Departments of Environmental Science and Engineering and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

Departments of Environmental Science and Engineering and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

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

Departments of Environmental Science and Engineering and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

Departments of Environmental Science and Engineering and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA