A three-year investigation of daily PM2.5 main chemical components in four sites: the routine measurement program of the Supersito Project (Po Valley, Italy)

Abstract The Supersito Project ( www.supersito-er.it ) has been active in the Emilia-Romagna region, southern part of the Po Valley (Italy), since 2011. Focal aim of the project is to enhance the knowledge on atmospheric aerosol and its impact on human health. In the framework of Supersito, major chemical components of daily PM2.5 were investigated over a period of more than three years at four sampling sites, representative of dissimilar territorial conditions: one rural background (SPC) and three urban background sites in the coastal (RN), central (MS) and inner area (PR) of the region. In all the sites, organic and elemental carbon and water soluble inorganic ions accounted for more than 70% of PM2.5 mass, during all seasons. Nitrate and organic carbon (OC) were the main components of winter PM2.5, while summer aerosol was mainly contributed by OC and sulphate. OC was dominated by primary sources, with a potentially important contribution from biomass burning, in winter, while secondary processes dominated OC production in summer. A substantial homogeneity was observed on a regional scale in terms of spatial distribution of pollutants, with EC only presenting significant differences between urban and rural areas during winters. Nonetheless, differences were observed between the coastal and the inner part of the region, with the former being systematically characterized by higher concentrations of carbonaceous compounds and lower concentrations of ammonium nitrate. The coastal area was likely influenced by the aged OC from the Po Valley outflow in addition to local sources, while the scarcity of local sources of ammonia limited the formation of ammonium nitrate. In the studied area, local and regional meteorology - mostly governed by geographical collocation and orography – was responsible for PM2.5 mass and composition no less than local and regional emission sources.