Isotopic constraints on sources, production, and phase partitioning for nitrate in the atmosphere and snowfall in coastal East Antarctica

Abstract Atmospheric samples and snowfall collected in coastal East Antarctica over two years are used to investigate the sources, production of atmospheric nitrate (NO 3 − ) and its link with snowfall NO 3 − based upon the isotopic composition of NO 3 − ( δ 15 N, δ 18 O, and Δ 17 O). Snowfall and the atmosphere show similar seasonal trends in concentrations and isotopic composition of NO 3 − . In summer, atmospheric NO 3 − is closely associated with snowpack emissions of NOx from photolysis of snow NO 3 − . In winter, linear relationships between δ 15 N and δ 18 O (or Δ 17 O) of NO 3 − in both snowfall and the atmosphere indicate mixing between stratospheric inputs and tropospheric sources contributing to NO 3 − , with stratospheric inputs contributing 55±21% of the atmospheric NO 3 − budget. The linear relationships suggest that the lower limits of δ 15 N, δ 18 O, and Δ 17 O of stratospheric-sourced NO 3 − are close to ∼18, ∼120, and ∼45‰, respectively. Concentration correlates well with the isotopic composition of NO 3 − in winter, indicating less variable contribution of tropospheric sources. A significant linear correlation between δ 18 O and Δ 17 O of NO 3 − suggests a mix of oxidation processes by O3 and H2O/OH which can influence NOx cycling and the production of NO 3 − . Lower values of Δ 17 O of atmospheric NO 3 − were observed during O3 depletion events in September, suggesting that oxygen isotopes of NO 3 − could be more sensitive to the changes in surface O3 compared to BrO concentrations. Oxygen isotopic composition of NO 3 − in snowfall is close to that of the atmosphere throughout the year, suggesting that snowfall NO 3 − can relay information on oxidative chemistry of NOx in the atmosphere. Snowfall δ 15 N is close in value to that in the atmosphere during winter, but ∼20‰ higher than that in the atmosphere during summer, possibly associated with seasonal changes in the gas-aerosol partitioning of atmospheric NO 3 − . This suggests that the interpretation of δ 15 N in snow needs to consider seasonal changes in sources and chemistry.