Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation
The photo-oxidation chemistry of isoprene (ISOP; C 5H 8) was studied in a continuous-flow chamber under conditions such that the reactions of the isoprene-derived peroxyl radicals (RO 2) were dominated by the hydroperoxyl (HO 2) pathway. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) with switchable H 3O + and NO + reagent ions was used for product analysis. The products methyl vinyl ketone (MVK; C 4H 6O) and methacrolein (MACR; C 4H 6O) were differentiated using NO + reagent ions. The MVK and MACR yields via the HO 2 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 2 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 HO 2 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 HO 2-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 HO 2 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.