Effects of sulfuric acid and ammonium sulfate coatings on the ice nucleation properties of kaolinite particles

The onset conditions for ice nucleation on H 2 SO 4 coated, (NH 4 ) 2 SO 4 coated, and uncoated kaolinite particles at temperatures ranging from 233 to 246 K were studied. We define the onset conditions as the relative humidity and temperature at which the first ice nucleation event was observed. Uncoated particles were excellent ice nuclei; the onset relative humidity with respect to ice (RH i ) was below 110% at all temperatures studied, consistent with previous measurements. H 2 SO 4 coatings, however, drastically altered the ice nucleating ability of kaolinite particles, increasing the RH i required for ice nucleation by approximately 30%, similar to the recent measurements by Möhler et al. [2008b]. (NH 4 ) 2 SO 4 coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K. The differences between H 2 SO 4 and (NH 4 ) 2 SO 4 coatings may be explained by the deliquescence and efflorescence properties of (NH 4 ) 2 SO 4 . These results support the idea that emissions of SO 2 and NH 3 may influence the ice nucleating properties of mineral dust particles.