Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

now at: Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Remote Sensing Laboratories, Department of Geography, University of Zurich, Zurich, Switzerland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Helsinki University Centre for Environment, P.O. Box 27, University of Helsinki, 00014 Helsinki, Finland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Department of Forest Sciences, P.O. Box 27, University of Helsinki, 00014 Helsinki, Finland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Department of Plant Physiology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Tartu, Estonia

Pacific Northwest National Laboratory, Atmospheric Sciences and Global Change Division, Richland, WA 99352, USA

Department of Forest Sciences, P.O. Box 27, University of Helsinki, 00014 Helsinki, Finland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland

Division of Atmospheric Sciences, Department of Physics, P.O. Box 48, University of Helsinki, 00014 Helsinki, Finland