Temporal evolution and particle size distribution of aerosol constituents collected in Northern Tunisia (Boukornine) under Sirocco wind circulations F. Ellouz, M. Masmoudi, K. Medhioub & C. Azri
Arabian Journal of Geosciences ISSN 1866-7511 Arab J Geosci DOI 10.1007/s12517-013-1085-0
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Author's personal copy Arab J Geosci DOI 10.1007/s12517-013-1085-0
ORIGINAL PAPER
Temporal evolution and particle size distribution of aerosol constituents collected in Northern Tunisia (Boukornine) under Sirocco wind circulations F. Ellouz & M. Masmoudi & K. Medhioub & C. Azri
Received: 11 March 2013 / Accepted: 22 August 2013 # Saudi Society for Geosciences 2013
Abstract Aerosols samples were collected from Boukornine, Northern Tunisia in order to examine the temporal evolution of the principal constituents of the Tunisian aerosols under Sirocco wind circulations. High constituent concentrations registered during the period of Sirocco showed the contribution of the Sahara in terms of loading aerosols with particulate matters. The mass size distribution of the principal constituents of the aerosols under Sirocco wind circulations was obtained from measurements carried out with an inertial cascade impactor. These distributions of the aerosol showed a bimodal structure for the majority of the elements except sulfur and sodium. The mass size distribution has a significant fraction of their mass concentration in the fine particle size range. This may be explained by the effect of wind wakes and probably selective disintegration.
Keywords Aerosols . Sirocco . Sahara . Particle size distribution . Tunisia
F. Ellouz (*) : M. Masmoudi : C. Azri Département de Géologie, Faculté des Sciences de Sfax, Sfax, Tunisia e-mail:
[email protected] K. Medhioub Institut Préparatoire aux Etudes d’Ingénieurs de Sfax (IPEIS), Sfax, Tunisia
Introduction North Africa is the strongest and most persistent source of dust in the world and nearly a half of desert dust deposited in the world’s oceans derives from the Sahara (Prospero et al. 2002; Goudie and Middleton 2001; Middelton and Goudie 2001; Al-Dousari et al. 2012; Mokadem et al. 2013). The occurrence of dust outbreaks in the Mediterranean Sea has a marked seasonal behavior and is mainly driven by intense cyclones. These cyclones are generated from the South of the Atlas mountains by the thermal contrast of cold marine Atlantic air and warm continental air crossing North Africa during spring and summer (Rodriguez et al. 2001; Escudero et al. 2005; Antoine and Nobileau 2006). Mineral dust has been widely studied for its impact on the radiative balance through absorption and scattering of sunlight (IPCC 2001; Sokolik et al. 2001), besides, it may act as a cloud condensation nucleus (Levin et al. 1996). It may also be a site for heterogeneous reactions (Dentener et al. 1996). The deposition of dust in marine regions is a source of nutrients for the phytoplankton, due to the content of iron oxides and phosphorous (Falkowski et al. 1998). Moreover, dust particles represent a component of the suspended particulate matter recorded in air quality monitoring networks (Lelieveld 2002). Tunisia is fully subjected to the Saharan influences and the Saharan wind named Sirocco which is mostly frequent in the spring and summer (Bousnina 1990;
Author's personal copy Arab J Geosci
midity and an important elevation cially in the summer period. The present study examines the the principal components of the northern Tunisia (Boukornine) in geochemical behavior and size situations.
