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Free keywords:
molecular-hydrogen
meteoric precipitation
atmospheric hydrogen
stable hydrogen
methoxyl groups
d/h ratios
cycle
stratosphere
photolysis
chemistry
Abstract:
Differences in isotopic composition between the various sources of H-2 are large, but only a few measurements have been carried out to constrain them. Two conflicting values have been published for H-2 from biomass burning. Both rely on the assumption that the isotopic composition of H-2 should scale with the isotopic composition of the precipitation at the location where the biomass grew. Here we test this hypothesis using 18 wood samples collected from various locations around the globe. The sample locations cover a range of delta D content of H-2 in precipitation, from below -120 parts per thousand in Siberia and Canada to -15 parts per thousand in Zimbabwe. The results confirm the predicted dependence of the H-2 isotopic composition on the precipitation in the sampling region. The water content itself is found to at most slightly affect the results. Furthermore, delta D of H-2 depends strongly on combustion efficiency. Thus, the isotopic composition of H-2 from biomass burning shows a strong variability around the globe and between different stages of a fire. It is suggested that, rather than a global bulk number, global models that attempt to reproduce the spatial and temporal distribution of delta D in H-2 should incorporate explicitly the variability of delta D(H-2) from biomass burning on delta D in precipitation.
