TCEQ Interoffice Memorandum 

To: David Brymer, Director 

Air Quality Division 

From: ¥-Daniel Menendez, Team Leader 

Air Dispersion Modeling Team (ADMT) 

Air Permits Division 

Date: June 5, 2015 

Subject: Mocleling Analysis of Sulfur Dioxide for Major Oak Power, LLC -Twin Oaks 

(RN100226570) 

1. Project Identification Information 

On March 2, 2015, the U.S. District Court for the Northern District of California accepted 

as an enforceable order an agreement between the Environmental Protection Agency 

(EPA) and Sierra Club aod Natural Resources Defense Council to resolve litigation 

concerning the deadline for completing !-hour so2 designations. 

On March 20, 2015, the EPA notified the Texas Commission on Environmental Quality 

(TCEQ) of Texas power plant sources that meet the criteria established in the court's 

order. In addition, the letter advised that the EPA will use the most recent available 

information when making designation and boundary decisions. The EPA requested 

updated recommendations aod supporting information by September 18, 2015. 

Due to time aod resource constraints, the TCEQ conducted limited refined screening 

modeling to support attainment designations near selected sources. 

2. Modeling Results 

The predicted maximum ground level concentration (GLCmax) is 84 [.Lg/m3

, aod this 

represents a three-year average of the high, fourth high (or 991

h percentile) maximun1 

daily !-hour concentrations. The location of the GLCmax is approximately 1300 meters 

from the two modeled stacks to the north. Table !lists the location of the predicted 

GLCmax. The location is in the Universal Traosverse Mercator (UTM) Zone 14 North, 

North American Datum of 1983 (NAD83) coordinate system. 

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TCEQ Interoffice Memorandum 

719635 

Background concentrations for S02 were obtained from the EPA Aerometric Information 

Retrieval System (AIRS) monitor 483091037located at 4472 Mazanec Road, Waco 

McLennm1 County. The tln·ee-ycar average (2012-2014) of the 99'11 percentile of the 

annual distribution of the maximum daily 1-hour concentrations was used for the !-hour 

value. The monitor is located approximately 70 kilometers north-northwest of the site, 

which is in the direction of the predominMt wind flow for the region. TCEQ emissions 

inventory data from 2013 were reviewed Md there were not any emissions of S02 

reported within 20 kilometers of the Twin Oaks site. ]1Jere are approximately 360 tons of 

S02 emissions from sources located within 20 kilometers of the monitor site. The use of 

this monitor is reasonable given the location of the monitor Md the magnitude of 

emissions near the monitor site relative to the Twin Oaks site. 

The nearest source, located approximately 21 kilometers away, to the Twin Oaks site is 

the Oak Grove Stemn Electric Station. In 2013, the Oak Grove Steam Electric Station 

reported 6950 tons of S02. Although this source would be unlikely lo cause a significm1t 

concentration gradient in the vicinity of the Twin Oaks site, it was included in the 

modeling since there are no sources near the monitor with a similar magnitude of 

reported emissions. 

3. Model Used and Modeling Techniques 

AERMOD (Version 14134) was used in a refined screening mode. For refined screening, 

National Weather Service (NWS) meteorological raw input data are used with 

generalized surface characteristics of the application site. 

A. Land Use 

A land use/land cover analysis was performed using AERSURFACE consistent 

with guidance given in the AERMOD Implementation Guide (March 19, 2009). 

The recommended input data, the National Land Cover Data 1992 archives 

(NLCD92), were used for this Malysis. 

The AERSURFACE analysis conducted of the area surrounding the Twin Oaks site 

resulted in a calculated albedo of 0.17 a calculated Bowen ratio of 0.70, and a 

calculated surface rotJghness length of 0.043 meters. These values were used to 

develop the meteorological data set for this analysis. 

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TCEQ Interoffice Memorandum 

Terrain elevations within the modeling domain were determined using AERMAP 

(Version Ill 03). The input data used for this analysis were National Elevation Data 

(NED) seamless data obtained from the United States Geological Survey (USGS). 

B. Meteorological Data 

Meteorological data for years 2012-2014 were used in the analysis. Raw surface 

and upper air meteorological data were processed using AERMET (Version 14134). 

Surface Station and ID: College Station, TX (Station#: 3904) 

Upper Air Station and ID: Fort Worth, TX (Station#: 3990) 

Meteorological Dataset: 2012-2014 

Profile Base Elevation: I oo·meters 

These meteorological stations are the nearest stations to the Twin Oaks site and are 

representative to use given that there are no significant terrain barriers between the 

site and the meteorological stations. 

C. Receptor Grid 

The receptor grid used in the modeling analysis consisted of receptors with 100 

meter spacing along the fence line out to one kilometer, receptors with 500 meter 

spacing out to five kilometers, and receptors with one kilometer spacing out to 50 

kilometers. The purpose of the receptor grid was to determine representative 

maximum ground-level concentrations. 

D. Building Wake Effects (Downwash) 

Input data to BPIP-Prime (Version 04274) were developed from a plot plan 

submitted dming previous permitting actions. The building locations were validated 

by ADMT using aerial photography. 

4. Modeling Emissions Inventory 

Source locations were validated by ADMT using aerial photography. The emission 

source coordinates are in the UTM Zone 14 North, NAD83 coordinate system. 

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TCEQ Interoffice Memorandum --------------------------------------

Table 2. On-Property Point Source Parameter Information 

1-1 Ul 719731.92 3442182.39 103.63 437.98 24.57 3.8 

2-1 U2 719768.86 3442220.88 103.63 451.32 23.16 3.8 

An hourly emissions file (TwinOaks_OakGrove_emissions.txt) for years 2012-2014 was 

developed for the modeling analysis. The emissions data were obtained from the EPA's 

Acid Rain Program database. The modeled emission rates represent the actual !-hour 

average emission rates reported for the two modeled stacks. 

As in section 2, the TCEQ emissions inventory data from 2013 were reviewed to 

determine if any nearby sources should be modeled. The nearest source, the Oak Grove 

Steam Electric Station, is located within approximately 21 kilometers and reported 6950 

tons of S02 in 2013. The emissions data for the Oak Grove Steam Electric Station were 

obtained from the EPA's Acid Rain Program database and included in the hourly 

emissions file. The modeled emission rates represent the actuall-hour average emission 

rates reported for the Oak Grove Steam Electric Station. 

With respect to any additional nearby sources of S02, Twin Oaks is a relatively isolated 

source. TCEQ emissions inventory data :from 2013 were reviewed and the next closest 

sources are located approximately 29-35 kilometers east of the site with a total of 69 tons 

of S02 emissions. Given the distance and magnitude of emissions, these sources would 

not be expected to cause significant concentration gradients in the vicinity of Twin Oaks 

and are represented in the modeling via monitored background concentrations. 

5. Modeling Files Listing 

CLLFWD2012_2014.SFC AERMET surface output file 

CLLFWD2012_2014.PFL AERMET profile output file 

Twin oaks 2012-2014.DTA AERMOD input file 

Twin oaks 2012-20!4.GRF AERMOD plot file 

Twin_oaks_2012-2014.LST AERMOD output file 

Twin oaks 2012-20!4.SUM AERMOD summary file 

Twin Oaks_ OakGrove _emissions. txt Hourly emissions file 

bpip.PIP BPIP-PR!ME input file 

bpip.SO BPIP-PRIME output file 

bpip.SUM BPIP-PRIME summary file 

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