Investigating isoprene photo-oxidation at low-NOx conditions using PTR-TOF-MS

Recent studies have shown that gas-phase isoprene chemistry plays an important role in the formation of secondary organic aerosol as well as in HOx radical recycling, but many uncertainties remain in the mechanisms governing these processes. Photo-oxidation of isoprene was investigated in the Harvard Environmental Chamber at steady-state mode using H2O2 as the primary HOx source. The concentrations of isoprene, H2O2 and NO, with typical values of 10-80 ppb, 10 ppm, and <0.15 ppb, respectively, were precisely controlled such that the isoprene peroxy radical chemistry was dominated by the HO2 pathway. There are few other laboratory studies that specifically probe this reaction channel, which is important in remote regions of the atmosphere with low NOx levels. A proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) equipped with switchable reagent ion capability (SRI, including H3O+, NO+ and O2+) was used to measure isoprene and its oxidation products. Isomeric ketones and aldehydes, such as the two major isoprene oxidation products methyl vinyl ketone (MVK, C4H6O) and methacrolein (MAC, C4H6O), which cannot be separated using the normal H3O+ ionization, were successfully differentiated using NO+ at optimized drift tube conditions. The NO+ ion reacted with MAC to give mainly dehydrogenated cations C4H5O+ and a small amount of C4H6O.NO+ cluster ions, whereas it reacted with MVK to give only C4H6O.NO+ cluster ions. This feature was used to separately quantify MVK and MAC yields from isoprene oxidation. Since the production of MVK and MAC via the HO2 pathway may be accompanied by HOx recycling, the quantified yields of MVK and MAC are valuable for understanding possible HOx recycling mechanisms in remote regions of the atmosphere with low NOx levels. The extensive information on molecular formulas and chemical functionality provided by the TOF and SRI capabilities is also being used to facilitate the identification of other oxidation products of isoprene.