Role of Vehicular Catalytic Converter Temperature in Emission of Pollutants: An Assessment Based on Isotopic Analysis of CO2 and N2O.

Vehicular catalytic converters are used to regulate, reduce, and convert toxic and environmentally unfriendly compounds in exhaust gases into relatively inert and less harmful chemical species. The efficiency, however, is largely affected by the operating temperature of the converter which is set by the hot exhaust gas released from the combustion chamber. A major gas released during combustion is CO2, and its multiply substituted isotopocule, namely, 13C16O18O, provides a window of opportunity to probe directly the effective temperature of the converter in operation. Here, we report multiple isotopic measurements in exhaust CO2 (δ13C, δ17O, δ18O, and Δ47) of diesel (trucks and buses) and gasoline (sedans, trucks, and two-wheel motorcycles)-powered vehicles. For investigating the efficiency of a converter in reducing toxic compounds, we studied NOx processes through isotopic analysis of the exhaust N2O. We found that the degree of N2O reduction to N2 in gasoline-powered vehicles is high when the temperature is above 200 °C (inferred by Δ47). In contrast, diesel-powered vehicles produce N2O in abundance, probably a consequence of selective catalytic reduction of NOx, and the reduction efficiency depends on the converter temperature. In other words, the catalytic converters act as sinks and sources of N2O to the atmosphere in gasoline- and diesel-operated vehicles, respectively. We also report a new set of field data by measuring the isotopic compositions of CO2 and N2O in the Hsuehshan tunnel, a ∼13 km long highway tunnel in Taiwan. Elevated N2O concentrations inside the tunnel indicate that the emission of N2O by heavy-duty diesel vehicles is much higher compared to the reduction by gasoline-operated passenger cars, making the vehicular exhausts a net source of N2O to the atmosphere. The combined study of clumped isotopes and N2O concentration in exhaust gases suggests that it is useful to probe the operational temperature of catalytic converters and monitor the pollution level in operation, thus providing an opportunity for manufacturers to optimize the catalytic efficiency to reduce the level of toxic pollutants to the environment.