Drop size-dependent S(IV) oxidation in chemically heterogeneous radiation fogs

Six radiation fog episodes were sampled in the Central Valley of California during winter 1998/1999. Drop size-resolved fog samples were sampled using a size-fractionating Caltech active strand cloudwater collector (sf-CASCC) . The sf-CASCC collects a large fog drop sample, comprised mainly of drops larger than 17 mum diameter, and a small fog drop sample, comprised mainly of drops with diameters between 4 and 17 mum. The fog pH was found to vary between approximately pH 5.3 and 6.8, with the pH of the large fog drop sam ple typically several tenths of a pH unit higher than the simultaneously co llected small fog drop sample. At these high pH values, dissolved sulfur di oxide can be rapidly oxidized by a variety of chemical pathways and also ca n react quickly with dissolved formaldehyde to form hydroxymethanesulfonate . The amount of sulfate produced by aqueous-phase oxidation during each fog episode was determined by application of a tracer technique. The ratio of large: small drop S(IV) oxidation was compared with theoretically predicted ratios of large: small drop S(IV) oxidation rates. Although the higher pH of the large fog drops should promote more rapid S(IV) oxidation by ozone, finite rates of mass transport into the large drops and an increasing rate of complexation of S(IV) by formaldehyde at high pH combine to depress theo retically predicted rates of aqueous sulfate production in large fog drops below rates expected for small fog drops. This prediction is supported by t he tracer results that indicate the concentration of sulfate resulting from aqueous-phase S(W) oxidation in small drops generally exceeded the concent ration formed in large drops. These findings stand in sharp contrast to obs ervations in acidic clouds at Whiteface Mountain, New York, where hydrogen peroxide was determined to be the dominant S(IV) oxidant and the rate of S( IV) oxidation was found to be independent of drop Size. (C) 2001 Elsevier S cience Ltd. All rights reserved.