Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions

Dust particles coated with soluble materials, such as sulfate, are frequently observed in the Mediterranean. Thus far, the processes responsible for the sulfate coating of dust particles have still not been identified. One possible explanation is that the formation of the sulfate-coated aerosols is related to cloud processing of dust particles. In this process the scavenging of aerosol particles and gases, such as SO2, O-3, and H2O2, by the droplets and the subsequent impaction scavenging of mineral dust particles followed by evaporation could release into the atmosphere dust particles coated with soluble materials. These modified particles can serve as giant cloud condensation nuclei and thus can have a significant impact on the microphysical development of other clouds. Using an air parcel model with detailed microphysics, it is shown that cloud processing of dust particles is a possible effective pathway to form soluble coatings on these particles. Furthermore, the simulations show that after one or two cycles of particles through convective clouds the contribution of gas uptake by drops and subsequent liquid phase oxidation add considerable mass of soluble material to particles in the size range of 0.05 mu m. On the other hand, this process adds about 1 order of magnitude less mass to the larger particles as compared to the contribution made by coagulation of drops containing soluble aerosols.