Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane delta C-13(CH4) isotopic signatures were in the range -55 to -49 parts per thousand for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 +/- 1 parts per thousand from Upper Congo and Zambezi wetlands. Very similar delta C-13(CH4) signatures were measured over the Amazonian wetlands of NE Bolivia (around -59 parts per thousand) and the overall delta C-13(CH4) signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 +/- 2 parts per thousand. These results were more negative than expected. For African cattle, delta C-13(CH4) values were around -60 to -50 parts per thousand. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, delta C-13(CH4) values were around -28 parts per thousand. By contrast, African C4 tropical grass fire delta C-13(CH4) values were -16 to -12 parts per thousand. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had delta C-13(CH4) around -37 to -36 parts per thousand. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric delta C-13(CH4) values predicted by global atmospheric models are highly sensitive to the delta C-13(CH4) isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa.