## W.A. Parish Electric Generating Station

Thompsons, Texas

Evaluation of Compliance wi th the 1-hour NAAQS for SO 2



## December 15, 2015

Conducted by:

Steven Klafka, P.E., BCEE

Wingra Engineering, S.C.

Madison, WisconsinEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 2

1. Introduction

Wingra Engineering, S.C. was hi red by Sierra Club to conduct an air modeling impact analysis to help the U.S. Environmental Prot ection Agency (USEPA), state an d local air agencies identify
facilities that are likely causing exceedances of the 1-hour su lfur dioxide (SO 2 ) national ambient air quality standard (NAAQS). This doc ument describes the results and procedures for an evaluation
conducted for the W.A. Parish E lectric Genera ting Station locat ed in Thompsons, Texas.

To ensure the modeling analysis reflected the cumulative concen tration of SO 2 emissions, it included emissions from the Rhodia, Inc . Houston Plant, which is an addi tional source of SO 2 emissions
located within 50 kilometers of the W.A. Parish Electric Genera ting Station.

This report updates th e modeling results presented in an earlie r report dated September 17, 2015. The analysis was updated to use building downwash, and actual h ourly emissions, stack temperatures and exit velocities for the W.A. Parish Electric G enerating Station as provided by
USEPA.

The dispersion modeling analysis predicted ambient air concentr ations for comparison with the 1-
hour SO 2 NAAQS. The modeling was performe d using the most recent versi on of AERMOD, AERMET, and AERMINUTE, with data provided to Sierra Club by reg ulatory air agencies or obtained through other publicly-a vailable sources as documented below. The analysis was
conducted in adherence to all ava ilable USEPA guidance for eval uating source impacts on
attainment of the 1-hour SO 2 NAAQS via aerial dispersion m odeling, including the AERMOD Implementation Guide; USEPA’s A pplicability of Appendix W Model ing Guidance for the 1-hour
SO 2 National Ambient Air Qual ity Standard, August 23, 2010; modeli ng guidance promulgated by USEPA in Appendix W to 40 CFR Part 51; USEPA’s March 2011 Model ing Guidance for SO 2 NAAQS Designations; 1 and, USEPA’s December 2013 SO 2 NAAQS Designations Technical
Assistance Document. 2

It was determined that based on eit her current allowable emissi ons or measured actual emissions, the
W.A. Parish Electric Generati ng Station is estimated to create downwind SO 2 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. pdfEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 3

2. Compliance with the 1-hour SO 2 NAAQS

2.1 1-hour SO 2 NAAQS

The 1-hour SO 2 NAAQS takes the form of a t hree-year average of the 99 th percentile of the annual distribution of daily maximum 1- hour concentrations, which cann ot exceed 75 parts per billion (ppb). 3 Compliance with this standar d was verified using USEPA’s AERM OD air dispersion model, which produces air concentr ations in units of μg/m 3 . The 1-hour SO 2 NAAQS of 75 ppb equals 196.2 μg/m 3 , and this is the value used for determining whether modeled im pacts exceed the NAAQS. 4 The 99 th percentile of the annual dist ribution 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. Paris h Electric Generating Station an d 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 c urrent allowable emissions or measured actual emissions, the W.A. Parish Electric Generating Stati on is estimated to create downw ind SO 2 concentrations which exceed the 1-hour NAAQS.

More specifically, the modeling results presented in Tables 1 a nd 2, show exceedances of the NAAQS by the plant’s a llowable and actual emissions. “Allowable ” is the peak emission rate from each unit as approved by the curre nt air quality operation perm it for the facility. For the Parish plant,
“actual” emissions are the measured emissions for each hour bet ween 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 averag e of those reported to the Texas Commission on Environmental Quality for calendar year 2014.

Besides using actual hourly emissi ons, this report incorporated actual hourly stack temperatures, exit velocities, building and structure dimensions and aerodynamic d ownwash effects into the modeling parameters for the W.A. Paris h Generating Station. These parame ters were provided by the USEPA.

The emissions from Rhodia, Inc. s ignificantly contribute to the ambient SO 2 concentration in the area impacted by W.A. Parish El ectric Generating Station.

3 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010.

