Molecular hydrogen (H<sub>2</sub>) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H<sub>2</sub>

Abstract

Abstract. Molecular hydrogen (H 2 ), its isotopic signature (deuterium/hydrogen, δD), carbon monoxide (CO), and other compounds were studied in the exhaust of a passenger car engine fuelled with gasoline or methane and run under variable air-fuel ratios and operating modes. H 2 and CO concentrations were largely reduced downstream of the three-way catalytic converter (TWC) compared to levels upstream, and showed a strong dependence on the air-fuel ratio (expressed as lambda, λ). The isotopic composition of H 2 ranged from δD=−140‰ to δD = −195‰ upstream of the TWC but these values decreased to −270‰ to −370‰ after passing through the TWC. Post-TWC δD values for the fuelrich range showed a strong dependence on TWC temperature with more negative δD for lower temperatures. These effects are attributed to a rapid temperature-dependent H-D isotope equilibration between H 2 and water (H 2 O). In addition, post TWC δD in H 2 showed a strong dependence on the fraction of removed H 2 , suggesting isotopic enrichment during catalytic removal of H 2 with enrichment factors (ε) ranging from −39.8‰ to −15.5‰ depending on the operating mode. Our results imply that there may be considerable variability in real-world δD emissions from vehicle exhaust, which may mainly depend on TWC technology and exhaust temperature regime. This variability is suggestive of a δD from traffic that varies over time, by season, and by geographical location. An earlier-derived integrated pure (end-member) δD from anthropogenic activities of −270‰ (Rahn et al., 2002) Correspondence to: M. K. Vollmer (martin.vollmer@empa.ch) can be explained as a mixture of mainly vehicle emissions from cold starts and fully functional TWCs, but enhanced δD values by >50‰ are likely for regions where TWC technology is not fully implemented. Our results also suggest that a full hydrogen isotope analysis on fuel and exhaust gas may greatly aid at understanding process-level reactions in the exhaust gas, in particular in the TWC.

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

“…At the bottom of the table are estimates by different authors of the global average χ (H 2 ) and δD(H 2 ). Vollmer et al (2010) indicate that this source signature may be lower for modern cars and certain driving conditions. Results from a polluted station in the Netherlands also indicate this .…”

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

“…Tropospheric H 2 is enriched in deuterium with δD ∼ +130 ‰, (Gerst and Quay, 2001;Rhee et al, 2006;Rice et al, 2010;Batenburg et al, 2011) compared to surface emissions from fossil fuel combustion and biomass burning (δD approximately −200 ‰ and −300 ‰, respectively) (Gerst and Quay, 2001;Rahn et al, 2002;Röckmann et al, 2010a;Vollmer et al, 2010). 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).…”

H<sub>2</sub> vertical profiles in the continental boundary layer: measurements at the Cabauw tall tower in The Netherlands

“…Our results are (as expected from the uncorrected results) larger than these other estimates. Vollmer et al (2010) studied emissions from a passenger car engine, and showed that the absolute quantities of H 2 and CO emitted, and the H 2 /CO ratio depend on the operational regime. They found that exhaust H 2 concentrations (after catalyst) decrease below the atmospheric H 2 concentrations in fuel lean functioning conditions; if this is the general rule, then fluent traffic should not have a large impact on the atmospheric H 2 mole fractions (while still affecting atmospheric CO).…”