Effects of Surface Wetness on the Evolution and Vertical Transport of Submicron Particles

Abstract

Simulations have been carried out with a numerical model describing air chemistry, aerosol microphysics, and turbulent mixing, in order to study the behavior of fine sulfate particles in the atmospheric surface layer over wet surfaces. Achievement of local equilibrium of sulfuric acid vapor between the gas phase and particles is rapid and can overpower turbulent mixing in controlling local particle size distributions. Numerical results clearly indicate that in regions of relative humidifies lower than about 80% the large submicron particles increase in number at the expense of concentrations of smaller particles. Simulations that incorporate turbulent mixing and surface dry deposition above wet surfaces show rapid change of vertical flux with height and tend to product downward fluxes for particles larger than 0.1 ?m in radius and upward fluxes for particles smaller than about 0.05 ?m in radius. This tendency has been seen in fluxes measured by eddy correlation at heights of several meters above surfaces of varying wetness. However, the model has not reproduced certain other observations, such as strong upward fluxes of the larger particles above the sea and persistence of upward fluxes of the smaller particles above coniferous forests for several hours after surface wetness disappears. In addition, the numerical indications that fluxes of the smaller particles tend to change direction within a meter of a wet surface have yet to be supported by field experiments.