4 The ppb to μg/m 3 conversion is found in the source code to AERMOD v. 15181, sub routine Modules. The conversion calculation is 75/0.3823 = 196.2 μg/m 3

. 5 http://ampd.epa.gov/ampd/Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 4

Air quality impacts presente d in Tables 1 and 2 are based on tw o background concentrations. The first concentration is 7.8 μg/m 3 . This is the 2011-13 design value for El Paso County, Texas—th e lowest measured background concentr ation in the state. The sec ond concentration is 23.5 μg/m 3

. 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 measur ed design value in the Houston area, it is a conservative estimate and likely under-predicts the actual ambient SO 2 concentrations in adjacent Fort Bend County. See
Section 5 for further discussi on of the background concentratio ns used for this analysis.

Table 1 - SO 2 Modeling Results Using El Paso County Background

Emission Rates Averaging Period 99 th Percentile 1-hour Daily Maximum (μg/m 3 ) 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 - SO 2 Modeling Results Using Harris County Background

Emission Rates Averaging Period 99 th Percentile 1-hour Daily Maximum (μg/m 3 ) 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 SO 2 December 15, 2015 Page 5

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

Table 3 - Modeled SO 2 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 Tot al 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 re ductions from current allowable rates necessary to achieve com pliance with the 1-hour NAAQS. T his assumes a one-hour averaging period for the emission rate and that the emission rate is bind ing 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 wer e not evaluated. During these periods, decreased gas velocities and temperatures may lead to greater ambient impacts at ground level. Further, the hypothetic al emission limitation in Table 4 would allow W.A. Parish Electric Generating Station to consume t he entire NAAQS, leaving little to no room for any other source of
SO 2 in the area. No margin of safet y has been included in the hypo thetical emission limitation.

Table 4 - Required Emission Reductions from W.A. Parish Electric Generating Station for Compliance with the 1-hour NAAQS for SO 2

Background Concentration Monitor Acceptable Impact (NAAQS - Background) 99th Percentile 1-hour Daily Max (μg/m 3 ) 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 t aken from Federal Operating P ermit, Regulated Entity Number: RN100888312, March 20, 2012. Rhodia allowable emissions taken from Federal O perating Permit, Regulated Entity Number: RN100220581, June 28, 2012.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 6

Predicted exceedances of the 1-hour NAAQS for SO 2 based on allowable emissions extend throughout the region to a maxim um distance of 50 kilometers.

Figure 1 shows the extent of NAA QS violations based on allowabl e emissions from both sources using the 7.8 μg/m 3 background concentration from El Paso County. Figure 2 shows th e extent of NAAQS violations based on actual hourly emissions from both sou rces using the 7.8 μg/m 3 background concentration from El Paso County.

Figure 3 shows the extent of NAA QS violations based on allowabl e emissions from both sources using the 23.5 μg/m 3 background concentration from Ha rris County. Figure 4 shows the extent of NAAQS violations based on actual hourly emissions from both sou rces using the 23.5 μg/m 3 background concentration from Harris County.

2.3 Conservative Modeling Assumptions

A dispersion modeling analysis requires the selection of numero us parameters which affect the predicted concentrations. For the enclosed analysis, several pa rameters were selected which underpredict facility impacts.

Assumptions used in this modeli ng analysis which likely under-e stimate concentrations include the following:

 Allowable emissions are base d on a limitation with an averaging period which is greater than the 1-hour average used for the SO 2 air quality standard. Emissions and impacts during any 1-hour period may be higher than assumed for the modeling analy sis.

 Except for the W.A. Parish Electric Generating Station, no cons ideration 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 r ate, and temperature were provided by EPA and incorporat ed into the analysis.

 Except for the W.A. Parish Ele ctric Generating Station, no cons ideration of building or structure downwash. These downwas h effects typically increase p redicted concentrations near the facility. For W.A. Par ish, building structure and dow nwash parameters were provided by EPA and incorporat ed into the analysis.

