Recent Isotopic Evidence for Elevated Vehicular NOx Emission to Atmospheric Nitrate Formation in Chinese Megacities

As the primary component of air pollution, nitrogen oxides (NOx = NO + NO2) are responsible for the formation of ozone and fine particle matter (PM2.5), which together pose a threat to the environment and human health worldwide. Coal combustion is assumed to be a prominent emitter of atmospheric NOx in urban environments on the basis of nitrogen isotopic constraints (δ15N), but recently detailed NOx emission inventories are inconsistent with the extant isotopic estimates. To resolve this disagreement, we compiled and dissected the previously reported δ15N compositions of particulate nitrate in Chinese megacities over recent winter seasons from 2013 to 2017. The results show that the δ15N value of nitrate (δ15N-NO3 –) decreased from North China to South China in general (e.g., from +13.8 ± 5.0‰ in Beijing to +2.7 ± 2.5‰ in Chengdu), but little variation was observed in any given city during this period (e.g, from +11.5 ± 5.0‰ in 2018 to +13.8 ± 5.0‰ in 2014 in Beijing). More interestingly, the initial average δ15N-NOx values (from −5.8 to −2.4‰) compared well among all the urban cities studied here, derived from the corresponding δ15N-NO3 – values by correcting for the nitrogen isotopic fractionation effects. These results point toward the importance of NOx emissions from vehicular exhaust to nitrate formation in urban areas, which is estimated to account for 67.4 ± 10.1% on the basis of a Bayesian mixing model. The estimate agreed with reported data of local detailed emission inventory models (e.g., multiresolution emission inventory for China). Our results highlight that strengthening vehicle emission standards would benefit NOx abatements and improve the urban air quality in the near future.