Stable isotopic measurements of atmospheric CO (
13CO, C
18O) were carried out at three season in a middle size city, Mainz in Germany. CO stable isotopic compositions of car exhaust fueled by gasoline, diesel and natural gas were also measured and showed different isotopic compositions (δ
18O vs. V-SMOW = 22.3 ± 0.3‰, 10.9 ± 0.8‰, 20.1 ± 1.0‰ and δ
13C vs. V-PDB = -29.9 ± 0.3‰, -22.2 ± 1.1‰, -51.3 ± 1.5‰, respectively). The
18O/
16O ratios of atmospheric CO in Mainz show a clear linear correlation with the reciprocal concentration of CO. This reveals the contribution of CO emitted from a single pollution source (δ
18O = 20.7 ± 0.5‰ V-SMOW), whose isotopic composition is close to that of CO from a gasoline engine combustion. From this value and car exhaust measurements, about 14 ± 7% of CO in Mainz air is estimated to originate from diesel fuel engine combustion. The oxygen isotopic composition can be used as a good indicator of contamination from car exhausts. On the other hand, the
13C/
12C ratios of CO do not show a clear relationship with the reciprocal concentration of CO.
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