Contact freezing experiments of kaolinite particles with cloud droplets

Contact freezing experiments were conducted with the Collision Ice Nucleation Chamber (CLINCH) with cloud droplets of 26 mu m in diameter in a laminar flow. Size-selected kaolinite particles were used as ice nuclei with mobility equivalent diameters of 400 and 800 nm. The influence of the aerosol concentration and the particle size on the measured frozen fraction of droplets was investigated. A custom-made depolarization detector (Ice Optical Detector, IODE) was used to distinguish between liquid droplets and ice crystals. With this instrumentation, the frozen fraction of droplets was measured at temperatures down to 235 K. It was found that contact freezing is more efficient as compared to immersion freezing. An ice nuclei concentration higher than 100 cm(-3) (at our experimental conditions) is necessary to observe ice formation because of contact freezing. It was not possible to see the influence of the aerosol concentration, but it was found that particles of 800 nm in diameter were better ice nuclei compared to particles of 400 nm, with a difference in the onset freezing temperature of 7 degrees C (assuming a similar collision rate).