NH3-enhanced formation mechanism of highly oxygenated organic molecules from the photooxidation of aromatic and biogenic volatile organic compounds

초록

Highly oxygenated organic molecules (HOMs) are key intermediates in the atmospheric oxidation of volatile organic compounds (VOCs), yet their formation mechanisms remain incompletely understood. In this study, we investigated the role of ammonia (NH₃) in enhancing HOM formation during secondary organic aerosol (SOA) formation from both aromatic (toluene, m-xylene, ethylbenzene) and biogenic (α-pinene, isoprene) VOCs under controlled smog chamber conditions. Orbitrap electrospray ionization mass spectrometry (ESI-MS) was used to identify HOMs. We observed that the presence of NH₃ significantly increased both the number (by up to 369 %) and abundance (by 158–1437 %) of HOM species, particularly from aromatic precursors. NH₃ also promoted the formation of more oxidized, low-volatility HOMs that partitioned predominantly into the particle phase. These effects were precursor-dependent, with biogenic VOCs showing weaker or even suppressed enhancements. Volatility and oxidation state analyses further suggest that NH₃ facilitates the formation of highly oxygenated, nitrogen-containing species (e.g., CHON and CHONS compounds), thereby altering both the chemical composition and physical properties of SOA. Overall, our results highlight the significant and multifaceted role of NH₃ in atmospheric oxidation chemistry of organics and its influence on HOM formation pathways during VOC photooxidation. © 2025

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