Abstract
The aim of this work was to improve the understanding of the OH-radical initiated oxidation of aromatic hydrocarbons (benzene, toluene, p-xylene (BTX) and 1,3,5-trimethylbenzene (TMB)). These mechanisms are considered a major uncertainty in state-of-the-art photochemical models as they are used to predict photooxidant formation from urban air. Differential Optical Absorption Spectroscopy (DOAS) was employed in a systematic outdoor smog-chamber study at the European Photo Reactor (EUPHORE) located at the CEAM-Institute, Valencia/Spain. The available DOAS system was improved for this purpose.
The yields of ring-retaining products (phenol from benzene, phenol-type and aldehyde-type compounds from p-xylene and TMB) and glyoxal (from BTX) were investigated. The phenol yield (FPHEN =53%) was found more than two times higher than presently available literature values. Further, the bicycloalkyl-radical pathway was identified as a major pathway from BTX. It was demonstrated that the results of this study are representative for the atmosphere. Deviations from the the degradation pathways of BTX and TMB were further observed under conditions of high NOx (e.g. several ppm). The results of this work indicate that the representations of aromatics in photochemical models need to be updated. The results further indicate that the contribution of aromatic hydrocarbons to the formation of photooxidants (i.e. ozone) is underestimated today.