1. Fine Particle and Aerosol Technology Laboratory, University of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland, TTS Research, P.O. Box 5 FI-05201 Rajamäki, Finland, and VTT, Technical Research Centre of Finland, Fine Particles, P.O. Box 1000, FI-02044 VTT, Espoo, Finland