1. Prof. Dr. Karsten Levsen Fraunhofer Institute of Toxicology and Aerosol Research Nikolai-Fuchs-Str. 1 D-30625 Hannover, West Germany, and Prof. Dr. Gerold Wünsch Institute of Inorganic Chemistry, University of Hannover Callinstr. 9 D-30167 Hannover, West Germany