Secondary reactions of aromatics-derived oxygenated organic molecules lead to plentiful highly oxygenated organic molecules within an intraday OH exposure

Highly oxygenated organic molecules (HOMs) can participate in new particle formation (NPF) and enhance growth of newly formed particles partially because of their low volatility. Previous studies have shown formation of HOMs via autoxidation reactions of RO2 intermediates generated by OH-initiated oxidation of anthropogenic volatile organic compounds (VOCs). It was also suggested that multi-generation OH oxidation could be an important source for aromatics-derived HOMs. However, our understanding of the generation of aromatics-derived HOMs is still insufficient, especially of their formation mechanisms, which determine molar yields of HOMs and are essential to the establishment of global chemical box models related to HOMs. In this study, with a potential aerosol mass oxidation flow reactor (PAM OFR), two series of OH-initiated oxidation experiments of 1,3,5-trimethylbenzene (1,3,5-TMB) were conducted to investigate the formation of aromatics-derived HOMs. In the first series, the evolution of oxidation products of 1,3,5-TMB in an OH exposure range of (0.5-5.0) x 1010 molecules cm-3 s, equivalent to an OH exposure of 0.7-6.9 h at an OH concentration ([OH]) of 2x106 molecules cm-3, was investigated by a nitrate-based chemical ionization mass spectrometer and a Vocus proton-transfer-reaction mass spectrometer, indicating significant secondary OH chemistry during the aging of stabilized first-generation oxygenated products within an intraday OH exposure and formation of various HOMs with lower double-bond equivalence (DBE). In addition, organonitrates, formed after the introduction of NOx into the reaction systems, further confirmed the existence of such secondary reactions. The second series of experiments was conducted with same residence time but much lower [OH], which also shows the generation of multi-generation HOMs with an [OH] as low as 1.06x107 molecules cm-3 for 53 s, i.e., an OH exposure of around 5.86x108 molecules cm-3 s. Our study suggests the important role of secondary OH chemistry in the oxidation of aromatics if these oxygenated products survived long enough in the ambient atmosphere and elucidates detailed formation mechanisms of certain HOM products.