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Contribution of Saharan-dust to particulate matter (PM) levels affects most of Europe, being particularly significant in the Southern European regions. This condition impacts both daily and annual PM average values. Obviously, most important effects on PM exceedances are observed in polluted areas and large cities (e.g., Rodriguez et al., 2001, Escudero et al., 2007, Gobbi et al. 2007, Mitsakou et al., 2008, Querol et al. 2009).

While a wide literature exists documenting episodes of Saharan dust transport towards the Mediterranean and Europe (e.g., Basart et al., 2009, and references therein), just a few studies are available providing statistically significant results on the impact of Saharan dust on the particulate matter loads in Rome (e.g., Gobbi et al, 2006, Gobbi et al., 2007, Perrino et al., 2009). In the year 2001, an average 30% of the Mediteranean sea region was affected by the presence of dust transported from the Sahara desert. In the central Tyrrhenian region (where Rome belongs) Saharan dust showed an average column mass of 20 mg/m2 in winter and of 50 mg/m2 in the remaining seasons (Barnaba et al., 2004).

In the period 2001-2004, the ISAC-CNR polarization lidar observed Saharan dust plumes to cross over Rome on about 28% of the time, with minimum occurrence in winter. Dust was observed to reach the ground on ~17% of the time (Gobbi et al., 2006, Gobbi et al., 2007). Most (80%) of the observed events lasted less than four days, averaging to ~3.1 days. Typical altitude range of the dust plumes was 0-7 km, with centre of mass at ~2.5 km. These events were accompanied by average ground PM10 increases ranging between 12 and 19 μg/m3. Since the PM10 yearly average of many traffic stations in Rome is close to 40 μg/m3 , these events can both cause the surpassing of such yearly average and increase above the legal threshold the number of yearly exceedances of background stations (Gobbi et al., 2007).

Satellite observations are very effective at capturing the horizontal spreading of Saharan advections (Barnaba et al., 2004; Hatzianastassiou et al., 2009) but they cannot detect the altitude of the dust clouds, i.e., if the cloud is really affecting the ground PM. Such information is effectively provided by polarization lidars, i.e., laser radars capable of an altitude-resolved discrimination of spherical vs. non-spherical aerosols. One of such systems has been designed and operated at ISAC CNR Rome since 1999 (e.g., Gobbi et al., 2000). Still, today's polarization lidars are mainly non-operational, expensive research-type instruments. The possibility of manufacturing cheap, operational polarization lidars and scatter a number of them on the territory would represent a breakthrough in the detection and quantification of atmospheric aerosol layers (e.g., the unique tracking of the Eyjafjallajökull volcano plume by the network of non-polarization CHM15k ceilometers of the German Weather Service, Flentje et al., 2010). This is one of the major goals of DIAPASON.

The EU Air Quality Directive (2008/50/EC) allows Member States to subtract contributions from natural sources before comparing concentrations of PM in air to the relevant limit values set by the directive itself. In the year 2010, the European Commission issued guidelines to help assessing the contribution of natural particles to PM levels. Mineral particles from dry regions are among these natural contributions. While setting out a Saharan desert dust-detection methodology, the EU guidelines also consider the need to improve it. The DIAPASON project intends to strengthen such methodology by means of innovative, technologies, capable of attesting the presence of Saharan dust advections and assessing their effects on the observed particulate matter (PM) levels.