Secondary Organic Aerosol Formation from Isoprene Photooxidation

Environ. Sci. Technol., 40 (6), 1869 -1877, 2006. 10.1021/es0524301 S0013-936X(05)02430-2
Web Release Date: February 15, 2006

Secondary Organic Aerosol Formation from Isoprene Photooxidation

Jesse H. Kroll, Nga L. Ng, Shane M. Murphy, Richard C. Flagan, and John H. Seinfeld*

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

Received for review December 3, 2005

Revised manuscript received January 13, 2006

Accepted January 18, 2006

Abstract:

Recent work has shown that the atmospheric oxidation of isoprene (2-methyl-1,3-butadiene, C₅H₈) leads to the formation of secondary organic aerosol (SOA). In this study, the mechanism of SOA formation by isoprene photooxidation is comprehensively investigated, by measurements of SOA yields over a range of experimental conditions, namely isoprene and NO_x concentrations. Hydrogen peroxide is used as the radical precursor, substantially constraining the observed gas-phase chemistry; all oxidation is dominated by the OH radical, and organic peroxy radicals (RO₂) react only with HO₂ (formed in the OH + H₂O₂ reaction) or NO concentrations, including NO_x-free conditions. At high NO_x, yields are found to decrease substantially with increasing [NO_x], indicating the importance of RO₂ chemistry in SOA formation. Under low-NO_x conditions, SOA mass is observed to decay rapidly, a result of chemical reactions of semivolatile SOA components, most likely organic hydroperoxides.

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