Horizontal Dust Flux Research Articles

Summertime Taklimakan dust structure

Vertical dust structures and meteorological conditions over the Taklimakan Desert during the summertime periods (July–September) in 2006 and 2007 were investigated using satellite data including the CALIPSO lidar (CALIOP) measurements and the Weather Research and Forecasting (WRF) model simulation. Summertime convective velocity simulated by WRF was greater than 3 m/s, and the convective time scale was about 1000 s (ca. 15 min). We examined 42 CALIPSO paths (24 daytime and 18 nighttime paths) of data acquired under convective dusty conditions. The dust layer thickness derived from the CALIOP measurements reaches 3000–4000 m, which is approximately equal to the Tarim Basin depth. This thickness shows a good correlation with the WRF simulated depth of the convective boundary layer (BL). The dust remains suspended during both daytime and nighttime; the CALIOP average dust extinction coefficients in the BL are respectively 0.151 ± 0.102 km−1 and 0.128 ± 0.079 km−1 for daytime and nighttime. Finally, we estimated the dust amount transported from the BL to the free atmosphere. Typically, 10–20 Gg/day of dust (assuming area of 600 km × 300 km as a main part of the Taklimakan Desert) is transported from the Taklimakan Desert by vertical mixing. The daily horizontal dust flux above the BL was estimated to be 40–50 Gg/day over the Taklimakan Desert.

The implications for dust emission modeling of spatial and vertical variations in horizontal dust flux and particle size in the Bodélé Depression, Northern Chad

The Bodélé Depression has been confirmed as the single largest source of atmospheric mineral dust on Earth. It is a distinctive source because of its large exposure of diatomite and the presence of mega‐barchan dunes. Direct measurements of horizontal dust flux and particle size were made to investigate dust emission processes and for comparison with mechanisms of emission assumed in current dust models. More than 50 masts, with traps mounted on each, were located across and downwind of three barchans in 56 km2 study area of the eastern Bodélé. The size‐distribution of surface material is bi‐modal; there are many fine dust modes and a mixed mineralogy with a particle density three times smaller than quartz. Horizontal fluxes (up to 70 m above the playa) of particles, up to 1000 μm in diameter, are produced frequently from the accelerated flow over and around the barchans, even in below‐threshold shear conditions on the diatomite playa. Our data on dust sizes do not conform to retrievals of dust size distributions from radiance measurements made in the same area. Dust emission models for the region may need to be revised to account for: saltators in the Bodélé, which are a mixture of quartz sand and diatomite flakes; the great spatial and vertical variation in the abundance, mass and density of dust and abraders; and the patterns of surface erodibility. All of these have important local effects on the vertical dust flux and its particle sizes.

Effects of grazing and topography on dust flux and deposition in the Xilingele grassland, Inner Mongolia

The dynamics of dust emission and deposition in the grasslands of Inner Mongolia were investigated during a measuring campaign from April to May 2005 in grazed and un-grazed plots. Both processes are determined by the grazing intensity, whereas dust deposition rates are modified additionally by the topography. Because grazing intensity influences the height and density of the vegetation, it could therefore be measured through the surface roughness length ( z 0). Almost all strong winds come regularly from the northwest, which give rise to the distinction between exposed windward and mostly sheltered leeward slopes. Dust deposition and dust remobilization are merged processes, which are difficult to separate during dust storms. Airborne sediments that originate from various source areas (supra-regional dust storms and local wind erosion) were distinguished by comparing vertical transport profiles. The average horizontal dust flux measured during the measuring campaign for below the height of 1 m was between 180 and 239 g m −1 width and day. The average proportion of material transported by local wind erosion amounted to only 5% in grazed plots. Evidence of dust emission was found at all grazed sites (up to 0.8 g m −2 d −1) while ungrazed sites seemed well protected. The dust deposition rates on grazed and ungrazed sites were on average 1.3 and 2.4 g m −2 d −1, respectively. Leeward slopes had 29–33% higher deposition rates than windward slopes, summits and plane positions.

