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Abstract.To identify the relative contribution of local versus regional sources of particulate matter (PM) in the Greater Athens Area (GAA), simultaneous 24-h mass and chemical composition measurements of size segregated particulate matter (PM 1 , PM 2.5 and PM 10 ) were carried out from September 2005 to August 2006 at three locations: one urban (Goudi, Central Athens, "GOU"), one suburban (Lykovrissi, Athens, "LYK") in the GAA and one at a regional background site (Finokalia, Crete, "FKL").The two stations in the GAA exceeded the EU-legislated PM 10 limit values, both in terms of annual average (59.0 and 53.6 µg m −3 for Lykovrissi and Goudi, respectively) and of 24-h value. High levels of PM 2.5 and PM 1 were also found at both locations (23.5 and 18.6 for Lykovrissi, while 29.4 and 20.2 µg m −3 for Goudi, respectively).Significant correlations were observed between the same PM fractions at both GAA sites indicating important spatial homogeneity within GAA. During the warm season (April to September), the PM 1 ratio between GAA and FKL ranged from 1.1 to 1.3. On the other hand this ratio was significantly higher (1.6-1.7) during the cold season (October to March) highlighting the role of long-range transport and local sources during the warm and cold seasons respectively. Regarding the coarse fraction no seasonal trend was observed for both GAA sites with their ratio (GAA site/FKL) being higher than 2 indicating significant contribution from local sources such as soil and/or road dust.Chemical speciation data showed that on a yearly basis, ionic and crustal mass represent up to 67-70 % of the gravimetrically determined mass for PM 10 samples in the GAA Correspondence to: N. Mihalopoulos (mihalo@chemistry.uoc.gr) and 67 % for PM 1 samples in LYK. The unidentified mass might be attributed to organic matter (OM) and elemental carbon (EC), in agreement with the results reported by earlier studies in central Athens. At all sites, similar seasonal patterns were observed for nss-SO 2− 4 , a secondary compound, indicating significant contribution from regional sources in agreement with PM 1 observations. The contribution of local sources at both GAA sites was also estimated by considering mass and chemical composition measurements at Finokalia as representative of the regional background. Particulate Organic Matter (POM) and EC, seemed to be the main contributor of the local PM mass within the GAA (up to 62 % in PM 1 ). Dust from local sources contributed also significantly to the local PM 10 mass (up to 33 %).

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