The deuterium anomaly in stratospheric molecular hydrogen

Abstract

Molecular hydrogen (H_2) is the second most abundant reduced gas in the atmosphere (after methane) and has a globally averaged mixing ratio of ~530 ppbv. Its recognized sources include: photochemical oxidation of CH_4 and nonmethane hydrocarbons (NMHC); biomass burning, fossil fuel burning, nitrogen fixation, and ocean degassing. The stable isotopic composition of H_2 can help quantify the relative importance of these various sources and balancing sinks. The average δD_(SMOW) of H_2 in the surface atmosphere is ca. + 100 ‰ - strongly D enriched compared to that measured for atmospheric CH_4 δD_(SMOW) ~ -90 ‰), expected for NMHC's, and known for combustion sources of H_2 δD_(SMOW) of ca. -200 to -300 ‰) [Gerst and Quay, 2001; Rahn et al., 2002]. The DIH ratios of H_2 from ocean degassing and nitrogen fixation are poorly known but are likely to have δD_(SMOW) values of ca. - 700 ‰, in near thermodynamic equilibrium with local H_2O. Thus, all major sources of atmospheric H_2 are hundreds of permit poorer in D than H_2 itself. Either the loss processes (photo-oxidation and/or soil uptake) must discriminate against reaction with HD and/or isotopic fractionation associated with photo-oxidation of CH_4 must favor production of HD over HH. Our current understanding of the global H_2 budget depends strongly on the exact contribution of each of these effects to observed D enrichment.

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