Gas-to-particle conversion in the atmosphere: I. Evidence from empirical atmospheric aerosols

Condensable vapours such as sulphuric acid form aerosol in the atmosphere b y the competing mechanisms of condensation on existing aerosol and the nucl eation of new aerosol. Observational and theoretical evidence for the relat ive magnitudes of the competing processes is reviewed, and a number of gene ral conclusions are made. Condensation is sensitive to the sticking probabi lity of sulphuric acid molecules on aerosol particles, but there is now goo d evidence that it should be close to unity. In this case, equilibration ti mescales between acid vapour and the aerosol in most of the atmosphere are of the order of minutes or less, so that the acid concentration on such tim escales given simply by the production rate times the equilibration time. W hen the acid concentration exceeds a threshold, nucleation will occur. The atmospheric aerosol therefore follows a history of initial formation in a n ucleation burst followed by growth and coagulation with final removal by pr ecipitation. This leads to the inverse correlation between aerosol number c oncentration and mass concentration found by Clarke (1992. Journal of Atmos pheric Chemistry 14, 479-488) in the free troposphere. Binary homogeneous n ucleation of sulphuric acid/water droplets, for which various simplified ra tes are compared, may dominate in such regions, but other mechanisms are po ssible elsewhere. A detailed analysis is performed of the number concentrat ions, removal rates, and masses of the components of the different types of global aerosols proposed empirically by Jaenicke (1993. Tropospheric Aeros ols, Aerosol-Cloud-Climate Interaction. Academic Press, New York). There is a striking correlation between number concentrations in the nucleation and accumulation modes; and the giant aerosol mode, which if it is present dom inates the mass, has little effect on the gas-to-particle conversion proces s. The mass of the atmospheric aerosol is therefore uncorrelated with the m agnitude of molecular aerosol removal by condensation. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.