Parameterization of In-Cloud Aerosol Scavenging Due to Atmospheric Ionization: Part 3. Effects of Varying Droplet Radius

摘要

A new set of simulations of collision rate coefficients for aerosol particles with droplets (in-cloud scavenging) extends to a droplet radius of 15 μm, with short-range effects on collisions treated as an increment of droplet effective collision radius (ECR). The simulations are made with our Monte Carlo trajectory model for midtropospheric (~5 km) conditions. The use of ECR allows the results to be expressed as a modified analytic solution (MAS) for the rate coefficients, incorporating both long-range and short-range effects. For smaller particles with radius less than 0.2 μm, the increment of ECR is only caused by the particle charge inducing a short-range image electric force, and results of the MAS match quite well that of our trajectory model. For larger particles, the short-range effects become complex, including particle size effects (weight, intercept, and flow around the particle), with image forces induced by both droplet and particle charges. An ECR based on these factors gives average errors between the MAS and the trajectory model of less than 10%, with smaller errors for larger droplets. We have now completed the parameterizations of rate coefficients for midtropospheric conditions that can be applied to cloud models. One application would be to test whether electrically induced changes in scavenging rates, which have been proposed to significantly affect cloud processes dependent on condensation nucleus concentration and contact ice nucleation, can explain observed cloud responses to changes in current flow in the global electric circuit.