Atmos. Chem. Phys., 10, 2691–2708, 2010 www.atmos-chem-phys.net/10/2691/2010/ © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. Atmospheric Chemistry and Physics Atmospheric nitrogen budget in Sahelian dry savannas C. Delon 1 , C. Galy-Lacaux 1 , A. Boone 2 , C. Liousse 1 , D. Serc ¸a 1 , M. Adon 1,3 , B. Diop 4 , A. Akpo 5 , F. Lavenu 6,† , E. Mougin 6 , and F. Timouk 6 1 Laboratoire d’A´ erologie, Universit´ e de Toulouse and CNRS, Toulouse, France 2 CNRM/GMME, M´ et´ eo-France, Toulouse, France 3 Laboratoire de Physique de l’Atmosphere, Abidjan, C ˆ ote d’Ivoire 4 Universit´ e de Bamako, Mali 5 Universit´ e Abomey Calavi, Cotonou, B´ enin 6 CESBIO, Toulouse, France † deceased Received: 30 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 30 June 2009 Revised: 5 March 2010 – Accepted: 9 March 2010 – Published: 23 March 2010 Abstract. The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized nitrogen compounds. In this study, a first attempt at esti- mating the Sahel nitrogen budget for the year 2006 is made, through measurements and simulations at three stations from the IDAF network situated in dry savanna ecosystems. Dry deposition fluxes are estimated from measurements of NO 2 , HNO 3 and NH 3 gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are cal- culated from NH + 4 and NO − 3 concentrations in samples of rain. Emission fluxes are estimated including biogenic emis- sion of NO from soils (an Artificial Neural Network module has been inserted into the ISBA-SURFEX surface model), emission of NO x and NH 3 from domestic fires and biomass burning, and volatilization of NH 3 from animal excreta. Un- certainties are calculated for each contribution of the budget. This study uses original and unique data from remote and hardly-ever-explored regions.The monthly evolution of oxi- dized N compounds shows that emission and deposition in- crease at the beginning of the rainy season because of large emissions of biogenic NO (pulse events). Emission of ox- idized compounds is dominated by biogenic emission from soils (domestic fires and biomass burning of oxidized com- pounds account for 0 to 13% at the most at the annual scale, depending on the station), whereas emission of NH 3 is dom- inated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accounts Correspondence to: C. Delon (claire.delon@aero.obs-mip.fr) for 47% of the total estimated deposition flux, for both oxidized and reduced compounds. The average estimated wet plus dry deposition flux in dry savanna ecosystems is 7.5±1.8 kgNha −1 yr −1 , with approximately 30% attributed to oxidized compounds, and the rest attributed to NH x . The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgNha −1 yr −1 , dominated by NH 3 volatilization (72–82%) and biogenic emission from soils (11–17%), de- pending on the applied volatilization rate of NH 3 . While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH 3 emission from volatilization. When scaled up from the 3 measurement sites to the Sa- helian region (12 ◦ N:18 ◦ N, 15 ◦ W:10 ◦ E), the estimated to- tal emission ranges from 2(±0.9) to 3(±1.4) TgNyr −1 , de- pending on the applied volatilization rate of NH 3 and esti- mated total deposition is 1.8(±0.4) TgNyr −1 . The dry sa- vanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget. 1 Introduction Nitrogen is a key compound both as a nutrient for plants and animals and as an atmospheric pollutant. In the atmosphere, several nitrogen trace compounds are present, such as NO, NO 2 , HNO 3 ,N 2 O and NH 3 , as well as particulate and aque- ous forms such as NO − 3 and NH + 4 . At the global scale, the reactive N cycle has been widely impacted by human ac- tivities, notably for food production. Indeed, the creation of reactive nitrogen has increased by 120% since 1970 and Published by Copernicus Publications on behalf of the European Geosciences Union.