Cloud chemical heterogeneity and its influence on aqueous sulfur(IV) oxidation

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, total sulfur (IV), hydrogen peroxide, formaldehyde, hydroxymethanesulfonate (HMS) and trace metals iron and manganese were measured. 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. No obvious pattern of drop size-dependence of hydrogen peroxide concentrations was observed. Trace metal concentrations were found to vary with drop size in clouds and fogs sampled at a variety of U.S. locations. 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 hydrogen ion and trace metal concentrations in cloud and fog drops is expected to influence in-cloud S(IV) oxidation rates. Effects of chemical heterogeneity on overall in-cloud S(IV) oxidation rates will largely depend on contributions of the different oxidation paths. About 84 percent of the samples are calculated to experience little enhancement in S(IV) oxidation, due to the dominance of the H...