Measurements of ice nucleation by mineral dusts in the contact mode.

Formation of ice in Earth's atmosphere at temperatures above approximately -20 °C is one of the outstanding problems in cloud physics. Contact nucleation has been suggested as a possible mechanism for freezing at relatively high temperatures; some laboratory experiments have shown contact freezing activity at temperatures as high as -4 °C. We have investigated Arizona Test Dust and kaolinite as contact nuclei as a function of size and temperature and find that the fraction of submicron particles that are active as contact ice nuclei is less than 10-3 for -18 °C and greater. We also find that the different dusts are quite distinct in their effectiveness as contact nuclei; Arizona Test Dust catalyzed freezing in the contact mode at all mobility diameters we tested a -18 °C whereas kaolinite triggered freezing only for mobility diameters of 1000 and 500 nm at that temperature. [ABSTRACT FROM AUTHOR]