Methylperoxy self-reaction: products and branching ratio between 223 and 333 K

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

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.

Near-infrared kinetic spectroscopy of the HO2 and C2H5O2 self-reactions and cross reactions

The self-reactions and cross reactions of the peroxy radicals C2H5O2 and HO2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO2, and UV absorption monitored C2H5O2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221-296 K. The Arrhenius expression determined for the cross reaction, k2(T) = (6.01(-1.47)(+1.95)) x 10(-13) exp((638 +/- 73)/T) cm3 molecules(-1) s(-1) is in agreement with other work from the literature. The measurements of the HO2 self-reaction agreed with previous work from this lab and were not further refined. The C2H5O2 self-reaction is complicated by secondary production of HO2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO2. The Arrhenius expression for the self-reaction rate constant is k3(T) = (1.29(-0.27)(+0.34)) x 10(-13)exp((-23 +/- 61)/T) cm3 molecules(-1) s(-1), and the branching fraction value is alpha = 0.28 +/- 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO2 self-reactions are required.