9712603 Collett The majority of the global sulfur dioxide oxidation is believed to occur in clouds. The primary mechanisms are oxidation by ozone, hydrogen peroxide, and molecular oxygen in the presence of trace metals, such as iron and manganese. Recent measurements and cloud chemistry models indicate that cloud drop composition varies with drop size. This chemical heterogeneity among cloud drop populations has the potential to significantly influence the rate of aqueous phase sulfur oxidation, resulting in faster sulfate production in large drops. The average oxidation rate in a chemically heterogeneous cloud drop population could significantly exceed the rate expected from the average drop composition. This project, to be conducted jointly by groups at Colorado State University and at the State University of New York at Albany/ New York State Department of Health, will test the hypothesis that sulfate production does not vary with drop size when hydrogen peroxide is the dominant oxidant, but can vary significantly with drop size when oxidation by ozone or trace metal catalyzed oxidation are dominant. The study will apply a tracer technique which was recently developed by L. Husain to measure sulfate production in cloud drops. The ratios of sulfate to a conservative aerosol tracer (Se, As, or Sb) will be measured both in pre-cloud aerosol and cloud water, indicating the proportion of the sulfate that was produced in the cloud. Large and small cloud drops will be collected using a size-fractionating cloud collector. Measurements of droplet composition and gas phase concentrations of sulfur dioxide, ozone, and hydrogen peroxide will also be made to permit prediction of sulfur oxidation rates as a function of drop size for comparison with rates determined by the tracer technique. Two environments (Whiteface Mountain, New York, and Bakersfield, California) have been selected for study to ensure that conditions will be encountered where hydrogen peroxide both is and is not the dominant oxida nt.
