Ultrafine particles (UFP), the nano fraction of airborne particulate matter, are recognised as a major health risk factor in the WHO Guidance, considered to cause serious environmental effects and have a significant climate impact. The most important emission sector is transport of all kinds through direct particle emissions from vehicles, ships and aircraft engines but also by producing volatile organic pollutants which are converted in the atmosphere through photochemical reactions. UFPs health effects are costantly demonstrated at all scales through indoor and ambient exposure. A further interest in UFP’s results from their specific role in atmospheric processes such as cloud formation and precipitation and in climate. In particular, UfPs contribute to the Short-Lived Climate Pollutants (SLCPs), in particular black carbon (BC) and organic aerosols. The relation between UFP and human health and that of UFP and climate are both areas of active research and cross-links between these fields are more and more found nowadays. EFCA is committed to promote a “ONE-ATMOSPHERE” approach to the framing of air/climate protection policies and to the standard/metric of UfPs in cooperation with international organizations. Therefore, the subtitle of the symposium series: “air quality and climate” reflects this constant development. However, the present policies to decrease exposure to particulate matter make use of the mass-based metrics PM10 and PM2.5, which do not properly represent all risks for human health. EFCA is therefore in favour of the development of a fraction-by-fraction approach on particulate matter, both with respect to size and chemical composition including Black Carbon particles as an additional metric in the Air Quality Directive. The organizers trust that EFCA‘s 9th Ultrafine Particles Symposium 2024 again feature the most recent scientific progress in the field and so contribute to policy-relevant developments which improve the dialogue with policymakers in Europe and in the UNECE region. EFCA and KIT, ...