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Abstract:
The importance of wind-blown mineral dust for cloud droplet formation is studied by considering i) the adsorption of water on the surface of insoluble particles, ii) the particle coating by soluble material (due to atmospheric aging) which augments cloud condensation nuclei (CCN) activity, and iii) the effect of dust on inorganic aerosol concentrations through thermodynamic interactions with mineral cations. The ECHAM5/MESSy Atmospheric Chemistry (EMAC) model is used to simulate the composition of global atmospheric aerosol; the ISORROPIA-II thermodynamic equilibrium model treats the interactions of K+-Ca2+-Mg2+-NH4+-Na+-SO42−-NO3−-Cl−-H2O aerosol with gas-phase inorganic constituents. Dust is considered a mixture of inert material with reactive minerals; emissions are calculated online by taking into account the soil particle size distribution and chemical composition of different deserts worldwide. The impact of dust on droplet formation is treated through the "unified dust activation parameterization" that considers the inherent hydrophilicity from adsorption and acquired hygroscopicity from soluble salts during aging. Our simulations suggest that the presence of dust increases cloud droplet number concentrations (CDNC) over major deserts (e.g., up to 20 % over the Sahara and Taklimakan Deserts) and decreases CDNC over polluted areas (e.g., up to 10 % over southern Europe and 20 % over northeastern Asia). This leads to a global net decrease of CDNC by 11 %. The adsorption activation of insoluble aerosols and the mineral dust chemistry are shown to be equally important for the cloud droplet formation over the main desserts, e.g., by considering these effects CDNC increases by 20 % over the Sahara. Remote from deserts the application of adsorption theory is critically important since the increased water uptake by the large aged dust particles (i.e., due to the added hydrophilicity by the soluble coating) reduce the maximum supersaturation and thus the cloud droplet formation from the smaller anthropogenic particles (e.g., CDNC decreases by 10 % over southern Europe and 20 % over northeastern Asia by applying adsorption theory). The global average CDNC decreases by 10 % by considering adsorption activation, while changes are negligible when accounting for the mineral dust chemistry. Sensitivity simulations indicate that CDNC is also sensitive to the mineral dust mass and inherent hydrophilicity, and not to the chemical composition of the emitted dust.
