Gerald Gentleman Station 

Sutherland, Nebraska 

Evaluation of Compliance with the 1-hour NAAQS for S02 

September 17, 2015 

Conducted by: 

Steven Klajka, P.E., BCEE 

Wingra Engineering, S.C. 

Madison, Wiscon.'iill 
Sep

Emluation of Compliance with the /-hour i\'AA.QSfor S01 

tember 17, 2015 

Page 2 

l. Introduction 

he

Win

lp 

g

the 

ra E

U.S. 

ngine

Environmen

ering, S.C. was hired by Sierra Club to conduct an air mode ling impact analysis to 

facilities 

tal Protection Agency (USEPA), state and local air agencies identify 

A

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

ir Quality Standard (NAAQS). This document describes 

eva

the results and procedures for an 

luation conducted for the Gerald Gentleman Station located in Sutherland, Nebraska. 

The 

hour 

di

S0

sper

2 NAAQS. 

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

AERMET, 

The modeling was performed using the most recent version of AERMOD, 

obtained 

and AERMINUTE, with data provided to Sierra Club by regulatory air agencies or 

through other publicly-avai

condu

lable sources as documented below. The analysis was 

cted in adherence 

atta

to all available US EPA guidance for evaluating source impacts on 

Impl

inm

eme

ent 

nt

o f the 1-hour S0 2 NAAQS via aerial dispersion modeling, including the AERMOD 

S02 Nationa

ation Guide; USEPA's Applicability of Appendix W Modeli ng Guidance for the !-hour 

USEPA 

l Ambient Air Quality Standard, August 23. 20 I 0; modeling guidance promulgated by 

NAAQS 

in 

Des

Appendix 

ignations; 

W to 40 CFR Part 5 1; USEPA's March 20 11 Modeling Guidance for S02 1 

Assis

and USEPA 's December 2013 S02 NAAQS Designations Technical 

tance Document.2 

2. Compliance with the 1-bour S02 NAAQS 

2.1 1-hour S02 NAAQS 

di

The 1-hour S02 NAAQS takes the fonn of a three-year average of the 99th percentile of the annual 

(ppb).

stribution 

3 Compliance 

of daily maximum !-hour concentrations, wh ich cannot exceed 75 parts per billion 

which produces 

with this standard was verified using USEPA's AERMOD air dispersion model, 

air concentrations in units of llg/m3

196.2 

• The !-hour S02 NAAQS of 75 ppb equals 

llg/m3

NAAQS

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

.

4 

correspond

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

s to the fourth-h ighest value at each receptor for a given year. 

1 http://www.epa.gov/scramOO 1/so2_modeling_guidance 2 

.htm 

3 U

http:

SE

//

PA, 

www.epa

Ap

.gov/oaqpsOO I /sulfurdioxide/pdfs/S0 2ModelingT AD.pdf 

Augus

plicability of Appendix W Modeling Guidance for the 1-hour S02 National Ambient Air Quality Standard, 

t 23, 2010. 

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

calculation is 75/0.3823 = 96.2~Jg/m • 
£1'(1/ualion of Compliance ll'ilh 1he /-hour :\:.JAQSfor S01 

Seplember 17. 2015 

Page 3 

2.2 Modeling Results 

Modeling results for Gerald Gentleman Station are summarized in Table I. It was determined that 

based on either current allowable emissions or measured actual emissions. the Gerald Gentleman 

Station is estimated to create downwind S02 concentrations which exceed the !-hour NAAQS. 

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

emissions cause exceedences of the 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 I. 2012 and December 31, 20 14 as taken from US EPA Air 

Atfarkets Program Data.5 

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

. This is the 

20 ll-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 I - S01 Modeling Results for Gerald Gentleman Station Modeling Analysis 

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

) Complies with 

Emission Rates Period Impact Background Total NAAQS AAQS? 

