1. Chemical Sciences Division, National Oceanic and Atmospheric Administration (NOAA), Earth System Research Laboratory, Boulder, CO 80305, USA.
2. Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80305, USA.
3. Global Monitoring Division, NOAA, Earth System Research Laboratory, Boulder, CO, USA.
4. Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, USA.
5. NASA Langley Research Centre, Hampton, VA, USA.
6. Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS–Institut National des Sciences de l'Univers, Université de Versailles St Quentin, Université de Paris 6, France.