Effect of Aerosol Composition on Cloud Droplet Size Distribution: A Numerical Study

Abstract

Numerical computations have been performed to study the effect on the characteristics of a condensationally produced droplet size distribution of a variability in the kind and fraction of soluble material among atmospheric aerosol particles. The model aerosol used in the study is assumed to be comprised only of nuclei containing 1, 10, 50 and 90% soluble material and to have a particle size distribution equal to the mean of the model continental and maritime aerosol size distributions described by Junge and McLaren. The soluble constituent of the nuclei is taken to be ammonium sulfate. Droplet size distributions are computed for a height of 150 m above cloud base. The computations show that the droplet size distribution resulting from condensation upon a population of mixed nuclei (i.e., nuclei composed of both soluble and insoluble components) is not broader than that produced by condensation upon a population of pure salt nuclei. Further, the dispersion of cloud droplet sizes is found to be quite insensitive to variations in the percentages of the different nucleus types. Cloud droplet number density, however, decreases as the percentages of the more insoluble nuclei increase. Computations for the cases of droplet growth upon aerosols composed entirely of ammonium sulfate or sodium chloride show that the droplet size distribution is also insensitive to the chemical composition of the soluble material in the nuclei. The dispersion coefficients of the computed droplet distributions are in reasonable accord with measurements made near the base of small, non-precipitating cumuli by Fitzgerald and Warner. The effect of the variations in aerosol composition on the supersaturation spectrum of cloud condensation nuclei is also discussed.