Saharan dust episodes in Italy: influence on PM10 daily limit value (DLV) exceedances and the related synoptic.

The relevance of dust outbreaks from the Sahara into Italy was evaluated for the period from 2001 to 2007. To this end, days affected by Saharan dust were recognized at four PM(10) background stations belonging to the Regional Agencies for Environmental Protection (ARPA) monitoring networks (one working in the 2005-2007 period only). The analysis included computation of back trajectories, complemented with satellite images and meteorological charts. Results from other models are also collected for comparison. Saharan Dust contribution to PM(10) was calculated referring to a method developed for Iberian monitoring networks. PM(10) exceeding the daily limit value (DLV) of the European Union air quality standard were reviewed also. The provenience of air masses yielding the exceedances was also recognized, allowing to distinguish the Atlantic-European episodes from the (Atlantic)-Saharan ones. The European episodes are due to anthropogenic pollution that affected mainly northern Italy in winter; instead, the Saharan ones are due to dust outbreaks that affected mainly southern Italy in summer. Central Italy is characterised by a transitional situation that is experienced from north to south. A number of dust affected days with DLV exceedance (from 4 to 108 at the considered stations) were identified and grouped into 55 Saharan dust episodes. Analysis showed that DLV exceedances due to Saharan dust advection vanish at all stations when dust contributions are subtracted. A synoptic analysis based on the meteorological charts was carried out allowing a view of Saharan dust transport into Italy for each meteorological season. Notwithstanding that summer is the season most influenced by Saharan episodes at all of the stations, the analysis showed a low-high system of anomalies on the geopotential height distribution causing south-southwesterly flow carrying dust into Italy during the other seasons.