Aging of savanna biomass burning aerosols: Consequences on their optical properties

During the FOS-DECAFE experiment at Lamto, Ivory Coast, in January 1991, various ground studies were undertaken simultaneously in order to investigate the physical and chemical characteristics of smoke emitted by savanna biomass burning. Here we present sunphotometer ground-based results which allow the measurements of the spectral optical depth between 450 and 850 nm, the atmospheric water vapour content and the particle size distribution spectrum. The carbonaceous content of the savanna biomass burning aerosols is also investigated. This is the first time that the physical characteristics of particles emitted by savanna plumes are obtained from ground-field studies. All the results suggest that a rapid aging of the smoke occurs first hundred metres from the savanna fire èmission source. They show a relationship between the optical properties of smoke and the chemical aging of the aerosols primarily due to particle growth and a loss of organic material relative to the black carbon content.