Dual-modelling-based source apportionment of NOx in five Chinese megacities: Providing the isotopic footprint from 2013 to 2014

[Display omitted] •The N and O isotopic values both have a rising tendency as the temperature drops.•Proportion for the OH pathway correlates significantly with latitude all year round.•Coal combustion is the primary source of NOx at Chinese megacities, especially in winter.•Residential coal combustion is the major source for the high PM2.5 pollution. In China, nitrate (NO3−) becomes the main contributor to fine particles (PM2.5) because the emissions of its precursor, nitrogen oxides (NOx), were not recognized and controlled well in recent years. In this work, sources, conversion, and geographical origin of NOx were interpreted combining the isotopic information (δ15N and δ18O) of NO3− and dual modelling at five Chinese megacities (Beijing, Shanghai, Guangzhou, Wuhan and Chengdu) during 2013–2014. Results showed that the δ15N-NO3− values (n = 512) ranged from −12.3‰ to +22.9‰, and the average δ18O-NO3− value was +83.4‰ ± 17.2‰. The isotopic compositions both had a rising tendency as ambient temperature dropped, attributing largely to the source changes. Bayesian model indicated the percentage for the OH pathway of NOx conversion had a clear seasonal variation with a higher value during summer (58.0% ± 9.82%) and a lower value during winter (11.1% ± 3.99%); it was also significantly correlated with latitude (p