With state-of-the-art particle instruments, the nPETS consortium monitored and sampled the ultrafine transport-generated particles from shipping, road, rail, and aviation in the field and controlled laboratory environments. This includes aged and fresh aerosols and primary and secondary components. Characterisation of the emissions includes linking their sizes, chemical compositions, and morphologies to their specific emission sources. All quantification and monitoring have been performed with the same type of instruments to ensure comparable measurement results. The characterisation also includes toxicology studies based on sampled particles from filters, in situ cell exposure with an Air Liquid Interface system that can mimic the interactions of airborne particles with lung cells and a zebrafish embryo model.
Ultrafine particle emissions have been identified in all field studies, i.e. street canyons, urban backgrounds, harbours, airports, and train stations. The 2021 Air Quality Guidelines from the WHO include both recommended low and high limit values. Most measured ultrafine particle number concentration levels are above the high-level limit, and all are above the low-level limit. The cleanest environments were urban background sites and subways, while airports and road sites including road tunnels were the most polluted environments. Further, at all sites we observed secondary particle formations and at some locations they were dominating. Toxic effects were linked to car and ship emissions and were higher in the cold season than in the warm season. Chemical analyses followed by correlation and source apportionment analyses identified PAHs and combustion processes as the main drivers of the observed toxic effects.
All studied non-exhaust sources—including mechanical brakes, clutches, tire-to-road interactions, railway wheel-to-rail contacts, and electric power systems for trains—were found to produce ultrafine particle emissions. Gaseous emissions from brake wear were identified as an unexpected source of secondary particulate matter.
Lab testing revealed significant differences in the chemical composition of ultrafine particles compared to larger particles with diameters up to 2.5 micrometers. Based on this study, it can be concluded that chemical data from coarser non-exhaust emission fractions should not be used to assess the toxicological behavior of nanoparticle emissions generated by the same sources.
The nPETS project also conducted extensive testing on five modern vehicles, all of which were confirmed to be low emitters and compliant with Euro 6 limits. However, testing of a Euro 6d diesel vehicle, including during diesel particle filter regeneration, revealed a significant impact of regeneration events, with over 95% of the total particle number emitted during these periods. Preliminary analysis further suggested a decrease in cell viability linked to regeneration particles.
Similar to the toxicology results from the nPETS field campaigns, the findings from the laboratory campaigns were also converted into toxicology scores. The greatest variation was observed in car disc brake friction materials, with results positioned between those of tailpipe exhaust emission.
A database of measurement results has been compiled and made publicly available, representing a significant and unique contribution by nPETS. In addition, regular collaboration with parallel EU funded projects (LEON-T and ULTRHAS) focusing on particle emissions has facilitated interdisciplinary engagement, resulting in the forthcoming publication of a jointly authored policy paper.
