Influence of dust composition on cloud droplet formation

Previous studies suggest that interactions between dust particles and clouds are significant; yet the conditions where dust particles can serve as cloud condensation nuclei (CCN) are uncertain. Since major dust components are insoluble, the CCN activity of dust strongly depends on the presence of minor components. However, many minor components measured in dust particles are overlooked in cloud modeling studies. Some of these compounds are believed to be products of heterogeneous reactions involving carbonates. In this study, we calculate Kohler curves (modified for slightly soluble substances) for dust particles containing small amounts of K+, Mg2+, or Ca2+ compounds to estimate the conditions where reacted and unreacted dust can activate. We also use an adiabatic parcel model to evaluate the influence of dust particles on cloud properties via water competition. Based on their bulk solubilities, K+ compounds, MgSO4•7H2O, Mg(NO3)2•6H2O, and Ca(NO3)2•4H2O are classified as highly soluble substances, which enable activation of fine dust. Slightly soluble gypsum and MgSO3•6H2O, which may form via heterogeneous reactions involving carbonates, enable activation of particles with diameters between about 0.6 and 2 μm under some conditions. Dust particles>2 μm often activate regardless of their composition. Only under very specialized conditions does the addition of a dust distribution into a rising parcel containing fine (NH4)2SO4 particles significantly reduce the total number of activated particles via water competition. Effects of dust on cloud saturation and droplet number via water competition are generally smaller than those reported previously for sea salt. Large numbers of fine dust CCN can significantly enhance the number of activated particles under certain conditions. Improved representations of dust mineralogy and reactions in global aerosol models could improve predictions of the effects of aerosol on climate.