Deuterium Enrichment in Stratospheric Molecular Hydrogen

Molecular hydrogen (H₂) is the second most abundant reduced gas in the atmosphere (after methane) with a globally averaged mixing ratio of ~ 530 ppbv. Its largest source is believed to be photochemical oxidation of methane (C H₄) and non-methane hydrocarbons (NMHCs); other recognized sources include biomass burning, fossil fuel burning, nitrogen fixation, and ocean degassing. As with other atmospheric trace gases, the stable isotopic content of H₂ has the potential to help quantify various aspects of its production and destruction. The average deuterium content of H₂ (expressed as δD_H2) is enriched by ~110 ‰ relative to Vienna Standard Mean Ocean Water while CH₄ in the troposphere, the precursor for photochemical H₂ production, is depleted by ~ 90 ‰ relative to V-SMOW and similar values are expected for NMHCs. Both natural and anthropogenic combustion sources of H₂ have been shown to be depleted in deuterium by 200 to 300 ‰ (Gerst and Quay, 2001; Rahn et al., 2002), and the ocean and N₂ fixation sources are expected to be in near thermodynamic equilibrium with local H₂O and should have deuterium levels of ~-700 ‰ (Rahn et al., 2002). In order to offset these deuterium depleted sources and account for the observed tropospheric δD_H2, the balancing loss processes must discriminate against reaction with HD and/or the total fractionation associated with CH₄ oxidation and the subsequent reactions leading to H₂ must favor production of deuterated H₂. We have analyzed a suite of stratospheric air samples in order to investigate the photochemical processes influencing the deuterium content of H₂. While the mixing ratio of H₂ is nearly constant, the deuterium content increases such that δD=440 ‰ in samples with a stratospheric mean age of ~6 years. The constant mixing ratio results from the fact that production due to CH₄ oxidation and loss due to H₂ oxidation are approximately equal. The observed trend in δD of stratospheric H₂ can only be accounted for by an enrichment in the ratio of D to H of H₂ relative to that in precursor CH₄ in addition to the enrichment due to the slower oxidation of deuterated H₂. We calculate the fractionation associated with this enrichment to be α_Total=1.54. As with other trace gases, in situ photochemical processes and the return flux of air from the stratosphere must be accounted for to explain tropospheric observations. Gerst, S., and P. Quay, J. Geophys. Res., 106, 5021-5031, 2001. Rahn, T., N. Kitchen, and J. M. Eiler, Geochim. Cosmochim. Acta, 66, 2475-2481, 2002.