Also at Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland

Paul Scherrer Institut, Laboratory for Radio- and Environmental Chemistry, Villigen, Switzerland

Also at Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland

Paul Scherrer Institut, Laboratory for Radio- and Environmental Chemistry, Villigen, Switzerland

Present address: Center for Atmospheric Sciences, Scripps Institution of Oceanography, University of California, San Diego, USA

Max-Planck-Institut für Chemie, Abt. Chemie der Atmosphäre, Mainz, Germany

Paul Scherrer Institut, Laboratory for Radio- and Environmental Chemistry, Villigen, Switzerland