Calibration of the MDCO dust collector and of four versions of the inverted frisbee dust deposition sampler

Geomorphology 82 (2006) 360 – 375 www.elsevier.com/locate/geomorph

Calibration of the MDCO dust collector and of four versions of the inverted frisbee dust deposition sampler Mamadou Sow a , Dirk Goossens a,b,⁎, Jean Louis Rajot c a

b

Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR-CNRS 7583, Université Paris 12, 61 av. du Général de Gaulle, F-94010 Créteil, France Physical and Regional Geography Research Group, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium c Institut de Recherche pour le Développement, B.P. 11416, Niamey, Niger Accepted 25 May 2006 Available online 13 July 2006

Abstract Wind tunnel experiments were conducted to determine the efficiency of sediment samplers designed to measure the deposition of aeolian dust. Efficiency was ascertained relative to a water surface, which was considered the best alternative for simulating a perfectly absorbent surface. Two types of samplers were studied: the Marble Dust Collector (MDCO) and the inverted frisbee sampler. Four versions of the latter catcher were tested: an empty frisbee, an empty frisbee surrounded by an aerodynamic flow deflector ring, a frisbee filled with glass marbles, and a frisbee filled with glass marbles and surrounded by a flow deflector ring. Efficiency was ascertained for five wind velocities (range: 1–5 m s− 1) and eight grain size classes (range: 10–89 μm). The efficiency of dust deposition catchers diminishes rapidly as the wind speed increases. It also diminishes as the particles caught become coarser. Adding a flow deflector ring to a catcher substantially improves the catcher's efficiency, by up to 100% in some cases. The addition of glass marbles to a catcher, on the other hand, does not seem to increase the efficiency, at least not at wind velocities inferior to the deflation threshold. For higher velocities the marbles protect the settled particles from resuspension, keeping them in the catcher. The following five parameters determine the accumulation of aeolian dust in a catcher: the horizontal dust flux, the weight of the particles, atmospheric turbulence, resuspension, and the dust shadow effect created by the catcher. The final accumulation flux depends on the combination of these parameters. The catchers tested in this study belong to the best catchers currently in use in earth science and have been the subject of various aerodynamic studies to improve their efficiency. Nevertheless the catching efficiency remains low, in the order of 20–40% for wind speeds above 2 m s− 1. Other catchers suffer from the same low efficiencies. There is, thus, evidence to believe that dust deposition rates published in the aeolian literature and obtained by collecting the sediment in a catcher largely underestimate the true deposition. The errors are considerable, of the order of 100% and more. A reconsideration of the literature data on aeolian dust deposition measured by catchers is, therefore, required. © 2006 Elsevier B.V. All rights reserved. Keywords: Aeolian dust; Dust deposition; Dust accumulation; Sediment catcher

1. Introduction ⁎ Corresponding author. Physical and Regional Geography Research Group, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Heverlee, Belgium. Tel.: +32 16 32 64 36; fax: +32 16 32 29 80. E-mail address: [email protected] (D. Goossens). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.05.013

The deposition of atmospheric dust has been recognized as an important environmental process worldwide (Goossens and Riksen, 2004). Correct

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estimations of the amounts of dust settling on the Earth's surface are required in many disciplines. Examples include: the calculation of aeolian dust balances in arid and semi-arid regions (Rajot, 2001), the study of iron deposition in the oceans (Jickells et al., 2005), and water and soil pollution caused by the deposition of contaminating or toxic particles (Schulz, 1992; PeligBa et al., 2001; Arimoto et al., 2005). On the other hand, the deposition of airborne sediment has been identified as an important source of nutrients on the continents (Beavington and Cawse, 1979; Herut and Krom, 1996; Sterk et al., 1996) as well as in the oceans (Gao et al., 2003; Boyd et al., 2004). Deposition of atmospheric dust also influences the process of soil formation (McFadden et al., 1987; Reheis et al., 1995; Reynolds et al., 2001). It affects vegetation, qualitatively as well as quantitatively (Farmer, 1993; Liblik et al., 2003). It even affects human-made constructions such as buildings (Erell and Tsoar, 1999), building materials (Lefèvre and Ausset, 2002) and the functioning of solar collectors (Hasan and Sayigh, 1992; El-Shobokshy and Hussein, 1993). Correct data on atmospheric dust deposition are also required to test the theoretical dust models used in atmospheric and soil research. Despite its great significance, measuring the deposition of atmospheric dust remains a problematic procedure (Goossens and Offer, 2000; Wiggs et al., 2002). Most measuring devices currently available perturb the airflow and, consequently, the dust flow. Another problem is that, to date, no international standard for measuring dust deposition has been adopted. National standards exist in several countries (see Hall et al., 1994 for some examples), but many of these have not been absolutely calibrated. The proposed ISO standard (International Standards Organisation, 1991), a cylindrical container 20 cm in diameter and 40 cm deep derived from the Norwegian NILU sampler, is not very efficient and has not been retained in earth science research. There are two approaches to quantifying the deposition of atmospheric dust: theoretical calculations and experimental measurements. A good summary of the major dry deposition models currently available is provided in a study by Wesely and Hicks (2000). It should be noted that many of these models were derived for the deposition of gases, and are not necessarily applicable to macroscopic dust particles. Examples of theoretical techniques are (Seinfeld and Pandis, 1998): the gradient method, the inferential method, the eddy correlation method, and the eddy accumulation method. It has not yet become sufficiently clear whether these techniques are truly reliable, and to

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what degree they predict identical deposition values for a given situation. Experimental studies usually show a wide range in dust deposition values, depending on which method is used (see Goossens, 2005 for some examples). The second approach measures the deposition of dust by using either surrogate surfaces or sediment catchers. Surrogate surfaces are surfaces that are supposed to imitate the original surface. They have been widely used in wind tunnels because they are easy to install and can be incorporated in the original surface, thus avoiding perturbations of the flow. Examples of surrogate surfaces described in the literature are: water or antifreeze (Smith and Twiss, 1965; McTainsh, 1980; Bücher, 1986), glass marbles (Ganor, 1975; Offer et al., 1992), moist filter paper (Goossens and Offer, 1993), sticky surfaces such as oiled paper (Clements et al., 1963) or liquid paraffin (Hall and Upton, 1988), grass (Chamberlain, 1967), moss (Rühling and Taylor, 1971; Clough, 1975), paper (Goossens and Offer, 1994), snow (Dovland and Eliassen, 1976), glass (Dawes and Slack, 1954; Goossens, 2005), plastic (Gregory, 1961), towelling material laid on aluminium sheets (Chamberlain, 1967), metal (Goossens, 2005), artificial grass (Chamberlain, 1967), etc. Although handy in wind tunnels, surrogate surfaces are usually not recommendable in field experiments because of practical and instrumental problems. This is especially true during long-term measurements, or when the deposition has to be measured above ground level. Dust deposition in the field is usually measured by means of sediment catchers. Many catcher types have been described, varying from simple constructions such as ordinary household buckets to complex instruments supplied with aerodynamic devices to minimize the perturbations of the airflow (Clements et al., 1963; Köhler and Fleck, 1963; Clough, 1975; Ganor, 1975; Ralph and Barrett, 1976, Skärby, 1977; Goodman et al., 1979; Bücher, 1986; Hall and Waters, 1986; Hall and Upton, 1988; Reheis, 1990; Goossens and Offer, 1990; Orange et al., 1990; Pye, 1992; Offer et al., 1992; Goossens and Offer, 1993; Hall et al., 1994; Reheis et al., 1995; Littmann, 1997; Erell and Tsoar, 1999; Goossens et al., 2001; Palumbo et al., 2004; Marx and McGowan, 2005; Würtz et al., 2005). Despite these efforts, the dust deposition values reported in the literature should be regarded with care. Many catchers have not been adequately calibrated, and for those that have been calibrated the efficiencies have often been determined in relative terms, against other catchers (Goossens and Offer, 1994) or under too simplified conditions (Wiggs et al., 2002). Detailed studies on dust

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sampler efficiencies are sparse but do exist, e.g. Hall and Upton (1988) and Hall et al. (1994). Many factors influence the efficiency of a dust deposition catcher: the shape and size of the catcher, the deposition surface inside the catcher, the airflow (wind speed, wind direction, level of turbulence), and the characteristics of the sediment (mainly, but not exclusively, its grain size distribution). Due to the combination of these factors the catcher's efficiency varies with the conditions at the time of measurement. This makes calibrating a dust deposition sampler very timeconsuming. This study investigates the efficiency of two dust deposition samplers: the Marble Dust Collector (MDCO) and the inverted frisbee sampler. The MDCO was chosen because of its frequent use in earth science studies. The frisbee was selected because its aerodynamics have already been studied (Hall et al., 1994; Wiggs et al., 2002), with a result that the catcher has been improved considerably over the last few years. Efficiencies were determined for various grain sizes and wind speeds. For the MDCO catcher, which is rectangular, we also measured efficiency as a function of wind direction. Of course, the catchers investigated in this study represent only a small selection of the numerous types of dust samplers currently available, but they permit reliable measurements in the field, even in uncomfortable conditions. The study has two goals. First, we aim to present reliable efficiency values for the samplers investigated. Secondly, we want to demonstrate that the efficiency of most dust deposition samplers currently in use is low. This has important consequences for the dust deposition data published in the literature, which may largely underestimate the true deposition. 2. Description of the samplers 2.1. The MDCO The MDCO is based on an original concept by Ganor (1975), who used glass marbles to collect settling dust. The collector consists of a rectangular plastic tray 52.5 cm long, 31.5 cm wide and 10.0 cm high with a marble filter at the top (Fig. 1). The filter is made of two layers of marbles 1.5 cm in diameter, which are stored in a sieve container on top of the plastic tray (mesh diameter of the sieve openings: 0.5 cm). Dust settles on and between the marbles and is washed (by natural rain or by the operator) into the plastic tray. It can be collected via a small outlet underneath the tray. The great advantage of the marble filter is that, due to the extremely low

Fig. 1. The MDCO sampler. Scale indication: the sampler is 52.5 cm long and 31.5 cm wide.

microroughness of the marbles, there is no outsplash of dust from the collector, even during the heaviest rains (Goossens et al., 2001). This makes the MDCO a very handy collector in humid climates. The marble filter also acts as a dust trap, protecting the particles that have penetrated into it from resuspension. 2.2. The inverted frisbee sampler Hall and colleagues, who tried to reduce the aerodynamic blockage created by conventional dust samplers, used an inverted frisbee to collect settling dust. They originally used a commercial version of the classic “World Class Frisbee” (Hall and Waters, 1986; Hall and Upton, 1988), but in their subsequent work they abandoned the classic shape by replacing it with more sophisticated forms to attain optimum aerodynamic properties and a minimum resistance to sediment blow-out (Hall et al., 1993; Hall et al., 1994). They also added a flow deflector ring to the sampler to improve its aerodynamics. In a recent study, Wiggs et al. (2002) tested a slightly enlarged version of the original Hall et al. (1994) sampler. The sampler we used has the same size as the one tested by Wiggs et al. (2002). It consists of a circular stainless steel collecting bowl, 30 cm in diameter and 3.6 cm deep, surrounded by an aerodynamically shaped aluminium deflector ring with an inner diameter of 38.4 cm and outer diameter of 64.0 cm (Fig. 2). To facilitate the collection of the dust particles, a hole, 1 cm in diameter, was drilled in the centre of the collecting bowl. Dust is collected by washing the bowl with distilled water or, alternatively, dust deposition can be directly measured by weighing the bowl before and after the experiment (the central hole is closed with a plug in that case).

