Ultrafine particles in the planetary boundary layer were considered to be climate and health relevant. The climate effect is based on their ability to act as cloud condensation nuclei with an impact on cloud microphysics and rainfall distribution, their health effect is due to the high mobility and deep penetration into the lungs where chemical components bound to the aerosols can be deposited. Although a network for monitoring of ultrafine particles (GUAN) now exists for several years in Germany the knowledge about sources and distribution in the planetary boundary layer is still limited. Airborne measurements were performed using small aircraft flying low and slow to characterize ultrafine particle number concentrations all over Germany from the Bavarian Alps towards the Northern Sea, to identify the major sources and to estimate their source strength and contribution to the overall ultrafine particle budget. Dominating single sources identified were all related to burning and processing of sulphur containing fossil fuel, highlighting the role of sulphur compounds for the generation of ultrafine particulate matter.