A laboratory study of the efficiency with which aerosol particles are scavenged by snow flakes

Abstract

An evaluation is made of presently available efficiencies with which aerosol particles are collected by snow crystals and aggregates of snow crystals (snow flakes). In this evaluation we incorporated our own recent experiments, carried out with naturally as well as with laboratory grown snow flakes inside a large walk-in cold chamber. Our evaluation shows that snow flakes are considerably more efficient in scavenging aerosol particles than single snow crystals. This result is supported by the recent wind tunnel studies of Lew et al. (1986, J. atmos. Sci. 43, 2410–2417) on the uptake of supercooled drops by snow flakes, and by the theoretical studies of Redkin (1973, Hydrodynamics and Thermodynamics of Aerosols (edited by Voloshichok and Sedunov), pp. 220–238. Wiley, New York), who both attributed the high collection efficiency of snow flakes to a filtering effect involving the open mesh structure of the snow flakes. This conclusion can be understood on the basis of an analysis of the available collection efficiencies of aerosol particles by single snow crystals and snow flakes in terms of the Stokes number. The analysis shows that scavenging of aerosol particles of diameter larger than 0.2 μm by snow crystals larger than 1 mm in diameter is controlled by inertial capture. This mechanism plays only a very minor role in the capture of aerosol particles by snow flakes.