Electric charge modulation of aerosol scavenging in clouds: Rate coefficients with Monte Carlo simulation of diffusion

Abstract

The effects of electric charge on the scavenging by droplets of aerosol particles, particularly cloud condensation nuclei, appear to have a significant effect on cloud properties and climate. The effects on scavenging are calculated here in a trajectory model relative to the falling droplets, with a Monte Carlo simulation of particle diffusion along the trajectory. The charges are in the range 5-100 elementary charges, consistent with the measured and modeled deposition of small amounts of charge by the flow everywhere of atmospheric electric current density (part of the global circuit) through clouds. Particles move through a corrected Stokes flow field, under the influence of gravitational, phoretic, electric, and drag forces while diffusing. For the general situation of like charges on droplets and aerosol particles, the model simulates the long-range electrical repulsive force that has a net effect of reducing the collision rates for small particles (radii less than about 0.1 μm) below the values for combined Brownian diffusion and phoretic forces. It also simulates the net effect for larger particles of the short-range attractive electric forces in increasing the rate coefficients above the Brownian and phoretic values. Initial validation is provided by treating the case of ventilation in pure Brownian diffusion, for which new and more accurate ventilation coefficients are obtained as a function of the Pclet number. These are applicable to combined diffusion and phoretic scavenging with pure inverse square scavenging forces. The rate constants obtained for the electrical effects, comparable to those for Brownian and phoretic scavenging process, lead to time scales for scavenging in clouds of a few hours, consistent with observations of significant electrical effects on clouds associated with modulation of the global circuit. Copyright 2010 by the American Geophysical Union.