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

The nitrogen stable isotopic composition in nitrate (delta N-15-NO3-) measured in ice cores from low-snow-accumulation regions in East Antarctica has the potential to provide constraints on past ultraviolet (UV) radiation and thereby total column ozone (TCO) due to the sensitivity of nitrate (NO3-) photolysis to UV radiation. 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 delta N-15-NO3- and NO3- mass concentrations. As NO 3 undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of delta N-15-NO3- and air-snow transfer modelling are necessary 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- mass concentration and delta N-15-NO3- in the atmosphere, skin layer (operationally defined as the top 5 mm of the snowpack), 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 on the East Antarctic Plateau. Additionally, we apply the conceptual 1D model of TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS) to assess the impact of NO3- recycling on delta N-15-NO3- and NO3- mass concentrations archived in snow and firn. We find clear evidence of NO3- photolysis at DML and confirmation of previous theoretical, field, and laboratory studies that UV photolysis is driving NO3- recycling and redistribution at DML. Firstly, strong denitrification of the snowpack is observed through the delta N-15-NO3- signature, which evolves from the enriched snowpack (-3 parts per thousand to 100 parts per thousand), to the skin layer (-20 parts per thousand to 3 parts per thousand), to the depleted atmosphere (-50% to -2 parts per thousand), corresponding to mass loss of NO3- from the snowpack. Based on the TRANSITS model, we find that NO3- is recycled two times, on average, before it is archived in the snowpack below 15 cm and within 0.75 years (i.e. below the photic zone). Mean annual archived delta N-15-NO3- and NO3- mass concentration values are 50 parts per thousand and 60 ng g(-1), respectively, at the DML site. We report an e-folding depth (light attenuation) of 2-5 cm for the DML site, which is considerably lower than Dome C. A reduced photolytic loss of NO3- at DML results in less enrichment of delta N-15-NO3- than at Dome C mainly due to the shallower e-folding depth but also due to the higher snow accumulation rate based on TRANSITS-modelled sensitivities. Even at a relatively low snow accumulation rate of 6 cm yr(-1) (water equivalent; w.e. ), the snow accumulation rate at DML is great enough to preserve the seasonal cycle of NO3- mass concentration and delta N-15-NO3-, in contrast to Dome C where the depth profiles are smoothed due to longer exposure of surface snow layers to incoming UV radiation before burial. TRANSITS sensitivity analysis of delta N-15-NO3- at DML highlights that the dominant factors controlling the archived delta N-15-NO3- signature are the e-folding depth and snow accumulation rate, with a smaller role from changes in the snowfall timing and TCO. Mean TRANSITS model sensitivities of archived delta N-15-NO3- at the DML site are 100% for an e-folding depth change of 8 cm, 110% for an annual snow accumulation rate change of 8.5 cm yr(-1) w.e., 10% for a change in the dominant snow deposition season between winter and summer, and 10% for a TCO change of 100DU (Dobson units). Here we set the framework for the interpretation of a 1000-year ice core record of delta N-15-NO3- from DML. Ice core delta N-15-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 delta N-15-NO3- records.