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.

Atmospheric nitrate (NO3−) pollution has become an obstacle to efforts to further reduce fine particulate (PM2.5) concentration in North China. However, there have been limited long‐term measurements of NO3− and isotopic knowledge (δ15N, δ18O) on the driving factors during NO3− changes. Here, we report observations of 10 voyages from 2014 to 2019 conducted in the Bohai Sea, a typical background area in North China. The results show that the average proportion of NO3− in PM2.5 increased from 0.08 to 0.16 over the study period. The δ15N–NO3− ranged from −4.1‰ to +20.5‰, with a significant annual decline (p < 0.01), especially in winter. The average δ18O–NO3− was +72.6 ± 13.5‰, and a Monte Carlo calculation revealed that the contribution of the •OH pathway in the NO3− formation declined by 27.4% in winter, implying an increase in O3 pollution. Coal combustion remained the most important contributor to NO3− (46.6 ± 15.9%), but its contribution showed a significant downward trend (p < 0.01), consistent with the control of disperse coal use in North China. Enhancement of atmospheric oxidation and the unexpected large increase in contribution of microbial processes were found to be the main causes of the increasingly serious NO3− pollution in North China. In addition, a spike in the contribution of coal combustion in 2018 indicates that the coal‐control policy needs to be reinforced.

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