Emission standard upgrades decouple nitro-PAH mass from toxicity in real-world fleets

Vehicular emissions are a major urban source of nitrated polycyclic aromatic hydrocarbons (NPAHs), yet the impact of recent emission standard upgrades on NPAH mass and toxicity remains unclear. We conducted two comprehensive tunnel campaigns in Guangzhou (2014 and 2019) to quantify changes in PM_2.5-bound parent PAHs and NPAHs from real-world vehicle fleets. Fleet-average mass-based emission factors (EFs) for total PAHs and total NPAHs decreased by 74.8 % (45.22-11.39 μg km^-1 veh^-1) and 57.8 % (7.06-2.98 μg km^-1 veh^-1), respectively. In contrast, the total carcinogenic potency of the NPAH mixture, expressed as benzo[a]pyrene toxic equivalents (TEQ_BaP), increased by 28.3 % (2.69-3.45 μg km^-1 veh^-1). This divergence reflects a compositional shift in the traffic fleet: in 2019 diesel vehicles constituted only 3.3 % of traffic but accounted for 91.0 % of NPAH emissions with enrichment in high-potency congeners such as dinitropyrenes. These real-world measurements demonstrate that mass-only metrics for regulated (PAHs) and unregulated (NPAHs) emissions can be misleading, and highlight the need for toxicity-weighted indicators and diesel-targeted control strategies to manage NPAH hazards in modern fleets.