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

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- (delta N-15, delta O-18 and Delta O-17). 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 delta N-15 and delta O-18 (or Delta O-17) 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 delta N-15, delta O-18 and Delta O-17 of stratospheric-sourced NO3- are close to similar to 18, similar to 120, and similar to 45 parts per thousand, respectively. Concentration correlates well with the isotopic composition of NO3- in winter, indicating less variable contribution of tropospheric sources. A significant linear correlation between delta O-18 and Delta O-17 of NO3- suggests a mix of oxidation processes by O-3 and H2O/OH which can influence NOx cycling and the production of NO3-. Lower values of Delta O-17 of atmospheric NO3- were observed during O-3 depletion events in September, suggesting that oxygen isotopes of NO3- could be more sensitive to the changes in surface O-3 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 delta N-15 is close in value to that in the atmosphere during winter, but similar to 20 parts per thousand 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 delta N-15 in snow needs to consider seasonal changes in sources and chemistry. (C) 2021 Elsevier B.V. All rights reserved.