Experimental Study on the Contact Freezing of Supercooled Micro-Droplets in Electrodynamic Balance

The aim of this work, was to investigate the heterogeneous ice nucleation of supercooled micro-droplets induced by contact with an ice nucleating particle (INP). The contact freezing is poorly quantified but can be potentially important for understanding precipitation mechanisms via cloud glaciation in mixed clouds. The contact freezing results are compared to results of freezing experiments in the immersion mode. The same INP is ice active at higher temperatures in the contact mode than in the immersion mode. An electrodynamic balance was used to levitate single, supercooled micro-droplets of deionized water. The droplet is exposed to a flow of mobility-selected aerosol particles. The injection of droplets and the detection of freezing events is automated. In this experiment, it is possible to calculate the collision rate separately, and the probability of freezing on a single contact. Furthermore, immersion freezing experiments are performed in the same experimental setup, and a direct comparison between immersion and contact mode is possible. As ice nucleating particles, some relevant atmospheric minerals (hematite, feldspar, illite, kaolinite), cellulose, SNOMAX®, birch pollen washing water and a mixture of illite and birch pollen washing water have been investigated. The resulting contact freezing probabilities show a temperature, particle size, and material dependency. For comparison of immersion and contact freezing results an Ice Nucleating Active Site (INAS) density has been calculated. With exclusion of SNOMAX® and birch pollen washing water, all particles have been shown to be more ice active in the contact freezing mode than in immersion mode