Gasoline Vehicular NO _<i>x</i> Emissions and Isotopic Signatures Revealed by Chassis Dynamometer Measurements

Nitrogen oxides (NOx) emitted from gasoline vehicles are major contributors to urban air pollution, yet accurately quantifying their absolute emissions and relative contributions remains challenging. Here, we present comprehensive measurements of NOx emission factors (EFs) and nitrogen isotopic compositions (δ15N-NOx) from controlled chassis dynamometer experiments on light-duty gasoline vehicles spanning China-3 to China-6 emission standards (based on European standards). The results reveal a pronounced technology-driven decline in emissions, with fleet-averaged hot-start NOx EFs decreasing by more than 90%, from 99.3 ± 104.9 mg km–1 for China-3 to only 7.0 ± 5.6 mg km–1 for China-6 vehicles under the Worldwide Harmonized Light Vehicles Test Cycle. In contrast, δ15N-NOx exhibits a nonlinear evolution, peaking at +9.7 ± 4.6% for China-5 before declining to +0.4 ± 4.6% for China-6, primarily due to on-catalyst NOx-NH3 isotope exchange. These measurements underpin two dynamic modeling frameworks for EFs and isotopic signatures, enabling the derivation of city- and year-specific parameters that refine emission inventories and constrain relative contribution of vehicular NOx. Our findings resolve long-standing biases from static assumptions and establish a consistent foundation for assessing how evolving vehicle technologies may influence atmospheric NOx burdens.