Kinetics and Product Branching Ratio Study of the CH₃O₂ Self-Reaction in the Highly Instrumented Reactor for Atmospheric Chemistry

Abstract

The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH3O2) using the conversion of CH3O2 into methoxy radicals (CH3O) by excess NO, followed by the detection of CH3O, has been used to study the kinetics of the self-reaction of CH3O2. Fourier transform infrared (FTIR) spectroscopy has been employed to determine the products methanol and formaldehyde of the self-reaction. The kinetics and product studies were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) in the temperature range 268–344 K at 1000 mbar of air. The product measurements were used to determine the branching ratio of the reaction channel forming methoxy radicals, r CH3O. A value of 0.34 ± 0.05 (errors at 2σ level) was determined for r CH3O at 295 K. The temperature dependence of r CH3O can be parametrized as r CH3O = 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}. An overall rate coefficient of the self-reaction of (2.0 ± 0.9) × 10–13 cm3 molecule–1 s–1 at 295 K was obtained by the kinetic analysis of the observed second-order decays of CH3O2. The temperature dependence of the overall rate coefficient can be characterized by k overall = (9.1 ± 5.3) × 10–14 × exp((252 ± 174)/T) cm3 molecule–1 s–1. The found values of k overall in the range 268–344 K are ∼40% lower than the values calculated using the recommendations of the Jet Propulsion Laboratory and IUPAC, which are based on the previous studies, all of them utilizing time-resolved UV–absorption spectroscopy to monitor CH3O2. A modeling study using a complex chemical mechanism to describe the reaction system showed that unaccounted secondary chemistry involving Cl species increased the values of k overall in the previous studies using flash photolysis to initiate the chemistry. The overestimation of the k overall values by the kinetic studies using molecular modulation to generate CH3O2 can be rationalized by a combination of underestimated optical absorbance of CH3O2 and unaccounted CH3O2 losses to the walls of the reaction cells employed.

Calculated and Empirical Values of Vibronic Transition Dipole Moments of Reactive Chemical Intermediates for Determination of Concentrations

“…In these reports, the self-reaction rate constant of CH 3 O 2 always has been taken near the recommended value , of k SR = 3.5 × 10 –13 cm 3 molecule –1 s –1 at 298 K (note that because of rapid follow-up reactions the effective, or observed, rate constant, k OB , is 4.8 × 10 –13 cm 3 molecule –1 s –1 for the value of k SR above.) However, a recent report by Onel et al gives a value of k SR = 2.1 × 10 –13 cm 3 molecule –1 s –1 , 40% lower than the recommended value. Correspondingly, this would lead to a reduction of 40% in the cross sections quoted above.…”

Kinetics of the cross‐reaction of CH3O2 + HO2 radicals measured in the Highly Instrumented Reactor for Atmospheric Chemistry

“…Using a kinetic model consisting of the central reactions, where all rate coefficients are fixed except for that of reaction (8), combined with the initial radical concentrations of both radicals, k 8 can be obtained. The reactions included in the model are summarized in Table 1 for the fit of the data at T = 295 K using IUPAC and JPL recommendations, 12,13 the rate coefficient and branching ratio ( α ) of the self‐reaction of CH 3 O 2 , reaction (6), ( k 6 and α 6b = k 6b / k 6 ) from Onel et al., 11 and the experimentally determined HO 2 heterogeneous loss to the walls from an HO 2 self‐reaction experiment performed in HIRAC. Using different initial CH 4 and CH 3 OH concentrations, a range of initial concentrations, [CH 3 O 2 ] 0 and [HO 2 ] 0 , were generated for each radical decay over the course of the experiments.…”

“…The overall rate coefficient for reaction (6) was found to be ∼40% lower than the recommended values by the International Union of Pure and Applied Chemistry (IUPAC) 12 and the Jet Propulsion Laboratory (JPL) 13 . Onel et al 11 . suggested that the overestimation by the previous studies 14–21 was caused by secondary chemistry in some experiments and a combination of overestimation of optical properties (absorption coefficients) with unaccounted CH 3 O 2 heterogenous losses in other experiments.…”

“…CH 2 O formed in channel (6a) is toxic via inhalation and its photolysis provides a source of HO 2 , and channel (6b) leads to the formation of HO 2 radicals, as described above in reaction (4). The rate coefficient, k 6 , and the branching ratio of the channel forming CH 3 O radicals, α 6b = k 6b / k 6 , has recently been remeasured by Onel et al 11 . They found k 6 = (2.0 ± 0.4) × 10 −13 cm 3 molecule −1 s −1 and α 6b = 0.34 ± 0.02 at 295 K as well as elucidating the temperature dependence of the rate coefficient and branching ratio.…”

Advances in Atmospheric Chemical and Physical Processes

“…The initial products of oxidant/VOC reactions are often organic peroxy radicals, and studies of these species are represented in this issue. Studies of bimolecular peroxy radical reactions, including self-reactions of CH 3 O 2 and of CH 3 C­(O)­OO are presented, , as is a study of the impacts of autoxidation on the formation of highly oxygenated species (HOMs) following reaction of ozone with α-pinene . The reaction of ozone with VOCs produces Criegee intermediates.…”