Unregulated delayed emissions dominate ultrafine particles from modern diesel vehicles.

Diesel exhaust is a major source of urban particulate matter (PM), especially ultrafine particles, with implications for air quality and public health. Current studies and emission standards mainly target directly emitted, non-volatile particles, whereas temperature-sensitive particles formed or transformed during post-tailpipe cooling remain poorly constrained. Here, we operationally define delayed particulate matter (DPM) as a thermally removable particle fraction and quantify it using temperature-resolved measurements from 30 to 300 °C, combined with chassis-dynamometer and tunnel observations. China V and China VI diesel vehicles exhibited comparable cycle-average particle number emission factors (1.84-4.90 ×1011 km-1), but showed distinct size peaks (33 versus 63 nm), indicating the contribution of DPM. DPM accounted for 46.2% of China V particle number emissions (1.30 ×1011 km-1) and increased 39-fold in China VI vehicles under ultra-high-speed operation (5.11 ×1012 km-1). Episodic diesel particulate filter (DPF) regeneration increased ultrafine particle emissions by 104-fold, with a peak of 1.4 × 1014 km-1, and was accompanied by higher levels of low-volatility organic components. China VI vehicles also emitted abundant low-volatility oxygenated semi-/intermediate-volatility organic compounds (S/IVOCs), including ketones and esters, consistent with enhanced DPM formation potential. Tunnel observations further showed persistent ultrafine particle emissions from diesel-related sources, supporting the fleet-scale relevance of laboratory-derived DPM features under near-source conditions. These findings provide experimental evidence that DPM represents a measurable temperature-sensitive ultrafine fraction in modern diesel exhaust that is not fully captured by conventional non-volatile particle measurements.