Abstract
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/.
Highlights
Measurement programs play a critical role in air pollution and atmospheric chemistry studies
The sites are located to represent a transect of the African ecosystems, i.e., dry savanna-wet savanna-forest
For each type of African ecosystem, we studied the long term time series (1) to characterize the levels of gaseous surface concentrations, (2) to document the seasonal cycles according to the atmospheric sources of gases
Summary
Measurement programs play a critical role in air pollution and atmospheric chemistry studies. Long term measurement programs are crucial to documenting changes in atmospheric composition and long term monitoring allows the characterization of seasonal and interannual variability This is important in tropical regions where seasonal cycles related to natural and anthropogenic emissions sources of gases and particles are marked, resulting from seasonality of biomass burning, soil and vegetation emissions. Within the IDAF framework, several studies of the chemical composition of precipitation representative of African ecosystems have been published These studies have helped to quantify the wet deposition of biogeochemically important elements and to estimate the contributions of different atmospheric sources (Galy-Lacaux and Modi, 1998; Galy-Lacaux et al, 2001; Sigha et al, 2003; Yoboue et al, 2005; Mphepya et al, 2004, 2006; Galy-Lacaux et al, 2009). Realistic dry deposition velocities need to be determined for each specific site and species in order to estimate dry deposition fluxes
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