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Abstract
The acid rain problem is mainly attributed to anthropogenic sulfur dioxide and, to a lesser extent, nitrogen oxide emissions.The transformation of sulfur dioxide to acid sulfate is the major cause of atmospheric acid precipitation. It is generally agreed that the gas phase oxidation including photochemical oxidation by O3 and H2O2 ,which are produced in the atmosphere by photochemical reactions , and oxidation by O2 in aqueous phase catalysed by dissolved trace metal ions and by suspended particulate matter are major contributors to acid precipitation [1-5]. The oxidation of sulfur dioxide has been one of the most frequently studied reactions in aqueous atmospheric droplets. Three reaction pathways are considered to be dominantly responsible for oxidation of SO2 in atmospheric water droplets. These are the oxidation of dissolved SO2 by H2O2, O3 and O2 in the presence of transition metal ions as catalysts [6-7]. The role of O3 becomes more important above pH 6 [8]. Oxidation by molecular oxygen may also be important if cloud water contains sufficient amount of transition metal ions (e.g., Fe, Mn) for autocatalytic reactions to occur [9].