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An analysis of saltation data led us to modify the theory of P. R. Owen using a soil-related parameter ‘A’ that gave us the possibility of expressing limitation of sand grains of saltation-size in the underlying soil. The value of ‘A’ was set equal to the ratio of the horizontal flux of saltating particles to Owen's function of wind, times air density divided by gravitational acceleration. Values of A can be used to: (1) characterize the efficiency of the wind to move sand by saltation for different soil textures and aggregations; and (2) to make practical predictions of sand movement based on the condition of the surface soil. Values for A in a range from 1 to 10 are usually associated with supply-unlimited saltation and are usually associated with loose, sandy-textured soils. Values for A in a range from 0·25 to 1 are associated with finer soils that contain more silt and clay. The range of A values between 0 and 0·25 usually reflects finer textured soils that are packed, aggregated, or crusted. A decrease of A to a smaller value is a sign of supply limitation and usually to the soil changing from a looser state to a more aggregated state or more depleted state. Likewise, an increase of A usually corresponds to soil changing from an aggregated state to a looser state. Copyright © 2008 John Wiley & Sons, Ltd.

Major, minor, and trace element concentrations were determined by particle-induced x-ray emission (PIXE) in 118 aeolian sediment samples collected at six heights in seven locations along a 1.2 km transect during three sequential dust storms at Owens (dry) Lake, California, USA. Na and S concentrations covaried with each other (and inversely with Si and Ca), increased with height, and decreased with distance downwind and time. Mg, Al, Si, K, Mn, Fe, and Sr concentrations at northerly sites varied with height and location as opposed to nearly constant concentrations at southerly locations. Volumetric particle-size distribution (PSD) for each sample was determined via laser diffraction. PSDs of the collected sediment reflected a trimodal distribution:63% of samples peaked at 20-50 μm (silt), 11% at 50-100 μm (very fine sand) and 26% at 100-250 μm (fine sand). Most silty samples occurred during the first two events. Significant differences in element concentrations existed in relation to the PSDs. Na and S concentrations were proportional to the submicron to silt particle fraction during each event. Al, Ti, Mn, K, Fe, and Rb concentrations correlated with 100-500 μm (fine/medium sand) particles in the first two events and a wider PSD range 250-1000 μm (coarse sand) in the third event. The results suggest sodium sulfate aerosol emission during the first windstorm, while subsequent saltation-dominated events released more aluminosilicate minerals containing higher trace metal concentrations. These combined techniques reveal particle size/chemical fractionation and spatial variability of sediment properties during resuspension at aeolian 'hotspots,' with implications to geochemical cycling and aerosol source/receptor relationships.

Past research has shown that the most important areas for active sand movement in the northern part of the Chihuahuan Desert are mesquite-dominated desert ecosystems possessing sandy soil texture. The most active sand movement in the mesquite-dominated ecosystems has been shown to take place on elongated bare soil patches referred to as “streets”. Aerodynamic properties of mesquite streets eroded by wind should be included in explaining how mesquite streets are more emissive sand sources than surrounding desert land. To understand the effects of wind properties, we measured them at two flat mesquite sites having highly similar soil textures but very different configurations of mesquite. The differences in wind properties at the two sites were caused by differences of size, orientation, and porosity of the mesquite, along with the presence of mesquite coppice dunes (sand dunes stabilized by mesquites growing in the dune and on its surface) found only at one of the two sites. Wind direction, u * (friction velocity), z 0 (aerodynamic roughness height) and D (zero plane displacement height) were estimated for 15-m tower and 3-m mast data. These aerodynamic data allowed us to distinguish five categories with differing potentials for sediment transport. Sediment transport for the five categories varied from unrestricted, free transport to virtually no transport caused by vegetation protection from wind forces. In addition, “steering” of winds below the level of the tops of mesquite bushes and coppice dunes allowed longer parallel wind durations and increased wind erosion for streets that aligned roughly SW–NE.

Aeolian processes are tightly linked to soil and vegetation change in arid and semi-arid systems at multiple spatial and temporal scales. Wind influences patterns of vegetation and soil within the landscape, and these patterns control wind erosion at patch to landscape scales. Aggregated at larger scales, patterns in soil and vegetation distributions influence global distributions of dust and its biogeochemical impacts. Understanding the controls on aeolian processes is therefore important not only in understanding the biogeochemistry and land cover patterns in dryland environments, but also in understanding global land cover, climate, and biogeochemistry. Although the microscopic physics that control aeolian processes are well understood, the controls on these processes in real landscapes are poorly constrained, particularly for structurally complex plant communities such as shrub-invaded grasslands. This paper reviews the controls on aeolian processes and their consequences at plant-interspace, patch-landscape, and regional-global scales. Based on this review, we define the requirements for a cross-scale model of wind erosion in structurally complex arid and semi-arid ecosystems.

