Modeling the Extreme Deuterium Enrichment in Stratospheric Hydrogen

Observations of the isotopic composition of molecular hydrogen in whole air samples collected by the NASA ER-2 aircraft showed extreme enrichment in deuterium of up to 440 per mil on the V-SMOW isotope scale [Rahn et al., Nature, 2003]. In this study, we use the Lawrence Livermore National Laboratory 2D model to investigate the extreme deuterium enrichment in stratospheric H₂ and find that kinetic isotope effects (KIEs) in the oxidation of H₂ in the stratosphere account for over 60% of the enrichment while production of heavy H₂ from CH₄ oxidation accounts for the remainder. The sensitivity of the H₂ isotopic composition to uncertainties in the CH₄ oxidation pathway branching ratios, formaldehyde oxidation KIEs, formaldehyde photolysis isotope effects, and the H₂ oxidation KIEs is explored, and recommendations for needed laboratory and field measurements based on these model sensitivities are given. Finally, we investigate the global H₂ isotope budget using model estimates of δ D-H₂ produced from CH₄ oxidation in both the stratosphere and troposphere.