Parameterization of aerosol scavenging due to atmospheric ionization

Abstract

A new approach to parameterizing the modulation of aerosol scavenging by electric charges on particles and droplets gives improved accuracy and is applied over an extended range of droplet and particle radii relevant to cloud microphysical processes. The base level scavenging rates for small particles are dominated by diffusion and for large particles by intercept, weight, and flow effects. For charged particles encountering uncharged droplets, in all cases there is an increase in the scavenging rates, due to the image force. For dropletswith charges of opposite sign to those of the particle charge, the rates are further increased, due to the Coulomb force, whereas for droplet with charges of the same sign, the rates are decreased. Increases above the base level (electroscavenging) predominate for the larger particles and occur in the interior of clouds even when no space charge (net charge) is present. Decreases below the base level (electroantiscavenging) occur for same-sign charges with smaller particles. The rates for uncharged droplets are parameterized, and the effect of charges on the droplets then parameterized as a departure from those rates. The results are convenient for incorporation in models of clouds which include detailed microphysics, to model the electrically induced reductions and increases in cloud condensation nucleus and ice forming nucleus concentrations and size distributions and contact ice nucleation rates that affect coagulation and precipitation and cloud albedo. Implications for effects on weather and climate, due both to externally and internally induced variability in atmospheric ionization, are outlined.