The Formation of Sulfates and the Enhancement of Cloud Condensation Nuclei in Clouds

Abstract

The production of ammonium sulfate by oxidation of dissolved sulfur dioxide in cloud droplets in a wave cloud situation, and the resulting enhancement of cloud condensation nuclei (CCN) released from the cloud on evaporation, have been calculated. The condensational growth of droplets formed on 75 initial CCN sizes is considered simultaneously with the production of sulfates via the Scott-Hobbs mechanism in the droplets in an air parcel moving through a wave cloud. The results show that significant increases in the concentrations of CCN active at 0.5% supersatutation can be produced by SO2 oxidation in wave clouds with short “flow-through” times (4 min) and with concentrations of SO2 and NH3 typical of unpolluted air (1 and 3 ppb, respectively). The sulfate production is found to decrease as the SO2 concentration rises above 10 ppb. This effect is due to the limited buffering capacity of the NH3. The results of the calculations indicate that the in-cloud production of ammonium sulfate can probably explain previous observations of higher than ambinent CCN concentrations in air from evaporating clouds, and that the rate of production of ammonium sulfate in clouds is sufficiently fast that it is probably the major worldwide source of these particles.