1. Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, California Institute of Technology, Pasadena, California, University of California, Berkeley, California, NOAA Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, Jet Propulsion Laboratory, Pasadena, California, and Atomospheric Enivironment, Downsview, Ontario, Canada