HETEROGENEOUS AND HOMOGENEOUS OXIDATION OF SO2 IN THE REMOTE MARINE ATMOSPHERE

A photochemical computer model was used to simulate production and removal of S-containing species in the remote marine atmosphere. The results of aircraft and shipboard trace gas and aerosol measurements, performed during the 1988 Coordinated Air-Sea Experiment/Western Atlantic Ocean Experiment (CASE/WATOX) were used for the model input. The modeling revealed that homogeneous oxidation can not explain the existence of excess sulfate on large particles. Thus, it must be supplemented by an additional process in order to obtain a balance between sources and sinks of sulfur species. A proposed heterogeneous pathway consisting of SO2 condensation on and oxidation in aerosol particles was tested. With the addition of the heterogeneous processes, and assuming that the rate-determining step is condensation of SO2 on large particles by molecular diffusion, it was possible to simulate the experimentally observed concentrations of all sulfur species. Results show that the upper limit for rate constants (dictated by diffusion theory) was 50-1500 times larger than the rate constants used in the model for the condensation process. The balance between sources and sinks, using this model, requires that nearly 60% of the observed oxidation should occur via the heterogeneous channel at a rate of about 0.015 h-1. The 24h average homogeneous rate was approximately 0.01 h-1.