Methyl halide emissions from savanna fires in southern Africa

The methyl halides, methyl chloride (CH3Cl), methyl bromide (CH3Br), and methyl iodide (CH3I), were measured in regional air samples and smoke from savanna fires in southern Africa during the Southern Africa Fire-Atmosphere Research Initiative-92 (SAFARI-92) experiment (August-October 1992). All three species were significantly enhanced in the smoke plumes relative to the regional background. Good correlations were found between the methyl halides and carbon monoxide, suggesting that emission was predominantly associated with the smoldering phase of the fires. About 90% of the halogen content of the fuel burned was released to the atmosphere, mostly as halide species, but a significant fraction (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 methyl halide species. The emission ratios decreased in the sequence CH3Cl > 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 atmospheric budget of CH3Cl and CH3Br. For tropospheric CH3I, on the other hand, fires appear to be a minor source. Our results suggest that pyrogenic emissions of CH3Cl and CH3Br need to be considered as significant contributors to stratospheric ozone destruction.