Experimental and theoretical investigation of the reaction of RO2 radicals with OH radicals: Dependence of the HO2 yield on the size of the alkyl group

The HO2 yield in the reaction of peroxy radicals with OH radicals has been determined experimentally at 50Torr helium by measuring simultaneously OH and HO2 concentration time profiles, following the photolysis of XeF2 in the presence of different hydrocarbons and O-2. The following yields have been obtained: phi CH 3O2=(0.90 +/- 0.1), phi C2H5O2=(0.75 +/- 0.15), phi C3H7O2=(0.41 +/- 0.08), and phi C4H9O2=(0.15 +/- 0.03). The clear decrease in HO2 yield with increasing size of the alkyl moiety can be explained by an increased stabilization of the trioxide adduct, ROOOH. This has been confirmed by ab initio and Rice-Ramsperger-Kassel-Marcus master equation calculations. Extrapolation of the experimental results to atmospheric conditions shows that the stabilized adduct, ROOOH, is the nearly exclusive product of the reaction between OH radicals and peroxy radicals containing more than three C-atoms. The fate and possible impact of these species is completely unexplored so far.