Atmospheric Chemistry and Physics Time dependence of immersion freezing: an experimental study on size selected kaolinite particles

Abstract. The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets in-creased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freez-ing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher tempera-tures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a dis-tribution of contact angles is able to reproduce the ice nucle-ation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typi-cal mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of−1 K temperature shift for an increase in times scale by one order of magnitude. This suggests that temperature is more important than time.