Seasonal trends of dry and bulk concentration of nitrogen compounds over a rain forest in Ghana

[摘要] African tropical forests of the equatorial belt might receive significantinput of extra nitrogen derived from biomass burning occurring in the northsavanna belt and transported equatorward by northeastern winds. In order totest this hypothesis an experiment was set up in a tropical rain forest inthe Ankasa Game Reserve andNini-Suhien National Park (Ghana) aimed atquantifying magnitude and seasonal variability of concentrations of Ncompounds, present as gas and aerosol (dry nitrogen) or in the rainfall (bulknitrogen), over the studied forest; and relating their seasonal variabilityto trends of local and regional winds and rainfall and to variations of fireevents in the region. Three DELTA systems, implemented for monthlymeasurements of NO₂, were mounted over a tower at 45 m height, 20 mabove 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 analyzeNH₄⁺, NO₃⁻ and ion concentration). The tower was providedwith a sonic anemometer to estimate local wind data. The experiment startedin October 2011 and data up to October 2012 are presented. To interpret theobserved seasonal trends of measured compounds, local and regional meteo dataand regional satellite fire data were analyzed. The concentration of Ncompounds significantly increased from December to April, during the drierperiod, peaking from December to February when NE winds (the Harmattan) weremoving dry air masses over the west-central African region, and theIntertropical Convergence Zone (ITCZ) was at its minimum latitude over theEquator. This period also coincided with fire peaks in the whole region. Onthe contrary, N concentration in gas, aerosol and rain decreased from May toOctober when prevalent winds arrived from the sea (southeast), during themonsoon period. Both ionic compositions of rain and analysis of local winddirection showed a significant and continuous presence of see breeze at site.The ionic composition of rainwater resulted much closer to seawater andpoorer in N compounds from May to October.

[学科分类] 地球化学与岩石