1. Aerosol Physics Laboratory, Institute of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland, Emission Control, VTT Technical Research Centre of Finland, P.O.Box 1000, FI-02044 VTT, Finland, Department of Technology, Helsinki Polytechnic, P.O. Box 4020, FI-00099 Helsinki, Finland, and Department of Physical Sciences, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland