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

Abstract

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 <i>δ</i>¹³C_CH₄ isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar <i>δ</i>¹³C_CH₄ signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall <i>δ</i>¹³C_CH₄ signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was -59 ± 2‰. These results were more negative than expected. For African cattle, <i>δ</i>¹³C_CH₄ values were around -60 to -50‰. 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, <i>δ</i>¹³C_CH₄ values were around -28‰. By contrast, African C4 tropical grass fire <i>δ</i>¹³C_CH₄ values were -16 to -12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had <i>δ</i>¹³C_CH₄ around -37 to -36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric <i>δ</i>¹³C_CH₄ values predicted by global atmospheric models are highly sensitive to the <i>δ</i>¹³C_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. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.