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

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 (delta N-15 and delta O-18) of NO3- and dual modelling at five Chinese megacities (Beijing, Shanghai, Guangzhou, Wuhan and Chengdu) during 2013-2014. Results showed that the delta N-15-NO3- values (n = 512) ranged from -12.3 parts per thousand to +22.9 parts per thousand, and the average delta O-18-NO3- value was +83.4 parts per thousand +/- 17.2 parts per thousand. 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 center dot 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 < 0.01). Coal combustion was the most important source of NOx (31.1%-41.0%), which was geographically derived from North China and other south-central developed regions implied by Potential Source Contribution Function (PSCF). Apart from Chengdu, mobile sources was the second largest contributor to NOx. This source was extensive but uniformly distributed all around the typical urban agglomerations of China. Biomass burning and microbial processes shared similar source areas, mostly originating from the North China Plain and Sichuan Basin. Based on the NOx features, we infer that residential coal combustion was the primary source of heavy PM2.5 pollution in Chinese megacities. Controlling the source categories of these regional priorities would help mitigate atmospheric pollution in these areas