Rate constants of CH₃O₂ + NO₂ CH₃O₂NO₂ and C₂H₅O₂ + NO₂ C₂H₅O₂NO₂ reactions under atmospheric conditions

Abstract

Peroxy (RO2) radicals derived from short‐chain alkanes were detected by an improved technique involving combined fluorescence assay by gas expansion and laser‐induced fluorescence (FAGE‐LIF) and chemical conversion under atmospheric conditions (298 K and 1 bar). The suitability of the system for measuring RO2 kinetics was tested using the CH3O2 + NO2 + M ⇄ CH3O2NO2 + M reaction. CH3O2 radicals were generated by the OH + CH4 → CH3 + H2O reaction and subsequent O2 addition. The CH3O2 radicals were monitored using the LIF intensity of the OH radicals, which were converted from CH3O2 radicals by chemical reaction with NO and O2. The decay rates of CH3O2 radicals were measured at various NO2 concentrations, and the bimolecular rate constant was determined to be kCH3O2 = (3.9 ± 0.3) × 10−12 cm3 molecule−1 s−1. The result is consistent with the literature value, thus demonstrating the validity of the improved system. We also constructed a fitting equation for the simultaneous determination of the forward and reverse reaction rates because the thermal decomposition of CH3O2NO2 was not negligible. From the global fitting analysis using the fitted equation, the decomposition rate of CH3O2NO2 was determined to be kCH3O2NO2 = 2.0 ± 0.6 s−1. Furthermore, we investigated the C2H5O2 + NO2 + M ⇄ C2H5O2NO2 + M reaction. The forward reaction rate was approximately 1.7 times faster than that of the CH3O2 + NO2 + M reaction: kC2H5O2 = (6.5 ± 0.4) × 10−12 cm3 molecule−1 s−1, and the reverse reaction rate was 3.6 ± 0.7 s−1. These results agree with the previous estimates.