BLACK CARBON FORMATION BY SAVANNA FIRES - MEASUREMENTS AND IMPLICATIONS FOR THE GLOBAL CARBON-CYCLE

During a field study in southern Africa (Southern African Fire-Atmosph ere Research Initiative (SAFARI-92)), black carbon formation was quant ified in the residues of savanna fires. The volatilization ratios of C , H, N, and S were determined by measuring their contents in the fuel and residue loads on six experimental sites. The volatilization of sul fur (86+/-8%) was significantly higher than previously reported. Volat ilization of H, N, and S was significantly correlated with that of car bon, enabling us to estimate their volatilization during savanna fires by extrapolation from those of carbon. By partitioning the residues i n various fractions (unburned, partially burned, and ash), a strong co rrelation between the H/C ratio in the residue and the formation of bl ack carbon was obtained. The ratio of carbon contained in ash to carbo n contained in the unburned and partially burned fraction is introduce d as an indicator of the degree of charring. As nitrogen was enriched in the residue, especially in the ash fraction of > 0.63 mm, this indi cator may be useful for an assessment of nutrient cycling. We show tha t the formation of black carbon is dependent on the volatilization of carbon as well as the degree of charring. The ratio of black carbon pr oduced to the carbon exposed to the fire in this field study (0.6-1.5% ) was somewhat lower than in experimental fires under laboratory condi tions (1.0-1.8%) which may be due to less complete combustion. Tile av erage ratio of black carbon in the residue to carbon emitted as CO2 ra nged from 0.7 to 2.0%. Using these ratios together with various estima tes of carbon exposed or emitted by savanna fires, the worldwide black carbon fonnation was estimated to be 10-26 Tg C yr(-1) with more than 90% of the black carbon remaining on the ground. Tile formation of th is black carbon is a net sink of biospheric carbon and thus of atmosph eric CO2 as well as a source of O-2.