of temperature, espetemporal evolution of aerosol registered in order to identify its under Sirocco wind
Data and site description Study area The site of Boukornine is located on the North African coast along the Mediterranean Sea (36°43′N and 10°18′E) at 40 m above the sea level. The study area is about 2 km from the coast and surrounded from the West by the Boukornine mountains (Fig. 1). The mountains are formed of limestone rocks and produce large amounts of sand which are carried by winds. Sampling program and analysis procedures
Fig. 1 Location of the sampling site
Awad and Mashat 2013). It blows from the south, south west, or deviates to the south east and generally precedes a slightly active cold front. Its turbulent movement is frequently responsible for the localized lifts of fine sand particles in the atmosphere (Azri et al. 2002). It is associated with a remarkable drop of relative hu-
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Fig. 2 Temporal evolution of the total concentration of aerosol constituents during June 2007
Forty samples of atmospheric particles were collected in Boukornine area during June 2007 by a total filtration of the air on a daily basis at hourly rate of 180 l and by cascade impactor system (EGAI, 80) which was operated at the nominal flow rate of 1 m3/h using nucleopore polycarbonate membranes (0.4 μm in porosity). This kind of impactor allows the collection of aerosol particles within the 0.1–25 μm size range (the size cut of diameters are about 8.75, 3.9, 1.95, and 0.65 μm for stages 1–5, respectively). All the samples were analyzed by XRF (SIEMENS SRS 303). According to the procedure described by Quisefit et al. (1996), the concentrations of 11 chemical elements were determined: Al, Si, Ca, Ti, Fe, Mg, Mn, K, Na, Cl, and S.
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Author's personal copy Arab J Geosci Fig. 3 Concentrations of the analyzed constituents of the aerosols
The temporal evolution of aerosol constituents was interpreted on the basis of the meteorological synoptic situation (NOAA ARL data 2007), air HYSPLIT back trajectory analysis, and local meteorological variables continuously measured at the monitoring station given by the Tunisian National Institute of Meteorology.
Results and discussion Temporal evolution of the aerosol constituents in June 2007 Chemical analysis of the aerosol matter sampled from June 2007 showed that the concentrations of different elements are
Fig. 4 Synoptic pressure surface systems: L low pressures over southeast of Algeria; H: high pressure over east of the Mediterranean Sea
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Fig. 5 Synoptic map at 500 hPa in 24 June 2007
characterized by a marked variability in time. The total concentrations of the analyzed chemical elements were shown to vary between 95 and 470 μg/m3 (Fig. 2). This temporal variation supports the idea that the rapid change at this station was not only due to local causes but also to the long-range transport of dust. Figure 2 showed the rapid and abrupt increase of the total concentration of analyzed elements during the 3-day period (from 23 to 25 June). These high concentrations are coincident with Sirocco days. The aerosol elementa-
Fig. 6 Diurnal variations of temperature and relative humidity during the study period
ry analysis at Boukornine area showed that the concentrations of the aerosol constituents were found to be very important during this Sirocco period. They exceed by three to nine times those of the aerosol constituents sampled in situations without sirocco. This clearly shows the effect of Sirocco winds in enriching the Tunisian background aerosol with particulate matters, especially silicon (silicon concentrations fluctuate between 250 and 330 μg/m3; Fig. 3). The significant silicon concentrations in the aerosols at the studied site may be related
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Fig. 7 Back trajectories calculated at 1200 UTC and at 500, 1000 and 2500m arrival height, for 25 June 2007
to the additional local enrichment from the sand dunes bordering the coast. This phenomenon may also be enhanced by the characteristics of the local air flow. In fact, the turbulent winds improve the rise of dust, especially silica, highly influenced by the saltation phenomenon (Pye 1987). In addition, in Meloni et al. (2007), we assumed that an area is active in mobilizing dust particles if the surface wind at the point identified by the entrainment condition would be larger than 7 m/s. So, the relatively high wind velocities (averages are between 7 and 10 m/s in the study area) reinforce earth dust entrainment in the air. Moreover, the Boukornine site is near a mountain so, particles cannot disperse easily. The blocking effect of the mountainous
chain reinforces particle concentrations by the phenomenon of wind wake created upstream and downstream of these lofty chains facing the main Sirocco wind flow direction. These geomorphologic features were shown to cause wind wake cavities, where particle concentrations can be multiplied by a factor of 10 (Azri et al. 2009). Meteorological context Saharan air masses reach the study area when the synoptic situation is characterized by a combination of both cyclone and anticyclone systems. In 24 June 2007, low pressures located over southeast of Algeria and an anticyclone placed
Author's personal copy Arab J Geosci Fig. 8 Mass size distribution of different elements for aerosols < 25 µm collected by impactor at Boukornine area in days without Sirocco (a) and in Sirocco days (b)
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over Central Mediterranean basin caused a southwest regime at the surface (Fig. 4) and also at higher altitudes (at 500 hPa). This situation is characterizing North Africa (Tunisia, Algeria, and Libya; Fig. 5), which resulted in substantial increases of the temperatures over the study area (maximum temperatures reached 46 °C) and a remarkable drop of relative humidity (Fig. 6). During the “post-Sirocco” days, when the anticyclone moved towards the East of the Mediterranean basin, predominant currents from the northwest, northeast, and eastern sectors were established.
