Secondary organic aerosols (SOA) contribute to urban air pollution and originate from the oxidation of volatile organic compounds (VOCs). As vehicle emissions of VOCs decline through regulatory controls, volatile chemical products (VCPs) are becoming a relevant contributor to urban SOA. VCP VOCs can introduce new chemical functionality to urban VOC emissions, including organo-halogens and oxygenates, which leads to more complex oxidation pathways than traditional urban SOA precursors. Our current incomplete understanding of functionalized VOC oxidation chemistry limits our ability to explain and predict urban SOA production. The Secondary organic aerosol Chamber Experiments for Non-Traditional Species (SCENTS) project explored OH-initiated oxidation mechanisms of oxygenated VCP VOCs using online high-resolution mass spectrometry observations of both gas and particle phase chemical composition.
Benzyl alcohol (C₇H₈O) is an oxygenated aromatic emitted from an array of personal care products and industrial solvents and produces SOA with literature mass yields of 5-90%. However, the range of reported aerosol mass yields indicates a high degree of uncertainty about its importance to urban SOA and its oxidation mechanism. We focus on the characterization of benzyl alcohol’s oxidation mechanism, with the goal of understanding which pathways control SOA formation. Using real-time chemical ionization mass spectrometry, we can observe formation kinetics and time-dependent aerosol composition. Over the equivalent of 25 hours photochemical aging (assuming an OH concentration of 1.5 x10⁶ molecules cm^-3) in a 10m³chamber, we identify several oxidation pathways: gas-phase formation of nitroaromatics and oxygenated aromatics, and multiphase formation of highly oxygenated molecules. We observe aerosol mass yield up to 35% (under atmospherically relevant conditions of ~55% humidity and ~30 ppb NO_x), with oxidation products throughout the oxidation mechanism contributing to aerosol formation. Bicyclic peroxide compounds contribute to SOA mass at low photochemical ages from 1-10 hours, while highly oxygenated ring-opening products contribute at higher photochemical ages, between 15-25 hours. Time-dependent benzyl alcohol SOA yields observed during SCENTS are the result of dynamic multiphase chemistry that converts benzyl alcohol first to functionalized ring-retaining species, and ultimately to highly oxygenated ring-opening products that accumulate in the aerosol phase.

Matthew Coggon

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