ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-241-2011 Evidence for heterogeneous chlorine activation in the tropical UTLS von Hobe M. 1 Grooß J.-U. 1 Günther G. 1 Konopka P. 1 Gensch I. 1 Krämer M. 1 Spelten N. 1 Afchine A. 1 Schiller C. 1 Ulanovsky A. 2 Sitnikov N. 2 Shur G. 2 Yushkov V. 2 Ravegnani F. 3 Cairo F. 3 Roiger A. 4 Voigt C. 4 Schlager H. 4 Weigel R. 5 Frey W. 5 Borrmann S. 5 Müller R. 1 Stroh F. 1 Forschungszentrum Jülich GmbH, Institute for Energy and Climate Research – Stratosphere (IEK-7), Jülich, Germany Central Aerological Observatory, Dolgoprudny, Moscow, Russia Institute of Atmospheric Sciences and Climate CNR-ISAC, Bologna, Italy Deutsches Zentrum für Luft und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany Johannes Gutenberg Universität Mainz, Institute of Atmospheric Physics, Mainz, Germany 12 01 2011 11 1 241 256 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/11/241/2011/acp-11-241-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/241/2011/acp-11-241-2011.pdf

Airborne in-situ observations of ClO in the tropics were made during the TROCCINOX (Aracatuba, Brazil, February 2005) and SCOUT-O<sub>3</sub> (Darwin, Australia, November/December 2005) field campaigns. While during most flights significant amounts of ClO (≈10–20 parts per trillion, ppt) were present only in aged stratospheric air, instances of enhanced ClO mixing ratios of up to 40 ppt – significantly exceeding those expected from gas phase chemistry – were observed in air masses of a more tropospheric character. Most of these observations are associated with low temperatures or with the presence of cirrus clouds (often both), suggesting that cirrus ice particles and/or liquid aerosol at low temperatures may promote significant heterogeneous chlorine activation in the tropical upper troposphere lower stratosphere (UTLS). In two case studies, particularly high levels of ClO observed were reproduced by chemistry simulations only under the assumption that significant denoxification had occurred in the observed air. However, to reproduce the ClO observations in these simulations, O<sub>3</sub> mixing ratios higher than observed had to be assumed, and at least for one of these flights, a significant denoxification is in contrast to the observed NO levels, suggesting that the coupling of chlorine and nitrogen compounds in the tropical UTLS may not be completely understood.

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