Dry deposition velocities of atmospheric aerosols as inferred by applying a particle dry deposition parameterization to a general circulation model

A particle dry deposition parameterization is applied to a general circulation model to carry out a diagnostic study of global scale dry deposition velocities of ambient aerosols. Bulk deposition velocities are calculated for observed nuclei, accumulation, and coarse modes using two models, one with a more complete description of surface processes. Seasonal as well as diurnal effects are analyzed. The spatial and temporal variability of deposition velocities depends in a complex fashion on both microphysical (particle size and characteristics of the surface elements) and meteorological (surface wind speed and atmospheric stability near the surface) variables. For individual snapshots, mass-average dry deposition velocities are found to lie in the range of 0.01-0.2 cm/s for the nuclei mode. 0.001-0.05 cm/s for the accumulation mode, and 1.25-5 cm/s for the coarse mode. When averaged over long-term simulations, the corresponding values are 0.015–0.15 cm/s, 0.0024.03 cm/s, and 1.25-3.5 cm/s, respectively. Averaged deposition velocities are larger over vegetated areas than over snow, bare ground, and ocean surfaces by a factor of about 1.5-3 and seem also to be sensitive to different vegetation types. Over oceans, the deposition rates tend to mainly correlate with the surface wind speed patterns. Over land areas, however, the effect of the surface thermal response to insolation on convective mixing in the boundary layer becomes important, especially in conditions of small mean solar zenith angle. In this case, the deposition velocity shows a marked diurnal trend, being larger during daytime than during nighttime by a factor of 1.5-2.5. The seasonal variation of surface wind speed and insolation, thus, affects the deposition patterns. Diffusiophoresis related to water evaporation over ocean surfaces is shown to possibly play an important rde in inhibiting deposition of accumulation-mode particles.