Disaggregating the contribution of local dispersion and long-range transport to the high PM10 values measured in a Mediterranean urban environment

The EU daily PM10 limit-value of 50 μg m− 3 is frequently breached in the center of Athens, Greece. A total of 852 daily exceedances were recorded in the city-center during the 6-year period 2001-2006. These exceedances were more frequent in winter, followed by spring and autumn. For the needs of the study, the PM10 episodes (i.e., concentrations 30% above the median value) were grouped in two categories: (a) the City-Center episodes and (b) the Wider Metropolitan Area episodes. It was assumed that City-Center episodes occurred when elevated PM10 values were measured in the city-center exclusively, whereas Wider Metropolitan Area episodes occurred when high PM10 concentrations were also measured in the suburbs. Then back-trajectory cluster analysis was performed in an attempt to associate high PM10 levels with local dispersion or long-range transport. The City-Center episodes were associated according to the origin of air parcels with six types of air-masses (slow and fast moving from northern, northeasterly and southern directions, or stagnated around Athens) and were mainly due to traffic or other local sources, whereas the Wider Metropolitan Area episodes were mainly associated with air-masses coming from southern directions and were linked to long-range transport. On the whole, the analysis provided evidence of Sahara-dust events and sea-spray transportation.