Abstract
Abstract. This study aims to determine the mineral contribution to PM10 in the central Mediterranean Sea, based on 7 yr of daily PM10 samplings made on the island of Lampedusa (35.5° N, 12.6° E). The chemical composition of the PM10 samples was determined by ion chromatography 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 the PM10 sampling. The average PM10 concentration at Lampedusa over the period June 2004–December 2010 is 31.5 μg m−3, with low interannual variability. The annual means are below the EU annual standard for PM10, but 9.9% of the total number of daily data exceeds the daily threshold 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 the contribution of Al, Si, Fe, Ti, non-sea-salt (nss) Ca, nssNa, and nssK oxides in samples in which PIXE data were available. Cases in which crustal content exceeded the 75th percentile of the crustal oxide content distribution were identified as elevated dust events. Using this threshold, we obtained 175 events. Fifty-five elevated dust events (31.6%) displayed PM10 higher than 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 (corresponding to 49% of PM10) during an intense Saharan dust event. The crustal content estimated from a single tracer, such as Al or Ca, is in good agreement with the one calculated as the sum of the metal oxides. Conversely, larger crustal contents are derived by applying the EU guidelines for demonstration and subtraction of exceedances in PM10 levels due to high background of natural aerosol. The crustal aerosol amount and contribution to PM10 showed a very small seasonal dependence; conversely, the dust columnar burden displays an evident annual cycle, with a strong summer maximum (monthly average aerosol optical depth at 500 nm up to 0.28 in June–August). We found that 71.3% of the dust events identified from optical properties over the atmospheric column display a high dust content at the ground level. Conversely, the remaining 28.7% of cases present a negligible or small impact on the surface aerosol composition due to the transport processes over the Mediterranean Sea, where dust frequently travels above the marine boundary layer, especially in summer. Based on backward trajectories, two regions, one in Algeria–Tunisia, and one in Libya, are identified as main source areas for intense dust episodes occurring mainly in autumn and winter. Data on the bulk composition of mineral aerosol arising from these two source areas are scarce; results on characteristic ratios between elements show somewhat higher values of Ca / Al and (Ca + Mg) / Fe (2.5 ± 1.0, and 4.7 ± 2.0, respectively) for Algeria–Tunisia than for Libyan origin (Ca / Al = 1.9 ± 0.7 and (Ca + Mg) / Fe = 3.3 ± 1.1).
Highlights
Mineral aerosol is produced by wind erosion and resuspension in arid and semiarid regions and contributes by about 45 % to the total atmospheric aerosol load (Duce at al., 1991)
The annual means are below the EU annual standard for PM10, but 9.9 % of the total number of daily data exceeds the daily threshold value established by the European Commission for PM (50 μg m−3, European Community, EC/30/1999)
The PM10 annual mean is quite high, it is below the EU annual PM10 standard for PM10 (40 μg m−3); 112 days exceed the daily threshold value established by the European Commission for PM (50 μg m−3, European Community, EC/30/1999)
Summary
Mineral aerosol is produced by wind erosion and resuspension in arid and semiarid regions and contributes by about 45 % to the total atmospheric aerosol load (Duce at al., 1991). Estimates of global dust emission range from 1000 to 3000 Tg yr−1 (Zender et al, 2004). Dust may greatly increase the atmospheric levels of PM, adversely affecting air quality. This effect is especially relevant in southern and eastern Europe (Escudero et al, 2005, 2007; Pederzoli et al, 2010; Gerasopoulos et al, 2006, Dayan et al, 1991) due to the transport processes from Africa and the Arabian Peninsula and to the relatively low precipitation, which causes a long residence time of PM in the Mediterranean atmosphere (Querol et al, 2009). Intense dust transport episodes may cause health impacts due to the high levels of PM, with which transport of anthropogenic pollutants may be associated (Erel et al, 2006)
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