Modeling Dispersion of Buoyant Emissions from a Low Level Source in an Urban Area

A tracer study was conducted in July 2008 in the vicinity of a 625 kW gas fired power plant located in Palm Springs, CA. The tracer, SF6, was released with the exhaust gases, and sampled at 50 locations at distances ranging from 60 m to 2 km from the stack. The exhaust stack stands 2.3 m high above the flat roof of a 7 m high building surrounded by one storey residences. There were seven experiments, four during the night, and three during the daytime, during which tracer was released for 6 consecutive hours. Meteorological measurements were made with sonic anemometers at 3 m and 11 m from the ground. These measurements indicated that the wind speeds were low, around 1 m/s, but the turbulent intensities were around 0.5. The meteorological measurements were used in AERMOD, a state-of-the-art dispersion model developed by the USEPA (Cimorelli et al. 2005), to estimate concentrations, which were compared with measured tracer concentrations. This evaluation indicated that AERMOD could provide adequate estimates of concentrations during the daytime hours. However, during the nighttime hours, concentrations were underestimated. This paper presents results from sensitivity studies conducted to explain the discrepancies between model estimates and observations during the night. It turns out that ground-level concentrations are sensitive to the vertical structure of the boundary layer during the night.

Reference:

Cimorelli, A. J., Perry, S. G., Venkatram, A., Weil, J. C., Paine, R., Wilson, R. B., Lee, R. F., Peters, W. D. and Brode, R. W. (2005) AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General Model Formulation and Boundary Layer Characterization. Journal of Applied Meteorology. 44(5):682-693.