Airborne measurements of fire emission factors for African biomass burning sampled during the MOYA campaign

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 ofWest 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 gkg..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.