Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation

Other literature type English OPEN
Liu, Y. J. ; Herdlinger-Blatt, I. ; McKinney, K. A. ; Martin, S. T. (2013)

The photo-oxidation chemistry of isoprene (ISOP; C<sub>5</sub>H<sub>8</sub>) was studied in a continuous-flow chamber under conditions such that the reactions of the isoprene-derived peroxyl radicals (RO<sub>2</sub>) were dominated by the 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 via the HO<sub>2</sub> pathway were (3.8 ± 1.3)% and (2.5 ± 0.9)%, respectively, at +25 &deg;C and < 2% relative humidity. The respective yields were (41.4 ± 5.5)% and (29.6 ± 4.2)% via the NO pathway. Production of MVK and MACR via the HO<sub>2</sub> pathway implies concomitant production of hydroxyl ((6.3 ± 2.1)%) and hydroperoxyl ((6.3 ± 2.1)%) radicals, meaning a HO<sub>x</sub> recycling of (12.6 ± 4.2)% given that HO<sub>2</sub> was both a reactant and product. Other isoprene oxidation products, believed to be mostly organic hydroperoxides, also contributed to the ion intensity at the same mass-to-charge (<i>m/z</i>) ratios as the MVK and MACR product ions for HO<sub>2</sub>-dominant conditions. These products were selectively removed from the gas phase by placement of a cold trap (−40 &deg;C) inline prior to the PTR-TOF-MS. When incorporated into regional and global chemical transport models, the yields of MVK and MACR and the concomitant HO<sub>x</sub> recycling reported in this study can improve the accuracy of the simulation of the HO<sub>2</sub> reaction pathway of isoprene, which is believed to be the fate of approximately half of atmospherically produced isoprene-derived peroxy radicals on a global scale.
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