PARTICULATE CONTENT OF SAVANNA FIRE EMISSIONS

As part of the FOS-DECAFE experiment at Lamto (Ivory Coast) in January 1991, various aerosol samples were collected at ground level near pre scribed fires or under local background conditions, to characterize th e emissions of particulate matter from the burning of savanna vegetati on. This paper deals with total aerosol (TPM) and carbon measurements. Detailed trace element and polycyclic hydrocarbon data are discussed in other papers presented in this issue. Near the fire plumes, the aer osols from biomass burning are primarily of a carbonaceous nature (C% similar to 70% of the aerosol mass) and consist predominantly of submi cron particles (more than 90% in mass.) They are characterized by thei r organic nature (black to total carbon ratio Cb/Ct in the range 3-20% ) and their high potassium content (K/Cb similar to 0.6). These aeroso ls undergo aging during their first minutes in the atmosphere causing slight alterations in their size distribution and chemical composition . However, they remain enriched in potassium (K/Cb = 0.21) and pyrene, a polycyclic aromatic hydrocarbon, such that both of these species ma y be used as tracers of savanna burning aerosols. We show that during this period of the year, the background atmosphere experiences severe pollution from both terrigenous sources and regional biomass burning ( 44% of the aerosol). Day-night variations of the background carbon con centrations suggest that fire ignition and spreading occur primarily d uring the day. Simultaneous TPM and CO2 real-time measurements point t o a temporal and spatial heterogeneity of the burning so that the rati o of the above background concentrations (Delta TPM/Delta CO2) varies from 2 to 400 g/kg C. Smoldering processes are intense sources of part icles but particulate emissions may also be important during the rapid ly spreading heading fires in connection with the generation of heavy brown smoke. We propose emission factor values (EF) for aerosols from the savanna biomass burning aerosols: EF (TPM)= 11.4 +/- 4.6 and 69 +/ - 25 g/kg Cd, plant and EF(Ct) = 7.4 +/- 3.4 and 56 +/- 16 g C/kg C-dr y plant for flaming and smoldering processes respectively. In these es timates, the range of uncertainty is mostly due to the intra-fire vari ability. These values are significantly lower than those reported in t he literature for the combustion of other types of vegetation. But due to the large amounts of vegetation biomass being burnt in African sav annas, the annual flux of particulate carbon into the atmosphere is es timated to be of the order of 8 Tg C, which rivals particulate carbon emissions from anthropogenic activities in temperate regions.