Long term measurements of sulfur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in Africa using passive samplers

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 to study long term gases concentrations (1998–2007) by determining the ambient concentration of the inorganic gases, i.e., SO2, NO2, HNO3, NH3 and O3 using passive samplers at seven remote sites in West and Central Africa. Sites are representative of a great African ecosystem and are located according a transect: dry savannas-wet savannas-forests with Banizoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d'Ivoire), Zoetele (Cameroon) and Bomassa (Congo). The validation and inter-comparison studies conducted with the IDAF passive samplers assure the quality and the control of the measurement technique to show the accuracy of the measurements. For each type of African ecosystems, we have studied the long term data series to document the levels of gaseous surface concentrations and the seasonal and interannual variations analyzed as a function of emission sources variations. We have tried to compare West and Central African gases concentrations to the 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, HNO3 from 0.2±0.1 to 0.5±0.2 ppb. Annual mean O3 and SO2 concentrations are lower for all the 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 dry savannas processes involved in gases emission is presented in this work, explaining the high concentrations of all gases measured on the three dry savannas sites. For all gases, seasonal concentrations are higher in the wet season of dry savannas. Conversely, concentrations are higher in the dry season of wet savannas. In forest, we measure no significant difference between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at: http://www.obs-mip.fr/idaf/.