METHANE, CARBON-MONOXIDE AND LIGHT NONMETHANE HYDROCARBON EMISSIONS FROM AFRICAN SAVANNA BURNINGS DURING THE FOS DECAFE EXPERIMENT/

Atmospheric samples from savanna burnings were collected in the Ivory Coast during two campaigns in January 1989 and January 1991. About 30 nonmethane hydrocarbons from C-2 to C-6, carbon monoxide, carbon dioxi de and methane were measured from the background and also at various d istances from the burning. Concentrations in the fire plume reached pp mv levels for C-2-C-4 hydrocarbons, and 5300, 500 and 93 ppmv for CO2, CO and CH4 respectively. The excess in the mixing ratios of these gas es above their background level is used to derive emission factors rel ative to CO and CO2. For the samples collected immediately in the fire plume, a differentiation between high and low combustion efficiency c onditions is made by considering the CO/CO2 ratio. Ethene (C2H4), acet ylene (C2H2), ethane (C2H6) and propene (C3H6) are the major NMHC prod uced in the flaming stage, whereas a different pattern with an increas ing contribution of alkanes is observed in samples typical of post fla ming processes. A strong correlation between methane and carbon monoxi de suggests that these compounds are produced during the same stage of the combustion. In samples collected at a distance from the fire and integrated over a period of 30 minutes, the composition is very simila r to that of flaming. Delta NMHC/Delta CO2 is of the order of 0.7%, De lta CH4 Delta CO2 of the order of 0.4% and Delta CO/Delta CO2 of the o rder of 6.3%. From this study, a global production by African savanna fires is derived: 65 Tg of CO-C, 4.2 Tg of CH4-C and 6.7 Tg of NMHC-C, Whereas acetylene can be used as a conservative tracer of the fire pl umes, only ethene, propene and butenes can be considered in terms of t heir direct photochemical impact.