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

• Main Authors: Burger, Jessica Mary, Granger, Julie, Joyce, Emily, Hastings, Meredith Galanter, Spence, Kurt Angus McDonald, Altieri, Katye Elisabeth
• Format: Text
• Language: English
• Published: 2021
• Subjects: Southern Ocean; Weddell; Weddell Sea; Sea ice
• Online Access: https://doi.org/10.5194/acp-2021-519; https://acp.copernicus.org/preprints/acp-2021-519/

Atmospheric nitrate originates from the oxidation of nitrogen oxides (NO x = NO + NO 2 ) and impacts both tropospheric chemistry and climate. NO x sources, cycling, and NO x 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 (δ 15 N and δ 18 O) 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 δ 15 N-NO 3 − decreased with latitude from −2.7 ‰ to −43.1 ‰. The decline in δ 15 N with latitude is attributed to changes in the dominant NO x 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 NO x sources or equilibrium isotopic fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a δ 15 N 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 δ 18 O-NO 3 − was always less than 70 ‰, indicating that daytime processes involving OH are the dominant NO x oxidation pathway during summer. Unusually low δ 18 O-NO 3 − 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 δ 18 O-NO 3 − is therefore attributed to increased exchange of NO with peroxy radicals, which have a low δ 18 O, relative to ozone, which has a high δ 18 O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important NO x 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.