Solubility of single chemical compounds from an atmospheric aerosol in pure water

The water-solubility differentiates atmospheric particles into cloud condensation nuclei (CCN) and interstitial aerosol. This interaction between aerosol and cloud influences the climate in different known and hypothetical ways [Hansson et al. (1994) In Proceedings of EUROTRAC Symposium'94, (edited by Borrell P. M.), p. 984. SPB Academic Publishing, The Hague; Schwartz et al. (1995) In Aerosol Forcing of Climate (edited by Charlson R. J. and Heintzanberg J.), p. 251. Wiley, London]. Some authors studied the fraction of water-soluble amounts with particle size [Meszaros (1968) Tellus 20, 443; Winkler (1974) Meteorol. Rdsch. 27, 129; Fuzzi et al. (1988) Journal of geophysics Research 93, 1114); Svenningsson et al. (1992) Tellus 44B, 556; (1994) Journal of Atmospheric Chemistry 19, 129). As the water-solubility (or hygroscopicity) is a property of chemical compounds it is of great interest to identify the chemical composition of aerosols and of the dissolved fraction. Using an optimal combination of analytical methods, the elemental and species concentrations were determined in the original aerosol as well as in the resulting solutions. The fraction of soluble species were calculated from the measured values and three groups were found: with high (50-73%), medium (20-26%), and low (2%) dissolved fraction. In a further series of experiments the rate constants of dissolution of various species were determined. These can be arranged in the order of decreasing rate of dissolution: the alkaline and alkaline earth elements K+ approximate to Mg2+ > NH4+ > Na+ > Ca2+, the anions NO3- approximate to SO42- approximate to Cl-, and for the iron oxidation states Fe3+ much greater than Fe2+. A photolytic reduction of Fe3+ to Fe2+ was also observed. For Mg2+, Ca2+, and SO42- a dependance of the rate constants for dissolution on time was discovered. This can be explained by the presence of different chemical compounds in the aerosol. The discussion of the results with regard to the time scale of the formation of cloud or fog droplets shows that the slightly soluble inventory of a solid aerosol of the alkaline and alkaline earth elements, as well as of the anions, is dissolved into the aqueous phase in a relatively short time interval of 10-700 s. On the other hand, Fe3+ is dissolved in a time period of 3000-4500 s, whereas Fe2+ appears in the solution in a time period of up to 7000 s. (C) 1997 Elsevier Science Ltd.