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Ice nucleating properties of soot, Kaolinite, & Goethite at conditions relevant for the lower troposphere

University of British Columbia

Although soot and mineral dust are both ubiquitous in the Earth's atmosphere, their contribution to the formation of the ice phase in lower tropospheric clouds remains poorly understood. This thesis investigates the ice nucleating properties of a range of soot and mineral dust species in the deposition mode - below liquid water saturation - and at temperatures ranging from 239 to 258 K. It is found that under these experimental conditions soot exhibits no ice nucleating abilities whereas Kaolinite and Goethite are found to be effective ice nuclei. In experiments involving soot, ice nucleation was only observed at ~ 243 K on a few occasions. However, even at these temperatures the relative humidity with respect to ice (RH[sub i]) was close to water saturation when ice nucleation was observed, suggesting water nucleation may have occurred first followed by ice nucleation during the condensation process. The ice nucleating abilities of soot exposed to atmospherically-relevant quantities of ozone were also investigated. Even after an exposure equivalent to 80 ppb O3 at atmospheric pressure for 13.7 days, soot exhibited poor ice nucleating abilities. In the case of Kaolinite and Goethite, ice particles consistently formed below ~ 252 K and at RH[sub i] well below water supersaturation, suggesting deposition nucleation is the dominant mode of ice formation under these experimental conditions. The ice nucleating abilities of soot and mineral dust were quantified by determining the heterogeneous nucleation rate coefficient, J[sub het]; heterogeneous nucleation theory was employed to determine the contact angle for formation of an ice germ on the surface of soot, Kaolinite, and Goethite. The experimentally-determined J[sub het] was used to estimate the number density of ice particles that might form under given conditions of temperature, RH[sub i], and particle number density for soot and mineral dust in the atmosphere. Whereas soot alone cannot account for the number density of ice nuclei observed in field studies, these measurements show Kaolinite and Goethite may significantly contribution to the formation of the ice phase in lower tropospheric clouds. This is consistent with recent field measurements and some recent laboratory data.

Text

2010-01-05

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http://hdl.handle.net/2429/17515

Chemistry

University of British Columbia

UBCV

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