Allowable 1-hour 809.7 88.9 898.6 196.2 No 

Actual 1-hour 128.6 88.9 2 17.5 196.2 0 

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

Table 2- Modeled S02 Emissions from Gerald Gentleman Station 6 

Stack Unit A I towable Em iss ions 

ID lD 

2-hour Average 

(lbs/hr) 

SO l Unit I 18.422.5 

S02 Unit 2 20,770.5 

Stack Total All Units 39,193.0 

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

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

5 http://ampd.epa.gov/ampdl 6 Allowable emissions for Unit 1-1 are based on the 2-hour average 2.5 lbs/rnmbtu limitation in Pem1it to Operate issued 

by the Nebraska Department of Environmental Quality. August 26. 2002. 
Emluation of Compliance with the !-hour NAAQSfor S0 2 

September 17. 2015 · 

Page .J 

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 em ission limitation in Table 3 

would allow Gerald Gentleman 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 Gerald Gellfleman Stationfor Compliance witlt tlte 

1-ltour 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 Dai ~ Max Allowable Emissions (lbs/hr) Emission Rate 

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

107.3 87% 5,193.8 0.33 

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

throughout the region to a maximum distance of 50 kilometers. 

Figure I 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 S02 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 I 00% load. Stack parameters such as exit 

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

dispersion and increasing predicted air quality impacts. 
Evaluation ofCompliance with the 1-hour.NAAQSfor S01 

Seplember 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 NAA QS for S01 

September 17, 2015 

Page 6 

0 

0 

.;;· 

~ 0 

0 

fl. 

'f; "' 

0 

0 

0 

~ 

" 0 

~-

"' .. 

~ "' .. 

0 

0 

,, 0 

~ 

" n 

0 

~ ., 

.. 

.. ·~ 

" 

~ 

27!-000 281»JQ 285000 290000 29::.000 :100000 30f.QOO 310000 31:.000 3~0000 325000 330000 

UTM East(mJ 

·. · 

I 

335000 34000•) 3-1:.000 :.soooo '~;.ooo ltlOOOO 

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

---1-------·11-------f~~~~~~-'-------' 

196 300 400 500 600 700 

Figure 1- Regional View of Impacts Based on Allowable Emissions 
Ewluation ofCompliance with the /-hour NAAQSfor S01 

September 17. 2015 

Page 7 

0 J 0. 

~ 

§ 

s-

-:; 

317000 316000 

Gerald Gentleman Station 

)1WOQ noooo n•ooo j22000 

UTM East(mj 

325000 JZvOOO 

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

196 210 

Figure 2 - Regional View oflmpacts Based on Actual Emissions for the 2012-14 Period 
Emluation of Compliance with the /-hour NAAQSfor S01 

September 17. 2015 

Page8 

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 defau lts 

• 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 20 12-2014. 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 fonn ofthe l-hour S02 NAAQS.

8 

Please refer to Table I 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 5 1, November 9, 2005. 8 USEPA, Area Designations for the 20 10 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, 

Attachment 3, March 24, 20 II, pp. 24-26. 
Emluation ofCompliance ll'ith 1he /-hour NAAQSfor SOJ 

Septemher 17. 2015 

Page 9 

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 (GfS) 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 AERSURF ACE, approximately 0.18% 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/ 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 !-hour NAAQSfor S0 1 

Sep1ember 17. 2015 

Page /0 

4.2 Emission Rates and Source Parameters 

The mode ling analyses only considered S02 emissions from the facility. Off-site sources were not 

considered. Concentrations were predicted for the scenarios shown in Tables l and 2: 

I) allowable emissions based on the current permit issued by the regulatory agency, and 

2) actual hourly emissions measured each hour between January l, 201 2 and December 3 1, 

20 14 as taken from USEPA Air Markets Program Data.

10 

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

Table 4 - Facility Stack Parameters and Emissions 

11 

Stack SOl S02 

Description Boiler I Bo iler 2 

X Coord. [m] 320187 320202 

Y Coord. [m] 4550011 4549893 

Base Elevation [m] 953.56 956.95 

Release Height [m] 167.64 167.64 

Gas Exit Temperature [°K] 427.594 422.039 

Gas Exit Velocity [m/s] 24.42 24.09 

Inside Diameter [m] 8.534 8.534 

Allowable Emission Rate [g/s] 2,32 1 2,6 17 

Actual Em iss ion 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 I 00% 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 Nebraska Public Power District, Year2010 Air Emissions Inventory, Facility ID 34385, March 28,20 11. 
Em!uation ojCompliance with the /-hour NAAQSfor S01 

September 17. 2015 

Page II 

4.3 Building Dimensions 

No building dimensions or prior downwash evaluations were ava ilable. Therefore this modeling 

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

4.4 Receptors 

For Gerald Gentleman Station, three receptor grids were employed: 

I. A I 00-meter Cartesian receptor grid centered on Gerald Gentleman Station and extending out 

5 kilometers. 

