BEHAVIOR OF S(IV) AND FORMALDEHYDE IN A CHEMICALLY HETEROGENEOUS CLOUD

S(IV), H2O2, HCHO, and hydroxymethanesulfonate (HMS) concentrations in cloudwater and fogwater were measured as a function of drop size in s amples collected at five locations across the United States. Samples w ere collected with two cloud samplers capable of partitioning the clou d drop size spectrum into two or three independent drop size fractions . Although sample composition varied widely between sites, S(IV) was p redominantly complexed with formaldehyde as HMS at all locations. HMS concentrations, however, comprised only a; small fraction of total for maldehyde. Formaldehyde concentrations were generally higher in small drops than in large drops. Theoretical calculations suggest that aqueo us formaldehyde should be in equilibrium with the gas phase; however, the equilibrium cannot explain the drop size dependence of formaldehyd e concentrations. Variations in drop acidity and concentrations of for maldehyde and oxidants across the drop size spectrum can influence the relative importance of S(IV) complexation and oxidation asa function of drop size. Complexation of free S(IV) with formaldehyde can compete with S(IV) oxidation when the drop pH is high (approximately 6 or gre ater) and formaldehyde concentrations are high. Three case studies wit h quite different drop chemistry reveal good agreement between the exp ected and observed fates of S(IV).