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

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 on seven remote sites representative of major African ecosystems. Dry deposition fluxes were estimated by the inferential method using on one hand surface measurements of gas concentrations (NO2, HNO3, NH3, SO2, and O3) and on the other hand simulated dry deposition velocities (Vd). Vd were calculated using the big-leaf model of Zhang et al. (2003b). In the model of deposition, surface and meteorological conditions specific to IDAF sites have been adapted in order to simulate Vd representative of major African ecosystems. The monthly, 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 ± 0.0 to 0.8 ± 0.2 kg N ha−1 yr−1 for NO2, from 0.7 ± 0.1 to 1.0 ± 0.3 kg N ha−1 yr−1 for HNO3, and from 2.3 ± 0.8 to 10.5 ± 5.0 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 (11.2–11.8 kg N ha−1 yr−1) than in wet and dry savannas (3.4–5.3 kg N ha−1 yr−1). NH3 dominated nitrogen dry deposition, representing 67–80% of the total. The annual mean dry deposition fluxes of ozone range between 11.3 ± 4.7 and 17.5 ± 3.0 kg ha−1 yr−1 in dry savannas, 17.5 ± 3.0 and 19.2 ± 2.9 kg ha−1 yr−1 in wet savannas, and 10.6 ± 2.0 and 13.2 ± 3.6 kg ha−1 yr−1 in forests. Lowest O3 dry deposition fluxes in forests are correlated to low measured O3 concentrations, lower of a factor of 2–3, compared to others ecosystems. Along the ecosystem transect, annual mean of SO2 dry deposition fluxes present 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.