Thinning semiarid forests amplifies wind erosion comparably to wildfire: Implications for restoration and soil stability

Semiarid forests across the western USA and elsewhere are being thinned to reduce risk from fire, restore previous ecological conditions, and/or salvage trees from recently burned areas. Prescriptions and monitoring for thinning generally focus on biotic characteristics of vegetation, like tree density, rather than abiotic characteristics of soils and their loss, which are usually only considered in association with water erosion. Recent studies indicate that sediment transport by wind in forests is substantial and can exceed water transport, yet forest wind erosion responses to tree thinning and/or burning are unknown. We measured wind-driven horizontal dust flux, a metric related to wind erosion, with respect to presence/absence of fire and/or thinning in a ponderosa pine ( Pinus ponderosa) forest in northern New Mexico, USA. Passive dust collectors at several sampling heights documented elevated dust fluxes at sites that were burned and/or thinned. Unexpectedly, thinned sites had erosion rates as large as burned sites, documenting significant restoration impacts on soil stability. Thinning and fire impacts on dust flux were correlated with remaining tree or ground cover. The results highlight that dust fluxes provide a readily measurable metric of soil stability that should be integrated into prescription and monitoring plans for forest restoration and thinning.

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

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.

From dust to dose: Effects of forest disturbance on increased inhalation exposure

Ecosystem disturbances that remove vegetation and disturb surface soils are major causes of excessive soil erosion and can result in accelerated transport of soils contaminated with hazardous materials. Accelerated wind erosion in disturbed lands that are contaminated is of particular concern because of potential increased inhalation exposure, yet measurements regarding these relationships are lacking. The importance of this was highlighted when, in May of 2000, the Cerro Grande fire burned over roughly 30% of Los Alamos National Laboratory (LANL), mostly in ponderosa pine (Pinus ponderosa) forest, and through areas with soils containing contaminants, particularly excess depleted and natural uranium. Additionally, post-fire thinning was performed in burned and unburned forests on about 25% of LANL land. The first goal of this study was to assess the potential for increased inhalation dose from uranium contaminated soils via wind-driven resuspension of soil following the Cerro Grande Fire and subsequent forest thinning. This was done through analysis of post-disturbance measurements of uranium air concentrations and their relationships with wind velocity and seasonal vegetation cover. We found a 14% average increase in uranium air concentrations at LANL perimeter locations after the fire, and the greatest air concentrations occurred during the months of April–June when wind velocities are highest, no snow cover, and low vegetation cover. The second goal was to develop a methodology to assess the relative contribution of each disturbance type towards increasing public and worker exposure to these resuspended soils. Measurements of wind-driven dust flux in severely burned, moderately burned, thinned, and unburned/unthinned forest areas were used to assess horizontal dust flux (HDF) in these areas. Using empirically derived relationships between measurements of HDF and respirible dust, coupled with onsite uranium soil concentrations, we estimate relative increases in inhalation doses for workers ranging from 15% to 38%. Despite the potential for increased doses resulting from these forest disturbances, the estimated annual dose rate for the public was <1 μSv yr−1, which is far below the dose limits for public exposures, and the upper-bound dose rate for a LANL worker was estimated to be 140 μSv yr−1, far below the 5×104 μSv yr−1 occupational dose limit. These results show the importance of ecosystem disturbance in increasing mobility of soil-bound contaminants, which can ultimately increase exposure. However, it is important to investigate the magnitude of the increases when deciding appropriate strategies for management and long-term stewardship of contaminated lands.

Thirteen years of aeolian dust dynamics in a desert region (Negev desert, Israel): analysis of horizontal and vertical dust flux, vertical dust distribution and dust grain size

