Insight into the Variability of the Nitrogen Isotope Composition of Vehicular NOx in China

Abstract

Nitrogen isotope (δ¹⁵N) monitoring is a potentially powerful tool in tracing atmospheric nitrogen oxides (NO_x); however, the isotopic fingerprint of vehicle exhaust remains poorly interpreted. This deficiency limits our understanding of the origin of atmospheric haze pollution, especially in China. In this study, we systemically explored the δ¹⁵N-NO_x fingerprints of various vehicle exhausts (n = 137) in China. The δ¹⁵N-NO_x values of vehicle exhausts ranged from -18.8‰ to +6.4‰, presenting a significant correlation with NO_x concentrations (p < 0.01). The highest δ¹⁵N-NO_x values were observed for liquefied petroleum gas vehicles (-0.1 ± 1.8‰), followed by gasoline vehicles (-7.0 ± 4.8‰) and diesel vehicles (-12.7 ± 3.4‰), all of which displayed a rising trend as emissions standards were continuously updated. The δ¹⁵N-NO_x values under working conditions followed the trend warm start (-5.9 ± 5.0‰) > driving (-7.3 ± 5.9‰) > cold start (-9.2 ± 2.7‰). By establishing a suitable model for assessing representative δ¹⁵N-NO_x values, the δ¹⁵N-NO_x values of various vehicles, including different fuel types with different emission standards, were evaluated. A model of δ¹⁵N-NO_x associated with motor vehicle data was developed, which estimated the national δ¹⁵N-NO_x value of vehicle emissions to be -12.6 ± 2.2‰, but there was considerable variation among different target areas in China.