Oxygen-18 study of nonaqueous-phase oxidation of sulfur dioxide

Abstract

In a study of the mechanisms of atmospheric sulfate formation, oxygen isotope ratios were measured in sulfates and in the SO 2 and water vapors from which they were formed, in the absence of liquid water. In a 3-ℓ glass chamber, SO 2 and water vapor of various 18O contents were isotopically equilibrated, and then air oxidation of the SO 2 to sulfate was performed by four different methods: high-voltage discharges, NO 2 addition, gamma irradiation and adsorption on activated charcoal. Isotopic equilibration between SO 2 and water vapor proceeded rapidly, resulting in a strong dependence of the δ 18O of the sulfate on that of the water vapor. Oxidation of SO 2 on dry charcoal occurred through the apparent formation of 9-oxygen, 2-sulfur, chemisorbed molecules which decomposed to sulfate in leach water. The δ 18 O SO 2− 4 vs δ 18 O H 2 O relationships observed for these four nonaqueous-phase oxidations of SO 2 to sulfate, together with those in three previously reported aqueous-phase oxidations ( Fe 3+-catalyzed air oxidation, charcoal-catalyzed air oxidation and H 2 O 2 oxidation), were compared to sulfate in rain and snow collected at Argonne, IL. The δ 18 O of sulfate in precipitation water was significantly higher than could be accounted for by any of the several oxidation reactions that were investigated as possible pathways in the formation of secondary sulfates in the atmosphere, either singly or in combination.