 Except for Rhodia, Inc., no consideration of off-site sources. Other off-site sources of SO 2 will increase the predicted impacts.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 7

Figure 1 - View of Impacts Due to Allowable Emissions from Bo th Facilities Using an El Paso County Background of 7.8 μg/m 3Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 8

Figure 2 - View of Impacts Due to Ac tual Emissions from Both Facilities Using an El Paso County Background of 7.8 μg/m 3Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 9

Figure 3 - View of Impacts Due to Allowable Emissions from Both Facilities Using a Harris County Background of 23.5 μg/m 3Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 10

Figure 4 - View of Impacts Due to Actu al Emissions from Both Facilities Usin g a Harris County Background of 23.5 μg/m 3Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 11

3. Modeling Methodology

3.1 Air Dispersion Model

The modeling analysis used U SEPA’s AERMOD program, v. 15181. A ERMOD, as available from the Support Center for Regulator y Atmospheric Modeling (SCRAM) website, was used in conjunction with a third-par ty modeling software program, AERMOD View , sold by Lakes
Environmental Software.

3.2 Control Options

The AERMOD model was run with t he following control options:

 1-hour average air concentrations

 Regulatory defaults

 Flagpole receptors

To reflect a representative i nhalation 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 det ermine if the modeled facilit y 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 ne arby city and a default surfac e roughness of 1.0 meter. Methods
described in Section 4.1 were used t o determine whether rural o r urban dispersion coefficients were appropriate for the modeling analysis.

3.3 Output Options

The AERMOD analysis was based on three years of recent meteorol ogical data. The modeling analyses used one run with three ye ars of sequential meteorolog ical data from 2012-2014. Consistent with USEPA’s Modeling Guidance for SO 2 NAAQS Designations, AER MOD provided a table of fourth-high 1-hour SO 2 impacts concentrations consiste nt with the form of the 1-hour SO 2 NAAQS. 9

Please refer to Table 1 for the modeling results.

8 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on 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 Di oxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 24-26.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 12

4. Model Inputs

4.1 Geographical Inputs

The “ground floor” of all air dis persion modeling analyses is e stablishing a coordinate system for identifying the geographical loc ation of emission sources and r eceptors. These geographical
locations are used to determine local characteristics (such as land use and elevation), and also to ascertain source to receptor di stances and relationships.

The Universal Transverse Merca tor (UTM) NAD83 coordinate system was used for identifying the easting (x) and northing (y) coordi nates of the modeled sources and receptors. Stac k locations were obtained from facility permits and prior modeling files provide d by the state regulatory agency. The
stack locations were then veri fied using aerial photographs.

The facility was evaluated to de termine if it should be modeled using the rural o r urban dispersion coefficient option in AERMOD. A Geographic Information System (GIS) was used to determine whether rural or urban dispersi on coefficients apply to a site. Land use within a three-kilometer radius circle surrounding the f acility was considered. USEPA gu idance states that urban dispersion
coefficients are used if more t han 50% of the area within 3 kil ometers has urban land uses. Otherwise, rural dispersion c oefficients are appropriate. 10

USEPA’s AERSURFACE v. 13016 was use d to develop the meteorologi cal data for the modeling analysis. This model was also us ed to evaluate surrounding land use within 3 kilometers. Based on the output from the AERSURFAC E, approximately 1.5% of surroundi ng land use around the modeled facility was of urban l and use types including Type 21 – Low Intensity Residential, Type 22 – High Intensity Residential and Type 23 – Commercial / Indu strial / Transportation.

This is less than the 50% value c onsidered appropriate for the use of urban dispersion coefficients. Based on the AERSURFACE analysis, i t was concluded that the rur al option would be used for the modeling summarized in this re port. Please refer to Section 4. 5.3 for a discussion of the AERSURFACE analysis.

10 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, No vember 9, 2005, Section 7.2.3.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 13

4.2 Emission Rates and Source Parameters

The modeling analysis considered SO 2 emissions from the W.A. Paris h Electric Generating Station and Rhodia, Inc. Other off-site s ources were not considered. Co ncentrations were predicted for the
scenarios shown in Tables 1 and 2:

1) allowable emissions based on t he current permit issued by th e regulatory agency, and

2) actual hourly emissions from the W.A. Parish Station were me asured each hour between January 1, 2012 and December 31, 2014 as taken from USEPA Air Markets Program Data

. 11 For the Rhodia plant, actual emi ssions are the annual average o f those reported to the Texas Commission on Environmental Qua lity for calendar year 2014.

