Ultrafine particles (UFPs) are a major air quality concern because their small diameter (<100 nm) allows them to reach the lungs' alveolar regions, causing adverse health effects. Civil aviation and airports are important sources of UFPs in urban areas. In this study, a one-month intensive measurement campaign in November 2022, 1 km downwind of Zurich Airport, revealed that high UFP number concentrations up to 300 000 cm-3 originate solely from aircraft operations. These emissions are