<p>In spite of the important advances in the science of aerosols and air quality, important scientific and environmental challenges remain unsolved, especially those related to source apportionment of the specific components of atmospheric particulate matter (PM), atmospheric processes influencing aerosols, and the associated climate and health impacts. Moreover, ultrafine particle (UFP) studies are growing, they are still insufficient and much needed. Furthermore, there is a clear lack of information and guidance on UFP measurement, especially in smaller ranges. In addition, it is widely recognised that exposure to PM negatively impacts human health (WHO, 2021). In 2016, ambient air pollution accounted for almost seven million premature deaths per year (WHO, 2016), as derived from the aggravation of cardiovascular and respiratory diseases and cancers. Several studies have also shown that UFP can deeply penetrate the respiratory system, thus causing respiratory and cardiovascular diseases in humans (Cassee et al., 2019).</p>
<p>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 RI-URBANS project. The main objective was to describe the phenomenology of urban ultrafine particles in Europe with a significant air quality focus. The varying lower size detection limits made it difficult to compare PN concentrations (PNC), particularly PN<sub>10-25</sub>, 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<sub>10-25</sub>, suggesting significant PNC contributions from nucleation, fumigation and shipping.</p>
<p>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<sub>25-800</sub>, was positively correlated with BC, NO<sub>2</sub>, 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.</p>
<p>WHO, 2021. Ambient (outdoor) air pollution. 22 September 2021, https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health</p>
<p>WHO, 2016. Ambient air pollution: a global assessment of exposure and burden of disease. World Health Organization., 121 pp, https://apps.who.int/iris/handle/10665/250141</p>
<p>Cassee F., Morawska L., Peters A. (Eds)., 2019. The White Paper on Ambient Ultrafine Particles: evidence for policy makers. &#8216;Thinking outside the box&#8217; Team, October 2019, 23 pp, https://efca.net/files/WHITE%20PAPER-UFP%20evidence%20for%20policy%20makers%20(25%20OCT).pdf</p>