This paper presents PM 10 fugitive dust emission factors for a range of vehicles types and examines the influence of vehicle and wake characteristics on the strength of emissions from an unpaved road. Vertical profile measurements of mass concentration of the passing plumes were carried out using a series of 3 instrumented towers. PM 10 emission fluxes at each tower were calculated from knowledge of the vertical mass concentration profile, the ambient wind speed and direction, and the time the plume took to pass the towers. The emission factors showed a strong linear dependence on speed and vehicle weight. Emission factors (EF = grams of PM 10 emitted per vehicle kilometer traveled) ranged from approximately EF = 0.8 x (km h -1 ) for a light (∼1200 kg) passenger car to EF = 48 x (km h -1 ) for large military vehicles (∼18000 kg). In comparison to emission estimates derived using US EPA AP-42 methods the measured emission factors indicate larger than estimated contributions for speeds generally > 10-20 km h -1 and for vehicle weights > 3000 kg. The size of a wake created by a vehicle was observed to be dependent on the size of the vehicle, increasing roughly linearly with vehicle height. Injection height of the dust plume is least important to long-range transport of PM 10 under unstable conditions and most important under stable atmospheric conditions.

Kinetic energy from the oscillatory impacts of the grass stalk against a stationary object was measured with a kinetic energy measuring device. These energy inputs were measured as part of a resuspension experiment of uniform latex microspheres deposited on a single rye grass seed pod in a wind tunnel. The experiment was designed to measure resuspension from aerodynamic (viscous and turbulent) mechanisms compared to that from mechanisms from mechanical resuspension resulting from the oscillatory impact of the grass hitting a stationary object. The experiment was run for deposited spherical latex particles with diameters from 2 to 8.1 μm. Wind tunnel tests were run for wind speeds from 2 to 18.5 m s −1 and a turbulence intensity (root-mean-square fluctuation wind speed/mean wind speed) of 0.1. Our experiments showed the following: • Threshold mechanical energy input rates increased from 0.04 to 0.2 μJ s −1 for resuspension of spherical polystyrene latex particles from 2 to 8.1 μm diameter. • Kinetic energy flux generated by mechanical impact of the wind-driven oscillating grass was found to be highly sensitive to slightly different placements and grass morphology. • The kinetic energy input by impaction of the grass against a stationary cylinder is roughly proportional to the kinetic energy flux of the wind.

This work was motivated by the need to better reconcile emission factors for fugitive dust with the amount of geologic material found on ambient filter samples. The deposition of particulate matter with aerodynamic diameter less than or equal to 10 microm (PM10), generated by travel over an unpaved road, over the first 100 m of transport downwind of the road was examined at Ft. Bliss, near El Paso, TX. The field conditions, typical for warm days in the arid southwestern United States, represented sparsely vegetated terrain under neutral to unstable atmospheric conditions. Emission fluxes of PM10 dust were obtained from towers downwind of the unpaved road at 7, 50, and 100 m. The horizontal flux measurements at the 7 m and 100 m towers indicated that PM10 deposition to the vegetation and ground was too small to measure. The data indicated, with 95% confidence, that the loss of PM10 between the source of emission at the unpaved road, represented by the 7 m tower, and a point 100 m downwind was less than 9.5%. A Gaussian model was used to simulate the plume. Values of the vertical standard deviation sigma(z) and the deposition velocity Vd were similar to the U.S. Environmental Protection Agency (EPA) ISC3 model. For the field conditions, the model predicted that removal of PM10 unpaved road dust by deposition over the distance between the point of emission and 100 m downwind would be less than 5%. However, the model results also indicated that particles larger than 10 microm (aerodynamic diameter) would deposit more appreciably. The model was consistent with changes observed in size distributions between 7 m and 100 m downwind, which were measured with optical particle counters. The Gaussian model predictions were also compared with another study conducted over rough terrain and stable atmospheric conditions. Under such conditions, measured PM10 removal rates over 95 m of downwind transport were reported to be between 86% and 89%, whereas the Gaussian model predicted only a 30% removal. One explanation for the large discrepancy between measurements and model results was the possibility that under the conditions of the study, the dust plume was comparable in vertical extent to the roughness elements, thereby violating one of the model assumptions. Results of the field study reported here and the previous work over rough terrain bound the extent of particle deposition expected to occur under most unpaved road emission scenarios.

