Microphysics of aqueous droplets in clouds and fogs as applied to PM-fine modeling

Clouds and fogs transform atmospheric gases through gas- and liquid-phase chemistry, and cause the production of PM-fine aerosols. The aqueous-phase concentrations of pollutants in the clouds and fogs depend on gas-phase concentrations, their solubility, and mass transfer rates. Therefore, it is important to study cloud microphysics and dynamics to understand in what portions of an aerosol size distribution oxidation occurs during a supersaturation event, when and how it occurs, and as a result understand why chemical composition depends on droplet size. The growth rate of individual cloud condensation nuclei to cloud droplets is governed by the mass transfer equation which is traditionally expressed in terms of first moment change, i.e., rate of change of cloud droplet diameters. Since droplet aqueous chemistry is directly related to the liquid water mass, the third moment more fully elucidates their growth behavior. By viewing cloud activation in these terms, it is observed that some sections of the aerosol size distribution trade water thus limiting the time that a section may experience the aqueous chemistry. Three characteristic times explain the trading of water between different sections during the cloud's lifetime, and why transport to particles is fast or slow in different distributions. (C) 2001 Elsevier Science Ltd. All rights reserved.