The reaction of the OH radical with isoprene, CH (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k = (9.90 ± 0.09) × 10 cm molecule s) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + CH ⇌ HOCH equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C and C to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol for the C adduct). Addition to the less stable C and C adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k = (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k( T, [ M]). An analytical representation of k( T, [ M]) and k( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k = (9.5 ± 0.2) × 10( T/298 K) + (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s, significantly extend the measured temperature range of R1.

Abstract. OH and HO2 radicals are closely coupled in the atmospheric oxidation and combustion of volatile organic compounds (VOCs). Simultaneous measurement of HO2 yields and OH kinetics can provide the ability to assign site-specific rate coefficients that are important for understanding the oxidation mechanisms of VOCs. By coupling a fluorescence assay by gaseous expansion (FAGE) laser-induced fluorescence (LIF) detection system for OH and HO2 with a high-pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-1st-order loss processes up to ∼100 000 s−1 and to determine HO2 yields via time-resolved measurements. This time resolution allows discrimination between primary HO2 from the target reaction and secondary production from side reactions. The apparatus was characterized by measuring yields from the reactions of OH with H2O2 (1:1 link between OH and HO2), with C2H4∕O2 (where secondary chemistry can generate HO2), with C2H6∕O2 (where there should be zero HO2 yield), and with CH3OH∕O2 (where there is a well-defined HO2 yield). As an application of the new instrument, the reaction of OH with n-butanol has been studied at 293 and 616 K. The bimolecular rate coefficient at 293 K, (9.24±0.21)×10-12 cm3 molec.−1 s−1, is in good agreement with recent literature, verifying that this instrument can measure accurate OH kinetics. At 616 K the regeneration of OH in the absence of O2, from the decomposition of the β-hydroxy radical, was observed, which allowed the determination of the fraction of OH reacting at the β site (0.23±0.04). Direct observation of the HO2 product in the presence of oxygen has allowed the assignment of the α-branching fractions (0.57±0.06) at 293 K and (0.54±0.04) at 616 K, again in good agreement with recent literature; branching ratios are key to modelling the ignition delay times of this potential “drop-in” biofuel.

Literature rate coefficients for the prototypical radical–radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO2 radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H2O2 reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k 1,298K ∼1 × 10–11 cm3 molecule–1 s–1, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k 1,H2O, 298K = (2.17 ± 0.09) × 10–28 cm6 molecule–2 s–1, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k 1,298K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD­(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD­(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k 1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO2 → HCl + O2. The implications of these results in atmospheric models are discussed.

While forward and reverse rate constants are frequently used to determine enthalpies of reaction and formation, this process is more difficult for pressure-dependent association/dissociation reactions, especially since the forward and reverse reactions are usually studied at very different temperatures. The problems can be overcome by using a data-fitting procedure based on a master equation model. This approach has been applied to existing experimental pressure-dependent forward and reverse rate coefficients for the reaction C 2 H 4 + H ⇄ C 2 H 5 (k 1 , k −1 ) using the MESMER code to determine Δ f H 0 ⊖ C 2 H 5 from the enthalpy of the reaction. New measurements of k 1 , k −1 were included in analysis. They are based on laser flash photolysis with direct observation of H atom time profiles by vacuum ultraviolet laser-induced fluorescence under conditions where the approach to equilibrium could be observed. Measurements were made over the temperature range 798−828 K and with [He] from 2.33 to 7.21 × 10 18 molecule cm −3 . These data were then combined with a wide range of existing experimental data with helium as the bath gas (112 measurements of k 1 and k −1 , covering the temperature range 285−1094 K, and [He] = 7.1 × 10 15 −1.9 × 10 19 molecule cm −3 ) and fitted using the master equation solver MESMER. The required vibrational frequencies and rotational constants of the system were obtained from ab initio calculations, and the activation threshold for association (ΔE thresh ), enthalpy of reaction (Δ r H 0 ⊖ ), imaginary frequency (υ imag ), and helium energy-transfer parameters (⟨ΔE⟩ d,298 (T/298) n ) were optimized. The resulting parameters (errors are 2σ) are ΔE thresh = 11.43 ± 0.34 kJ mol −1 , Δ r H 0 ⊖ = −145.34 ± 0.60 kJ mol −1 , υ imag = 730 ± 130 cm −1 , ⟨ΔE⟩ d,298 = 54.2 ± 7.6 cm −1 , and n = 1.17 ± 0.12. A value of Δ f H 298.15 ⊖ (C 2 H 5 ) = 120.49 ± 0.57 kJ mol −1 is obtained by combining Δ r H 0 ⊖ with standard enthalpies of formation for H and C 2 H 4 and making the appropriate temperature corrections. The dependence of these parameters on how the internal rotor and CH 2 inversion modes are treated has been explored. The experimental data for other bath gases have been analyzed, and data sets compatible with the potential energy surface parameters determined above have been identified. The parameters are virtually identical but with slightly smaller error limits. Parameterization of k 1 , k −1 using the Troe formalization has been used to investigate competition between ethyl decomposition and reaction with oxygen under combustion conditions.

Abstract. Multi-oxygenated volatile organic compounds are important markers of air pollution and precursors of ozone and secondary aerosols in both polluted and remote environments. Herein, their accurate determination was enhanced. The approach was based on an automated system for active sampling and on-fibre derivatization coupled with the gas chromatography–mass spectrometry (GC–MS) technique. The method capability was determined for different compound families, such as aldehydes, ketones, α-dicarbonyls, hydroxy-aldehydes, hydroxy-ketones, and carboxylic acids. A good accuracy (<7 %) was demonstrated from the results compared to Fourier-transform infrared spectroscopy (FTIR). Limits of detection (LODs) of 6–100 pptV were achieved with a time resolution lower than 20 min. The developed method was successfully applied to the determination of multi-oxygenated compounds in air samples collected during an intercomparison campaign (EUROCHAMP-2020 project). Also, its capability and accuracy for atmospheric monitoring was demonstrated in an isoprene ozonolysis experiment. Both were carried out in the high-volume outdoor atmospheric simulation chambers (EUPHORE, 200 m3). In summary, our developed technique offers near-real-time monitoring with direct sampling, which is an advantage in terms of handling and labour time for a proper quantification of trace levels of atmospheric multi-oxygenated compounds.