Chemistry Department, Portland State University, Portland, OR, 97207-0751, USA

University of Colorado, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, 80309, USA

NOAA Earth System Research Laboratory, Chemical Science Division, Boulder, CO, 80305, USA

now at: Aerodyne Research, Inc. Billerica, MA, 01821, USA

CARTEL, Département de géomatique appliqueé, Université de Sherbrooke, Sherbrooke, Quebec, J1K 2R1, Canada

NOAA Pacific Marine Environment Laboratory, 7600 Sand Point Way NE, Seattle, WA, 98115, USA

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

Earth and Atmospheric Sciences Department, University of Houston, Houston, TX, 77204-5007, USA