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Fig. 2. The frisbee sampler. The photograph shows the version with marbles and a flow deflector ring.

To quantify the effect of the flow deflector ring, we tested the frisbee sampler (1) without ring, and (2) with the ring. In addition we also tested the effect of adding glass marbles to the collector. Four versions of the frisbee collector were thus tested: • frisbee without marbles and without a flow deflector ring • frisbee without marbles but with a flow deflector ring • frisbee with marbles but without a flow deflector ring • frisbee with marbles and with a flow deflector ring. Fig. 2 shows the collector with the ring and with the marbles. 3. Experimental procedure 3.1. Facilities and sediment All wind tunnel experiments were carried out in the closed-return wind tunnel of the Laboratory for Experimental Geomorphology at the Katholieke Universiteit Leuven, Belgium. The tunnel contains two test channels. All experiments were conducted in the large

channel, which is 7.6 m long, 1.2 m wide and 0.6 m high. During the experiments, dust was added to the air current by connecting an Engelhardt laboratory dust cloud producer to the tunnel. This apparatus ensures a continuous feed of dust particles to the airflow, and allows the operator to adjust dust discharge. The dust was added to the flow in the return section of the tunnel via a small but wide orifice in the tunnel roof. During its passage through the tunnel, the dust was fully dispersed over the tunnel section. The exact distribution of the dust at the location where the experiments were carried out was accurately measured (see Section 3.2) so that the appropriate corrections could be made when calculating the deposition. A detailed technical description of the wind tunnel and the dust cloud producer can be found in a technical report by Goossens and Offer (1988). The dust used in the experiments was prepared from calcareous loam collected at Korbeek-Dijle, Belgium. The loam was dried, ground and sieved through a 125μm sieve. In the sifting, some of the finest grains were lost in small dust clouds. The remaining sediment was composed of 4% clay (< 2 μm), 83% silt (2–63 μm) and 13% sand (> 63 μm). There were no particles >104 μm.

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Fig. 3. Grain-size distribution of the dust used in the tests.

The median grain diameter of the sediment was 39 μm. All these data were obtained by analysing the sediment in a water solution with a Malvern grain size analyser, type S (Malvern Instruments Ltd., Malvern, UK). Fig. 3 shows the typical grain size distribution curve. 3.2. Procedure During a test, the sampler was placed in the centre of the wind tunnel at a fetch of 600 cm. Installation was such that the centre of the sampler's inlet was always at a height of 30 cm above the tunnel floor. The fetch was not prepared in any special way, and given these conditions, the depth of the boundary layer in the tunnel is of the order of 7 cm, i.e., significantly below 30 cm. All tests were thus carried out in the free-stream region of the tunnel. Five wind velocities were selected at 1, 2, 3, 4 and 5 m s− 1. The velocities were measured with an accuracy of 0.01 m s− 1 using a digital Furness FC016 manometer. No higher speeds could be installed in the test section used, but 3–5 m s− 1 is an acceptable range for the average wind speed in most deserts, especially in the lowermost layers of the atmosphere (Offer and Goossens, 1990; Zangvil et al., 1991). Wind speeds were measured with a standard Pitot tube, at the location where the samplers were later installed. During the tests, the dust cloud machine produced a dust discharge of 13.0 kg h− 1. At the sampler's location, airborne dust concentration was approximately 1 g m− 3, slightly varying with wind velocity. As the MDCO sampler is rectangular in shape, three sampler orientations relative to the wind were tested: 0° (sampler parallel to the wind), 90° (sampler perpendicular to the wind) and 45° (intermediate orientation). There was no need to test multiple orientations for the frisbee sampler as all four frisbee versions tested are circular.

Three independent test runs were carried out at each velocity for each sampler. Before each run a protective cover was placed over the sampler to keep it free from dust. Once the correct wind speed had been attained, the cover was removed and the experiment started. Each run lasted 3 min. After the run, the wind tunnel was turned off and the protective cover was immediately replaced on the sampler to prevent dust from entering the trap. The sampler was then removed from the tunnel. For the MDCO, the marble filter was carefully washed with water and the solution was stored in a pot. The amount of dust collected by the sampler was determined by weighing the residue after evaporating the water in an oven. For the frisbee collector dust deposition was determined by weighing the inner collecting bowl (with or without marbles) before and after each experiment. All measurements were done with a precision of 0.01 g. To correct the results for small accidental deviations in the dust concentration values, a BSNE trap (Fryrear, 1986) was installed in the wind tunnel during each run, at the same fetch and height where the test samplers were installed (600 cm and 30 cm, respectively) but well outside their wake to avoid interference (Fig. 4A). The BSNE was also protected by a cover before and after each experiment. All deposition fluxes measured by the test samplers were later corrected for dust concentration differences that could have occurred during the experiments. As already pointed out by Hall and Upton (1988), Shao et al. (1993) and Goossens et al. (2000), in a wind tunnel the streamwise particle flux density at the inlet of a catcher is usually not representative of the average over the inlet of another catcher with a different inlet size and/or shape. The reason is that both particle concentration and wind speed vary with height and width (and particle concentration also with the fetch length) in the wind tunnel. The non-linearity of both variations further complicates the problem. The efficiency of catchers with different collecting surfaces can, thus, only be compared after the results have been recalculated to conditions of identical average particle flux over the inlet area. To determine the correction factors we measured, for all five wind speeds, the lateral variations of the particle deposition flux in the wind tunnel at a fetch of 600 cm and a height of 30 cm (i.e., at the samplers' position). A metal plate 120 cm long, 20 cm wide and 0.3 cm thick and oriented in the lateral direction was used for this purpose. The plate was placed immediately downwind of a wooden plate 120 cm wide, 289 cm long and 0.3 cm thick, which was installed at a height of 30 cm above the wind tunnel

