Sources and evolution of ultrafine particles were investigated both horizontally and vertically in the large urban agglomerate of Barcelona, Spain. Within the SAPUSS project (Solving Aerosol Problems by Using Synergistic Strategies), a large number of instruments was deployed simultaneously at different monitoring sites (road, two urban background, regional background, urban tower 150 m a.s.l., urban background tower site 80 m a.s.l.) during a 4 week period in September–October 2010. Particle number concentrations (<i>N</i><sub>>5 nm</sub>) are highly correlated with black carbon (BC) at all sites only under strong vehicular traffic influences. By contrast, under cleaner atmospheric conditions (low condensation sink, CS) such correlation diverges towards much higher <i>N</i>/BC ratios at all sites, indicating additional sources of particles including secondary production of freshly nucleated particles. Size-resolved aerosol distributions (<i>N</i><sub>10–500</sub>) as well as particle number concentrations (<i>N</i><sub>>5 nm</sub>) allow us to identify three types of nucleation and growth events: (1) a regional type event originating in the whole study region and impacting almost simultaneously the urban city of Barcelona and the surrounding urban background area; (2) a regional type event impacting only the regional background area but not the urban agglomerate; (3) an urban type event which originates only within the city centre but whose growth continues while transported away from the city to the regional background. Furthermore, during these clean air days, higher <i>N</i> are found at tower level than at ground level only in the city centre whereas such a difference is not so pronounced at the remote urban background tower. In other words, this study suggests that the column of air above the city ground level possesses the optimal combination between low CS and high vapour source, hence enhancing the concentrations of freshly nucleated particles. By contrast, within stagnant polluted atmospheric conditions, higher <i>N</i> and BC concentrations are always measured at ground level relative to tower level at all sites. Our study suggests that the city centre of Barcelona is a source of non-volatile traffic primary particles (29–39% of <i>N</i><sub>>5 nm</sub>), but other sources, including secondary freshly nucleated particles contribute up to 61–71% of particle number (<i>N</i><sub>>5 nm</sub>) at all sites. We suggest that organic compounds evaporating from freshly emitted traffic particles are a possible candidate for new particle formation within the city and urban plume.