Resuspension of uniform latex micro spheres deposited on a single seed pod of field rye grass stalk and head was investigated experimentally in a wind tunnel. The experiment was designed to distinguish aerodynamic (viscous and turbulent) mechanisms from mechanical resuspension resulting from the oscillatory impact of the grass hitting a stationary object. The experiment was run for deposited spherical latex particles with diameters from 2 to 10 μm. Wind tunnel tests were run for wind speeds from 2 to 18.5 m s−1 and a turbulence intensity (root-mean-square fluctuation wind speed/mean wind speed) of 0.1. Our experiments showed the following for our test of concept experiment: • Resuspension particle flux increases when mechanical impacts occur. • Mechanical resuspension dominated for 2 μm particles over purely aerodynamic resuspension, but for larger particles aerodynamic mechanisms were roughly equally effective in resuspending particles.

Wind erosion of the dried bed (playa) of Owens Lake, California is an extremely intense source of mineral aerosol, transporting dust hundreds of kilometers downwind to critical ecological areas and several cities. A dust-producing site on the playa was studied over a four-year period to document the processes associated with aerosol emission. The playa takes on a variety of sedimentary forms and phases with surface crusts of differing susceptibilities to wind erosion. The sediments are classed into three general categories based on appearance: soft (saline), loose with drifting sand (salt–silt–clay), and hard and clean (silt–clay). Sediment samples were collected over a two-year period as the study site cycled through all three crust types, and the samples were crushed and analyzed by PIXE. The results indicate that visual appearance and sedimentary structure does not correlate with elemental composition. All sediment types contain significant concentrations of various elements including sodium, calcium and silicon. Potentially toxic trace elements are also found in the sediments. All sediment types contain lead and/or arsenic in tens of parts per million, as well as various other heavy metals. Pb and As levels do not clearly correlate with salt content or sediment type. Arsenic levels may be slightly higher in the crusts with loose material present and potentially lower in the clean hard crusts, while Pb was least frequently detected in the samples with loose material. Future research will add mineralogical and stable isotope analyses to correlate with the PIXE data.

A model for the emission of PM10 dust has been constructed using the concept of a threshold friction velocity which is dependent on surface roughness. Surface roughness in turn was correlated with geomorphology or soil properties for Kuwait, Iraq, part of Syria, Saudi Arabia, the United Arab Emirates and Oman. The PM10 emission algorithm was incorporated into a Lagrangian transport and dispersion model. PM10 air concentrations were computed from August 1990 through August 1991. The model predicted about the right number of dust events over Kuwait (events occur 18% of the time). The model results agreed quantitatively with measurements at four locations in Saudi Arabia and one in Kuwait for one major dust event (>1000 μg/m3). However, for smaller scale dust events (200–1000 μg/m3), especially at the coastal sampling locations, the model substantially over-predicted the air concentrations. Part of the over-prediction was attributed to the entrainment of dust-free air by the sea breeze, a flow feature not represented by the large-scale gridded meteorological data fields used in the model computation. Another part of the over-prediction was the model's strong sensitivity to threshold friction velocity and the surface soil texture coefficient (the soil emission factor), and the difficulty in accurately representing these parameters in the model. A comparison of the model predicted PM10 spatial pattern with the TOMS satellite aerosol index (AI) yielded a spatial pattern covering a major portion of Saudi Arabia that was quite similar to the observed AI pattern.

A site at Owens Dry Lake was observed for more than 4 years. The site was a vegetation-free saline playa where the surface formed “ephemeral crusts,” crusts that form after rainfall. Sometimes these crusts were destroyed and often a layer of particles on the crust would engage in vigorous aeolian activity. Three “phases” of active sand drifting are defined as almost no movement (extreme supply limitation), loose particles on crust with some degree of sand drift (moderate supply limitation), and unlimited source movement corresponding to a destroyed surface crust (unlimited supply). These “phases” occurred 45, 49, and 6% of the time, respectively. The accumulation of loose particles on the crust was mostly the result of in situ formation. Crusted sediments with loose particles on top can exhibit mass flux rates about the same as for noncrusted sediments. Crusted sediments limit or eliminate sand drift in two conditions: for rough crusts that effect a sufficiently high threshold friction velocity (above the wind friction velocity) and for limited amounts of loose particles on the crust where particle supply is less than would be transported in normal saltation for a thick sandy surface. These “supply-limited” cases are similar to wind erosion of limited spilled material on a hard concrete surface. We quantified “supply limitation” by defining a “potential” or “supply unlimited” sand drift function Q = AG where A represents supply limitation that decreases as the particle source is depleted. Here Q is the mass of sand transported through a surface perpendicular to the ground and to the wind and having unit width during time period t, and G = ∫ u*(u2* − u2*t) dt for u* > u*t. G is integrated for the same time period t as for Q, u* is the friction velocity of the wind, and u*t is the threshold friction velocity of the wind. Hard crusts (usually formed in the summer) tended to show almost no change of threshold friction velocity with time and often gave total protection from wind erosion. Rough crusts provided sufficient protection expressed as high threshold friction velocities. For these high threshold friction velocities, aeolian activity was greatly reduced or practically prevented. The softest crusts, usually formed in the winter, provided much less protection and sometimes were destroyed by the wind. Following this destruction the “potential” or “supply unlimited” sand drift would be observed.

