Formation and photochemical aging of secondary organic aerosols from NO3 oxidation of phenolic compounds

Phenolic compounds containing at least one hydroxyl functional group on the aromatic ring constitute a significant fraction of volatile organic compounds precursors in urban cities, representing a wide range of environmental, climate, and health effects. Daytime OH oxidation and nighttime NO₃ oxidation reactions are their main loss pathways. However, compared to OH oxidation, although NO₃ reactions demonstrate higher chemical reactivities and potentially higher SOA yields, less is known about SOA formation from NO₃ oxidation of phenolic compounds and their roles in next daytime OH oxidation aging. Here, we conducted NO₃ oxidation experiments of phenol and o-cresol in SAPHIR-STAR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber-Stirred Atmospheric flow Reactor) and photochemical aging experiments in SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). In both chamber experiments, an online EESI-ToF-MS (Extractive ElectroSpray Ionization Time-of-Flight Mass Spectrometer) was used for direct measurement of SOA composition on near-molecular level. Combining gas measurement using NO₃^- CI-ToF-MS (Chemical Ionization Time-of-Flight Mass Spectrometer), we showed a full picture of gas-phase products formation, their partitioning to particle phase, and photochemical aging of particle phase products. In contrast to OH oxidation reactions, highly oxygenated organic molecules (O≥6) contribute a small fraction of both gas (<1%) and of particle (<10%) phase products. The main particle-phase products (monomers and accretion products) show different time series during OH oxidation aging process. Overall, our experiments help understand SOA formation and photochemical aging from NO₃ oxidation of phenolic compounds and provide fundamental data support for accurate assessment of their roles in urban air quality, climate, and health effects.