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Fig. 4. Experimental set-up for the three types of wind tunnel experiments carried out in this study.

floor (Fig. 4B). The plates divide the wind tunnel section in two equal vertical sections without creating substantial blockage (recall that the height of the wind tunnel's test channel is 60 cm). This ensures that the lateral corrections of the dust deposition flux can be accurately determined. The variations in the longitudinal dust deposition flux were determined in a similar way. All measured fluxes were later corrected appropriately. There was no need to correct for vertical variations in the dust flux since all catchers measured the deposition at the same height (30 cm). To determine the absolute efficiency of the samplers, all measured deposition fluxes should be compared to the “true” deposition flux, defined in this study as the deposition flux on a perfectly absorbent surface. In the case of the deposition of particles, such surfaces do not occur in the natural environment (they may occur in the case of deposition of gases). Due to the rebound of impacting grains and/or resuspension, some of the settling particles will not remain on the surface but will leave it after deposition. Thus, a dust “deposition” sampler does not measure deposition but accumulation, i.e. only the dust that has been retained by it (Goossens, 2001).

Although, for particles, there is no such thing as a perfectly absorbent surface, liquids, for instance water, provide a good alternative. Previous studies (e.g. Goossens and Offer, 1994; Goossens, 2005) consistently confirm the high particle retention capacity of liquids. Therefore, the efficiencies of the samplers tested in this study will always be calculated relative to a water surface. Though not 100% perfect, it is reasonable to assume that they offer sufficient approximation of the true values. To measure the dust deposition flux for a water surface a 20 cm × 20 cm wide and 2 cm deep metal container was installed in the wind tunnel at fetch 600 cm and height 30 cm, immediately downwind of the 120 cm × 289 cm wooden plate (Fig. 4C). The container was filled with distilled water until its surface was level with that of the wooden plate. It was surrounded by 0.1 cm thin metal plates to avoid border effects. The amount of dust caught by the water surface (5 wind speeds, 3 replicas for each wind speed) was determined by collecting the water and the dust in a pot, evaporating the water in an oven and then weighing the dust on an analytical balance. Here again, the results were

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although slightly lower efficiencies are observed for a frisbee with marbles compared to a frisbee without marbles. Note also that, somewhat surprisingly, the efficiency of an empty frisbee (without marbles and without ring) is generally lower than that of an MDCO despite the latter's far less aerodynamic shape. To investigate the efficiencies as a function of particle size, all collected sediments were analysed by the Malvern instrument. The efficiencies were calculated for the following grain size classes: 10–19 μm, 19–31 μm, 31–41 μm, 41–48 μm, 48–56 μm, 56–

Fig. 5. Efficiency curves for the total sediment (all grain size classes included) vertical bars indicate standard deviation: (A) MDCO sampler, (B) frisbee sampler.

corrected for the longitudinal and lateral variations in the dust flux in the wind tunnel. 4. Results Fig. 5 shows, for the MDCO and the frisbees, the efficiencies as a function of wind speed. All efficiencies are for the total sediment (all grain-size classes together). For the MDCO (Fig. 5A) the efficiencies are high when the wind speed is low, but they rapidly decrease with increasing wind speed. At wind velocities ≥ 3 m s− 1 they have already dropped below 20%. Note also the stabilization of the curves above about 4 m s− 1, and the effect of the collector's orientation with respect to the wind. Highest efficiencies are obtained for an MDCO parallel to the wind (position 0°), lowest efficiencies for an MDCO perpendicular to the wind (position 90°). For the frisbee (Fig. 5B) the efficiency curves resemble those of the MDCO. Here again, stabilization occurs above a wind speed of approximately 4 m s− 1. The effect of adding a flow deflector ring to the frisbee is significant; the efficiencies double when the ring is added. The effect of adding marbles is much smaller,

Fig. 6. Efficiency of the MDCO sampler as a function of grain size: (A) position 0° (MDCO parallel to the wind), (B) position 45° (MDCO at 45° to the wind), (C) position 90° (MDCO perpendicular to the wind). To avoid overloading the pictures no error bars are shown.

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66 μm, 66–76 μm, and 76–89 μm. The class 89– 104 μm was not taken into consideration because its proportion in the total sediment was very low, about 1% or less. Also, the class < 10 μm was not considered because of the risk of undesired dispersion of the grains (recall that all grain size analyses were done in water). Inspection of the sediment under a microscope revealed that, for the dust used in the tests, dispersion of the sediment will be low for the grains > 10–15 μm (only very little aggregation was observed for these grains), and that it should remain within reasonable limits for the smaller grains because of the generally low percentage of the finest particles (less than 4% clay and less than 5% of particles < 10 μm after spontaneous dispersion of the sediment in water). To avoid any problem, the < 10 μm class was not considered. Fig. 6 shows the efficiency curves for the MDCO. The trend of the curves is identical to that of the total sediment (Fig. 5A), with high efficiencies at low wind speeds and low efficiencies at high wind speeds. The effect of particle size is prominent at all collector orientations; the efficiency systematically decreases

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with increasing particle size up to a grain diameter of approximately 50 μm. For coarser dust the efficiency no longer depends on the grain size. The frisbee runs (Fig. 7) display similar trends. Another conclusion is that particle size has a much smaller effect on a frisbee with marbles than on a frisbee without marbles. This remains true whether or not the frisbee has a flow deflector ring. 5. Interpretation and discussion Experimental studies on the collection efficiency of dust deposition samplers are sparse, especially those investigating efficiency as a function of grain size. For the MDCO collector, no such study exists at the moment. Data are available for the British Standard deposit gauge (Ralph and Barrett, 1984) and also for two inverted frisbees: an uncoated one and a second coated with liquid paraffin to prevent resuspension of deposited particles (Hall and Upton, 1988). It should be noted that the frisbees used in Hall and Upton's study were of the type “World Class Frisbee” (not the modified Hall et al. (1994) frisbee, which served as prototype for the

Fig. 7. Efficiency of the frisbee sampler as a function of grain size: (A) frisbee without marbles and without flow deflector ring, (B) frisbee without marbles but with flow deflector ring, (C) frisbee with marbles but without flow deflector ring, (D) frisbee with marbles and with flow deflector ring. To avoid overloading the pictures no error bars are shown.

