The reactions between OH radicals and hydrogen halides (HCl, HBr, HI) have been studied between 298 and 460 K by using a discharge flow-electron paramagnetic resonance technique. The rate constants were found to be (298 K) = (7.9 ± 1.3) × 10 cm molecule s with a weak positive temperature dependence, (298-460 K) = (1.04 ± 0.2) × 10 cm molecule s, and (298 K) = (3.0 ± 0.3) × 10 cm molecule s, respectively. The homogeneous nature of these reactions has been experimentally tested.

Levels of the stabilized Criegee Intermediate (sCI), produced via the ozonolysis of unsaturated volatile organic compounds (VOCs), were estimated at two London urban sites (Marylebone Road and Eltham) and one rural site (Harwell) in the UK over the period of 1998-2012. The steady-state approximation was applied to data obtained from the NETCEN (National Environmental Technology Centre) database, and the levels of annual average sCI were estimated to be in the range of 30-3000 molecules cm for UK sites. A consistent diurnal cycle of sCI concentration is estimated for the UK sites with increasing levels during daylight hours, peaking just after midday. The seasonal pattern of sCI shows higher levels in spring with peaks around May due to the higher levels of O. The ozone weekend effect resulted in higher sCI in UK urban areas during weekend. The sCI data were modeled using the information provided by the Air Quality Improvement Research Program (AQIRP) and found that the modeled production was five- to six-fold higher than our estimated data, and therefore the estimated sCI concentrations in this study are thought to be lower estimates only. Compared with nighttime, 1.3- to 1.8-fold higher sCI exists under daytime conditions. Using the levels of sCI estimated at Marylebone Road, globally the oxidation rates of NO + sCI (22.4 Gg/yr) and SO + sCI (37.6 Gg/yr) in urban areas can increase their effect in the troposphere and potentially further alter the oxidizing capacity of the troposphere. Further investigations of modeled sCI show that CHCHOO (64%) and CHOO (13%) are dominant among all contributing sCI at the UK sites.

A comprehensive and hierarchical optimization of a joint hydrogen and syngas combustion mechanism has been carried out. The Kéromnès et al. (, 2013, 160, 995-1011) mechanism for syngas combustion was updated with our recently optimized hydrogen combustion mechanism (Varga et al., , 2015, 35, 589-596) and optimized using a comprehensive set of direct and indirect experimental data relevant to hydrogen and syngas combustion. The collection of experimental data consisted of ignition measurements in shock tubes and rapid compression machines, burning velocity measurements, and species profiles measured using shock tubes, flow reactors, and jet-stirred reactors. The experimental conditions covered wide ranges of temperatures (800-2500 K), pressures (0.5-50 bar), equivalence ratios ( = 0.3-5.0), and C/H ratios (0-3). In total, 48 Arrhenius parameters and 5 third-body collision efficiency parameters of 18 elementary reactions were optimized using these experimental data. A large number of directly measured rate coefficient values belonging to 15 of the reaction steps were also utilized. The optimization has resulted in a H/CO combustion mechanism, which is applicable to a wide range of conditions. Moreover, new recommended rate parameters with their covariance matrix and temperature-dependent uncertainty ranges of the optimized rate coefficients are provided. The optimized mechanism was compared to 19 recent hydrogen and syngas combustion mechanisms and is shown to provide the best reproduction of the experimental data.

Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second-order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D), decamethylcyclopentasiloxane (D), and dodecamethylcyclohexasiloxane (D) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D and D with the OH radical are 1.9 × 10 (95% confidence interval (CI): (1.7-2.2) × 10) and 2.6 × 10 (CI: (2.3-2.9) × 10) cm molecule s, respectively, which are 1.9× and 1.7× faster than previous measurements. Our measured rate constant for D is 2.8 × 10 (CI: (2.5-3.2) × 10) cm molecule s and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D were 33% higher than for D (CI: 30-37%), whereas the rates for D were only 8% higher than for D (CI: 5-10%). The activation energies of the reactions of D, D, and D with OH were not statistically different and had a value of 4300 ± 2800 J/mol.

The bimolecular rate coefficients [Formula: see text] and [Formula: see text] were measured using the relative rate technique at (297 ± 3) K and 1 atmosphere total pressure. Values of (2.7 ± 0.7) and (4.0 ± 1.0) × 10 cm molecule s were observed for [Formula: see text] and [Formula: see text], respectively. In addition, the products of [Formula: see text] and [Formula: see text] gas-phase reactions were investigated. Derivatizing agents -(2,3,4,5,6-pentafluorobenzyl)hydroxylamine and -bis (trimethylsilyl)trifluoroacetamide and gas chromatography mass spectrometry (GC/MS) were used to identify the reaction products. For [Formula: see text] reaction: hydroxyacetaldehyde, 3-hydroxypropanal, 4-hydroxybutanal, butoxyacetaldehyde, and 4-(2-oxoethoxy)butan-2-yl nitrate were the derivatized products observed. For the [Formula: see text] reaction: benzaldehyde ((CH)C(=O)H) was the only derivatized product observed. Negative chemical ionization was used to identify the following nitrate products: [(2-butoxyethoxy)(oxido)amino]oxidanide and benzyl nitrate, for [Formula: see text] and [Formula: see text], respectively. The elucidation of these products was facilitated by mass spectrometry of the derivatized reaction products coupled with a plausible 2-butoxyethanol or [Formula: see text] reaction mechanisms based on previously published [Formula: see text] gas-phase mechanisms.