The 2017-2019 hourly particle number size distributions (PNSD) from 26 sites in Europe and 1 in the US were evaluated focusing on 16 urban background (UB) and 6 traffic (TR) sites in the framework of Research Infrastructures services reinforcing air quality monitoring capacities in European URBAN & industrial areaS (RI-URBANS) project. The main objective was to describe the phenomenology of urban ultrafine particles (UFP) in Europe with a significant air quality focus. The varying lower size detection limits made it difficult to compare PN concentrations (PNC), particularly PN_10-25, from different cities. PNCs follow a TR > UB > Suburban (SUB) order. PNC and Black Carbon (BC) progressively increase from Northern Europe to Southern Europe and from Western to Eastern Europe. At the UB sites, typical traffic rush hour PNC peaks are evident, many also showing midday-morning PNC peaks anti-correlated with BC. These peaks result from increased PN_10-25, suggesting significant PNC contributions from nucleation, fumigation and shipping. Site types to be identified by daily and seasonal PNC and BC patterns are: (i) PNC mainly driven by traffic emissions, with marked correlations with BC on different time scales; (ii) marked midday/morning PNC peaks and a seasonal anti-correlation with PNC/BC; (iii) both traffic peaks and midday peaks without marked seasonal patterns. Groups (ii) and (iii) included cities with high insolation. PNC, especially PN_25-800, was positively correlated with BC, NO₂, CO and PM for several sites. The variable correlation of PNSD with different urban pollutants demonstrates that these do not reflect the variability of UFP in urban environments. Specific monitoring of PNSD is needed if nanoparticles and their associated health impacts are to be assessed. Implementation of the CEN-ACTRIS recommendations for PNSD measurements would provide comparable measurements, and measurements of <10 nm PNC are needed for full evaluation of the health effects of this size fraction.

There is a body of evidence that ultrafine particles (UFP, those with diameters ≤ 100 nm) might have significant impacts on health. Accordingly, identifying sources of UFP is essential to develop abatement policies. This study focuses on urban Europe, and aims at identifying sources and quantifying their contributions to particle number size distribution (PNSD) using receptor modelling (Positive Matrix Factorization, PMF), and evaluating long-term trends of these source contributions using the non-parametric Theil-Sen's method. Datasets evaluated include 14 urban background (UB), 5 traffic (TR), 4 suburban background (SUB), and 1 regional background (RB) sites, covering 18 European and 1 USA cities, over the period, when available, from 2009 to 2019. Ten factors were identified (4 road traffic factors, photonucleation, urban background, domestic heating, 2 regional factors and long-distance transport), with road traffic being the primary contributor at all UB and TR sites (56-95 %), and photonucleation being also significant in many cities. The trends analyses showed a notable decrease in traffic-related UFP ambient concentrations, with statistically significant decreasing trends for the total traffic-related factors of -5.40 and -2.15 % yr^-1 for the TR and UB sites, respectively. This abatement is most probably due to the implementation of European emissions standards, particularly after the introduction of diesel particle filters (DPFs) in 2011. However, DPFs do not retain nucleated particles generated during the dilution of diesel exhaust semi-volatile organic compounds (SVOCs). Trends in photonucleation were more diverse, influenced by a reduction in the condensation sink potential facilitating new particle formation (NPF) or by a decrease in the emissions of UFP precursors. The decrease of primary PM emissions and precursors of UFP also contributed to the reduction of urban and regional background sources.

