1. Department of Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 12, 223 62, Lund, Sweden;
2. Department of Physics, Division of Nuclear Physics, Lund University, Lund, Sweden;
3. Department of Physics, Section of Tropospheric Aerosols, Leibniz-Institute for Tropospheric Research, Leipzig, Germany;
4. Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014 University of Helsinki, Finland;
5. Institute of Environmental Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia