Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Cooperative Institute for Research in the Environmental Sciences, Boulder, CO 80309, USA

now at Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA

Institute for Atmosphere and Climate Science, ETH, Zurich 8092, Switzerland

Institute for Atmosphere and Climate Science, ETH, Zurich 8092, Switzerland

Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Cooperative Institute for Research in the Environmental Sciences, Boulder, CO 80309, USA

Department of Chemistry, University of Delaware, Newark, DE 19716, USA

Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Department of Chemistry, University of Delaware, Newark, DE 19716, USA

Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Cooperative Institute for Research in the Environmental Sciences, Boulder, CO 80309, USA

Earth System Research Laboratory, NOAA, Boulder, CO 80305, USA

Cooperative Institute for Research in the Environmental Sciences, Boulder, CO 80309, USA

Departments of Mechanical and Aeronautical Engineering, Civil and Environmental Engineering, and Land, Air and Water Resources, University of California, Davis, CA 95616, USA