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Published May 31, 2016 | Supplemental Material + Published
Journal Article Open

Isoprene photochemistry over the Amazon rainforest

Abstract

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO_2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4–0.6. This result implies a ratio of the reaction rate of ISOPOO with HO_2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.

Additional Information

© 2016 National Academy of Sciences. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved April 13, 2016 (received for review December 23, 2015). Published online before print May 16, 2016. We thank John Crounse and Paul Wennberg for supporting GEOS-Chem simulations. We thank Ronald Cohen and Yi Li for helpful discussions. Institutional support was provided by the Central Office of the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA), the National Institute of Amazonian Research (INPA), and Amazonas State University (UEA). We acknowledge the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a user facility of the United States Department of Energy, Office of Science, sponsored by the Office of Biological and Environmental Research, and support from the Atmospheric System Research (ASR) program of that office. Funding was obtained from the United States Department of Energy (DOE), the Amazonas State Research Foundation (FAPEAM), the São Paulo Research Foundation (FAPESP), the Brazilian Scientific Mobility Program (CsF/CAPES), and the United States National Science Foundation (NSF). The research was conducted under Scientific License 001030/2012-4 of the Brazilian National Council for Scientific and Technological Development (CNPq). Author contributions: Y.L., P.A., F.N.K., A.H.G., A.B.G., A.O.M., R.A.F.S., K.A.M., and S.T.M. designed the research; Y.L., J.B., M.R.D., J.C.R., R.S., K.H.B., S.D., S.K., S.R.S., and T.B.W. performed the research; Y.L., K.A.M., and S.T.M. analyzed the data; and Y.L., K.A.M., and S.T.M. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1524136113/-/DCSupplemental.

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Published - PNAS-2016-Liu-6125-30.pdf

Supplemental Material - pnas.1524136113.sapp.pdf

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August 22, 2023
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