1. Research Group EnVOC, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium; Institute of Environmental Engineering, Ecological Systems Design, ETH Zurich, HIL G 35.2, CH-8093, Switzerland; Department of Environmental Science, Institute for Wetland and Water Research, Faculty of Science, Radboud University Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands