Air-snow transfer of nitrate on the East Antarctic Plateau - Part 2: An isotopic model for the interpretation of deep ice-core records
Erbland, J. ; Savarino, J. ; Morin, Samuel ; France, J. L. ; Frey, M. M. ; King, M. D.
Unraveling the modern budget of reactive nitrogen on the Antarctic Plateau is critical for the interpretation of ice-core records of nitrate. This requires accounting for nitrate recycling processes occurring in near-surface snow and the overlying atmospheric boundary layer. Not only concentration measurements but also isotopic ratios of nitrogen and oxygen in nitrate provide constraints on the processes at play. However, due to the large number of intertwined chemical and physical phenomena involved, numerical modeling is required to test hypotheses in a quantitative manner. Here we introduce the model TRANSITS (TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow), a novel conceptual, multi-layer and one-dimensional model representing the impact of processes operating on nitrate at the air-snow interface on the East Antarctic Plateau, in terms of concentrations (mass fraction) and nitrogen (?15N) and oxygen isotopic composition (17O excess, ?17O) in nitrate. At the air-snow interface at Dome C (DC; 75° 06' S, 123° 19' E), the model reproduces well the values of ?15N in atmospheric and surface snow (skin layer) nitrate as well as in the ?15N profile in DC snow, including the observed extraordinary high positive values (around +300 ?) below 2 cm. The model also captures the observed variability in nitrate mass fraction in the snow. While oxygen data are qualitatively reproduced at the air-snow interface at DC and in East Antarctica, the simulated ?17O values underestimate the observed ?17O values by several per mill. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO2 as well as by our lack of understanding of the NOx chemistry at Dome C. The model reproduces well the sensitivity of ?15N, ?17O and the apparent fractionation constants (15?app, 17Eapp) to the snow accumulation rate. Building on this development, we propose a framework for the interpretation of nitrate records measured from ice cores. Measurement of nitrate mass fractions and ?15N in the nitrate archived in an ice core may be used to derive information about past variations in the total ozone column and/or the primary inputs of nitrate above Antarctica as well as in nitrate trapping efficiency (defined as the ratio between the archived nitrate flux and the primary nitrate input flux). The ?17O of nitrate could then be corrected from the impact of cage recombination effects associated with the photolysis of nitrate in snow. Past changes in the relative contributions of the ?17O in the primary inputs of nitrate and the ?17O in the locally cycled NO2 and that inherited from the additional O atom in the oxidation of NO2 could then be determined. Therefore, information about the past variations in the local and long-range processes operating on reactive nitrogen species could be obtained from ice cores collected in low-accumulation regions such as the Antarctic Plateau.
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