Multigenerational oxidation of benzyl alcohol in the atmosphere

Volatile organic compounds (VOC) have significant impact on air quality as they oxidize and form condensable vapors, which are important sources of secondary organic aerosol (SOA). As vehicle emissions are becoming increasingly regulated, other sources of VOC are increasing in relevance. One of these sources are volatile chemical compounds (VCPs), which include cleaning agents, personal care products, pesticides and adhesives (McDonald et al., 2018).  However, quantifying the SOA yields from VCP emissions is challenging as the atmospheric chemistry of many of the compounds is still unclear.

In this work, we use quantum chemical calculations to study the multigenerational atmospheric oxidation of benzyl alcohol, an aromatic hydrocarbon often found in VCPs. Its sources include cosmetics, inks and dyes, pharmaceuticals and flowers. The atmospheric oxidation of benzyl alcohol produces SOA in high yields (Charan et al. 2020), but the formation mechanisms are largely unknown. Fast intra-molecular reactions are needed for condensable vapor formation, but the double-ringed intermediates, bicyclic peroxy radicals (BPRs), in aromatic oxidation make these reactions slow. However, recently ipso-BPR, where the OH radical has added to a substituted carbon, have been shown to be unstable, leading to ring-open products that rapidly form condensable vapors through intra-molecular reactions (Iyer et al., 2023). Furthermore, geminal diol BPRs, where OH has added to an OH substituted carbon, have been shown to be highly unstable as well, leading to condensable vapors (Ojala et al., 2025).

Based on our calculations, the high SOA yield measured from benzyl alcohol oxidation is likely in part due to the ipso-BPR of benzyl alcohol and geminal diol BPR from hydroxybenzyl alcohol, which is a first-generation phenolic product of benzyl alcohol. The oxidation products phenol and catechol also likely to contribute to the total SOA yield. Our results provide key insights into the multigenerational atmospheric oxidation of benzyl alcohol, showing the potential pathways to condensable vapors.

References

Charan, S. M. et al. (2020) Secondary organic aerosol yields from the oxidation of benzyl alcohol. Atmospheric chemistry and physics. 20 (21), 13167–13190.

Iyer, S. et al. (2023) Molecular rearrangement of bicyclic peroxy radicals is a key route to aerosol from aromatics. Nature communications. 14 (1), 4984.

McDonald, B. C. et al. (2018) Volatile chemical products emerging as largest petrochemical source of urban organic emissions. Science. 359 (6377), 760–764.

Ojala, A. et al. (2025) Secondary organic aerosol formation from sequential oxidation of toluene and cresols. [Preprint] Available from: https://doi.org/10.21203/rs.3.rs-7621262/v1