1. ETH Zürich Department of Environmental Sciences, Universitätsstr. 16, 8092 Zürich, Switzerland, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, Canda M3H 5T4, and Center for Applied Geoscience (ZAG), Eberhard-Karls-University Tübingen Wilhelmstrasse 56, D-72074 Tübingen, Germany