1. Department of Environmental Science, Fine Particle and Aerosol Technology Laboratory, University of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland, VTT Technical Research Centre of Finland, Fine Particles, P.O. Box 1000, FI-02044 VTT, Espoo, Finland, Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, FI-70211 Kuopio, Finland, and Department Physics, University of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland