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

Molecular hydrogen (H-2), its isotopic signature (deuterium/hydrogen, delta 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, lambda). The isotopic composition of H-2 ranged from delta D=-140 parts per thousand to delta D =-195 parts per thousand upstream of the TWC but these values decreased to -270% to -370 parts per thousand after passing through the TWC. Post-TWC delta D values for the fuel-rich range showed a strong dependence on TWC temperature with more negative delta D for lower temperatures. These effects are attributed to a rapid temperature-dependent H-D isotope equilibration between H-2 and water (H2O). In addition, post TWC delta 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 (epsilon) ranging from -39.8 parts per thousand to -15.5 parts per thousand depending on the operating mode. Our results imply that there may be considerable variability in real-world delta D emissions from vehicle exhaust, which may mainly depend on TWC technology and exhaust temperature regime. This variability is suggestive of a delta D from traffic that varies over time, by season, and by geographical location. An earlier-derived integrated pure (end-member) delta 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 delta D values by >50 parts 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.