Relating plume spread to meteorology in urban areas

This paper examines relationships between dispersion and meteorology measured in a field study conducted in Barrio Logan, a suburb of San Diego, during 5 days of the period 21 August 2001–31 August 2001. The mean building height in Barrio Logan is about 4 m. The tracer, SF6, was released at a height of 5 m from a shipyard on the shoreline, and the concentrations of the tracer were sampled on 4 arcs at 200, 500, 1000, and 2000 m from the source during ten hours of the day starting at 10 am. The meteorological conditions that governed dispersion were measured using sonic anemometers and a SODAR with a range of 200 m in the vertical. It turned out that ground-level concentrations at the receptors used in this study were governed by the meteorological variables in the urban boundary layer above the urban roughness sublayer (RSL). In this region between 15 and 150 m above ground-level, the horizontal and vertical turbulent intensities were relatively uniform. This uniformity in turbulent intensities allowed the formulation of simple expressions for horizontal and vertical plume spreads that could be used in a Gaussian dispersion model. These expressions are similar to those proposed by Briggs (ERL, ARL USAEC Report ATDL-106, U.S. Atomic Energy Commission Oak Ridge, Tennessee, 1975) and Hanna et al. (Atmos. Environ. 37 (2003) 5069) to model dispersion in St. Louis and Salt Lake City, respectively. However, the application of these dispersion curves requires information on the meteorology of the boundary layer. It might be possible to use measurements above the average building height (4 m in our case) to infer these boundary layer properties. The dispersion model based on boundary layer meteorological information explained about 63% of the variance of the maximum observed concentrations on each sampling arc, and 60% of these concentrations was within a factor of two of the corresponding model estimates. It was necessary to account for initial plume spread caused by building effects to explain concentrations on the 200 and 500 m arcs.