Nebraska Public Power District

Sulfur Dioxide 1-Hour NAAQS Designations Modeling

for

Gerald Gentleman Station (Facility ID #34385)

Sutherland, Nebraska

For submittal to:

Lisa Alam

c/o Records Management

Nebraska Department of Environmental Quality

Suite 400, The Atrium

1200 N Street

P.O. box 98922

Lincoln, Nebraska 69509-8922

Prepared by: 

HDR Engineering, Inc.

September 4, 2015
GGS 1-Hour SO2 Modeling 1 September 2015

INTRODUCTION

This modeling report summarizes dispersion modeling of Gerald Gentleman Station to demonstrate 

attainment with the sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) for the 1-hour 

averaging period, which is equal to 75 parts per billion (ppb) or approximately 196.5 micrograms per 

cubic meter (μg/m3

). This modeling has been completed for submission to the Nebraska Department of 

Environmental Quality (NDEQ) for review and approval. The results of the dispersion modeling analysis 

will be used by the NDEQ to formulate recommendations to EPA on the NAAQS designations.

MODELING PROTOCOL

The dispersion modeling results summarized in this report were obtained using the EPA’s AERMOD 

dispersion modeling system, using the modeling procedures and data as described in a modeling protocol 

developed in collaboration with NDEQ, as provided in Attachment 1 to this report. In addition to this 

report, this submittal to NDEQ includes a compact disk/DVD containing all the data files listed at the end 

of the modeling protocol in Attachment 1.

MODEL INPUTS

This modeling report contains figures detailing the modeled sources, receptors, background SO2

concentration and meteorological data locations, and the resulting 1-hour SO2 modeled impacts. The SO2

modeling was conducted in accordance to the attached detailed dispersion modeling protocol.
GGS 1-Hour SO2 Modeling 2 September 2015

Figure 1 displays the source locations of the two point sources included in the model, which are the coalfired boiler stacks for Gerald Gentleman Station (GGS) Unit 1 (GGS1) and GGS Unit 2 (GGS2).

Figure 1: Source Locations
GGS 1-Hour SO2 Modeling 3 September 2015

Figure 2 displays the receptor locations as specified in the attached dispersion modeling protocol in 

accordance with NDEQ guidance. Note that receptors are not included over water bodies in accordance 

with EPA guidance, and therefore, receptors have been excluded over the waters of Sutherland Reservoir.

Figure 2: Receptor Locations
GGS 1-Hour SO2 Modeling 4 September 2015

Figure 3 displays the NPPD facility location in relation to the background SO2 monitor location (Trego 

County, Kansas) and meteorological data stations for surface data (Imperial, Nebraska) and upper-air data 

(North Platte, Nebraska). 

Figure 3: Site, Monitor, and Meteorological Station Locations.
GGS 1-Hour SO2 Modeling 5 September 2015

The stack parameter inputs for GGS1 and GGS2 are provided in Table 1. The stack temperature and 

velocity are placeholder design values. Actual temperature and velocity data were input to the AERMOD

model on an hourly basis with an hourly emission file that contains emission rates, stack temperatures and 

exit velocities for both stacks for each hour of the model simulation, which spanned the years 2012-2014. 

Table 1: Source Input Data for Modeling Analysis

UTM Coordinates

Base 

Elevation

Stack 

Height

Stack 

Temperature

Stack 

Exit 

Velocity

Stack 

Source ID X Y Diameter

(m) (m) (m) (m) (Deg. K) (m/s) (m)

Unit 1 320171.7 4550011 954.1 168.56 432 23.14 8.54

Unit 2 320192.2 4549894 957.5 167.64 417 25.62 8.54

The hourly emissions data files input to AERMOD were based on continuous emissions and stack 

parameter monitoring data, which have been collected and reported consistent with Acid Rain program 

monitoring procedures as provided in 40 CFR 75. The hourly emissions data file (including stack exhaust 

temperature and velocity data) is provided electronically on the CD/DVD accompanying this report.

