Ice nucleating properties of bare and sulfuric acid coated mineral dust particles

Ice nucleation properties of different dust minerals coated with soluble material are not well understood. We determined the ice nucleation ability of bare and sulfuric acid coated mineral dust particles as a function of temperature (-25 to -35 degC) and relative humidity with respect to water (RHw; 75 to 110%). We examined five different mineral dust species: Arizona Test Dust (ATD), illite, montmorillonite, quartz and kaolinite, which were dry dispersed, size-selected at 150 nm and exposed to sulfuric acid vapors. The condensed sulfuric acid per particle was estimated from the cloud condensation nuclei measurements. The fraction of dust particles nucleating ice at various temperatures and RHw was determined using ice nucleation chamber. At water-subsaturated conditions, compared to bare dust particles, we found that only coated dust particles showed a reduction of ice nucleation ability. At water-supersaturated conditions, however, we did not observe the effect of coating among all dust species, i.e., the bare and coated dust particles had similar nucleating properties. These results suggest that chemical reactions between the dust surface and sulfuric acid vapor play an important role, and also indicate that these reactions do not impair the ice nucleation properties of dust particles permanently. We further examined the patterns of X-rays diffracted from bare and coated dust particles, and we observed that coated dust particles were amorphous compared to bare particles which were crystalline. This concludes that crystal structures play an important role in ice nucleation.