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

Molecular hydrogen (H ₂), its isotopic signature (deuterium/hydrogen, δ D), 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 δ D = −140‰ to δ D = −195‰ upstream of the TWC but these values decreased to −270‰ to −370‰ after passing through the TWC. Post-TWC δ D values for the fuel-rich range showed a strong dependence on TWC temperature with more negative δ D for lower temperatures. These effects are attributed to a rapid temperature-dependent H-D isotope equilibration between H ₂ and water (H ₂O). In addition, post TWC δ D 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 δ D emissions from vehicle exhaust, which may mainly depend on TWC technology and exhaust temperature regime. This variability is suggestive of a δ D from traffic that varies over time, by season, and by geographical location. An earlier-derived integrated pure (end-member) δ D 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 δ D 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.