Fine particulate concentration data resulting from several monitoring campaigns performed in the city of Milan at urban sites with different exposure to the emission sources are presented. Low volume PM2.5 and PM1 samplers are utilised together with a low volume optical analyser, enabling the intercomparison between the measurements obtained by the gravimetrical and the optical method. The concentration levels observed at the different sites are compared in order to point out intra-site seasonal differences and inter-site differences for corresponding seasons of the year. These different concentration levels are analysed and explained considering the exposure to the primary emissions and accounting for the role of meteorology. PM10, PM2.5 and PM1 are described in terms of the distribution of 1-h concentration data and their relative mass fractions are determined. In order to assess the significance of secondary sources of fine particulate, a PM2.5 high volume sampler is utilised for the collection of dust-loaded filters to be analysed for chemical characterisation. The composition of PM2.5 emission from traffic is investigated by analysing 24- h samples from an urban tunnel site (TU): data on carbonaceous species, organic carbon (OC) and elemental carbon (EC), are obtained and the ratio between these species is evaluated for real traffic emissions. Secondary organic aerosol (SOA) contribution to PM2.5 mass in ambient air is assessed by means of the primary OC/EC ratio approach, based on chemical data of the filters from the TU. Organic and inorganic secondary production in the outdoor atmosphere is contributing for about 75% of PM2.5 mass in winter and 40% in summer: as a consequence, effective long-term actions, still controlling the emissions of primary pollutants too, are required for air quality standards attainment and the potentiality of short-term interventions, as traffic restriction, appears quite limited.