Model Clearinghouse Information Storage and Retrieval System
Record Details

Issue:

The Texas Commission on Environmental Quality TCEQ has submitted a 1-Hour 
sulfur dioxide SO2 Attainment Demonstration State Implementation Plan SIP 
for the Rusk-Panola nonattainment area that includes modeling that uses an 
alternative model that TCEQ requested be approved instead of utilizing the 
regulatory version of the AERMOD Modeling System available at the time v21112. 
The alternative model is a formulation of AERMOD v21112 with code changes to 
modify the way highly buoyant plumes that penetrate the boundary layer are 
treated within the model.

In convective conditions, the regulatory version of AERMOD has a 3-plume 
treatment for stack emissions: direct, indirect, and penetrated components. 
The direct and indirect plumes remain within the mixed/convective layer. The 
penetrated plume is the portion of the plume that is sufficiently buoyant to 
break through the elevated inversion into the stable layer aloft which has 
less vertical mixing than the mixed/convective layer. EPA’s formulation in 
AERMOD does account for the penetrated plume, but the assertion by AECOM and 
TCEQ is EPA’s formulation prematurely mixes the penetrated plume back into 
the convective layer resulting in penetrated plume impacts adding to the 
receptors on the ground in its predictions too early and repeats this behavior 
for hours leading up to the actual interception of the penetrated plume by 
the rising convective mixed layer. AECOM and TCEQ provided information 
indicating this behavior has been observed by investigators associated with 
field studies where the model is found to overpredict ground-level 
concentration events due to the penetrated plume component and make those 
predictions too early in the day.

EPA Region 6 has conducted a thorough review of the request and has proposed 
to approve the use of AERMOD-HBP as an alternative model to conduct the air 
quality modeling analysis as part of TCEQ’s attainment demonstration SIP for 
the 1-Hour SO2 Rusk-Panola nonattainment area. In the concurrence request 
memo from EPA Region 6 to the Model Clearinghouse on July 11, 2024, EPA 
Region 6 proposed approval of an AERMOD-HBP alternative model for the 1-Hour 
SO2 Attainment Demonstration SIP for the Rusk-Panola nonattainment area 
pursuant to Appendix W, Sections 3.2.2b2, 3.2.2d, and 3.2.2e.  EPA 
Region 6 also provided modeling files and analysis files electronically and 
a technical support document AERMOD-HBP TSD summarizing their review. EPA 
Region 6 included, the TCEQ’s May 24, 2021, alternative model request and 
non-copywrited supporting documents submitted by TCEQ, and clarifying 
documentation provided by Vistra’s contractor AECOM to EPA on May 1, 2024. 
EPA Region 6’s analysis concluded that AERMOD-HBP performs better than the 
regulatory version of AERMOD in this case-specific situation.

C/H Response:

the Model Clearinghouse fully concurs with EPA Region 6 proposed approval of 
a using AERMOD-HBP for the modeling to demonstrate attainment in the TCEQ’s 
a 1-Hour SO2 Attainment Demonstration SIP for the Rusk-Panola nonattainment 
area based on the alternative model justification package provided by TCEQ 
with supplemental material provided by AECOM and the technical review 
documentation provided by EPA Region 6. The Model Clearinghouse encourages 
EPA Region 6 to respond to TCEQ and to the docket of the proposed Limited 
Approval/Limited Disapproval of the State Implementation Plan with a letter 
of alternative model approval, as appropriate. The information associated with 
the EPA Region 6 alternative model approval and the Model Clearinghouse 
concurrence should be available for comment during the appropriate public 
comment period for this SIP limited approval and limited disapproval action.

Given the relatively limited data available for the analysis of AERMOD-HBP, 
the Model Clearinghouse recommends caution and careful review before additional 
alternative model considerations of AERMOD-HBP in other projects in this area 
or elsewhere. This case-specific Model Clearinghouse concurrence does not 
constitute a generic approval of AERMOD-HBP for any other applications. The 
Model Clearinghouse and the AERMOD Model Development Team will continue to 
evaluate potential model formulation concerns with AERMOD’s treatment of 
penetrated plumes and may consider future regulatory updates as additional 
representative monitoring data is collected at a variety of facilities and 
subsequent model performance evaluations are conducted with appropriate 
scientific peer-review.

Memo:  Model Clearinghouse Review of an Alternative Model Approach, AERMOD-HBP, in 
Support of Clean Air Act CAA SO2 Nonattainment SIP Attainment Demonstration 
Modeling – 1-Hour SO2 Rusk-Panola Nonattainment Area
        Dated: July 24, 2024
        From:  George Bridgers, Model Clearinghouse Director
                 Air Quality Modeling Group, Air Quality Assessment Division, 
                 Office of Air Quality Planning and Standards
        To:    Erik Snyder, Lead Regional Air Quality Modeler
                 SO2  Regional Haze Section, Air and Radiation Division
                 EPA Region 6, Dallas, TX
        Thru:  Guy Donaldson, Branch Chief
                 State Planning  Implementation Branch, Air and Radiation Division 
                 EPA Region 6, Dallas, TX
                 
Memo:  Concurrence Request for Approval of Alternative Model AERMOD-HBP, in Support 
of Clean Air Act CAA SO2 Nonattainment SIP Attainment Demonstration Modeling
        Dated: July 11, 2024
        From:  Erik Snyder, Lead Regional Air Quality Modeler
                 SO2  Regional Haze Section, Air and Radiation Division
                 EPA Region 6, Dallas, Texas
        Thru:  Guy Donaldson, Branch Manager
                 State Planning  Implementation Branch, Air and Radiation Division
                 EPA Region 6, Dallas, Texas
        To:    George Bridgers, Model Clearinghouse Director
                 Air Quality Modeling Group, Air Quality Assessment Division, 
                 Office of Air Quality Planning and Standards

Memo:  Technical Support Document - EPA Region 6’s Review of TCEQ’s Alternative Model 
Request of AERMOD with Highly Bouyant Plume Treatment HBP
        Dated: July 2024

Memo:  Supporting Materials Submitted by TCEQ and AECOM
        Dated: May 2021