Ice nucleation on mineral dust particles: Onset conditions, nucleation rates and contact angles

An optical microscope coupled to a flow cell was used to investigate the onset conditions for ice nucleation on five atmospherically relevant minerals at temperatures ranging from 233 to 246 K. Here we define the onset conditions as the humidity and temperature at which the first ice nucleation event was observed. Kaolinite and muscovite were found to be efficient ice nuclei in the deposition mode, requiring relative humidities with respect to ice (RHi) below 112% in order to initiate ice crystal formation. Quartz and calcite, by contrast, were poor ice nuclei, requiring relative humidities close to water saturation before ice crystals would form. Montmorillonite particles were efficient ice nuclei at temperatures below 241 K but were poor ice nuclei at higher temperatures. In several cases, there was a lack of quantitative agreement between our data and previously published work. This can be explained by several factors including the mineral source, the particle sizes, the surface area available for nucleation, and observation time. Heterogeneous nucleation rates (J(het)) were calculated from the measurements of the onset conditions (temperature and RHi) required from ice nucleation. The Jhet values were then used to calculate contact angles (theta) between the mineral substrates and an ice embryo using classical nucleation theory. The contact angles measured for kaolinite and muscovite ranged from 6 degrees to 12 degrees, whereas for quartz and calcite, the contact angles ranged from 25 degrees to 27 degrees. The reported Jhet and q values may allow for a more direct comparison between laboratory studies and can be used when modeling ice cloud formation in the atmosphere.