X. Rao et Jl. Collett, BEHAVIOR OF S(IV) AND FORMALDEHYDE IN A CHEMICALLY HETEROGENEOUS CLOUD, Environmental science & technology, 29(4), 1995, pp. 1023-1031
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).