PREDICTING ODOR IMPACTS FOR AVERAGING PERIODS OF LESS THAN ONE HOUR USING THE PROPOSED AERMOD AND CALPUFF MODELS

The Industrial Source Complex, Version 3, (ISC3) dispersion is recommended by the U.S. Environmental Protection Agency (EPA) for most air quality modeling analyses to demonstrate compliance with the National Ambient Air Quality Standards. This model has also been the workhorse for odor studies where odor thresholds are defined with averaging periods of less than one hour. To incorporate the advancements on atmospheric and computer sciences, EPA proposed that the models AERMOD and CALPUFF replace ISC3 in air quality modeling studies. This paper compares the three dispersion models and suggests how the proposed models might be used in odor modeling with averaging periods of less than one-hour.Various schemes have been proposed to convert the concentrations predicted by ISC3 to averaging periods of less than one hour. The dispersion coefficients used in the ISC3 model are based on field measurements with a 10-minute sample period. To provide additional conservatism in odor studies, peak-to-mean ratios were developed to scale 1-hour average concentrations to shorter averaging periods. While these schemes have been helpful in conducting odor impact assessments, the classification of ambient turbulence levels into stability categories is not used in the models proposed by EPA.The models proposed by EPA greatly improve the prediction of air quality impacts and compliance with air quality standards. Applying these models to characterize short-term odor impacts required careful application of peak-to-mean ratios or detailed characterization of the meteorological environment. The guidance provided in this paper would benefit wastewater treatment plant managers and planners that are involved in assessing compliance with odor threshold limits.