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| Titel |
Long term precipitation chemistry and wet deposition in a remote dry savanna site in Africa (Niger) |
| VerfasserIn |
C. Galy-Lacaux, D. Laouali, L. Descroix, N. Gobron, C. Liousse |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 5 ; Nr. 9, no. 5 (2009-03-03), S.1579-1595 |
| Datensatznummer |
250007000
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| Publikation (Nr.) |
 copernicus.org/acp-9-1579-2009.pdf |
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| Zusammenfassung |
| Long-term precipitation chemistry have been recorded in the rural area of
Banizoumbou (Niger), representative of a semi-arid savanna ecosystem. A
total of 305 rainfall samples ~90% of the total annual rainfall)
were collected from June 1994 to September 2005. From ionic chromatography,
pH major inorganic and organic ions were detected. Rainwater chemistry is
controlled by soil/dust emissions associated with terrigeneous elements
represented by SO42−, Ca2+, Carbonates, K+ and
Mg2+. It is found that calcium and carbonates represent ~40%
of the total ionic charge. The second highest contribution is nitrogenous,
with annual Volume Weighed Mean (VWM) for NO3−
and NH4+ concentrations of 11.6 and 18.1 μeq.l−1,
respectively. This is the signature of ammonia sources from animals and
NOx emissions from savannas soil-particles rain-induced. The mean
annual NH3 and NO2 air concentration are of 6 ppbv and 2.6 ppbv,
respectively. The annual VWM precipitation concentration of sodium and
chloride are both of 8.7 μeq.l−1 which reflects the marine
signature of monsoonal and humid air masses. The median pH value is of 6.05.
Acidity is neutralized by mineral dust, mainly carbonates, and/or dissolved
gases such NH3. High level of organic acidity with 8μeq.l−1
and 5.2 μeq.l−1 of formate and acetate were also found. The
analysis of monthly Black Carbon emissions and Fraction of Absorbed
Photosynthetically Active Radiation (FAPAR) values show that both biogenic
emission from vegetation and biomass burning could explain the
rainfall organic acidity content. The interannual variability of the VWM
concentrations around the mean (1994–2005) is between ±5% and ±30%
and mainly due to variations of sources strength and rainfall
spatio-temporal distribution. From 1994 to 2005, the total mean wet
deposition flux in the Sahelian region is of 60.1 mmol.m−2.yr−1
±25%. Finally, Banizoumbou measurements are compared to other
long-term measurements of precipitation chemistry in the wet savanna of
Lamto (Côte d'Ivoire) and in the forested zone of Zoétélé
(Cameroon). The total chemical loading presents a maximum in the dry savanna
and a minimum in the forest (from 143.7, 100.2 to 86.6 μeq.l−1),
associated with the gradient of terrigeneous sources. The wet deposition
fluxes present an opposite trend, with 60.0 mmol.m−2.yr−1 in
Banizoumbou, 108.6 mmol.m−2.yr−1 in Lamto and 162.9
mmol.m−2.yr−1 in Zoétélé, controlled by rainfall
gradient along the ecosystems transect. |
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