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 (NO3−) and its link with snowfall NO3− based upon the isotopic composition of NO3− (δ15N, δ18O, and Δ17O). Snowfall and the atmosphere show similar seasonal trends in concentrations and isotopic composition of NO3−. In summer, atmospheric NO3− is closely associated with snowpack emissions of NOx from photolysis of snow NO3−. In winter, linear relationships between δ15N and δ18O (or Δ17O) of NO3− in both snowfall and the atmosphere indicate mixing between stratospheric inputs and tropospheric sources contributing to NO3−, with stratospheric inputs contributing 55±21% of the atmospheric NO3− budget. The linear relationships suggest that the lower limits of δ15N, δ18O, and Δ17O of stratospheric-sourced NO3− are close to ∼18, ∼120, and ∼45‰, respectively. Concentration correlates well with the isotopic composition of NO3− in winter, indicating less variable contribution of tropospheric sources. A significant linear correlation between δ18O and Δ17O of NO3− suggests a mix of oxidation processes by O3 and H2O/OH which can influence NOx cycling and the production of NO3−. Lower values of Δ17O of atmospheric NO3− were observed during O3 depletion events in September, suggesting that oxygen isotopes of NO3− could be more sensitive to the changes in surface O3 compared to BrO concentrations. Oxygen isotopic composition of NO3− in snowfall is close to that of the atmosphere throughout the year, suggesting that snowfall NO3− can relay information on oxidative chemistry of NOx in the atmosphere. Snowfall δ15N 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 NO3−. This suggests that the interpretation of δ15N in snow needs to consider seasonal changes in sources and chemistry.