Temporal variability of mineral dust in southern Tunisia : analysis of 2 years of PM10 concentration, aerosol optical depth, and meteorology monitoring

International audienceThe south of Tunisia is a region very prone to wind erosion. During the last decades, changes in soil management have led to an increase in wind erosion. In February 2013, a ground-based station dedicated to the monitoring of mineral dust (that can be seen in this region as a proxy of the erosion of soils by wind) was installed at the Institut des Régions Arides (IRA) of Médenine (Tunisia) to document the temporal variability of mineral dust concentrations. This station allows continuous measurements of surface PM10 concentration (TEOM™), aerosol optical depth (CIMEL sunphotometer), and total atmospheric deposition of insoluble dust (CARAGA automatic sampler). The simultaneous monitoring of meteorological parameters (wind speed and direction, relative humidity, air temperature, atmospheric pressure, and precipitations) allows to analyse the factors controlling the variations of mineral dust concentration from the sub-daily to the annual scale. The results from the two first years of measurements of PM10 concentration are presented and discussed. In average on year 2014, PM10 concentration is 56 µg/m3. However, mineral dust concentration highly varies throughout the year: very high PM10 concentrations (up to 1,000 µg/m3 in daily mean) are frequently observed during wintertime and springtime, hardly ever in summer. These episodes of high PM10 concentration (when daily average PM10 concentration is higher than 240 µg/m3) sometimes last several days. By combining local meteorological data, air-masses trajectories, sunphotometer measurements, and satellite imagery, the part of the high PM10 concentration due to local emissions and those linked to an advection of dusty air masses by medium and long range transport from the Sahara desert is quantified

Optimization of photoluminescence of Eu³⁺ doped YVO<inf>4</inf> nanoparticles by green microwave synthesis

A detailed investigation of the europium-doped yttrium orthovanadate (YVO4:Eu3+) phosphorus prepared by fast and facile microwave synthesis was undertaken. The phase purity, morphology, particle distribution analysis, and photoluminescence were all thoroughly examined. The photoluminescence (PL) properties of the as-synthesized YVO4:Eu3+ nanostructures depend greatly on the synthesis parameters. The PL intensity of the nanomaterials increased when the Eu concentration, holding time, and amount of water used in the prepared phosphors were adjusted. The optimal europium doping concentration was 7 mol% for temperature holding times of 60 min, and 5 ml of water was used as the solvent. The emission intensity of Eu3+-doped YVO4 phosphors can be rationally modified by simply changing the pH of the solution or by employing different solvents. The phosphors studied were produced as nanoparticles with a very intense emission spectrum, making them good candidates for fluorescent lamps and light-emitting diodes (LEDs).“This work was supported by the European Structural and Invest ment Funds in the FEDER Component through the Operational Competitiveness and Internationalization Programme (COMPETE 2020) under Advanced Decision Making in productive systems through Intelligent Networks (ADM.IN) Project 055087 (POCI-01-0247-FEDER-055087).

Proceedings of the international conference on integrated environmental Management for sustainable development. Vol. 3. Environmental and health assessment

Southern Tunisia is a region very prone to wind erosion because of its soil features, and the development of mechanized agriculture. Moreover, this region is located downwind the Sahara, which is the main source of mineral dust in the world. For these reasons, dust haze is frequently observed in this region. If some authors have already documented air quality in the northern part of Tunisia, no equivalent studies have ever been conducted for southern regions even though Dahech & Beltrando (2012) highlighted the potential negative impact of mineral dust on air quality in Sfax. This is why a ground-based station dedicated to the monitoring of mineral dust was installed at the Institut des Régions Arides (IRA) of Médenine to document the temporal variability of mineral dust concentrations in southern Tunisia. We present here the results from the two first years of measurements of meteorology, PM10 concentration, and direct solar radiation

Influence of Atmospheric Stability on the Size Distribution of the Vertical Dust Flux Measured in Eroding Conditions Over a Flat Bare Sandy Field

