Abstract. Sub-50 nm particles originating from traffic emissions pose risks to human health due to their high lung deposition efficiency and potentially harmful chemical composition. We present a modelling study using an updated EUCAARI number emission inventory, incorporating a more realistic, empirically justified particle size distribution (PSD) for sub-50 nm particles from road traffic. We present experimental PSDs and CO2 concentrations, measured in a highly trafficked street canyon in Helsinki, Finland, as an emission factor particle size distribution (EFPSD), which was then used in updating the EUCAARI inventory. We applied the updated inventory in a simulation using the regional chemical transport model PMCAMx-UF over Europe for May 2008 to test the effect of updated emissions in regional and local scales and in contrast to atmospheric new particle formation (NPF). Updating the inventory increased simulated average total particle number concentrations by only 1 %, although the total particle number emissions were increased to a 3-fold level. The concentrations increased up to 11 % when only 1.3–3 nm-sized particles (nanocluster aerosol, NCA) were considered. These values indicate that the effect of updating overall is insignificant in a regional scale during this photochemically active period, during which the fraction of the total particle number originating through atmospheric NPF processes was 91 %. These simulations give a lower limit for the contribution of traffic to the aerosol levels. Nevertheless, the situation is different when examining the effect of the update spatially or temporally, or when focusing to the chemical composition or the origin of the particles. For example, daily average NCA concentrations increased by a factor of several hundreds or thousands in some locations on certain days. Overall, the most significant effects–reaching several orders of magnitude–from updating the inventory are observed when examining specific particle sizes (especially 7–20 nm), particle components, and specific urban areas. While the model still has a tendency to predict more sub-50 nm particles compared to the observations, the most notable underestimations in the concentrations of sub-10 nm particles are, after updating, overcome and the simulated distributions now agree better with the data observed at locations having high traffic densities. The findings of this study highlight the need to consider emissions, PSDs, and composition of sub-50 nm particles from road traffic in studies focusing on urban air quality. Updating this emission source brings the simulated aerosol levels particularly in urban locations closer to observations, which highlights its importance for calculations of human exposure to nanoparticles.
Particle size distribution and particle mass measurements at urban, near-city and rural level in the Copenhagen area and Southern Sweden
