1. Nanosafety Research Center, Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, FI-00250 Helsinki, Finland
2. Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland
3. Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
4. Department of Physics, Faculty of Science, University of Jordan, Amman, 11942 Jordan
5. Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland