Secondary organic aerosol formation from sequential oxidation of toluene and cresols

Cresols are both primary emissions and products of toluene oxidation in the atmosphere. Their subsequent reactions, along with those of their primary oxidation products methylcatechols, lead to the formation of condensable vapors known as highly oxygenated organic molecules (HOM), which contribute to secondary organic aerosol (SOA) formation. SOA constitutes a significant fraction of atmospheric aerosol, impacting both climate and human health. However, quantifying the contribution of sequential oxidation of toluene to SOA is hampered by an incomplete understanding of how cresols and methylcatechols form condensable vapors. Here, we present flow reactor oxidation experiments demonstrating the rapid (sub-second) formation of HOM with up to 10 oxygen atoms from the three cresol isomers (ortho-, para- and meta-cresol) under atmospheric conditions. Our quantum chemical calculations resolve the underlying autoxidation mechanisms, revealing that initial OH additions to the hydroxyl containing carbon atom lead to geminal diol intermediates that are critical to the rapid formation of HOM from cresols and 3-methylcatechol. Incorporating these autoxidation mechanisms into SOA simulations shows that they account for approximately 38% of toluene SOA composition, and 61% to 88% of ortho-cresol SOA composition. These results indicate strong multi-generational contributions to toluene and cresol SOA, which should be considered when assessing their climate and health impacts.