Clouds out of Pores: Redefining Deposition Nucleation

Cirrus clouds play an important role in modulating the Earth’s radiation budget and subsequently impact climate. The amount of heating that cirrus clouds contribute to the climate system depends on their height, thickness and the concentration and size of the ice crystals within them. There are two pathways for ice crystal formation in the atmosphere, either homogeneously or heterogeneously. Homogeneous freezing occurs when supercooled cloud droplets or solution droplets cool to temperatures below -38 ˚C and freeze. In contrast, heterogeneous freezing involves an ice nucleating particle which acts to lower the energy required for the formation of ice. For cirrus clouds, heterogeneous freezing can either occur when an ice nucleating particle is immersed/coated by a solution or through deposition nucleation. Deposition nucleation is defined as the direct transition of water molecules from the gas phase to the ice phase without the involvement of the bulk liquid water phase. Thus, deposition nucleation occurs in regions of air that are subsaturated with respect to water and supersaturated with respect to ice. However, it is possible for liquid water to exist in narrow cavities or pores below water saturation due to the Kelvin effect, questioning the occurrence of deposition nucleation. Indeed, recent studies have shown that the freezing efficiency of particles associated with cirrus cloud formation have a strong dependence on the homogeneous freezing temperature of water even at conditions below water saturation.

To test the role of water condensed in narrow pores on ice nucleation occurring at relative humidities below water saturation, we exposed nonporous and mesoporous silica with well-defined pore diameters to varying temperatures and supersaturations with respect to ice in a cloud chamber. The porous samples showed 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 based on the Kelvin effect and subsequent freezing. Thus, the results indicate that water condensed in pores is likely responsible for ice nucleation below water saturation. Ultimately, rendering deposition nucleation irrelevant for cirrus cloud formation.

With the new understanding that pores play an important role in cirrus cloud formation, we implement a new parametrization for cirrus cloud formation in the Community Earth System Model.