Does the Homogeneous Ice Nucleation Initiate in the Bulk Volume or at the Surface of Super-Cooled Water Droplets? A Review

Gianni Santachiara,Franco Belosi

doi:10.4236/acs.2014.44058

Abstract

The formation of ice in clouds can occur through primary processes, either homogeneously or heterogeneously triggered by aerosol particles called ice nuclei, as well as through secondary processes. The homogeneous ice nucleation process involves only pure water or solution droplets. Homogeneous freezing is crucial for the microphysics in the formation of high-altitude cirrus and polar stratospheric clouds, and also in the glaciation of thunderclouds, at temperatures below about 235 K. Nucleation rates in supercooled water have been measured using different experimental techniques: expansion cloud chambers, water-in-oil emulsions, levitation methods, free falling droplets, supersonic nozzles, field measurements, and molecular dynamics simulations. An important question concerns the possibility that the nucleation process in supercooled water can occur not only in the interior volume of the droplet, but even at or close to its surface. Even if there is no conclusive evidence, the majority of experimental and theoretical results suggest that the contribution of surface nucleation increases with decreasing radius of the supercooled droplets, and the surface (or sub-surface) nucleation rate is prevalent for droplets with radius lower than about 5 μm. If homogeneous freezing initiates at the droplet surface, the freezing rate should depend on the droplet size, and even a slight contamination by molecules within the surface layer could hamper the rate of the nucleation process.

Highlights

The formation of ice in clouds can occur through primary processes (nucleation of ice from the liquid or waterHow to cite this paper: Santachiara, G. and Belosi, F. (2014) Does the Homogeneous Ice Nucleation Initiate in the Bulk Volume or at the Surface of Super-Cooled Water Droplets? A Review
The present paper focuses on a bounded issue, i.e. the problem of surface and/or volume homogeneous nucleation of supercooled water droplets
The microphysical model based on experimental measurements in the study of Kuhn et al [32] could explain the results of DeMott and Rogers [21], who obtained nucleation rates, based on volume nucleation, higher than values reported in experiments performed at the same temperature [17] [18] [20]

Summary

Introduction

The formation of ice in clouds can occur through primary processes There is a spread of over three orders of magnitude in the values obtained at 238 K [17] [18] [21] [22] For some time, it has been unclear whether these large discrepancies were due solely to systematic differences in the experimental techniques and/or errors, or to some deficiency in the classical volume-based approach. The possibility of a surface nucleation process is an important point, as this process would alter the concentration and size of the ice crystals produced, as compared to volume-only nucleation scenario This has important consequences for bulk parameterizations in numerical models, which typically employ temperature—and composition—dependent formulations for the homogeneous nucleation rate coefficient, based solely on the classical, volume approach [23] [24]. Any contribution from a surface-based process would alter the predicted characteristics of ice formation and growth, and in turn, the predicted cloud microphysical and radiative properties [25]

Theoretical Approach

Reported Data

Discussion