Ice nucleation in the contact mode: Temperature and size dependence for selected dusts
What initiates ice formation in the atmosphere for temperatures greater than about −20°C? The contact mode, in which an aerosol particle catalyzes freezing at the air-water interface, has been suggested as a possible candidate for primary ice formation at high temperatures; previous work has suggested that it is active at temperatures as much as five degrees higher than the immersion/condensation mode. We will discuss measurements of ice nucleation in the contact mode by Arizona Test Dust, kaolinite, and volcanic ash, which cast doubt upon contact nucleation as a pathway to primary ice production in the atmosphere at high temperatures. Our measurements show that less than one in a thousand Arizona Test Dust particles with a diameter of 1 micron catalyzes freezing of water in the contact mode at −18°C. Kaolinite and volcanic ash were less effective; in fact the only substance we have tested which has an efficiency approaching one in ten in the contact mode for any temperature greater than −20°C is Snomax™, a commercially available form of the bacteria,P. syringae Our measurements also suggest that the smallest Arizona Test Dust particles we investigated (electrical mobility diameter of 62.5 nm) are more effective as contact nuclei. Differences in the morphology of the dust as a function of size may be responsible for the difference.