Abstract
Abstract. This work is part of the IDAF program (IGAC-DEBITS-AFRICA) and is based on the long-term monitoring of gas concentrations (1998–2007) established at seven remote sites representative of major African ecosystems. Dry deposition fluxes were estimated by the inferential method using on the one hand surface measurements of gas concentrations (NO2, HNO3, NH3, SO2 and O3) and on the other hand modeled exchange rates. Dry deposition velocities (Vd) were calculated using the big-leaf model of Zhang et al. (2003b). The bidirectional approach is used for NH3 surface–atmosphere exchange (Zhang et al., 2010). Surface and meteorological conditions specific to IDAF sites have been used in the models of deposition. The seasonal and annual mean variations of gaseous dry deposition fluxes (NO2, HNO3, NH3, O3 and SO2) are analyzed. Along the latitudinal transect of ecosystems, the annual mean dry deposition fluxes of nitrogen compounds range from −0.4 to −0.8 kg N ha−1 yr−1 for NO2, from −0.7 to −1.0 kg N ha−1 yr−1 for HNO3 and from −0.7 to −8.3 kg N ha−1 yr−1 for NH3 over the study period (1998–2007). The total nitrogen dry deposition flux (NO2+HNO3+NH3) is more important in forests (−10 kg N ha−1 yr−1) than in wet and dry savannas (−1.6 to −3.9 kg N ha−1 yr−1). The annual mean dry deposition fluxes of ozone range between −11 and −19 kg ha−1 yr−1 in dry and wet savannas, and −11 and −13 kg ha−1 yr−1 in forests. Lowest O3 dry deposition fluxes in forests are correlated to low measured O3 concentrations, lower by a factor of 2–3, compared to other ecosystems. Along the ecosystem transect, the annual mean of SO2 dry deposition fluxes presents low values and a small variability (−0.5 to −1 kg S ha−1 yr−1). No specific trend in the interannual variability of these gaseous dry deposition fluxes is observed over the study period.
Highlights
Deposition of chemical species onto the Earth’s surface plays an essential role in controlling the concentration of gases and aerosols in the troposphere
Our study presented an original database of 10 yr of measurements (1998–2007) of five important atmospheric gases (NO2, HNO3, NH3, O3, SO2) obtained within the IDAF African network at seven sites in west and central Africa
NO2, HNO3, SO2 and O3 were considered to be net deposited, while surface–atmosphere exchange of NH3 is considered to be bidirectional for all ecosystems
Summary
Deposition of chemical species onto the Earth’s surface plays an essential role in controlling the concentration of gases and aerosols in the troposphere. Within the framework of IDAF, several studies of precipitation chemical composition representative of great African ecosystems have been recently published (Galy-Lacaux and Modi, 1998; Galy-Lacaux et al, 2001, 2009; Al-Ourabi and Lacaux, 2002; Lacaux et al, 1993, 2003; Sigha et al, 2003; Yoboue et al, 2005; Mphepya et al, 2004; 2006; Laouali et al, 2012) To complement these studies, it is appropriate to study and quantify dry deposition fluxes. Dry deposition fluxes are estimated using the inferential method, which is a combination of gaseous concentration measurements and modeling of deposition velocities according to the resistance analogy (Wesely, 1989; Zhang et al, 2003b; and references therein). The monthly, seasonal and annual mean variations of gaseous dry deposition fluxes are analyzed
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