The D/H content of methane emitted from biomass burning

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