1. NASA Goddard Space Flight Center, Greenbelt, Maryland.
2. Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado.
3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
4. Laboratory for Atmospheric and Space Physics and Department of Atmospheric and Oceanic Sciences, University of Colorado at Boulder, Boulder, Colorado.