1. Cooperative Institute for Research in Environmental Sciences University of Colorado Boulder Boulder Colorado USA
2. Chemical Sciences Division NOAA Earth System Research Laboratory Boulder Colorado USA
3. Global Monitoring Division NOAA Earth System Research Laboratory Boulder Colorado USA
4. Now at the National Institute of Standards and Technology Gaithersburg Maryland USA
5. Department of Climate and Space Sciences and Engineering University of Michigan Ann Arbor Michigan USA
6. Department of Energy Resources Engineering Stanford University Stanford California USA
7. Department of Civil and Environmental Engineering Stanford University Stanford California USA
8. Department of Land, Air, and Water Resources University of California Davis California USA