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Published August 7, 2009 | Supplemental Material
Journal Article Open

Unexpected Epoxide Formation in the Gas-Phase Photooxidation of Isoprene


Emissions of nonmethane hydrocarbon compounds to the atmosphere from the biosphere exceed those from anthropogenic activity. Isoprene, a five-carbon diene, contributes more than 40% of these emissions. Once emitted to the atmosphere, isoprene is rapidly oxidized by the hydroxyl radical OH. We report here that under pristine conditions isoprene is oxidized primarily to hydroxyhydroperoxides. Further oxidation of these hydroxyhydroperoxides by OH leads efficiently to the formation of dihydroxyepoxides and OH reformation. Global simulations show an enormous flux -— nearly 100 teragrams of carbon per year -— of these epoxides to the atmosphere. The discovery of these highly soluble epoxides provides a missing link tying the gas-phase degradation of isoprene to the observed formation of organic aerosols.

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© 2009 American Association for the Advancement of Science. Received for publication 2 March 2009. Accepted for publication 24 June 2009. We thank X. Levine, H. O. T. Pye, and the Harvard GEOS CHEM team (Daniel J. Jacob, principal investigator) for their help in setting up the GEOS-CHEM model; A. J. Kwan, A. W. Chan, P. S. Chhabra, and N. Eddingsaas for experimental assistance; J. D. Surratt for providing the speciation of the SOA resulting from BEPOX reactive uptake; and J. Lane, I. Maxwell-Cameron, and S. Jørgensen for helpful discussions regarding the quantum calculations. F.P. was partially supported by the William and Sonya Davidow fellowship. J.D.C. thanks the EPA Science to Achieve Results (STAR) Fellowship Program (FP916334012) for providing partial support. The mass spectrometer used in this study was purchased as part of a major research instrumentation grant from the National Science Foundation (ATM-0619783). Assembly and testing of the CIMS instrument was supported by the Davidow Discovery Fund. The numerical simulations for this research were performed on Caltech's Division of Geological and Planetary Sciences Dell Cluster. This work was supported by the Office of Science (Biological and Environmental Research), U.S. Department of Energy grant DE-FG02-05ER63983, U.S. Environmental Protection Agency STAR agreement RD-833749, and the Marsden Fund administrated by the Royal Society of New Zealand. The TC4 and ARCTAS campaigns were supported by NASA grants NNX07AL33G and NNX08AD29G. This work has not been formally reviewed by the EPA. The views expressed in this document are solely those of the authors, and the EPA does not endorse any products or commercial services mentioned in this publication. Supporting Online Material www.sciencemag.org/cgi/content/full/325/5941/730/DC1 Materials and Methods Figs. S1 to S9 Tables S1 to S9 References

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