Highly Oxygenated Ring-Retaining and Ring-Opening Products Drive Aerosol Formation from the OH-Initiated Oxidation of Benzyl Alcohol

To understand the reaction pathways and products that drive secondary organic aerosol (SOA) formation from the oxidation of benzyl alcohol, we use online mass spectrometry to follow the formation and loss kinetics of oxidation products in both gas and particle phases. Over 20 h of equivalent photochemical aging (assuming [OH] = 1.5 × 106 molecules cm–3), we observe the formation of oxygenated aromatic and nitroaromatic compounds and highly oxygenated ring-retaining and ring-opening products. In the gas phase, we detect the formation of hydroxybenzyl alcohol (18–24% molar yield), benzaldehyde (14–47% yield), and phenol (3–7% yield); however, their early-generation oxidation products cannot explain rapid SOA formation with peak aerosol mass yields [20(±8)–37(±15)%] at 2–4 h of photochemical aging. The peroxy radical (RO2) bimolecular lifetime in our chamber is 2–3 s against reaction with NO and HO2, which suggests that RO2 fate is a competition between the unimolecular H-shift (lifetime of ∼0.01–1.1 s) and NO, HO2, and RO2 bimolecular reactions. In the particle phase, bicyclic peroxide, polyol, and carbonyl products peak within the first 10 h of photochemical aging, while highly oxygenated ring-opening products, including decomposition and accretion products, primarily accumulate in the aerosol after 10 h of photochemical aging. Observed product distributions demonstrate that the bicyclic peroxy radical’s fate is central to explaining SOA formation from benzyl alcohol.