Chemical Heterogeneity Among Cloud Drop Populations and its Effects on Sulfur Oxidation

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9509596 Collett Cloud chemistry models utilizing explicit treatments of cloud microphysics predict that cloud drop composition varies with drop size. Recent experimental observations confirm that large and small cloud drops typically have quite different chemical compositions. Chemical heterogeneity among cloud drop populations can influence removal rates for individual solutes, and can also enhance rates of sulfur oxidation relative to rates predicted from average cloud water properties. Clouds are major contributors to global atmospheric sulfate production. Developing an accurate picture of the atmospheric production of sulfate therefore requires an accurate understanding of the rate of sulfate production in real, chemically heterogeneous clouds. The primary objective of this project is to characterize variations in cloud drop chemistry across the drop size spectrum and determine how much sulfate production in clouds is enhanced by chemical heterogeneity among cloud drop populations. Data will be collected through simultaneous measurements of the chemical composition of multiple drop size classes. Cloud water will be collected with two cloud collectors used in previous work (two-stage, and three-stage, respectively), as well as a new five-stage instrument to be constructed as part of this project. Major ions, pH, free and complexed sulfur (IV), oxidants, and catalysts will be measured and used to characterize sulfur oxidation rates as a function of drop size. Measurements of the size-dependent aerosol composition and of soluble trace gases also will be made to determine the influence of particle and gas scavenging on the drop size-dependence of the cloud water composition.