International audienceIn spite of their importance for the modeling of the atmospheric cycle of mineral dust, measurements of the intensity and size-distribution of the dust emission flux produced by wind erosion in natural conditions remain rare. During the WIND-O-V's (WIND erOsion in presence of sparse Vegetation) 2017 experiment, 8 major erosion events having occurred on a sandy flat field of southern Tunisia were documented. Consistent with the small size (90 µm) of the erodible sand grains and the low aerodynamic roughness length (Z 0 < 0.079 cm), the threshold for wind erosion was low (= 22 cm s-1). The classical gradient method was applied to assess the size-resolved vertical dust flux, and the stability of the atmosphere quantified by the means of the Richardson number (Ri) as well as of its shear stress () and thermal gradient () components. The vertical dust flux increased with following a power law but the number size- distribution of the dust flux was found to be significantly richer in submicron particles in thermally unstable than in stable periods. This challenges the usual assumption that, independently of their size, the particles smaller than 10 µm follow equally the movements of the air masses in which they are embedded and that the thermal stratification of the surface layer does not affect the size-distribution of the surface flux when measured a few meters above the ground. Finally, we propose a simple empirical method for taking this influence of the thermal instability into account

Aerodynamic parameters over an eroding bare surface: reconciliation of the law of the wall and eddy covariance determinations

International audienceAssessing accurately the surface friction velocity is a key issue for predicting and quantifying aeolian soil erosion. This is usually done either indirectly from the law of the wall (LoW) of the mean wind velocity profile or directly from eddy covariance (EC) of the streamwise and vertical wind velocity fluctuations. However, several recent experiments have reported inconsistency between friction velocities deduced from both methods. Here we reinvestigate the determination of aerodynamic parameters (friction velocity and surface roughness length) over an eroding bare surface and look at the possible reasons for observing differences on these parameters following the method. For that purpose a novel field experiment was performed in South Tunisia under the research program WIND-O-V (WIND erOsion in presence of sparse Vegetation). We find no significant difference between friction velocities obtained from both law of the wall and EC approaches when the friction velocity deduced from the EC method was extrapolated to the surface. Surface roughness lengths show a clear increase with wind erosion, with more scattered values when deduced from the EC friction velocity. Our measurements further suggest an average value of the von Karman constant of 0.407±0.002, although individual wind events lead to different average values due probably to the definition of the ground level or to the stability correction. Importantly, the von Karman constant was found independent of the wind intensity and thus of the wind soil erosion intensity. Finally, our results lead to several recommendations for estimating the aerodynamic parameters over bare surface in order to evaluate saltation and dust fluxes

Dissimilarity between dust, heat, and momentum turbulent transports during aeolian soil erosion

International audienceMeasuring accurately size-resolved dust flux near the surface is crucial for better quantifying dust losses by semiarid soils. Dust fluxes have been usually estimated from the flux-gradient approach, assuming similarity between dust and momentum turbulent transport. This similarity has, however, never been verified. Here we investigate the similarity between dust (0.3 to 6.0 m in diameter), momentum, and heat fluxes during aeolian erosion events. These three fluxes were measured by the Eddy Covariance technique during the WIND-O-V (WIND erOsion in presence of sparse Vegetation's) 2017 field experiment over an isolated erodible bare plot in South Tunisia. Our measurements confirm the prevalence of ejection and sweep motions in transporting dust as for heat and momentum. However, our measurements also reveal a different partition of the dust flux between ejection and sweep motions and between eddy time scales compared to that of momentum and heat fluxes. This dissimilarity results from the intermittency of the dust emission compared to the more continuous emission (absorption) of heat (momentum) at the surface. Unlike heat emission and momentum absorption, dust release is conditioned by the wind intensity to initiate sandblasting. Consequently, ejection motions do not carry dust as often as heat and low momentum from the surface. This dissimilarity diminishes with increasing wind intensity as saltation patterns, and thus dust emission through sandblasting, become spatially more frequent. Overall, these findings may have implications on the evaluation of dust flux from techniques based on similarity with momentum or heat turbulent transport