MODEL RESULTS

Modeled design impact for the 3-year average 1-hour SO2 concentration is summarized in Table 2 and 

presented visually in Figure 4. The 4th high 3-year average concentrations are compared with the 1-hour 

SO2 NAAQS of 75 parts per billion (ppb) or approximately 196.5 micrograms per cubic meter (μg/m3

). 

The modeled concentration is the combined impact of Units 1 and 2. The background 1-hour SO2

concentration of 9 μg/m3 was added to the modeled concentration. The modeled plus background 

concentration is well below the NAAQS as shown in Table 2. 

Table 2: Modeled Impacts (with background of 9 μg/m3

)

Pollutant

Averaging 

Period Years

UTM Coordinates Modeled 

Concentration

Modeled Plus 

Easting Northing Background NAAQS

(m) (m) (μg/m3) (μg/m3) (μg/m3)

SO2 1-hour 2012-14 321900 4548800 135.8 144.8 196.5
GGS 1-Hour SO2 Modeling 6 September 2015

Figure 4: Modeled Result vs. SO2 Standard

Figure 5 is a contour plot of the modeled 1-hour design concentrations (not including background), 

representing the average of the daily maximum 4th high values over the 3 years of meteorological data. 

The black box surrounding the contour lines indicates the extent of the receptor grid. The contour levels 

are: Orange: 100 μg/m3

, Yellow: 110 μg/m3, and Pink: 120 μg/m3. The location of the maximum 4th high 

3-year average modeled (without background) concentration (135.8 μg/m3

) is displayed for reference. 

0

50

100

150

200

250

SO2 Modeling Result SO2 Standard

SO2

μg/m3

Standard

Modeled Result

Background
GGS 1-Hour SO2 Modeling 7 September 2015

Figure 5: Contour Plot of SO2 1-Hour Design Concentrations
GGS 1-Hour SO2 Modeling 8 September 2015

CONCLUSION

This modeling analysis shows that the design maximum SO2 impact from GGS, when added to the 

estimated background concentration of 9 μg/m3

, would cause a total maximum impact of 144.8 μg/m3, 

which is well below (under 75%) of the SO2 1-hour NAAQS of 196.5 μg/m3 (75 ppb). 
Attachment 1
ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ Facility ID #34385, 07/15/2015

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HDR Engineering, Inc.

Sulfur Dioxide 1-Hour NAAQS Designations Modeling Protocol

Nebraska Public Power District, Gerald Gentleman Station (NDEQ Facility ID #34385)

Sutherland, NE

July 15, 2015

INTRODUCTION

This modeling protocol addresses proposed dispersion modeling for the Nebraska Public Power 

District (NPPD) Gerald Gentleman Station (GGS), in Lincoln County, near Sutherland, Nebraska. 

The protocol summarizes the information that will be used to conduct dispersion modeling with 

respect to the National Ambient Air Quality Standard (NAAQS) for 1-hour average sulfur dioxide 

(SO2) concentration, which is equal to 75 parts per billion (ppb) or approximately 196.5 micrograms 

per cubic meter (μg/m3

)

This protocol has been prepared for 1-hour SO2 State Implementation Plan (SIP) dispersion modeling 

and is being submitted to the Nebraska Department of Environmental Quality (NDEQ) for review and 

approval. The results of the dispersion modeling analysis will be used by the NDEQ to formulate 

recommendations to EPA on the NAAQS attainment/nonattainment area designations for SIP

purposes. EPA will review and use this information, along with any available monitoring data, to 

propose and finalize attainment/nonattainment designations for affected areas with respect to the 1-

hour SO2 NAAQS.

MODELING SOFTWARE

The following EPA modeling software will be used for this analysis. 

• AERMOD (Version 14134) 

• BPIP-Prime (Version 04274) 

• AERMAP (Version 11103)

The AERMOD model will be executed using the rural dispersion mode, given the predominantly 

rural character of the land surrounding the subject facility.

