A tracer study was conducted in July 2008 in the vicinity of a 650 kW gas fired power plant located in Palm Springs, CA. The tracer, SF6, was released with the exhaust gases, and sampled at 49 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 11 m from the ground. The measurements indicated that the wind speeds were low, around 2 m/s, but the lateral turbulent intensities were around 0.5. The meteorological measurements were used in AERMOD, a state-of the- art dispersion model developed by the USEPA, to estimate concentrations, which were compared with measured tracer concentrations. The 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 details, such as vertical structure, of the stable boundary layer.
展开▼