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frisbees tested here), but the general trends can be compared. Although the tested sediments were also somewhat coarser (between 50 and 183 μm) than in our experiments, Hall and Upton's efficiency curves behave identically to ours: they show a clear drop in efficiency with increasing wind speed, a tendency towards stabilization above a certain speed (4–5 m s− 1 for the coated frisbee, faster for the uncoated frisbee), and with one exception the efficiencies are lower as the particles become coarser. The results of the British Standard deposit gauge are similar, with a stabilization at about 4 m s− 1 for particles 87 μm in size (no smaller particles were tested in Ralph and Barrett's (1984) study). Our results thus confirm those by Ralph and Barrett (1984) and Hall and Upton (1988). Wiggs et al. (2002) performed wind tunnel experiments with a frisbee comparable to the one used in our tests. These experiments were done with very finegrained silica flour (median diameter: 7.6 μm). Although the tests were carried out under somewhat simplified conditions they also showed a drop of the efficiency with the wind speed. No tests were done with other sediments. To understand the behaviour of the efficiency curves it is necessary to know the parameters (or phenomena) that play a role in the accumulation process. The accumulation flux in a catcher depends on the following five parameters: the horizontal dust flux above the catcher, the weight of the particles, the level of turbulence near the catcher's inlet, erosion (resuspension) of particles deposited in the catcher, and the size of the dust shadow zone near the catcher. The horizontal dust flux is important because it guarantees the supply of particles able to settle in the catcher. The weight of the particles is important because it determines the rate of gravitational settling. A high level of turbulence is also beneficial, especially for particles smaller than approximately 30–50 μm whose deposition is significantly affected by turbulence (Hunt and Barrett, 1989). Resuspension of particles deposited in the catcher will diminish the accumulation rate, thus affecting the catcher's final efficiency. The last factor, the dust shadow zone, is especially important in the case of light to moderate wind speeds. Because catchers act as an obstacle to the flow they create a dust shadow zone around and above them (see Goossens, 1988 for more details). The low airborne dust concentrations in this zone directly diminish the amount of particles available for deposition in the catcher. The combined effect of these five parameters explains the efficiency patterns in Figs. 5–7. The decrease of efficiency with increasing wind speed is

explained by the dust shadow effect (for low to moderate wind speeds) and the resuspension phenomenon (at sufficiently high wind speeds). Wind speeds need not be high to create a dust shadow, some tens of cm s− 1 or even less is enough (Goossens, 1994). In turbulent flows the perturbation of the flow caused by an obstacle usually diminishes with the square of the wind's velocity (see Goossens, 1987). Fig. 8 shows the normalized drag coefficient curve in the wind speed interval 0.5–12.0 m s− 1, calculated with Goossens' (1987) formula for a spheroid that is 30 cm in diameter, 3.6 cm deep and parallel to the flow (same dimensions and position as for the frisbee tested in this study). The shape of our frisbee deviates somewhat from a spheroid; the drag coefficient curve of the frisbee will, thus, differ from the one shown in Fig. 8, but it is reasonable to assume that its shape will be similar. Since the drag coefficient is a measure of the aerodynamic perturbation (caused by an obstacle) of the flow near that obstacle, it is reasonable to assume that the frisbee's effect on the dust flow will also behave similar. It decreases with the wind speed, but should stabilize somewhere around 4 m s− 1. The increasing length (along the wind) of the dust shadow explains the rapid decrease of the collector's efficiency below about 4 m s− 1. Below such wind speeds the catcher is only partially immersed in the dust shadow it creates; many particles will, thus, lodge in the catcher. As the wind speed increases, the dust shadow becomes longer, a larger proportion of the catcher lies within the dust shadow, and fewer particles settle in the catcher. The decrease in efficiency will continue until the catcher has become totally immersed in the dust shadow, or until the shadow's length along the wind has become constant. In both cases the efficiencies evolve towards a constant value. This may explain the stabilization of the curves in Figs. 5–7.

Fig. 8. Normalized drag coefficient curve for a spheroid with a diameter of 30 cm, a height of 3.6 cm and installed parallel to the flow.

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Particle resuspension may occur when wind speeds are sufficiently high. Tests performed by Hall and Upton (1988) suggest that, for a frisbee collector and for particles <100 μm, resuspension will start at a wind speed of approximately 5 m s− 1, i.e., the maximum speed in our tests. A control experiment was performed in the Leuven wind tunnel with an empty frisbee (no marbles, no flow deflector ring). The frisbee was covered by a thin, homogeneous layer of dust, and the wind tunnel was run for several minutes (recall that, in the efficiency tests, the experiments lasted for only 3 min). No significant erosion was observed for wind speeds below 5 m s− 1, so the efficiency curves in Figs. 5–7 are not substantially affected by resuspension. For instrumental reasons, no efficiency tests could be conducted in the large wind tunnel section for wind speeds > 5 m s− 1; hence no data could be collected for wind speeds beyond this value. For wind speeds above the erosion threshold resuspension will occur, and the stabilization of the efficiency curves will come to an end. However, the efficiency of a catcher will not drop to zero until the accumulation limit has been reached. At this limit the amount of dust eroded by the wind has become equal to the amount of dust deposited in the catcher and the accumulation has dropped to zero (see Goossens, 2001 for more information). A test was performed in the small wind tunnel section with an empty frisbee (the catcher in our study most susceptible to erosion) to determine the accumulation limit. This test showed that the accumulation limit had not yet been reached at a wind velocity of 12 m s− 1 for the dust used in the experiments, hence the efficiency curves obtained in our study will not drop to zero before reaching this wind speed value. That the catchers in this study are more efficient for fine particles rather than coarse particles can be explained by the size of the dust shadow zone (and, in the case of sufficiently high wind speeds, by the deflation curve). Goossens (2006) recently investigated how the dust shadow created by obstacles in the flow changes as a function of particle size. It was found that shadow length increases as particle size increases, up to approximately 50 μm, after which it remains more or less constant. This may explain why efficiency falls with particle size, and why it becomes independent of particle size for grains > 50 μm. In the case of high wind speeds the dust in the collector may also become subject to resuspension. For sediment <80–100 μm the deflation threshold increases as particle size decreases (Pye, 1987). Fine dust is, thus, less easily

