1. Norwegian Institute for Air Research, P.O. Box 100, NO-2027, Kjeller, Norway
2. Department of Chemistry, University of Oslo, P.O. Box 1033, NO-0315, Oslo, Norway
3. Department of Environmental Science, Stockholm University, SE-106 91, Stockholm, Sweden
4. Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada