HR: 17:30h
AN: A54B-07    [Abstracts]
TI: Heterogeneous Nucleation of Ice on Mineral Dust Studied With a New Laboratory Technique
AU: * Knopf, D A
EM: knopf@chem.ubc.ca
AF: Institute for Atmospheric and Climate Science, IACETH, ETH Zurich, ETH-Hoenggerberg HPP, Zurich, ZH 8093 Switzerland
AU: Koop, T
EM: thomas.koop@uni-bielefeld.de
AF: Institute for Atmospheric and Climate Science, IACETH, ETH Zurich, ETH-Hoenggerberg HPP, Zurich, ZH 8093 Switzerland
AB: Mineral dust particles may contribute significantly to the indirect aerosol effect by inducing the formation of cirrus ice clouds. Recently, the NASA-CRYSTALFACE campaign has shown that mineral dust particles emerging from the Saharian desert can act as heterogeneous ice nuclei. Using a new experimental technique, we present the results of a study of the heterogeneous nucleation of ice for the following particle systems: mineral dust particles, effloresced (NH$_4$)$_2$SO$_4$/H$_2$O-particles containing mineral dust particles, pure solid (NH$_4$)$_2$SO$_4$/H$_2$O-particles, and H$_2$SO$_4$-coated mineral dust particles.\The new experimental apparatus allows us to control the temperature of the particles between 190--300 K and to adjust the relative humidity (RH) between 0 % and 100 % with respect to water. The setup is designed in a way so that supersaturation with respect to ice can be achieved. The instrument is calibrated by observing known phase transitions and melting points of crystals. The phase changes of the aerosol particles are observed by optical microscopy and Raman spectroscopy.\The experiments show that the investigated particle systems are very efficient ice nuclei, inducing ice formation between 100--115 % RH with respect to ice in a temperature range of 197--240 K. Below 240 K only deposition mode freezing is observed, i.\ e.\ the formation of ice directly from the gas phase. \The experiments show that mineral dust particles can be preactivated, i.\ e.\ nucleation of ice is facilitated if the particle was previously involved in ice nucleation. Mineral dust particles in the preactivated state nucleate ice at 10--30 % lower RH compared to the non-preactivated ones.\The experimental results suggest another nucleation pathway for the formation of cirrus ice clouds. These findings may explain the low RH-values observed for the onset of upper tropospheric cloud formation in field studies.
DE: 4801 Aerosols (0305)
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting