OXIDATION OF SO-2 IN THIN WATER FILMS CONTAINING NH3

The reaction between NH3 and SO2 in thin water films was studied with a focus on the formation of sulfate. The reactions were conducted at 5 and 60% relative humidities or partial pressure of water vapor and at 0.04, 0.16 or 0.20 atm O2 for temperatures between 20 and 50-degrees-C using a flow reactor and static flask reactors. In the presence of water vapor, the oxidation of SO2 to sulfate was observed only when NH3 was present. The product was primarily (NH4)2SO4. Conversion of as much as 81 % of the SO2 to sulfate was observed after 10 min of reaction when the initial concentrations of SO2 and NH3 were 1 ppm and the relative humidity was 60%. Conversion was only 1-3% when NH3 and/or water vapor were absent from the system. The surface of the reactor was found to influence the reaction by providing a place for a film of liquid water to form, and for the first several hours of reaction in the film, the rate of sulfate formation was observed to be faster than oxidation in liquid water. In general, these results suggest that thin water films on environmental surfaces play a greater role in the deposition of atmospheric gases and the accumulation of their reaction products on surfaces than previously considered.