Air–snow transfer of nitrate on the East Antarctic Plateau – Part 2: An isotopic model for the interpretation of deep ice-core records
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 m...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/12079/2015/acp-15-12079-2015.pdf |
| Summary: | 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<sup>15</sup>N) and oxygen
isotopic composition (<sup>17</sup>O excess, Δ<sup>17</sup>O) in nitrate. At the
air–snow interface at Dome C (DC; 75° 06' S, 123° 19' E),
the model reproduces well the values of δ<sup>15</sup>N in atmospheric and
surface snow (skin layer) nitrate as well as in the δ<sup>15</sup>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 Δ<sup>17</sup>O values underestimate the
observed Δ<sup>17</sup>O values by several per mill. This is
explained by the simplifications made in the description of the atmospheric
cycling and oxidation of NO<sub>2</sub> as well as by our lack of understanding of
the NO<sub><i>x</i></sub> chemistry at Dome C. The model reproduces well the sensitivity
of δ<sup>15</sup>N, Δ<sup>17</sup>O and the apparent fractionation
constants (<sup>15</sup>ϵ<sub>app</sub>, <sup>17</sup><i>E</i><sub>app</sub>) 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<sup>15</sup>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 Δ<sup>17</sup>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 Δ<sup>17</sup>O in
the primary inputs of nitrate and the Δ<sup>17</sup>O in the locally
cycled NO<sub>2</sub> and that inherited from the additional O atom in the
oxidation of NO<sub>2</sub> 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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| ISSN: | 1680-7316 1680-7324 |