Direct observation of the particle-phase bicyclic products from OH-initiated oxidation of 1,3,5-trimethylbenzene under NOx-free conditions

The OH initiated oxidation of 1,3,5-trimethylbenzene (1,3,5-TMB) under NOx-free conditions has been investigated by using a home-made vacuum ultraviolet (VUV) photoionization aerosol mass spectrometer complemented by high-level theoretical computations. The chemical composition of the nascent particles from the oxidation reaction is measured on-line and the high mass O-2-bridged bicyclic compounds in the particle phase are directly observed and determined, with the aid of the deuterated 1,3,5-TMB experiments to confirm the species' assignment. To illuminate the formation of the O-2-bridged bicyclic compounds, the potential energy surfaces of the reaction of the bicyclic peroxy radical (BPR) with the hydroperoxy radical (HO2), and of the self reaction of the BPR radical, have been theoretically calculated and then their reaction mechanisms are discussed in detail. This study provides direct evidence to support the essential role of the O-2-bridged bicyclic compounds in the particle formation from the OH initiated oxidation of 1,3,5-TMB.