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

Measurements of CO2, CO, N2O and CH4 mole fractions, O-2/N-2 ratios and the stable isotopes C-13 and O-18 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 C-13 in CO2 reflects the isotopic composition of the fuel. O-18 in CO2 is slightly depleted compared to the O-18 in atmospheric O-2, and shows significant variability. In contrast, the delta C-13 values of CO show that significant fractionation takes place during CO destruction in the catalytic converter. C-13 in CO is enriched by 3 parts per thousand compared to the C-13 in the fuel burnt, while the O-18 content is similar to that of atmospheric O-2. We compute a fractionation constant of (-2.7 +/- 0.7)parts per thousand for C-13 during CO destruction. The N2O: CO2 average ratio of (1.8 +/- 0.2) x 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) x 10(-2) ppb: ppm. The O-2 : CO2 ratios of (-1.47 +/- 0.01) ppm: ppm are very close to the expected, theoretically calculated values of O-2 depletion per CO2 enhancement.