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 (NO2, NH3, HNO3) dry deposition fluxes estimated by considering nitrogen compound concentrations at the monthly scale and modeling average monthly dry deposition velocities, (b) particulate PM10 (pNO3 −, pNH4 +) dry deposition fluxes calculated using the same inferential method and (c) wet deposition (WD) fluxes including ions concentration measurements (NO3 −, NH4 +) in rainwater combined with rainfall amount. We demonstrate for the first time the monthly and annual decreasing trends for dry nitrogen deposition of N-NO2 (−2.33% month−1 and −2.54% yr−1) and N-NH3 (−2.55% month−1 and −2.89% yr−1), but increasing trends for dry deposition of N-HNO3 (+1.00% month−1) and WD of N-NO3 − (+1.67% month−1 and +2.13% yr−1) and N-NH4 + (+2.33% month−1 and +3.36% yr−1). Dry season N-NO2 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.
Highlights
Reactive nitrogen (Nr) compounds in the oxidized (NOy) and the reduced (NHx) forms play a central role in the chemistry of the atmosphere as well as in the functioning of marine, freshwater and terrestrial ecosystems (Dentener et al 2014)
This study aims for the first time to assess long-term statistical trends of atmospheric and seasonal dynamic of N depositions in a regionally representative African wet savanna ecosystem, using a 16 year dataset representing the 2000–2015 period
Results based on 10 years of study has shown that atmospheric concentrations NH3 are higher yearround than that of NO2 (Adon et al 2010) leading to generally acid rain in Lamto (Yoboue et al 2005)
Summary
Reactive nitrogen (Nr) compounds in the oxidized (NOy) and the reduced (NHx) forms play a central role in the chemistry of the atmosphere as well as in the functioning of marine, freshwater and terrestrial ecosystems (Dentener et al 2014). The deposition of atmospheric nitrogen (N) is one of the major processes for removing N pollutants in the air, and one of the most important steps in the biogeochemical cycle of nitrogen and an important input to the ecosystem (Wei 2018). Both climate change and N deposition from air pollution may provoke biodiversity loss (Bobbink et al 2010, Rao et al 2010). Global Nr deposition assessments are available to determine the critical levels of nitrogen in atmospheric deposition for temperate ecosystems (Bobbink et al 2010, Lamarque et al 2013, Vet et al 2014, Ackerman et al 2019), but in situ measurements recorded on the African continent remain scarce, scattered (Reis et al 2016) and during relatively short periods of time (no more than 10 years)
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