Cloud chemical heterogeneity and its influence on aqueous sulfur (IV) oxidation
Date
1997-06
Authors
Rao, Xin, author
Collett, Jeffrey L., Jr., author
Journal Title
Journal ISSN
Volume Title
Abstract
Differences in chemical composition among cloud and fog drops of diverse sizes were investigated at several locations across the United States. Chemical species including acidity, sulfur (IV), hydrogen peroxide, formaldehyde, hydroxymethanesulfonate (HMS) and trace metals iron and manganese were measured. The study examined coastal stratus and stratocumulus clouds in southern California and northern Oregon, frontal and orographic clouds at Mt. Mitchell, North Carolina and Whiteface Mountain, New York, and radiation fogs in California's San Joaquin Valley. Samples were collected with three cloud samplers capable of partitioning the cloud drop size spectrum into two or three independent drop size fractions. Measurements of pH variations within natural cloud drop populations reveal that small drops are often more acidic than large drops. Differences between small and large cloud drop acidities as large as two pH units were observed, although differences were generally below one pH unit. The chemical heterogeneity can significantly enhance oxidation of sulfur dioxide to sulfate within clouds, relative to oxidation rates predicted from the average cloudwater composition. Trace metal concentrations were found to vary with drop size in clouds and fogs sampled at a variety of U.S. locations. Significantly higher concentrations of total iron and manganese were found in large drops in clouds sampled at Mt. Mitchell, North Carolina, and along the southern California coast, while small drops were often enriched in concentrations of iron and manganese in fogs sampled in California's San Joaquin Valley and coastal clouds sampled at Angora Pk., Oregon. Iron speciation measurements in San Joaquin Valley fogs revealed that dissolved iron in small fog drops was present almost entirely as Fe (III). The observed size dependence of trace metal concentrations in cloud and fog drops is expected to influence in-cloud S(IV) oxidation rates as well. Effects of chemical heterogeneity on overall in-cloud S(IV) oxidation rates will largely depend on contributions of the different oxidation paths. Errors in predicting sulfur oxidation rates based on average cloud drop compositions are smallest when abundant hydrogen peroxide is present, for example in the summer clouds at La Jolla Pk., California and at Mt. Mitchell, North Carolina. About 84 percent of the samples are calculated to experience little enhancement in S(IV) oxidation, due to the dominance of the02 path. Approximately 9 percent of the samples are calculated to experience oxidation rate enhancement between I0 and 30%, while 7 percent of the samples are calculated to experience oxidation rate enhancement of30% or more. Effects of chemical heterogeneity on enhancements in sulfur oxidation rates are likely to be strong when (I) hydrogen peroxide concentrations are low, for example the radiation fog in California's San Joaquin Valley, where the calculated enhancement factors range from 1.10 to 1.65, or (2) the droplet pH is high enough to support rapid S(IV) oxidation by ozone and metal-catalyzed S(IV) autooxidation, for example in relatively pristine environments like Angora Pk., Oregon, where the calculated enhancement factors range from 1.02 to 2.0. We expect real clouds to contain more than two chemically distinct drop populations. A wide distribution of drop compositions can support even faster sulfur oxidation rates.
Description
June 1997.
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Subject
Acid deposition
Cloud physics
Oxidation
Sulfur dioxide