HO2 formation from the OH plus benzene reaction in the presence of O2

In this study we investigated the secondary formation of HO2 following the benzene + OH reaction in N-2 with variable O-2 content at atmospheric pressure and room temperature in the absence of NO. After pulsed formation of OH, HOx (= OH + HO2) and OH decay curves were measured by means of a laser-induced fluorescence technique (LIF). In synthetic air the total HO2 yield was determined to be 0.69 +/- 0.10 by comparison to results obtained with CO as a reference compound. HO2 is expected to be a direct product of the reaction of the intermediately formed OH-benzene adduct with O-2. The HO2 yield is slightly greater than the currently recommended yield of the proposed HO2 co-product phenol (similar to 53%). This hints towards other, minor HO2 forming channels in the absence of NO, e. g. the formation of epoxide species that was proposed in the literature. For other test compounds upper limits of HO2 yields of 0.10 (isoprene) and 0.05 (cyclohexane) were obtained, respectively. In further experiments at low O-2 concentrations (0.06-0.14% in N-2) rate constants of (2.4 +/- 1.1) x 10(-16) cm(3) s(-1) and (5.6 +/- 1.1) x 10(-12) cm(3) s(-1) were estimated for the OH-benzene adduct reactions with O-2 and O-3, respectively. The rate constant of the unimolecular dissociation of the adduct back to benzene + OH was determined to be (3.9 +/- 1.3) s(-1). The HO2 yield at low O-2 was similar to that found in synthetic air, independent of O-2 and O-3 concentrations indicating comparable HO2 yields for the adduct + O-2 and adduct + O-3 reactions.