Vehicle emissions of greenhouse gases and related tracers from a tunnel study: CO : CO 2, N 2O : CO 2, CH 4 : CO 2, O 2 : CO 2 ratios, and the stable isotopes 13C and 18O in CO 2 and CO
Measurements of CO 2, CO, N 2O and CH 4 mole fractions, O 2 / N 2 ratios and the stable isotopes 13C and 18O in CO 2 and CO have been performed in air samples from the Islisberg highway tunnel (Switzerland). The molar CO : CO 2 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 CO 2 reflects the isotopic composition of the fuel. 18O in CO 2 is slightly depleted compared to the 18O in atmospheric O 2, and shows significant variability. In contrast, the δ 13C 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 O 2. We compute a fractionation constant of (−2.7 ± 0.7)‰ for 13C during CO destruction. The N 2O : CO 2 average ratio of (1.8 ± 0.2) × 10 −2 ppb:ppm is significantly lower than in past studies, showing a reduction in N 2O emissions likely related to improvements in the catalytic converter technology. We also observed small CH 4 emissions, with an average CH 4 : CO 2 ratio of (4.6 ± 0.2) × 10 −2 ppb:ppm. The O 2 : CO 2 ratios of (−1.47 ± 0.01) ppm:ppm are very close to the expected, theoretically calculated values of O 2 depletion per CO 2 enhancement.