Omaha Public Power District

Sulfur Dioxide 1-Hour NAAQS Designations Modeling

for

Nebraska City Station (Facility ID #58343)

Nebraska City, 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.

August 2015
NCS 1-Hour SO2 Modeling 1 Aug. 2015

INTRODUCTION

This modeling report summarizes dispersion modeling 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 

reviewed and approved by 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.
NCS 1-Hour SO2 Modeling 2 Aug. 2015

Figure 1 displays the source locations of the two point sources included in the model, which are the coalfired boiler stacks for Nebraska City Unit 1 (NC1) and Nebraska City Unit 2 (NC2).

Figure 1: Source Locations
NCS 1-Hour SO2 Modeling 3 Aug. 2015

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

accordance with NDEQ guidance. 

Figure 2: Receptor Locations
NCS 1-Hour SO2 Modeling 4 Aug. 2015

Figure 3 displays the OPPD facility in relation to the background SO2 monitor location and 

meteorological data stations for surface data (Falls City, Nebraska) and upper-air data (Topeka, Kansas). 

Figure 3: Site, Monitor, and Meteorological Station Locations.

The stack parameter inputs for NC1 and NC2 at OPPD are provided in Table 1. The stack temperature 

and velocity are placeholder average 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)

NC1 265387 4500392 281.9 213.36 415.37 30.02 7.19

NC2 265447 4500318 281.9 121.92 347.04 24.29 7.01
NCS 1-Hour SO2 Modeling 5 Aug. 2015

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 files (including stack 

exhaust temperature and velocity data) are 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 4

th 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 264750 4505000 69.5 78.5 196.5

Figure 4: Modeled Result vs. SO2 Standard

0

50

100

150

200

250

SO2 Modeled Result SO2 Standard

SO2 

μg/m3

Standard

Modeled Result

Background
NCS 1-Hour SO2 Modeling 6 Aug. 2015

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

representing the average of the daily maximum 4

th 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: 50 μg/m3, Yellow: 55 μg/m3, and Pink: 60 μg/m3. The location of the maximum 4th high 3-

year average modeled (without background) concentration (69.5 μg/m3) is displayed for reference. 

Figure 5: Contour Plot of SO2 1-Hour Design Concentrations

50 μg/m3

55 μg/m3

60 μg/m3
NCS 1-Hour SO2 Modeling 7 Aug. 2015

CONCLUSION

This modeling analysis shows that the SO2 impact from NCS, when added to the estimated background 

concentration of 9 μg/m3, would cause a total maximum impact of 78.5 μg/m3, which is well below 

(under 50%) of the SO2 1-hour NAAQS of 196.5 μg/m3 (75 ppb).