Unappreciated Role of Photochemical Aging of Biogenic Organic Nitrates in Atmospheric Ozone Formation

Biogenic organic nitrate compounds (BONs) are major components of atmospheric secondary organic aerosols (SOA) and significantly impact the production of O₃ through photochemical aging. However, the photolysis mechanisms and roles of BONs, especially the highly oxygenated molecules (HOMs-BONs), in the formation of the O₃ remain unclear. By combining lab chamber experiments, numerical model simulation, and field observation, here we show that under our experimental conditions, the O₃ formation rate through α-pinene-ONs photolysis is ten times greater than that of isoprene-ONs. This enhancement arises from stronger intramolecular interactions between the carbonyl and nitrooxy functional groups in α-pinene-ONs, which promote ultraviolet absorption and accelerate photolytic decomposition. The photolysis pathways of those BONs are different. While isoprene-ONs undergo photolysis through an initial cleavage of the O-NO₂ bond followed by RO fragmentation, α-pinene-ONs undergo multigeneration photolysis commencing with C(O)-C bond cleavage and subsequent O-NO₂ bond dissociation. We found that these BONs are actually oxidized by OH radicals via H abstraction from the peroxyhydroxyl group (-OOH) rather than the traditional thought from aldehyde (C(O)-H) or hydroxyl groups (-OH). Currently, the contribution of BONs photolysis to O₃ formation is significantly underestimated in models, mainly due to neglecting the photolysis role of HOMs-BONs. Our study is the first to reveal a non-negligible contribution of α-pinene-ONs to atmospheric ozone formation, which should be accounted for by photochemical models in the future.