METHYL HALIDE EMISSIONS FROM SAVANNA FIRES IN SOUTHERN AFRICA

The methyl halides, methyl chloride (CH3Cl), methyl bromide (CH3Br), a nd methyl iodide (CH3I), were measured in regional air samples and smo ke from savanna fires in southern Africa during the Southern Africa Fi re-Atmosphere Research Initiative-92 (SAFARI-92) experiment (August-Oc tober 1992). All three species were significantly enhanced in the smok e plumes relative to the regional background. Good correlations were f ound between the methyl halides and carbon monoxide, suggesting that e mission was predominantly associated with the smoldering phase of the fires. About 90% of the halogen content of the fuel burned was release d to the atmosphere, mostly as halide species, but a significant fract ion (3-38%) was emitted in methylated form. On the basis of comparison with the composition of the regional background atmosphere, emission ratios to carbon dioxide and carbon monoxide were determined for the m ethyl halide species. The emission ratios decreased in the sequence CH 3Cl > CH3Br > CH3I. Extrapolation of these results in combination with data from other types of biomass burning, e.g. forest fires, suggests that vegetation fires make a significant contribution to the atmosphe ric budget of CH3Cl and CH3Br. For tropospheric CH3I, on the other han d, fires appear to be a minor source. Our results suggest that pyrogen ic emissions of CH3Cl and CH3Br need to be considered as significant c ontributors to stratospheric ozone destruction.