Btyan W. Shaw, Ph.D., P.E., Chairman Toby Baker, Commissioner Jon Nierrnann, Commissioner Richard A. Hyde, P .E., Executive Director 

### TEXAS COMMISSION ON ENVIRONMENTAL QUALITY

Mr. Erik M. Snyder Protecting Texas by Reducing andPreventing Pollution November 30, 2015
Environmental Protection Agency, Region 6 1445 Ross Avenue Suite 1200 Mail Code: 6PD Dallas, TX 75202-2733 Dear Mr. Snyder: Thankyou for holding a conference call on November 6, 2015 to discuss modeling performedbytheTexas Commission on Environmental Quality (TCEQ) in support ofour September 18, 2015 updated recommendation regarding designations for the 2010 Sulfur Dioxide (S02) National Ambient Air Quality Standard (NAAQS). In our discussions, you requested supplemental modeling to evaluate the sensitivity of A,ERMOD to changes in meteorological files based onthelocation used to determine surface characteristics. The modeling submittedby the 'l'CEQ on September 18, 2015 used AERSURFACE to calculate generalized surface characteristics ofthe areas surrounding the emission sources. The resulting meteorological input data was considered to be adequately representative for the project site andmodeling domain, andto satisfythe intent ofsection 7.2: 

### Surface Characteristics andRepresentativeness ofthe S02 Modeling Technical Assistance Document. Although the TCEQ does not agree that characteristics ofa meteorological tower mustbe used, supplemental modeling was performed for the Twin Oaks, Coleto Creek, and Tolk facilities to resolve the United States Environmental Protection Agency's (EPA) concerns aboutthe representativeness ofthe meteorological data used in our analyses. The supplemental modeling was performed using the same model setup as the modeling analysis submittedtothe EPA on September 18, 2015, except that in developing the meteorological data, AERSURF ACE-calculated surface characteristics atthe meteorological tower location were usedinstead ofAERSURFACE-calculated surface characteristics atthe project site. The mostrecent versions ofAERMOD andAERMET (version 15181) were used in the modeling exercises. Table 1: 

### Surface Characteristic Comparison andTable 2:

### AERMODPredictedMaximum GroundLevel Concentrations (GLCmax) compare the TCEQ's modeling results using meteorological data developed using AERSURFACE-calculated surface characteristics at the project site to the supplemental modeling results using data developed using AERSURF ACE-calculated surface characteristics atthe meteorological tower location.

P.O. Box 13087 • Austin, Texas 78711-3087 • 512-239-1000 • tceq.texas.gov How is our customer service? tceq.texas.govjcustomersurveyMr. Erik M. Snyder Page 2 November 30, 2015

Twin Oaks 0.17

Coleta 0.17 Creek Talk 0.18 Table 1: Surface Characteristic Comparison

0.70 0.043 0.17 0.67 0.069
0.68 o.os8 0.17 0.78 o.os8
0.79 0.122 0.18 0.61 0.090
Table 2: AERMOD PredictedMaximum Ground Level Concentrations (GLCmax)

Twin Oaks 84 83

Coleta 84 85 Creek Talk 150 158 represents average 99th maximum concentrations. As shown inTable 2, the changes tothe GLCmax are minimal between thetwo modeling demonstrations. As a result, the overall conclusions from the previous modeling are not changed, and all three locations are predicted tobe in compliance with the 2010 one-hour S02NAAQS. Thankyou for the opportunityto provide this supplemental information to clarify the modeling information provided by the TCEQ in support ofdesignations recommended by the State ofTexas. Ifyou would like copies ofthe modeling files orhave specific questions aboutthe modeling, please contact Daniel Menendez at 512-239-1537 or

**Daniel.Menendez@tc~texas.gov.** Otherwise, please contact me at 512-239-1295 or at Steve.Hagle@tceq.texas.go.Y,_or David Brymer, Director ofthe Air Quality Division, at 512- 239-1725 or at DavidJ.ITymer@tceq.texas.gov, with any questions. Sincerely,



# ~~ Steve Hagle, P.E., Deputy Director Office ofAir Texas Commission on Environmental Quality