1. Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, U.K., NILU, Norwegian Institute for Air Research, P.O. Box 100, N-2027 Kjeller, Norway, and Department of Environmental Chemistry, Institute of Chemical and Environmental Research (ICER-CSIC), Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain