THE EFFECT OF STREAMWISE DIFFUSION ON GROUND-LEVEL CONCENTRATIONS

This paper examines the effect of streamwise diffusion on the distribu tion of mean scalar concentrations associated with surface releases in the atmospheric surface layer. We use a two-dimensional (2D) (streamw ise-vertical) Lagrangian stochastic model to simulate dispersion. Befo re applying the model, we test its validity by comparing model results of mean concentration and streamwise mass flux density with wind tunn el data reported by Raupach and Legg (1983, Journal of Fluid Mechanics 136, 111-137). The results from simulations conducted with the 2D sto chastic model indicated that: (1) inclusion of streamwise dispersion i n a model provides a plausible explanation for the ''-2'' asymptote ob served in Project Prairie Grass for the fall-off of ground-level conce ntrations under unstable conditions, and (2) streamwise diffusion play s a minor role in dispersion under stable conditions. (C) 1998 Elsevie r Science Ltd. All rights reserved.