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

• Main Authors: V. H. L. Winton, A. Ming, N. Caillon, L. Hauge, A. E. Jones, J. Savarino, X. Yang, M. M. Frey
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/5861/2020/acp-20-5861-2020.pdf

<p>The nitrogen stable isotopic composition in nitrate (<span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="4c315b3ea451cf26923ad12993612b33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00001.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00001.png"/></svg:svg></span></span>) 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 (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a186e28964d6ae507e65dbc91f8b1f71"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00002.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00002.png"/></svg:svg></span></span>) 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 <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="91b2e19ca239409a7665981c17575147"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00003.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00003.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a33a7d42b70ca1fe513ac92c5832eec2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00004.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00004.png"/></svg:svg></span></span> mass concentrations. As <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a02883d0956e7dc256b9fe9fffa70b09"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00005.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00005.png"/></svg:svg></span></span> undergoes a number of post-depositional processes before it is archived in ice cores, site-specific observations of <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="b3512ed4eb493ff037a5c39221523c47"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00006.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00006.png"/></svg:svg></span></span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="dd23f13eb24280cbe650be4567ce8571"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00007.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00007.png"/></svg:svg></span></span> mass concentration and <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="737339a8d3517116341490f01d8cfecf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00008.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00008.png"/></svg:svg></span></span> in the atmosphere, skin layer (operationally defined as the top 5&thinsp;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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="9712381780fcc4de6c4d72f703a8771c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00009.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00009.png"/></svg:svg></span></span> recycling on <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ecc3e6dd5af0ffb1da8bfbfcb16b8e8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00010.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00010.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="48a6d5724cc017ced9c974ab9a81c03a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00011.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00011.png"/></svg:svg></span></span> mass concentrations archived in snow and firn. We find clear evidence of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="822fcc3376206f5298bc14405cca7022"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00012.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00012.png"/></svg:svg></span></span> photolysis at DML and confirmation of previous theoretical, field, and laboratory studies that UV photolysis is driving <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M18" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="7dd3c683c0655cd2a5c1ed2d08ea01e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00013.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00013.png"/></svg:svg></span></span> recycling and redistribution at DML. Firstly, strong denitrification of the snowpack is observed through the <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="827b0fe0e97f70953101fc9e20cd0031"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00014.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00014.png"/></svg:svg></span></span> signature, which evolves from the enriched snowpack (<span class="inline-formula">−3</span>&thinsp;‰ to 100&thinsp;‰), to the skin layer (<span class="inline-formula">−20</span>&thinsp;‰ to 3&thinsp;‰), to the depleted atmosphere (<span class="inline-formula">−50</span>&thinsp;‰ to <span class="inline-formula">−20</span>&thinsp;‰), corresponding to mass loss of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="361af9fc60c163df468716a068868655"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00015.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00015.png"/></svg:svg></span></span> from the snowpack. Based on the TRANSITS model, we find that <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="59e8efc9900af362c4e31d9012378c85"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00016.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00016.png"/></svg:svg></span></span> is recycled two times, on average, before it is archived in the snowpack below 15&thinsp;cm and within 0.75 years (i.e. below the photic zone). Mean annual archived <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="da99f0b0265c157ce21f9580f34f8fd2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00017.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00017.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="f406d9210c9988b6f1f99fbfd13290fc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00018.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00018.png"/></svg:svg></span></span> mass concentration values are 50&thinsp;‰ and 60&thinsp;ng&thinsp;g<span class="inline-formula">⁻¹</span>, respectively, at the DML site. We report an <span class="inline-formula"><i>e</i></span>-folding depth (light attenuation) of 2–5&thinsp;cm for the DML site, which is considerably lower than Dome C. A reduced photolytic loss of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M32" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="1617f129b271622bc1f8b44f3237e7d7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00019.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00019.png"/></svg:svg></span></span> at DML results in less enrichment of <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="983e472baa2b1950fa497a555dcc7b2e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00020.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00020.png"/></svg:svg></span></span> than at Dome C mainly due to the shallower <span class="inline-formula"><i>e</i></span>-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&thinsp;cm&thinsp;yr<span class="inline-formula">⁻¹</span> (water equivalent; w.e.), the snow accumulation rate at DML is great enough to preserve the seasonal cycle of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M37" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="06178b8a1ccf121783e8e34310fe8913"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00021.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00021.png"/></svg:svg></span></span> mass concentration and <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="0ed2522f87147883b53f48851745fd62"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00022.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00022.png"/></svg:svg></span></span>, 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 <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M41" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="12193b9fdcecd5060489ac774923195d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00023.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00023.png"/></svg:svg></span></span> at DML highlights that the dominant factors controlling the archived <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M43" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="2bdfa65311a7f5f5135cb65d40e792b4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00024.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00024.png"/></svg:svg></span></span> signature are the <span class="inline-formula"><i>e</i></span>-folding depth and snow accumulation rate, with a smaller role from changes in the snowfall timing and TCO. Mean TRANSITS model sensitivities of archived <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M46" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="0b2342e6bcb051a73d8b6141e44df4e2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00025.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00025.png"/></svg:svg></span></span> at the DML site are 100&thinsp;‰ for an <span class="inline-formula"><i>e</i></span>-folding depth change of 8&thinsp;cm, 110&thinsp;‰ for an annual snow accumulation rate change of 8.5&thinsp;cm&thinsp;yr<span class="inline-formula">⁻¹</span>&thinsp;w.e., 10&thinsp;‰ for a change in the dominant snow deposition season between<span id="page5862"/> winter and summer, and 10&thinsp;‰ for a TCO change of 100&thinsp;DU (Dobson units). Here we set the framework for the interpretation of a 1000-year ice core record of <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M50" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="894d1464517d96b44bb1c3820f6003d0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00026.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00026.png"/></svg:svg></span></span> from DML. Ice core <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M52" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d8cd9552c7c4785ac20013eed83c16b2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00027.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00027.png"/></svg:svg></span></span> 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 <span class="inline-formula"><i>δ</i>¹⁵N</span>-<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M54" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="12b3efeee90d492a66890d92e4bfda63"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-5861-2020-ie00028.svg" width="25pt" height="16pt" src="acp-20-5861-2020-ie00028.png"/></svg:svg></span></span> records.</p>