Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, USA

now at: Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, USA

Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, USA

now at: Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA

Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA

Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Institute of Environmental Physics, University of Heidelberg, INF 229, 69120 Heidelberg, Germany