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

<p>The nitrogen stable isotope composition (<span class="inline-formula"><i>δ</i>¹⁵</span>N) of nitrogen oxides (NO<span class="inline-formula">_<i>x</i>)</span> is a powerful indicator of source apportionment of atmospheric NO<span class="inline-formula">_<i>x</i></span>; however, <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values emitted from ships have not been reported, affecting the accuracy of source partitioning of atmospheric NO<span class="inline-formula">_<i>x</i></span> in coastal zones with a lot of vessel activity. In addition, <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values from ship emissions could also be important for source apportionment of atmospheric nitrogen deposition in remote ocean regions. This study systemically analysed the <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> variability and main influencing factors of ship emissions. The results showed that <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values from ships, which were calculated by weighting the emission values from the main engine and auxiliary engine of the vessel, ranged from <span class="inline-formula">−</span>35.8 ‰ to 2.04 ‰ with a mean <span class="inline-formula">±</span> standard deviation of <span class="inline-formula">−</span>18.5 <span class="inline-formula">±</span> 10.9 ‰. The <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values increased monotonically with the ongoing tightening of emission regulations, presenting a significantly negative logarithmic relationship with NO<span class="inline-formula">_<i>x</i></span> concentrations (<span class="inline-formula"><i>p</i>&lt;0.01</span>). The selective catalytic reduction (SCR) system was the most important factor affecting changes in <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values, followed by the ship category, fuel types, and operation states of ships. Based on the relationship between <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> 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 <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values from ship emissions in the international merchant fleet was evaluated. These simulated <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values can be used to select suitable <span class="inline-formula"><i>δ</i>¹⁵</span>N–NO<span class="inline-formula">_<i>x</i></span> values for a more accurate assessment, including the contribution of ship-emitted exhaust to atmospheric NO<span class="inline-formula">_<i>x</i></span> and its influence on atmospheric nitrate (NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="12pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="5cff292f284d562dc6d0310a4128574e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-12851-2023-ie00001.svg" width="12pt" height="16pt" src="acp-23-12851-2023-ie00001.png"/></svg:svg></span></span> air quality and nitrogen deposition studies.</p>