ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-10-5707-2010 Molecular hydrogen (H<sub>2</sub>) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H<sub>2</sub> Vollmer M. K. 1 Walter S. 2 Bond S. W. 1 Soltic P. 3 Röckmann T. 2 Empa, Swiss Federal Laboratories for Materials Science and Research, Laboratory for Air Pollution and Environmental Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland Institute for Marine and Atmospheric research Utrecht, Utrecht University, Princetonplein 5, 3508TA Utrecht, The Netherlands Empa, Swiss Federal Laboratories for Materials Science and Research, Laboratory of I. C. Engines, Überlandstrasse 129, 8600 Dübendorf, Switzerland 29 06 2010 10 12 5707 5718 Copyright: © 2010 M. K. Vollmer et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/10/5707/2010/acp-10-5707-2010.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/10/5707/2010/acp-10-5707-2010.pdf

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