1. GKSS Research Centre, Department of Environmental Chemistry, Institute for Coastal Research, Max-Planck-Str. 1, DE-21502 Geesthacht, Germany, University of Lueneburg, Faculty of Environmental Sciences, Institute for Ecology and Environmental Chemistry, Scharnhorststr. 1, DE-21335 Lueneburg, Germany, Norwegian Institute for Air Research (NILU), The Polar Environmental Centre, NO-9296 Tromso, Norway, and Department of Applied Environmental Science (ITM), Stockholm University, SE-10691 Stockholm, Sweden