Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA

now at: Energy Storage Division, NEXT ENERGY EWE-Forschungszentrum für Energietechnologie e.V., 26129 Oldenburg, Germany

soon at (September 2012): United States Agency for International Development, Washington, DC, 20523 USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

now at: Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA 94720, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

now at: School of Chemical and Biomolecular Engineering and School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA

Copenhagen Center for Atmospheric Chemistry, Department of Chemistry, University of Copenhagen, 2100 Copenhagen Ø, Denmark

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA

Division of Geology and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA