1. Institute of Terrestrial Ecology, Swiss Federal Institute of Technology Zürich (ETHZ), Grabenstrasse 11a, CH-8952 Schlieren, Switzerland, Department of Rural Engineering, Air and Soil Pollution Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), GR Ecublens, CH-1015 Lausanne, Switzerland, and Swiss Federal Institute of Forest, Snow, and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland