The importance of alkyl nitrates and sea ice emissions to atmospheric NO x sources and cycling in the summertime Southern Ocean marine boundary layer

Burger, Jessica M.; Granger, Julie; Joyce, Emily; Hastings, Meredith G.; Spence, Kurt A. M.; Altieri, Katye E.

Atmospheric nitrate originates from the oxidation of nitrogen oxides inline-formula M3inlinescrollmathml ( chem normal NO x = chem normal NO + chem normal NO normal 2 ) 93pt13ptsvg-formulamathimg5750f2f575feb9c155d643b1c38773c6 acp-22-1081-2022-ie00001.svg93pt13ptacp-22-1081-2022-ie00001.png and impacts both tropospheric chemistry and climate. inline-formulaNOx sources, cycling and inline-formulaNOx 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 pre-industrial atmosphere. Here, we measured the isotopic composition (inline-formulaδ15N and inline-formulaδ18O) of atmospheric nitrate in coarse-mode (inline-formula>1inline-formulaµm) aerosols collected in the summertime marine boundary layer of the Atlantic Southern Ocean from 34.5 to 70inline-formula S and across the northern edge of the Weddell Sea. The inline-formulaδ15Ninline-formula M12inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg737339a8d3517116341490f01d8cfecf acp-22-1081-2022-ie00002.svg25pt16ptacp-22-1081-2022-ie00002.png decreased with latitude from inline-formula−2.7 ‰ to inline-formula−42.9 ‰. The decline in inline-formulaδ15N with latitude is attributed to changes in the dominant inline-formulaNOx 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 inline-formulaNOx sources or equilibrium isotope fractionation. Using air mass back trajectories and an isotope mixing model, we calculate that oceanic alkyl nitrate emissions have a inline-formulaδ15N signature of inline-formula M19inlinescrollmathml - normal 21.8 ± normal 7.6 58pt10ptsvg-formulamathimgc1949f6e2c58afc807f9f69473c4c66c acp-22-1081-2022-ie00003.svg58pt10ptacp-22-1081-2022-ie00003.png  ‰. 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 inline-formulaδ18Oinline-formula M21inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg8a28d0a06c83cc478fcb3953dfc5e6ea acp-22-1081-2022-ie00004.svg25pt16ptacp-22-1081-2022-ie00004.png was always less than 70 ‰, indicating that daytime processes involving OH are the dominant inline-formulaNOx oxidation pathway during summer. Unusually low inline-formulaδ18Oinline-formula M24inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimga226654730cbc5374d88051445c2d9d9 acp-22-1081-2022-ie00005.svg25pt16ptacp-22-1081-2022-ie00005.png 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 inline-formulaδ18Oinline-formula M26inlinescrollmathml chem normal NO normal 3 - 25pt16ptsvg-formulamathimg59e8efc9900af362c4e31d9012378c85 acp-22-1081-2022-ie00006.svg25pt16ptacp-22-1081-2022-ie00006.png is therefore attributed to increased exchange of NO with peroxy radicals, which have a low inline-formulaδ18O, relative to ozone, which has a high inline-formulaδ18O. This study reveals that the mid- and high-latitude surface ocean may serve as a more important inline-formulaNOx 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.

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Burger, Jessica M. / Granger, Julie / Joyce, Emily / et al: The importance of alkyl nitrates and sea ice emissions to atmospheric NOx sources and cycling in the summertime Southern Ocean marine boundary layer. 2022. Copernicus Publications.

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