1. National Oceanic and Atmospheric Administration, Environmental Laboratories, Aeronomy Laboratory, Boulder, CO 80303 and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309
2. Climate and Diagnostic Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303
3. National Oceanic and Atmospheric Administration, Environmental Laboratories, Aeronomy Laboratory, Boulder, CO 80303 and Cooperative Institute for Research in Environmental Sciences and Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309