Preprints
https://doi.org/10.5194/acp-2021-703
https://doi.org/10.5194/acp-2021-703

  27 Aug 2021

27 Aug 2021

Review status: this preprint is currently under review for the journal ACP.

The nitrate radical (NO3) oxidation of alpha-pinene is a significant source of secondary organic aerosol and organic nitrogen under simulated ambient nighttime conditions

Kelvin H. Bates1,2, Guy J. P. Burke1, James D. Cope1, and Tran B. Nguyen1 Kelvin H. Bates et al.
  • 1Department of Environmental Toxicology, University of California Davis, Davis CA 95616, USA
  • 2Center for the Environment, Harvard University, Cambridge, MA 02138, USA

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.

Kelvin H. Bates et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on acp-2021-703', Jessica Haskins, 07 Sep 2021
  • RC1: 'Comment on acp-2021-703', Anonymous Referee #1, 06 Oct 2021
  • RC2: 'Comment on acp-2021-703', Anonymous Referee #2, 21 Oct 2021

Kelvin H. Bates et al.

Kelvin H. Bates et al.

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Short summary
The main nighttime sink of α-pinene, a hydrocarbon abundantly emitted by plants, is reaction with NO3 to form nitrooxy peroxy radicals (nRO2). Using uniquely designed chamber experiments, we show that this reaction is a major source of organic aerosol when nRO2 react with other nRO2, and forms a nitrooxy hydroperoxide when nRO2 react with HO2. Under ambient conditions, these pathways are key loss processes of atmospheric reactive nitrogen in areas with mixed biogenic and anthropogenic influence.
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