A baseline urban dispersion model evaluated with Salt Lake City and Los Angeles tracer data

A simple baseline urban dispersion model is suggested for use in simulating near-surface releases of tracer chemicals in the urban canopy layer. The model is based on the Gaussian plume or puff model, accounting for low wind speeds, nearly neutral stabilities, large turbulence intensities, and large initial mixing in urban areas. The performance characteristics of this baseline model can be easily determined and used for comparisons with more complex models. Two urban tracer data sets are used to demonstrate the baseline modelʹs performance—the Salt Lake City (SLC) Urban 2000 data set, and the Los Angeles (LA) 2001 data set. The focus of the comparisons is on the maximum concentration, Cmax, on a given monitoring arc, normalized by the emission rate, Q. The Cmax/Q observations follow some straightforward similarity relations, such as a decrease with downwind distance, x, raised to the power −1.5 to −2.0, and a lack of dependence on wind speed during nighttime light wind scenarios when wind speeds are less than about 1.5 m/s. The predictions of the simple baseline model are shown to agree with the observations from the 30 experimental trials in SLC and LA within a factor of about two to three.