CCN Ability of Asian Mineral Dust Particles and Their Effects on Cloud Droplet Formation

To understand the cloud condensation nuclei (CCN) ability of Asian mineral dust particles, we investigated the activation spectrum (fraction) and the hygroscopicity parameter kappa of the certified reference materials for Asian mineral dust (CAD) particles. The CAD particles were dispersed in dry conditions using a rotating brush generator and then measured with a cloud condensation nuclei counter and a condensation particle counter. The concentration ratios of CCN to condensation nuclei (CN) of the CAD particles were 0.7 at supersaturation (SS) = 0.2% and 0.8 at SS = 0.4%. This suggests that Asian mineral dust particles, by themselves, may effectively act as CCN. The mean hygroscopic parameter kappa of the CAD particles estimated from the relationship between the measured critical supersaturation and dry diameter was 0.014. Using this kappa-value, we simulated the change in the size distribution from CCN to droplets during adiabatic ascent by a parcel model. The simulation suggested that the CAD (low hygroscopicity) particles do not grow large enough to promote collision-coalescence in the early stage of cloud formation. The hygroscopic parameter kappa of internally mixed particles of CAD and ea salt was calculated by using the kappa-Kohler theory, and their activation and subsequent condensation growth processes during adiabatic ascent were also simulated with the parcel model. The simulation results suggest that the hygroscopicity of Asian mineral dust particles was considerably enhanced, and droplets activated on them grew much larger than those activated on pure mineral dust particles.