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 (C20H32N2O8-13). 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.
Highlights
NO3 radicals generated from N2O5, and the resulting secondary organic aerosol (SOA) aged in the absence of external stimuli
Implications The composition of NO3-derived α-pinene SOA is dominated by dimers formed through RO2-RO2 reactions under the experiment conditions in this study
Pye et al (2015) modeled the atmospheric conditions during nighttime chemistry in the southeastern US showing that the reactivity of RO2 radicals in the atmosphere will react with either other RO2 radicals (~40%) 380 or HO2 radicals (60%) making up the difference, with little reactivity with NO3 radicals
Summary
Organic aerosol in the atmosphere can have important impacts on human health and climate (Jimenez et al, 2009). Claflin and Ziemann (2018) speculated about the importance of dimer formation within the particle-phase itself for β-pinene + NO3 SOA. We will employ the 55 extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) along with a chemical ionization mass spectrometer with filter inlet for gases and aerosols (FIGAERO-CIMS) to elucidate the particle-phase composition with high chemical and temporal resolution and uncover the changes occurring therein. Even though the FIGAERO-CIMS and EESI-ToF were not present for experiments 4 and 5, these experiments are included to confirm the changes in particle mass concentration with dark aging at lower mass concentrations are consistent with the measurements at 90 higher mass concentration. The wall loss-corrected mass was divided by the uncorrected mass to obtain the wall loss correction factor that was applied to the EESI-ToF and FIGAERO-CIMS data to correct for particle wall loss, during experiments 1-3
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