ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-95-2004 The role of transition metal ions on HO<sub>x</sub> radicals in clouds: a numerical evaluation of its impact on multiphase chemistry Deguillaume L. 1 Leriche M. 1 Monod A. 2 Chaumerliac N. 1 Laboratoire de Météorologie Physique (LaMP), CNRS, Université Blaise Pascal, 24 av. des Landais, 63177 Aubière, Cedex, France Laboratoire Chimie et Environnement, Université de Provence, Case 293, place Victor Hugo,13331 Marseille Cedex 3, France 26 01 2004 4 1 95 110 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/4/95/2004/acp-4-95-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/95/2004/acp-4-95-2004.pdf

A new modelling study of the role of transition metal ions on cloud chemistry has been performed. Developments of the Model of Multiphase Cloud Chemistry (M2C2; Leriche et al., 2001) are described, including the transition metal ions reactivity emission/deposition processes and variable photolysis in the aqueous phase. The model is then applied to three summertime scenarios under urban, remote and marine conditions, described by Ervens et al. (2003).<br> <br> Chemical regimes in clouds are analyzed to understand the role of transition metal ions on cloud chemistry and especially, on H<sub>x</sub>O<sub>y</sub> chemistry, which consequently influences the sulphur and the VOCs chemistry in droplets. The ratio of Fe(II)/Fe(III) exhibits a diurnal variation with values in agreement with the available measurements of Fe speciation. In the urban case, sensitivity tests with and without TMI chemistry, show an enhancement of OH concentration in the aqueous phase when TMI chemistry is considered.