[1] Lab. fur Atmospharenphys. (LAPETH), ETH Zürich, HPP Hönggerberg, 8093 Zürich, Switzerland
[2] Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany
[3] Department of Chemistry, Centre for Atmospheric Science, University of Cambridge, Cambridge CB2 1EW, Germany