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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 10 Sep 2019

Submitted as: research article | 10 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica

V. Holly L. Winton1, Alison Ming1, Nicolas Caillon2, Lisa Hauge1, Anna E. Jones1, Joel Savarino2, Xin Yang1, and Markus M. Frey1 V. Holly L. Winton et al.
  • 1British Antarctic Survey, Cambridge, CB3 0ET, UK
  • 2University of Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, F-38000 Grenoble, France

Abstract. The nitrate (NO3) isotopic composition δ15N-NO3 of polar ice cores has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO), in addition to the oxidising capacity of the ancient atmosphere. However, understanding the transfer of reactive nitrogen at the air-snow interface in Polar Regions is paramount for the interpretation of ice core records of δ15N-NO3 and NO3 mass concentrations. As NO3 undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ15N-NO3 and air-snow transfer modelling are necessary in order to understand and quantify the complex photochemical processes at play. As part of the Isotopic Constraints on Past Ozone Layer Thickness in Polar Ice (ISOL-ICE) project, we report new measurements of NO3 concentration and δ15N-NO3 in the atmosphere, skin layer (operationally defined as the top 5 mm of the snow pack), and snow pit depth profiles at Kohnen Station, Dronning Maud Land (DML), Antarctica. We compare the results to previous studies and new data, presented here, from Dome C, East Antarctic Plateau. Additionally, we apply the conceptual one-dimensional model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of photochemical processes that drive the archival of δ15N-NO3 and NO3 in the snow pack. We find clear evidence of NO3 photolysis at DML, and confirmation of our hypothesis that UV-photolysis is driving NO3 recycling at DML. Firstly, strong denitrification of the snow pack is observed through the δ15N-NO3 signature which evolves from the enriched snow pack (−3 to 100 ‰), to the skin layer (−20 to 3 ‰), to the depleted atmosphere (−50 to −20 ‰) corresponding to mass loss of NO3 from the snow pack. Secondly, constrained by field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO3 mass concentrations, the TRANSITS model is able to reproduce our δ15N-NO3 observations in depth profiles. We find that NO3 is recycled three times before it is archived (i.e., below the photic zone) in the snow pack below 15 cm and within 0.75 years. Archived δ15N-NO3 and NO3 concentration values are 50 ‰ and 60 ng g−1 at DML. NO3 photolysis is weaker at DML than at Dome C, due primarily to the higher DML snow accumulation rate; this results in a more depleted δ15N-NO3 signature at DML than at Dome C. Even at a relatively low snow accumulation rate of 6 cm yr−1 (water equivalent; w.e.), the accumulation rate at DML is great enough to preserve the seasonal cycle of NO3 concentration and δ15N-NO3, in contrast to Dome C where the profiles are smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ15N-NO3 at DML highlights that the dominant factors controlling the archived δ15N-NO3 signature are the snow accumulation rate and e-folding depth, with a smaller role from changes in the snowfall timing and TOC. Here we set the framework for the interpretation of a 1000-year ice core record of δ15N-NO3 from DML. Ice core δ15N-NO3 records at DML will be less sensitive to changes in UV than at Dome C, however the higher snow accumulation rate and more accurate dating at DML allows for higher resolution δ15N-NO3 records.

V. Holly L. Winton et al.
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Status: final response (author comments only)
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V. Holly L. Winton et al.
V. Holly L. Winton et al.
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Short summary
We report new measurements of nitrate concentration and its nitrogen isotope (δ15N-NO3) in atmospheric aerosol and surface snow at Kohnen, East Antarctica. Results show that the dominant factors controlling δ15N-NO3 preserved in firn are snow accumulation rate, light attenuation in snow, and with a smaller role changes in snowfall timing and total column ozone. Potential reconstruction of the past ozone layer from ice cores requires independent constraints of these other parameters.
We report new measurements of nitrate concentration and its nitrogen isotope (δ15N-NO3) in...