Preprints
https://doi.org/10.5194/acp-2021-519
https://doi.org/10.5194/acp-2021-519

  06 Jul 2021

06 Jul 2021

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

The importance of alkyl nitrates and sea ice emissions to atmospheric NOx sources and cycling in the summertime Southern Ocean marine boundary layer

Jessica Mary Burger1, Julie Granger2, Emily Joyce3, Meredith Galanter Hastings3, Kurt Angus McDonald Spence1, and Katye Elisabeth Altieri1 Jessica Mary Burger et al.
  • 1Department of Oceanography, University of Cape Town, Rondebosch, 7701, South Africa
  • 2Department of Marine Sciences, University of Connecticut, Groton, 06340, USA
  • 3Department of Earth, Environmental and Planetary Sciences and Institute at Brown for Environment and Society, Brown University, Providence, RI, 02906, USA

Abstract. Atmospheric nitrate originates from the oxidation of nitrogen oxides (NOx = NO + NO2) and impacts both tropospheric chemistry and climate. NOx sources, cycling, and NOx to nitrate formation pathways are poorly constrained in remote marine regions, especially the Southern Ocean where pristine conditions serve as a useful proxy for the preindustrial atmosphere. Here, we measured the isotopic composition (δ15N and δ18O) of atmospheric nitrate in coarse-mode (> 1 μm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5° S to 70° S, and across the northern edge of the Weddell Sea. The δ15N-NO3 decreased with latitude from −2.7 ‰ to −43.1 ‰. The decline in δ15N with latitude is attributed to changes in the dominant NOx sources: lightning at the low latitudes, oceanic alkyl nitrates at the mid latitudes, and photolysis of nitrate in snow at the high latitudes. There is no evidence of any influence from anthropogenic NOx sources or equilibrium isotopic fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ15N signature of −22.0 ‰ ± 7.5 ‰. Given that measurements of alkyl nitrate contributions to remote nitrogen budgets are scarce, this may be a useful tracer for detecting their contribution in other oceanic regions. The δ18O-NO3 was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NOx oxidation pathway during summer. Unusually low δ18O-NO3 values (less than 31 ‰) were observed at the western edge of the Weddell Sea. The air mass history of these samples indicates extensive interaction with sea ice covered ocean, which is known to enhance peroxy radical production. The observed low δ18O-NO3 is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ18O, relative to ozone, which has a high δ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NOx source than previously thought, and that the ice-covered surface ocean impacts the reactive nitrogen budget as well as the oxidative capacity of the marine boundary layer.

Jessica Mary Burger et al.

Status: open (extended)

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  • RC1: 'Comment on acp-2021-519', Anonymous Referee #1, 16 Jul 2021 reply

Jessica Mary Burger et al.

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The importance of alkyl nitrates and sea ice emissions to atmospheric NOx sources and cycling in the summertime Southern Ocean marine boundary layer. Jessica Burger, Julie Granger, Emily Joyce, Meredith Hastings, Kurt Spence, Katye Altieri https://doi.org/10.5281/zenodo.5006983

Jessica Mary Burger et al.

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Short summary
The nitrogen (N) isotopic composition of atmospheric nitrate in the Southern Ocean (SO) marine boundary layer (MBL) reveals the importance of oceanic alkyl nitrate emissions as a source of reactive N to the atmosphere. The oxygen isotopic composition suggests peroxy radicals contribute up to 63 % to NO oxidation and that nitrate forms via the OH pathway. This work improves our understanding of reactive N sources and cycling in a remote marine region, a proxy for the pre-industrial atmosphere.
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