1. Finnish Meteorological Institute, FI-00101 Helsinki, Finland
2. Laboratory of Atmospheric Processes and their Impacts, ENAC, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
3. Institute of Chemical Engineering Sciences, Foundation for Research and Technology Hellas (FORTH/ICE-HT), 26504 Patras, Greece
4. Department of Applied Physics, University of Eastern Finland, Kuopio 70211, Finland