A consistent molecular hydrogen isotope chemistry scheme based on an independent bond approximation

Abstract

Abstract. The isotopic composition of molecular hydrogen (H2) produced by photochemical oxidation of methane (CH4) and Volatile Organic Compounds (VOCs) is a key quantity in the global isotope budget of (H2). The many individual reaction steps involved complicate its investigation. Here we present a simplified structure-activity approach to assign isotope effects to the individual elementary reaction steps in the oxidation sequence of CH4 and some other VOCs. The approach builds on and extends the work by Gerst and Quay (2001) and Feilberg et al. (2007b). The description is generalized and allows the application, in principle, also to other compounds. The idea is that the C-H and C-D bonds – seen as reactive sites – have similar relative reaction probabilities in isotopically substituted, but otherwise identical molecules. The limitations of this approach are discussed for the reaction CH4+Cl. The same approach is applied to VOCs, which are important precursors of H2 that need to be included into models. Unfortunately, quantitative information on VOC isotope effects and source isotope signatures is very limited and the isotope scheme at this time is limited to a strongly parameterized statistical approach, which neglects kinetic isotope effects. Using these concepts we implement a full hydrogen isotope scheme in a chemical box model and carry out a sensitivity study to identify those reaction steps and conditions that are most critical for the isotope composition of the final H2 product. The reaction scheme is directly applicable in global chemistry models, which can thus include the isotope pathway of H2 produced from CH4 and VOCs in a consistent way.

Reassessing the variability in atmospheric H2 using the two‐way nested TM5 model

“…The two‐way nested setup of the TM5 model [ Krol and Houweling , ] was recently enhanced by implementing a H 2 isotope chemistry scheme [ Pieterse and Krol , ], an H 2 emission inventory adopted from the project for Global and regional Earth system Monitoring using Satellite and in situ data [GEMS: Schultz and Stein , ], a soil moisture dependent deposition parametrization [ Sanderson and Collins , ], and a stratospheric parametrization for H 2 and HD [ Rahn and Eiler , ; McCarthy and Boering , ; Pieterse and Krol , ]. Our previous study [ Pieterse and Krol , ] was primarily focussed on the introduction and global evaluation of the new H 2 isotope chemistry scheme.…”

“…The resulting framework was also used to evaluate the stable H 2 isotope budgets previously reported by Gerst and Quay [], Rahn and Kitchen [], Rahn and Eiler [], and Rhee and Brenninkmeijer []. A full H 2 isotope chemistry scheme was recently implemented in the TM5 model [ Pieterse et al ., , ] and used to further constrain the global budget of H 2 with δ [ D H 2 ] measurements. Both studies showed that the modeled tropospheric δ [ D H 2 ] is very sensitive to the values of the isotopic composition of stratospheric molecular hydrogen that is heavily enriched in deuterium [ Rahn and Eiler , ; Röckmann and Rhee , ].…”

H2 clumped isotope measurements at natural isotopic abundances

“…Recently, it became possible to analyze the D content of H 2 (δD) in small atmospheric samples at natural isotopic abundances . These measurements enabled the study of different components of the H 2 cycle and helped constrain the atmospheric H 2 budget, which as a consequence is currently relatively well understood on the large scale . Although H 2 isotope measurements have proven useful, they are difficult to perform, and only few labs had or have this capability.…”