Abstract
Abstract. African tropical forests of the equatorial belt might receive significant input of extra nitrogen derived from biomass burning occurring in the north savanna belt and transported equatorward by northeastern winds. In order to test this hypothesis an experiment was set up in a tropical rain forest in the Ankasa Game Reserve and Nini-Suhien National Park (Ghana) aimed at quantifying magnitude and seasonal variability of concentrations of N compounds, present as gas and aerosol (dry nitrogen) or in the rainfall (bulk nitrogen), over the studied forest; and relating their seasonal variability to trends of local and regional winds and rainfall and to variations of fire events in the region. Three DELTA systems, implemented for monthly measurements of NO2, were mounted over a tower at 45 m height, 20 m above forest canopy to sample gas (NH3, NO2, HNO3, HCl, SO2) and aerosol (NH4+, NO3−, and several ions), together with three tanks for bulk rainfall collection (to analyze NH4+, NO3− and ion concentration). The tower was provided with a sonic anemometer to estimate local wind data. The experiment started in October 2011 and data up to October 2012 are presented. To interpret the observed seasonal trends of measured compounds, local and regional meteo data and regional satellite fire data were analyzed. The concentration of N compounds significantly increased from December to April, during the drier period, peaking from December to February when NE winds (the Harmattan) were moving dry air masses over the west-central African region, and the Intertropical Convergence Zone (ITCZ) was at its minimum latitude over the Equator. This period also coincided with fire peaks in the whole region. On the contrary, N concentration in gas, aerosol and rain decreased from May to October when prevalent winds arrived from the sea (southeast), during the monsoon period. Both ionic compositions of rain and analysis of local wind direction showed a significant and continuous presence of see breeze at site. The ionic composition of rainwater resulted much closer to seawater and poorer in N compounds from May to October.
Highlights
Tropical forests have a key role in terrestrial carbon cycling, acting as the most important global terrestrial C sink and C reservoir
The movement of air masses in West Africa is a function of the position of the Intertropical Convergence Zone (ITCZ), which separates the hot and dry continental air coming from the Sahara from the cooler, humid maritime air masses originating from the equatorial Atlantic Ocean (Lebel et al, 2009)
A direct proof of N origin cannot be provided, this study shows that there is a clear seasonality in the atmospheric concentration of N compounds over this West African rainforest, which is coherent with the dynamics of local and regional meteorological drivers and fire occurrences
Summary
Tropical forests have a key role in terrestrial carbon cycling, acting as the most important global terrestrial C sink and C reservoir. The significant increase of tropical deforestation rates observed in the last century (Houghton and Hackler, 2006) as well as the increased use of land for animal breeding (FAOSTAT database), most often accompanied by the practice of burning in managed pastures, has most likely increased the input of Nr in tropical regions. This phenomenon alone or in combination with increased atmospheric concentration of CO2 might have stimulated forest growth. Little atmospheric concentration data in tropics are available, in particular in the African continent (Martins, 2007; Adon, 2010; Delon, 2012; Sigha, 2003) which might make it possible to quantify N depositions and to validate and implement chemical-transport model to test the fire hypothesis and to better forecast regional air quality (Elbern et al, 2007; Carmichael et al, 2008; Chen et al, 2010; Curier et al, 2012)
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