Ultrafine particles in the atmosphere may have important climate and health effects. As they are below visible size and not visible for remote sensing techniques, the majority of observations thus come from ground-based measurements. Some of those observations indicate elevated sources for ultrafine particles. Here we present for the first time airborne measurements of number concentration and size distributions of ultrafine particles along defined flight paths across Germany, allowing to derive background concentrations and to identify major single sources. A significant impact of fossil fuel–related emissions on background and maximum concentrations was found. Maxima reaching up to 90 000 particles cm−3 were encountered in plumes of single large sources extending over more than 200 km. Modelling shows that about 10–40 % of Germany were continuously affected by such plumes. Regional-scale transport and boundary layer dynamics were identified as major factors controlling spatial and temporal patterns of size and number distributions.