The nitrate radical (NO<sub>3</sub>) oxidation of alpha-pinene is a significant source of secondary organic aerosol and organic nitrogen under simulated ambient nighttime conditions

Abstract

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.

Secondary Organic Aerosol Mass Yields from NO3 Oxidation of α-Pinene and Δ-Carene: Effect of RO2 Radical Fate

“…α-Pinene is the most abundant biogenic monoterpene globally, but previously measured SOA mass yields from its NO 3 oxidation are inconsistent, ranging from 0 to 25%. − This latter and highest yield was found at low temperature, 5 °C, and >100 μg m –3 background OA. The generally lower SOA mass yields from α-pinene + NO 3 are in stark contrast with the higher values from reaction of NO 3 with other monoterpenes …”

“…43−45 Recent work on the NO 3 + α-pinene system probing RO 2 fate dependence showed large yields under certain conditions, which they attributed to dominance of RO 2 + RO 2 reactions, with an estimated SOA mass yield of ∼65% for that pathway and negligible SOA for other RO 2 fates. 31 Given the uncertainties in atmospheric RO 2 fate, we examine how these different fates affect SOA formation from NO 3 oxidation of terpenes. Specifically, we sought to determine if the observed anomalously low α-pinene SOA yield was unique to particular RO 2 fates or chamber artifacts, by performing experiments across three different putative RO 2 fates, RO 2 + NO 3 , RO 2 + RO 2 , and RO 2 + HO 2 , and using a range of inorganic and organic seed aerosol concentrations (Figure 1).…”

“…Mao et al found nighttime ambient concentrations of HO 2 to be ∼5 pptv at a ponderosa pine plantation, which highlights that RO 2 + HO 2 may be an important pathway in the ambient nighttime atmosphere. Unfortunately, most previous experiments studying BVOC + NO 3 generally did not consider RO 2 + HO 2 reactions, although a few recent studies have. ,,− Recent work has reopened the question of the products of the RO 2 + RO 2 reactions, showing that the rates of formation of dialkyl peroxides (ROOR), which have been previously assumed negligible, are in fact rapid for several RO 2 radicals produced from the OH radical initiated oxidation of 1,3,5-trimethylbenzene . Because these ROOR products have substantially lower volatility relative to their precursors, this reaction pathway can rapidly lead to condensable products.…”

“…Because these ROOR products have substantially lower volatility relative to their precursors, this reaction pathway can rapidly lead to condensable products. Additional recent work in the NO 3 + β-pinene and Δ-carene systems has shown that RO 2 + RO 2 gas-phase reactions in combination with particle-phase oligomerization can underpin large SOA mass yields from monoterpenes. − Recent work on the NO 3 + α-pinene system probing RO 2 fate dependence showed large yields under certain conditions, which they attributed to dominance of RO 2 + RO 2 reactions, with an estimated SOA mass yield of ∼65% for that pathway and negligible SOA for other RO 2 fates …”

Gas- and Particle-Phase Products and Their Mechanisms of Formation from the Reaction of Δ-3-Carene with NO3 Radicals

“…Although in a low NO x atmosphere these RO 2 • radicals could also react with HO 2 or NO 3 radicals or isomerize (reaction with NO is unlikely since NO rapidly titrates NO 3 radicals), all the bimolecular reactions can lead to many of the same products, 21 and the formation of low-volatility ROOR dimers through RO 2 • + RO 2 • reactions is currently a topic of much interest. 22 For example, photochemical modeling by Bates et al 23 designed to mimic nighttime chemistry during the 2013 SOAS campaign indicated that ∼20% of RO 2 • radical reactions occurred with other RO 2 • radicals and that ROOR products were formed with a branching ratio of ∼18%. Conducting experiments in this regime also greatly simplifies the development of reaction mechanisms and can provide a useful point from which to perturb the chemistry into other, less well understood regimes, under controlled conditions.…”