1. Laboratory for Air Pollution and Environmental Technology, Swiss Federal Laboratories for Materials Testing and Research, Empa, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland, Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology ETH, CH-8092 Zurich, Switzerland, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom, Laboratory of Radio and Environmental Chemistry, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland