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

The onset conditions for ice nucleation on H2SO4 coated, (NH4)(2)SO4 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 (RHi) was below 110% at all temperatures studied, consistent with previous measurements. H2SO4 coatings, however, drastically altered the ice nucleating ability of kaolinite particles, increasing the RHi required for ice nucleation by approximately 30%, similar to the recent measurements by Mohler et al. [2008b]. (NH4)(2)SO4 coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K. The differences between H2SO4 and (NH4)(2)SO4 coatings may be explained by the deliquescence and efflorescence properties of (NH4)(2)SO4. These results support the idea that emissions of SO2 and NH3 may influence the ice nucleating properties of mineral dust particles. Citation: Eastwood, M. L., S. Cremel, M. Wheeler, B. J. Murray, E. Girard, and A. K. Bertram (2009), Effects of sulfuric acid and ammonium sulfate coatings on the ice nucleation properties of kaolinite particles, Geophys. Res. Lett., 36, L02811, doi: 10.1029/2008GL035997.