Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

now at: NASA Goddard Institute for Space Studies and the Earth Institute, Columbia University, New York, New York, USA

Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

Department of Chemical Engineering, California Institute of Technology, Pasadena, California, USA

Aerodyne Research, Inc., Billerica, Massachusetts, USA

School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA

now at: Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong,China

School of Engineering and Applied Sciences and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA

Center for Atmospheric Sciences, University of California, Berkeley, California, USA

now at: Department for Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA