Experimental Investigations of Ice in Supercooled Clouds. Part II: Scavenging of an Insoluble Aerosol

Abstract

An experimental study of aerosol scavenging by ice growing in supercooled clouds was conducted with a continuous flow cloud chamber. Techniques for detecting insoluble (latex) submicron particles in individual ice crystals were developed. The effects of microphysical parameters on the scavenging process were examined quantitatively. Measurements of the aerosol scavenging rates were documented as functions of cloud temperature, liquid water content, and the diameters (0.109 ?m and 0.551 ?m) of the nearly monodisperse aerosol particles. Scavenging data were acquired at temperatures of ?6°, ?8°, ?11.5°, and ?14°C. The liquid water contents of the supercooled clouds were varied from ?0.3 to 6 g m?3, while the maximum dimensions of the ice crystals ranged from about 50 to 300 ?m. The scavenging data agree with some previously published theoretical and experimental results and expand the empirical database available for understanding the mechanisms of scavenging. It was found that the presence of liquid water reduced the aerosol removal rates, particularly for crystals growing in the habit transition region near ?8°C. It is hypothesized that the retardation effect is due to enhancement of the thermophoretic forces arising from more rapid vapor deposition and latent heat release at higher liquid water contents. The scavenging efficiency at a given liquid water content, however, was not found to depend significantly on the growth habit of the ice crystal. The data, particularly regarding the dependence of the scavenging rates on liquid water content, appear to resolve an important conflict in the literature regarding the relative roles of thermophoresis and diffusiophoresis in the scavenging of submicron particles by ice crystals growing in supercooled clouds.