The effect of cloud-processing of aerosol particles on clouds and radiation

A detailed spectral microphysics and scavenging model coupled to a dynamic framework describing a medium-sized convective cloud has been used to simulate the evolution of different clouds forming precipitation sized drops in a marine air mass. The resulting drop spectra entered a radiation code to yield the up- and downdwelling radiative fluxes, the cloud optical depth and the cloud albedo. If we start from the scenario that in a perturbed marine environment the number of small aerosol particles has doubled, this can increase the albedo of a cloud forming in this air mass of about 5% with respect to clouds forming in unperturbed conditions. This capacity to increase the cloud albedo, however, is not persistent. The cloud itself changes the particle spectrum. The smallest aerosol particles are reduced by impaction scavenging. The particles between 0.01 μm and 0.1 μm are depleted due to the fact that they serve as CCN and grow through in-cloud processes. Here, our studies have shown, however, that growth due to absorption and oxidation of gases (e.g., SO₂) plays a minor róle and that collision and coalescence of drops is the dominant growth mechanism in a region with low gas concentrations like the remote oceans. As a result, an aerosol particle spectrum which has gained small particles due to an enhanced production of new particles seems to relax back to a spectrum similar to the previous undisturbed after cycling through some repeated cloud events.