Description:    The evaluation of the contribution of natural sources to PM 10 and PM 2.5 concentrations is a priority especially for the countries of European south strongly influenced by Saharan dust transport events. Daily PM 2.5 concentrations and composition were monitored at an urban site at 14 m above ground level, at the National Technical University of Athens campus from February to December 2010. The typical dust constituents Si, Al, Fe, K, Ca, Mg, and Ti were determined by wavelength dispersive X-ray fluorescence spectrometry (WDXRF). Sulfur, a tracer of anthropogenic origin and major constituent of PM 2.5 , was determined by both WDXRF and ionic chromatography. The contribution of dust and sulfates in PM 2.5 was calculated from the analytical determinations. An annual mean of 20 μg/m 3 was calculated from the mean daily PM 2.5 concentrations data. Twenty-two per cent of daily concentrations of PM 2.5 reached or exceeded the EU annual target concentration of 25 μg/m 3 . The exceedances occurred during 13 short periods of 1–4 days. Back-trajectory analysis was performed for these periods in order to identify the air masses origin. From these periods, ten periods were associated to Saharan dust transport events. The most intense dust transport event occurred between February 17th and 20th and was responsible for the highest recorded PM 2.5 concentration of 100 μg/m 3 where the dust contribution in PM 2.5 reached 96 %. The other dust transport events were less intense and corresponded to less pronounced enhancements of PM 2.5 concentrations, and their contribution ranged from 15 to 39 % in PM 2.5 concentrations. Air masses originated from northwest Africa while the influence of central Sahara was quite smaller. Content Type Journal Article Pages 1-14 DOI 10.1007/s11270-012-1373-4 Authors E. Remoundaki, Laboratory of Environmental Science and Engineering, School of Mining and Metallurgical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 15780 Zografou, Greece A. Papayannis, Laser Remote Sensing Laboratory, National Technical University of Athens, Heroon Polytechniou 9, 15780 Zografou, Greece P. Kassomenos, Laboratory of Meteorology, Department of Physics, University of Ioannina, University Campus, GR 45110, Ioannina, Greece E. Mantas, Laboratory of Environmental Science and Engineering, School of Mining and Metallurgical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 15780 Zografou, Greece P. Kokkalis, Laser Remote Sensing Laboratory, National Technical University of Athens, Heroon Polytechniou 9, 15780 Zografou, Greece M. Tsezos, Laboratory of Environmental Science and Engineering, School of Mining and Metallurgical Engineering, National Technical University of Athens, Heroon Polytechniou 9, 15780 Zografou, Greece Journal Water, Air, & Soil Pollution Online ISSN 1573-2932 Print ISSN 0049-6979 Journal Volume Volume 224 Journal Issue Volume 224, Number 1