Aristide Akpo

Dry and Wet Atmospheric Nitrogen Deposition in West Central Africa

This work is part of the IDAF (IGAC/DEBITS/AFrica) programme which started in 1995 with the establishment of 10 measurement sites representative of major African ecosystems. The objectives of the programme are to study wet and dry deposition fluxes, to identify the relative contribution of natural and anthropogenic sources and factors regulating these fluxes. This study presents an estimation of the atmospheric nitrogen (N) deposition budget in Africa based on a long term monitoring measurements database including gaseous, precipitation and aerosols chemical composition. Annual nitrogen fluxes including wet and dry processes are estimated to be around 6 kg N ha−1 year−1, 6.5 kg N ha−1 year−1 and 13 kg N ha−1 year−1 respectively over dry savanna, humid savanna and over the forest.

Interannual Variability of the Atmospheric Nitrogen Budget in West African Dry Savannas

Surface emission and deposition fluxes of reactive nitrogen compounds have been studied in three sites of West Africa during the year 2006, representative of dry savannas ecosystem, and part of the IDAF network: Agoufou (Mali, 15.3°N, 1.4°W), Banizoumbou (Niger, 13.3°N, 2.4°E) and Katibougou (Mali, 12.5°N, 7.3°W). Dry deposition fluxes are calculated from surface measurements of NO2, HNO3 and NH3 concentrations (from passive samplers) and simulated deposition velocities, and wet deposition fluxes are calculated from NH4 + and NO3 − concentration in samples of rain. Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NOx and NH3 from biomass burning and domestic fires, and volatilization of NH3 from animal excreta. From these 3 sites, the average deposition flux, attributed to dry savanna ecosystems in Sahel, is 7.5 (± 1.8) kg N ha−1 year−1, and the average emission flux is from 8.5 ± 3.8 kg N ha−1 year−1. This budget is dominated by the NH3 contribution (due to volatilization of animal excreta). Biogenic emissions from soils are the second most important contribution in emission fluxes. Total estimated deposition is 1.84 ± 0.44 Tg N year−1 and the estimated emission is 2.0 ± 0.9 Tg N year−1 for the Sahel region. Limited interannual variability (between 2002 and 2007) in precipitation is responsible for small variability in local emission sources and hence deposition fluxes.