Size distributions were obtained for airborne particles in the saltation layer for cases where the ratio of the vertical flux of suspended particles to the horizontal flux of particles of all sizes is known. The data can be used to test theories of emissions of suspended particles. All of our sampled erodible soils with textures of sand, loamy sand and loam had saltation-grain mean particle sizes in the range 117–160 µm. Our clay soil had saltation mean particle sizes ranging from 560 to 584 µm. In the light of these data, we interpret the results found previously as supporting sandblasting rather than direct aerodynamic entrainment. The data are consistent with sandblasting theories of Shao et al. and of Alfaro. Copyright © 1999 John Wiley & Sons, Ltd.

A joint United States/Russian/French collaborative experiment was undertaken in March 1993 and March 1996. Projects LODE I and II (Lake Owens Dust Experiments) took place on the anthropogenically desertified playa (dry lakebed) and surrounding regions of Owens Lake, in east-central California. One of the five parts of Project LODE was to determine relationships between optical depth and flux of dust emitted from the dry lake. Project LODE II included subsequent dust plume measurements and size distributions obtained through April 1996, to further refine the flux measurements for distinct mineral aerosol source regions at Owens Lake. Size distributions of dust aerosol were determined and aerosol optical depths were calculated from sunphotometer solar extinction measurements taken downwind in plumes coming from the emissive areas of Owens Lake. This source was visually observed for 10 measured dust storms. The plume mass was calculated to be 1·5 × 109 g using ground-based measurements and ≥1·6 × 109 g from satellite data. Project LODE II results were found to be consistent with LODE I results for the south end of the playa, but flux values were found to be reduced for the northeastern portion of the playa by comparison. Vertical flux values estimated by sunphotometry were found to be consistent with values estimated via a micrometeorological method. Copyright © 1999 John Wiley & Sons, Ltd.

Our understanding of the physical mechanisms of wind erosion and dust emission stimulated by disturbance of surface soils is not adequate, even though it is a problem leading to desertification in the arid zone. Simulations of the influence of surface soil disturbance ratios on wind erosion and sedimentary characteristics of an eroded loessal sandy loam soil were conducted in an indoor wind tunnel. The results showed that the grain-size composition of the drifting soil was finer than, though similar to, the original soil, as the surface disturbance ratio increased. Wind erosion on the loessal sandy loam soil was selective. The eroded sediments were poorly sorted and became poorer with increases in the disturbance ratio and wind speed. The relative proportion of suspension population increased with increasing disturbance ratio. Several parameters such as the pattern of the frequency distribution curves, skewness and kurtosis reflected the dependence of the sedimentary characteristics of the drifting sediments on the eroding source soil. Soil erosion and dust emission by wind in drylands can be reduced by reducing unnecessary mechanical disturbance during erosion seasons.

Biological soil crusts, consisting of cyanobacteria, green algae, lichens, and mosses, are important in stabilizing soils in semi-arid and arid lands. Integrity of these crusts is compromised by compressional disturbances such as foot, vehicle, or livestock traffic. Using a portable wind tunnel, we found threshold friction velocities (TFVs) of undisturbed crusts well above wind forces experienced at these sites; consequently, these soils are not vulnerable to wind erosion. However, recently disturbed soils or soils with less well-developed crusts frequently experience wind speeds that exceed the stability thresholds of the crusts. Crustal biomass is concentrated in the top 3 mm of soils. Sandblasting by wind can quickly remove this material, thereby reducing N and C inputs from these organisms. This loss can result in reduced site productivity, as well as exposure of unprotected subsurface sediments to wind and water erosion. Actions to reduce impacts to these crusts can include adjustments in type, intensity, and timing of use.

