A parametric investigation of electrical effects on aerosol scavenging by droplets over large fires

Abstract

Our nucleation model of strong up-drafts over large fires (such as those ignited by nuclear blasts) show that many droplets in the size range of 1–10 μm are rapidly formed when the cloud becomes supersaturated. In this study we calculate aerosol/droplet collection kernels for electrical parameters commonly seen in cloud air. These kernels, which do not include Brownian diffusion, are used with aged droplet distributions to determine instantaneous scavenging rates of unwettable aerosols with cloud droplets. We find that within a portion of this parameter space the aerosol may be removed from the cloud air with time constants substantially below 1 h over the entire aerosol size range of 0.025–0.5 μm. For much of the parameter space, E-field enhanced attachment of charged aerosol is more rapid than that for Brownian diffusion. The aging of the droplet distribution incorporates the detailed microphysics of nucleation and condensation in a Lagrangian parcel with forced up-draft of 50 m s −1. We assume an initial aerosol distribution that is lognormal about a median radius of 0.05 μm with a geometric standard deviation of 2. Initially there are 10 6 insoluble aerosol particles per cubic centimeter of air with one-half of them being wettable and the other half being non-wettable. We present our results as a set of curves showing collection kernels and instantaneous scavenging rates vs aerosol radius and electric field strengths. The collection kernel curves are parametrically displayed for droplets of radii 1, 3 and 10 μm and electrical parameters typical of cumulus and thunderstorm clouds. The instantaneous scavenging curve vs aerosol radius is displayed as a function of electrical parameters.