Saharan dust aerosol over the central Mediterranean Sea: optical columnar measurements vs. aerosol load, chemical composition and marker solubility at ground level

Abstract

This study aims at the determination of the mineral contribution to PM₁₀ in the central Mediterranean Sea on the basis of 7 yr of PM₁₀ chemical composition daily measurements made on the island of Lampedusa (35.5° N, 12.6° E). Aerosol optical depth measurements are carried out in parallel while sampling with a multi-stage impactor, and observations with an optical particle counter were performed in selected periods. Based on daily samples, the total content and soluble fraction of selected metals are used to identify and characterize the dust events. The total contribution is determined by PIXE (particle-induced X-ray emission) while the composition of the soluble fraction by ICP-AES (inductively coupled plasma atomic emission spectroscopy) after extraction with HNO₃ at pH 1.5. <br><br> The average PM₁₀ concentration at Lampedusa calculated over the period June 2004–December 2010 is 31.5 &mu;g m^&minus;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 exceed the daily threshold value established by the European Commission for PM (50 &mu;g m^&minus;3, European Community, EC/30/1999). <br><br> 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 with crustal content exceeding the 75th percentile of the crustal oxide content distribution were identified as dust events. Using this threshold we identify 175 events; 31.6% of them (55 events) present PM₁₀ higher than 50 &mu;g m^&minus;3, with dust contributing by 33% on average. <br><br> The annual average crustal contribution to PM₁₀ is 5.42 &mu;g m^&minus;3, reaching a value as high as 67.9 &mu;g m^&minus;3, 49% of PM₁₀, during an intense Saharan dust event. The crustal aerosol amount and contribution to PM₁₀ shows 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 495.7 nm up to 0.28 in June–August). We found that 71.3% of the events identified from optical properties over the air column display a high dust content at the ground level. Conversely, the remaining 28.7% of cases present a negligible or small impact of dust on the surface aerosol composition due to the Saharan dust transport processes over the Mediterranean that frequently occur above the marine boundary layer especially in spring, summer and early autumn. <br><br> The solubility of different elements presents a large variability. In general, the solubility is lower in Saharan dust samples than for non-Saharan dust events. The solubility also displays a marked dependency on size for Saharan dust samples. Crustal markers show two relative maxima ...

Study of atmospheric aerosols by IBA techniques: The LABEC experience

“…During the AIRUSE campaign, a maximum value about 20 lg/m 3 was reached, corresponding to approximately 60% of the PM10 mass, while in days not affected by Saharan dust, the concentration of the crustal component was only few lg/m 3 . Detailed examples of this approach are reported in [9,30,31].…”

Natural Sources of Nanoparticles

“…The same studies [34,36] identified nanoparticles of compounds resulting from reactions of their components with anthropogenic aerosols, especially ammonium sulphate (NH 4 HSO 4 ). Samples collected at high altitudes were associated with air masses coming from Europe and travelling over North African desert areas.…”