Dry deposition of nitrogen compounds (NO₂, HNO₃, NH₃), sulfur dioxide and ozone in west and central African ecosystems using the inferential method

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 (V_d) 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.