Effects of Image Charges on the Scavenging of Aerosol Particles by Cloud Droplets and on Droplet Charging and Possible Ice Nucleation Processes

Abstract

Previous calculations of the rate at which falling droplets in clouds collide with aerosols have led to the conclusion that except in thunderclouds any electrical charges on the aerosols or droplets have little effect on the collision rate. However, it had been assumed that the aerosols would have only a few elementary charges on them, whereas it is now known that at the tops of nonthunderstorm clouds the evaporating droplets may have several hundred elementary charges on them and that much of this charge remains on the residual aerosol for 5 min or so after the evaporation. Also, most previous calculations neglected image charge forces that provide strong attraction at close range even when droplet and aerosol have charges of the same sign and of comparable magnitude. The authors present numerical calculations showing that electrical effects dominate collision rates for charged evaporation aerosols. The calculations are for the size range of 0.1- to 1.0-?m radius with the collision efficiency compared to that for phoretic and Brownian effects being greater by up to a factor of 30 greater for droplets from 18.6- to 106-?m radius with relative humidity in the range 95%?100% and only 50 elementary charges on the aerosol. The results imply that electrical effects can be important for the scavenging of evaporation aerosol particles in the size range of the Greenfield gap. The authors call this process ?electroscavenging.? Electroscavenging of charged particles, when the particles are mostly of the same sign, is a previously unrecognized droplet charging process. Electroscavenging also provides a pathway for contact ice nucleation when charged aerosol particles from evaporated charged droplets collide with supercooled droplets. Ice nucleation can occur because aerosol particles from the evaporation of cloud droplets have been found to be more effective as ice forming nuclei than other aerosol particles that have not been processed through droplets.