Atmos. Chem. Phys. Discuss., 10, 3021-3051, 2010
www.atmos-chem-phys-discuss.net/10/3021/2010/
doi:10.5194/acpd-10-3021-2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Molecular hydrogen (H2) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H2
M. K. Vollmer1, S. Walter2, S. W. Bond1, P. Soltic3, and T. Röckmann2
1Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Air Pollution and Environmental Technology, Uberlandstrasse 129, 8600 Dubendorf, Switzerland
2Institute for Marine and Atmospheric research Utrecht, Utrecht University, Princetonplein 5, 3508TA Utrecht, The Netherlands
3Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory of I. C. Engines, Uberlandstrasse 129, 8600 Dubendorf, Switzerland

Abstract. 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‰ 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 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 (H2O). 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 (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) 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.

Citation: Vollmer, M. K., Walter, S., Bond, S. W., Soltic, P., and Röckmann, T.: Molecular hydrogen (H2) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H2, Atmos. Chem. Phys. Discuss., 10, 3021-3051, doi:10.5194/acpd-10-3021-2010, 2010.
 
Search ACPD
Discussion Paper
    XML
    Citation
    Final Revised Paper
    Share