2. A 500-meter Cartesian receptor grid centered on Gera ld Gentleman Station and extending out 

I 0 kilometers. 

3. A I ,000-meter Cartesian receptor grid centered on Gerald Gentleman Station and extending 

out 50 kilometers. 50 kilometers is the ~'< imum distance accepted by USEPA for the use of 

the A EIUAOD 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 I arc-second (30 

meter) resolution NED files. The USEPA software program AERMAP v. Ill 03 is used for these 

tasks. 

4.5 Meteorological Data 

To improve the accuracy of the modeling analysis, recent meteoro logical data for the 201 2-201 4 

period were prepared using the USEPA's program AERM ET which creates the model-ready surface 

and profile data fi les required by AER.MOD. Required data inputs to AERMET included surface 

meteorological measurements, twice-daily soundings of upper air measurements, and the 

microrneteorological 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 !-hour S02 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.l.( I), November 9, 

2005. 

JJ US EPA, A rca Designations for the 20 I 0 Revised Primary Sui fur Diox ide National A mbicnt Air Quality Standards. 

Attachment 3, March 24, 20 II , p. 19. 
Era/uarion of Compliance wirh rile /-hour XA.-IQSfor S01 

Seprember 17. 2015 

Page 12 

modeling analyses. The USEPA software program AERMET v. 14 134 is used for these tasks. 

4.5.1 Surface Meteorology 

Surface meteorology was obtained for North Platte Regional Airport located near the Gerald 

Gentleman Station. Integrated Surface Hourly (ISH) data for the 2012-20 14 period were obtained 

from the National Climatic Data Center (NCDC). The ISH surface data was processed through 

AERMET Stage I, 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 rad ioed back include: air pressure, height, temperature, dew point, wind speed, 

and wind direction. The upper air data were processed through AERMET Stage I, which performs 

data extraction and quality control checks. 

For Gerald Gentleman Station, the concurrent 20 12-20 14 upper air data from twice-daily radiosonde 

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

Platte, Nebraska measurement station. These data are in Forecast Systems Laboratory (FSL) format 

and were downloaded in ASCll text format from NOAA's FSL website.

14 All reporting levels were 

downloaded and processed with AERMET. 

4.5.3 AERSURFACE 

AERSURF ACE 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. 130 16 was used to deve lop 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 I 0 kilometer by I 0 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 wi nter 

14 Avai lable at: http://esrl.noaa.gov/raobs/ 
Evaluation of Compliance with the !-hour NA AQSfor S01 

September 17. 2015 

Page 13 

months havi ng continuous snow cover. 

4.5.4 Data Review 

Missing meteorological data were not fil led as the data fi le met USEPA's 90% data comp leteness 

requ irement. 15 The AERMOD output fi le shows there were 0.53% missing data. 

To confirm the representativeness of the airport meteorological data, the surface characteristics of 

the a irport data collection site and the mode led source location were compared. Since the North 

Platte Regiona l Airport is located c lose to Gera ld Gentleman Station, this meteorological data set 

was considered appropriate for this modeling analysis. 

16 Additionally, this weather station provided 

high quality surface measurements fo r the most recent 3-year time, and had similar land use, surface 

characteristics, terra in 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 fo rm of the !-hour S0 2 standard, based on the 99th 

percentile of the ann ual distribution of da ily maximum 1-hour concentrations averaged across the 

number of years mode led, the back!!round fo urth-hi ghest daily maximum !-hour S0 2 concentration 

was added to the modeled fourth-highest daily max imum 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 fi les from the programs used fo r this mode ling analysis are available to regulatory agenc ies. 

These inc lude ana lyses 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 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, 20 II , pp. 20-23 .. 18 USEPA, Applicability of Appendix W Modeling Guidance for the 1-hour S02 National Ambient Air Quality Standard, 

August 23.2010, p. 3. 

IQ http://www .epa.gov/ai rtrends/val ues.hm1l