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| Titel |
Seasonal variations in stable isotope ratios of atmospheric nitrate in a tropical boundary layer environment (Cape Verde Observatory). |
| VerfasserIn |
Joel Savarino, Samuel Morin, Joseph Erbland, Francis Grannec, Matthew Patey, Eric Achterberg |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038683
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| Zusammenfassung |
| Atmospheric nitrate (NO3-) is the end-product of nitrogen oxides (NOx = NO + NO2)
oxidation, thus its production is sensitive to the oxidative state of the atmosphere.
Unfortunately, revealing this complex chemical activity using only nitrate concentration
appears to be an extremely difficult task if not impossible. More sophisticated approaches are
thus needed to extract from this ubiquitous atmospheric species information on the chemical
functioning of our atmosphere and its interaction with the climate.
Isotopic investigations have proven to be instrumental in deciphering sources and
processes affecting nitrate. Combining the analysis of the two oxygen stable isotope ratios
(17O/16O, 18O/16O) with the nitrogen stable isotope ratio (15N/14N) on the same sample is a
powerful tool to reveal unexpected processes happening in the air. 15N/14N ratio (expressed
in terms of δ15N) is used to trace NOx sources and mass transfers between reservoirs because
δ15N values are for the most part conserved during the chemical conversion of NOx to
nitrate. In contrast, the triple oxygen isotopic composition of nitrate provides original
information about the nature and the relative importance of NOx oxidation pathways,
ultimately leading to atmospheric nitrate. This interpretation results directly from the fact that
ozone possesses a unique and unconventional isotope composition (Î17O - δ17O -
0.52 Ã δ18O) which is transferred to others atmospheric species during chemical
attacks.
Despite recent successes in using the isotope composition of nitrate for deciphering
atmospheric chemical processes in polar and mid latitude regions, the strong oxidative
conditions prevailing in the sub tropical and tropical regions has been ignored hitherto.
Forgetting to document these regions is particularly detrimental for modellers who want to
use these new isotopic markers to place new constrains on their chemical and physical
codes.
In order to partially fill this gap, we present a full seasonal cycle of the nitrate isotope
systematic at the Cap Verde (lat. 16° 85’N, lon. 24° 87’ W) characteristic of a tropical
oceanic boundary layer. Both Î17O and δ15N display a marked seasonal cycle in
opposing phase. Î17O values are maximum in winter (- 30 permil) and minimal in
summer (- 26 permil) while δ15N display the opposite trend with maximum in
summer (-2.9 permil) and minimum in winter (-8.8 permil). No correlation is found
with the nitrate atmospheric loading for any of the isotope ratio. Î17O seasonal
variations correlate with the monthly averaged daytime depletion of ozone whose
seasonal variation have been attributed mainly to halogen oxides. This observation
suggests that part of the maximum nitrate oxygen anomaly (Î17O) could result from
halogen chemistry in the marine boundary layer via probably the strong NOx oxidant
BrO. For δ15N, a broad correlation is found with the atmospheric concentration
of NO and solar radiation intensity, confirming that δ15N of nitrate retranscribes
principally a source and/or a photochemical equilibrium effect between NO and
NO2. |
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