Sheldon Station 

Hallam, Nebraska 

Evaluation of Compliance with the 1-hour NAAQS for S02 

September 17, 2015 

Co11ducted by: 

Steve11 Klajka, P.E., BCEE 

Wi11gra E11gilleeri11g, S.C. 

Madiso11, Wisco11si11 
Evaluation of Compliance with the 1-hour NAAQS for S01 

September 17, 2015 

Pagel 

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 (USEP A), state and local air agencies identify 

facilities that are likely causing exceedences of the 1-hour sulfur dioxide (S02) National Ambient 

Air Quality Standard (NAAQS). This document describes the results and procedures for an 

evaluation conducted for the Sheldon Station located in Hallam, Nebraska. 

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

hour S02 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 USEP A guidance for evaluating source impacts on 

attainment of the 1-hour S02 NAAQS via aerial dispersion modeling, including the AERMOD 

Implementation Guide; USEPA's Applicability of Appendix W Modeling Guidance for the 1-hour 

S02 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 S02 

NAAQS Designations; 1 and USEPA's December 2013 S0 2 NAAQS Designations Technical 

Assistance Document.2 

2. Compliance with the 1-hour 802 NAAQS 

2.1 1-hour SOz NAAQS 

The 1-hour S0 2 NAAQS takes the form of a three-year average of the 991

h -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~g/m . The 1-hour S02 NAAQS of75 ppb equals 

196.2 g/ , and this is the value used for determining whether modeled impacts exceed the 

NAAQS.4 The 99th-percentile of the annual distribution of daily maximum 1-hour concentrations 

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

1 http://www.epa.gov/scramOO 1/so2 _ modcling_guidance.htm 

2 http://www .epa.gov/oaqpsOO 1/sulfurdioxide!pdfs/S02ModelingT AD .pdf 3 USEPA, Applicability of Appendi.x W Modeling Guidance for the !-hour S02 National Ambient Air Quality Standard, 

August 23, 2010. 

4 The ppb to J.1g/m3 conversion is found in the source code to AERMOD v. 14134, subroutine Modules. The conversion 

calculation is 75/0.3823 = 196.2 J.lg/m3

. 
Evaluation of Compliance with the 1-hour NAAQS for S01 

September 17, 2015 

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2.2 Modeling Results 

Modeling results for Sheldon Station are summarized in Table 1. It was determined that based on 

either current allowable emissions or measured actual emissions, the Sheldon Station is estimated to 

create downwind S02 concentrations which exceed the 1-hour NAAQS. 

More specifically, the modeling results presented in Table 1, show the plant's allowable and actual 

emissions cause exceedances ofthe NAAQS. "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 US EPA Air 

Markets Program Data.5 

Air quality impacts in Nebraska are based on a background concentration of 88.9 Jlg/m3

. This is the 

2011-13 design value for Douglas County, Nebraska - 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 - S02 Modeling Results for Sheldon Station Modeling Analysis 

Averaging 99th Percentile !-hour Daily Maximum (f.1g/m3

) Complies with Emission Rates Period Impact Background Total NAAQS NAAQS? 

Allowable 1-hour 841.2 88.9 930.1 196.2 No 

Actual !-hour 119.7 88.9 208.6 196.2 No 

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

Table 2 -Modeled S02 Emissions from Sheldon Station 6 

Stack Unit Allowable Emissions 

ID ID 2-hour Average 

(lbs/hr) 

SOl Unit 1 3,435.0 

S02 Unit2 3,435.0 

Stack Total All Units 6,870.0 

5 http://ampd.epa.gov/ampd/ 6 Allowable emissions are based on the 2-hour average 2.5 lbs/mmbtu limitation in Title V Air Contaminant Source 

Operating Permit issued by the City of Lincoln, Nebraska, December 6, 2010. 
Evaluation of Compliance with the 1-hour NAAQS for S01 

September 17, 2015 

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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 even 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 Sheldon Station to consume the entire NAAQS, leaving little to no room for any other 

source of S02 in the area. No consideration has been given to other sources and no margin of safety 

has been included in the hypothetical emission limitation. 

Table 3 - Required Emission Reductions from Sheldon Station for Compliance with the 1-hour 

NAA QS for S02 

Acceptable Impact Required Required Required 

(NAAQS - Background) Total Facility Total Facility 

99th Percentile Reduction Based on 

Total Facility 1-hour Average Emission Rate 1-hour Daill Max Allowable Emissions (lbslhr) Emission Rate 

(J.tg/m) (%) (lbs/mmbtu) 

107.3 87% 876.3 0.32 

Predicted exceedences of the 1-hour NAAQS for S02 based on allowable emissions extend 

throughout the region to a maximum distance of 42 kilometers. 

