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Abstract

Experiments within a large aerosol chamber lead to a better understanding of the formation processes of solid particles under simulated atmospheric conditions. In- vestigations of particle composition and associated changes under well controlled conditions accomplished with an aerosol composition mass spectrometer. Particles are separated from ambient gas using an aerodynamic lens and a di erentially pum- ped chamber system. The particles are evaporated and analyzed by a quadrupol mass spectrometer. Adiabatic cooling processes, which are observed in atmosphe- ric lee waves and which are responsible for the formation of cirrus clouds in the upper troposphere and polar stratospheric clouds (PSC's) in the stratosphere, can be simulated within the aerosol chamber by expansion cooling. During this process an ice coated wall produces conditions of super saturation for the aerosols with re- spect to ice and creates the possibility of homogeneous ice nucleation. Therefore, freezing experiments were performed with liquid binary H2SO4/H2O and ternary HNO3/H2SO4/H2O aerosol systems in a temperature range between 185 to 238 K. The onset of the induced phase transition during a lee wave simulation was measu- red and the corresponding ice saturation ratio at this moment was found to be 1; 65 at 189K for the binary system and 1; 62 at 184K for the ternary system. With these measurements the validity of model calculations for ice saturation ratios could be con rmed.