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/.

Chemical and aerosol characterisation of the troposphere over West Africa during the monsoon period as part of AMMA

“…There is, however, a strong vegetation gradient around 12-13 • N (Janicot et al, 2008), with substantial tree cover to the south and scrubland and bare soil to the north. A similar pattern for O 3 has been observed at surface IDAF (IGAC-DEBITS-AFRICA) sites during the wet season, which shows mixing ratios to be around 10 ppbv higher over dry savannas in the north compared to wet savannas to the south (Adon et al, 2010).…”

Nitrogen emission and deposition budget in West and Central Africa

“…Dry deposition fluxes of HNO 3 are very low compared to NO 2 and NH 3 with values ranging between 0.7 (±0.01) and 1.0 (±0.01) kgN ha −1 yr −1 for all the ecosystems. This result is correlated to very low HNO 3 concentrations measured in all the stations (Adon et al 2010). Total dry deposition fluxes of gases (sum of the three dry deposition gases fluxes) are presented in table 1 and indicates a total of 4−5.3 kgN ha −1 yr −1 in dry savannas, 3.4−4.6 kgN ha −1 yr −1 in wet savannas to 11.2−11.8 kgN ha −1 yr −1 in forested ecosystems.…”

“…Measurements of wet deposition are made by collection and analysis of rain using wet-only samplers, opening and closing the sampler at the beginning and end of rainfall events. Wet deposition is (2012), Vet et al (2014) and also because of the availability of a combined database composed of rain chemistry, gas concentrations, dry deposition velocity calculations and environmental parameters (Adon et al 2010(Adon et al , 2013 during this period. In this paper, a transect of ecosystem from dry savanna to humid savanna and forest is studied using all the IDAF regionalrepresentative sites located pairs by pairs on each ecosystem.…”

Exploring the challenges confronting the west Africa climate system

“…One of the advantages of satellite observation over ground observation is the enabled overview of the meteorological performance over any location. For example, the ozone distribution over West Africa showed significant south-north gradient with lower and higher values over forested regions and the north of 12•N respectively (Adon et al, 2010); the emission of emit large amounts of biogenic volatile organic compounds (VOCs), NOx and O3 are observed over the boundary layer in the Sahelian climate zones (Ferreira et al, 2010;Stewart et al, 2008;Jaegle et al, 2004); the activities of the mesoscale convective systems (MCS) over the northern edge of the West African ITCZ in Sahel climate zone can be monitored (Mohr, 2004). Though there had been successful satellite exploration in WA, the need for ground-trotting is eminent.…”