1. Department of Chemistry and Institute for Atmospheric and Earth System Research (INAR), University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
2. Aerosol Physics Laboratory, Physics Unit, Tampere University, FI-33101 Tampere, Finland
3. Department of Physics and Institute for Atmospheric and Earth System Research (INAR), University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland