Computational study of cresol autoxidation: Initial steps in secondary organic aerosol formation

Aromatic compounds, especially BTEX-compounds (benzene, toluene, ethylbenzene and xylene) and their derivatives can have an impact on the climate and human health through secondary organic aerosol (SOA) formation. They primarily originate from anthropogenic sources, such as vehicle emissions, industrial processes and solvent evaporation. These volatile organic compunds (VOCs) in the atmosphere can autoxidize, which is a sequential process of intramolecular reactions of peroxy radicals (molecules with R-O-O· structure) followed quickly by O₂ additions. This leads to low-volatility products with multiple oxygen-containing functional groups, called highly oxygenated organic molecules (HOMs).^[1] These molecules can condense irreversibly to form and grow SOA, which have an impact on the climate through the scattering and absorption of sunlight and acting as seeds for cloud formation.

The oxidation process for aromatics is initiated by OH-radicals, which leads quickly to bicyclic peroxy radical (BPR) intermediates in significant yields. BPRs retain the initial 6-membered ring, but add an additional endoperoxide bridge, consisting of two oxygen atoms, that connects to carbon atoms on both sides of the initiating OH-addition site. BPRs are sterically hindered, and their autoxidation is therefore slow, preferring to undergo bimolecular reactions with e.g. NO in polluted environments. Recently, a unimolecular pathway for ring-opening of BPRs was reported, leading to HOM formation in even sub-second timescales^[2]. This opens up a pathway for aromatics to lead to SOA, even in non-polluted environments.

In this work, the autoxidation of cresols is studied. The cresol pathway is a major source of overall SOA for BTEX-compounds, as it is expected to account for up to 40% of toluene-related SOA formation^[3]. ωB97XD/aug-cc-pVTZ-level of theory is used for geometry optimization with single point energy calculations done at ROHF-ROCCSD(T)-F12a/cc-pVDZ-F12-level. A thorough conformer sampling is done at a lower level of theory, and multi-conformer transition state theory (MC-TST) is used for rate calculations with Eckart-tunneling correction. For cresols, the OH-addition can happen at six different sites with different yields, leading to different chemistry. Preliminary results for ortho-cresol suggest that sites 3, 1 and 2 relative to the methyl group have the highest yields in descending order. Slow ring breakage is seen for the two highest yield addition sites, whereas fast ring breakage is seen for the BPR formed from 2-position addition. This is compatible with previous results for the ring-breaking of other substituted aromatic compounds^[2]. The results of this study shed light on the SOA formation processes in the atmosphere. This will improve current models of SOA formation, which are known to have inconsistencies^[1].

^[1] Nault et al. Secondary organic aerosols from anthropogenic volatile organic compounds contribute substantially to air pollution mortality. Atmospheric Chemistry and Physics, 2021.
^[2] Iyer et al. Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics. Nature communications, 2023.
^[3] Schwantes et al. Formation of highly oxygenated low-volatility products from cresol oxidation. Atmospheric Chemistry and Physics, 17(5):3453–3474, 2017.