ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-7-1381-2007 Modelling iodide – iodate speciation in atmospheric aerosol: Contributions of inorganic and organic iodine chemistry Pechtl S. 1 3 Schmitz G. 2 von Glasow R. 1 4 Institute for Environmental Physics, University of Heidelberg, Heidelberg, Germany Faculté des Sciences Appliquées, Université Libre de Bruxelles, Bruxelles, Belgium now at: Deutsches Patent- und Markenamt, Munich, Germany now at: School of Environmental Sciences, University of East Anglia, Norwich, UK 28 02 2007 7 5 1381 1393 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/7/1381/2007/acp-7-1381-2007.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/1381/2007/acp-7-1381-2007.pdf

The speciation of iodine in atmospheric aerosol is currently poorly understood. Models predict negligible iodide concentrations but accumulation of iodate in aerosol, both of which is not confirmed by recent measurements. We present an updated aqueous phase iodine chemistry scheme for use in atmospheric chemistry models and discuss sensitivity studies with the marine boundary layer model MISTRA. These studies show that iodate can be reduced in acidic aerosol by inorganic reactions, i.e., iodate does not necessarily accumulate in particles. Furthermore, the transformation of particulate iodide to volatile iodine species likely has been overestimated in previous model studies due to negligence of collision-induced upper limits for the reaction rates. However, inorganic reaction cycles still do not seem to be sufficient to reproduce the observed range of iodide – iodate speciation in atmospheric aerosol. Therefore, we also investigate the effects of the recently suggested reaction of HOI with dissolved organic matter to produce iodide. If this reaction is fast enough to compete with the inorganic mechanism, it would not only directly lead to enhanced iodide concentrations but, indirectly via speed-up of the inorganic iodate reduction cycles, also to a decrease in iodate concentrations. Hence, according to our model studies, organic iodine chemistry, combined with inorganic reaction cycles, is able to reproduce observations. The presented chemistry cycles are highly dependent on pH and thus offer an explanation for the large observed variability of the iodide – iodate speciation in atmospheric aerosol.

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