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eroded than coarse dust, resulting in higher accumulation efficiency for the former than for the latter. To investigate the effect of adding marbles and/or a flow deflector ring to the frisbee sampler we calculated the ratio of the efficiency of the catcher with marbles (ring) to the efficiency of an empty frisbee (no marbles and no ring) (Fig. 9). The graphs show the average for all grain size classes and frisbee types studied. The marble curve (lower curve in the figure) shows that only for very low winds (1 m s− 1 or less) does a layer of marbles reinforce the accumulation of dust. An empty frisbee accumulates more dust at higher wind speeds. Adding a flow deflector ring to the frisbee sampler, on the other hand, is advantageous at all wind speeds but the positive effect of the ring tends to stabilize at wind velocities higher than approximately 3 m s− 1. No information was obtained for wind speeds >5 m s− 1. However, wind tunnel experiments by Wiggs et al. (2002) suggest that the positive effect of the ring extends to wind velocities of at least 12.5 m s− 1. There is also little doubt that, for wind speeds above the deflation threshold, a marble filter will benefit the accumulation because the particles trapped in and underneath the filter will be well protected against redeflation. In most regions on the globe the average wind velocity is of the order of 2–3 m s− 1 or more, varying from area to area. Fig. 5 shows that in such circumstances the efficiency of dust catchers is low, on the order of 20% or less. Of course the catchers tested in this study represent only a small selection of the catchers currently in use, but there is little evidence that the other catchers will be more efficient because many of them are not aerodynamically shaped. Even for a frisbee surrounded by a specially designed flow deflector ring (probably the most aerodynamic dust

Fig. 9. Effect of a marble layer (lower curve) and a flow deflector ring (upper curve) on the efficiency of the frisbee sampler.

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Table 1 Uncorrected and corrected dust deposition fluxes for a field experiment in Niger in April–May 2005 Period

Dust deposition flux (10− 6 g m− 2 s− 1) Uncorrected

MDCO 16–19 April 2005 19–23 April 2005 23–27 April 2005 10–13 May 2005 13–16 May 2005 16–20 May 2005 20–23 May 2005 23–27 May 2005

4.89 2.71 3.12 4.40 2.59 7.47 5.26 10.31

Average efficiency

Corrected 8.55 4.21 6.89 10.56 5.93 10.80 9.60 22.70

0.57 0.64 0.45 0.42 0.44 0.69 0.55 0.45

Frisbee with marbles and flow deflector ring 16–19 April 2005 5.20 9.77 19–23 April 2005 2.50 4.13 23–27 April 2005 4.53 9.02 10–13 May 2005 5.98 8.99 13–16 May 2005 3.21 5.99 16–20 May 2005 6.43 10.30 20–23 May 2005 4.30 9.00 23–27 May 2005 7.96 18.75

0.53 0.61 0.50 0.67 0.54 0.62 0.48 0.42

Catcher efficiency was calculated every 5 min; the table shows the integrated results for the periods indicated in the first column.

catcher currently available) the collection efficiency rarely exceeds 40% for winds above 2 m s− 1. This conclusion has far-reaching consequences. It shows that dust deposition rates reported in the literature and obtained by collecting the sediment in a catcher largely underestimate the true deposition. The errors are large, on the order of several hundred percent; errors of 300% are common and errors of 500% and more are not unusual. A revision of the dust deposition data obtained with catchers will not be easy because the efficiency of most catchers is not known, but it is clear that many values should be at least doubled or tripled. Table 1 illustrates the need for correcting dust deposition data obtained by catchers. The table shows deposition data collected during a field experiment in Niger in April–May 2005. The experiment aimed at evaluating several techniques and catchers for measuring the deposition of atmospheric dust and included, among other catchers, the MDCO and the frisbee with marbles and a flow deflector ring (more details on the experiment will be given in a subsequent publication). The need for correcting original deposition values is obvious from the table. To conclude the study, we present formulae that calculate the efficiency of an MDCO or a frisbee when

the grain size composition of the sediment and the wind's speed and direction are known. For the frisbee the equation: E ¼ au6 þ bu5 þ cu4 þ du3 þ eu2 þ fu þ g

ð1Þ

where: E = efficiency, u = wind speed (m s− 1), calculates very accurately the efficiencies measured during the wind tunnel tests. The formula is applicable to any wind speed between 0 and 7 m s− 1; for wind speeds > 7 m s− 1 and up to at least 10 m s− 1 the efficiencies approximate those at 7 m s− 1. The numerical values of coefficients a–g are given in Table A in Appendix A. For the MDCO the formula (Eq. (2)) is more complex because it should also include a term for the orientation of the sampler relative to the wind. The formula: E ¼ p½cosð2HÞ þ qcosð4HÞ þ r

ð2Þ

where: E = efficiency; H = orientation of the MDCO (rad); H = 0 for an MDCO parallel to the flow and H = π/2 for an MDCO perpendicular to the flow, accurately calculates the efficiencies measured during the wind tunnel tests. Table B in Appendix A shows how the coefficients p, q and r should be calculated. The formula is applicable to any wind speed between 1 and 7 m s− 1; for wind speeds >7 m s− 1 and up to at least 10 m s− 1 efficiency approximates that at 7 m s− 1. Fig. 10 shows the calculated efficiency (Eqs. (1) and (2)) versus the efficiency measured in the wind tunnel, for all combinations of wind speed, grain size class and catcher orientation investigated. A close-toperfect agreement is observed for all catchers examined.

