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Volume 10, issue 12
Atmos. Chem. Phys., 10, 5707–5718, 2010
https://doi.org/10.5194/acp-10-5707-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 10, 5707–5718, 2010
https://doi.org/10.5194/acp-10-5707-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

  29 Jun 2010

29 Jun 2010

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 M. K. Vollmer et al.
  • 1Empa, Swiss Federal Laboratories for Materials Science and Research, Laboratory for Air Pollution and Environmental Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
  • 2Institute for Marine and Atmospheric research Utrecht, Utrecht University, Princetonplein 5, 3508TA Utrecht, The Netherlands
  • 3Empa, Swiss Federal Laboratories for Materials Science and Research, Laboratory of I. C. Engines, Überlandstrasse 129, 8600 Dübendorf, 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.

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