Data on mass-levels of PM10 measured at regional background sites across the Mediterranean Basin, available from Airbase (European Environmental Agency) and from a few aerosol research sites, are compiled. PM10 levels increase from north to south and west to east of the Basin. These variations are roughly coincident with the PM10 African mineral dust load. However, when subtracting the African dust from mean PM10 levels using a consistent methodology, the PM10 background levels are still 5-10 μg m-3 higher in the Eastern Basin (EMB) when compared with those in the Western (WMB), mainly due to the higher anthropogenic and sea spray loads. As regards for the seasonal trends, these are largely driven by the occurrence of African dust events, resulting in a spring-early summer maximum over the EMB, and a clear summer maximum in the WMB, although in this later region the recirculations of aged air masses play an important role. Furthermore, a marked seasonal trend is still evident when subtracting the African dust load. This is characterised by a high summer maximum (driven by low precipitation, high insolation) and a winter minimum (intense synoptic winds). Important inter-annual variations in the dust contribution are detected, more evident in the southern sites. These differences are generally associated with the occurrence of extreme dust events. Generally, the years with higher dust contributions over the EMB correspond with lower contributions over the WMB, and vice versa. The characterization of individual particles, collected in both basins during African dust events, by scanning electron microscopy reveals only slight differences between them. This fact probably reflects the high degree of mixture of mineral dust from different sources before the transport towards the receptor sites. © 2009 Elsevier Ltd. All rights reserved.

This study was supported by the Ministry of Science and Innovation (CGL2005-03428-C04-03/CLI, CGL2007-62505/CLI, GRACCIE- CSD2007-00067), the European Union (6th framework CIRCE IP, 036961, EUSAAR RII3-CT-2006-026140). The authors would also like to acknowledge NASA/Goddard Space Flight Center, SeaWIFS-NASA Project, University of Athens, Navy Research Laboratory-USA and the Barcelona Super-Computing Centre for their contribution with TOMS maps, satellite images, SKIRON dust maps, NAAPs aerosol maps, and DREAM dust maps, respectively. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.arl.noaa.gov/ready.html) used in this publication. We would like express our gratitude to Jesús Parga for his technical support. Finally, we would like to express our gratitude to Airbase-EEA for allowing free access to ambient PM levels recorded at a large number of sites in Europe, some of which have been used in this study.

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