Effects of Wind Shear on Pollution Dispersion

摘要

Using an accurate numerical method for simulating advection and diffusion of pollution puffs, it is demonstrated that relatively small changes in wind speeds or direction with height induces appreciable horizontal dispersion that cannot be accurately simulated using classical puff diffusion models. Using idealized winds, it is shown that individual puffs of pollution preferentially disperse in the horizontal direction aligned with the mean wind shear vector in the atmospheric planetary boundary layer (PBL). Even if wind directions are constant in the PBL, the speed shear in the lowest constant flux surface layer induces appreciable horizontal dispersion along the mean wind direction. Using a more realistic and typical observed late afternoon PBL wind profile, it is shown that a Lagrangian puff model cannot reasonably simulate the observed transport and dispersion of a puff of pollution for a 3-4 hour period. These results suggest that shear-related dispersion effects are probably the dominant mechanism of horizontal dispersion in the atmosphere. For assessing long-range dispersion of atmospheric pollutants, the common assumption that pollutants disperse as spherical “puffs” is inherently inaccurate since the effects of horizontal dispersion along the direction of ambient wind shear experienced by a dispersing pollution puff are not accurately accounted for. If a puff model is used to assess long range transport and dispersion, it is demonstrated that if peak concentrations are correctly simulated, then the overall area impacted by pollution will be severely skewed, and there will be many areas where concentrations are severely over- or under-estimated. If larger horizontal puff dispersions are assumed so as to encompass the actual impact area, then peak concentrations calculated by Lagrangian puff model are severely underestimated. These results suggest that horizontal dispersion of pollutants should be proportional to the wind shear across the vertical depth of the atmosphere containing pollutants.