Air–snow transfer of nitrate on the East Antarctic Plateau – Part 2: An isotopic model for the interpretation of deep ice-core records

Abstract

Abstract. 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 (δ 15 N) and oxygen isotopic composition ( 17 O excess, 17 O) in nitrate. At the airsnow interface at Dome C (DC; 75• 06 S, 123• 19 E), the model reproduces well the values of δ 15 N in atmospheric and surface snow (skin layer) nitrate as well as in the δ 15 N 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 17 O values underestimate the observed 17 O values by several per mill. This is explained by the simplifications made in the description of the atmospheric cycling and oxidation of NO 2 as well as by our lack of understanding of the NO x chemistry at Dome C. The model reproduces well the sensitivity of δ 15 N, 17 O and the apparent fractionation constants ( 15 ε app , 17 E app ) 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 δ 15 N 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 17 O 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 17 O in the primary inputs of nitrate and the 17 O in the locally cycled NO 2 and that inherited from the additional O atom in the oxidation of NO 2 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 lowaccumulation regions such as the Antarctic Plateau.

The impact of snow nitrate photolysis on boundary layer chemistry and the recycling and redistribution of reactive nitrogen across Antarctica and Greenland in a global chemical transport model

“…Dry-deposition velocities for coarsemode aerosols (radii between 1 and 10 mm) are calculated based on aerosol size and hydroscopic growth as described in Zhang et al (2001). Aerosol deposition to snow and ice surfaces is described by Fisher et al (2011). For smaller aerosols (radii less than 1 µm), dry deposition velocities are calculated with a standard resistance-in-series scheme (Wang et al, 1998;Wesely, 1989).…”

Analysis of nitrate in the snow and atmosphere at Summit, Greenland: Chemistry and transport

“…This minimal loss fits with isotopic observations of nitrate at Summit thus far. However, recent modeling of the isotopic composition of nitrate under conditions of postdepositional photolytic loss at Dome C on the East Antarctic ice sheet [Erbland et al, 2015] suggests that a significant amount of recycling of NO 3 À can take place locally ( Figure 1, arrows a, c, and d)-i.e., NO 3 À is photolyzed and NO x escapes the snow, this NO x reacts in the gas phase above the snow and is either transported away (Figure 1, arrow b) or redeposited locally as NO 3 À (Figure 1, arrows c and d). If this process was important at Summit as well, the δ 15 N of NO 3 À in the snow should reflect both be photolyzed in surface snow (arrow a) releasing NO x to the atmosphere above, which can be transported away (arrow b) or reacted with local oxidants to regenerate NO 3 À (arrow c).…”

Spatial variations in snowpack chemistry, isotopic composition of NO<sub>3</sub><sup>−</sup> and nitrogen deposition from the ice sheet margin to the coast of western Greenland

“…Hence Erbland et al (2015) define "NO − 3 recycling" as the net effect of NO − 3 photolysis (producing NO x ), atmospheric re-processing and the local redeposition (wet or dry) and export of products. Recycling does not result in net removal from the snowpack but can progressively modify isotopic signatures of the NO − 3 .…”