Electroscavenging in clouds with broad droplet size distributions and weak electrification

Abstract

Electrical, thermophoretic, diffusophoretic and gravitational forces have been included in a self-consistent way in new trajectory calculations of the scavenging of charged aerosol particles by cloud droplets. The important effect of the electrical force is a short-range attraction due to the interaction between the charge on the aerosol particle and the image charge that it induces on the droplet. It is stronger than phoretic forces with only a few tens of elementary charges on aerosol particles of radii 0.1 to 1.0 μm, interacting with cloud droplets of radii greater than 15 μm, and for relative humidity of order 98%. Under these conditions the electrical forces result in collision efficiencies typically an order of magnitude greater than that due to phoretic forces alone. The short-range attraction is insensitive to the sign or magnitude of the charge on the droplet, for larger droplets in weakly electrified clouds. Under typical conditions, aerosol particles arise from evaporation of charged droplets (evaporation residues) and retain the droplet charge, which decays with a time constant of order 15 min. During the time the charge is retained the electrically enhanced scavenging occurs. Some of the evaporation residues may act as ice-forming or condensation nuclei, so there are implications for contact ice nucleation, droplet size distributions, precipitation and cloud cover, as well as for cloud chemistry. We call this electrically enhanced scavenging electroscavenging, and it is strongest for broad droplet size distributions (extending past 15-μm radius). Applying calculated electroscavenging rates to charged evaporation residues, and assuming a fraction of them to have ice nucleation properties consistent with the limited measurements and theory available, we compare for illustrative purposes the rates of primary production of ice, in the form of droplets frozen by contact nucleation, with the concentrations of ice particles from deposition nucleation. Using a set of measured droplet size distributions that are broad or bimodal we find, for temperatures between 0 and −15 °C and in regions of clouds where mixing and evaporation are occurring, that the two production rates are of comparable magnitude.