Upgrading Diesel Vehicle After-Treatment Technology Significantly Changed the Composition of Reactive Nitrogen in Exhaust Emissions.

The progressive implementation of advanced after-treatment systems in diesel vehicles has fundamentally reshaped the emission characteristics of reactive nitrogen compounds (RNCs). To elucidate these changes, we systematically investigated RNC transformations across light-duty diesel vehicles under China's IV, V, and VI emission standards, employing a synergistic approach combining chassis dynamometer testing and controlled experiments. Our analysis reveals a fundamental shift in nitrogen speciation: NO accounted for more than 80% of reactive nitrogen emissions under China IV vehicles, whereas NH3 and N2O became more evident in China VI vehicles. Controlled experiments showed that diesel oxidation catalysts promoted the oxidation of NO to NO2, thereby increasing the NO2/NOx ratio under several operating conditions, selective catalytic reduction systems induced significant NH3 slip, and ammonia slip catalysts promoted N2O production. Furthermore, SHAP interpretation of tree-based models identified emission standard, exhaust temperature, and vehicle speed as the dominant drivers of RNC speciation. We further propose a mechanistic framework to explain the unintended redistribution of reactive nitrogen caused by after-treatment upgrades. These findings provide critical insights for developing integrated emission-control strategies and advancing urban reactive nitrogen management.