Stack parameters and emissions use d for the modeling analysis a re summarized in Table 4. When modeling the actual hourly emissi ons from the W.A. Parish Elect ric Generating Station, hourly stack exit temperatures and velocitie s were based on information prov ided 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] 439.817 439.817 416.483 416.483 344.2 61 310.928

Gas Exit Velocity [m/s] 25.908 25.908 25.298 25.298 18.898 9.44 9

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 reg ulatory agency documents and
databases identified in Secti on 2.2. Except for the W.A. Parish Electric Generating Station actual hourly emissions scenario, the ana lysis was conducted based on 100% operating load using
maximum exhaust flow rates and t emperatures. For the allowable scenarios and for the Rhodia, Inc. facility, operation at less than f ull capacity loads was not co nsidered. 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 SO 2 December 15, 2015 Page 14

impacts since stack parameters such as exit flow rate and tempe rature are typically lower at less than full load, reducing pollutant dis persion and increasing predict ed air quality impacts. Stack location, height and diameter were verifi ed using aerial photographs, and flue gas flow rate and temperature were verified using com bustion calculations.

4.3 Building Dimensions

Building and structure dimensions were provided by USEPA for th e W.A. Parish Electric Generating Station, but were not a vailable for Rhodia. Therefor e 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 gri ds were employed:

1. A 100-meter Cartesian receptor grid centered on W.A. Parish Ele ctric Generating Station and extending out 5 kilometers.
2. A 500-meter Cartesian receptor grid centered on W.A. Parish Ele ctric Generating Station and extending out 10 kilometers. 3. A 1,000-meter Cartesian receptor grid centered on W.A. Parish E lectric Generating Station and extending out 50 kilometers. 50 kilometers is the maximum d istance accepted by USEPA for the use of the AERMOD dispersion model. 13

A flagpole height of 1.5 meters w as used for all these receptor s.
Elevations from stacks and recep tors were obtained from Nationa l Elevation Dataset (NED) GeoTiff data. GeoTiff is a binary file tha t includes data descriptors a nd geo-referencing information necessary for extract ing terrain elevations. These elevations w ere extracted from 1 arc-second (30 meter) resolution NED files. The U SEPA 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 c oncentrations occurred off facil ity property so eliminating on-site receptors would not change the modeling results. While USEPA te chnical guidance suggests receptors should not be placed where ambient monitors cannot be located, such as above water bodies, the concentrations predic ted above water bodies still s how potential exposure to the general public.

13 USEPA, Revision to the Guideline on Air Quality Models: Adopti on 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.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 15

4.5 Meteorological Data

To improve the accuracy of the modeling analysis, recent meteor ological data for the 2012-2014 period were prepared using the U SEPA’s program AERMET which cre ates the model-ready surface and profile data files requir ed by AERMOD. Required data inpu ts to AERMET included surface meteorological measurements, t wice-daily soundings of upper air measurements, and the micrometeorological parameters surface roughness, albedo, and B owen ratio. One-minute ASOS
data were available so USEPA met hods were used to reduce calm a nd missing hours. 14 The USEPA software program AERMINUTE v. 15272 is used for these tasks. AE RMET was used to randomize
wind directions for NWS wind data if one-minute ASOS wind speed s were not available for any periods.

This section discusses how the meteorological data was prepared for use in the 1-hour SO 2 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 Stati on. Integrated Surface Hourly ( ISH) data for the 2012-2014 period
were obtained from the National C limatic Data Center (NCDC). The ISH surface data was processed through AERMET Stage 1, which performs data extractio n and quality control checks.

The height of the meteorological tower for this station is 7.6 meters, lower than th e standard height
of 10 meters. The lower tower hei ght was taken into account whe n processing the weather data using
AERMET.

4.5.2 Upper Air Data

Upper-air data are collected by a “ weather balloon” that is rel eased twice per day at selected
locations. As the balloon is rel eased, it rises through the at mosphere, and radios the data back to the surface. The measuring and transmitting device is known as eit her a radiosonde, or rawindsonde.
Data collected and radioed back i nclude: 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 Genera ting Station, the concurrent 201 2-2014 upper air data from twicedaily radiosonde measurements obtai ned at the most representati ve location were used. This

14 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, p. 19.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 16

location was the Lake Charles, L ouisiana measurement station. T hese data are in Forecast Systems Laboratory (FSL) format and were downloaded in ASCII text forma t from NOAA’s FSL website. 15 All reporting levels were downl oaded and processed with AERMET.

