Relationship of Size and Activity for AgI Smoke Particles

Abstract

The ability of AgI particles to nucleate ice by the freezing mechanism was measured with the Goetz aerosol centrifuge as a function of particle size, temperature and time. When the cooled centrifuge deposit was exposed to a puff of supersaturated nitrogen, each particle was immersed in a supercooled droplet which, if it contained an active AgI particle, froze and formed a visible ice crystal. The fraction of active particles was found by comparing the number of ice crystals with the size distribution of all the particles on the deposit. The mean nucleation rate per particle depended on particle size for a given temperature, which suggested that the surface of the particles was smooth with respect to the size of the critical ice embryos. The measured rates were about a factor of 1014 ± 16 smaller than predicted by Fletcher's theory. Activity spectra, calculated from the measured rates by numerically integrating the activities of all the particles in log-normal size distributions, showed the importance of the nucleation time lag at temperatures warmer than about ?12°C or when the AgI generator produces very small particles.