Friction threshold velocities (FTVs) were determined for biological soil crusts in diAerent stages of recovery. Particles on the surface of crusts that had been relatively undisturbed for at least 20 years were found to have significantly higher FTVs than those that had been disturbed 5, 10 or 1 years previously (376, 87, and 46 cm sec ˇ1 , respectively). FTV’s for crust breakage was also much higher for undisturbed crusts when compared to the previously disturbed crusts (573, 148, and 88 cm sec ˇ1 , respectively). All crusted surfaces were more stable than bare sand, which had an FTV of 16 cm sec ˇ1 . Disturbance treatments were then applied to the three crustal classes. Disturbance significantly reduced the FTVs of all classes by 73‐92 per cent. Comparing crustal FTVs with mean and high monthly wind speeds found in this region, it was observed that only crusts that had been undisturbed for approximately 20 years or more were able to protect soil surfaces from wind gusts expected on the average of once a month. Other crustal classes, as well as all disturbance treatments, had FTVs lower or equal to that of commonly occurring winds in this region. Because most of the crustal biomass occurs in the top 0.3 mm of soils, even slight soil loss can negatively influence stability and nutrient inputs to this ecosystem. #1997 John Wiley & Sons, Ltd.

A field experiment at Owens (dry) Lake, California, tested whether and how the relative profiles of airborne horizontal mass fluxes for >50-μm wind-eroded particles changed with friction velocity. The horizontal mass flux at almost all measured heights increased proportionally to the cube of friction velocity above an apparent threshold friction velocity for all sediment tested and increased with height except at one coarse-sand site where the relative horizontal mass flux profile did not change with friction velocity. Size distributions for long-time-averaged horizontal mass flux samples showed a saltation layer from the surface to a height between 30 and 50 cm, above which suspended particles dominate. Measurements from a large dust source area on a line parallel to the wind showed that even though the saltation flux reached equilibrium ∼650 m downwind of the starting point of erosion, weakly suspended particles were still input into the atmosphere 1567 m downwind of the starting point; thus the saltating fraction of the total mass flux decreased after 650 m. The scale length difference and ratio of 70/30 suspended mass flux to saltation mass flux at the farthest down wind sampling site confirm that suspended particles are very important for mass budgets in large source areas and that saltation mass flux can be a variable fraction of total horizontal mass flux for soils with a substantial fraction of

Variability in airborne particles larger than sand flux for distance scales larger than 200 m was observed during wind erosion episodes on the northeast side of a dry lake bed (Owens Lake). Measurements were made during erosion episodes on a ∼3-km line of wind measuring and sand flux collecting instruments. Data were selected for winds that (1) aligned with the instrument line and (2) had mean speeds cubed at 4-m heights at the beginning, middle, and end of the line that differed by less than 5% (i.e., mean wind speeds differed by less than 1.7%). Four mechanisms were determined to cause the large-scale differences in the mass flux profiles. In order of their importance, the mechanisms are as follows: (1) change of the drag coefficient (or the ratio u*/U, where u* is wind friction velocity and U is mean wind speed from place to place), this is a measure of variability in roughness height; (2) particle limitation (depletion of the loose “available” erodible material on the surface); (3) variation of the threshold friction velocity; and (4) the Owen effect (the increase of u*/U with U).

The vertical flux of particles smaller than 10 μm for a saline playa surface, the particle size composition of which was classified as loam-textured, was estimated for a highly wind-erodible site on the playa of Owens (dry) Lake in California. The ratio of this vertical flux to the horizontal flux of total airborne material through a surface perpendicular to the soil and to the wind, Fa/qtot, is 2.75×10−4 m−1. This is consistent with that ratio for sand-textured soils and suggests that the binding energy and size of saltating particles for the tested surface material at Owens Lake is of the same order as that for sandier soils. The horizontal mass flux of saltating grains, q, in the reported wind erosion event is 51.3% of the total horizontal mass flux qtot. Therefore the ratio of Fa/q is 5.4×10−4 m−1.

A physical model was developed to explain threshold friction velocities u*t for particles of the size 60–120 μm lying on a rough surface in loose soils for semiarid and arid parts of the United States. The model corrected for the effect of momentum absorption by the nonerodible roughness. For loose or disturbed soils the most important parameter that controls u*t is the aerodynamic roughness height z0. For physical crusts damaged by wind the size of erodible crust pieces is important along with the roughness. The presence of cyanobacteriallichen soil crusts roughens the surface, and the biological fibrous growth aggregates soil particles. Only undisturbed sandy soils and disturbed soils of all types would be expected to be erodible in normal wind storms. Therefore disturbance of soils by both cattle and humans is very important in predicting wind erosion as confirmed by our measurements.

A better understanding of the effects of precipitation and source area on blowing dust in the Sonoran Desert has been sought through the study of 1190 dust episodes occurring during the 1948–78 time period at Blythe, California, and Yuma, Arizona. Threshold mean hourly wind speeds (MHWSs) increase directly with prior precipitation in proportion to the inhibiting effect of the vegetation canopy, which blooms following periods of increased precipitation. Because of the time required for the vegetation canopy to fully develop and the persistence of the vegetation canopy once developed, correlation between the threshold MHWS and precipitation is highest for 4–6-month windows of total precipitation prior to each dust event at both stations. Many dust events associated with unusually low MHWSs are clustered in time, and these events can be correlated with interstate highway construction and soil preparation for new irrigation projects. Since threshold MHWSs for blowing dust lie well below the recorded ...

