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Y. J. Liu; I. Herdlinger-Blatt; K. A. McKinney; S. T. Martin (2013)
Publisher: Copernicus Publications
Journal: Atmospheric Chemistry and Physics
Languages: English
Types: Article
Subjects: Geophysics. Cosmic physics, Chemistry, DOAJ:Earth and Environmental Sciences, QD1-999, GE1-350, G, DOAJ:Environmental Sciences, Physics, Geography. Anthropology. Recreation, Environmental sciences, QC1-999, QC801-809
The photo-oxidation chemistry of isoprene (C<sub>5</sub>H<sub>8</sub>) was studied in a continuous-flow chamber under conditions such that the reactions of isoprene-derived peroxyl radicals (RO<sub>2</sub>) were dominated by hydroperoxyl (HO<sub>2</sub>) pathway. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) with switchable H<sub>3</sub>O<sup>+</sup> and NO<sup>+</sup> reagent ions was used for product analysis. The products methyl vinyl ketone (MVK; C<sub>4</sub>H<sub>6</sub>O) and methacrolein (MACR; C<sub>4</sub>H<sub>6</sub>O) were differentiated using NO<sup>+</sup> reagent ions. The MVK and MACR yields were 4.3 ± 0.4% and 3.2 ± 0.3%, respectively, for HO<sub>2</sub>-dominant conditions at +25 &deg;C and < 2% relative humidity. The respective yields were 41.1 ± 2.2% and 28.8 ± 1.2% for NO-dominant conditions. The yields for HO<sub>2</sub>-dominant conditions imply a concomitant yield (i.e., recycling factor) of hydrogen oxide radicals (HO<sub>x</sub>) of 15 ± 0.7% from the reaction of isoprene-derived RO<sub>2</sub> with HO<sub>2</sub>. Other isoprene oxidation products, believed to be organic hydroperoxides, also contributed to the ion intensity at the same mass-to-charge (<i>m</i>/<i>z</i>) ratios as the MVK and MACR product ions, and these products were selectively removed from the gas phase using a variable temperature cold trap (−40 °C) in front of the PTR-TOF-MS. These hydroperoxide products were absent for NO-dominant conditions. When incorporated into regional and global chemical transport models, the yields of MVK and MACR and concomitant HO<sub>x</sub> yields reported in this study will improve the accuracy of simulations of the HO<sub>2</sub> reaction pathway of isoprene, which has been shown to make a significant contribution to the total reactivity of isoprene-derived RO<sub>2</sub> radicals on a global scale.

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