1School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA
2Institute of Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria
3Department of Chemistry, Amherst College, Amherst, Massachusetts, USA
4Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA
Abstract. The photo-oxidation chemistry of isoprene (C5H8) was studied in a continuous-flow chamber under conditions such that the reactions of isoprene-derived peroxyl radicals (RO2) were dominated by 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 were 4.3 ± 0.4% and 3.2 ± 0.3%, respectively, for HO2-dominant conditions at +25 °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 HO2-dominant conditions imply a concomitant yield (i.e., recycling factor) of hydrogen oxide radicals (HOx) of 15 ± 0.7% from the reaction of isoprene-derived RO2 with HO2. Other isoprene oxidation products, believed to be organic hydroperoxides, also contributed to the ion intensity at the same mass-to-charge (m/z) 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 HOx yields reported in this study will improve the accuracy of simulations of the HO2 reaction pathway of isoprene, which has been shown to make a significant contribution to the total reactivity of isoprene-derived RO2 radicals on a global scale.