Importance of sub-23 nm particles in traffic environments: Particle number emission factors and extrathoracic deposition doses

New research suggests that exposure to ultrafine particles (UFPs; particle diameter d_p < 100 nm) is particularly harmful to brain health. One pathway into the body is via deposition in the respiratory system, where the smallest UFPs deposit efficiently in human extrathoracic airways. Traffic is a major source of these particles, yet sub-23 nm (d_p < 23 nm) particles are currently unregulated in engine emission testing worldwide, including the stringent requirements of the European Union, nor are there requirements for ambient monitoring. In this study, we report size-resolved particle number emission factors (EFs) for traffic and estimates of extrathoracic dose rates of sub-23 nm particles. The EFs and dose rates are based on measurements conducted in different urban environments, including roads, tunnels, an airport, and a riverside, in two Central European cities (Düsseldorf and Prague) from March to April 2022. A key difference between the cities is that Düsseldorf has a low-emission zone in its central area and a newer vehicle fleet compared to Prague. Overall, traffic-influenced sites had large EFs for sub-23 nm particles. In the highway and tunnel environments, EFs of particles with d_p > 2.5 nm were between 2 and 18 times greater than the EFs of particles with d_p > 23 nm. Near the airport, the EF of particles with d_p > 23 nm was already high, being 2–9 times higher than in other environments. The number concentrations of sub-23 nm particles varied significantly within the studied cities, and dose rates (measured in billions of particles per hour) differed by up to a factor of ten or more depending on the location.