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

Abstract. The photo-oxidation chemistry of isoprene (ISOP; C₅H₈) was studied in a continuous-flow chamber under conditions such that the reactions of the isoprene-derived peroxyl radicals (RO₂) were dominated by the hydroperoxyl (HO₂) pathway. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) with switchable H₃O⁺ and NO⁺ reagent ions was used for product analysis. The products methyl vinyl ketone (MVK; C₄H₆O) and methacrolein (MACR; C₄H₆O) were differentiated using NO⁺ reagent ions. The MVK and MACR yields via the HO₂ 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 HO₂ pathway implies concomitant production of hydroxyl ((6.3 ± 2.1)%) and hydroperoxyl ((6.3 ± 2.1)%) radicals, meaning a HO_x recycling of (12.6 ± 4.2)% given that HO₂ 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 (m/z) ratios as the MVK and MACR product ions for HO₂-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_x recycling reported in this study can improve the accuracy of the simulation of the HO₂ 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.