Particle-phase processing of α-pinene NO₃ secondary organic aerosol in the dark

Abstract. The NO₃ radical represents a significant night-time oxidant present in or downstream of polluted environments. There are studies that investigated the formation of secondary organic aerosol (SOA) from NO₃ radicals focusing on yields, general composition, and hydrolysis of organonitrates. However, there is limited knowledge about how the composition of NO₃-derived SOA evolves as a result of particle phase reactions. Here, SOA was formed from the reaction of α-pinene with NO₃ radicals generated from N₂O₅, and the resulting SOA aged in the absence of external stimuli. The initial composition of NO₃-derived α-pinene SOA was slightly dependent upon the concentration of N₂O₅ injected (excess of NO₃ or excess of α-pinene), but was largely dominated by dimer dinitrates (C₂₀H₃₂N₂O_8-13). Oxidation reactions (e.g. C₂₀H₃₂N₂O₈ C₂₀H₃₂N₂O₉ C₂₀H₃₂N₂O₁₀ etc...) accounted for 60–70 % of the particle phase reactions observed. Fragmentation reactions and dimer degradation pathways made up the remainder of the particle-phase processes occurring. The exact oxidant is not known, though suggestions are offered (e.g. N₂O₅, organic peroxides, or peroxy-nitrates). Hydrolysis of −ONO2 functional groups was not an important loss term during dark aging under the relative humidity conditions of our experiments (58–62 %), and changes in the bulk organonitrate composition were likely driven by evaporation of highly nitrogenated molecules. Overall, 25–30 % of the particle-phase composition changes as a function of particle-phase reactions during dark aging representing an important atmospheric aging pathway.