Investigation of OH-reactivity budget in the isoprene, α-pinene and m-xylene oxidation with OH under high NOx conditions

The present study aimed to investigate OH-reactivity change in the OH-initiated photo-oxidation of isoprene, α-pinene and m-xylene under high NOx conditions in the smog chamber at the National Institute for Environmental Studies, Japan. In all OH-initiated photo-oxidation experiments, first-generation oxidation products measured by Fourier-transform infrared spectroscopy (FTIR) could not fully explain the OH reactivity of the products measured by the laser photolysis/laser-induced fluorescence technique, and ∼20%, ∼40%, and ∼70% of OH reactivity attributed to these volatile organic compound (VOC) products, respectively, were missing. A master chemical mechanism (MCM) calculation demonstrated that the observed OH reactivity during photo-oxidation could be explained with secondary VOC products. It was found that vapor wall loss (VWL) and aerosol formation (AF) strongly affected the OH reactivity when the photochemical reaction was prolonged, i.e., multiple oxidations were involved. The MCM calculation showed that aromatics have almost twice the productivity of OH reactivity attributed to secondary VOC products, Ps_VOC, compared with isoprene and α-pinene at the beginning of the period not affected by VWL/AF. The evaluation of the missing OH reactivity from O3 oxidation and the sequential multiple oxidations of secondary products still involves an uncertainty that originates from an incomplete knowledge of VWL/AF, and thus, further investigation is required to understand OH-reactivity evolution in both chamber and ambient observations. The error in photolysis rate and possibly unconsidered reactions of secondary products, such as the pinonaldehyde case, also presents potential uncertainty in the OH reactivity prediction and thus requires evaluation.