Modeling the photochemical origins of the extreme deuterium enrichment in stratospheric H2

The isotopic composition of H2 produced by methane oxidation (“δDhν”) is an important yet poorly constrained term in the global H2 isotope budget. Box model analyses of the extreme deuterium enrichment in stratospheric H2 demonstrated empirically that δDhν is much larger than the initial δD of CH4, a conclusion that qualitatively resolved major discrepancies between the global H2 concentration and isotope budgets. However, the box model studies necessarily assumed that the isotopic fractionation factor for the conversion of CH4 to H2 remains constant throughout the stratosphere and that δDhν for the troposphere is equal to, or can be easily extrapolated from, the stratospheric value. Here, we use a 2‐D chemical‐radiative‐transport model to investigate these assumptions by determining the sensitivity of the isotopic composition of H2 (δD‐H2) and δDhν to known and unknown isotope effects in the elementary steps of the photochemical production and destruction of H2. Our results show that four categories of isotopic fractionation, (1) kinetic isotope effects (KIEs) for CH4 and H2 oxidation, (2) H versus D ion for CH3D oxidation to H2 or HD, (3) KIEs for CH2O oxidation, and (4) isotope effects for CH2O photolysis, all play significant but varying roles in determining δD‐H2 and δDhν in the stratosphere and troposphere. Furthermore, we show that calculated δDhν values vary significantly with latitude and altitude, leading to larger uncertainties in δDhν than previously estimated. Using these sensitivities, we also identify the laboratory experiments, theoretical calculations, and observations most needed to reduce uncertainties in the magnitude of δDhν and, hence, the global H2 isotope budget.