6. Conclusions The most important conclusion of this study is that the collection efficiency of the dust samplers currently in use in earth science research is low, usually below 50% and in many cases even much lower. A collector's efficiency depends on the collector type, the sediment grain size and the wind characteristics. The catchers tested in this study represent only a small selection of the catchers currently in use, but there is little evidence that those other catchers are substantially more efficient because they are seldom aerodynamic. Five parameters determine the dust accumulation flux in a catcher: the horizontal dust flux, the weight of the particles, atmospheric turbulence, resuspension of

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Fig. 10. Calculated (Eqs. (1) and (2)) versus experimentally-measured efficiencies, for (A) the MDCO sampler; (B–E) the 4 frisbee samplers tested in this study.

particles deposited in the catcher, and the dust shadow created by the catcher. The final accumulation flux depends on the combination of these parameters. At low wind velocities the role of resuspension is negligible or absent. The other parameters exert an effect at all wind velocities. A catcher will collect sediment at all wind velocities up to the accumulation limit. Beyond this threshold speed the erosion rate exceeds the deposition rate, resulting in zero accumulation in the catcher. The accumulation limit depends on the characteristics

(especially the grain size) of the sediment and on the type of catcher, but most dust catchers currently used in earth science studies will accumulate dust up to wind velocities of at least 10 m s− 1 or more. However, at such high velocities there is little efficiency. The wind tunnel results reported here show that flow deflector rings may substantially improve ability of dust samplers to collect efficiently. For the four frisbee samplers tested efficiency doubled when a flow deflector ring was added to the sampler. The addition

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of marbles to a collector does not seem to increase efficiency, at least not at wind speeds up to 5 m s− 1. There is little doubt that at higher velocities (above the deflation threshold) a marble filter will benefit the accumulation because the particles trapped in and underneath the filter will be well protected against redeflation. Marble filters are also advantageous in humid climates, where they considerably reduce the amount of splash. Finally, they significantly help reduce the effects particle size exerts on the efficiency of a catcher. Marbles are thus a good solution to

diminish bias in the particle size distribution of collected dust. Acknowledgement This study was carried out in the framework of the EU project AMMA (African Monsoon Multidisciplinary Analyses) and was co-financed by the Centre National de la Recherche Scientifique, France. Receipt of a CNRS research fellowship for D. Goossens is greatly acknowledged.

Appendix A Table A Numerical values for coefficients a–g in Eq. (1) Grain size class

a

b

c

d

0.067052593 0.039526307 0.000482301 − 0.004153056 − 0.017544336 − 0.008637679 − 0.001946533 0.014153415

−0.282162916 −0.147575772 −0.023288213 −0.026600871 0.019231585 −0.019221155 −0.049202781 −0.124726466

Frisbee without marbles and with flow deflector ring 10–19 μm 0.000433572 −0.011309688 19–31 μm 0.000725470 −0.016571989 31–41 μm 0.000512891 −0.011662181 41–48 μm 0.000367835 −0.008583734 48–56 μm 0.000259330 −0.006022293 56–66 μm 0.000159625 −0.003820773 66–76 μm 0.000107789 −0.002731420 76–89 μm 0.000368035 −0.008857262

0.110458751 0.141668131 0.099559929 0.075982378 0.053435159 0.035176390 0.026834728 0.081829790

Frisbee with marbles and without flow deflector ring 10–19 μm 0.000255163 −0.005978909 19–31 μm − 0.000003829 0.000157537 31–41 μm − 0.000186525 0.004252771 41–48 μm − 0.000198838 0.004388285 48–56 μm − 0.000228302 0.004880936 56–66 μm − 0.000232439 0.005047216 66–76 μm − 0.000205149 0.004412196 76–89 μm − 0.000154213 0.003200440 Frisbee with marbles and with flow deflector ring 10–19 μm 0.000064766 −0.001844342 19–31 μm 0.000019000 −0.000178788 31–41 μm − 0.000032745 0.001309793 41–48 μm − 0.000040919 0.001574053 48–56 μm − 0.000065349 0.002184059 56–66 μm − 0.000056598 0.001886744 66–76 μm − 0.000060419 0.001975215 76–89 μm 0.000000778 0.000338548

Frisbee without marbles and without flow deflector ring 10–19 μm 0.000290774 −0.007221394 19–31 μm 0.000229331 −0.004932561 31–41 μm − 0.000007649 0.000122584 41–48 μm − 0.000057472 0.001026888 48–56 μm − 0.000139026 0.002746365 56–66 μm − 0.000095604 0.001747262 66–76 μm − 0.000063318 0.001005637 76–89 μm 0.000000359 −0.000626630

e

f

g

0.525970773 0.300653904 0.198059924 0.281096844 0.221950613 0.302569000 0.370401537 0.536019197

−0.543354976 −0.580487539 −0.715823735 −0.879553901 −0.876428827 −0.941911739 −1.005053478 −1.139073492

1.017187451 1.005620227 1.002223651 1.004480948 1.006564743 1.004011411 1.005592634 1.018134304

−0.485011925 −0.546124908 −0.389719627 −0.316594605 −0.227506893 −0.158435834 −0.131516730 −0.362947860

0.873594587 0.886600504 0.696697556 0.648688526 0.505158469 0.393982065 0.367602414 0.810346330

−0.507584205 −0.568306130 −0.675911455 −0.790612492 −0.746599896 −0.699787680 −0.718827158 −1.008894250

