Effects of sulfate coatings on the ice nucleation properties of a biological ice nucleus and several types of minerals

An optical microscope coupled to a flow cell was used to study the ice nucleation properties of uncoated and coated mineral dust and SNOMAX (a proxy for biological ice nucleators made from cells of Pseudomonas syringae) at temperatures ranging from 234 to 247 K. We define the onset conditions as the relative humidity (RH) and temperature at which the first ice nucleation event was observed. The results show that H2SO4 coatings modified the ice nucleation properties of all the minerals studied. For kaolinite and illite, the acid coatings increased the RH over ice (RHi) required for ice nucleation by similar to 30% RHi; for montmorillonite and quartz, the acid coatings increased the RHi by similar to 20% RHi. NH4HSO4 coatings also influenced the ice nucleation properties of kaolinite particles. In addition, our results indicate that SNOMAX is a reasonably good ice nucleus, having onset values between 110 to 120% RHi. In contrast to the mineral studies, sulfuric acid coatings did not hinder the ice nucleating ability of SNOMAX particles. Combined, our results support the idea that anthropogenic emissions of SO2 and NH3 may influence the ice-nucleating properties of mineral dust particles. From our laboratory data, we also determined contact angles (theta) between the heterogeneous nuclei and ice embryos according to classical nucleation theory to parameterize the laboratory data for inclusion in atmospheric models. The data show that for uncoated ice nuclei the contact angles are small (below similar to 20 degrees), but for mineral particles coated with sulfuric acid, the contact angles are larger (above similar to 60 degrees).