Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation
Other literature type
Liu, Y. J.
McKinney, K. A.
Martin, S. T.
(issn: 1680-7324, eissn: 1680-7324)
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 °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 °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.