OH-initiated oxidation of benzene - Part I. Phenol formation under atmospheric conditions

The phenol yield from the OH-radical initiated oxidation of benzene was studied in two simulation chambers: (1) the large-volume outdoor chamber EUPHORE at CEAM, Valencia, Spain and ( 2) an indoor chamber at NIES, Tsukuba, Japan. In the first study two spectroscopic techniques, i.e. differential optical absorption spectroscopy ( DOAS) and Fourier transform infra-red spectroscopy ( FTIR) were used to simultaneously measure phenol and benzene. The second study used only FTIR spectroscopy to monitor both compounds. Six different types of OH-radical sources were employed and initial concentrations for benzene and NOx were varied by about a factor of 400 and four orders of magnitude, respectively. The high sensitivity of DOAS towards phenol allowed experiments with initial benzene concentrations similar to those found in the polluted atmosphere. With respect to the NOx concentrations and light conditions employed, the experiments are representative of the atmospheric boundary layer. The phenol yield was determined to be Phi(phenol) = (53.1 +/- 6.6)%, which is about twice the value reported in the literature to date. The high phenol yield was found to remain essentially constant for NOx levels of up to several 10 ppb, which are rarely exceeded in the atmosphere. It was also found to be independent of the oxygen concentration under these conditions. With increasing concentrations of NOx ( >100 ppb) the phenol yield was found to decrease. The data could be adequately described if in addition to the kinetics of the aromatic-OH adduct reactions with O-2 two reactions involving NOx (i.e. benzene-OH + NO2 and benzene-OH-O-2 + NO) were considered. The temperature dependence of Phi(phenol) was studied over a limited temperature range of DeltaT = 20 K. The results indicate that the major part, if not all of the phenol is formed directly from the reaction of the benzene OH adduct with oxygen. No evidence was found for phenol formation via the photolysis of benzene oxide/oxepin. The atmospheric relevance of the results is discussed.