Kinetics of the cross-reaction of CH₃O₂ + HO₂ radicals measured in the Highly Instrumented Reactor for Atmospheric Chemistry

The sensitive Fluorescence Assay by Gas Expansion (FAGE) method has been used to detect methyl peroxy (CH3O2) and hydroperoxyl (HO2) radicals after their conversion by titration with excess NO to methoxy (CH3O) and hydroxyl (OH) radicals, respectively, to study the kinetics of the reaction of CH3O2 + HO2 radicals. The rate coefficient of the reaction was measured in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) at 1000 mbar of synthetic air at T = 268-344 K, selectively detecting both radicals. Using a numerical model to fit both CH3O2 and HO2 radical temporal decays globally at each temperature investigated, rate coefficients for the reaction have been obtained. The room temperature rate coefficient was found to be k(CH3O2 +HO2)(295 K) = (4.6 +/- 0.7) x 10(-12) molecule(-1) cm(3) s(-1) (2 sigma errors) and the temperature dependence of the rate coefficient can be characterized in Arrhenius form by k(CH3O2 + HO2)(268 K T < 344 K) = (5.1 +/- 2.1) x 10(-13) x exp((637 +/- 121)/T) cm(3) molecule(-1) s(-1). The rate coefficients obtained here are 14%-16% lower than the literature recommended values with an uncertainty which is reduced significantly compared to previous reports.