Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland Departement of Geological Sciences, Central Washington University, Ellensburg, Washington, USA Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Harbor, Michigan, USA Key Laboratory of Tibetan Environmental Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Bejing, China State Key Laboratory of Cryospheric Sciences, Chinese Academy of Sciences, Lanzhou, China School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing, China Climate Change Institute, University of Maine, Orono, Maine, USA