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This study aims to determine the mineral contribution toPM10 in the central Mediterranean Sea, based on 7 yr of daily PM10samplings made on the island of Lampedusa (35.5° N, 12.6° E). The chemical composition of the PM10 samples was determined by ionchromatography for the main ions, and, on selected samples, by particle-induced X-ray emission (PIXE) for the total content of crustal markers.Aerosol optical depth measurements were carried out in parallel to thePM10 sampling. The average PM10 concentration at Lampedusa over the period June2004–December 2010 is 31.5 μg m−3, with low interannualvariability. The annual means are below the EU annual standard forPM10, but 9.9% of the total number of daily data exceeds the dailythreshold value established by the European Commission for PM(50 μg m−3, European Community, EC/30/1999). The Saharan dust contribution to PM10 was derived by calculating thecontribution of Al, Si, Fe, Ti, non-sea-salt (nss) Ca, nssNa, and nssK oxidesin samples in which PIXE data were available. Cases in which crustal contentexceeded the 75th percentile of the crustal oxide content distribution wereidentified as elevated dust events. Using this threshold, we obtained 175events. Fifty-five elevated dust events (31.6%) displayed PM10 higherthan 50 μg m−3, with dust contributing by 33% on average. The crustal contribution to PM10 has an annual average value of 5.42 μg m−3,and reaches a value as high as 67.9 μg m−3 (correspondingto 49% of PM10) during an intense Saharan dust event. The crustal content estimated from a single tracer, such as Al or Ca, is ingood agreement with the one calculated as the sum of the metal oxides.Conversely, larger crustal contents are derived by applying the EU guidelinesfor demonstration and subtraction of exceedances in PM10 levels due tohigh background of natural aerosol. The crustal aerosol amount andcontribution to PM10 showed a very small seasonal dependence;conversely, the dust columnar burden displays an evident annual cycle, with astrong summer maximum (monthly average aerosol optical depth at 500 nm up to0.28 in June–August). We found that 71.3% of the dust events identifiedfrom optical properties over the atmospheric column display a high dustcontent at the ground level. Conversely, the remaining 28.7% of casespresent a negligible or small impact on the surface aerosol composition dueto the transport processes over the Mediterranean Sea, where dust frequentlytravels above the marine boundary layer, especially in summer. Based on backward trajectories, two regions, one in Algeria–Tunisia, and onein Libya, are identified as main source areas for intense dust episodesoccurring mainly in autumn and winter. Data on the bulk composition of mineralaerosol arising from these two source areas are scarce; results oncharacteristic ratios between elements show somewhat higher values ofCa / Al and (Ca + Mg) / Fe (2.5 ± 1.0, and 4.7 ± 2.0, respectively) forAlgeria–Tunisia than for Libyan origin (Ca / Al = 1.9 ± 0.7 and(Ca + Mg) / Fe = 3.3 ± 1.1)

atmospheric chemistry and physics

Saharan dust aerosol over the central Mediterranean Sea: PM&amp;lt;sub&amp;gt;10&amp;lt;/sub&amp;gt; chemical composition and concentration versus optical columnar measurements