Peak-to-Mean Ratios for Odor Impacts: A Tracer Study Revisited

From September 20 to 25, 1992, a field tracer study was performed to assess the site specific peak-to-mean ratios appropriate for assessing odors from a wastewater treatment plant. The results of this study were presented in several papers in the early 1990's. In the past 10 years, the advancements in dispersion models have provided new tools for assessing the fate and transport of air pollutants. This paper will compare the results from the tracer study to the predicted impacts from new or improved dispersion models.The field tracer study consisted of releasing an inert tracer gas, sulfur hexafluoride (SF6), from each of the two existing scrubber stacks. Two types of instruments, a real time continuous analyzer and a time-average syringe sampler, were used to collect samples in the ambient air immediately downwind of the source. The ScienTech Inc. LBF#3 Precision Tracer Gas Analyzer, consisting of a fast response detector, was used to provide continuous real time, onsite measurement of the SF6 concentrations released from the emission points. The real time measurements were made with a response frequency of 3 Hz. Ten separate field tracer experiments were performed.As with any real life experiment, a number of factors contribute to the dispersion of pollutants in the ambient environment. At the wastewater treatment plan where the tracer study was performed, there were surrounding buildings and structures, elevated terrain and a coastal boundary layer. The ability of dispersion models to better characterize the surface boundary layer, building wake and cavity effects, and impacts in elevated terrain has greatly improved. The results of the tracer study will be compared to several new or improved dispersion models.The models to be evaluated include:Second-order Closure Integrated PUFF model (SCIPUFF) to assess the impacts of puff releases over various averaging periods. A variation of SCIPUFF called SCICHEM which includes the Prime building downwash algorithm.Industrial Source Complex . Short Term Version 3 (ISCST3) to provide an initial basis for comparison. A variation of the ISC3 model which includes the Prime building downwash algorithm will also be evaluated.The AERMOD model incorporates the Prime downwash algorithm, has improved treatment of the planetary boundary layer and improved treatment of elevated terrain features.