USE OF STABLE SULFUR ISOTOPE SYSTEMATICS FOR EVALUATING OXIDATION REACTION PATHWAYS AND IN-CLOUD SCAVENGING OF SULFUR-DIOXIDE IN THE ATMOSPHERE

Sulfur dioxide injected into the atmosphere is most likely oxidized in to sulfate. Two major oxidation pathways are possible: 1) a homogeneou s pathway involving gas reaction with hydroxyl radicals and 2) a heter ogeneous pathway involving aqueous dissolution or aerosol reactions (F igure 1.). The relative importance of these reaction pathways conditio ns is controversial. Sulfur isotope ratios can be used to quantify the relative importance of these reaction pathways. However, its applicat ion was severly hampered by the fact that the isotope fractionation fa ctor for the homogeneous pathway was not known (Caron et. al., 1986; N riagu et. al., 1987). A significant isotope fractionation in the homog eneous SO2 oxidation is identified for the first time using an ab init io quantum mechanical calculation. By using the sulfur isotope fractio nation factors we demonstrate a technique that uses measurements of th e sulfur isotope ratio in gaseoUS SO2, aerosol SO4 and sulfate in wet precipitation to quantify the relative importance of the homogeneous a nd heterogeneous reaction pathways as well as the in-cloud scavenging of sulfur dioxide for a set of isotopic observations at New Haven, CT, USA.