Abstract. The nitrogen stable isotopic composition in nitrate (δ15N-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 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 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 δ15N-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 δ15N-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 δ15N-NO3- signature, which evolves from the enriched snowpack (−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 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 δ15N-NO3- and NO3- mass concentration values are 50 ‰ 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 δ15N-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 δ15N-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 δ15N-NO3- at DML highlights that the dominant factors controlling the archived δ15N-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 δ15N-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 100 DU (Dobson units). 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.

<p><strong>Abstract.</strong> The nitrate (NO<sub>3</sub><sup>−</sup>) isotopic composition δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> mass concentrations. As NO<sub>3</sub><sup>−</sup> undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 NO<sub>3</sub><sup>−</sup> concentration and δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> in the snow pack. We find clear evidence of NO<sub>3</sub><sup>−</sup> photolysis at DML, and confirmation of our hypothesis that UV-photolysis is driving NO<sub>3</sub><sup>−</sup> recycling at DML. Firstly, strong denitrification of the snow pack is observed through the δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 NO<sub>3</sub><sup>−</sup> from the snow pack. Secondly, constrained by field measurements of snow accumulation rate, light attenuation (e-folding depth) and atmospheric NO<sub>3</sub><sup>−</sup> mass concentrations, the TRANSITS model is able to reproduce our δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> observations in depth profiles. We find that NO<sub>3</sub><sup>−</sup> 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 δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and NO<sub>3</sub><sup>−</sup> concentration values are 50 ‰ and 60 ng g<sup>−1</sup> at DML. NO<sub>3</sub><sup>−</sup> photolysis is weaker at DML than at Dome C, due primarily to the higher DML snow accumulation rate; this results in a more depleted δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> signature at DML than at Dome C. Even at a relatively low snow accumulation rate of 6 cm yr<sup>−1</sup> (water equivalent; w.e.), the accumulation rate at DML is great enough to preserve the seasonal cycle of NO<sub>3</sub><sup>−</sup> concentration and δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup>, in contrast to Dome C where the profiles are smoothed due to stronger photochemistry. TRANSITS sensitivity analysis of δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> at DML highlights that the dominant factors controlling the archived δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> from DML. Ice core δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> 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 δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> records.</p>

Abstract. Polar researchers spend enormous costs transporting snow and ice samples to home laboratories for simple analyses in order to constrain annual layer thicknesses and identifying accumulation rates of specific sites. It is well known that depositional noise, incurred from wind drifts, seasonally-biased deposition, melt layers and more, can influence individual snow and firn records and that multiple cores are required to produce statistically robust time series. Thus at many sites core samples are measured in the field for densification, but the annual accumulation and the content of chemical impurities are often represented by just one core to reduce transport costs. We have developed a portable Light weight in Situ Analysis (LISA) box for ice, firn and snow analysis capable of constraining annual layers through the continuous flow analysis of melt water conductivity and peroxide under field conditions. The box can run using a small gasoline-generator and weighs less than 50 kg. The LISA box was tested under field conditions at the deep ice core drilling site EastGRIP in Northern Greenland. Analysis of the top 2 metres of snow from 7 sites in Northern Greenland (Figure 1) allowed the reconstruction of regional snow accumulation patterns for the period 2015–2019.

Abstract. There are enormous costs involved in transporting snow and ice samples to home laboratories for “simple” analyses in order to constrain annual layer thicknesses and identify accumulation rates of specific sites. It is well known that depositional noise, incurred from factors such as wind drifts, seasonally biased deposition and melt layers can influence individual snow and firn records and that multiple cores are required to produce statistically robust time series. Thus, at many sites, core samples are measured in the field for densification, but the annual accumulation and the content of chemical impurities are often represented by just one core to reduce transport costs. We have developed a portable “lightweight in situ analysis” (LISA) box for ice, firn and snow analysis that is capable of constraining annual layers through the continuous flow analysis of meltwater conductivity and hydrogen peroxide under field conditions. The box can run using a small gasoline generator and weighs less than 50 kg. The LISA box was tested under field conditions at the East Greenland Ice-core Project (EastGRIP) deep ice core drilling site in northern Greenland. Analysis of the top 2 m of snow from seven sites in northern Greenland allowed the reconstruction of regional snow accumulation patterns for the 2015–2018 period (summer to summer).

No abstract