title: 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 (HO2) 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 HO2 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.
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138;
authors.forename: Yingjun
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Applied Physics, University of São Paulo, São Paulo 05508, Brazil;
authors.forename: Joel
authors.initials: J
authors.lastname: Brito
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706;
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Dorris
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706; Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138;
authors.forename: Jean C
authors.initials: JC
authors.lastname: Rivera-Rios
authors.affiliation: Department of Earth System Science, University of California, Irvine, CA 92697;
authors.forename: Roger
authors.initials: R
authors.lastname: Seco
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Kelvin H
authors.initials: KH
authors.lastname: Bates
authors.affiliation: Department of Applied Physics, University of São Paulo, São Paulo 05508, Brazil;
authors.forename: Paulo
authors.initials: P
authors.lastname: Artaxo
authors.affiliation: Department of Chemistry, Universidade do Estado do Amazonas, Manaus, AM 69050, Brazil;
authors.forename: Sergio
authors.initials: S
authors.lastname: Duvoisin
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; Department of Chemistry, University of Wisconsin-Madison, Madison, WI 53706; Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138;
authors.forename: Frank N
authors.initials: FN
authors.lastname: Keutsch
authors.affiliation: Department of Earth System Science, University of California, Irvine, CA 92697;
authors.forename: Saewung
authors.initials: S
authors.lastname: Kim
authors.affiliation: Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720;
authors.forename: Allen H
authors.initials: AH
authors.lastname: Goldstein
authors.affiliation: Department of Earth System Science, University of California, Irvine, CA 92697; Pacific Northwest National Laboratory, Richland, WA 99354;
authors.forename: Alex B
authors.initials: AB
authors.lastname: Guenther
authors.affiliation: Instituto Nacional de Pesquisas da Amazonia, Manaus, AM 69067, Brazil;
authors.forename: Antonio O
authors.initials: AO
authors.lastname: Manzi
authors.affiliation: Department of Meteorology, Universidade do Estado do Amazonas, Manaus, AM 69050, Brazil;
authors.forename: Rodrigo A F
authors.initials: RA
authors.lastname: Souza
authors.affiliation: Department of Environmental and Climate Sciences, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Springston
authors.affiliation: Department of Environmental and Climate Sciences, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Watson
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; kamckinney@seas.harvard.edu scot_martin@harvard.edu.
authors.forename: Karena A
authors.initials: KA
authors.lastname: McKinney
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138; Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138 kamckinney@seas.harvard.edu scot_martin@harvard.edu.
authors.forename: Scot T
authors.identifier: 0000-0002-5372-6214
authors.initials: ST
authors.lastname: Martin
mesh_terms: D000171:Acrolein / Q000031:analogs & derivatives / Q000032:analysis; D000393:Air Pollutants / Q000032:analysis*; D001272:Atmosphere; D002070:Butadienes / Q000737:chemistry* / Q000528:radiation effects; D002074:Butanones / Q000032:analysis; D005609:Free Radicals / Q000032:analysis*; D045782:Hemiterpenes / Q000737:chemistry* / Q000528:radiation effects; D006801:Humans; D009569:Nitric Oxide / Q000737:chemistry*; D010084:Oxidation-Reduction; D010420:Pentanes / Q000737:chemistry* / Q000528:radiation effects; D010545:Peroxides / Q000737:chemistry; D010777:Photochemistry; D065947:Rainforest
chemical_list: D000393:Air Pollutants; D002070:Butadienes; D002074:Butanones; D005609:Free Radicals; D045782:Hemiterpenes; D010420:Pentanes; D010545:Peroxides; C005059:isoprene; D009569:Nitric Oxide; D000171:Acrolein; C039175:methacrylaldehyde; C057920:3-buten-2-one
keywords: Amazon; isoprene photochemistry; organic hydroperoxides