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

Abstract

In this study we investigated the secondary formation of HO 2 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, HO x (= OH + HO 2 ) and OH decay curves were measured by means of a laser-induced fluorescence technique (LIF). In synthetic air the total HO 2 yield was determined to be 0.69 AE 0.10 by comparison to results obtained with CO as a reference compound. HO 2 is expected to be a direct product of the reaction of the intermediately formed OH-benzene adduct with O 2 . The HO 2 yield is slightly greater than the currently recommended yield of the proposed HO 2 co-product phenol (B53%). This hints towards other, minor HO 2 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 HO 2 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 AE 1.1) Â 10 À16 cm 3 s À1 and (5.6 AE 1.1) Â 10 À12 cm 3 s À1were 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 AE 1.3) s À1. The HO 2 yield at low O 2 was similar to that found in synthetic air, independent of O 2 and O 3 concentrations indicating comparable HO 2 yields for the adduct + O 2 and adduct + O 3 reactions.

Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals

“…11) in the radical source as suggested by Peeters et al (2009) and da Silva et al (2010) within the time between formation and detection of RO 2 radicals (20 ms). This upper limit would be consistent with the limit of the rate constant for the 1,6-H-shift given by Peeters et al (2009), which converts to a lifetime of 120 ms, but does not fit to results obtained by Nehr et al (2011), who investigated prompt HO 2 formation on a time scale of one second. However, the same authors lowered recently the estimate for the rate constant to 0.1 s −1 (Peeters and Muller, 2010).…”

“…However, in contrast to alkoxy peroxy radicals, decomposition and isomerization appear to be the dominant pathways, except for the HOCH 2 CH 2 O radical (from ethene plus OH), for which decomposition and reaction with O 2 can be competitive (Atkinson, 1997a). Decomposition rate constants were determined to be on the order of 10 4 to 10 6 s −1 (Atkinson, 1997a,b;Orlando et al, 1998;Vereecken et al, 1999). Decomposition leads to the formation of a hydroxyalkyl radical, which then reacts rapidly and solely with O 2 forming a carbonyl compound plus HO 2 (Atkinson, 1997b;Orlando et al, 2003).…”

Experimental budgets of OH, HO<sub>2</sub>, and RO<sub>2</sub> radicals and implications for ozone formation in the Pearl River Delta in China 2014

“…Like in the Wangdu campaign, chemical modulation was applied on several occasions to test whether the OH measurements obtained normally by laser wavelength modulation are perturbed by artificial OH formation in the detection cell. Such kind of in-terference has been detected in some LIF instruments by applying chemical modulation in field campaigns (Mao et al, 2012;Novelli et al, 2014;Feiner et al, 2016). The chemical modulation system used at Wangdu and Heshan consisted of a flow tube in front of the OH measurement cell and allowed the scavenging of ambient OH by addition of propane in the sampled air flow.…”

“…Another possible reason for the high OH observations could be an interference in the OH measurements. Artifacts of up to 80 % have been reported when OH was measured by laser-induced fluorescence (LIF) in forest environments (Mao et al, 2012;Hens et al, 2014;Novelli et al, 2014;Feiner et al, 2016). The interference was presumably caused by oxidation products of biogenic VOCs in the measuring system and was quantified by chemical modulation of ambient OH before the measured air enters the device.…”

Measurement of interferences associated with the detection of the hydroperoxy radical in the atmosphere using laser-induced fluorescence

“…For OH + VOC reactions that lead to the production of HO 2 with a yield of y (OH + benzene and toluene for example; Klotz et al, 1998;Nehr et al, 2011), the OH to RO 2 conversion efficiency (C OH+VOC ) must be multiplied by the overall yield (1 − y) of RO 2 radicals produced from the OH + VOC reaction. Taking this yield into account, the signals due to RO 2 and HO 2 radicals become…”