As part of a joint Russian/American dust-storm experiment, GOES-VISSR (Geostationary Operational Environmental Satellite, Visible-Infrared Spin-Scan Radiometer), data from a visible-band satellite image of a large dust storm emanating from Owens Lake, California were acquired on March 10 and 11, 1993. The satellite data were calibrated to targets of known ground reflectance factors and processed with radiative transfer techniques to yield aerosol (dust) optical depth at those stages of the dust storm when concurrent ground-based measurements of optical depth were made. Calibration of the satellite data is crucial for comparing surficial changes in remotely sensed data acquired over a period of time from the same area and for determining accurate concentrations of atmospheric aerosols using radiative transfer techniques. The calibration procedure forces the distribution of visible-band, DN (digital number) values, acquired on July 1, 1992, at 1731 GMT from the GOES-VISSR sensor over a large test area, to match the distribution of visible-band, DN values concurrently acquired from a Landsat MSS (Multispectral Scanner) sensor over the same test area; the Landsat MSS DN values were directly associated with reflectance factors measured from ground targets. The calibrated GOES-VISSR data for July 1, 1992, were then used to calibrate other GOES-VISSR data acquired on March 10 and 11, 1993, during the dust storm. Uncertainties in location of ground targets, bi-directional reflectance and atmospheric attenuation contribute an error of approximately ±0.02 in the satellite-inferred ground reflectance factors. On March 11 at 1031 PST the satellite-received radiances during the peak of the storm were 3 times larger than predicted by our radiative transfer model for a pure clay dust plume of infinite optical depth. This result supported ground-based measurements that the plume at that time was composed primarily of large salt grains, probably sodium sulfate, which could not be properly characterized in our radiative transfer model. Further, the satellite data showed that the salt fell out of the plume within 35 km from the source. Finer-grained, clay dust was observed to extend beyond the salt-laden plume and was the major component of the dust plume after 1131 PST, when erosion of the salt crust on Owens Lake ceased. By 1331 and 1401 PST satellite-inferred, optical depths compared favorably with measurements concurrently acquired at the ground. Uncertainties in bi-directional reflectance, atmospheric attenuation, and locating ground points in the satellite data manifest errors between the inferred and measured optical depths in the range of 20 to 50%; these errors would be much greater without the calibration of the GOES-VISSR data. Changes in satellite-inferred reflectance factors over the lake bed during the course of the storm showed that 76 km 2 of the surface was disrupted during the March 11 storm, suggesting as much as 76 × 10 3 m 3 of crustal material were displaced for each millimeter of several estimated to have been moved during the storm; an unknown fraction of the displaced material was suspended. The satellite data also showed dust fallout on mountain snowfields. Whereas fallout may have removed most of the salt, satellite data acquired at 1631 PST, when the plume had a large brightness contrast with the ground, showed that it covered over 2500 km 2 and contained at least 1.6 × 10 9 g of sediment. For such a small source area, the dust represents a substantial contribution to the regional and global load of aerosols.

The increase of soil mass flux with distance downwind, the fetch effect for wind erosion, has been observed and reported on since 1939. This model incorporates the following three mechanisms. (1) The ‘avalanching’ mechanism in which one particle moving downwind would dislodge one or more particles upon impact with the surface. The result of a chain of such events is an increase of mass flux with distance. (2) The ‘aerodynamic feedback’ effect, suggested by P. R. Owen, in which the aerodynamic roughness height is increased by saltation of particles; the resulting increased momentum flux increases saltation. These increases define a positive feedback loop with respect to distance downwind. (3) The ‘soil resistance’ mechanism, which is largely an expression of the change with distance of threshold velocity. Change of threshold velocities may be caused by inhomogeneities of the soil or progressive destruction of aggregates and crust in the direction of saltation fetch. An experiment was run in March 1993 at Owens Lake to test this model. Detailed measurements of wind profiles and mass fluxes were taken on a line parallel to the wind direction. These data support the proposed three-mechanism model.

