CHEMICAL-COMPOSITION DIFFERENCES IN FOG AND CLOUD DROPLETS OF DIFFERENT SIZES

The distribution of acidity and solute concentration among the various droplet sizes in a fog or cloud and the effect of the evaporation-condensation cycle on the composition and size distribution of atmospheric aerosol is studied. Significant total solute concentration differences can occur in aqueous droplets inside a fog or cloud. For the fog simulated here, during the period of dense fog, the solute concentration in droplets larger than 10 μm diameter increased with size, in such a way that droplets of diameter 20 μm attain a solute concentration that is a factor of 3.6 larger than that in the 10 μm droplets. Droplets on which most of the liquid water condenses have access to most of the reacting medium for in situ S(IV) oxidation and are therefore preferentially enriched in sulfate. The gas and aqueous-phase chemical processes result in an increase of the total solute mass concentration nonuniform over the droplet spectrum for a mature fog. These chemical processes tend to decrease the total solute mass concentration differences among the various droplet sizes. Low cooling rates of the system also tend to decrease these concentration differences while high cooling rates have exactly the opposite effect. The mass/size distribution of the condensation nuclie influences quantitatively, but not qualitatively, the above concentration differences. © 1990.