Cities show high concentrations of ultrafine particles due to multiple emission sources such as traffic and industry. To analyse turbulent urban surface–atmosphere exchange of particles, we quantified multi-annual size-resolved particle number fluxes in Berlin, Germany. The site was a net source of particles with a dominant contribution of traffic-related emission, especially very small particles < 30 nm. Particle fluxes clearly varied as a function of anthropogenic activity and urban land use.