ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-979-2011 Modelling multi-phase halogen chemistry in the coastal marine boundary layer: investigation of the relative importance of local chemistry vs. long-range transport Lowe D. 1 Ryder J. 1 2 Leigh R. 3 4 Dorsey J. R. 1 McFiggans G. 1 Centre for Atmospheric Sciences, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK Centre for Ecology and Hydrology, Midlothian, EH26 0QB, UK Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK now at: Crichton Carbon Centre, Crichton University Campus, Dumfries, DG1 4ZL, UK 02 02 2011 11 3 979 994 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/11/979/2011/acp-11-979-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/979/2011/acp-11-979-2011.pdf

Measurements of significant concentrations of IO, I<sub>2</sub> and BrO in a semi-polluted coast environment at Roscoff, in North-West France, have been made as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign undertaken in September 2006. We use a one-dimensional column model, with idealised I<sub>2</sub> emissions predicted using macroalgael maps and tidal data from the littoral area surrounding Roscoff, to investigate the probable causes for these observations. The coupled microphysical and chemical aerosol model simulates mixed-phase halogen chemistry using two separate particle modes, seasalt and non-seasalt, each comprising of eight size-sections. This work confirms the finding of a previous study that the BrO measurements are most likely caused by unknown, local sources. We find that the remote observations of IO and I<sub>2</sub> are best replicated using the I<sub>2</sub> recycling mechanism suggested by previous studies, but that such a mechanism is not wholly necessary. However in-situ measurements of I<sub>2</sub> can only be explained by invoking an I<sub>2</sub> recycling mechanism. We suggest that focussed observations of the changes in NO<sub>x</sub> and NO<sub>y</sub> concentrations, as well as changes in the nitrate fraction of the non-seasalt aerosol mode, in the presence of I<sub>2</sub> bursts could be used to determine the atmospheric relevance of the predicted I<sub>2</sub> recycling mechanism.

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