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| Titel |
Comprehensive isotopic composition of nitrate in Antarctic snow and atmospheric aerosol samples: Towards interpretation of the polar ice core record |
| VerfasserIn |
William Vicars, Joseph Erbland, Joel Savarino |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046447
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| Zusammenfassung |
| Nitrate (NO3-) is the final product of the oxidation of atmospheric reactive nitrogen (NOx)
and is one of the most abundant ions present in polar ice and snow. Stable isotope ratios of
NO3- obtained from deep ice cores may serve as useful proxies for paleo-atmospheric
conditions, such NOx removal pathways and the role of ozone (O3) in overall oxidation
activity. However, strong post-depositional processing (photolysis of NO3- and
desorption/evaporation of HNO3) has been shown to complicate the interpretation of
long-term ice core records of nitrate. Analysis of NO3- obtained from a deep Antarctic ice
core suggests that nitrogen stable isotopes are strongly impacted by surface processing, while
the oxygen signal seems to be at least partially preserved. A primary goal of this study is to
establish the atmosphere/surface snow transfer of the NO3- isotopic signal, particularly the
Δ17O anomaly, which reflects the relative importance of different NO3- formation
mechanisms.
Here we report measurements of the nitrogen (δ15N) and triple oxygen (δ17O and δ18O)
isotopic composition of NO3- and use them to infer the origin and fate of NOX and NO3- in
the Antarctic atmosphere and surface snow on a seasonal basis. Surface snow (“skin layer,”
first 3-4 mm) and aerosol samples were collected at Dome C, Antarctica (75Ë 06’S, 123Ë
21’E, altitude 3233 m a.s.l.) throughout 2009. Preliminary results indicate that the seasonal
variations in the oxygen isotopic composition of NO3- in the skin layer closely follow
those of atmospheric NO3-, suggesting that the upper-most snow layer acts as
an integrator of the atmospheric signal. In the winter, the Δ17O (NO3-) of both
surface snow and aerosols reach maximum values (40-45 per mil), reflecting an
increased interaction of NOx with stratospheric ozone (O3), which possesses a strongly
positive oxygen isotope anomaly. The lowest Δ17O (NO3-) values (25-30 per mil)
for surface snow and aerosols are observed in the late spring and early summer
when O3 concentrations are at a seasonal low and atmospheric NO3- production
occurs predominantly through reaction of NOx with the hydroxyl radical (OH).
Conversely, skin layer δ15N (NO3-) values are consistently enriched by10-20 per mil
over the atmospheric signal throughout most the year. This observation indicates
fractionation of NO3- isotopes during post-depositional processing, which is known to
preferentially remove lighter isotopologues from the snowpack; however, the persistence
of this offset through the winter months cannot be explained at the present time.
For both the Δ17O and δ15N of NO3-, an approximate equilibrium between the
atmosphere and skin layer is achieved in the late spring/early summer, when atmospheric
NO3- concentrations reach their annual maxima (> 100 ng m-3). This strongly
suggests that NO3- removal and subsequent recycling is very intense and most likely
composed of more than one cycle (i.e., removal, atmospheric processing, redeposition or
possibly export). Information on these cycles is critical to the development of a
conceptual view of NO3- recycling at the snow surface on the Antarctic plateau. |
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