13 research outputs found
GĂ©osciences au Cameroun
L'étude des apports atmosphériques en milieu forestier sud-camerounais (suivant un transect ouest-est) est menée depuis 1989 par le Centre de Recherches Hydrologiques du Cameroun et l'ORSTOM dans le cadre du Programme d'Etude de l'Environnement de la Géosphère Intertropicale (PEGI) de l'ORSTOM-INSU-CNRS. Les résultats d'analyse chimique des eaux pluviales, prélevées à partir des pluviomètres à lectures directes en plexiglas, ont permis d'établir des bilans géochimiques à l'exutoire des bassins versants. Le suivi spatio-temporel des apports atmosphériques depuis l'océan Atlantique jusqu'à la frontière camerouno-centrafricaine a contribué non seulement à une meilleure connaissance de leur qualité chimique, mais aussi à différencier les sources des éléments, de même qu'à déterminer leurs impacts environnementaux. Ainsi, les teneurs en chlore et en sodium décroissent de Kribi vers Yokadouma-Moloundou, soulignent leur origine marine tandis que celles de potassium, calcium, magnésium et sulfates augmentent, indiquant ainsi d'autres sources (terrigènes, biogéniques et anthropiques éventuellement). Les mesures de pH effectuées sur les pluies collectées à Zoétélé (Nsimi) en 1996 à l'aide d'un préleveur à ouverture automatique montrent que les pluies en milieu forestier sud-camerounais sont à tendance acide (pH moyen = 4,9), acidité imputable principalement aux acides formique (13%) et acétique (6%) qui représentent environ 20% de la charge totale dissoute. Les premières conclusions de ce travail ont ouvert de nouveaux axes de recherche, notamment l'étude des gaz et des aérosols, suite à l'intégration du site de Zoétélé dans le réseu IGAC (International Global Atmospheric Chemistry)/DEBITS (Deposition of Biogeochemically Important Trace Species)-Africa (IDAF). (Résumé d'auteur
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Satellite mapping of rain-induced nitric oxide emissions from soils
We use space-based observations of NO2 columns from the Global Ozone Monitoring Experiment (GOME) to map the spatial and seasonal variations of NOx emissions over Africa during 2000. The GOME observations show not only enhanced tropospheric NO2 columns from biomass burning during the dry season but also comparable enhancements from soil emissions during the rainy season over the Sahel. These soil emissions occur in strong pulses lasting 1–3 weeks following the onset of rain, and affect 3 million km2 of semiarid sub-Saharan savanna. Surface observations of NO2 from the International Global Atmospheric Chemistry (IGAC)/Deposition of Biochemically Important Trace Species (DEBITS)/Africa (IDAF) network over West Africa provide further evidence for a strong role for microbial soil sources. By combining inverse modeling of GOME NO2 columns with space-based observations of fires, we estimate that soils contribute 3.3 ± 1.8 TgN/year, similar to the biomass burning source (3.8 ± 2.1 TgN/year), and thus account for 40% of surface NOx emissions over Africa. Extrapolating to all the tropics, we estimate a 7.3 TgN/year biogenic soil source, which is a factor of 2 larger compared to model-based inventories but agrees with observation-based inventories. These large soil NOx emissions are likely to significantly contribute to the ozone enhancement originating from tropical Africa.Engineering and Applied Science
Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers
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 SO2, NO2, HNO3, NH3 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 concentrations to results obtained in other parts of the world. Results show that the annual mean concentrations of NO2, NH3, HNO3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite similar concentrations. Annual mean NO2 concentrations vary from 0.9+/-0.2 in forests to 2.4+/-0.4 ppb in the dry savannas, NH3 from 3.9+/-1.4 to 7.4+/-0.8 ppb and HNO3 from 0.2+/-0.1 to 0.5+/-0.2 ppb. Annual mean O-3 and SO2 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: http://medias.obs-mip.fr/idaf/