Air - December 20, 2013

Ohio EPA Engineering Guide No. 69.

On October 10, 2013 Ohio EPA solicited comments on an update to Engineering Guide No. 69 (EG No. 69). Comments were due on November 1, 2013. EG No. 69 provides guidance on conducting air dispersion modeling. The most recent official update to EG No. 69 was back in 2003. Ohio EPA is proposing two significant changes to EG No. 69.

Beginning on January 1, 2014 AERSCREEN must be used instead of SCREEN3. EG No. 69 still allows the use of AERMOD for more complex air quality modeling projects. AERSCREEN is a streamlined version of AERMOD but like its predecessor SCREEN3 can only be used to predict offsite concentrations from a single egress point. The AERSCREEN model is more sophisticated and complex than the SCREEN3 model and as a result requires additional information not previously required by SCREEN3. AERSCREEN requires information about building and stack orientation, an appropriate terrain elevation file (i.e., a DEM or NED file), and information about ambient air temperatures and wind speeds. For sources located in urban areas, the population of the city must also be known. GT estimates that it will take us twice as long to setup an AERSCREEN modeling run as it would a SCREEN3 modeling run. It will also be less likely that the average environmental manager will be capable of running AERSCREEN without outside assistance.

The AERSCREEN model is not without its fair share of advantages, however. Unlike SCREEN3, AERSCREEN model has the capability to conduct NO2 modeling using the Plume Volume Molar Ratio Method (PVMRM) or the Ozone Limiting Method (OLM). Use of PVMRM and OLM is particularly useful when trying to demonstrate compliance with the new more stringent 1-hr NO2 NAAQS. AERSCREEN allows the use of AERSURFACE files, discrete receptor files and can model more complex buildings. AERSCREEN can generate line graphs and flow sector graphs to allow for visual representations of the results.

Ohio EPA is also proposing a significant update to Table 3 of EG No. 69. Table 3 of EG No. 69 summarizes the NAAQS, PSD Significant Emission Rates, PSD Significant Impact Increments, Class II PSD Increments, Ohio Significant Emission Rates and Ohio Acceptable Incremental Impacts. Most of the changes to Table 3 are not controversial and reflect the various changes to the NAAQS since 2003. Ohio EPA is proposing to increase the Ohio Modeling Significant Emission Rate in Table 3 for PM10 from 10 tpy to 15 tpy and for SO2 from 25 tpy to 40 tpy and for NO2 from 25 tpy to 40 tpy. Ohio EPA is proposing to lower the Ohio Acceptable Incremental Impact for lead from 0.375 µg/m3 to 0.0375 µg/m3.

Contact Ron Hansen at (614) 794-3570 ext. 21 if you have any questions.