Dr. Sergio Guerra enjoyed the opportunity to present at the 55th annual conference of the Pacific Northwest International Section of the Air & Waste Management Association, which was held November 4–6 in Stevenson, Washington, in the spectacular Columbia River Gorge. Dr. Guerra chaired and presented at the Dispersion Modeling session of the conference which was held during Session 8 on Friday, November 6.
Although Dr. Ron Petersen prepared a presentation, he was unable to attend in person and Sergio gave the talk in his place.
Following are summaries of the presentations that CPP’s air permitting leaders delivered at PNWIS 2015.
Strategies to Deal with Monitored Exceedances when AERMOD Can’t be Used
In this presentation, Dr. Petersen used the paper mill in Rhinelander, Wisconsin, as a case study that required using innovative wind tunnel modeling methods to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS) in situations for which AERMOD, the Environmental Protection Agency’s (EPA’s) preferred tool, is unreliable.
In this specific case, AERMOD predicted compliance at a monitoring station not far from the mill, while the monitoring station itself measured sulfur dioxide concentrations twice that predicted by AERMOD. It turned out that a cornering vortex flow (embed link for corner vortex post), which AERMOD does not account for, was increasing downdraft and inhibiting the plume rise necessary to push pollutants up and away from ground level.
CPP’s engineers determined the actual Good Engineering Practice (GEP) stack height using wind tunnel testing of a 1:240 scale model (will we include pics?). The results demonstrated conclusively that a corner vortex was responsible for the measured concentrations, which AERMOD could not simulate. Using these results, CPP was able to recommend a creditable GEP stack height of 90 meters, a full 25 meters taller than the height computed using the EPA GEP formula height. AERMOD was then run without building downwash effects at the 90 m GEP stack height with a wind tunnel determined building downwash factor applied to the emission rate that varied from 1 to 1.5 depending upon wind speed. In the end, compliance with NAAQS was demonstrated at the 90 m GEP stack height using this innovative EPA approved approach. More information can be found in the following paper: Evaluation of Monitored SO2 NAAQS Exceedances Due to the Corner Vortex.
Complying with EPA’s Updated Guidance for SO2 Designations
Dr. Guerra’s presentation discussed the EPA’s updated sulfur dioxide (SO2) designations, the first of which were issued in August 2013. These designations are being incrementally staged over the coming years and are expected to be in place by the end of 2020. The result of the new designations is that large sources of SO2 must either perform dispersion modeling or ambient monitoring.
Modeling is the preferred solution, but limitations in the model can lead to overestimates of the actual pollutant concentrations. But combining traditional AERMOD techniques with an Equivalent Building Dimension (EBD) approach delivers a more realistic simulation of how pollutants behave. The EBD technique makes use of wind tunnel testing to improve the inputs to AERMOD and obtain less conservative results.
Dr. Guerra also covered a case study that includes the use of the Monte Carlo technique and the use of a reasonable background concentration to combine with AERMOD predicted concentrations. The use of these methods is in harmony with the probabilistic nature of the NAAQS and can help demonstrate compliance through dispersion modeling analyses, while still being protective of the NAAQS. These advanced techniques allow owners and operators to meet the 1-hour sulfur dioxide requirements, as well as other NAAQS, in an accurate, cost-effective manner. More details on this case study are available in the December 2014 issue of the EM Magazine, “Innovative Dispersion Modeling“.