An updated estimation of the stable carbon and oxygen isotopic compositions of automobile CO emissions

We estimate up-to-date values of the average isotopic compositions of CO emitted from automobiles. In the estimation, we determined the isotopic compositions of CO in tail pipe exhaust for four gasoline automobiles and two diesel automobiles under varying conditions of both idling and running. While the dependence on the automobile manufacturer is little, each automobile equipped with functional catalytic converter exhibits a large temporal δ 13C and δ 18O variation. They tend to show 13C and 18O enrichment in accordance with the reduction of CO in exhaust, suggesting that the functional catalytic converter in engines enhances the δ 13C and δ 18O values of CO from tail pipes through a kinetic isotope effect during CO destruction. Assuming that automobiles run a modeled driving cycle, we estimated the average δ 13C and δ 18O of CO to be −23.8±0.8‰ PDB and +25.3±1.0‰ SMOW, respectively, for recent gasoline automobiles, and −19.5±0.7‰ PDB and +15.1±1.0‰ SMOW, respectively, for recent diesel automobiles. While the δ 13C and δ 18O values of recent gasoline automobiles coincide well with the isotopic compositions of source CO in present trunk road atmosphere estimated in this study, those are +4–+6‰ ( δ 13C) and +1–+3‰ ( δ 18O) higher than those reported previously and also those emitted from old, non-catalyst automobiles determined in this study. Recent improvements in functional catalytic converters have enhanced and will enhance the δ 13C and δ 18O values of CO from automobiles.