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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 highway tunnel Islisberg (Switzerland). The CO:CO2 ratios, with an average of
(4.15±0.34) 5 ppb : ppm, are lower than reported by 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 13C values of CO show that significant
fractionation takes place during CO destruction in the catalytic converter. 13C in CO is
10 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 (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 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.