A qualitative comparison of secondary organic aerosol yields and composition from ozonolysis of monoterpenes at varying concentrations of NO₂

Abstract. The effect of NO₂ on secondary organic aerosol (SOA) formation from ozonolysis of α-pinene, β-pinene, Δ³-carene, and limonene was investigated using a dark flow-through reaction chamber. SOA mass yields were calculated for each monoterpene from ozonolysis with varying NO₂ concentrations. Kinetics modeling of the first-generation gas-phase chemistry suggests that differences in observed aerosol yields for different NO₂ concentrations are consistent with NO₃ formation and subsequent competition between O₃ and NO₃ to oxidize each monoterpene. α-Pinene was the only monoterpene studied that showed a systematic decrease in both aerosol number concentration and mass concentration with increasing [NO₂]. β-Pinene and Δ³-carene produced fewer particles at higher [NO₂], but both retained moderate mass yields. Limonene exhibited both higher number concentrations and greater mass concentrations at higher [NO₂]. SOA from each experiment was collected and analyzed by HPLC-ESI-MS, enabling comparisons between product distributions for each system. In general, the systems influenced by NO₃ oxidation contained more high molecular weight products (MW > 400 amu), suggesting the importance of oligomerization mechanisms in NO₃-initiated SOA formation. α-Pinene, which showed anomalously low aerosol mass yields in the presence of NO₂, showed no increase in these oligomer peaks, suggesting that lack of oligomer formation is a likely cause of α-pinene's near 0 % yields with NO₃. Through direct comparisons of mixed-oxidant systems, this work suggests that NO₃ is likely to dominate nighttime oxidation pathways in most regions with both biogenic and anthropogenic influences. Therefore, accurately constraining SOA yields from NO₃ oxidation, which vary substantially with the volatile organic compound precursor, is essential in predicting nighttime aerosol production.