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 5 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 temper10
ature 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.