Measurements of cloud condensation nuclei (CCN), aerosol size distribution and non-refractory chemical composition were performed from 16 to 31 October 2012 in the São Paulo Metropolitan Area (SPMA), Brazil. CCN measurements were performed at 0.23, 0.45, 0.68, 0.90 and 1.13 % water supersaturation and were subsequently compared with the Köhler theory, considering the chemical composition. Real-time chemical composition has been obtained by deploying, for the first time in the SPMA, an aerosol chemical ionization monitor (ACSM). CCN closure analyses were performed considering internal mixtures. Average aerosol composition during the studied period yielded (arithmetic mean ± standard deviation) 4.81 ± 3.05, 3.26 ± 2.10, 0.30 ± 0.27, 0.52 ± 0.32, 0.37 ± 0.21 and 0.04 ± 0.04 µg m −3 for organics, BC, NH 4 , SO 4 , NO 3 and Cl, respectively. Particle number concentration was 12 813 ± 5350 cm −3 , with a dominant nucleation mode. CCN concentrations were on average 1090 ± 328 and 3570 ± 1695 cm −3 at SS = 0.23 % and SS = 1.13 %, respectively. Results show an increase in aerosol hygroscopicity in the afternoon as a result of aerosol photochemical processing, leading to an enhancement of both organic and inorganic secondary aerosols in the atmosphere, as well as an increase in aerosol average diameter. Considering the bulk composition alone, observed CCN concentrations were substantially overpredicted when compared with the Köhler theory (44.1 ± 47.9 % at 0.23 % su-persaturation and 91.4 ± 40.3 % at 1.13 % supersaturation). Overall, the impact of composition on the calculated CCN concentration (N CCN) decreases with decreasing supersatu-ration, partially because using bulk composition introduces less bias for large diameters and lower critical supersatu-rations, defined as the supersaturation at which the cloud droplet activation will take place. Results suggest that the consideration of only inorganic fraction improves the calculated N CCN. Introducing a size-dependent chemical composition based on filter measurements from previous campaigns has considerably improved simulated values for N CCN (average over-prediction error 14.8 ± 38.6 % at 0.23 % supersaturation and 3.6 ± 21.6 % at 1.13 % supersaturation). This study provides the first insight on aerosol real-time composition and hygro-scopicity at a site strongly impacted by emissions of a unique vehicular fleet due to the extensive biofuel usage.