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In September 2018, the Texas Commission on Environmental Quality (TCEQ) released its revised Air Quality Modeling Guidelines (APDG 6232). The revised modeling guidance aligns with the recent changes to the U.S. EPA Guideline on Air Quality Models (40 CFR Part 51 Appendix W), effective May 22, 2017.

With the revisions to its modeling guidance, TCEQ incorporates the April 17, 2018 EPA memorandum regarding the (non-binding) Significant Impact Levels (SILs) for particulate matter with a diameter less than 2.5 microns (PM2.5), superseding the May 20, 2014 EPA guidance. Based on a new analytical approach, the revised EPA guidance affirms the previous 24-hour PM2.5 SIL of 1.2 mg/m3 and recommends the use of an annual PM2.5 SIL of 0.2 mg/m3 in lieu of the previous annual SIL of 0.3 mg/m3. As before, SILs will continue to require justification.

As of the September 2018 modeling guidance release, TCEQ is requiring that secondary PM2.5 impacts be evaluated for every project (minor or major) that requires primary PM2.5 impacts to be evaluated. This represents a change in guidance where previously only major NSR projects with secondary PM2.5 precursor emissions (i.e., NOx and SO2) that exceed their respective Significant Emission Rates required this evaluation. In projects where no primary PM2.5 emissions increases are represented but increases in precursor emissions occur, no secondary PM2.5 impacts analysis will be required.

The revised modeling guidance addresses the assessment of secondary pollutants (i.e., secondary PM2.5 and ozone) with the adoption of a two-tier approach. Tier 1 involves using appropriate and technically credible relationships between emissions and ambient impacts developed from existing modeling studies deemed sufficient for evaluating a project's impact. EPA is in the process of finalizing Modeled Emission Rates for Precursors (MERPs) as a Tier 1 demonstration tool for secondary PM2.5 and ozone, which include MERPs for several hypothetical Texas sources (MERPs included in the revised TCEQ modeling guidance are based on the draft PEA MERP guidance). There are five modeled hypothetical sources located in Texas that can be used to derive a MERP, but selection must be justified (e.g., worst-case or most representative). TCEQ recommends using worst-case Texas MERPs whenever possible. If using other values, they must be in Texas (or New Mexico for projects in West Texas) and appropriately justified.

The use of MERPs in recent projects have resulted in only small increases in PM2.5 impacts resulting from the addition secondary PM2.5; therefore, the need to perform a Tier 2 evaluation (case-by-case application of complex photochemical grid modeling) is expected to be rare.

The revised modeling guidance also addresses changes to the EPA's Tier 2 option of NOx to NO2 conversion. Previously, the Tier 2 option used the Ambient Ratio Method (ARM) approach wherein either NOx emission rates or modeled NOx impacts were adjusted to NO2 by the application of a factor of 0.8 for the 1-hour averaging period or 0.75 for the annual averaging period. The September 2018 guidance requires the use of the EPA's ARM2 option in AERMOD to automatically adjust NOx emission rates to NO2 impacts when the modeling is executed.

The revised TCEQ modeling guidance also includes other minor changes.

If you have questions regarding how the revised air dispersion modeling guidance may impact your permitting plans, contact the Trinity office nearest you by calling (800) 229-6655 for more information.