Robert Lesclaux

Atmospheric chemistry of small organic peroxy radicals

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up-to-date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self-consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three-dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.

Peroxy radical kinetics resulting from the OH-initiated oxidation of 1,3-butadiene, 2,3-dimethyl-1,3-butadiene and isoprene

The laser flash photolysis/UV absorption spectrometry technique has been used to investigate the kinetics of the peroxy radical permutation reactions (i.e. self and cross reactions) arising from the OH-initiated oxidation of isoprene (2-methyl-1,3-butadiene), and of the simpler, but related conjugated dienes, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene. The results of the two simpler systems are analysed to provide values of the rate coefficients for the 6 peroxy radical permutation reactions of the three types of isomeric peroxy radical produced in each system (T = 298 K, P = 760 Torr). The rate coefficients are all significantly larger than values estimated previously by extrapolation of structure-reactivity relationships based on the kinetics of a limited dataset of simpler radicals containing similar structural features. The results are discussed in terms of trends in self and cross reaction reactivity of primary, secondary and tertiary peroxy radicals containing combinations of allyl, β-hydroxy and δ-hydroxy functionalities. Since the peroxy radicals formed in these systems are structurally very similar to those formed in the isoprene system, the kinetic parameters derived from the results of the simpler systems are used to assist the assignment of kinetic parameters to the 21 permutation reactions of the six types of isomeric peroxy radical generated in the isoprene system. Kinetic models describing the OH-initiated degradation of all three conjugated dienes to first generation products in the absence of NOx are recommended, which are also consistent with available end product studies. The model for isoprene is considered to be a further improvement on that suggested previously for its OH-initiated oxidation in the absence of NOx. The mechanism is further extended to include chemistry applicable to ‘NOx-present’ conditions, and calculated product yields are compared with those reported in the literature.

Kinetics of the reaction of HO2 with CH3C(O)O2 in the temperature range 253–368 K

The kinetics of the reaction between the HO2 and CH3C(O)O2 radicals has been studied in the gas phase at atmospheric pressure between 253 and 368 K. Flash photolysis was used to produce the radicals which were detected by their absorption in the UV. The two reactive channels for this reaction are CH3C(O)O2+HO2→CH3C(O)OOH+O2 (1a) and→CH3C(O)OH+O3 (1b). The overall rate constant measurement gave a value of k1 = (4.3±1.2)×10−13 exp[(1040±100)/T] cm3 molecule−1 s−1; the branching ratio k1b/(k1a+k1b) was found to be independent of temperature and equal to 0.33±0.07.

Flash photolysis studies of the reaction of NH2 radicals with NO

Abstract The kinetics of the reaction NH 2 + NO → N 2 + H 2 O were studied, using a conventional flash photolysis system. A value of k 1 = (1.1 ± 0.2) × 10 10 & mole −1 s −1 was obtained at room temperature and in the pressure range 2–700 torr in the presence of nitrogen. A slight negative temperature coefficient was observed between 300 and 500 K, equivalent to a negative activation energy of 1.05 ± 0.2 kcal mole −1 .

Gas phase oxidation of benzene: Kinetics, thermochemistry and mechanism of initial steps

A new investigation of the primary steps of the benzene oxidation, involving complementary experimental and theoretical approaches, is presented. The reactions of the OH-adduct (hydroxy-cyclohexadienyl radical c-C6H6-OH) were investigated using laser flash photolysis and producing OH radicals by H2O2 photolysis at 248 nm. It is confirmed that the adduct is in equilibrium with the corresponding peroxy radical RO2, near atmospheric conditions, the measured equilibrium constant being: Kc,2b = (2.62 ± 0.24) × 10−19 cm3 molecule−1 at 295 K, with the temperature dependent expression: ln(Kc,2b/cm3 molecule−1) = −63.29 + 6049/T, obtained by using the calculated entropy of reaction. The rate constant of the association reaction yielding RO2 is: k2b = (1.31 ± 0.12) × 10−15 cm3 molecule−1 s−1. Calculated data are in agreement with those values. In addition, data analysis shows that the reaction c-C6H6-OH + O2 involves an irreversible loss of radical species, yielding phenol and other oxidation products, with the global rate constant: kloss = (2.52 ± 0.40) × 10−16 cm3 molecule−1 s−1. Quoted errors are statistical (2σ), the possible total errors on the above values being estimated at around 40%. By comparison with the kloss value, the rate constant for phenol formation, calculated using a combination of DFT and ab initio CCSD(T) methods, corresponds to a phenol yield of about 55%, in reasonable agreement with experimental observations. Thermochemical and kinetic parameters have been calculated for the formation and for the reactions of the two RO2 stereoisomers, cis and trans. They show that the observed equilibrium must involve the trans isomer, which is more stable and is formed more rapidly than the cis isomer. Calculations show that the only possible reactions of peroxy radicals, under atmospheric conditions, is cyclisation yielding a bicyclic radical. However, cyclisation of the RO2(trans) is calculated to be too slow, compared to the rate of the irreversible radical loss, whereas it is very fast in the case of the cis isomer and can lead readily to oxidation products. On the basis of those results, a reaction mechanism is proposed for the first steps of benzene oxidation, consistent with all experimental and theoretical data, and which accounts for the principal oxidation products observed.

