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

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 Bani- zoumbou (Niger), Katibougou and Agoufou (Mali), Djougou (Benin), Lamto (Cote d’Ivoire), Zoetele (Cameroon) and Bo- massa (Congo). The strict control of measurement tech- niques as well as the validation and inter-comparison stud- ies 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 concen- trations. 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 NO 2 , NH 3 , HNO 3 measured in dry savannas are higher than those measured in wet savannas and forests that have quite simi- lar concentrations. Annual mean NO 2 concentrations vary from 0.9 ± 0.2 in forests to 2.4 ± 0.4 ppb in the dry savan- nas, 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 concen- trations 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, con- centrations are higher in the dry season in the wet savan- nas. In forested regions, we measure no significant differ- ence between wet and dry seasons. This unique database of long term gases concentrations monitoring is available at: http://medias.obs-mip.fr/idaf/