Ice Nucleation by Aerosol Particles: Experimental Studies Using a Wedge-Shaped Ice Thermal Diffusion Chamber

The principle and feasibility of a new wedge-shaped ice thermal diffusion chamber are demonstrated and the method of estimating the supersaturation in the chamber is shown. Ice nucleation studies were carried out using the chamber and relatively large aerosol particles freely suspended in air. In the studies, the temperature and the supersaturation were independently controlled without substrate. All natural and artificial, organic and inorganic nuclei studied exhibited condensation-freezing as well as deposition modes of ice formation. Conditions for their occurrence and modes of their activation were identified. A forbidden zone for condensation-freezing nucleation above water saturation was found and its temperature dependency, and the extremely steep rise of nucleation rate when the supersaturation increased beyond it, were determined. A unique deposition nucleation behavior near but below water saturation was confirmed with freely suspended aerosol particles. In contrast with deposition and condensation-freezing nucleations, contact nucleation was identified as a slow and therefore long-lasting process except for metaldehyde. The contact nucleation rate of metaldehyde was found to be about 103 times larger than that of AgI. Our results reaffirmed that ice nucleation is a function of temperature as well as supersaturation.