ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-11-5183-2011 Retrievals of chlorine chemistry kinetic parameters from Antarctic ClO microwave radiometer measurements Kremser S. 8 1 Schofield R. 2 Bodeker G. E. 1 9 Connor B. J. 3 Rex M. 2 Barret J. 4 Mooney T. 4 Salawitch R. J. 5 Canty T. 5 Frieler K. 6 Chipperfield M. P. 7 Langematz U. 8 Feng W. 7 National Institute of Water and Atmospheric Research, Lauder, New Zealand Stiftung Alfred-Wegener Institut (AWI), Forschungsstelle Potsdam, Potsdam, Germany BC Consulting Ltd., Alexandra, New Zealand Stony Brook University, Stony Brook, New York, USA Department Atmosphere & Ocean Science, University of Maryland, Maryland, USA Potsdam Institute for Climate Impact Research (PIK), Potsdam, Germany Institute for Climate & Atmospheric Science, School of Earth & Environment, University of Leeds, Leeds, UK Freie Universität Berlin, Berlin, Germany now at: Bodeker Scientific, Alexandra, New Zealand 01 06 2011 11 11 5183 5193 Copyright: © 2011 S. Kremser et al. 2011 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/11/5183/2011/acp-11-5183-2011.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/11/5183/2011/acp-11-5183-2011.pdf

Key kinetic parameters governing the partitioning of chlorine species in the Antarctic polar stratosphere were retrieved from 28 days of chlorine monoxide (ClO) microwave radiometer measurements made during the late winter/early spring of 2005 at Scott Base (77.85° S, 166.75° E). During day-time the loss of the ClO dimer chlorine peroxide (ClOOCl) occurs mainly by photolysis. Some time after sunrise, a photochemical equilibrium is established and the ClO/ClOOCl partitioning is determined by the ratio of the photolysis frequency, <i>J</i>, and the dimer formation rate, <i>k</i><sub>f</sub>. The values of <i>J</i> and <i>k</i><sub>f</sub> from laboratory studies remain uncertain to a considerable extent, and as a complement to these ongoing studies, the goal of this work is to provide a constraint on that uncertainty based on observations of ClO profiles in the Antarctic. First an optimal estimation technique was used to derive <i>J</i>/<i>k</i><sub>f</sub> ratios for a range of <i>K</i><sub>eq</sub> values. The optimal estimation forward model was a photochemical box model that takes <i>J</i>, <i>k</i><sub>f</sub>, and <i>K</i><sub>eq</sub> as inputs, together with a priori profiles of activated chlorine (ClO<sub>x</sub> = ClO+2×ClOOCl), profiles of ozone, temperature, and pressure. JPL06 kinetics are used as a priori in the optimal estimation and for all other chemistry in the forward model. Using the more recent JPL09 kinetics results in insignificant differences in the retrieved value of <i>J</i>/k<sub>f</sub>. A complementary approach was used to derive the optimal kinetic parameters; the full parameter space of <i>J</i>, <i>k</i><sub>f</sub>, <i>K</i><sub>eq</sub> and ClO<sub>x</sub> was sampled to find the minimum in differences between measured and modelled ClO profiles. Furthermore, values of <i>K</i><sub>eq</sub> up to 2.0 times larger than recommended by JPL06 were explored to test the sensitivity of the <i>J</i>/<i>k</i><sub>f</sub> ratio to changes in <i>K</i><sub>eq</sub>. The results show that the retrieved <i>J</i>/<i>k</i><sub>f</sub> ratios bracket the range of 1.23 to 1.97 times the <i>J</i>/<i>k</i><sub>f</sub> value recommended by JPL06 over the range of <i>K</i><sub>eq</sub> values considered. The retrieved <i>J</i>/<i>k</i><sub>f</sub> ratios lie in the lower half of the large uncertainty range of <i>J</i>/<i>k</i><sub>f</sub> recommended by JPL06 and towards the upper portion of the smaller uncertainty range recommended by JPL09.

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