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Clouds composed of both ice particles and supercooled liquid water dropletsexist at temperatures above ~236 K. These mixed phase clouds, whichstrongly impact climate, are very sensitive to the presence of solidparticles that can catalyse freezing. In this paper we describe experimentsto determine the conditions at which the clay mineral kaolinite nucleatesice when immersed within water droplets. These are the first immersion modeexperiments in which the ice nucleating ability of kaolinite has beendetermined as a function of clay surface area, cooling rate and also atconstant temperatures. Water droplets containing a known amount of claymineral were supported on a hydrophobic surface and cooled at rates ofbetween 0.8 and 10 K min<sup>−1</sup> or held at constant sub-zero temperatures.The time and temperature at which individual 10–50 μm diameter dropletsfroze were determined by optical microscopy. For a cooling rate of 10 K min<sup>−1</sup>, themedian nucleation temperature of 10–40 μm diameterdroplets increased from close to the homogeneous nucleation limit (236 K) to240.8 ± 0.6 K as the concentration of kaolinite in the droplets wasincreased from 0.005 wt% to 1 wt%. This data shows that theprobability of freezing scales with surface area of the kaoliniteinclusions. We also show that at a constant temperature the number of liquiddroplets decreases exponentially as they freeze over time. The constantcooling rate experiments are consistent with the stochastic, singular andmodified singular descriptions of heterogeneous nucleation; however,freezing during cooling and at constant temperature can be reconciled bestwith the stochastic approach. We report temperature dependent nucleationrate coefficients (nucleation events per unit time per unit area) forkaolinite and present a general parameterisation for immersion nucleationwhich may be suitable for cloud modelling once nucleation by other importantice nucleating species is quantified in the future

Heterogeneous freezing of water droplets containing kaolinite particles