Hydrogen isotope fractionation in the photolysis of formaldehyde

Abstract

Abstract. Experiments investigating the isotopic fractionation in the formation of H 2 by the photolysis of CH 2 O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in the H 2 produced is 500(±20)‰ with respect to the parent CH 2 O. We also observed that complete photolysis of CH 2 O under atmospheric conditions produces H 2 that has virtually the same isotope ratio as that of the parent CH 2 O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH 2 O+hν→CHO+H) as compared to the molecular channel (CH 2 O+hν→H 2 +CO) of the photolysis of CH 2 O in order to balance the relatively small isotopic fractionation in the competing reaction of CH 2 O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH 2 O. When CH 2 O is in photochemical steady state, the isotope ratio of the H 2 produced is determined not only by the isotopic fractionation occurring during the photolytical production of H 2 (α m ) but also by overall fractionation for the removal processes of CH 2 O (α f ), and is represented by the ratio of α m /α f . Applying the isotopic fractionation factors relevant to CH 2 O photolysis obtained in the present study to the troposphere, the ratio of α m /α f varies from ∼0.8 to ∼1.2 depending on the fraction of CH 2 O that reacts with OH and that produces H 2 . This range of α m /α f can render the H 2 produced from the photochemical oxidation of CH 4 to be enriched in D (with respect to the original CH 4 ) by the factor of 1.2-1.3 as anticipated in the literature.

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).…”

“…The first step in the chain of reactions that leads from CH 4 to H 2 is the abstraction of an H atom from CH 4 , and the last step is photolysis of the intermediate product formaldehyde (HCHO). Both these steps proceed more slowly for the deuterated than for the non-deuterated species (Sander et al, 2006;Feilberg et al, 2007;Rhee et al, 2008;Röckmann et al, 2010b), but these effects are offset by D-enriching effects in the other reactions in the chain (Nilsson et al, 2007). For the total process, stratospheric (δD) source signatures between +130 and +370 ‰ were reported (Rahn et al, 2003;Röckmann et al, 2003;Rhee et al, 2006a).…”

Isotope effect in the formation of H<sub>2</sub> from H<sub>2</sub>CO studied at the atmospheric simulation chamber SAPHIR

“…H 2 emitted from different sources usually carries a distinct isotope composition and the kinetic isotope effects in the two sinks differ strongly Quay, 2000, 2001;Rahn et al, 2003Rahn et al, , 2002aRhee et al, 2004Rhee et al, , 2005Rhee et al, , 2008Brenninkmeijer et al, 2003;Röckmann et al, , 2010Vollmer et al, 2010). In the case of hydrogen the isotope effects are particularly large due to the 100% relative mass difference between 1 H and 2 H (deuterium, in the following denoted D).…”

Temporal and spatial variability of the stable isotopic composition of atmospheric molecular hydrogen: observations at six EUROHYDROS stations

“…This has led to new constraints on the isotopic signatures of the most important sources and sinks. The isotopic composition of H 2 from CH 4 oxidation was first examined by measurements in the stratosphere (Rahn et al, 2003;Röckmann et al, 2003;Rhee et al, 2006a), but also individual steps in the oxidation sequence have been investigated, especially photolysis of formaldehyde (HCHO) (Feilberg et al, 2007;Mar et al, 2007;Nilsson et al, 2007;Rhee et al, 2008;Röckmann et al, 2010b). Furthermore, more information on the main surface sources, biomass burning and fossil fuel combustion (Röckmann et al, 2010a;Vollmer et al, 2010) and the largest sink, uptake in soil (Rahn et al, 2002a;Gerst and Quay, 2001;Rice et al, 2011), has become available.…”