W.A. Parish Electric Generating Station 

Thompsons, Texas 

Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

Conducted by: 

Steven Klafka, P.E., BCEE 

Wingra Engineering, S.C. 

Madison, Wisconsin
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 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 W.A. Parish Electric Generating Station located in Thompsons, Texas. 

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

emissions from the Rhodia, Inc. Houston Plant, which is an additional source of SO2 emissions 

located within 50 kilometers of the W.A. Parish Electric Generating Station. 

This report updates the modeling results presented in an earlier report dated September 17, 2015. 

The analysis was updated to use building downwash, and actual hourly emissions, stack 

temperatures and exit velocities for the W.A. Parish Electric Generating Station as provided by 

USEPA. 

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

 

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

W.A. Parish Electric Generating Station is estimated to create downwind SO2 concentrations which 

exceed the 1-hour NAAQS. 

 

 

1

 http://www.epa.gov/scram001/so2_modeling_guidance.htm 2

 http://www.epa.gov/oaqps001/sulfurdioxide/pdfs/SO2ModelingTAD.pdf 
Evaluation of Compliance with the 1-hour NAAQS for SO2

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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 

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

2.2 Modeling Results 

Modeling results for W.A. Parish Electric Generating Station and Rhodia, Inc. are summarized in 

Tables 1 and 2. Results are provided for each source alone, and for both sources combined. It was 

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

Parish Electric Generating Station is estimated to create downwind SO2 concentrations which exceed 

the 1-hour NAAQS. 

More specifically, the modeling results presented in Tables 1 and 2, 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. For the Parish plant, 

“actual” emissions are the measured emissions for each hour between January 1, 2012, and 

December 31, 2014, as taken from USEPA Air Markets Program Data.

5

 For the Rhodia plant, actual 

emissions are the annual average of those reported to the Texas Commission on Environmental 

Quality for calendar year 2014. 

Besides using actual hourly emissions, this report incorporated actual hourly stack temperatures, exit 

velocities, building and structure dimensions and aerodynamic downwash effects into the modeling 

parameters for the W.A. Parish Generating Station. These parameters were provided by the USEPA. 

The emissions from Rhodia, Inc. significantly contribute to the ambient SO2 concentration in the 

area impacted by W.A. Parish Electric Generating Station. 

 

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. 15181, subroutine Modules. The conversion 

calculation is 75/0.3823 = 196.2 μg/m3. 5

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

December 15, 2015 

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Air quality impacts presented in Tables 1 and 2 are based on two background concentrations. The 

first concentration is 7.8 μg/m3

. This is the 2011-13 design value for El Paso County, Texas—the 

lowest measured background concentration in the state. The second concentration is 23.5 μg/m3

. 

This is the lowest measured design value for 2011-2013 from the nine ambient monitors located in 

Harris County, Texas (i.e., Monitor Site ID 482010046).6 Because this background concentration 

corresponds to the lowest measured design value in the Houston area, it is a conservative estimate 

and likely under-predicts the actual ambient SO2 concentrations in adjacent Fort Bend County. See 

Section 5 for further discussion of the background concentrations used for this analysis. 

Table 1 - SO2 Modeling Results Using El Paso County Background 

Emission Rates Averaging 

Period 

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

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 

W.A. Parish 

1,564.3 7.8 1,572.1 196.2 No 

Actual 207.7 7.8 215.5 196.2 No 

Allowable 

Rhodia, Inc. 

312.2 7.8 320.0 196.2 No 

Actual 89.2 7.8 97.0 196.2 Yes 

Allowable 

All 

1,564.8 7.8 1,572.6 196.2 No 

Actual 207.7 7.8 215.5 196.2 No 

Table 2 - SO2 Modeling Results Using Harris County Background 

Emission Rates Averaging 

Period 

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

) Complies with 

NAAQS? Impact Background Total NAAQS 

Allowable 

W.A. Parish 

1,564.3 23.5 1,587.8 196.2 No 

Actual 207.7 23.5 231.2 196.2 No 

Allowable 

Rhodia, Inc. 

