1. National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL), R/E/CG1, 325 Broadway, Boulder, CO 80303, USA, and LMCE-DSM Commissariat a I'Energie Atomique, L'Orme des Meurisiers, 91191, Gif sur Yvette Cedex, France
2. NOAA/CMDL, R/E/CG1, 325 Broadway, Boulder, CO 80303, USA.
3. NOAA/CMDL, R/E/CG1, 325 Broadway, Boulder, CO 80303, USA, and Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA.
4. INSTAAR, and Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA.
5. Commonwealth Scientific and Industrial Research Organisation, Division of Atmospheric Research, Mordialloc, Victoria 3195, Australia