Joel Wong, Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Department of Environmental Systems Science, Zurich, Switzerland, Nicolas Gruber, Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland and Matthias Munnich, Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Environmental Systems Science, Zurich, Switzerland