312.2 23.5 335.7 196.2 No 

Actual 89.2 23.5 112.7 196.2 Yes 

Allowable 

All 

1,564.8 23.5 1,588.3 196.2 No 

Actual 207.7 23.5 231.2 196.2 No 

 

 

6

 http://www.epa.gov/airtrends/values.html. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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The emissions used for the modeling analysis are summarized in Table 3. 

Table 3 - Modeled SO2 Emissions 7

Stack 

ID 

Unit 

ID 

Allowable Emissions 

(lbs/hr) 

Actual Emissions 

(lbs/hr) 

S05 Unit 5 22,200 USEPA CEM 2012-14 

S06 Unit 6 22,200 USEPA CEM 2012-14 

S07 Unit 7 20,100 USEPA CEM 2012-14 

S08 Unit 8 20,100 USEPA CEM 2012-14 

W.A. Parish Facility Total 84,600 USEPA CEM 2012-14 

R101 Sulfur Burning Unit 8 325.0 134.3 

R104 Spent Acid Burning 325.0 75.2 

Rhodia, Inc. Facility Total 650.0 209.5 

Based on the modeling results, Table 4 provides the 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, given the 

conservative aspects of this modeling protocol, it is extremely likely that this limit is too high to 

protect the NAAQS. 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 4 would allow W.A. Parish Electric 

Generating Station to consume the entire NAAQS, leaving little to no room for any other source of 

SO2 in the area. No margin of safety has been included in the hypothetical emission limitation. 

Table 4 - Required Emission Reductions from W.A. Parish Electric Generating Station for 

Compliance with the 1-hour NAAQS for SO2 

Background 

Concentration 

Monitor 

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) 

El Paso County 188.4 88.0% 10,185.7 0.36 

Harris County 172.7 89.0% 9,336.9 0.33 

 

7

 W.A. Parish allowable emissions taken from Federal Operating Permit, Regulated Entity Number: RN100888312, 

March 20, 2012. Rhodia allowable emissions taken from Federal Operating Permit, Regulated Entity Number: 

RN100220581, June 28, 2012. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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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 

using the 7.8 μg/m3 background concentration from El Paso County. Figure 2 shows the extent of 

NAAQS violations based on actual hourly emissions from both sources using the 7.8 μg/m3 

background concentration from El Paso County. 

Figure 3 shows the extent of NAAQS violations based on allowable emissions from both sources 

using the 23.5 μg/m3 background concentration from Harris County. Figure 4 shows the extent of 

NAAQS violations based on actual hourly emissions from both sources using the 23.5 μg/m3 

background concentration from Harris County. 

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. 

 Except for the W.A. Parish Electric Generating Station, 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. For W.A. Parish, actual hourly emissions, exit flow rate, and temperature were 

provided by EPA and incorporated into the analysis. 

 Except for the W.A. Parish Electric Generating Station, no consideration of building or 

structure downwash. These downwash effects typically increase predicted concentrations 

near the facility. For W.A. Parish, building structure and downwash parameters were 

provided by EPA and incorporated into the analysis. 

 Except for Rhodia, Inc., no consideration of off-site sources. Other off-site sources of SO2

will increase the predicted impacts.
Evaluation of Compliance with the 1-hour NAAQS for SO2

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Figure 1 - View of Impacts Due to Allowable Emissions from Both Facilities Using an El Paso County Background of 7.8 μg/m3
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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Figure 2 - View of Impacts Due to Actual Emissions from Both Facilities Using an El Paso County Background of 7.8 μg/m3

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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Figure 3 - View of Impacts Due to Allowable Emissions from Both Facilities Using a Harris County Background of 23.5 μg/m3

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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Figure 4 - View of Impacts Due to Actual Emissions from Both Facilities Using a Harris County Background of 23.5 μg/m3

 
Evaluation of Compliance with the 1-hour NAAQS for SO2

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3. Modeling Methodology 

3.1 Air Dispersion Model 

The modeling analysis used USEPA’s AERMOD program, v. 15181. 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

December 15, 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 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 1.5% 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. 
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December 15, 2015 

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4.2 Emission Rates and Source Parameters 

The modeling analysis considered SO2 emissions from the W.A. Parish Electric Generating Station 

and Rhodia, Inc. Other 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 from the W.A. Parish Station were measured each hour between 

January 1, 2012 and December 31, 2014 as taken from USEPA Air Markets Program Data.

11

For the Rhodia plant, actual emissions are the annual average of those reported to the Texas 

Commission on Environmental Quality for calendar year 2014. 

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

modeling the actual hourly emissions from the W.A. Parish Electric Generating Station, hourly stack 

exit temperatures and velocities were based on information provided by USEPA. 

Table 4 – Facility Stack Parameters and Emissions 12

Stack S05 S06 S07 S08 H101 H104 

Description Unit 5 Unit 6 Unit 7 Unit 8 Sulfur Burning 

Unit No. 8 

Spent Acid 

Burning Unit 

X Coord. [m] 244421 244481.03 244563 244562 280255 280210 

Y Coord. [m] 3263932 3263835.95 3263679 3263584 3289880 3290054

Base Elevation [m] 21.91 21.78 21.45 21.36 8.97 9.63 

Release Height [m] 182.88 182.88 152.4 152.4 39.62 64.92 

Gas Exit Temperature [°K] 439.817 439.817 416.483 416.483 344.261 310.928 

Gas Exit Velocity [m/s] 25.908 25.908 25.298 25.298 18.898 9.449 

Inside Diameter [m] 7.315 7.315 6.706 6.858 2.134 0.762 

Allowable Emission Rate [g/s] 2,797 2,797 2,533 2,533 40.95 40.95 

Actual Emission Rate [g/s] - - - - 16.92 9.475 

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

databases identified in Section 2.2. Except for the W.A. Parish Electric Generating Station actual 

hourly emissions scenario, the analysis was conducted based on 100% operating load using 

maximum exhaust flow rates and temperatures. For the allowable scenarios and for the Rhodia, Inc. 

facility, operation at less than full capacity loads was not considered. This assumption under-predict 

 

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

12 W.A. Parish stack parameters taken from Environ, 2018 Base Case CAMx Simulation, Texas Haze Evaluation, 

Appendix A: Stack Parameters of Major Units at the Selected 38 Facilities, September 7, 2013. Rhodia stack parameters 

and actual emissions for 2014 taken from Texas CEQ, EIQ Report, Eco Services Operations LLC, August 12, 2015. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

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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. 

4.3 Building Dimensions 

Building and structure dimensions were provided by USEPA for the W.A. Parish Electric 

Generating Station, but were not available for Rhodia. Therefore this modeling analysis did address 

the effects of downwash for W.A. Parish, but not for Rhodia. 

4.4 Receptors 

For W.A. Parish Electric Generating Station, three receptor grids were employed: 

1. A 100-meter Cartesian receptor grid centered on W.A. Parish Electric Generating Station and 

extending out 5 kilometers. 

2. A 500-meter Cartesian receptor grid centered on W.A. Parish Electric Generating Station and 

extending out 10 kilometers. 

3. A 1,000-meter Cartesian receptor grid centered on W.A. Parish Electric 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. 

The receptor grids were not changed to remove receptors located on source property or above water 

bodies. The highest predicted concentrations occurred off facility property so eliminating on-site 

receptors would not change the modeling results. While USEPA technical guidance suggests 

receptors should not be placed where ambient monitors cannot be located, such as above water 

bodies, the concentrations predicted above water bodies still show potential exposure to the general 

public. 

