Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 2 2004 10.5194/acpd-4-1887-2004 http://www.atmos-chem-phys-discuss.net/4/1887/2004/ http://www.atmos-chem-phys-discuss.net/4/1887/2004/acpd-4-1887-2004.html http://www.atmos-chem-phys-discuss.net/4/1887/2004/acpd-4-1887-2004.pdf 1887 1909 2004-03-25 Heterogeneous freezing of single sulphuric acid solution droplets: laboratory experiments utilising an acoustic levitator M. Ettner ettner@mpch-mainz.mpg.de S. K. Mitra S. Borrmann Institute for Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany Max Planck Institute for Chemistry, Mainz, Germany The heterogeneous freezing temperatures of single binary sulphuric acid solution droplets were measured in dependency of acid concentration down to temperatures as low as −70°C. In order to avoid influence of supporting substrates on the freezing characteristics, the droplets were suspended by means of an acoustic levitator. The droplets contained immersed particles of graphite, kaolin or montmorillonite in order to study the influence of the presence of such contamination on the freezing temperature. The radii of the suspended droplets spanned the range between 0,4 and 1,1 mm and the concentration of the sulphuric acid solution varied between 5 and 25 weight percent. The presence of the particles in the solution raises the freezing temperature with respect to homogeneous freezing of these solution droplets. The pure solution droplets can be supercooled up to 40° below the ice-acid solution thermodynamic equilibrium curve. Depending on the concentration of sulphuric acid and the nature of the impurity the polluted droplets froze between −11°C and −35°C. The experimental set-up, combining a deep freezer with a movable ultrasonic levitator and suitable optics, proved to be a useful approach for such investigations on individual droplets.