A model for the reversible washout of sulfur dioxide, ammonia and carbon dioxide from a polluted atmosphere and the production of sulfates in raindrops

Abstract

A model has been developed to describe the washout of trace atmospheric gases and the production of acids in rain. The model is applied to washout of sulfur dioxide, carbon dioxide and ammonia and incorporates reversible mass transfer of the trace gases, ionic equilibria of the compounds in solution and catalyzed oxidation of the dissolved sulfur species to sulfates. The significance of ammonia and carbon dioxide on the raindrops' capacity for sulfur and on sulfate production based solely on bisulfite oxidation are explored in detail. The influence of raindrop size, rainfall intensity, cloud-base height, the presence of oxidation catalyzing compounds in the atmosphere and the initial composition of the raindrops as they enter the polluted atmospheric layer on the detailed chemical composition of rain at ground level and the time scale for gaseous sulfur dioxide removal are evaluated. Illustrative model simulations demonstrate that when simultaneous mass transfer of SO 2 and NH 3 are involved, the effective driving force for the transport of both gases can be reduced. This results in much longer relaxation times for the absorption process, and may shift conditions to a point where mass-transfer is the rate controlling step in the SO 2 oxidation process. Carbon dioxide was found to have little effect on the concentration transients or the approach to equilibrium composition of raindrops as they fall through the mixed layer. Ammonia, however, considerably increases rain's capacity for sulfur and may cause the chemical composition of even small droplets (light rainfalls) to be mass-transfer rate limited in extremum conditions. In addition, the effect of a background non-volatile base (such as NaOH) on the transient composition of rain was found to be quite different than the effect of a volatile base, (NH 3) g. The presence of a non-volatile base in the atmosphere may be causitive to occurrence of a bimodal pH distribution in raindrop size spectra.