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Measurements of cloud condensation nuclei (CCN), aerosol size distributionand non-refractory chemical composition were performed from 16 to 31 October2012 in the São Paulo Metropolitan Area (SPMA), Brazil. CCN measurementswere performed at 0.23, 0.45, 0.68, 0.90 and 1.13% watersupersaturation and were subsequently compared with the Köhler theory,considering the chemical composition. Real-time chemical composition hasbeen obtained by deploying, for the first time in the SPMA, an aerosol chemicalionization monitor (ACSM). CCN closure analyses were performed consideringinternal mixtures. Average aerosol composition during the studied period yielded (arithmeticmean~± 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−3for organics, BC, NH4, SO4, NO3 and Cl,respectively. Particle number concentration was 12 813 ± 5350 cm−3,with a dominant nucleation mode. CCN concentrations were onaverage 1090 ± 328 and 3570 ± 1695 cm−3 atSS = 0.23% and SS = 1.13%, respectively. Results show an increase in aerosol hygroscopicity in the afternoon as aresult of aerosol photochemical processing, leading to an enhancement ofboth organic and inorganic secondary aerosols in the atmosphere, as well asan increase in aerosol average diameter. Considering the bulk composition alone, observed CCN concentrations weresubstantially overpredicted when compared with the Köhler theory (44.1 ± 47.9%at 0.23% supersaturation and 91.4 ± 40.3% at1.13% supersaturation). Overall, the impact of composition on thecalculated CCN concentration (NCCN) decreases with decreasingsupersaturation, partially because using bulk composition introduces lessbias for large diameters and lower critical supersaturations, defined as thesupersaturation at which the cloud droplet activation will take place.Results suggest that the consideration of only inorganic fraction improvesthe calculated NCCN. Introducing a size-dependent chemical composition based on filtermeasurements from previous campaigns has considerably improved simulatedvalues for NCCN (average overprediction error 14.8 ± 38.6% at0.23% supersaturation and 3.6 ± 21.6% at 1.13%supersaturation). This study provides the first insight on aerosol real-timecomposition and hygroscopicity at a site strongly impacted by emissions of aunique vehicular fleet due to the extensive biofuel usage

atmospheric chemistry and physics

Measured and modelled cloud condensation nuclei (CCN) concentration in São Paulo, Brazil: the importance of aerosol size-resolved chemical composition on CCN concentration prediction