Road traffic emissions are often considered the main source of ultrafine particles (UFP, diameter smaller than 100 nm) in urban environments. However, recent studies worldwide have shown that – in high-insolation urban regions at least – new particle formation events can also contribute to UFP. In order to quantify such events we systematically studied three cities located in predominantly sunny environments: Barcelona (Spain), Madrid (Spain) and Brisbane (Australia). Three long-term data sets (1–2 years) of fine and ultrafine particle number size distributions (measured by SMPS, Scanning Mobility Particle Sizer) were analysed. Compared to total particle number concentrations, aerosol size distributions offer far more information on the type, origin and atmospheric evolution of the particles. By applying k-means clustering analysis, we categorized the collected aerosol size distributions into three main categories: " Traffic " (prevailing 44–63 % of the time), " Nucleation " (14–19 %) and " Background pollution and Specific cases " (7–22 %). Measurements from Rome (Italy) and Los Angeles (USA) were also included to complement the study. The daily variation of the average UFP concentrations for a typical nucle-ation day at each site revealed a similar pattern for all cities, with three distinct particle bursts. A morning and an evening spike reflected traffic rush hours, whereas a third one at midday showed nucleation events. The photochemically nucle-ated particles' burst lasted 1–4 h, reaching sizes of 30–40 nm. On average, the occurrence of particle size spectra dominated by nucleation events was 16 % of the time, showing the importance of this process as a source of UFP in urban environments exposed to high solar radiation. Nucleation events lasting for 2 h or more occurred on 55 % of the days, this extending to > 4 h in 28 % of the days, demonstrating that atmospheric conditions in urban environments are not favourable to the growth of photochemically nucleated particles. In sum-Published by Copernicus Publications on behalf of the European Geosciences Union. 5930 M. Brines et al.: Traffic and nucleation events as main sources of ultrafine particles mary, although traffic remains the main source of UFP in urban areas, in developed countries with high insolation urban nucleation events are also a main source of UFP. If traffic-related particle concentrations are reduced in the future, nu-cleation events will likely increase in urban areas, due to the reduced urban condensation sinks.