[Zurück]

T.M. Seifried, P. Baloh, F. Weiss, H. Grothe:
"Nucleation and Morphology of Ices";
Poster: 30/80 Meeting, Chemobrionics COST meeting, Granada, Spain; 11.03.2019 - 13.03.2019.

For the Earth's climate system ice clouds are of major importance. On the one hand they can act as a cooling factor by reflecting parts of the solar radiation, and on the other hand they can cause warming by absorbing solar radiation and by trapping the outgoing blackbody radiation. These effects are not well understood for making exact predictions about the overall radiation balance of the Earth. In fact, clouds and aerosol particles are known as one of the largest uncertainties of the radiation balance in the report of the International Panel on Climate Change IPCC.Here, water-oil emulsions were used to mimiccloud droplets and their freezing behavior. Aqueous droplets containing citric acid as a odel compound for carboxylic acids in cirrus clouds. Different concentrations of citric acid were prepared and observed using the following methods: a) X-Ray Diffraction for the phase composition, b) Environmental Scanning Electron Microscopy (ESEM) for the morphologies of the samples, c) Raman Spectroscopy for the chemical compositions of the droplets.Citric acid alters the freezing behavior of water in a way that Cubic Ice Ic (i.e. stacking faults in the hexagonal ice structure Ih) crystallizes. Ic occurs at elevated citric acid concentrations at the expense of Ih. At very high concentrations the droplets do not crystallize and stay amorphous. This is interesting asit was not expected that Ic can form under these conditions. Similar experiments were also carried out with malonic, succinic, glutaric, and tartaric acid. Even though none of these carboxylic acids occur in the atmosphere in relevant amounts, predictionsfor the influence of carboxylic acids in general can be derived.

https://publik.tuwien.ac.at/files/publik_279623.pdf