1Laboratoire d'Aérologie, UMR 5560, Université Paul-Sabatier (UPS) and CNRS, Toulouse, France
2Laboratoire de Physique de l'Atmosphère, Université de Cocody, Abidjan, Cote d'Ivoire
3School of Chemistry, North-West University, Potchefstroom 2520, South Africa
4Département de Physique, Université d'Alep, Alep, Syria
5Département de Physique, Université Abdou Moumouni, Faculté des Sciences, Niamey, Niger
6Département de Physique, Université de Bamako, Bamako, Mali
7Centre de Recherches Hydrologiques,Yaoundé, Cameroon
8Département de physique, Université Abomey Calavi, Cotonou, Benin
9Direction Générale de la Recherche Scientifique et Technologique, Brazzaville, Congo
10Centre d'études spatiales de la biosphère, UMR 5126, Toulouse, France
11Laboratoire des Mécanismes et Transferts en Géologie, UMR 5563 UR154 UPS/CNRS/IRD, Toulouse, France
Received: 18 Dec 2009 – Discussion started: 12 Feb 2010
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 SO2, NO2, HNO3, NH3 and O3, 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 O3 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/.
Revised: 09 Jul 2010 – Accepted: 26 Jul 2010 – Published: 10 Aug 2010
Adon, M., Galy-Lacaux, C., Yoboué, V., Delon, C., Lacaux, J. P., Castera, P., Gardrat, E., Pienaar, J., Al Ourabi, H., Laouali, D., Diop, B., Sigha-Nkamdjou, L., Akpo, A., Tathy, J. P., Lavenu, F., and Mougin, E.: Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers, Atmos. Chem. Phys., 10, 7467-7487, doi:10.5194/acp-10-7467-2010, 2010.