The D/H content of methane emitted from biomass burning

Abstract

The D/H content of methane emitted from biomass burning was measured using samples of smoke collected from large-scale laboratory combustion experiments and from fires typical of slash burning of primary forest and of pasture burning in the Brazilian Amazon. In laboratory experiments, the mean δD of methane emitted from Pinus ponderosa needle combustion (δD(CH4(bb))) was -233 ± 2‰. In Brazilian fires, δD(CH4(bb)) was -210 ± 16‰. The measured D/H of the fuel biomass ranged from -59 to -125‰ and indicated a significant hydrogen isotope fractionation (ε(D)(BURN)) during combustion. For the laboratory experiments, ε(D)(BURN) (=(D/H)(CH4)/(D/H)(Biomass)-1) 1000 = -124‰ and in the field studies, ε(D)(BURN) ranged from -130 to -180‰. These results contrast those for δ13C which showed no fractionation during combustion, consistent with earlier studies. The measurements presented here suggest that a relatively narrow range exists for the δD of CH4 emitted from biomass burning on a global scale and that δD is an independent tracer for atmospheric CH4. Combining δD(CH4(bb)) ~ -210‰ with literature values for the strength and δD of the major CH4 sources, we estimate a δD for the global CH4 source of -283 ± 13‰. This result provides a constraint for the D/H kinetic isotope effect associated with removal of atmospheric CH4 by atmospheric hydroxyl radicals.