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 δ¹³C_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 δ¹³C_CH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around -59‰) and the overall δ¹³C_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, δ¹³C_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, δ¹³C_CH4 values were around -28‰. By contrast, African C4 tropical grass fire δ¹³C_CH4 values were -16 to -12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ¹³C_CH4 around -37 to -36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ¹³C_CH4 values predicted by global atmospheric models are highly sensitive to the δ¹³C_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)'.

Keywords: African air pollution; African biomass burning; African wetlands; aircraft surveys; atmospheric methane; methane isotopes.

Figure 1.

Keeling plot of (1/methane abundance) versus δ¹³C_CH4 for isotopic measurements of samples in a fire plume on FAAM flight C005 over the Senegal Casamance region. (Online version in colour.)

Figure 2.

Miller–Tans plots of samples collected in regional air during flights over Lake Kyoga (δ¹³C_CH4 −54.5  ±  1.4‰) and Lake Wamala (δ¹³C_CH4 −49.3 ± 0.9‰), Uganda. (Online version in colour.)

Figure 3.

(a) ZWAMPS FAAM flight C136, height, CO and methane transects across Bangweulu wetlands. Height is metres above the ground surface. (b) ZWAMPS FAAM flight, showing measured methane abundance advected over the Bangweulu wetlands. Transects at various heights above ground level, coloured by in situ methane concentration as per legend. Note the highest values are over the wetlands SE of the lake, not over the large shallow lake. (Online version in colour.)

Figure 4.

Lukanga swamp. Methane observations during flight transects at various heights above ground level. (Online version in colour.)

Figure 5.

δ¹³C_CH4 signature of outer tropical wetlands of the Southern Hemisphere. Miller–Tans plot for data from Zambia and Bolivia. The inferred δ¹³C_CH4 value is −59.3 ± 2.0‰. Plot includes aircraft-collected samples from the Upper Congo (Bangweulu) and Zambezi basin (Lukanga, Kafue) wetlands in Zambia and from the Mamore River and Llanos de Moxos wetlands in Bolivian Amazonia. (Online version in colour.)

Figure 6.

Global impact of changing the δ¹³C_CH4 source signature of methane emitted from tropical wetlands. Black line (upper line) is a model scenario optimized to NOAA observations with the tropical wetland source having a −55‰ δ¹³C_CH4 signature. Red line (lower line) shows the impact of changing the tropical wetland source δ¹³C_CH4 signature from −55‰ to −60‰ on the optimized model scenario, with nothing else varied. (Online version in colour.)