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

Abstract

Abstract. The isotopic composition of molecular hydrogen (H 2 ) produced by photochemical oxidation of methane (CH 4 ) and Volatile Organic Compounds (VOCs) is a key quantity in the global isotope budget of (H 2 ). 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 CH 4 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 CH 4 +Cl. The same approach is applied to VOCs, which are important precursors of H 2 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 H 2 product. The reaction scheme is directly applicable in global chemistry models, which can thus include the isotope pathway of H 2 produced from CH 4 and VOCs in a consistent way.

The stable isotopic signature of biologically produced molecular hydrogen (H<sub>2</sub>)

“…As originally proposed by Gerst and Quay (2001) from budget closure, the photochemical sources of H 2 are also enriched in deuterium with δD between ∼ +100 ‰ and +200 ‰, (Rahn et al, 2003;Röckmann et al, 2003Röckmann et al, , 2010bFeilberg et al, 2007;Nilsson et al, 2007Nilsson et al, , 2010Pieterse et al, 2009 (Novelli et al, 1999;Hauglustaine and Ehhalt, 2002;Ehhalt and Rohrer, 2009;Pieterse et al, 2011) the extreme deuterium depletion relative to ambient atmospheric H 2 makes it a quite important contributor to the isotope budget (Price et al 2007;Pieterse et al 2011). An increasing demand and anthropogenic biological production of H 2 by e.g., industrial fermentation of biogenic waste material is associated with an expected release to the atmosphere because of leakage during production, storage, transport and use.…”

The stable isotopic composition of molecular hydrogen in the tropopause region probed by the CARIBIC aircraft

“…Recently, isotope effects in the production processes of H 2 have been studied (Gerst and Quay, 2001;Rahn et al, 2002bRahn et al, , 2003Röckmann et al, 2003;Rhee et al, 2006a;Feilberg et al, 2007;Rhee et al, 2008;Röckmann et al, 2010a,b;Vollmer et al, 2010;Walter et al, 2011;Haumann et al, 2012), as well as the isotope effects in the H 2 uptake by soils (Gerst and Quay, 2001;Rahn et al, 2002a;Rice et al, 2011). Two chemical transport models have been adapted to incorporate the stable isotopic composition of H 2 , namely GEOS-CHEM (Price et al, 2007) and TM5 (Pieterse et al, 2009(Pieterse et al, , 2012, and many more δD data have become available for the validation of such models (Rice et al, 2010;Batenburg et al, 2011). However, for obvious practical reasons, most of these data were collected at ground level, and yield little information about processes higher up in the atmosphere, particularly in the Upper TroposphereLower Stratosphere (UTLS) region.…”

Meridional distribution of molecular hydrogen and its deuterium content in the atmosphere

“…Evidence of a substantial enrichment in the deuterium content of formaldehyde is from theoretical and box model considerations of the oxidative chain between hydrocarbons and formaldehyde [Gerst and Quay, 2001;Rahn et al, 2003;Röckmann et al, 2003;Feilberg et al, 2007;Pieterse et al, 2009]; direct observations of the enrichment in dD of stratospheric H 2 as a result of CH 4 oxidation [Rahn et al, 2003;Röckmann et al, 2003]; photochemical modeling of the dD of H 2 in the troposphere and stratosphere [Price et al, 2007;Mar et al, 2007]; and observations of formaldehyde enriched in deuterium up to 210‰ [Rice and Quay, 2009]. The photochemical enrichment of formaldehyde must be sufficient to overcome a −450‰ to −500‰ isotope effect occurring with the photolysis of formaldehyde [Feilberg et al, 2007;Rhee et al, 2008].…”