Data from a high resolution radiometer AVHRR (580–680 nm optical lengthwaves) installed on the “NOAA-11” satellite as well as TV (500–700 nm) and IR (8000–12000 nm) equipment of the Russia satellite “Meteor-2/16” were used to study the evolution of dust storms for 1–30 September 1989 in Tajikistan, Uzbekistan, Turkmenistan and Afghanistan. These data help to validate the hypothesis, that long-term dusted boundary layer (duration of the order of a day or more), but of comparatively not high optical density (4–10 km meteorological visibility range at the 20–50 km background), is formed after the northwest intrusions into a region of intensive cold fronts at the surface wind velocities of 7–15 m/s. Stability of dust clouds of vertical power to 3–3.5 km (up to an inversion level) is explained by an action of collective buoyancy factors at heating the dust particles of 2–4 μm in mean diameter by solar radiation. The more intensive intrusions stimulate a formation of simultaneously dust and water clouds. The last partially reduce the solar radiation (by the calculations of the order of 30–50%) and decrease the role of buoyancy factors. Thus, initiated is the intensive but short-term dusted boundary layer at horizontal visibility of 50–200 m.

The shrub-steppe area near Shaartuz, Tadzhik, S.S.R., is shown to be a net accumulator of dust despite being an occasional source of dust. For the accumulation of the dust to form the observed surface crust, a net deposition of about 290–490 g m−2 yr−1 of particles smaller than 20 μm is required, depending on the duration of the deposition period. The particles smaller than 20 μm are mixed with particles brought up from the sandy material below the surface crust by bioturbation and are incorporated into the surface crust. Measurements during the 16 and 20 September 1989 dust storms provided a total deposition of 41.1 g m−2 of particles smaller than 20 μm. Because 10–30 dust storms are observed at Shaartuz, the measured average dust storm deposition would yield 206–617 g m−2 yr−1. This range of deposition is of the order of that needed to provide a mass balance for the observed crust formation. Cryptogams (including algae, lichen, and moss) and rainwater are the main agents of incorporation of the aeolian dust into a stable soil crust. The role that the vascular plants played at the Shaartuz site was to reduce the rate of soil movement to levels where the cryptogamic crusting was possible. the observed mechanisms of dust deposition followed by crust incorporation are possibly an important processes in loess formation in Central Asia.

In September 1989 a joint U.S.S.R.-U.S. study of the causes and effects of desert dust on the environment was conducted in the Tadzhik S.S.R. in Soviet Central Asia. The objectives of the study included measurements of optical absorption by suspended material, both windblown dust and aerosol “black” carbon. This latter material is a combustion effluent, prevalent in emissions from poorly controlled burning, with a long atmospheric lifetime and a large cross-section for the absorption of visible radiation. The measurements obtained from the analysis of filter samples indicate that only during periods of active dust production was there a significant contribution of dust to total absorption. At other times, the presence of black carbon from local and regional sources accounted for approximately 90° of the total aerosol optical absorption. The conclusions are that fuel combustion may produce a greater optical impact on the atmosphere in less-developed areas of the world than that arising from the effects of desert dust production.

Aerosol samples from dust storms were collected in Tadzhikistan (Soviet Central Asia) during September 1989, as a part of the joint U.S.S.R.-U.S. Dust Experiment. Physico-chemical characteristics of deposited dust were compared with those of soil-derived dust collected in other regions. Particle mass-size distributions appear to be characterized by a common log-normal mode between 1 and 10 μm. Chemical composition of the sampled material shows that the dust is particularly rich in calcium and silicon and poor in iron. Estimated mineral composition of dust indicates that this enrichment in Ca and Si for the Soviet Asian dust must be related to high contents of carbonate and quartz, respectively. Different Fe/Al also suggest a specific chemical composition for clay minerals in the Asian dust.

Size distribution data obtained during the U.S.S.R.-U.S. dust experiment make it possible to propose a general conception about the size distribution of dust aerosols within the size range 0.005–100 μm. The microstructure for the optically active fraction of arid aerosol is approximated in the form of a log-normal distribution with parameters D = 3.5−6 μ m, δ 2 = 0.5−0.8, which can be used when estimating radiative calculations.

A model of deposition and transport was constructed for the Kafirnigan Valley, in Soviet Central Asia. Data, consisting of deposition measurements at Shaartuz, atmospheric columnar mass, aerosol concentrations, wind speed, optical scattering, and movement of soil, were collected for the dust storms of 16 and 20 September 1989. Results from the model were compared with measurements of total atmospheric columnar mass loading for the dust storm of 16 September. Although sensitivity of the model to dust layer height does not recommend the model for general use, the model has some merit in predicting transport and deposition for dust contained in a river valley.

Data were successfully obtained from an experiment during dust storms on 16 and 20 September 1989, in the Kafirnigan Valley, Tadzhik S.S.R. (Tadzhikistan). The principal purposes of the experiment were to provide data sets for modeling radiation transfer, as modified by desert dust, and to specify transport, modification, and deposition of a desert dust system. The experiment provided data for investigations of four problem areas concerning climatic effects of desert dust: (1) speciication of dust-producing source areas and meteorology, (2) specification of dust microphysics, (3) description of optical and climate effects of the desert dust, including local meteorological conditions, and (4) description of chemical composition of the desert dust.

