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

摘要

Measurements of CO, CO, NO and CH mole fractions,O / N ratios and the stable isotopes C and O inCO and CO have been performed in air samples from the Islisberghighway tunnel (Switzerland). The molar CO : CO ratios, with an averageof (4.15 ± 0.34) ppb:ppm, are lower than reported in previous studies,pointing to a reduction in CO emissions from traffic. The C inCO reflects the isotopic composition of the fuel. O in COis slightly depleted compared to the O in atmospheric O, andshows significant variability. In contrast, the δC values ofCO show that significant fractionation takes place during CO destruction inthe catalytic converter. C in CO is enriched by 3‰ compared to the C in the fuel burnt, while theO content is similar to that of atmospheric O. We compute afractionation constant of (−2.7 ± 0.7)‰ forC during CO destruction. The NO : CO average ratio of (1.8 ± 0.2) × 10 ppb:ppm is significantly lower than inpast studies, showing a reduction in NO emissions likely related toimprovements in the catalytic converter technology. We also observed smallCH emissions, with an average CH : CO ratio of (4.6 ± 0.2) × 10 ppb:ppm.The O : CO ratios of (−1.47 ± 0.01) ppm:ppm are very close to the expected, theoreticallycalculated values of O depletion per CO enhancement.