An improved model of reversible SO 2-washout by rain

Abstract

Removal of sulphur dioxide from the atmosphere by rain is an important feature of atmospheric sulphur budgets. The exchange of sulphur dioxide between a falling rain drop and air is complicated not only by the complex circulation inside the drop but also by the chemical reactivity of the gas with water. The latter characteristic results in a non-linear relationship between the liquid and gas phase SO 2 concentrations. This relationship is derived. An improved method of modelling reversible SO 2 exchange between a falling rain drop and air is presented, which takes into account phenomena associated with the solution chemistry of SO 2 (e.g. ion-enhanced diffusion) as well as the microphysical effects of internal circulation. The model is used to calculate the redistribution and washout of an SO 2 plume by raindrop spectra characteristic of drizzle and heavy rain. The fractional plume washout rate (% mm −1 rain) is inversely related to plume concentration and thickness. In a heavy rain (25 mm h −1) washout from a 1000 ppb(v) SO 2 plume of 20m thickness occurs at a rate of 56% h −1. The relative importance of gas phase and liquid phase mass transfer is dependent on SO 2 atmospheric concentrations. At plume concentrations lower than 300 ppb(v) gas phase control dominates. Beneath a plume, harmful ground level SO 2 concentrations are unlikely to occur as a result of reversible washout by rain.