Characterization of the nitrogen stable isotope composition (<i>δ</i>¹⁵N) of ship-emitted NO_<i>x</i>

Abstract

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 ‰. The δ15N–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 δ15N–NOx values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between δ15N–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 δ15N–NOx 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 (NO3-) air quality and nitrogen deposition studies.