 

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. 
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December 15, 2015 

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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. 15272 is used for these tasks. AERMET was used to randomize 

wind directions for NWS wind data if one-minute ASOS wind speeds were not available for any 

periods. 

This section discusses how the meteorological data was prepared for use in the 1-hour SO2 NAAQS 

modeling analyses. The USEPA software program AERMET v. 15181 is used for these tasks. 

4.5.1 Surface Meteorology 

Surface meteorology was obtained for Houston Sugarland Memorial Airport located near the W.A. 

Parish Electric 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. 

The height of the meteorological tower for this station is 7.6 meters, lower than the standard height 

of 10 meters. The lower tower height was taken into account when processing the weather data using 

AERMET. 

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 W.A. Parish Electric Generating Station, the concurrent 2012-2014 upper air data from twicedaily radiosonde measurements obtained at the most representative location were used. This 

 

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

December 15, 2015 

Page 16 

location was the Lake Charles, Louisiana 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. Current AERMOD guidance from USEPA recommends that 

surface roughness, albedo, and daytime Bowen ratio values be obtained for the region surrounding 

the meteorological data collection site rather than the location of the sources under evaluation.16 

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 

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.17 The AERMOD output file shows there were 0.48% 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 Houston 

Sugarland Memorial Airport is located close to W.A. Parish Electric Generating Station, this 

meteorological data set was considered appropriate for this modeling analysis. 18 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. 

 

15 Available at: http://esrl.noaa.gov/raobs/ 

16 USEPA, AERMOD Implementation Guide, 3.1.2 Methods for determining surface characteristics (01/09/08), August 

3, 2015, http://www3.epa.gov/ttn/scram/7thconf/aermod/aermod_implmtn_guide_3August2015.pdf 

17 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. 

18 USEPA, AERMOD Implementation Guide, March 19, 2009, pp. 3-4. 
Evaluation of Compliance with the 1-hour NAAQS for SO2

December 15, 2015 

Page 17 

Finally, TCEQ provides pre-processed meteorological data suitable for modeling for each county.19

For Fort Bend County, TCEQ recommends using data from the same surface and upper air stations 

used for this modeling analysis. The TCEQ data were not used for this project because they were not 

from the most recent years needed for this analysis and had not been processed using the latest 

versions of USEPA modeling software. 

5. Background SO2 Concentrations 

Background concentrations were determined consistent with USEPA’s Modeling Guidance for SO2

NAAQS Designations.20, 21 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.22 

Background concentrations were based on the 2011-13 design value measured by the ambient 

monitors located in Texas.23 The air quality impacts are based on two background concentrations. 

The first concentration is 7.8 μg/m3

, which is the 2011-13 design value for El Paso County, Texas—

the lowest measured background concentration in the state. 

The closest air quality monitors to the W.A. Parish Electric Generating Station are located in Harris 

County to the east. The second background concentration used for the modeling analysis is 23.5 

μg/m3

, which is the lowest measured design value for 2011-2013 from the nine ambient monitors 

located in adjacent Harris County, Texas (i.e., Monitor Site ID 482010046). The monitors in Harris 

County measured concentrations from 23.5 to 107.5 μg/m3 with an average of 59.9 μg/m3. Using the 

lowest concentrations accounts for the presence of background SO2 but is not excessively 

conservative. 

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. 

 

19 Texas Commission on Environmental Quality, Meteorological Data for Refined Screening with AERMOD, 

http://www.tceq.texas.gov/permitting/air/modeling/aermod-datasets.html, Last updated November 22, 2013. 

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

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

21 USEPA, SO2 NAAQS Designations Modeling Technical Assistance Document, Dec. 2013, section 8.1, pp 27-28. 22 USEPA, Applicability of Appendix W Modeling Guidance for the 1-hour SO2 National Ambient Air Quality Standard, 

August 23, 2010, p. 3. 

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