Nucleation of water drops by Brownian contact with AgI and other aerosols

Aitken‐sized (0.01μm) particles of AgI, CuI, and ‘contaminated’ Ag 2 O were able to nucleate various sized water droplets in free fall (20–80μm in radius) by a direct, diffusive (Brownian) contact mechanism. The AgI aerosol was found to be the most efficient nucleator, showing a probability of 100 per cent at −16°C, 50 per cent at about −14°C, and 1 per cent at −11°C. These nucleation characteristics for AgI can be modelled from heterogeneous nucleation theory if the value of the contact parameter is assumed to be 0.70. Ag 2 O, passing through carrier tubing previously contaminated with iodine, was found to be only a slightly less efficient nucleating agent than AgI. CuI was found to have a nucleating efficiency corresponding to that which would theoretically be expected from an aerosol with a contact parameter of 0.55. Aerosols of the ‘pure’ oxides of silver, platinum, and chromium were unable to cause nucleation (to within detectable limits) at temperatures as low as −18°C. Auxiliary experiments indicated that the AgI aerosol had a small, but finite (10 −5 ), nucleating efficiency at −5°C. Aitken‐sized particles of AgI did not lose their nucleating ability when captured at temperatures higher than 0°C. It is postulated that contact nucleation by the Brownian capture of Aitken particles may be an important enough process in the atmosphere to contribute significantly to the large amounts of ice which have been observed at relatively high (−10°C) temperatures in some clouds.