Dispersion length scales within and above the urban canopy

Analysis of field experiments on dispersion in Oklahoma City, Salt Lake City, London and St. Louis are presented. We also discuss lab experiments on two model urban canopies with small and large building aspect ratios to examine the physics of dispersion within the urban canopy. Measured concentration profiles, lateral and vertical, indicate that dispersion above the urban canopy is governed by turbulent length scales characteristic of atmospheric boundary layer turbulence. In contrast dispersion within the urban canopy is governed by urban canopy length scales. A model to describe both sets of results is developed. The model is based on a simple Gaussian formulation where the diffusion coefficients are determined by the theories of Taylor (1921) in the horizontal plane, and Hunt and Weber (1979) to account for the vertically inhomogeneous turbulence.