A tracer study was conducted in July 2008 in thevicinity of a 650 kW gas fired power plant locatedin Palm Springs, CA. The tracer, SF6, wasreleased with the exhaust gases, and sampled at49 locations at distances ranging from 60 m to 2km from the stack. The exhaust stack stands 2.3m high above the flat roof of a 7 m high buildingsurrounded by one storey residences. There wereseven experiments, four during the night, andthree during the daytime, during which tracer wasreleased for 6 consecutive hours. Meteorologicalmeasurements were made with sonicanemometers at 11 m from the ground. Themeasurements indicated that the wind speedswere low, around 2 m/s, but the lateral turbulentintensities were around 0.5. The meteorologicalmeasurements were used in AERMOD, a state-of the-art dispersion model developed by the USEPA,to estimate concentrations, which were comparedwith measured tracer concentrations. Theevaluation indicated that AERMOD could provideadequate estimates of concentrations during thedaytime hours. However, during the nighttimehours, concentrations were underestimated. Thispaper presents results from sensitivity studiesconducted to explain the discrepancies betweenmodel estimates and observations during thenight. It turns out that ground-level concentrationsare sensitive to the details, such as verticalstructure, of the stable boundary layer.
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