HR: 0800h
AN: A51E-0842    [Abstracts]
TI: Laboratory Investigations of Immersion Freezing in Concentrated Aqueous Solutions: the Role of Water Activity
AU: * Zobrist, B
EM: zobrist@env.ethz.ch
AF: Institute for atmospheric and climate science (IAC), ETH Zurich, Zurich, 8093 Switzerland
AU: Marcolli, C
EM: claudia.marcolli@env.ethz.ch
AF: Institute for atmospheric and climate science (IAC), ETH Zurich, Zurich, 8093 Switzerland
AU: Peter, T
EM: thomas.peter@ethz.ch
AF: Institute for atmospheric and climate science (IAC), ETH Zurich, Zurich, 8093 Switzerland
AU: Koop, T
EM: thomas.koop@uni-bielefeld.de
AF: Faculty of Chemistry, University of Bielefeld, Bielefeld, 33615 Germany
AB: Cirrus ice clouds have a large influence on the global radiative forcing and the dehydration of the upper troposphere and lower stratosphere. Efficient dehydration might occur through sedimentation of a few large ice particles initially formed by heterogeneous ice nucleation. At present, only a few types of ice nuclei have been investigated in laboratory studies at upper tropospheric conditions. In this study, immersion freezing in aqueous solution droplets was studied experimentally using a variety of ice nuclei. The investigated nuclei include inorganic amorphous or crystalline materials (AgI and SiO2 nano-particles), organic crystallites (dicarboxylic acids), as well as organic surfactants (nonadecanol). The nuclei were either suspended in the droplets or prepared as a monolayer at the droplet surface. Droplet sizes varied between ~5-1100 micrometer. Freezing was investigated in a temperature range of 180-273 K using either a differential scanning calorimeter or a cooling stage equipped with a CCD camera. The experiments revealed that nonadecanol monolayers and AgI nano-particles were effective ice nuclei in aqueous solutions. In contrast, SiO2 nano-particles led only to a very small increase of freezing temperatures when compared to the situation without added nuclei. Depending on dicarboxylic acid only a weak or no ice nucleation activation was found. The effect of solutes on the heterogeneous freezing points was very similar for all investigated nuclei, i.e., decreasing freezing temperatures with increasing solute concentration. Furthermore, the data were interpreted within a water activity based approach to ice nucleation. The analysis shows that, apart from nuclei specific parameters, water activity is an important parameter for describing heterogeneous ice nucleation in supercooled solution droplets. This shows the water activity based approach to be useful for parameterizing heterogeneous freezing in atmospheric applications.
DE: 0305 Aerosols and particles (0345, 4801)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting