Heterogeneous ice nucleation: Exploring the transition from stochastic to singular freezing behavior
Heterogeneous ice nucleation, a primary pathway for ice formation in the atmosphere, has been described al- ternately as being stochastic, in direct analogy with homoge- neous nucleation, or singular, with ice nuclei initiating freez- ing at deterministic temperatures. We present an idealized, conceptual model to explore the transition between stochas- tic and singular ice nucleation. This “soccer ball” model treats particles as being covered with surface sites (patches of finite area) characterized by different nucleation barriers, but with each surface site following the stochastic nature of ice embryo formation. The model provides a phenomeno- logical explanation for seemingly contradictory experimen- tal results obtained in our research groups. Even with ice nucleation treated fundamentally as a stochastic process this process can be masked by the heterogeneity of surface prop- erties, as might be typical for realistic atmospheric particle populations. Full evaluation of the model findings will re- quire experiments with well characterized ice nucleating par- ticles and the ability to vary both temperature and waiting time for freezing.