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

Abstract

SUMMARYAitken-sized (0.01pm) particles of AgI, CuI, and 'contaminated' Ag,O were able to nucleate various sized water droplets in free fall (20-80@ni 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,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 --1S"C. Auxiliary experiments indicated that the AgI aerosol had a small, but finite 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 postukated 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.

Ice nucleation efficiency of AgI: review and new insights

“…The analysis of immersion freezing studies in Fig. 2 suggests that freezing temperatures are higher for those studies where the particle is free to take a position on the surface of the droplet, as is the case for experiments performed with IMCA-ZINC or with a cloud chamber, as in the study by Sax and Goldsmith (1972).…”

“…Assuming a collision efficiency of ca. 0.3, around 100 particles would be captured by the droplet (note that this number is higher than the collection of only 1 particle estimated by Sax and Goldsmith, 1972). For immersion freezing experiments, the droplets passed the aerosol stream at T > 273 K before they were cooled to the target temperature.…”

“…At 98 % RH and temperatures of 262, 258 and 255 K, a small fraction of particles (< 0.005) was active in deposition mode. Sax and Goldsmith (1972) performed immersion and contact freezing experiments in a cloud chamber (see Tables 1 and 2). Results are shown as filled and open blue diamonds in Fig.…”

“…If ice nucleation on dissolving AgI particles is reduced, the experiments performed by Sax and Goldsmith (1972) should also be affected by dissolution because in these experiments the residence time in the chamber was on the same timescale as dissolution (about 4 s).…”

Contact freezing: a review of experimental studies

“…Phys., 13, 9745-9769, 2013 www.atmos-chem-phys.net/13/9745/2013/ drops. Langer et al (1978) determined the freezing probabilities of AgI particles using the mathematical expression from Sax and Goldsmith (1972). They found that the freezing probabilities increased with decreasing temperature from 259 K to 253 K. In addition, an aerosol particle size effect was clearly observed.…”