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Delon C.,CNRS Laboratory for Aerology | Galy-Lacaux C.,CNRS Laboratory for Aerology | Boone A.,Meteo - France | Liousse C.,CNRS Laboratory for Aerology | And 8 more authors.
Atmospheric Chemistry and Physics | Year: 2010

The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized nitrogen compounds. In this study, a first attempt at estimating the Sahel nitrogen budget for the year 2006 is made, through measurements and simulations at three stations from the IDAF network situated in dry savanna ecosystems. Dry deposition fluxes are estimated from measurements of NO2, HNO3 and NH3 gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are calculated from NH4 + and NO3− concentrations in samples of rain. Emission fluxes are estimated including biogenic emission of NO from soils (an Artificial Neural Network module has been inserted into the ISBA-SURFEX surface model), emission of NOx and NH3 from domestic fires and biomass burning, and volatilization of NH3 from animal excreta. Uncertainties are calculated for each contribution of the budget.

This study uses original and unique data from remote and hardly-ever-explored regions.The monthly evolution of oxidized N compounds shows that emission and deposition increase at the beginning of the rainy season because of large emissions of biogenic NO (pulse events). Emission of oxidized compounds is dominated by biogenic emission from soils (domestic fires and biomass burning of oxidized compounds account for 0 to 13% at the most at the annual scale, depending on the station), whereas emission of NH3 is dominated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accounts for 47% of the total estimated deposition flux, for both oxidized and reduced compounds. The average estimated wet plus dry deposition flux in dry savanna ecosystems is 7.5±1.8 kgNha -1 yr-1, with approximately 30% attributed to oxidized compounds, and the rest attributed to NHx. The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgN ha -1 yr-1, dominated by NH3 volatilization (72"82%) and biogenic emission from soils (11"17%), depending on the applied volatilization rate of NH3. While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH3 emission from volatilization.

When scaled up from the 3 measurement sites to the Sahelian region (12° N:18° N, 15° W:10° E), the estimated total emission ranges from 2(±0.9) to 3(±1.4) TgN yr-1, depending on the applied volatilization rate of NH 3 and estimated total deposition is 1.8(±0.4) TgNy -1. The dry savanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget. Source

Delon C.,CNRS Laboratory for Aerology | Galy-Lacaux C.,CNRS Laboratory for Aerology | Adon M.,CNRS Laboratory for Aerology | Adon M.,Laboratoire Of Physique Of Latmosphere | And 4 more authors.
Biogeosciences | Year: 2012

Surface emission and deposition fluxes of reactive nitrogen compounds have been studied in five sites of West Africa during the period 2002 to 2007. Measurements of N deposition fluxes have been performed in IDAF sites representative of main west and central African ecosystems, i.e., 3 stations in dry savanna ecosystems (from 15° N to 12° N), and 2 stations in wet savanna ecosystems (from 9° N to 6° N). Dry deposition fluxes are calculated from surface measurements of NO 2, HNO 3 and NH 3 concentrations and simulated deposition velocities, and wet deposition fluxes are calculated from NH4+ and NO3− concentration in samples of rain. Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NOx and NH 3 from biomass burning and domestic fires, and volatilization of NH 3 from animal excreta. This paper is a tentative to understand the eventual impact of the monsoon variability from year to year, with the natural variability of local sources, on the emission and deposition N fluxes, and to compare these evolutions between dry and wet savanna ecosystems. In dry savanna ecosystems where the rain season lasts mainly from June to September, the occurence of rain correlates with the beginning of emission and deposition fluxes. This link is less obvious in wet savanna ecosystems (wet season mainly from May to October), where the surface is less submitted to drastic changes in terms of water content. Whatever the location, the natural variability of rain from year to year does not exceed 15 %, and the variability of emission and deposition magnitude ranges between 15 % and 28 %. While quasi providing the same total N budget, and due to the presence of different types of soils and vegetation, wet and dry savanna do not present the same distribution in emission and deposition fluxes contributions: in dry savanna, the emission is dominated by ammonia volatilization, and the deposition is dominated by the dry contribution. In wet savanna, emission is equally distributed between ammonia volatilization, emissions from biomass burning and natural NO emissions from soils, and wet and dry deposition are equivalent. Due to the scarcity of available data on the African continent, and despite the numerous uncertainties resulting from the different calculations and assumptions, this work is a combination of data from different origins (surface measurements, satellite and modelling) to document the atmospheric Nitrogen cycle in tropical regions. © Author(s) 2012. Source

Ndoye S.,Laboratoire Of Physique Of Latmosphere | Ndoye S.,Paris-Sorbonne University | Capet X.,Paris-Sorbonne University | Estrade P.,Laboratoire Of Physique Of Latmosphere | And 5 more authors.
Journal of Geophysical Research C: Oceans | Year: 2015

The southern end of the Canary current system comprises of an original upwelling center that has so far received little attention, the Southern Senegal-Gambia Upwelling Center (SSUC). We investigate its dynamical functioning by taking advantage of favorable conditions in terms of limited cloud coverage. Analyses and careful examinations of over 1500 satellite images of sea surface temperature scenes contextualized with respect to wind conditions confirm the regularity and stability of the SSUC dynamical functioning (as manifested by the recurrence and persistence of particular SST patterns). The analyses also reveal subtle aspects of its upwelling structure: shelf break cooling of surface waters consistent with internal tide breaking/mixing; complex interplay between local upwelling and the Mauritanian current off the Cape Verde headland; complexity of the inner-shelf/mid shelf frontal transition. The amplitude of the diurnal cycle suggests that large uncertainties exist in the SSUC heat budget. The studies limitations underscore the need for continuous in situ measurement in the SSUC, particularly of winds. Key Points: Analysis of SST and wind reveals the stability of the SSUC dynamical functioning Analysis of SST reveals large amplitude of the nearshore diurnal cycle Improved SSUC dynamical description by identification of recurrent features © 2014. American Geophysical Union. All Rights Reserved. Source

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