Long-term atmospheric inorganic nitrogen deposition in West African savanna over 16 year period (Lamto, Côte d’Ivoire)

Abstract

We present a long term assessment trend of atmospheric inorganic nitrogen deposition in Sub Saharan Africa (2000–2015) using observational and model data. This work proposes a compilation of International Network to study Deposition and Atmospheric chemistry in Africa wet and dry nitrogen deposition fluxes collected at the wet savanna site of Lamto (Côte d’Ivoire). Total deposition calculation takes in account: (a) gaseous (NO _2 , NH _3 , HNO _3 ) dry deposition fluxes estimated by considering nitrogen compound concentrations at the monthly scale and modeling average monthly dry deposition velocities, (b) particulate PM10 (pNO _3 ^− , pNH _4 ^+ ) dry deposition fluxes calculated using the same inferential method and (c) wet deposition (WD) fluxes including ions concentration measurements (NO _3 ^− , NH _4 ^+ ) in rainwater combined with rainfall amount. We demonstrate for the first time the monthly and annual decreasing trends for dry nitrogen deposition of N-NO _2 (−2.33% month ^−1 and −2.54% yr ^−1 ) and N-NH _3 (−2.55% month ^−1 and −2.89% yr ^−1 ), but increasing trends for dry deposition of N-HNO _3 (+1.00% month ^−1 ) and WD of N-NO _3 ^− (+1.67% month ^−1 and +2.13% yr ^−1 ) and N-NH _4 ^+ (+2.33% month ^−1 and +3.36% yr ^−1 ). Dry season N-NO _2 deposition flux decline shows agreement with long term trend in NOx emissions by biomass burning. Increasing trends for wet N deposition signals a gradual increase of nitrogen fertilizers use in agricultural practices in the Lamto area. Results also show no significant trend in total N deposition over the 16 year study period explained by the compensation of decreasing and increasing trends for dry and wet N deposition, respectively. However, at the annual scale, the mean total N deposition flux is estimated to 10.3 ± 1.2 kgN ha ^−1 yr ^−1 over the 16 year period, indicating an increase of 8% compared to the period 2000–2007.

Keywords

• sub Saharan Africa
• wet savanna ecosystem
• inorganic nitrogen
• wet and dry depositions
• trends