Experimental study on the ice nucleation ability of size-selected kaolinite particles in the immersion mode

The recently developed immersion mode cooling chamber has been used as an extension of the Zurich ice nucleation chamber (ZINC) in order to measure the ice nucleation efficiency of size-selected kaolinite particles in the immersion mode. Particles with selected diameters of 200, 400, and 800 nm have been activated as cloud condensation nuclei in order to obtain water droplets with single immersed particles. After continuous cooling of the droplets to the experimental temperature in ZINC, the frozen fraction of the droplets was measured with a recently developed depolarization detector, the ice optical detector (IODE). Temperatures below -30 degrees C were necessary to freeze 50% of the droplets throughout the investigated size range. Although not very strong, a size dependence of the freezing efficiency has been observed. The median freezing temperature increases from -35 degrees C for 200 nm kaolinite particles to -33 degrees C for 800 nm particles. An experiment with 200 nm ammonium sulfate particles in the same temperature range resulted in no significant frozen fraction of the droplets. This suggests that the ice crystals observed in experiments with kaolinite particles nucleated heterogeneously. The temperature-dependent frozen fraction of water droplets has been fitted with different theoretical models. Comparison of the resulting fit curves with the data suggests that including the concept of active sites in the description of the ice nucleus surface is more appropriate than the approximation of a constant contact angle.