To test the role of pores on ice nucleation, we exposed mesoporous silica with well-defined pore diameters ranging from 2.3 to 9 nm to varying temperatures and supersaturations with respect to ice in the Zurich Ice Nucleation Chamber (Stetzer et al., 2008). The porous samples have an enhanced freezing behavior relative to nonporous samples, which is not reconcilable with deposition nucleation occurring as a direct transition from water vapor to ice. Furthermore, particle batches were synthesized with different concentrations of hydroxyl and trimethylsilyl groups, effectively altering the contact angle of the particle surface with respect to water. When accounting for contact angle and pore diameter, the onset relative humidity required for ice nucleation was consistent with the humidity predicted for pore filling and subsequent freezing. Thus, the results indicate that pore condensation and freezing is likely the mechanism responsible for ice nucleation below water saturation. Ultimately, rendering deposition nucleation obsolete for atmospherically relevant porous particles like dust and soot.
Marcolli, C.: Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities, Atmos Chem Phys, 14(4), 2071–2104, doi:10.5194/acp-14-2071-2014, 2014.
Stetzer, O., Baschek, B., Lüönd, F. and Lohmann, U.: The Zurich Ice Nucleation Chamber (ZINC)-A New Instrument to Investigate Atmospheric Ice Formation, Aerosol Sci. Technol., 42(1), 64–74, doi:10.1080/02786820701787944, 2008.