Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 4 1 2004 10.5194/acp-4-95-2004 http://www.atmos-chem-phys.net/4/95/2004/ http://www.atmos-chem-phys.net/4/95/2004/acp-4-95-2004.html http://www.atmos-chem-phys.net/4/95/2004/acp-4-95-2004.pdf 95 110 2004-01-26 The role of transition metal ions on HO_x radicals in clouds: a numerical evaluation of its impact on multiphase chemistry L. Deguillaume M. Leriche A. Monod N. Chaumerliac 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 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_xO_y 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.