1. Climate Monitoring and Diagnostics Laboratory, R/E/CG1, 325 Broadway, Boulder, Colorado 80303-3328, USA
2. AES Environment Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H-5T4
3. Mauna Loa Observatory, Box 275, Hilo, Hawaii 96720, USA
4. Geophysical Institute, University of Alaska, Fairbanks, Alaska, 99701, USA
5. CIRES, University of Colorado, Boulder, Colorado 80309-0216, USA