Characterization of emission factors of diesel trucks at a roadside at a causeway

Heavy-duty vehicles are a major contributor to on-road air pollution, largely because compression-ignition engines emit substantial amounts of nitrogen oxides and high numbers of fine and ultrafine particles (UFPs, <100 nm). UFPs are increasingly linked to cardiovascular diseases and neurological damages because they can deposit efficiently in alveoli region and may access the brain via olfactory pathways, respectively. However, UFP emissions are less regulated than other pollutants. Real-world particle emission factors for heavy-duty vehicles, particularly in the ultrafine range, remain poorly understood because they are usually quantified based on a specific engine under controlled lab conditions. It is also unclear how strongly fleet emissions are dominated by a small subset of high emitters. In this study, we address these gaps by quantifying real-world particle number EFs from a large population of heavy-duty vehicles under roadside conditions.

We conducted roadside measurements during three days (October–November 2025) on the causeway to Global Container Terminals Delta port in the Greater Vancouver Area, Canada, capturing exhaust plumes from thousands of passing vehicles using ambient air sampling rather than direct tailpipe measurements. The site was dominated by freight vehicle traffic. Therefore, the air pollutants were predominantly from trucks, with minimal interference from other sources due to its marine surroundings. Vehicles travel at relatively constant speeds along this straight roadway, such that the measurements primarily reflect steady-state driving conditions rather than a full range of operating modes (e.g., acceleration or transient conditions). Hence, this study provides insight into fleet variability under steady-state operating conditions but does not represent the full spectrum of real-world driving behavior. With the mobile laboratory, we measured CO2 and particle number concentrations with two water-based Condensation Particle Counters (CPCs; 2 nm and 7 nm particle diameter cut-offs), complemented by particle size distribution measurement using Fast Mobility Particle Sizer and co-pollutant/speciation measurements (NOx, organic carbon by aerosol mass spectrometer, light absorption properties of carbonaceous particles at 870nm and 405nm wavelengths). With more than a thousand plumes captured, individual plumes were identified in the time series and emission factors were derived from plume-integrated particle and CO₂ enhancements using a carbon-balance method. Preliminary analysis suggests that the particle number EF ranges from 10^14 to 10^17 per kg diesel, with the mean on the order of 10^15 per kg diesel. Notable imbalance in emissions was observed across the fleet, with approximately 20% of the vehicles accounting for more than 50% of particle number emissions, showing that high emitters have a disproportionate impact on UFPs. These findings provide insight into the consideration of UFPs in future monitoring and mitigation strategies.