1. Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland, Empa, Laboratory for Air Pollution/Environmental Technology, 8600 Dübendorf, Switzerland, and Hamburger Synchrotronstrahlungslabor at Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22603 Hamburg, Germany