Identifying gross particle emitters: Insights from periodic technical inspections of diesel vehicles

Periodic technical inspections (PTI) are used globally to ensure in-use vehicle fleet roadworthiness. Since 2022, five European countries have added a particle number test (PN-PTI) to PTI for diesel vehicles with particulate filters to identify high ultrafine particle emitters and mandate repairs. This study examines PN-PTI results from nearly two million Euro 5 and Euro 6 diesel vehicles inspected between 2022 and 2024 in the Flemish region of Belgium and the Netherlands and evaluates different PN limits. The analysis reveals lower failure rates in newer vehicles meeting more recent Euro standards and significant differences between brands. High mileage increases failure rates in older vehicles but has little effect on newer ones. Using three machine learning models, the study identifies first registration year as generally the most important feature associated with PN-PTI failure, followed by brand and mileage. Overall, PN-PTI emerges as an effective policy tool for mitigating gross particle emitters.