ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-7617-2011 HOCl and Cl<sub>2</sub> observations in marine air Lawler M. J. 1 Sander R. 2 Carpenter L. J. 3 Lee J. D. 4 von Glasow R. 5 Sommariva R. 5 Saltzman E. S. 1 Department of Earth System Science, University of California, Irvine, CA, USA Department of Atmospheric Chemistry, Max-Planck Institute for Chemistry, Mainz, Germany Department of Chemistry, University of York, York, UK National Centre for Atmospheric Science, University of York, York, UK School of Environmental Sciences, University of East Anglia, Norwich, UK 01 08 2011 11 15 7617 7628 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/11/7617/2011/acp-11-7617-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/7617/2011/acp-11-7617-2011.pdf

Cl atoms in the marine atmosphere may significantly impact the lifetimes of methane and other hydrocarbons. However, the existing estimates of Cl atom levels in marine air are based on indirect evidence. Here we present measurements of the Cl precursors HOCl and Cl<sub>2</sub> in the marine boundary layer during June of 2009 at the Cape Verde Atmospheric Observatory in the eastern tropical Atlantic. These are the first measurements of tropospheric HOCl. HOCl and Cl<sub>2</sub> levels were low in air with open ocean back trajectories, with maximum levels always below 60 and 10 ppt (pmol/mol), respectively. In air with trajectories originating over Europe, HOCl and Cl<sub>2</sub> levels were higher, with HOCl maxima exceeding 100 ppt each day and Cl<sub>2</sub> reaching up to 35 ppt. The increased Cl cycling associated with long distance pollutant transport over the oceans likely impacts a wide geographic area and represents a mechanism by which human activities have increased the reactivity of the marine atmosphere. Data-constrained model simulations indicate that Cl atoms account for approximately 15 % of methane destruction on days when aged polluted air arrives at the site. A photochemical model does not adequately simulate the observed abundances of HOCl and Cl<sub>2</sub>, raising the possibility of an unknown HOCl source.

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