Cloud Nucleation on Insoluble Particles

Abstract

The theory of heterogeneous nucleation on partially-wettable insoluble spherical particles as developed by Fletcher is used to obtain relations readily applicable in the meteorologically important limit of low supersaturation ratio S. Taking 1.03 as the probably absolute maximum value of S in natural clouds, it is found that activation of a particle demands that the cantact angle theta be less than 12deg. Data from surface chemistry indicate that this theta-limit seperates airborne particulates into a small class of active nucleating substances and a remaining braod class of nonactive cases. However, roughness effects (Wenzel's rule) may shift some from the latter to the former class, a trend possible offset under special conditions (e.g., over industrial areas) by adsorption of non-wetting layers. A simple physical basis for understanding nucleation enhancement in terms of reduced size of the critical embryo is suggested, and a rule for predicting the approximate size-limits for activation of fully wettable atmospheric particulates, but it is concluded that typicalclouds characterized by average values of maximum S(=1.001) can activate only silacates of size larger than a few microns. The most favourable conditions (maritime cumuli, strong updrafts) may lead to activation of dusts with maximum diameters just under 0.1microns contributing to the global precipitation-scavenging of the silicate dust component of the atmospheric aerosol, a contribution especially important for those in the gap between the particle sizes scavenged by Brownian diffusion on cloud droplets and sizes scavenged by aerodynamic impaction by raindrops.