Abstract. Molecular hydrogen (H 2 ), 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. 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 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 fuelrich 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 H 2 and water (H 2 O). In addition, post TWC δD 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 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) Correspondence to: M. K. Vollmer (martin.vollmer@empa.ch) 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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A B S T R A C T Measurements of H 2 at the high-altitude site of Jungfraujoch, Switzerland are reported upon for the period of August, 2005-November, 2009. The time series consists of measurements that are primarily representative of free tropospheric background conditions. Highest background H 2 mixing ratios were observed in May, while the lowest were observed in November. The mean seasonal H 2 peak-to-trough amplitude of 21 parts per billion (ppb, 10 −9 dry air mixing ratio) at Jungfraujoch was considerably less than at other stations at similar latitudes and the seasonal minimum in November was comparatively delayed. These differences are primarily attributed to a dampening and delay of the surface soil sink signal during its vertical propagation to the free troposphere. Excess (mixing ratio minus corresponding baseline value) H 2 ( H 2 ) and excess CO ( CO) displayed no significant correlation. This lacking correlation is attributed to H 2 removal by soil during transport to Jungfraujoch, thereby significantly altering the H 2 / CO ratio from traffic combustion sources, which is the largest source of anthropogenic H 2 influencing measurements at Jungfraujoch.

Abstract. Molecular hydrogen (H2), its stable isotope signature (δD), and the key combustion parameters carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) were measured from various combustion processes. H2 in the exhaust of gas and oil-fired heaters and of waste incinerator plants was generally depleted compared to ambient intake air, while CO was significantly elevated. These findings contradict the often assumed co-occurring net H2 and CO emissions in combustion processes and suggest that previous H2 emissions from combustion may have been overestimated when scaled to CO emissions. For the gas and oil-fired heater exhausts, H2 and δD generally decrease with increasing CO2, from ambient values of ~0.5 ppm and +130‰ to 0.2 ppm and −206‰, respectively. These results are interpreted as a combination of an isotopically light H2 source from fossil fuel combustion and a D/H kinetic isotope fractionation of hydrogen in the advected ambient air during its partial removal during combustion. Diesel exhaust measurements from dynamometer test stand driving cycles show elevated H2 and CO emissions during cold-start and some acceleration phases. While H2 and CO emissions from diesel vehicles are known to be significantly less than those from gasoline vehicles (on a fuel-energy base), we find that their molar H2/CO ratios (median 0.026, interpercentile range 0.12) are also significantly less compared to gasoline vehicle exhaust. Using H2/CO emission ratios, along with CO global emission inventories, we estimate global H2 emissions for 2000, 2005, and 2010. For road transportation (gasoline and diesel), we calculate 8.3 ± 2.2 Tg, 6.0 ± 1.5 Tg, and 3.8 ± 0.94 Tg, respectively, whereas the contribution from diesel vehicles is low (0.9–1.4%). Other fossil fuel emissions are believed to be negligible but H2 emissions from coal combustion are unknown. For residential (domestic) emissions, which are likely dominated by biofuel combustion, emissions for the same years are estimated at 2.7 ± 0.7 Tg, 2.8 ± 0.7 Tg, and 3.0 ± 0.8 Tg, respectively. For biomass burning H2 emissions, we derive a mole fraction ratio ΔH2/ΔCH4 (background mole fractions subtracted) of 3.6 using wildfire emission data from the literature and support these findings with our wood combustion results. When combining this ratio with CH4 emission inventories, the resulting global biomass burning H2 emissions agree well with published global H2 emissions, suggesting that CH4 emissions may be a good proxy for biomass burning H2 emissions.

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