Nanoparticle Emissions of Ageing Diesel Cars with DPF—A First European PTI-like Field Study

This paper investigates nanoparticle emissions from older diesel passenger cars in four European countries outside of PTI (periodic technical inspection) testing facilities, using a methodology similar to that of the PTI. Per opportunistic sampling and voluntary testing, 618 diesel cars with an average mileage of 163,348 km were measured between 2020 and 2022 on the streets around university campuses and in public areas in Brussels (Belgium), Aveiro (Portugal), Belgrade (Serbia) and Birkenfeld (Germany). Through subsequent research, 109 vehicles were found not to be equipped with a diesel particulate filter (DPF) and were separated. Portable nanoparticle detectors were used to measure PN emissions in a PTI-like approach at low idle. From the 509 analysed diesel cars equipped with DPF, a high share of 31% revealed PN tailpipe concentrations of over 250 kP/cm3 and would therefore have failed the PTI established in Germany. That rate is 2–9 times higher than that found in Belgian and German PTI PN measurements, possibly caused by the fact that vehicles are serviced immediately before the PTI. Moreover, 45% of all tested diesel cars with DPF emitted concentrations higher than 20 kP/cm3. This level was found, both in the literature and in our measurements, to be indicative of a DPF operating outside its normal performance range, taking into account the measurement uncertainty. Even among relatively new vehicles with DPF (up to mileages of 180,000 km), 13% emitted as much PN as diesel cars without DPF (1000 kP/cm3 or more). In Portugal and Serbia, a higher share of cars showed elevated emissions compared to Germany and Belgium. DPF technology can in principle greatly reduce particle emissions of diesel cars. However, its efficiency degrades with time and vehicle mileage. Several factors leading to higher PN emissions have been reported; however, they cannot be discriminated in the results. This is primarily because active filter regeneration generates particle emission peaks every several 100 km, which were found to be up to four orders of magnitude higher than normal. Within the fleet on the streets, up to 4% of all diesel cars equipped with a modern DPF can be found in active regeneration mode. While at present politics and carmakers focus on electrifying the new vehicle generation, the emission problem of around 69 million diesel cars with elevated PN emissions on European streets remains unresolved so far. In European countries with a particularly high share of diesel cars, an old vehicle fleet, and a low income level, emission problems may be exacerbated.