Abstract. In this paper we present the long term monitoring of ambient gaseous concentrations within the framework of the IDAF (IGAC-DEBITS-AFRICA) program. This study proposes for the first time an analysis of long-term inorganic gas concentrations (1998 to 2007) of SO 2 , NO 2 , HNO 3 , NH 3 and O 3 , determined using passive samplers at seven remote sites in West and Central Africa. Sites are representative of several African ecosystems and are located along a transect from dry savannas-wet savannas-forests with sites at Banizoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d'Ivoire), Zoetele (Cameroon) and Bomassa (Congo). The strict control of measurement techniques as well as the validation and inter-comparison studies conducted with the IDAF passive samplers assure the quality and accuracy of the measurements. For each type of African ecosystem, the long term data series have been studied to document the levels of surface gaseous concentrations. The seasonal and interannual variability have also been analyzed as a function of emission source variations. We compared the measured West and Central African gas Correspondence to: M. Adon (adonatma@yahoo.fr) concentrations to results obtained in other parts of the world. Results show that the annual mean concentrations of NO 2 , NH 3 , HNO 3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite similar concentrations. Annual mean NO 2 concentrations vary from 0.9±0.2 in forests to 2.4±0.4 ppb in the dry savannas, NH 3 from 3.9±1.4 to 7.4±0.8 ppb and HNO 3 from 0.2±0.1 to 0.5±0.2 ppb. Annual mean O 3 and SO 2 concentrations are lower for all ecosystems and range from 4.0±0.4 to 14.0±2.8 and from 0.3±0.1 to 1.0±0.2 ppb, respectively. A focus on the processes involved in gas emissions from dry savannas is presented in this work, providing explanations for the high concentrations of all gases measured at the three dry savannas sites. At these sites, seasonal concentrations of all gases are higher in the wet season. Conversely, concentrations are higher in the dry season in the wet savannas. In forested regions, we measure no significant difference between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at:

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Abstract. 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 NO 2 , HNO 3 and NH 3 gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are calculated from NH 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 NH 3 is dominated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accountsCorrespondence to: C. Delon (claire.delon@aero.obs-mip.fr) 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 NH x . The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgNha −1 yr −1 , dominated by NH 3 volatilization (72-82%) and biogenic emission from soils (11-17%), depending on the applied volatilization rate of NH 3 . While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH 3 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) TgNyr −1 , depending on the applied volatilization rate of NH 3 and estimated total deposition is 1.8(±0.4) TgNyr −1 . The dry savanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget.

Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NO x 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.

International audienceThe purpose of this study is to analyze a long term database of the chemical composition of precipitation at three African dry savanna sites in the Sahel. The precipitation samples were collected during the monsoon season at Agoufou (15°20′N, 01°29′W, Mali) from 2004 to 2006, Banizoumbou (13°31′N, 02°38′E, Niger) from 1994 to 2009 and Katibougou (12°56′N, 07°32′ W, Mali) from 1997 to 2008. pH and major inorganic and organic ions in precipitation were analyzed by ionic chromatography. A characterization of mean precipitation chemistry with the associated wet deposition fluxes for each species is presented. The first important result is that interannual variability of all volume-weighted mean (VWM) concentrations is low, ranging between ±5% and ±25%. Acidity in dry savannas is low and indicates the strong alkaline nature of the precipitation. The average annual pH at Agoufou is 6.28, 5.75 at Banizoumbou and 5.54 at Katibougou. This result is correlated with the important terrigenous contribution measured in the chemical content of precipitation, implying acidity neutralization by mineral species such as Ca2+ and NH4+. Mg2+ and K+ are found to play a minor role in neutralization. Enrichment factor calculations for Ca2+, SO42−, K+ and Mg2+ with respect to the sea reference reveal a significant influence of Saharan and Sahelian crustal sources. VWM concentrations of these species dominate the composition of measured precipitation. An estimation of the potential particulate and gas contribution to the total precipitation composition is given for each site: At Agoufou, the mean relative contribution in rainwater is 80% for particles and 20% for gases, while at the Banizoumbou and Katibougou sites, results indicate 70% for particles and 30% for gases. The high particulate phase contribution to precipitation emphasizes the importance of multiphase processes between gases and particles in the atmospheric chemistry typical of African semi-arid savanna ecosystems. The second highest contribution is nitrogenous, with high VWM concentrations of NO3− and NH4+ measured at the three sites. Monthly evolution of NO3− and NH4+ concentrations are studied in relation to gaseous emission sources in the Sahelian region, i.e. biogenic soil emission and ammonia sources from animals. The calculated wet nitrogen deposition flux presents a regular increase throughout the wet season at the three sites. Results suggest total mean nitrogen deposition fluxes of 1.80 kg N ha−1 yr−1 at Agoufou, 2.10 kg N ha−1 yr−1 at Banizoumbou, and 3.30 kg N ha−1 yr−1 at Katibougou. The marine contribution is lower, 23% at Agoufou, 17% at Banizoumbou and 13% at Katibougou. The last contribution concerns organic acidity, which ranges from 5% at Agoufou, 10% at Banizoumbou to 14% at Katibougou. Terrigenous and marine contributions present a negative gradient, whereas nitrogenous and organic contributions a positive gradient along the Sahelian transect defined by Agoufou-Banizoumbou-Katibougou