Redefining Deposition Nucleation as Pore Condensation and Freezing

Deposition nucleation is conventionally described as the formation of ice directly from the vapor phase, without the prior formation of a bulk liquid water phase. However, water is capable of condensing in narrow pores and cavities well below water saturation as predicted by the Kelvin effect. Therefore, it is difficult to rule out the possibility that condensed water in pores is responsible for ice nucleation in conditions below water saturation (Marcolli, 2014). In the atmosphere, porous particles are prevalent; they contribute to the largest fraction of airborne dust and are associated with anthropogenic aerosols like soot. As such, understanding the role of pores on ice nucleation is critical for understanding and predicting ice formation globally.

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.