Deuterium component of the global molecular hydrogen cycle

The deuterium (D) component of the global molecular hydrogen (H 2 ) cycle is discussed. Field measurements of the fractionation effect during soil uptake of atmospheric H 2 (α soil ) were made using an in situ static flux chamber, and an α soil of 0.943±0.024 (±1σ) was determined. The δD of H 2 emitted during combustion of pine needles and pine branches was experimentally measured at −330±50‰ (versus Vienna SMOW) and was 230±60‰ lower than the δD of the plant matter combusted. This hydrogen isotope fractionation effect yielded an estimate of −290±60‰ for the δD of H 2 emitted during global biomass burning. The δD of H 2 emitted by fossil fuel combustion in the Seattle, Washington, urban area was −196±10‰. On the basis of a global H 2 budget calculation, a δD of +130±70‰ is required for the photochemical H 2 source in order for the available isotopic measurements to agree with the global H 2 budget. A possible mechanism for a significant enrichment in the deuterium content of H 2 produced by the photochemical destruction of methane is given. We suggest that the currently available isotopic measurements are not in disagreement with the current view of the global H 2 budget.