Towards understanding aromatic SOA by studying the molecular level oxidations mechanisms of xylene isomers

Abstract

Aromatic compounds like xylene contribute significantly to the formation of tropospheric secondary organic aerosol (SOA) that have strong implications on health and on climate. The sources of this class of molecules are primarily anthropogenic, but biogenic sources of aromatics can be significant too. To form SOA, the volatile xylene needs to oxidize into low volatility aerosol precursors with multiple oxygen containing polar functional groups called highly oxygenated organic molecules (HOMs). It does this through the autoxidation mechanism, which is a sequential process involving peroxy radicals where each intra-molecular reaction step such as an H-atom shift is followed quickly by O2 addition. While laboratory measurements using the sensitive chemical ionization mass spectrometer (CIMS) instrument indicate rapid conversion of xylene to HOM, this is unsupported by established oxidation mechanisms. This is due to the assumed stability of the crucial bicyclic peroxy radical (BPR), an intermediate that is intrinsic to aromatic oxidation in general. Recently, we showed that the BPR associated with toluene oxidation can be unstable, and its decomposition is pivotal to the subsequent autoxidation mechanism that leads to HOM. [1]   Through investigating the autoxidation mechanisms of xylene in this work, we establish the importance of aromatic derived BPR decomposition to the formation of SOA. We combine theoretical modelling with sub-second HOM measurements using CIMS to develop the Aerosol Dynamics gas- and particle-phase chemistry model for laboratory CHAMber (ADCHAM) code [2] for xylene that is robust at reproducing the SOA mass yields we measure from our chamber experiments. We also show that the underlying autoxidation mechanisms are remarkably similar for many of the atmospherically dominant monocyclic aromatics, which opens the remarkable prospect of significantly improved model predictions of aromatic SOA even in the absence of theoretical and experimental data. [1] Iyer, S., Kumar, A., Savolainen, A. et al. Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics. Nat. Commun. 14, 4984 (2023). https://doi.org/10.1038/s41467-023-40675-2 [2] Roldin, P., Eriksson, A.C., Nordin, E.Z., Hermansson, E., Mogensen, D., Rusanen, A., Boy, M., Swietlicki, E., Svenningsson, B., Zelenyuk, A. and Pagels, J., 2014. Modelling non-equilibrium secondary organic aerosol formation and evaporation with the aerosol dynamics, gas-and particle-phase chemistry kinetic multilayer model ADCHAM. Atmospheric Chemistry and Physics, 14(15), pp.7953-7993.