Cloud studies seen from a physical chemist's perspective. Schwartz S. E. Proceedings of the Sixth European Symposium, Varese, Italy, 18-22 October, 1993 G. Angeletti and G. Restelli, Eds. Office of Official Publications of the European Commission, Luxembourg, Vol 2, pp. 891-900, 1994.

Substantial advances have been made in recent years in describing the rates of chemical reactions in clouds. Inputs to these calculations are concentrations of reagent species, solubilities of reactive gases, rate expressions for interphase mass transport, rate expressions for aqueous-phase reaction of the dissolved species, the liquid-water content of the cloud and its drop-size spectrum, and the composition of the cloudwater (especially pH) as it affects solubility and reaction rates. Armed with this information one can calculate the rates of reactions of interest. The question still remains as to the accuracy of such calculations. To answer this we need field measurements, of cloud properties and the concentrations of reagents--to permit evaluation of reaction rates--and of differences in composition that may be attributed to chemical reaction. However determination of rates and extents of in-cloud reactions presents a major challenge because of uncertainties associated with measurements of concentration changes. Even attribution of a measured composition change to a specific reaction is difficult. Another challenge to the field measurement community is determining the size dependence of cloudwater composition (especially pH) and the resultant difference between reaction rates based on bulk composition and actual reaction rates integrated over the cloud droplet spectrum. This paper reviews these issues and offers suggestions for field measurements to confirm model based-understanding of these processes.


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