210Po in savanna burning plumes

During the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991 aerosols samples were collected at ground level above fires in order to investigate the possibility of using²¹⁰Po as a tracer of biomass burning. The concentration of this radionuclide in plants is studied as a function of its content in soils and in the atmospheric background. It is shown that it depends strongly on the atmospheric content in²¹⁰Po, due to dry deposition of the aerosols. The mean concentration of plants at Lamto is found to be about 4.4 pCi of²¹⁰Po/gC during the fire season and falls down to less than 1pCi/gC outside this period. The budget of²¹⁰Po is evaluated taking into account its complete volatilization during the flaming phase, the (²¹⁰Po)_ash/(²¹⁰Po)_plants ratio, which is measured to be about 14% and the percentage of submicron particles in the plume, about 91%. The inferred flux of²¹⁰Po is 3850 Ci/yr for the African savanna, and 5800 Ci/yr for the global savanna. From this flux, fluxes of Ct and Cs are estimated to be 8.4 and 1.1 Tg of C/yr for the worldwide savanna.