Molecular hydrogen (H2) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H2 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
Received: 24 November 2009 – Published in Atmos. Chem. Phys. Discuss.: 05 February 2010 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.
Revised: 26 May 2010 – Accepted: 10 June 2010 – Published: 29 June 2010
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., 10, 5707-5718, doi:10.5194/acp-10-5707-2010, 2010.