1.042110337 1.014112420 1.013081807 1.009157592 1.006668135 1.004027679 1.006548715 1.016836250

0.054651711 − 0.001118187 − 0.035597454 − 0.034953184 − 0.037335744 − 0.039398163 − 0.033686998 − 0.022705495

−0.248940583 −0.009824746 0.122201781 0.107821419 0.107782159 0.118349321 0.093580654 0.046996557

0.620698435 0.153740090 − 0.058851390 0.007267178 0.032098198 0.009898648 0.061804910 0.151512208

−0.965940643 −0.665992415 −0.582612175 −0.681682122 −0.724080473 −0.708386873 −0.750977374 −0.809806755

1.007721741 1.001084163 0.998627161 1.004421157 1.008395234 1.003524082 1.003775769 1.006141656

0.019862222 − 0.003115359 − 0.019493787 − 0.022380711 − 0.027921503 − 0.024032381 − 0.024600039 − 0.008140620

−0.100950991 0.048728970 0.133254542 0.145363696 0.166209359 0.141693653 0.140937866 0.065558700

0.264920811 − 0.192654255 − 0.388287190 − 0.397348430 − 0.417047538 − 0.342333187 − 0.323441127 − 0.177832573

−0.512253234 0.016358007 0.159601426 0.117396659 0.074998001 −0.014246698 −0.066845145 −0.134856357

1.005491794 0.936881217 0.924131430 0.940926547 0.973551611 0.972925056 0.996300007 0.977774070

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373

Table B Calculation of coefficients p–r in Eq. (2). The wind velocity (u) should be expressed in m s− 1 Grain size class: 10–19 μm p = −0.0007312u6 + 0.0179021u5 − 0.1707926u4 + 0.7998553u3 − 1.909097u2 + 2.1796702u − 0.7589208 q = 0.003254u6 − 0.0812532u5 + 0.7914492u4 − 3.7704872u3 + 8.9850607u2 − 9.8238079u+ 4.6008771 r = 0.00033u6 − 0.0083723u5 + 0.0830726u4 − 0.4052823u3 + 1.0294318u2 − 1.5555221u + 2.0279614 Grain size class: 19–31 μm p = −0.0001487u6 + 0.0039875u5 − 0.0430022u4 + 0.2363859u3 − 0.6819261u2 + 0.8975262u − 0.1900518 q = 0.0011874u6 − 0.0267361u5 + 0.2292733u4 − 0.9326238u3 + 1.8279941u2 − 1.5175786u + 0.5077858 r = 0.0000704u6 − 0.0011765u5 + 0.004459u4 + 0.0178979u3 − 0.1133941u2 − 0.0988192u + 0.9562646 Grain size class: 31–41 μm p = −0.0003893u6 + 0.0098364u5 − 0.0988843u4 + 0.500032u3 − 1.3078237u2 + 1.5641027u − 0.4489484 q = 0.0011525u6 − 0.0245822u5 + 0.1964212u4 − 0.7251544u3 + 1.2310008u2 − 0.7651931u + 0.0718773 r = 0.0000679u6 − 0.001387u5 + 0.0109808u4 − 0.050245u3 + 0.2048164u2 − 0.7133766u + 1.174822 Grain size class: 41–48 μm p = −0.0003354u6 + 0.0083569u5 − 0.0825755u4 + 0.4081882u3 − 1.0319106u2 + 1.1570826u − 0.2542443 q = 0.0014286u6 − 0.0314525u5 + 0.2639643u4 − 1.0568822u3 + 2.0750595u2 − 1.7884813u + 0.4618608 r = −0.0002514u6 + 0.0060844u5 − 0.0567245u4 + 0.2469244u3 − 0.4381122u2 − 0.088025u + 0.9188296 Grain size class: 48–56 μm p = −0.0003839u6 + 0.0095027u5 − 0.0929896u4 + 0.4535455u3 − 1.1270835u2 + 1.2410075u − 0.2826763 q = 0.0008109u6 − 0.0182303u5 + 0.1562452u4 − 0.6388685u3 + 1.2855912u2 − 1.1439787u + 0.3163917 r = −0.000471u6 + 0.0111491u5 − 0.1018407u4 + 0.4408787u3 − 0.8449778u2 + 0.2762581u + 0.8195589 Grain size class: 56–66 μm p = −0.0003408u6 + 0.008414u5 − 0.0821892u4 + 0.4006571u3 − 0.9970171u2 + 1.1003705u − 0.2447915 q = 0.0011089u6 − 0.0252229u5 + 0.2193808u4 − 0.9120308u3 + 1.8558738u2 − 1.6347861u + 0.4165165 r = −0.0006631u6 + 0.0155548u5 − 0.1408579u4 + 0.6076432u3 − 1.1928163u2 + 0.5823348u + 0.7550838 Grain size class: 66–76 μm p = −0.0002656u6 + 0.0066135u5 − 0.0653224u4 + 0.3229905u3 − 0.8181548u2 + 0.9219298u − 0.2068569 q = 0.0012006u6 − 0.0267896u5 + 0.2272357u4 − 0.9102762u3 + 1.7382254u2 − 1.3410343u + 0.2334168 r = −0.0008862u6 + 0.0207805u5 − 0.1884012u4 + 0.8179367u3 − 1.6515778u2 + 1.0132845u + 0.6500999 Grain size class: 76–89 μm p = 0.0001831u6 − 0.004039u5 + 0.0328742u4 − 0.1157104u3 + 0.1369389u2 + 0.0696383u − 0.0800579 q = −0.0050413u6 + 0.0997494u5 − 0.6770816u4 + 1.4824707u3 + 2.2849432u2 − 12.9243352u + 12.4668909 r = −0.000542u6 + 0.0130571u5 − 0.1223308u4 + 0.5513871u3 − 1.1486929u2 + 0.6390925u + 0.7385311

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