4.5.3 AERSURFACE

AERSURFACE is a program that ex tracts surface roughness, albedo , and daytime Bowen ratio for an area surrounding a given locatio n. Current AERMOD guidance from USEPA recommends that surface roughness, albedo, and daytime Bowen ratio values be ob tained for the region surrounding the meteorological data collecti on site rather than the locatio n of the sources under evaluation. 16

AERSURFACE uses land use and la nd cover (LULC) data in the U.S. Geological Survey’s 1992 National Land Cover Dataset to extract the necessary micrometeo rological data. LULC data was used for processing meteorological data sets used as input to A ERMOD.

AERSURFACE v. 13016 was used to deve lop surface roughness, albe do, and daytime Bowen ratio values in a region surrounding the meteorological data collecti on 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 kilomet er area centered on the meteorological data collection si te. These micrometeorological data were processed for seasonal periods using 30-degree sectors . Seasonal moisture conditions w ere considered ave rage with winter months having no continuous snow cover.

4.5.4 Data Review

Missing meteorological data were not filled as the data file me t USEPA’s 90% data completeness requirement. 17 The AERMOD output file shows th ere were 0.48% missing data.

To confirm the representativene ss of the airport meteorological data, the surface characteristics of the airport data collection s ite and the modeled source locatio n were compared. Since the Houston Sugarland Memorial Airport is loc ated close to W.A. Parish Elec tric Generating Station, this meteorological data set was consid ered appropriate for this mod eling analysis. 18 This weather station provided high quality surf ace measurements for the most recent 3-year time, and had similar land use, surface characteristic s, terrain features and climate

.

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

16 USEPA, AERMOD Implementation Guide, 3.1.2 Methods for determin ing surface characteristic s (01/09/08), August 3, 2015, http://www3.e pa.gov/ttn/scram/7thco nf/aermod/aermod_im plmtn_guide_3August2015.pdf

17 USEPA, Meteorological Monitoring Guidance for Regulatory Modeli ng 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 SO 2 December 15, 2015 Page 17

Finally, TCEQ provides pre-proces sed meteorological data suitab le 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 fo r this project because they were not from the most recent years need ed for this analysis and had not been processed using the latest
versions of USEPA m odeling software.

5. Background SO 2 Concentrations

Background concentrations were determined consistent with USEPA ’s Modeling Guidance for SO 2 NAAQS Designations. 20, 21 To preserve the form of the 1-hour SO 2 standard, based on the 99 th percentile of the annual distri bution of daily maximum 1-hour c oncentrations averaged across the number of years modeled, the b ackground fourth-highest daily ma ximum 1-hour SO 2 concentration
was added to the modeled fourth-h ighest daily maximum 1-hour SO 2 concentration. 22

Background concentrations were based on the 2011-13 design valu e measured by the ambient
monitors located in Texas. 23 The air quality impacts are based on two background concentra tions. The first concentration is 7.8 μg/m 3 , which is the 2011-13 design valu e for El Paso County, Texas— the lowest measured background con centration in the state.

The closest air quali ty monitors to the W.A. Parish Electric Ge nerating Station are l ocated in Harris County to the east. The second b ackground concentration used fo r the modeling analysis is 23.5 μg/m 3 , which is the lowest measur ed design value for 2011-2013 from the nine ambient monitors located in adjacent Harris County, T exas (i.e., Monitor Site ID 482010046). The monitors in Harris County measured concentr ations from 23.5 to 107.5 μg/m 3 with an average of 59.9 μg/m 3

. Using the
lowest concentrations accounts for the presence of background S O 2 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 w ith AERSURFACE, AERMET, AERMAP , and AERMOD.

19 Texas Commission on Environmental Quality, Meteorological Data for Refined Screening with AERMOD, http://www.tceq.texas.gov/permit ting/air/modeling/aermod-datase ts.html, Last updated November 22, 2013.

20 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 20-23.

21 USEPA, SO 2 NAAQS Designations Modeling Te chnical Assistance Document, Dec . 2013, section 8.1, pp 27-28.

22 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010, p. 3.

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