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

Abstract

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− (δ15N, δ18O and Δ17O) 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 δ18O and Δ17O of NO3− is used to interpret important pathways for NO3− production. In the tropics, >59% of atmospheric NO3− is produced via OH oxidation of NO2, while the elevated oxygen isotopic ratios (δ18O and Δ17O) 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 Δ17O values in seawater NO3− (1.7 ± 1.0‰) provide direct evidence of uncycled atmospheric deposition contribution, with a calculated contribution of at least 2-3% to total surface NO3−. A Δ17O=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 δ15N (<−30‰) could lower δ15N found in sea ice, and this process could be isotopically important to evaluate nitrogen cycling in sea ice.