson 

1,046.6 47.1 1,093.7 196.2 No 

Actual 229.7 47.1 276.8 196.2 No 

Allowable 

Petersburg 

1,978.9 47.1 2,026.0 196.2 No 

Actual 726.5 47.1 773.6 196.2 No 

Allowable 

Both 

1,980.7 47.1 2,027.8 196.2 No 

Actual 726.9 47.1 774.0 196.2 No 

 

5

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

September 3, 2015 

Page 4 

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

Table 2 - Modeled SO2 Emissions from Gibson Generating Station 6

Stack 

ID 

Unit 

ID 

Allowable Emissions 

30-day Average 

 (lbs/hr) 

G01 Unit 1 5,875.0 

G02 Unit 2 5,875.0 

G03 Unit 3 8,845.5 

G04 Unit 4 3,538.2 

G05 Unit 5 6,490.0 

Gibson Subtotal 30,623.7 

P01 Unit 1 13,200.0 

P02 Unit 2 24,864.0 

P03 Unit 3 6,648.0 

P04 Unit 4 6,660.0 

Petersburg Subtotal 51,372.0 

Both Stations Total 81,995.7 

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. 

This assumes a one-hour averaging period for the emission rate, and that the emission rate is binding 

at all times. However, it is extremely likely that this limit is too high to protect the NAAQS, given 

the conservative aspects of this modeling protocol. 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 Gibson to consume the entire NAAQS, leaving little to no room for any other source of SO2 in 

the area. No consideration has been given to other sources aside from Petersburg Generating Station, 

and no margin of safety has been included in the hypothetical emission limitation. 

 

 

6

 For Gibson, allowable emissions are based on Indiana Department of Environmental Management, Part 70 Operating 

Permit Renewal No. T 051-27086-00013, June 8, 2009. The emission limitations for Units 1 to 5 are 1.0, 1.0, 1.5, 0.6 

and 1.1 lbs/mmbtu, respectively. For Petersburg, allowable emisisons are based on Indiana Department of Environmental 

Management, Part 70 Operating Permit Renewal No. T125-30045-00002, First Significant Modification No. 125-33773-

00002, January 10, 2014. The emission limitation for Units 1 to 2 is 6.0 lbs/mmbtu based on a 30-day rolling average. 

The emission limitation for Units 3 and 4 is 1.2 lbs/mmbtu based on a 3-hour average. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 3, 2015 

Page 5 

Table 3 - Required Emission Reductions from Gibson 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 92% 2,305.2 0.08 

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

throughout the region to a maximum distance of 50 kilometers. 

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. 

 Except for the Petersburg Generating Station, no consideration of off-site sources. These 

other sources of SO2 will increase the predicted impacts. 

2.4 Comparison with Existing Ambient Monitoring 

An ambient monitor for SO2 is operated approximately 2.7 km or 1.7 miles northeast of the Gibson 

Generating Station. The USEPA database for ambient monitoring measurements identifies Monitor 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 3, 2015 

Page 6 

#5180510002 as the Gibson Coal Road - PSI/ Duke Energy- Gibson Network/ E. Mt. Carmel 

monitor.7

 The 2012-14 design value measured at this monitor was 72 ppb or 188 μg/m3. 

Based on actual hourly emissions from the Gibson plant and other regional sources, the modeling 

analysis predicted a concentration at the monitor location of 229 μg/m3

. Minus the assumed 

background of 47.1 μg/m3 the predicted impact due to regional SO2 sources is 181.9 μg/m3. Since 

the predicted and actual measured design values are in close agreement, this comparison supports the 

accuracy of the modeling results presented in this report.

 

7 http://www.epa.gov/airtrends/pdfs/SO2_DesignValues_20122014_FINAL_8_3_15.xlsx 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 3, 2015 

Page 7

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

September 3, 2015 

Page 8

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

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

September 3, 2015 

Page 9

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.8

 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.9 

Please refer to Table 1 for the modeling results. 

 

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.

9 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 3, 2015 

Page 10 

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.10 

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 0.6% 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.

 

 

10 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 3, 2015 

Page 11 

4.2 Emission Rates and Source Parameters 

The modeling analyses only considered SO2 emissions from the Gibson and Petersburg facilities. 

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.

11

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

Table 4 – Facility Stack Parameters and Emissions 12

Facility Gibson Petersburg 

Stack S01 S02 S03 S04 S05 P01 P02 P03 P04 

Description Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 1 Unit 2 Unit 3 Unit 4 

X Coord. [m] 433023.73 433018.24 432936.33 432914.36 432867.49 477854.96 477855.47 477960 477982.44 

Y Coord. [m] 4247196.11 4247199.49 4247264.51 4247334.35 4247436.95 4264361.51 4264374.25 4264468.08 4264532.33 

Base Elevation [m] 125.1 125.41 124.34 124.47 124.54 127.58 128.26 130.86 130.81 

Release Height [m] 188.98 188.98 188.98 152.4 152.4 186.54 181.66 187.45 187.45 

Gas Exit Temperature [°K] 327.594 327.594 327.594 324.817 335.928 324.817 323.706 347.594 338.706 

Gas Exit Velocity [m/s] 21.464 21.464 21.464 21.398 21.398 22.469 21.72 26.727 25.391 

Inside Diameter [m] 7.62 7.62 7.62 7.163 7.163 4.654 6.279 6.706 6.706 

Allowable Emission Rate [g/s] 740.2 740.2 1,115 445.8 817.7 1,663 3,133 837.6 839.1 

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. 

 

 

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

12 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 3, 2015 

Page 12 

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 Gibson Generating Station, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on Gibson Generating Station and extending 

out 5 kilometers. 

2. A 500-meter Cartesian receptor grid centered on Gibson Generating Station and extending 

out 10 kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on Gibson Generating Station and extending 

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

the AERMOD dispersion model.13

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.14 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 

 

13 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. 

14 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 3, 2015 

Page 13 

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

4.5.1 Surface Meteorology 

Surface meteorology was obtained for Evansville Regional Airport located near the Gibson 

Generating 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 Gibson Generating 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 

Lincoln, Illinois measurement station. These data are in Forecast Systems Laboratory (FSL) format 

and were downloaded in ASCII text format from NOAA’s FSL website.15 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 

 

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

September 3, 2015 

Page 14 

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.16 The AERMOD output file shows there were 0.75% 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 Evansville 

Regional Airport is located close to Gibson Generating Station, this meteorological data set was 

considered appropriate for this modeling analysis. 17 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 Gibson County.18

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 

Indiana.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. 

 

16 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. 

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

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

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. 

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