The WIND-O-V field experiment: WIND erOsion in presence of sparse Vegetation

International audienceWind erosion in semiarid areas is a major threat for the soil productivity as it impoverishes soil in organic matters and nutrients. Compared to desert regions, these regions are characterized by sparse seasonal vegetation that impacts the erosion process. Semiarid areas face two major evolutions that may modify their wind soil erosion in the future: (1) climate change, with a modification of the amplitude and frequency of precipitations, affecting the surface vegetation cover, and (2) population growth, generating a considerable human pressure on the land use. Characterizing wind erosion in such complex regions is, therefore, crucial and challenging. In order to better predict the amount and composition of emitted dust from semiarid areas, a novel field experiment named WIND-O-V (wind erosion in presence of sparse vegetation) has been performed in 2017 and 2018 in South Tunisia. The originality of this experiment is (1) to cover successively a plot without and with sparse vegetation, and (2) to combine detailed measurements of wind dynamics (including turbulence), size-resolved saltation and dust fluxes, and erosion-flux compositional fractionation along the soil-saltation-suspension continuum. The experiment took place from March to May in the experimental range of the Institut des Régions Arides (IRA) of Médenine (Dar Dhaoui, 25 km east of Médenine). The site approximates a flat half-circle plot of 150 m radius where measurements were performed at the center of the circle in order to ensure a fetch of at least 150 m. In 2017, the surface has been tilled with a disc plough and levelled with a wood board in order to meet the conditions of an ideal flat bare soil without soil crust or ridges. In 2018, sparse vegetation consisting of barley tufts have been grown on the plot with a 3.3 m wide regular arrangement. Three types of measures were carried out. Meteorology: on a 9 m high mast erected at the center of the plot, turbulent velocity components and air temperature fluctuations were measured simultaneously at 1.0, 1.9, 3.0, and 4.1 m height using four 3D sonic anemometers sampling at 60, 50, 50, and 20 Hz, respectively. On the same mast, 7 cup anemometers (0.2, 0.6, 1.3, 1.8, 3.0, 4.0, 5.2 m) and 4 thermocouples (0.4, 1.6, 3.7, 5.0 m) were also installed to measure simultaneously at 0.1 Hz the mean horizontal wind velocity and temperature profiles, respectively. Three additional 2D sonic anemometers were installed in 2018 around a barley tuft to characterize the wind around the vegetation. Roughness length of the surface and friction velocity were computed on the bare plot case by comparing the Law-of-the-wall and Eddy-Covariance methods. Saltation flux: one vertical array of 5 sediment traps like Big Spring Number Eight (BSNE) was deployed to quantify the saltation flux and its size distribution. The modified BSNE had a 5 times wider opening area to collect larger sediment quantities, allowing sequential (in time) sampling of individual erosive events and guarantying the possibility of applying size resolved analyses. Saltation flux measurements with a better temporal resolution were thus associated with more stable friction velocity conditions. In 2018, 5 MWAC masts were added to measure the spatial variability of the flux due to the sparse vegetation. A Saltiphone and a camera gave information on the beginning, end, and duration of erosive events. Dust flux: for the first time size-resolved dust fluxes were estimated from both the traditional flux-gradient approach and the eddy covariance approach. For the first approach, mass and size resolved number concentrations were measured at two levels (2 and 4 m). To that purpose, two TEOM microbalances and two optical particle counters (WELAS Promo 2300) were used. Both sensor-types were connected to omnidirectional air sampling inlets. The WELAS monitored at 1 Hz the dust concentrations per size class (32 classes between 0.3 and 17 μm). For the second approach, a third WELAS was coupled to the 3 m high sonic anemometer in order to correlate the size-resolved dust concentration and the vertical wind velocity fluctuations. Finally, the chemical composition of dust fluxes was estimated from the sequential sampling of dust particles at two levels (2 and 4 m) with online filters equipped with inlets of different size cutoffs (20, 10, 2.5 and 1 μm)