Ultrafine particles (UFP, those with diameters ≤ 100 nm), have been reported to potentially penetrate deeply into the respiratory system, translocate through the alveoli, and affect various organs, potentially correlating with increased mortality. The aim of this study is to assess long-term trends (5-11 years) in mostly urban UFP concentrations based on measurements of particle number size distributions (PNSD). Additionally, concentrations of other pollutants and meteorological variables were evaluated to support the interpretations. PNSD datasets from 12 urban background (UB), 5 traffic (TR), 3 suburban background (SUB) and 1 regional background (RB) sites in 15 European cities and 1 in the USA were evaluated. The non-parametric Theil-Sen's method was used to detect monotonic trends. Meta-analyses were carried out to assess the overall trends and those for different environments. The results showed significant decreases in NO, NO₂, BC, CO, and particle concentrations in the Aitken (25-100 nm) and the Accumulation (100-800 nm) modes, suggesting a positive impact of the implementation of EURO 5/V and 6/VI vehicle standards on European air quality. The growing use of Diesel Particle Filters (DPFs) might also have clearly reduced exhaust emissions of BC, PM, and the Aitken and Accumulation mode particles. However, as reported by prior studies, there remains an issue of poor control of Nucleation mode particles (smaller than 25 nm), which are not fully reduced with current DPFs, without emission controls for semi-volatile organic compounds, and might have different origins than road traffic. Thus, contrasting trends for Nucleation mode particles were obtained across the cities studied. This mode also affected the UFP and total PNC trends because of the high proportion of Nucleation mode particles in both concentration ranges. It was also found that the urban temperature increasing trends might have also influenced those of PNC, Nucleation and Aitken modes.

Ultrafine particles (UFP, particles sized <100 nm) significantly impact health and the environment. However, their study is still a challenge, and a specific regulation is required. The measurement of UFP-PSD (Ultrafine Particles – Particle Size Distribution) and its application in air quality assessment is a significant goal of RI-URBANS. 2017-2019 hourly UFP-PSD data from 26 sites in urban Europe and one site in the US have been compiled and evaluated according to the instrumental and methodological approaches implemented; the comparison of urban concentrations across Europe; the identification of similarities and major differences; and the evaluation of relationships with other pollutants, such as BC, PMx and gaseous pollutants (SO2, NOx, O3, CO), and with meteorological parameters. The results of this study have recently been published by Trechera et al. (2023).To continue the study by Trechera et al. (2023) in UFP, source apportionment for the previous 27 urban sites is being analyzed with an extended period from 2009 to 2019. This study aims to identify and quantify sources contributing to UFP-PSD using Positive Matrix Factorization (PMF), comparing the differences between the sites. PMF is a widely used multivariate factor analysis tool to identify the source types that may be contributing to the sample using measured source profile information. According to a recent review on UFP source apportionment based on UFP-PSD measurements by Hopke et al. (2022), the typically reported sources of UFP include nucleation, several traffic sources (fresh to aged), domestic and residential heating, regional secondary inorganic aerosols (i.e., regional nitrate and sulfate), particles associated with oxidants as represented by O3 (i.e., regional secondary organic and inorganic aerosols) and other sources (such as biomass burning, urban background sources, industrial emissions, diverse sources, dust and unknown sources). In the near future we expect to publish the results of source apportionment for the datasets mentioned above. Cassee F., Morawska L., Peters A. (Eds)., 2019. The White Paper on Ambient Ultrafine Particles: evidence for policy makers. ‘Thinking outside the box’ Team, October 2019, 23 pp, https://efca.net/files/WHITE%20PAPER-UFP%20evidence%20for%20policy%20makers%20(25%20OCT).pdfHopke, P.K., Feng, Y., Dai, Q., 2022. Source apportionment of particle number concentrations: A global review. Sci. Total Environ. 819, 153104. https://doi.org/10.1016/j.scitotenv.2022.153104Paatero, P., and U. Tapper, 1994. Positive matrix factorization: A non-negative factor model with optimal utilization of error estimates of data values., Environmetrics, 5, 111–126, doi:10.1002/env.3170050203Rivas, I., Vicens, L., Basagaña, X., Tobías, A., Katsouyanni, K., Walton, H., Hüglin, C., Alastuey, A., Kulmala, M., Harrison, R.M., Pekkanen, J., Querol, X., Sunyer, J., Kelly, F.J., 2021. Associations between sources of particle number and mortality in four European cities. Environ. Int. 155. https://doi.org/10.1016/j.envint.2021.106662Trechera, P., Garcia-Marlès, M., Liu, X., et al., 2023. Phenomenology of ultrafine particle concentrations and size distribution across urban Europe. Environ. Int. Accepted (Publication in progress).