1. Department of Atmospheric and Oceanic Sciences and Cooperative Institute for Research in Environmental SciencesBoulder Colorado USA
2. Now at: NASA Goddard Institute for Space StudiesNew York New York USA
3. Now at: Earth and Environmental Systems InstitutePennsylvania State UniversityUniversity Park Pennsylvania USA
4. Department of Applied Mathematics and Cooperative Institute for Research in Environmental SciencesBoulder Colorado USA
5. Atmospheric Chemistry Observations & Modeling LaboratoryNational Center for Atmospheric ResearchBoulder Colorado USA.
6. Now at: College of Earth, Ocean, and Atmospheric SciencesOregon State UniversityCorvallis Oregon USA