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 atmosphere can be reproduced. Nearly all SOA from α-pinene + NO3 chemistry derives from the nRO2+ RO2 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 + NO3 under simulated nighttime conditions. 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+ RO2 and 45 % nRO2+ HO2), we estimate yields of 11 % SOA and 7 % particulate nitrate by mass and 26 % PNP by mole from α-pinene + NO3 in the ambient atmosphere. 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.
Highlights
Monoterpenes (C10 H16 ) are a major class of biogenic hydrocarbons
Molecular composition analysis shows that particulate nitrates are a large (60 %–70 %) portion of the secondary organic aerosol (SOA) and that dimer formation is the primary mechanism of SOA production from α-pinene + NO3 under simulated nighttime conditions
Molar yield of pinene nitrooxy hydroperoxide (PNP) from the nRO2 + HO2 pathway. Applying these laboratory constraints to model simulations of summertime conditions observed in the southeast United States, we estimate yields of 11 % SOA and 7 % particulate nitrate by mass and 26 % PNP by mole from α-pinene + NO3 in the ambient atmosphere
Summary
Monoterpenes (C10 H16 ) are a major class of biogenic hydrocarbons. less abundant than isoprene in terms of absolute emission flux of non-methane hydrocarbons, they have a disproportionate importance for the formation of secondary organic aerosol (SOA), accounting for half of the total fine aerosol globally (Zhang et al, 2018), and for nitrogen oxide (NO, NO2 , NO3 ) sequestration through the formation of gaseous and particle-phase organic nitrates (Pye et al, 2015). < 1 %, respectively) using NO2 + O3 as a source of NO3 and formaldehyde to promote HO2 chemistry From these observations, it is clear that chamber reaction conditions are highly influential in the observed SOA yields and that previous studies may have each probed different nRO2 fates. In light of the emerging appreciation for the importance of RO2 radical fate in designing chamber experiments (Nguyen et al, 2014a; Xu et al, 2019; Boyd et al, 2015; Teng et al, 2017; Crounse et al, 2013), we reinvestigate this reaction to probe the SOA yield and organic nitrate formation from α-pinene + NO3 from each relevant nRO2 reaction channel. We demonstrate a new HO2 formation route in the dark chamber that does not require carbon inputs and enables SOA yields to be more accurately measured when probing the RO2 + HO2 pathway from the α-pinene + NO3 reaction
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