Thermodynamic and Kinetic controls to the ice nucleation rate

Abstract

<p>Ice nucleation is a necessary step for the formation of ice clouds in the atmosphere. It has become clear that its correct representation is critical for the accurate description of atmospheric processes, and for the reliable prediction of the effect of anthropogenic activities on climate. This is accomplished in most cases using empirical models. Although a simple way to parameterize ice nucleation they provide limited information on the nature of ice formation and may not represent all atmospheric conditions.  Theoretical approaches used in cloud models are typically based on the Classical Nucleation Theory (CNT).  There is however a large uncertainty in key parameters of the theory which in most cases are fitted to reproduce observed rates. This talk details recent efforts to go beyond the formulation of CNT to describe ice nucleation. It shows that it is possible to define uncertain parameters like the ice-liquid interfacial tension and the activation energy over a pure thermodynamic basis, hence only as a function of the bulk thermodynamic properties of water. This approach is extended to describe heterogeneous ice nucleation mediated by immersed ice nucleating particles (INPs). It is shown that INPs that significantly reduce the work of ice nucleation also pose strong limitations to the growth of the nascent ice germs. This leads to the onset of a new ice nucleation regime, called spinodal ice nucleation, where the dynamics of ice germ growth instead of the ice germ size determines the nucleation rate. Nucleation in this regime is characterized by an enhanced sensitivity to particle area and cooling rate. Finally a new approach to extract intrinsic nucleation rates from droplet-freezing experiments is used to compare of predicted ice nucleation rates against experimental measurements, for a diverse set of species relevant to cloud formation. This comparison suggests that spinodal ice nucleation may be common in nature, and shows a considerable skill of the new theory in predicting measured ice nucleation rates. </p>