At Sede Boqer (northern Negev desert, Israel), aeolian dust dynamics have been measured during the period 1988–2000. This study focuses on temporal records of the vertical and horizontal dust flux, the vertical distribution of the dust particles in the atmosphere, and the grain size of the particles. This study extends results reported earlier on the characteristics of the airborne dust concentration, the accumulation of dust on the desert floor, and the high-magnitude dust events. Vertical dust flux showed a systematic trend over the year, with high values in spring and low values in autumn. Vertical flux was very low from 1989 to 1991 and from 1997 to 1998, and moderately high or high in the other years. Horizontal dust transport, which was quantified by means of the horizontal dust flux, was also highest in spring and lowest in autumn and in winter. During the period 1988–2000, the horizontal dust flux value oscillated systematically with a periodicity of 30 months. At Sede Boqer the amount of dust in the atmosphere always decreased with height. Vertical stratification was strongest during the first months of the year and lowest at the end of the year. Stratification was very low in 1989–1991, high in 1988 and between 1992 and 1996, and moderate in the other years. The size of the dust (quantified by means of the median grain diameter) was also highest in spring and lowest in autumn. Median grain diameter was around 30 μm in 1988–1989, in 1995–1996, and in 1999–2000; in the other years it was considerably lower, around 25 μm or less. The average annual vertical dust flux curve parallels the average annual wind speed curve. A similar relationship was observed between airborne dust concentration and vertical dust flux, but only for the background concentration. At Sede Boqer, a negative relationship exists between the amount of rainfall and the amount of vertical dust flux. This relationship is not only apparent during the rainy season itself, but also when the amount of rainfall during a rainy season is compared to the amount of vertical dust flux during the next dry season. The dust crust on the desert floor, which is much stronger after rainy seasons with high precipitation than after rainy seasons with low precipitation, is most probably responsible for the latter relationship.

Aeolian dust dynamics in agricultural land areas in Lower Saxony, Germany

AbstractThe dynamics of fine aeolian dust emitted from agricultural land was investigated over 15 months near Grönheim, Lower Saxony, Germany. The following aspects were studied: airborne dust concentration, the ratio of mineral versus organic dust, the vertical distribution of the particles in the atmosphere, horizontal and vertically integrated horizontal dust flux, vertical dust flux, dust deposition at ground level, grain‐size distribution of the mineral dust component, and vertical distribution of organic matter in the dust. Standard meteorological parameters (wind speed and direction, precipitation) were measured as well. Dust activity in Grönheim is high in spring (March–May) and autumn (October–November) and low to very low during the rest of the year. There is a strong relationship between the periods of tillage and the intensity of dust activity. Also, there is high dust activity during wind erosion events. For the year 1999, dust emission due to tillage was 6·6 times higher than dust emission due to wind erosion. A dust transport of 15·8 ton km−1 a−1 was calculated for the first 10 m of the atmosphere in 1999. Total dust transport (in the entire mixing layer) was estimated between 16 and 20 ton km−1 a−1. About 25–30 per cent of this dust is mineral dust, emitted from the fields during tillage or during wind erosion events. In spring and autumn there is a strong vertical stratification in the airborne sediment, with much (coarse) dust in the lower air layers and significantly less (and finer) dust at higher altitudes. In summer and winter, when there is no local dust production, there is no stratification: equal amounts of dust are transported at all heights. The stratification in spring and autumn is exclusively caused by the mineral part of the dust. The organic particles are much better mixed in the atmosphere because of their lower density. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Wind tunnel and field calibration of six aeolian dust samplers

The efficiency of six aeolian dust samplers was tested via wind tunnel experiments and field measurements. In the wind tunnel, four samplers designed to measure the horizontal dust flux and one sampler designed to measure the vertical dust flux (in the downward direction, i.e., deposition) were calibrated against an isokinetic reference sampler. The horizontal dust flux samplers were: the big spring number eight sampler (BSNE), the modified Wilson and Cooke sampler (MWAC), the suspended sediment trap (SUSTRA), and the wedge dust flux gauge (WDFG). Vertical deposition flux was measured using a marble dust collector (MDCO). A modified Sartorius SM 16711 dust sampler with adjustable flow rate (SARTORIUS) was used as isokinetic reference sampler. In the field experiments, the WDFG was replaced by a Sierra ultra high volume dust sampler (SIERRA). Wind tunnel calibrations were carried out at five wind velocities ranging from 1 to 5 m s −1. Field calibrations were conducted during seven periods of two weeks each. The most efficient samplers are the MWAC and the SIERRA, followed by the BSNE and the SUSTRA. The WDFG is more effective than the BSNE at velocities below 3 m s −1, but its efficiency drops quickly at higher wind speeds. The most recommendable sampler for field measurements is the BSNE, because its efficiency varies only very slightly with wind speed. In the absence of horizontal flux samplers, the MDCO collector can be used as an alternative to assess horizontal dust flux and airborne dust concentration provided the appropriate calibrations are made.