METEOROLOGICAL DATA

Meteorological data for this analysis will be provided by NDEQ in preprocessed format, based on the 

most recent versions of AERMET (Version 14134), AERMINUTE (Version 14337), and 

AERSURFACE (Version 13016).

The surface meteorological will be from Imperial, Nebraska and the upper air data will be from North 

Platte, Nebraska, which are considered representative of the Sutherland area. This analysis will use 

three years of meteorological data for the years 2012 through 2014.

POLLUTANT AND AVERAGING PERIOD

The AERMOD model will be executed for SO2 for 1-hour averages. By selecting SO2 as the 

pollutant and 1-hour as the averaging period, AERMOD will automatically average the results over 

the three years of meteorology. The model result for comparison with the 1-hour SO2 NAAQS of 

196.5 μg/m3 (75 parts per billion) will be the maximum of the 3-year average of the 4th highest (99th 
ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ Facility ID #34385, 07/15/2015

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HDR Engineering, Inc.

percentile) daily 1-hour maximum concentration, as automatically output by AERMOD from the 

multiyear (3-year) model run.

POINT SOURCES 

Emission points to be modeled for GGS will include only the Unit 1 and Unit 2 coal boiler stacks. 

The NDEQ and EPA have reviewed the other emissions sources in the region and determined that 

there are no nearby sources with large enough emissions to be included in a modeling analysis 

together with GGS. Thus, total impact for comparison with the NAAQS will consist of the Unit 1

and Unit 2 combined impact plus the background concentration (see below). The AERMOD output 

will be set up to produce source contributions for each unit, plus the total (“ALL” source group) 

concentrations.

The actual, hourly SO2 emissions measured by the continuous emissions monitoring system (CEMS) 

on the Unit 1and 2 stacks will be used for this analysis, by using the optional hourly emissions input 

file for input to AERMOD. The single hourly emissions file will correspond with the same period of 

record represented by the three year period of meteorological data (2012-2014) input to AERMOD. 

In addition to hourly SO2 emissions in grams/second, the hourly emissions file will include hourly 

average stack gas exhaust temperature and exhaust gas exit velocity. These additional hourly 

parameters will be based on measurements recorded by the same CEMS systems being used to track 

hourly SO2 emissions for each stack, in accordance with the routine monitoring requirements under 

40 CFR 60 (New Source Performance Standards) and 40 CFR 75 (Continuous Emission Monitoring 

under the Acid Rain program). 

In addition to the stack heights and the hourly emissions and stack parameters described above, the 

only other stack parameters needed by AERMOD are the stack exit diameters. The stack diameters to 

be input for Units 1 and 2 will be 28 feet (8.535 m), based on the current design of these stacks. 

BUILDING DOWNWASH INPUTS

The AERMOD input will include building downwash parameters calculated using the EPA’s 

Building Profile Input Program “PRIME” (BPIPPRIME) software (Version 04274). The 

BPIPPRIME input and output (I/O) files will be provided along with all the other modeling I/O files 

on CD with the final modeling report. 

TERRAIN ELEVATIONS

Terrain data will be processed to determine receptor elevations and “hill heights” for input to 

AERMOD using AERMAP, Version 11103. The AERMAP input will include terrain elevation data

from the National Elevation Dataset (NED). The NED data available on-line in 1 arc-second spacing 

from the US Geological Survey will be used for this analysis. The receptor grid (extent defined 

below) will include receptors only in UTM Zone 14. The NED data for this analysis will be based on 

North American Datum (NAD) 83 for horizontal locations and NAD88 for vertical locations 

(elevations). 

The NED terrain file downloaded from the USGS will be provided on CD along with all other model 

I/O files.
ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ Facility ID #34385, 07/15/2015

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HDR Engineering, Inc.

RECEPTOR GRID

Given there will be no nearby facilities included in this analysis, the receptor grid can be focused on 

just Gerald Gentleman Station. The receptor grid will include the following spacing on the fence 

lines and at downwind distances from the nearest fence lines.

