Chemical Transformation Involving Carbonyls and Carboxylic Acids Within Clouds

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Carbonyl compounds are photochemical precursors of ozone while carboxylic acids are major sources of acidity and therefore control in part pH dependent transformations in atmospheric droplets. Despite the recognized importance of these classes of compounds in atmospheric chemistry, much uncertainty exists concerning processes in their biogeochemical cycling. The proposed investigation will provide the first concurrent gas-phase and aqueous-phase (cloud) measurements of carbonyls and organic acids at a rural site. Three institutions, each bringing expertise in specific fields will perform a detailed study of in-cloud chemical transformations involving these compounds. This investigation will employ various meteorological and vapor- and aqueous-phase chemical measurements from an instrumented tower (1014 m elevation) situated in the Shenandoah National Park, Virginia. Intensive field sampling will be conducted over a one month period in April-May 1990, with later interpretation of the data facilitated by state-of-the-art computer modeling of in-cloud chemical and physical processes. The concurrent observations of carbonyls and organic acids will be examined for evidence of atmospheric oxidation of carbonyls to organic acids, with special focus on the in-cloud aqueous-phase oxidation of formaldehyde as a source of formic acid. The observed fractionations of individual compounds between the gas phase and the cloudwater phase will be compared to those expected from thermodynamic equilibrium. Two cloud droplet size fractions will be collected, which will document the size dependence of cloudwater concentrations. The results of this investigation will contribute substantially to our knowledge of important processes determining the chemical composition of the troposphere.