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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 CO 2 , and its multiply substituted isotopocule, namely, 13 C 16 O 18 O, provides a window of opportunity to probe directly the effective temperature of the converter in operation. Here, we report multiple isotopic measurements in exhaust CO 2 (δ 13 C, δ 17 O, δ 18 O, and Δ 47 ) 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 NO x processes through isotopic analysis of the exhaust N 2 O. We found that the degree of N 2 O reduction to N 2 in gasoline-powered vehicles is high when the temperature is above 200 °C (inferred by Δ 47 ). In contrast, diesel-powered vehicles produce N 2 O in abundance, probably a consequence of selective catalytic reduction of NO x , and the reduction efficiency depends on the converter temperature. In other words, the catalytic converters act as sinks and sources of N 2 O 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 CO 2 and N 2 O in the Hsuehshan tunnel, a ∼13 km long highway tunnel in Taiwan. Elevated N 2 O concentrations inside the tunnel indicate that the emission of N 2 O 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 N 2 O to the atmosphere. The combined study of clumped isotopes and N 2 O 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.

Air quality in the megacity Delhi is affected not only by local emissions but also by pollutants from crop residue burning in the surrounding areas of the city, particularly the rice straw burning in the post monsoon season. As a major burning product, gaseous CO 2 , which is rather inert in the polluted atmosphere, provides an alternative solution to characterize the impact of biomass burning from a new perspective that other common tracers such as particulate matters are limited because of their physical and chemical reactiveness. Here, we report conventional ([CO 2 ], δ 13 C, and δ 18 O) and unconventional (Δ 17 O) isotope data for CO 2 collected at Connaught Place (CP), a core area in the megacity Delhi, and two surrounding remote regions during a field campaign in October 18-20, 2017. We also measured the isotopic ratios near a rice straw burning site in Taiwan to constrain their end member isotopic compositions. Rice straw burning produces CO 2 with δ 13 C, δ 18 O, and Δ 17 O values of -29.02 ± 0.65, 19.63 ± 1.16, and 0.05 ± 0.02‰, respectively. The first two isotopic tracers are less distinguishable from those emitted by fossil fuel combustion but the last one is significantly different. We then utilize these end member isotopic ratios, with emphasis on Δ 17 O for the reason given above, for partitioning sources that affect the CO 2 level in Delhi. Anthropogenic fraction of CO 2 at CP ranges from 4 to 40%. Further analysis done by employing a three-component (background, rice straw burning, and fuel combustion) mixing model with constraints from the Δ 17 O values yields that rice straw burning contributes as much as ∼70% of the total anthropogenic CO 2 , which is more than double of the fossil fuel contribution (∼30%), during the study days., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)