• 50 meter spacing on the fence line

• 50 meter spacing from the fence to 1 kilometer from the fence

• 100 meter spacing from 1 kilometer to 2 kilometers from the fence

• 250 meter spacing from 2 kilometer to 5 kilometers from the fence

• 500 meter spacing from 5 kilometer to 7 kilometers from the fence

• 1000 meter spacing from 7 to 10 kilometers from the fence

The extent of this receptor grid is shown in the figure at the end of this protocol, and based on prior 

modeling experience, is expected to encompass areas of maximum 1-hour SO2 concentration. Note 

that in accordance with EPA guidance1

, receptors that would be over water (Sutherland Reservoir, 

just north of the facility) are excluded from analysis, given that it would not be practical to monitor at 

these locations.

Any hot spots in the 250 meter and coarser receptor spacing will be refined by performing a separate 

model run centered on the hot spot, with a 1000-meter by 1000 meter grid of 50-meter spacing 

centered on the highest impact receptor from the initial model run.

BACKGROUND CONCENTRATION

The background 1-hour SO2 concentration will be based on data from the rural monitor located at 

Cedar Bluff Reservoir (EPA Site ID number 201950001) in Trego County in western Kansas. This 

monitor is far from any nearby large SO2 sources, so is representative of background concentrations 

in rural areas of Nebraska exclusive of nearby major source impacts.

Given that there are apparent monitor operational problems evidenced in the data for the most recent 

year of complete data, 2014, the background concentration will be based on the next most recent three 

years of available data, those being 2011-2013. The table below shows the calculated average of the 

99th percentile daily maximum 1-hour value as 8.7 μg/m3 across the three years. Therefore, a 

background 1-hour SO2 concentration of 9 μg/m3 will be used for this analysis. 

Year

Daily Maximum 1-hour, 

99th Percentile

(ppm) (μg/m3)

2011 3 7.9

2012 4 10.5

2013 3 7.9

Average 3.3 8.7

 1 SO2 NAAQS Designations Modeling Technical Assistance Document, U.S. EPA Office of Air and Radiation 

Office of Air Quality Planning and Standards, Air Quality Assessment Division, December 2013,

DRAFT (“EPA TAD”)
ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ Facility ID #34385, 07/15/2015

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HDR Engineering, Inc.

MODELING REPORT 

A final modeling report will be submitted to NDEQ for review, describing modeling procedures 

(attaching this protocol), mitigation features (design changes), if any, proposed by the utility, and 

including all model and preprocessor input and output files on CD/DVD, with the exception of the 

meteorological data preprocessing files given that the NDEQ performed the meteorological data 

preprocessing. The data on CD/DVD will include all the hourly emissions (CEM) data files used to 

input actual emissions to AERMOD.

The modeling report will contain graphics displaying, at a minimum, 

• source locations,

• receptor locations,

• meteorological data locations,

• background monitor location, 

• contour plots displaying modeled design values (for general receptor grid and any refined 

grid model runs), and

• a bar chart showing background plus source impact for comparison with the NAAQS.

A copy of the final modeling files used to support the analysis will be provided on CD/DVD to:

Lisa Alam

c/o Records Management 

Nebraska Department of Environmental Quality

1200 "N" Street, Suite 400

P.O. Box 98922

Lincoln, Nebraska 68509-8922

The data and graphics files included on the CD/DVD will include as a minimum:

• AERMOD input and output files (source and receptor input data file, hourly emissions file, 

output listing file, and output graphics/plot file)

• Contour plot and bar chart graphics file(s)

• Source location graphic file (*.kml) from Google Earth

• Source, met data and background monitor location map/graphic

• AERMAP terrain data processor input file

• Preprocessed meteorological data (*.sfc and *.pfl) files provided by NDEQ

• BPIP-PRIME preprocessor input and output files
ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ Facility ID #34385, 07/15/2015

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HDR Engineering, Inc.

Receptor Grid for Gerald Gentleman Station SO2 Dispersion Modeling

Town of Sutherland NE