A methodology for evaluation of vertical dispersion and dry deposition of atmospheric aerosols

A new approach to treatment of vertical dispersion and dry deposition of atmospheric aerosols is suggested for primary application in mesoscale to global atmospheric transport models. The vertical exchange scheme extends the resistance analogy formulated earlier for gaseous species and fine aerosols. The approach is based on the exact solution of the steady-state equation for aerosol flux within a finite layer. The flux is expressed as a linear function of concentrations at the layer boundaries and accounts for the vertical inhomogeneity of the diffusion coefficient and the regular vertical velocity. The new dry deposition scheme accounts for physical properties of the air flow, surface and depositing particles. The flow is given by the vertical profile of exchange coefficient and characteristic velocity at the surface. The deposition rate to smooth surfaces is obtained via solution of the budget equation for particle mass. The transition from smooth to rough flow regime is considered. Rough surfaces are characterized by two length scales: the aerodynamic roughness and the "collection scale", introduced in this paper. The collection scale incorporates the effective size of collectors and a ratio of the airflow velocity at the top of the roughness elements to the friction velocity. The particles are described by their physical size, relaxation time and Brownian diffusivity. The scheme was developed basing exclusively on wind-tunnel and numerical experiments available from the literature, and reproduces them well. The data of outdoor experiments have noticeably larger uncertainties, which allowed only general evaluation of their agreement with the predictions.