Biogenic organic nitrate compounds (BONs) are major components of atmospheric secondary organic aerosols (SOA) and significantly impact O₃ production through photochemical aging. However, the photolysis mechanisms and roles of BONs especially the highly oxygenated molecules (HOMs-BONs) in O₃ formation remain unclear. By combining lab chamber experiments, numerical model simulation and field observation, here we show that O₃ formation rate through α-pinene-BONs photolysis is ten times greater than that of isoprene-BONs, attributed to the enhanced interactions between the carbonyl and nitrooxy groups in α-pinene-BONs, which foster ultraviolet absorption and expedite photolysis. The photolysis pathways of those BONs are different. While isoprene-BONs undergo photolysis through an initial cleavage of the O-NO₂ bond followed by RO fragmentation, α-pinene-BONs undergo multi-generation 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 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 role of HOMs-BONs. Our study is the first to reveal an important contribution of α-pinene-BONs to ozone formation in polluted atmosphere, which should be accounted for by photochemical models in future.

 

highly oxygenated organic nitrates, photochemical aging, nitrate radicals, air pollution, biogenic volatile organic compounds