Saharan dust aerosol over the central Mediterranean Sea: PM10 chemical composition and concentration versus optical columnar measurements 1Department of Chemistry, University of Florence, Sesto Fiorentino, Florence, 50019, Italy
21 Feb 2014
2ENEA, Laboratory for Earth Observations and Analyses, 92010, Lampedusa, Italy
3ENEA, Laboratory for Earth Observations and Analyses, 90141, Palermo, Italy
4Department of Physics and Astronomy, University of Florence and I.N.F.N., Florence, Via Sansone 1, 50019 Sesto F.no, Florence, Italy
5ENEA Laboratory for Earth Observations and Analyses, 00123, Rome, Italy
Received: 05 June 2013 – Published in Atmos. Chem. Phys. Discuss.: 14 August 2013 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).
Revised: 14 January 2014 – Accepted: 16 January 2014 – Published: 21 February 2014
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
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).
Citation: Marconi, M., Sferlazzo, D. M., Becagli, S., Bommarito, C., Calzolai, G., Chiari, M., di Sarra, A., Ghedini, C., Gómez-Amo, J. L., Lucarelli, F., Meloni, D., Monteleone, F., Nava, S., Pace, G., Piacentino, S., Rugi, F., Severi, M., Traversi, R., and Udisti, R.: Saharan dust aerosol over the central Mediterranean Sea: PM10 chemical composition and concentration versus optical columnar measurements, Atmos. Chem. Phys., 14, 2039-2054, doi:10.5194/acp-14-2039-2014, 2014.