Figure 1 shows the extent ofNAAQS violations based on allowable emissions. 

Figure 2 shows the extent ofNAAQS 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 SOz 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. 
Evaluation of Compliance with the 1-hour NAAQSfor S01 

September 17, 2015 

Page5 

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

increase predicted concentrations near the facility. 

• No consideration of off-site sources. These other sources of S02 will increase the predicted 

impacts. 
Evaluation of Compliance with the !-hour NAAQSfor S02 

September 17, 2015 

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1-hour average S02 concentrations (ug per cubic meter) -All colored areas exceed the NAAQS. 

196 400 600 800 

Figure 1 -Regional View of Impacts Based on Allowable Emissions 
Evaluation of Compliance with the 1 -hour NAAQS for S01 

September 17, 2015 

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1-hour average S02 concentrations (ug per cubic meter) - All colored areas exceed the NAAQS. 

196.2 205.0 

Figure 2- Regional View of Impacts Based on Actual Emissions for the 2012-14 Period 
Evaluation of Compliance with the 1-hour NAAQS[or S01 

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 ofthe 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 S02 NAAQS Designations, AERMOD provided a table of 

fourth-high 1-hour S02 impacts concentrations consistent with the form of the 1-hour S02 NAAQS.8 

Please refer to Table 1 for the modeling results. 

7 US EPA, 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, 201 1, pp. 24-26. 
Evaluation of Compliance with the 1 -hour NAAQS for S02 

September 1 7, 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 pennits 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. USEP A 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 AERSURF ACE, approximately 1.2% 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 I Industrial I Transportation. 

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

Based on the AERSURF ACE 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 

AERSURF ACE 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 NAAQSfor S02 

September 1 7, 2015 

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4.2 Emission Rates and Source Parameters 

The modeling analyses only considered S02 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, 2012 and December 31, 

2014 as taken from USEPAAir 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 SOl S02 

Description Boiler 1 Boiler 2 

X Coord. [m] 687602 687589 

Y Coord. [m] 4492175 4492215 

Base Elevation [m] 440.5 440.46 

Release Height [m] 53.04 53.04 

Gas Exit Temperature [°K] 424.261 424.261 

Gas Exit Velocity [ m/s] 22.25 22.25 

Inside Diameter [ m] 3.536 3.536 

Allowable Emission Rate [g/s] 432.8 476.9 

Actual Emission Rate [g/s] - -

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

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

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

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

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

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

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

calculations. 

10 http://ampd.epa.gov/ampd/ 11 Email, G. Bergstrom - Lincoln-Lancaster Co. Health Dept., Request for Title V Permit and Stack Paramaters for 

Sheldon Station - Hallam, Nebraska, January 18,201 2. 
Evaluation of Compliance with the 1-hour NAAQSfor S02 

September 17, 2015 

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4.3 Building Dimensions 

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

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

4.4 Receptors 

For Sheldon Station, three receptor grids were employed: 

1. A 1 00-meter Cartesian receptor grid centered on Sheldon Station and extending out 5 

kilometers. 

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

kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on Sheldon 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 gee-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 USEP A'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-minqte 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 SOz 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. I.( I), 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 I -hour NAAQS for S01 

September I 7, 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 Lincoln Municipal Airport located near the Sheldon 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 Sheldon 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 Omaha, 

Nebraska 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 AERSURF ACE 

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

an area surrounding a given location. AERSURF ACE 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. AERSURF ACE 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/raobsl 
Evaluation ofCompliance with the 1-hour NAAQSforS01 

September 17, 2015 

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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 0.33% 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 Lincoln 

Municipal Airport is located close to Sheldon Station, this meteorological data set was considered 

appropriate for this modeling analysis. 

16 Additionally, 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. 

5. Background S02 Concentrations 

Background concentrations were determined consistent with USEPA's Modeling Guidance for S02 

NAAQS Designations. 17 To preserve the form of the !-hour S02 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 S02 concentration. 18 Background 

concentrations were based on the 201 1-13 design value measured by the ambient monitors located in 

Nebraska. 19 

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 US EPA, 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, AER.i\ilOD Implementation Guide, March 19,2009, pp. 3-4. 17 USEPA, S02 NAAQS Designations Modeling Technical Assistance Document, Dec. 2013, section 8.1, pp 27-28; 

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

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

18 USEPA, Applicability of Appendix W Modeling Guidance for the !-hour S02 National Ambient Air Quality Standard, 

August 23, 2010, p. 3. 

19 http://www.epa.gov/airtrends/values.btml