Characterization of the nitrogen stable isotope composition (δ15N) of ship-emitted NOx

The nitrogen stable isotope composition (δ15N) of nitrogen oxides (NOx) is a powerful indicator of source apportionment of atmospheric NOx; however, δ15N–NOx values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NOx in coastal zones with a lot of vessel activity. In addition, δ15N–NOx values from ship emissions could also be important for source apportionment of atmospheric nitrogen deposition in remote ocean regions. This study systemically analysed the δ15N–NOx variability and main influencing factors of ship emissions. The results showed that δ15N–NOx values from ships, which were calculated by weighting the emission values from the main engine and auxiliary engine of the vessel, ranged from -35.8 ‰ to 2.04 ‰ with a mean ± standard deviation of -18.5 ± 10.9 ‰. Theδ15N–NOx values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NOx concentrations (p