Using stable isotopes to distinguish atmospheric nitrate production and its contribution to the surface ocean across hemispheres

Atmospheric samples and surface seawater collected on a Chinese Antarctic Research Expedition (CHINARE) transect are used to investigate sources and production of nitrate (NO3-) in the atmosphere and its contribution to the surface NO3- pool in the ocean. Most atmospheric NO3- is concentrated on intermediate size particles, and much higher concentrations were observed in the northern hemisphere than in the high southern latitudes. Isotopes of NO3-(delta N-15, delta O-18 and Delta O-17) suggest that elevated atmospheric NO3- in coastal areas was associated with human activities, while NO3- in the high southern latitudes tends to be influenced by precursor Antarctic snowpack emissions driven by photolysis. In general, no clear association was found between the isotopes of surface seawater and atmospheric NO3-, suggesting that the ocean is unlikely to be an important direct source of atmospheric NOx on this transect. A significant linear relationship between delta O-18 and Delta O-17 of NO3- is used to interpret important pathways for NO- 3production. In the tropics, >59% of atmospheric NO3- is produced via OH oxidation of NO2, while the elevated oxygen isotopic ratios (delta O-18 and Delta O-17) in the high southern latitudes suggest increased NO3- production via BrO and/or DMS pathways assuming a minor contribution of the N2O5 channel. In surface seawater, high NO3- concentrations are present in the coastal areas and in the Southern Ocean. In coastal areas of China, positive Delta O-17 values in seawater NO3-(1.7 +/- 1.0 parts per thousand) provide direct evidence of uncycled atmospheric deposition contribution, with a calculated contribution of at least 2-3% to total surface NO3-. A Delta O-17 = 0 was found everywhere else in seawater, suggesting that atmospheric deposition has a minimal presence in the surface NO3- pool. Near Antarctica, deposition of atmospheric NO3- with extremely low delta N-15(< - 30 parts per thousand) could lower delta N-15 found in sea ice, and this process could be isotopically important to evaluate nitrogen cycling in sea ice. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study found that atmospheric nitrate is mainly concentrated on intermediate size particles, with much higher concentrations observed in the northern hemisphere compared to high southern latitudes. Isotope analysis indicated that elevated atmospheric nitrate in coastal areas is associated with human activities, while in high southern latitudes, it is influenced by precursor Antarctic snowpack emissions.