African dust contributions to mean ambient PM 10 mass-levels across the Mediterranean Basin

Abstract

Data on mass-levels of PM 10 measured at regional background sites across the Mediterranean Basin, available from Airbase (European Environmental Agency) and from a few aerosol research sites, are compiled. PM 10 levels increase from north to south and west to east of the Basin. These variations are roughly coincident with the PM 10 African mineral dust load. However, when subtracting the African dust from mean PM 10 levels using a consistent methodology, the PM 10 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.