Elemental tracers for sulfur dioxide oxidation in cloud droplets

A tracer technique has been developed to quantify in-cloud oxidation of SO2. SO4 in cloud droplets originates from (1) scavenging of aerosols, and (2) absorption of SO2 followed by oxidation, primarily by H2O2. If a tracer element M is present as particulate with no gas-phase component, it can be shown that (SO4in) = [(SO4/M)cw(α/β)(SO4/M)aa](M)cw where SO4in = SO4 formed from in situ oxidation, α and β are the respective scavenging efficiencies of SO4 and M, cw and aa represent cloud water and aerosols, respectively. Measurements using Se as tracers at Whiteface Mountain, N.Y. yielded contributions of up to 50% from in situ oxidation. Arsenic was used as a tracer in one cloud event and yielded data in very good agreement with that of Se. Investigation on other trace elements is in progress. An accurate knowledge of in-cloud oxidation is essential in deciding the linearity/nonlinearity between regional SO2 emissions and acid deposition, which is critical in designing a strategy to restrict regional SO2 emissions to achieve goals in reducing acid deposition