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

The nitrogen stable isotope composition (delta N-15) of nitrogen oxides (NOx) is a powerful indicator of source apportionment of atmospheric NOx; however, delta N-15-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, delta N-15-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 delta N-15-NOx variability and main influencing factors of ship emissions. The results showed that delta N-15-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 parts per thousand to 2.04 parts per thousand with a mean +/- standard deviation of -18.5 +/- 10.9 parts per thousand. The delta N-15-NOx 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 delta N-15-NOx values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between delta N-15-NOx 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 delta N-15-NOx values from ship emissions in the international merchant fleet was evaluated. These simulated delta N-15-NOx values can be used to select suitable delta N-15-NOx values for a more accurate assessment, including the contribution of ship-emitted exhaust to atmospheric NOx and its influence on atmospheric nitrate (NO air quality and nitrogen deposition studies.