Molecular hydrogen (H2) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H2

• Published in: Atmospheric chemistry and physics Vol. 10; no. 12; pp. 5707 - 5718
• Main Authors: Vollmer, M K, Walter, S, Bond, S W, Soltic, P, Röckmann, T
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 15.06.2010; Copernicus Publications
• ISSN: 1680-7316; 1680-7324
• DOI: 10.5194/acp-10-5707-2010

Molecular hydrogen (H2 ), 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. H2 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 H2 ranged from δD = -140[per thousand] to δD = -195[per thousand] upstream of the TWC but these values decreased to -270[per thousand] to -370[per thousand] after passing through the TWC. Post-TWC δD values for the fuel-rich 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 H2 and water (H2 O). In addition, post TWC δD in H2 showed a strong dependence on the fraction of removed H2 , suggesting isotopic enrichment during catalytic removal of H2 with enrichment factors ([straight epsilon]) ranging from -39.8[per thousand] to -15.5[per thousand] 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[per thousand] (Rahn et al., 2002) can be explained as a mixture of mainly vehicle emissions from cold starts and fully functional TWCs, but enhanced δD values by >50[per thousand] 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.