Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel

Naval Research laboratory, Washington, D.C. 20375, USA

Institute for Atmospheric Physics, University of Mainz, Germany

Max Planck Institute for Chemistry, Mainz, Germany

now at: Umweltbundesamt, Dessau, Germany

Max Planck Institute for Chemistry, Mainz, Germany

Canadian Meteorological Centre, Meteorological Service of Canada, Montreal, Canada