Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland

Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland

now at: Department of Chemistry and Southern Ontario Centre for Atmospheric Aerosol Research, University of Toronto, 80 St. George Street, Toronto M5S 3H6, Ontario, Canada

Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland

now at: Laboratoire de Physique et Chimie de l'Environnement, CNRS, Université d'Orléans 45071 Orléans Cedex 2, France

Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland