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The Emilia-Romagna region and its Agency for Prevention and Environment are\ud running a project – called Supersito – the purpose of which is to gain further\ud knowledge about the components of fine and ultrafine particles in the\ud atmosphere. Supersito began the measurements at the end of 2011, in this paper\ud we summarize the preliminary results observed for the aerosol size distribution\ud and source apportionment of PM2.5 in Bologna’s urban background.\ud Results show that nitrates, sulphates and ammonium accounts for more than\ud 40% of the mass of PM2.5 in the cold season and for about 30% in the summer.\ud The carbonaceous fraction (organic aerosol plus elemental carbon) is about 40%\ud in both seasons.\ud PMF analysis of the data coming from the results of the mass composition\ud shows that important fractions of PM2.5 during the cold season come frombiomass burning for domestic heating. Other important factors observed are:\ud vehicular traffic, agriculture, industries, long-range transport and dust\ud resuspension. Finally, the aerosol distribution of submicron particles shows that\ud nucleation event days are very frequent; in particular, during the summertime\ud when they reached almost 25% of the measurement days