Abstract
Abstract. Airborne sampling of methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), and nitrous oxide (N2O) mole fractions was conducted during field campaigns targeting fires over Senegal in February and March 2017 and Uganda in January 2019. The majority of fire plumes sampled were close to or directly over burning vegetation, with the exception of two longer-range flights over the West African Atlantic seaboard (100–300 km from source), where the continental outflow of biomass burning emissions from a wider area of West Africa was sampled. Fire emission factors (EFs) and modified combustion efficiencies (MCEs) were estimated from the enhancements in measured mole fractions. For the Senegalese fires, mean EFs and corresponding uncertainties in units of gram per kilogram of dry fuel were 1.8±0.19 for CH4, 1633±171.4 for CO2, and 67±7.4 for CO, with a mean MCE of 0.94±0.005. For the Ugandan fires, mean EFs were 3.1±0.35 for CH4, 1610±169.7 for CO2, and 78±8.9 for CO, with a mean modified combustion efficiency of 0.93±0.004. A mean N2O EF of 0.08±0.002 g kg−1 is also reported for one flight over Uganda; issues with temperature control of the instrument optical bench prevented N2O EFs from being obtained for other flights over Uganda. This study has provided new datasets of African biomass burning EFs and MCEs for two distinct study regions, in which both have been studied little by aircraft measurement previously. These results highlight the important intracontinental variability of biomass burning trace gas emissions and can be used to better constrain future biomass burning emission budgets. More generally, these results highlight the importance of regional and fuel-type variability when attempting to spatially scale biomass burning emissions. Further work to constrain EFs at more local scales and for more specific (and quantifiable) fuel types will serve to improve global estimates of biomass burning emissions of climate-relevant gases.
Highlights
The atmospheric burdens of the greenhouse gases (GHGs) CO2, CH4, and N2O have been increasing since the onset of the Industrial Revolution
Chanton et al (2000) analysed biomass burning emissions via Keeling plot analysis (δ13C-CH4 vs. inverse CH4 mole fraction) from a range of fuel sources. They found that African grass burning emitted methane with δ13C-CH4 ranging between −17 ‰ and −26 ‰, whereas African woodland burning produced methane with a δ13C-CH4 ratio of approximately −30 ‰. For both near-field and far-field Methane Observations and Yearly Assessments (MOYA)-I flights, whole-air samples were taken of the biomass burning plumes sampled, as well as of the local background. δ13CCH4 isotopic ratios and mean CH4 mole fractions are determined from these whole-air samples
It is worth noting that CH4 emission factors (EFs) and corresponding modified combustion efficiencies (MCEs) for the far-field flights C006 and C007 are not included in Fig. 7, as the EFs from these flights are representative of multiple fires with a mixture of phases, whereas the near-field EFs are representative of single fires with a single combustion efficiency associated with them
Summary
The atmospheric burdens of the greenhouse gases (GHGs) CO2, CH4, and N2O have been increasing since the onset of the Industrial Revolution. The UK Natural Environment Research Council (NERC) Methane Observations and Yearly Assessments (MOYA) project is focused primarily on closing the global methane budget through new in situ observations and analysis of existing datasets This is being achieved (in part) through targeted field campaigns to constrain poorly quantified methane sources on local and regional scales, as well as the use of atmospheric chemical transport models, such as GEOSCHEM, to provide global estimates of methane emission trends (Bey et al, 2001; Holmes et al, 2013; Saunois et al, 2016). Emission factors (EFs) for CH4, CO2, N2O, and CO can be determined from the enhancement in trace gas mixing ratio observed when a biomass burning plume was intercepted These EFs were calculated for multiple fires observed in Senegal and Uganda. Comparisons are made between EFs determined in this study and EFs from Andreae (2019), who includes up to 50 studies reporting fire EFs and modified combustion efficiencies from multiple biomass burning types, such as tropical forest burning, savannah and grassland burning, and agricultural residue burning
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