UC Davis

The reaction of α -pinene with NO 3 is an important sink of both α -pinene and NO 3 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 [NO 3 ] conditions. Here, we characterize the SOA and organic nitrogen formation from α -pinene + NO 3 as a function of nitrooxy peroxy ( n RO 2 ) radical fates with HO 2 , NO, NO 3 , and RO 2 in an atmospheric chamber. We show that SOA yields are not small when the n RO 2 fate distribution in the chamber mimics that in the atmosphere, and the formation of pinene nitrooxy hydroperoxide (PNP) and related organonitrates in the ambient atmosphere can be reproduced. Nearly all SOA from α -pinene + NO 3 chemistry derives from the n RO 2 + RO 2 pathway, which alone has an SOA mass yield of 56 (±7) %. 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 + NO 3 under simulated nighttime conditions. Synergistic dimerization between n RO 2 and RO 2 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 n RO 2 + HO 2 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 NO 3 oxidation, leading to 20 % n RO 2 + RO 2 and 45 % n RO 2 + HO 2 ), we estimate yields of 11 % SOA and 7 % particulate nitrate by mass and 26 % PNP by mole from α -pinene + NO 3 in the ambient atmosphere. These results suggest that α -pinene + NO 3 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.