Air‐Sea Ammonia Fluxes Calculated From High‐Resolution Summertime Observations Across the Atlantic Southern Ocean

• Published in: Geophysical research letters 2021-05, Vol.48 (9), p.n/a
• Main Authors: Altieri, K. E., Spence, K. A. M., Smith, S.
• Format: Article
• Language: English
• Subjects: air‐sea flux; ammonia; Southern Ocean
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL091963

Oceanic ammonia emissions are the largest natural source of ammonia globally, but the magnitude of the air‐sea flux in remote regions with minimal human influence remains uncertain. Here, we measured the concentration of surface ocean ammonium and atmospheric ammonia gas every two hours across a latitudinal transect (34.5°S to 61°S) of the Atlantic Southern Ocean during summer. Surface ocean ammonium concentrations ranged from undetectable to 0.36 µM and ammonia gas concentrations ranged from 0.6 to 5.1 nmol m−3. Calculated ammonia fluxes ranged from −2.8 to −75 pmol m−2 s−1, and were consistently from the atmosphere into the ocean, even in regions where surface ocean ammonium concentrations were relatively high. As expected, temperature was the dominant control on the air‐sea ammonia flux across the latitudinal transect. However, a sensitivity analysis suggests that seasonality in the surface Southern Ocean nitrogen cycle may have a major influence on the direction of the ammonia flux. Plain Language Summary Ammonia is the most important basic gas in the atmosphere and it plays an important role in forming new aerosols and controlling the acidity of aerosols, with implications for climate. Ammonia emissions to the atmosphere have increased significantly over time due to human activities, primarily related to agriculture. There are very few regions where one can observe the atmosphere away from these human emissions, making it difficult to quantify natural processes. Our study investigated ocean ammonia fluxes, which are thought to be the largest natural source of ammonia globally, across the Atlantic Southern Ocean where human influences should be minimal. Previous studies have suggested that in this region, the ammonia should flux into the ocean because of the cold surface seawater, but direct measurements are scarce. We found that the flux was indeed into the ocean across the latitudinal transect, although the values were small. A sensitivity analysis suggests that the flux could easily reverse as a result of seasonal changes in surface ocean biological and chemical processes. More observations are needed in this remote region of the atmosphere, particularly during the inhospitable winter season, in order to understand the natural cycling of ammonia between the ocean and atmosphere. Key Points Air‐sea fluxes of ammonia were calculated from ship‐board observations every two hours across the Atlantic Southern Ocean Air‐sea fluxes were small and consistent