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

Abstract. Volatile organic compound (VOC) mixing ratios measured by five independent instruments are compared at a forested site dominated by ponderosa pine (Pinus Ponderosa) during the BEACHON-ROCS field study in summer 2010. The instruments included a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF-MS), a Proton Transfer Reaction Quadrupole Mass Spectrometer (PTR-MS), a Fast Online Gas-Chromatograph coupled to a Mass Spectrometer (GC/MS; TOGA), a Thermal Dissociation Chemical Ionization Mass Spectrometer (PAN-CIMS) and a Fiber Laser-Induced Fluorescence Instrument (FILIF). The species discussed in this comparison include the most important biogenic VOCs and a selected suite of oxygenated VOCs that are thought to dominate the VOC reactivity at this particular site as well as typical anthropogenic VOCs that showed low mixing ratios at this site. Good agreement was observed for methanol, the sum of the oxygenated hemiterpene 2-methyl-3-buten-2-ol (MBO) and the hemiterpene isoprene, acetaldehyde, the sum of acetone and propanal, benzene and the sum of methyl ethyl ketone (MEK) and butanal. Measurements of the above VOCs conducted by different instruments agree within 20%. The ability to differentiate the presence of toluene and cymene by PTR-TOF-MS is tested based on a comparison with GC-MS measurements, suggesting a study-average relative contribution of 74% for toluene and 26% for cymene. Similarly, 2-hydroxy-2-methylpropanal (HMPR) is found to interfere with the sum of methyl vinyl ketone and methacrolein (MVK + MAC) using PTR-(TOF)-MS at this site. A study-average relative contribution of 85% for MVK + MAC and 15% for HMPR was determined. The sum of monoterpenes measured by PTR-MS and PTR-TOF-MS was generally 20–25% higher than the sum of speciated monoterpenes measured by TOGA, which included α-pinene, β-pinene, camphene, carene, myrcene, limonene, cineole as well as other terpenes. However, this difference is consistent throughout the study, and likely points to an offset in calibration, rather than a difference in the ability to measure the sum of terpenes. The contribution of isoprene relative to MBO inferred from PTR-MS and PTR-TOF-MS was smaller than 12% while GC-MS data suggested an average of 21% of isoprene relative to MBO. This comparison demonstrates that the current capability of VOC measurements to account for OH reactivity associated with the measured VOCs is within 20%.

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 &deg;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 (HO_x) 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 (<i>m</i>/<i>z</i>) 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 HO_x 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

Abstract. Biogenic VOC emissions are often dominated by 2-methyl-1,3-butadiene (isoprene) and 2-methyl-3-buten-2-ol (232 MBO). Here we explore the possibility to selectively distinguish these species using NO + as a primary ion in a conventional PTR-MS equipped with an SRI unit. High purity of NO + (> 90 %) as a primary ion was utilized in laboratory and field experiments using a conventional PTR-TOF-MS. Isoprene is ionized via charge transfer leading to the major product ion C 5 H + 8 (> 99 %) (e.g. Spanel and Smith, 1998). 232 MBO undergoes a hydroxide ion transfer reaction resulting in the major product ion channel C 5 H + 9 (> 95 %) (e.g. Amelynck et al., 2005). We show that both compounds are ionized with little fragmentation (< 5 %) under standard operating conditions. Typical sensitivities of 11.1 ± 0.1 (isoprene) and 12.9 ± 0.1 (232 MBO) ncps ppbv −1 were achieved, which correspond to limit of detections of 18 and 15 pptv respectively for a 10 s integration time. Sensitivities decreased at higher collisional energies. Calibration experiments showed little humidity dependence. We tested the setup at a field site in Colorado dominated by ponderosa pine, a 232 MBO emitting plant species. Our measurements confirm 232 MBO as the dominant biogenic VOC at this site, exhibiting typical average daytime concentrations between 0.2-1.4 ppbv. The method is able to detect the presence of trace levels of isoprene at this field site (90-250 ppt) without any interference from 232 MBO, which would not be feasible using H 3 O + ionization chemistry, and which currently also remains a challenge for other analytical techniques (e.g. gas chromatographic methods).