Field Measurement of In-Cloud Sulfur Dioxide Oxidation

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Reactions and removal of tropospheric sulfur dioxide occur via a variety of pathways. The dominant gas phase reaction path, with the hydroxyl radical, is thought to be characterized by a sulfur dioxide half-life of 7-14 days. Reaction on aerosol surfaces, and in haze and cloud droplets results in the direct production of sulfate in cloud and precipitation water. Laboratory kinetic data suggest that aqueous phase reactions of dissolved sulfur dioxide with peroxides and ozone are the most efficient pathway by which sulfur dioxide is removed from the atmosphere. To complement laboratory data, and because of the need to understand processes which produce oxidized sulfur species in tropospheric clouds, field studies are recognized as important objectives of atmospheric chemistry research. Knowledge of the oxidants and processes responsible for the conversion of sulfur dioxide to sulfuric acid are essential if predictions of precipitation composition are to be made. The oxidation of sulfur dioxide dissolved in cloud droplets is thought to be affected by several factors which make it difficult to extrapolate laboratory studies of aqueous sulfur dioxide oxidation to natural cloud systems. In this research a set of unique experiments designed to measure the conversion of sulfur dioxide to sulfate in orographic clouds will be carried out. These measurements will be compared to a detailed cloud chemistry model using laboratory derived rate constants and cloud composition measurements as input parameters. The experimental strategy is to release pulses of sulfur dioxide into the cloud from a ground based generator and to collect cloud water as a rime ice deposit during plume detection at the mountain top observatory. This experimental design has been tested and shown to be capable of producing usable results. Plume transport times will be measured using several shorter pulses of an inert tracer during the sulfur dioxide releases.