Kinetics and Product Branching Ratio Study of the CH(3)O(2) Self-Reaction in the Highly Instrumented Reactor for Atmospheric Chemistry

[Image: see text] The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH(3)O(2)) using the conversion of CH(3)O(2) into methoxy radicals (CH(3)O) by excess NO, followed by the detection of CH(3)O, has been used to study the kinetics of the self-reaction of CH(3)O(2). 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) cm(3) molecule(–1) s(–1) at 295 K was obtained by the kinetic analysis of the observed second-order decays of CH(3)O(2). The temperature dependence of the overall rate coefficient can be characterized by k(overall) = (9.1 ± 5.3) × 10(–14) × exp((252 ± 174)/T) cm(3) 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 CH(3)O(2). 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 CH(3)O(2) can be rationalized by a combination of underestimated optical absorbance of CH(3)O(2) and unaccounted CH(3)O(2) losses to the walls of the reaction cells employed.