Heterogeneous freezing of water droplets containing kaolinite particles
Clouds composed of both ice particles and supercooled liquid water droplets exist at temperatures above ∼236 K. These mixed phase clouds, which strongly impact climate, are very sensitive to the presence of solid particles that can catalyse freezing. In this 5 paper we describe experiments to determine the rate at which kaolinite and montmoril- lonite nucleate ice when immersed within water droplets. These are the first immersion mode experiments in which the ice nucleating ability of individual minerals has been determined quantitatively. Water droplets containing a known amount of clay mineral were supported on a hydrophobic surface and cooled at a rate of 10Kmin−1. The 10 temperatures at which individual 10–40µm diameter droplets froze were determined by optical microscopy. As the concentration of kaolinite in the droplets was increased from 0.005wt% to 1wt% the median nucleation temperature increased from close to the homogeneous nucleation limit (236 K) to 240.8±0.6 K. We go onto show that the probability of freezing scales with surface area of the kaolinite inclusions rather than, 15 as is often assumed, the volume of the droplet. When droplets contained montmoril- lonite ice always nucleated at 245.8±0.6 K, independent of the mineral concentration. We report temperature dependent nucleation rates and present parameterisations for nucleation by these minerals which capture the surface area and cooling rate depen- dence of the nucleation rate. We show that our parameterisations produce significantly 20 different results to parameterisations employed in global models. These results also highlight the importance of understanding the ice nucleating properties of individual minerals rather than complex mixtures of minerals found in natural dusts and so-called test dusts.