Abstract. The reaction of α-pinene with NO3 is an important sink of both α-pinene and NO3 at night in regions with mixed biogenic and anthropogenic emissions; however, there is debate on its importance for secondary organic aerosol (SOA) and reactive nitrogen budgets in the atmosphere. Previous experimental studies have generally observed low or zero SOA formation, often due to excessive [NO3] conditions. Here, we characterize the SOA and organic nitrogen formation from α-pinene + NO3 as a function of nitrooxy peroxy (nRO2) radical fates with HO2, NO, NO3, and RO2 in an atmospheric chamber. We show that SOA yields are not small when the nRO2 fate distribution in the chamber mimics that in the atmosphere, and the formation of pinene nitrooxy hydroperoxide (PNP) and related organonitrates in the ambient can be reproduced. Nearly all SOA from α-pinene + NO3 chemistry derives from the nRO2 + nRO2 pathway, which alone has an SOA mass yield of 65 (±9) %. Molecular composition analysis shows that particulate nitrates are a large (60–70 %) portion of the SOA, and that dimer formation is the primary mechanism of SOA production from α-pinene + NO3 under simulated nighttime conditions. We estimate an average nRO2 + nRO2 → ROOR branching ratio of ~18 %. Synergistic dimerization between nRO2 and RO2 derived from ozonolysis and OH oxidation also contribute to SOA formation, and should be considered in models. We report a 58 (±20) % molar yield of PNP from the nRO2 + HO2 pathway. Applying these laboratory constraints to model simulations of summertime conditions observed in the Southeast United States (where 80 % of α-pinene is lost via NO3 oxidation, leading to 20 % nRO2 + nRO2 and 45 % nRO2 + HO2) , we estimate yields of 13% SOA and 9% particulate nitrate by mass, and 26 % PNP by mole, from α-pinene + NO3 in the ambient. These results suggest that α-pinene + NO3 significantly contributes to the SOA budget, and likely constitutes a major removal pathway of reactive nitrogen from the nighttime boundary layer in mixed biogenic/anthropogenic areas.
Is the gas-particle partitioning in alpha-pinene secondary organic aerosol reversible?