Laboratory studies of ice formation via deposition mode nucleation onto mineral dust and n-hexane soot samples

Laboratory studies are described whereby the heterogeneous ice nucleating ability of various dust samples was studied for particles mounted on a hydrophobic cold stage. Ice formation is observed using digital photography. The relative humidity with respect to ice (RHi) and temperature conditions of the flow system are validated by observing (NH4)(2)SO4 deliquescence. Four types of solid mineral samples, including authentic Saharan dust and commercial samples of alumina, silica, and montmorillonite, were investigated in the deposition freezing mode. The size of the dust particles ranged from 0.5 to 5 mu m, and the temperature range was from 263 to 218 K. With roughly 10 4 particles present on the cold stage, the onset for ice formation was observed at low RHs, between 102 and 108% RHi, for all samples and temperatures. This indicates that deposition mode nucleation is an efficient mode of ice formation, particularly under the cold temperatures prevalent in the cirrus regime. By contrast, ice deposition onto n-hexane soot particles was not nearly so efficient. Nucleation rates are calculated as a function of RHi from experiments conducted with Saharan dust, where we measured the dependence of the onset RHi on total dust surface area.