Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2

Molecular hydrogen (H-2) is the second most abundant trace gas in the atmosphere after methane(1) (CH4). In the troposphere, the D/H ratio of H-2 is enriched by 120parts per thousand relative to the world's oceans(2-4). This cannot be explained by the sources of H-2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning)(5,6). But the isotopic composition of H-2 from its single largest source-the photochemical oxidation of methane-has yet to be determined. Here we show that the D/H ratio of stratospheric H-2 develops enrichments greater than 440parts per thousand, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H-2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H-2 from CH4 concentrates D in the product H-2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H-2.