Freezing of water droplets colliding with kaolinite particles
1Department of Chemistry, Atmospheric Science, University of Gothenburg, 41296 Gothenburg, Sweden
2Copenhagen Centre for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark
3EPFL STI IMT LOA, Station 17, 1015 Lausanne, Switzerland
Abstract. Contact freezing of single supercooled water droplets colliding with kaolinite dust particles has been investigated. The experiments were performed with droplets levitated in an electrodynamic balance at temperatures from 240 to 268 K. Under dry conditions freezing was observed to occur below 249 K, while a freezing threshold of 267 K was observed at high relative humidity. The effect of relative humidity is attributed to an influence on the contact freezing process for the kaolinite-water droplet system, and it is not related to the lifetime of the droplets in the electrodynamic balance. Freezing probabilities per collision were derived assuming that collisions at the lowest temperature employed had a probability of unity. The data recorded at high humidity should be most relevant to atmospheric conditions, and the results indicate that parameterizations currently used in modelling studies to describe freezing rates are appropriate for kaolinite aerosol particles. Mechanisms for contact freezing are briefly discussed.