Kinetics and Product Branching Ratio Study of the CH 3 O 2 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 (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.