Role of the geminal diol pathway in organic aerosol formation from multigenerational aromatic oxidation

The atmospheric oxidation of aromatic hydrocarbons contributes to the formation of secondary organic aerosol (SOA). Phenolic compounds are aromatic products from the OH oxidation of primary aromatics such as benzene, toluene, xylenes and others. They are an important class of atmospheric trace gases as they are efficient SOA precursors, even more efficient than the parent aromatics (Nakao et al. 2011). As compounds oxidize, they accumulate oxygen-containing functional groups. This lowers the volatility of the compounds, and condensation to the particle phase is possible. Explicit oxidation mechanisms are needed in order to understand this process and to better model SOA formation in the atmosphere.

Current molecular scale understanding of how phenolics oxidize cannot explain the high SOA potentials of these compounds. To address this, we use quantum chemical methods to study the early steps of OH oxidation of phenol, cresol, catechol and methylcatechol, and show that the largely neglected geminal diol pathway is key to the rapid formation of highly oxygenated low-volatility products. OH addition to the OH-substituted carbon in the phenolic precursor leads to a geminal diol alkyl radical that can add O₂ and, over two rapid steps, lead to a geminal diol bicyclic peroxy radical (BPR). It has previously been shown that certain BPRs from primary aromatics such as toluene and xylene undergo molecular rearrangement reactions that break both structural rings at rate coefficients close to 1 s^-1. These ring broken peroxy radicals oxidize more efficiently, and the fast rate of the molecular rearrangement reaction makes the formation of SOA precursors competitive even under polluted conditions (Iyer et al. 2023). Remarkably, geminal diol BPRs undergo molecular rearrangement reactions that are about 3 orders of magnitude faster, directly producing less volatile peroxy radicals with carboxylic acid functionalities that are also efficient at oxidizing further. The fraction of the initial geminal diol alkyl radical goes from minor for phenol to the dominant fate for catechol, explaining the increasing SOA potential trend, benzene < phenol < catechol, observed in measurements. (Borrás et al. 2012)

Primary aromatics in the atmosphere are constantly oxidized and either directly lead to SOA or produce phenolic products with SOA potentials of their own. It has been known for some time that the SOA potentials of phenolics outcompete those of the parent aromatics, and this work provides the first molecular scale mechanisms that explain why. These mechanisms are crucial blueprints needed to model SOA yields at different stages of aromatic oxidation and help to characterize the multigenerational nature of SOA production from aromatic oxidation.

References:
Nakao, S. et al. (2011) Secondary organic aerosol formation from phenolic compounds in the absence of NOx. Atmos. Chem. Phys. 11, 10649–10660.

Iyer, S. et al. (2023) Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics. Nat Commun 14, 4984.
Borrás, E. Et al. (2012) L. A. Secondary organic aerosol formation from the photo-oxidation of benzene. Atmos. Environ. 47, 154–163.