Kinetics and Product Branching Ratio Study of the CH3O2Self-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 CH3O2into 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, rCH3O. A value of 0.34 ± 0.05 (errors at 2σ level) was determined for rCH3Oat 295 K. The temperature dependence of rCH3Ocan be parametrized as rCH3O= 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}. An overall rate coefficient of the self-reaction of (2.0 ± 0.9) × 10-13cm3molecule-1s-1at 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 koverall= (9.1 ± 5.3) × 10-14× exp((252 ± 174)/T) cm3molecule-1s-1. The found values of koverallin 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 koverallin the previous studies using flash photolysis to initiate the chemistry. The overestimation of the koverallvalues by the kinetic studies using molecular modulation to generate CH3O2can be rationalized by a combination of underestimated optical absorbance of CH3O2and unaccounted CH3O2losses to the walls of the reaction cells employed.