Heterogeneous atmospheric chemistry: Sulfate production in clouds, fogs, and aerosol

Aqueous-phase chemistry can play a major role in atmospheric physico-chemical processing of pollutants. Bulk aqueous-phase model simulations indicate that at least 30% of the ambient sulfate in the Los Angeles Basin is due to cloud and fog processing. A mathematical relationship is derived suggesting that bulk aqueous-phase chemistry models under-estimate the rate of sulfate production in clouds and fogs as compared to size-resolved aqueous-phase models. Bulk and size-resolved aqueous-phase models were compared to determine the magnitude of this difference. The ratio of the two model's predictions ranges from 1 (bulk and size-resolved models in agreement) to 30 (bulk model underpredicts actual sulfate production rate by a factor of 30). This error depends mainly on the availability of gas-phase species {dollar}rm(SOsb2, Osb3, Hsb2Osb2,{dollar} and NH{dollar}sb3),{dollar} the aerosol size/composition distribution, and the residence time of the air parcel in cloud containing air. High SO{dollar}sb2,{dollar} large pH differences in initial aerosol population, and short residence times in clouds are situations where a size-resolved model is required to accurately characterize the aqueous-phase chemistry. High {dollar}rm Hsb2Osb2{dollar} or NH{dollar}sb3{dollar} create conditions where both model types give similar results and therefore, a bulk model should be used.; Application of a size-resolved cloud processing model to the remote marine environment indicates that 25-50% of the non-sea-salt derived sulfate (NSS-sulfate) in the 0.9 to 16.0 {dollar}mu{dollar}m diameter range is due to cloud processing of aerosol emitted from the ocean surface. Cloud processing accounts for the majority of the NSS-sulfate produced in the sub-micron, 0.4-0.9 {dollar}mu{dollar}m size range. The initial alkalinity of the aerosol as it enters the cloud base is the strongest influence on the rate of sulfate production in-cloud assuming sufficient SO{dollar}sb2.{dollar}; A substantial improvement in atmospheric chemistry model's accuracy can be obtained by incorporating size-resolved cloud/fog physico-chemical models into current regional transport/gas-phase chemistry models. These models would then be better able to predict the effect of changing emissions on health and air quality.