ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-3-987-2003 Ice condensation on sulfuric acid tetrahydrate: Implications for polar stratospheric ice clouds Fortin T. J. 1 3 Drdla K. 2 Iraci L. T. 2 Tolbert M. A. 1 Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado, CIRES Building Room 318, Boulder, CO 80309-0216, USA NASA Ames Research Center, Earth Science Division, Moffett Field, CA 94035, USA Present address: Center for Science and Technology Policy Research, University of Colorado/CIRES, 1333 Grandview 09 07 2003 3 4 987 997 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/3/987/2003/acp-3-987-2003.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/3/987/2003/acp-3-987-2003.pdf

The mechanism of ice nucleation to form Type 2 PSCs is important for controlling the ice particle size and hence the possible dehydration in the polar winter stratosphere. This paper probes heterogeneous ice nucleation on sulfuric acid tetrahydrate (SAT). Laboratory experiments were performed using a thin-film, high-vacuum apparatus in which the condensed phase is monitored via Fourier transform infrared spectroscopy and water pressure is monitored with the combination of an MKS baratron and an ionization gauge. Results show that SAT is an efficient ice nucleus with a critical ice saturation ratio of S*<sub>ice</sub> = 1.3 to 1.02 over the temperature range 169.8-194.5 K. This corresponds to a necessary supercooling of 0.1-1.3 K below the ice frost point. The laboratory data is used as input for a microphysical/photochemical model to probe the effect that this heterogeneous nucleation mechanism could have on Type 2 PSC formation and stratospheric dehydration. In the model simulations, even a very small number of SAT particles (e.g., 10<sup>-3</sup> cm<sup>-3</sup>) result in ice nucleation on SAT as the dominant mechanism for Type 2 PSC formation. As a result, Type 2 PSC formation is more widespread, leading to larger-scale dehydration. The characteristics of the clouds are controlled by the assumed number of SAT particles present, demonstrating that a proper treatment of SAT is critical for correctly modeling Type 2 PSC formation and stratospheric dehydration.