Vehicle emissions of greenhouse gases and related tracers from a tunnel study: CO : CO₂, N₂O : CO₂, CH₄ : CO₂, O₂ : CO₂ ratios, and the stable isotopes ¹³C and ¹⁸O in CO₂ and CO

Abstract. Measurements of CO₂, CO, N₂O and CH₄ mole fractions, O₂ / N₂ ratios and the stable isotopes ¹³C and ¹⁸O in CO₂ and CO have been performed in air samples from the Islisberg highway tunnel (Switzerland). The molar CO : CO₂ ratios, with an average of (4.15 ± 0.34) ppb:ppm, are lower than reported in previous studies, pointing to a reduction in CO emissions from traffic. The ¹³C in CO₂ reflects the isotopic composition of the fuel. ¹⁸O in CO₂ is slightly depleted compared to the ¹⁸O in atmospheric O₂, and shows significant variability. In contrast, the δ¹³C values of CO show that significant fractionation takes place during CO destruction in the catalytic converter. ¹³C in CO is enriched by 3‰ compared to the ¹³C in the fuel burnt, while the ¹⁸O content is similar to that of atmospheric O₂. We compute a fractionation constant of (−2.7 ± 0.7)‰ for ¹³C during CO destruction. The N₂O : CO₂ average ratio of (1.8 ± 0.2) × 10⁻² ppb:ppm is significantly lower than in past studies, showing a reduction in N₂O emissions likely related to improvements in the catalytic converter technology. We also observed small CH₄ emissions, with an average CH₄ : CO₂ ratio of (4.6 ± 0.2) × 10⁻² ppb:ppm. The O₂ : CO₂ ratios of (−1.47 ± 0.01) ppm:ppm are very close to the expected, theoretically calculated values of O₂ depletion per CO₂ enhancement.