Editor's note: this post was contributed by Air Modeling Technical Manager Jen Beaulieu.
The Environmental Protection Agency (EPA) developed the Guideline on Air Quality Models in 1978 as part of its requirement under the Clean Air Act to standardize air quality modeling procedures. It aimed to help federal agencies, states, and industry prepare and review source permits in a systematic and clear way. In the Guideline, models for regulatory application and guidance for using these models is provided, as is a common baseline used for estimating air quality pollutant concentrations.
In July of 2010, five years after the last revisions to the Guideline, a petition was submitted on behalf of the Sierra Club to EPA, requesting that air quality models be established for ozone and PM2.5 (air pollution particles smaller than 2.5 micrometers) for Prevention of Significant Deterioration (PSD) permit applicants. When this petition’s request was granted in January 2012, the EPA pledged to review these models, incorporating new techniques and methods into the models as necessary. In the nearly four years since the 10th Conference on Air Quality Modeling EPA has continued to conduct research and model evaluations, developing changes to the AERMOD Modeling System to address technical concerns and improve its overall performance. The new revisions to the Guideline, based on this research, were published in the Federal Register on July 29, 2015, proposed at last year’s 11th Modeling Conference, and are intended to be finalized by this spring.
What you should know
EPA’s proposed changes to Appendix W of the Guideline are numerous and cover a variety of modeling topics. Highlights of the proposed changes include:
- The ability to create horizontal and capped point sources. This improves the current methodology for incorporating horizontal and/or capped point sources, which assumes there is no initial velocity associated with horizontal and/or capped stack.
- Tier 2 and Tier 3 NO2 modeling options. The EPA proposes to make the Tier 2 - Ambient Ratio Method 2 (ARM2) NO2/NOx ratio option and Tier 3 - Ozone Limiting Method (OLM) and Plume Volume Molar Ration Method (PVMRM2 options part of the regulatory default model for AERMOD.
- LOWWIND3 and ADJ_U* options. These are welcome changes that address the long-known issue of over-prediction during periods of low wind speeds.
- The designation of AERSCREEN as the preferred screening tool for modeling.
- A two-tiered approach for addressing single-source impacts of secondary formation of PM2.5 and O3, as the Sierra Club’s petition requested.
- The removal of the CALPUFF modeling system as the preferred model for long-range transport. Interestingly, the EPA proposed no alternative preferred model in its place to evaluate Class I ambient impacts of a new or modified source for distances beyond 50 kilometers in PSD permitting, Instead, it proposed an approach where long-range impacts are “screened” to determine if ambient impacts at or about 50 kilometers from the source are significant.
- The allowance of prognostic meteorological data in AERMOD modeling for cases when representative National Weather Service or comparable meteorological data is not available and the collection of 1-year of on-site meteorological data is either cost prohibitive or not feasible.
New versions of modeling systems
The EPA has also released new versions of AERMOD and AERMET executables (version 15181). These new versions allow for modelers to evaluate many of the enhancements to the model from the proposed Guideline revisions. Ultimately, the goal of these model enhancements is to improve model accuracy, which in some cases may lead to reductions in modeled concentrations. It should be noted that while the EPA plans to include the these changes to the model as regulatory default options in AERMOD once the proposed changes to Appendix W are finalized, right now they are still BETA options and would require approval from your regulatory authority prior to use. Two of the most notable proposed changes to AERMOD are the options to allow for more realistic predictions under low-wind conditions—an area where AERMOD has historically over-predicted—and changes to the default modeling options, which will simplify the use of advanced nitrogen dioxide (NO2) modeling techniques.
These changes offer air dispersion modelers the ability to obtain more accurate results. It will be very important for facilities that require air dispersion modeling to support an air permit application for a new emissions source or an emissions source that is being modified to understand how these proposed revisions might affect the permitting process. For more information, contact Jen Beaulieu, Air Modeling Technical Manager at firstname.lastname@example.org.