Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 4 2006 10.5194/acpd-6-7905-2006 http://www.atmos-chem-phys-discuss.net/6/7905/2006/ http://www.atmos-chem-phys-discuss.net/6/7905/2006/acpd-6-7905-2006.html http://www.atmos-chem-phys-discuss.net/6/7905/2006/acpd-6-7905-2006.pdf 7905 7944 2006-08-14 Understanding the kinetics of the ClO dimer cycle M. von Hobe R. J. Salawitch T. Canty H. Keller-Rudek G. K. Moortgat J.-U. Grooß R. Müller F. Stroh Forschungszentrum Jülich GmbH, Institute for Chemistry and Dynamics of the Geosphere (ICG-I), Jülich, Germany Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Max-Planck-Institute for Chemistry, Atmospheric Chemistry Division, Mainz, Germany Among the major factors controlling ozone loss in the polar winter is the kinetics of the ClO dimer catalytic cycle. The most important issues are the thermal equilibrium between ClO and Cl₂O₂, the rate of Cl₂O₂ formation, and the Cl₂O₂ photolysis rate. All these issues have been addressed in a large number of laboratory, field and theoretical studies, but large discrepancies between individual results exist and a self-consistent set of parameters compatible with field observations of ClO and Cl₂O₂ has not been identified. Here, we use thermodynamic calculations and unimolecular rate theory to constrain the ClO/Cl₂O₂ equilibrium constant and the rate constants for Cl₂O₂ formation and dissociation. This information is used together with available atmospheric data to examine Cl₂O₂ photolysis rates based on different Cl₂O₂ absorption cross sections. Good overall consistency is achieved using a ClO/Cl₂O₂ equilibrium constant recently suggested by Plenge et al. (2005), the Cl₂O₂ recombination rate constant reported by Nickolaisen et al. (1994) and Cl₂O₂ photolysis rates based on averaged absorption cross sections that are roughly intermediate between the JPL 2002 assessment and a laboratory study by Burkholder et al. (1990).