Chemistry within aqueous atmospheric aerosols and raindrops

Abstract

Hundreds of chemical compounds have been identified as constituents of atmospheric aerosols and raindrops, and there is substantial evidence for chemical transformations occurring among them. Since tropospheric aerosols have a water content of 30–50 wt % at typical relative humidities, aerosols and raindrops may be regarded (at least to a first approximation) as aqueous solutions. In this review, atmospheric measurements and solution chemistry data are combined to examine reactions and physical parameters of potential importance in aerosol and raindrop systems; much of the information is potentially applicable as well to lake and ocean surface waters. Among the conclusions are the following: (1) Solar radiation penetrates aqueous aerosols and raindrops, photodissociating several molecules and ions. H 2 O 2, formed in the gas phase and dissolved in the aerosol, photodissociates to produce OH ·, which is thus a common constituent of aqueous atmospheric systems and appears to play a role in many chemical processes. (2) The decomposition of atmospheric ozone dissolved in aqueous aerosols or raindrops is a second prolific source of HOx· radicals. (3) An analysis of the transition metal chemistry appropriate to these systems indicates that copper, manganese, and vanadium are potentially the most important homogeneous catalysts and that the concentrations of transition metals in aerosols are significant. (4) The oxidations of alkanes to alcohols, of alcohols to aldehydes, and of aldehydes to carboxylic acids result from chemical chains initiated by hydroxyl radicals or perhaps by organic photochemistry. (5) Rate parameters are available for many of the reactions of interest, particularly for the inorganic processes. Tabulations of relevant rate parameters, absorption coefficients, equilibrium constants, and aqueous solubilities are presented. (6) The tropospheric aerosol can be viewed schematically as an aqueous solution surrounding an insoluble core and covered by an organic surface film of varying integrity, thickness, and composition.