Hydrogen and carbon isotopic measurements of methane from agricultural combustion: Implications for isotopic signatures of global biomass burning sources

[ 1] The hydrogen and carbon isotope ratios of CH4 emitted from one bonfire and two laboratory combustions of agricultural residues were determined in this study as - 196 to - 262 parts per thousand for delta D and - 19.9 to - 35.1 parts per thousand for delta C-13. The isotopic difference between CH4 emitted from biomass burning and fuel biomass, that is, the apparent isotopic fractionation occurring during combustion (epsilon(Burn)), was within - 101 to - 174 parts per thousand for epsilon(D)(Burn) and + 6.9 to - 8.6 parts per thousand for epsilon(13C)(Burn). That difference varied according to burning conditions: flaming and smoldering. Variation in epsilon(Burn) is correlated with combustion efficiency (CE), defined here as the ratio of emitted CO2 to the sum of emitted CO2 and CO. In light of the previously reported global distributions of CE, the implied epsilon(Burn) values for global biomass burning sources were - 121 parts per thousand for epsilon(D)(Burn) and + 2.2 parts per thousand for epsilon(13C)(Burn). Using the relationship between epsilon(Burn) and CE and global distributions of CE and isotopic ratios of fuel biomass, we estimated the global isotopic signature for biomass burning sources as - 169 parts per thousand for delta D and - 23.6 parts per thousand for delta C-13.