A model of regional long-term average sulfur atmospheric pollution, surface removal, and net horizontal flux

Abstract

The Advanced Statistical Trajectory Regional Air Pollution (ASTRAP) model combines efficient calculation of long-term regional-scale concentrations and fluxes of pollutant sulfur with improved parameterizations of boundary-layer processes. The parameterizations include diurnal and seasonal variations of dry deposition velocities for SO 2 and sulfate, rate of transformation from SO 2 to sulfate, vertical structure of the planetary boundary layer, and emission rates. The deposition velocity variations simulate some recent experimental results indicating that the deposition velocity of sulfate particles may be of the same order as that for gaseous SO 2, not an order of magnitude less as often modeled. The transformation rate variation simulates a proposed photochemical effect. The stability variations simulate the cycle of nocturnal inversion formation, intensification, rise, and dissipation, as frequently observed in field investigations of the planetary boundary layer. The emission rate variation simulates the effects of such factors as daily patterns of electrical demand. An additional improved parameterization is the treatment of wet deposition as a function of the half power of the six-hourly precipitation amount, an empirical result. Budget studies of emissions from the eastern U.S. and Canada indicate that wet removal, dry removal, and horizontal transport out of the area are of similar magnitude during the summer months; during winter dry removal is only about one half as large as the other terms.