The phenoxy radical: UV spectrum and kinetics of gas-phase reactions with itself and with oxygen

Abstract The kinetics of the gas-phase reactions of the phenoxy radical with itself and with oxygen were investigated using flas photolysis coupled to UV absorption spectrometry at 760 Torr, using N 2 as the buffer gas. The UV absorption spectrum of the radical, which was determined from 220 to 300 nm, exhibits two intense bands at 232 and 280 nm, with cross section values of 4.0 and 1.1 × 10 −17 cm 2 molecule −1 , respectively. The rate expression for the self-reaction was determined from 280 to 423 K: k 1 ( T ) = (1.44 ± 0.16) × 10 −12 exp[(631 ± 37)K/ T ] cm 3 molecule −1 s −1 . No reaction could be detected between the phenoxy radical and molecular oxygen, with an upper limit for the rate constant estimated at 2 × 10 −18 cm 3 molecule −1 s −1 , for temperatures up to 500 K.

Kinetics of the reaction of CF3 with O2 over the temperature range 233–373 K

Abstract The rate constant of the reaction CF 3 + O 2 + M → CF 3 O 2 + M has been measured as a function of temperature in the fall-off region between 1 and 10 Torr, by laser photolysis and time-resolved mass spectrometry. The values of the fall-off parameters measured at room temperature are in good agreement with previous values and the temperature dependence of k 1(0) is reasonably well accounted for by the theoretical analysis proposed by Troe and co-workers. With M = N 2 k 1(0) = (1.9 ± 0.2) × 10 −29 ( T /298) (−4.7 ± 0.4) cm 6 molecule −2 s −1 . Reliable values of k 1(∞) cannot be derived from these low pressure measurements. The rate expression proposed, k 1(∞) = (9 ± 2) X 10 −12 ( T /298) (0±1) cm 3 molecule −1 s −1 , is consistent with a “broadening parameter” of the form F c = exp(− T /395) and previous values reported at room temperature.

Flash photolysis kinetic study of reactions of the BrO radical with BrO and HO2

Abstract The reaction between BrO and HO 2 radicals: BrO + HO 2 → HOBr + O 2 (1a) → HBr + O 3 (1b) was investigated using flash photolysis with UV absorption detection of the radicals, at 298 K and 760 Torr total pressure. BrO and HO 2 radicals were formed in the flash photolysis of Br 2 /O 3 /Cl 2 /CH 3 OH/O 2 /He mixtures. The self-reaction of BrO radicals was also studied: BrO + BrO → 2Br + O 2 (2a) → Br 2 + O 2 (2b). Kinetic simulations of decays of BrO radicals yielded k 1 = (3.4 ± 1.0) X 10 −11 cm 3 molecule −1 s −1 and k 2 = (3.1 ± 0.4) X 10 −12 cm 3 molecule −1 s −1 .

The rate constant for the HO2+HO2 reaction at elevated temperatures

Abstract The HO 2 +HO 2 reaction has been studied at atmospheric pressure between 298 and 777 K, using a new high-temperature flash photolysis/UV kinetic spectroscopy apparatus. An upward curvature of the Arrhenius plot is observed at temperatures above 600 K, supporting a recent suggestion that - in addition to the well-established low-temperature association complex mechanism - a direct abstraction reaction with a positive activation energy operates at combustion temperatures. Arrhenius parameters for this direct reaction are estimated.

Theoretical study on the atmospheric fate of carbonyl radicals: kinetics of decomposition reactions

The decomposition kinetics of a large set of representative R–CO carbonyl radicals has been studied theoretically. These radicals can either decompose into R + CO or, in the presence of oxygen, add to O2 to give acylperoxy radicals RC(O)O2. In this work, it is shown, by comparison to available experimental data, that reliable quantitative kinetic parameters for decomposition reactions can be obtained using DFT (B3LYP and BH&HLYP functionals) and ab initio G2(MP2) methods. Moreover, it has been demonstrated, by performing RRKM calculations, that the dissociation of carbonyl radicals under atmospheric conditions is not only governed by the height of the barriers, but that pressure effects can also play an important role and must be taken into account. Structure–activity relationships for the decomposition of R–CO radicals are presented according to the nature of the R group. The R–CO radicals, which are predicted to decompose under atmospheric conditions, contain chlorine or oxygen in the R group.