| dc.description.abstract | In this study, the isotopic composition of CO and of CO$_2$ and the CO:CO$_2$, CH$_4$:CO$_2$ and H$_2$:CO gas ratios in the exhaust of individual cars were investigated. This was done under idling and revving conditions, and for three cars in a full driving cycle on a test bench. The spread in the results, even within a single car, was large: for \delc in CO $\sim$ 0 to -60 \textperthousand, for \delo in CO $\sim$ +20 to +35 \textperthousand, and for all gas ratios several orders of magnitude. The results show an increase in the spread of isotopic values for CO compared to previous studies, suggesting that increasing complexity of emission regulations in cars might be reflected in the isotopic composition. When including all samples, we find a weighted mean for the \delc and \delo in CO of -28.7 $\pm$ 0.5 \perm \ and 24.8 $\pm$ 0.3 \perm \ respectively. This result is dominated by cold petrol cars. Our results suggest that in driving cycles where cold emissions are less important, both \delc and \delo would be expected to increase, which would result in isotopic values more in line with previous studies.
For the H$_2$:CO ratio, averaged over all cars, we found a value of 0.71 $\pm$ 0.31 ppb:ppb, in agreement with previous literature. The CO:CO$_2$ ratio, with a mean of 19.4 $\pm$ 6.8 ppb:ppm, and the CH$_4$:CO$_2$ ratio, with a mean of 0.26 $\pm$ 0.05 ppb:ppm, are both higher than is reported in recent literature. This is likely because our sampling distribution was biased towards cold cars, and therefore towards higher emission situations. In many ways the CH$_4$:CO$_2$ ratio was found to behave similarly to the CO:CO$_2$ ratio, suggesting that the processes affecting CO and CH$_4$ are similar.
Diesel cars behaved as a distinct group, with CO enriched in $^{13}$C and depleted in $^{18}$O compared to petrol cars. CO emissions from cold diesel cars were found to be significant, but reduced sharply in hot diesel cars.
The \delc values in CO$_2$ were close to the \delc expected from fuel, with no significant difference between petrol and diesel cars. The \delo values in CO$_2$ for petrol cars covered a range of 20 to 35 \perm, similar to the \delo of CO. The \delo values in CO$_2$ for diesel cars were closely centred around the \delo in atmospheric oxygen.
A set of samples taken in and near several parking garages showed good agreement with the individual car measurements. A \delc of -29.5 $\pm$ 0.4 \perm, a \delo of 25.9 $\pm$ 0.4 \perm (both for CO) and a H$_2$:CO ratio of 0.46 $\pm$ 0.06 ppb:ppm were found. This shows that, at least in parking garages, cold idling emissions play an important role. | |
