Total OH reactivity measurements for the OH-initiated oxidation of aromatic hydrocarbons in the presence of NOx

Abstract

The total OH reactivity of the secondary products formed from the OH-initiated oxidation of toluene, p-xylene, and 1,3,5-trimethylbenzene was directly measured in the presence of NOx using a laboratory environmental chamber in order to investigate unidentified reactive species in urban air. The total OH reactivity was also calculated from the concentrations of the reactants and products, which were monitored by Fourier-transform infrared spectroscopy. The difference between the measured and calculated OH reactivity, the so-called missing OH reactivity, comprised 58–81% of the total OH reactivity of the secondary products. These results suggest that the secondary products formed from the oxidation of aromatic hydrocarbons may be important candidates in accounting for the missing OH reactivity in the analyses of urban environments. The Master Chemical Mechanism (MCM) calculations were performed to predict the temporal variation in the total OH reactivity for the oxidation of aromatic hydrocarbons. The MCM calculations successfully reproduced the observed total OH reactivity when the particle and semi-volatile product concentrations were negligibly low. The MCM calculations were used to identify the missing secondary products. The results suggest that important components of the missing OH reactivity are unsaturated multifunctional products such as unsaturated dicarbonyls, unsaturated epoxydicarbonyls, and furanones.