Abstract. Atmospheric aerosols contribute some of the greatest uncertainties to estimates of global radiative forcing, and have significant effects on human health. New particle formation (NPF) is the process by which new aerosols of sub-2 nm diameter form from gas-phase precursors and contributes significantly to particle numbers in the atmosphere, accounting for approximately 50 % of cloud condensation nuclei globally. Here, we study summertime NPF in urban Barcelona in NE Spain. The rate of formation of new particles is seen to increase linearly with sulphuric acid concentration in a manner similar to systems studied in chamber studies involving sulphuric acid, water and dimethylamine (DMA), as well as sulphuric acid, water and the oxidation products of pinanediol. The sulphuric acid dimer : monomer ratio is significantly lower than that seen in experiments involving sulphuric acid and DMA in chambers, indicating that stabilization of sulphuric acid clusters by bases is weaker in this dataset than in chambers, and thus another mechanism, likely involving the plentiful highly oxygenated organic molecules (HOMs) is plausible. The high concentrations of HOMs arise largely from both alkylbenzene and monoterpene oxidation, with the former providing greater concentrations of HOMs due to significant local sources. The concentration of these HOMs shows a dependence on both temperature and precursor VOC concentration. New particle formation without growth past 10 nm is also observed, and on these days the highly oxygenated organic compound concentration is significantly lower than on days with growth, and thus high concentrations of low volatility oxygenated organics appear to be a necessary condition for the growth of newly formed particles in Barcelona. These results are consistent with prior observations of new particle formation in both chambers and the real atmosphere, and these results are likely representative of the urban background of many European Mediterranean cities.