Characterization of advection patterns by back trajectories The back trajectories realized by the air Hysplit Model (www.arl.NOAA.gov) have been used to infer source regions of the air masses advected over northeast Tunisia from 24 to 25 June 2007. The three levels back trajectories used in this study are shown in Fig. 7 and have been calculated at 1200 UTC for the Sirocco day with arrival height at 500, 1,000, and 2,500 m AGL. On 25 June (Fig. 7), back trajectories
Author's personal copy Arab J Geosci Table 1 Coarse and fine mass concentration shown at Boukornine area Fine particle concentration (μg/m3)
Coarse particle concentration (μg/m3)
Si Al Ca Fe
8.35 1.48 1.70 0.60
3.94 0.41 1.44 0.33
Mg Mn K S Cl Na
0.41 0.09 0.04 0.50 0.14 0.17
0.19 0.07 0.01 0.02 0.09 0.02
came from the Algerian Sahara, the Libyan Desert, and the Mediterranean basin before reaching the study area. Particle size distributions In order to obtain the mass size distributions of individual elements, it is necessary to invert the data of cascade impactors by an appropriate method. We used the data inversion technique described by Puttock (1981) which fits a sum of log normal distributions to impactor data. Puttock’s computational scheme converts stage loadings to size distribution using the detailed response functions of the impactor. This technique involves the calculation of the expected stages loadings from an assumed aerosol distribution and the impactor characteristics. Mass size distributions of different elements of Tunisian aerosols presented in Fig. 8 proved that the examined aerosol enrichment cannot be explained only by the local source but it was also shown to be affected by the Sirocco wind flow characteristics: Without Sirocco, the mass size distributions of the majority elements except Na and S are monomodal (Fig. 8b). So, in sirocco days (Fig. 8a), we note a bimodal distribution of almost the present elements at a study area except Na and Cl. The fine particle mode peak is in the 0.75–1.12 μm diameter range and rises to a second mode peak in the 16.7–17.7 μm range. The mass size distributions for all elements have a significant fraction of their mass concentration in the fine particle size range (Table 1). The fine particle concentration was 1.2–4 times higher than the coarse particle mass concentration for the crustal elements (Al, Si, Fe, Ti, Mg, and K). This result can be explained by the effect of wind wakes and probably selective disintegration. The mass size distributions for S and Na are distinctly different. These distributions are monomodal. They are characterized by the dominance of fine constituents (particle diameter less than 1 μm, d =0.74 μm for S, and d =1 μm for Na).
Conclusion The geochemical behavior of the Tunisian aerosols under Sirocco wind circulations was characterized by temporal evolution of the principal constituents of the aerosols (Si, Al, Ca, Fe, Ti, Mg, Mn, K, S, Na, and Cl) monitored at Boukornine which is an urban coastal and mountain site in Northern Tunisia. High constituent concentrations, exceeding three to nine times those of aerosols sampled during the period without Sirocco, were recorded. These high concentrations show the contribution of the Sahara in terms of loading aerosols with particulate matters especially silicon. The back trajectories realized by the air Hysplit Model have been used to infer source regions of the air masses advected over northeast Tunisia in Sirocco days. Back trajectories came from the Algerian Sahara, the Libyan Desert, and the Mediterranean basin before reaching the study area. Mass size distributions of the elements have been obtained from measurements carried out with an inertial impaction at the study area. A fitting procedure by data inversion has been applied to these data. The elemental size distributions in the range of 0.1–25 μm in diameter are bimodal for the majority of the constituents of the Tunisian aerosols except the sulfur and sodium elements whose distributions are monomodal. For the previous elements, the mass size distributions have a significant fraction of their mass concentration in the fine particle size range. This result can be explained by the effect of wind wakes and probably selective disintegration.
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