Rate constants of CH 3 O 2 + NO 2 ⇄ CH 3 O 2 NO 2 and C 2 H 5 O 2 + NO 2 ⇄ C 2 H 5 O 2 NO 2 reactions under atmospheric conditions

Peroxy (RO 2 ) 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 RO 2 kinetics was tested using the CH 3 O 2 + NO 2 + M ⇄ CH 3 O 2 NO 2 + M reaction. CH 3 O 2 radicals were generated by the OH + CH 4 → CH 3 + H 2 O reaction and subsequent O 2 addition. The CH 3 O 2 radicals were monitored using the LIF intensity of the OH radicals, which were converted from CH 3 O 2 radicals by chemical reaction with NO and O 2 . The decay rates of CH 3 O 2 radicals were measured at various NO 2 concentrations, and the bimolecular rate constant was determined to be kCH 3 O 2 = (3.9 ± 0.3) ×  10 −12  cm 3  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 CH 3 O 2 NO 2 was not negligible. From the global fitting analysis using the fitted equation, the decomposition rate of CH 3 O 2 NO 2 was determined to be kCH 3 O 2 NO 2 = 2.0 ± 0.6 s −1 . Furthermore, we investigated the C 2 H 5 O 2 + NO 2 + M ⇄ C 2 H 5 O 2 NO 2 + M reaction. The forward reaction rate was approximately 1.7 times faster than that of the CH 3 O 2 + NO 2 + M reaction: k C 2 H 5 O 2 = (6.5 ± 0.4)  ×  10 −12  cm 3  molecule −1  s −1 , and the reverse reaction rate was 3.6 ± 0.7 s −1 . These results agree with the previous estimates.