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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Burger, Jessica M., Granger, Julie, Joyce, Emily, Hastings, Meredith G., Spence, Kurt A. M., Altieri, Katye E.
• Format: Text
• Language: English
• Published: 2022
• Subjects: Southern Ocean; Weddell; Weddell Sea; Sea ice
• Online Access: https://doi.org/10.5194/acp-22-1081-2022; https://acp.copernicus.org/articles/22/1081/2022/

Atmospheric nitrate originates from the oxidation of nitrogen oxides <math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><mi mathvariant="normal">NO</mi></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow><mo>)</mo></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="93pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="5750f2f575feb9c155d643b1c38773c6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00001.svg" width="93pt" height="13pt" src="acp-22-1081-2022-ie00001.png"/></svg:svg> 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 pre-industrial 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 to 70 ∘ S and across the northern edge of the Weddell Sea. The δ 15 N – <math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="737339a8d3517116341490f01d8cfecf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00002.svg" width="25pt" height="16pt" src="acp-22-1081-2022-ie00002.png"/></svg:svg> decreased with latitude from −2.7 ‰ to −42.9 ‰. 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 isotope 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">21.8</mn><mo>±</mo><mn mathvariant="normal">7.6</mn></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="c1949f6e2c58afc807f9f69473c4c66c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00003.svg" width="58pt" height="10pt" src="acp-22-1081-2022-ie00003.png"/></svg:svg> ‰. 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 – <math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="8a28d0a06c83cc478fcb3953dfc5e6ea"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00004.svg" width="25pt" height="16pt" src="acp-22-1081-2022-ie00004.png"/></svg:svg> 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 – <math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a226654730cbc5374d88051445c2d9d9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00005.svg" width="25pt" height="16pt" src="acp-22-1081-2022-ie00005.png"/></svg:svg> 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 – <math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="59e8efc9900af362c4e31d9012378c85"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-1081-2022-ie00006.svg" width="25pt" height="16pt" src="acp-22-1081-2022-ie00006.png"/></svg:svg> 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.