Improved isotopic characterisation of methane emissions from biomass burning

Emissions of methane from combustion sources are typically distinguished by being enriched in ¹³C and ²H, causing a large isotopic shift to atmospheric methane δ¹³C and δD measurements downwind of fires.

The isotopic composition of the plant material being burnt has a strong effect on the isotopic composition of methane, with combustion of C4 plant material producing methane more enriched in ¹³C than C3 plant combustion. Characterisation of the bulk isotopic signature of methane emitted from large areas of biomass burning is required to improve our ability to use isotopes in global models and ascertain the extent to which fire emissions influence interannual variations in the methane budget.

Two approaches have been used to collect air samples from large areas of biomass burning for isotopic characterisation of methane emitted from the fires. In campaigns in Senegal, Uganda, Zambia and Finland, the UK’s FAAM research aircraft flew through fire plumes and onboard measurement of methane concentration allowed targeted sampling within the plumes. This work was carried out as part of the NERC highlight Global Methane Budget project (MOYA). Ground based sampling downwind of fires around Sydney, New South Wales in late 2019/early 2020 has allowed isotopic characterisation of those plumes. All air samples were measured by isotope ratio mass spectrometry at Royal Holloway University of London and Keeling plots used to identify source signatures, e.g. δ¹³C for fires in Senegal in March 2017 was -28.5 ± 0,8 , typical of C3 burning.

In this work we compare the isotopic signatures of methane from burning in these particular regions and discuss the extent to which the regional variability of the isotopic composition of fire emissions should be taken into account in global models using isotopes to constrain the global methane budget.