A study of PM2.5 and PM2.5-associated polycyclic aromatic hydrocarbons at an urban site in the Po Valley (Bologna, Italy)

PM2.5 and PAHs bound to PM2.5 were investigated in downtown Bologna, from January to June 2003, in order to determine the burden of the fine fraction in the aerosol of a typical urban environment of the Po Valley, a critical area in Northern Italy in terms of atmospheric pollution. The sampling campaign was divided into three parts: a winter sub-campaign, an intermediate campaign where PM2.5 and PM10 were simultaneously sampled and which identified PM2.5 as the major component of PM10, and a summer sub-campaign. Critical concentrations of both PM2.5 and PAHs were observed in winter time; for example, in January 2003 the mean value for the 24-h average PM2.5 concentration was 58 mu g/m(3), much higher than the annual arithmetic mean of 15 mu g/m(3) established by the US ambient air quality standard (NAAQS). Correspondingly, the mean value for benzo[a]pyrene (BAP) in PM2.5 was 1.79 ng/m(3), again higher than the annual mean of 1 ng/m(3), required by European regulations for BAP in PM10. In summer time the BAP concentration considerably decreases to 0.10 ng/m(3) as the likely effect of photolysis and dilution on a higher boundary layer; PM2.5 decreases too, but the mean concentration (22 mu g/m(3)) is still higher than the NAAQS value. Further analysis included TEM microscopy of collected particles and correlations between PM2.5, PAHs and gases (benzene, O-3, CO, NO2, SO2). All these observations identified on-road mobile sources as the main source of emissions and, in general, of the poor air quality level in the city of Bologna.