Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-3571-2006 http://www.atmos-chem-phys-discuss.net/6/3571/2006/ http://www.atmos-chem-phys-discuss.net/6/3571/2006/acpd-6-3571-2006.html http://www.atmos-chem-phys-discuss.net/6/3571/2006/acpd-6-3571-2006.pdf 3571 3609 2006-05-08 Oxalic acid as a heterogeneous ice nucleus in the upper troposphere and its indirect aerosol effect B. Zobrist C. Marcolli T. Koop B. P. Luo D. M. Murphy U. Lohmann A. Zardini U. K. Krieger T. Corti D. J. Cziczo S. Fueglistaler P. K. Hudson D. S. Thomson T. Peter Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland Department of Chemistry, Bielefeld University, Bielefeld, Germany Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Colorado, USA Heterogeneous ice freezing points of aqueous solutions containing various immersed solid dicarboxylic acids (oxalic, adipic, succinic, phthalic and fumaric) have been measured with a differential scanning calorimeter. The results show that only the dihydrate of oxalic acid (OAD) acts as a heterogeneous ice nucleus, with an increase in freezing temperature between 2-5 K depending on solution composition. In several field campaigns, oxalic acid enriched particles have been detected in the upper troposphere with single particle aerosol mass spectrometry. Simulations with a microphysical box model indicate that the presence of OAD may reduce the ice particle number density in cirrus clouds by up to ~50% when compared to exclusively homogeneous cirrus formation without OAD. Using the ECHAM4 climate model we estimate the global net radiative effect caused by this heterogeneous freezing to result in a cooling as high as -0.3 Wm<sup>-2</sub>.