Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 6 2009 10.5194/acpd-9-24281-2009 http://www.atmos-chem-phys-discuss.net/9/24281/2009/ http://www.atmos-chem-phys-discuss.net/9/24281/2009/acpd-9-24281-2009.html http://www.atmos-chem-phys-discuss.net/9/24281/2009/acpd-9-24281-2009.pdf 24281 24316 2009-11-12 Measurement and modelling of reactive halogen species over the tropical Atlantic Ocean A. S. Mahajan J. M. C. Plane j.m.c.plane@leeds.ac.uk H. Oetjen L. Mendes R. W. Saunders A. Saiz-Lopez C. E. Jones L. J. Carpenter G. B. McFiggans School of Chemistry, University of Leeds, Leeds LS2 9JT, UK Instituto Nacional de Meteorologia Geofisica (INMG), Delegação de São Vicente, Monte, CP 15, Mindelo, Cape Verde Laboratory of Atmospheric and Climate Science, Consejo Superior de Investigaciones Cientificas, 45007 Toledo, Spain Department of Chemistry, University of York, York, UK Centre for Atmospheric Science, School of Earth, Environment and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK Although reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 ppt, while the maximum observed IO mixing ratio was 3.1±0.4 ppt. The average values seen between 09:00–17:00 GMT were ~2.8 ppt for BrO and ~1.5 ppt for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O₃ onto the ocean surface in the presence of solar radiation. 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