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-NO_x variability and main influencing factors of ship emissions. The results showed that δ¹⁵N-NO_x 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 ‰. The δ¹⁵N-NO_x values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NOx concentrations (p<0.01). The selective catalytic reduction (SCR) system was the most important factor affecting changes in δ¹⁵N-NO_x values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between δ¹⁵N-NO_x values and emission regulations observed in this investigation, a mass-weighted model to compute accurate assessments over time was developed, and the temporal variation in δ¹⁵N-NO_x values from ship emissions in the international merchant fleet was evaluated. These simulated δ15N-NOx values can be used to select suitable δ15N-NOx values for a more accurate assessment, including the contribution of ship-emitted exhaust to atmospheric NOx and its influence on atmospheric nitrate (NO^3-) air quality and nitrogen deposition studies.