A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio Rt, the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present. The roughness density is quantified by the frontal area index λ. The prediction is Rt = (1 − mσλ)−½(1 + mβλ)−½, where β is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; σ is the basal-to-frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point. The prediction is well verified by four independent data sets.

Models of dust emissions by wind erosion (including winds associated with regional activity as well as dust devils) and vehicular disturbances of unpaved roads were developed, calibrated,and used to estimate alkaline dust emissions from elemental soil and road composition data. Emissions from tillage of soils were estimated from the work of previous researchers. The area of maximum dust production by all of those sources is the area of the old “Dust Bowl” of the 1930s (the panhandles of Texas and Oklahoma, eastern New Mexico and Colorado, and western Kansas). The areas of maximum alkaline dust production are the arid southwest, the “Dust Bowl,” and the midwestern-mideastern states from Iowa to Pennsylvania. Our calculations show that calcium is the dominant alkaline element produced by “open sources” (sources too great in extent to be controlled by enclosure or ducting). Although the largest dust mass source is wind erosion (by winds associated with regional activity and convective activity), the largest producer of the alkaline component is road dust because the abundance of alkaline materials in road coverings (which include crushed limestone) is significantly higher than for soils. Comparing the above estimated sources of alkaline material with inventories of SO2 and NOx emissions by previous investigators gives the rough approximation that alkaline emission rates are of the order of the SO2 + NOx emissions in the western United States and that they are much smaller than SO2 + NOx in the eastern United States. This approximation is substantiated by data on Ca/(SO4 + NO3) for wet deposition for National Atmospheric Deposition Program sites.

[1] The northern Chihuahuan Desert in New Mexico contains mesquite bushes and small coppice dunes as well as open areas lacking vegetation. Sandstorms are common in this area, gradually reshaping the flat grassland into a landscape of mesquite coppice dunes and bare open patches. During storms, complex airflows entrain sediment from the open areas, depositing it around downwind bushes and dunes. Understanding and quantifying these processes could help to clarify the ongoing process of desert formation. Sand flux patterns for eight storms occurring in April 2003 and April 2004 were predicted for a (60 m by 60 m) site on the basis of 297 10-min average velocity simulations using a semiempirical mass consistent diagnostic wind field model: Quick Urban & Industrial Complex version 3.5 (QUIC) used with a sand flux parameterization. The sand flux patterns were highly heterogeneous, varying with wind direction and differing between storms. Generally, the nonvegetated areas experienced high sand fluxes, while wake areas behind dunes experienced little or no sand flux. Sediment erosion and deposition patterns were calculated by taking the divergence of the sand flux. The open areas were the sources of the sediment, while the windward sides of the mesquite bushes and dunes were the primary deposition areas. The simulated sediment erosion and deposition magnitudes were qualitatively similar to an annual average from 45 years of measurements.

Estimates are made of the contribution of dust devils to the aerosol mass burden over the U.S. These estimates have been derived from experimental data generalized by using climatic and vegetation maps of the U.S. The area of maximum calculated dust production by dust devils in the southwestern U.S. Comparison of our calculated fluxes with background aerosol data shows general agreement. Comparison of alkaline dust emissions from wind erosion shows that dust devils provide a similar magnitude input of atmospheric dust.

International audience; Sandstorms are frequent in the northern Chihuahuan Desert in New Mexico, an area characterized by open areas lacking vegetation, individual mesquite bushes, and mesquite coppice dunes. Field measurements of sand fluxes and wind velocities over a two year period provided a description of the area-suggesting that the "streets", the flat, elongated, non-vegetated areas aligned with the dominant wind directions are the principal sources of wind-dispersed soil and dust. However, since soil erosion and dust movement depend on the pattern, strength, and gradients in the wind field, modeling soil erosion and dust movement requires a continuous wind velocity field. Consequently, air flow patterns at this site were simulated using a semi-empirical mass-consistent diagnostic wind field model: QUIC version 3.5 (Quick Urban & Industrial Complex). Two hundred and fifty-one simulations were run encompassing several dust storms occurring in April 2003. Wind velocity vectors were compared between the model and field data at three heights for six locations and were found to correlate well for a majority of the situations suggesting that the flow patterns are consistent throughout the domain. In particular , good agreement was found for wind speeds at 0.75 m, the height for which the model was tuned. However, it overestimated velocities at 1.5 m (10%) and 3.15 m (13%). Generally, the model successfully identified locations of the highest wind velocities and wind stresses, predominately found in "streets" aligned with the driving wind, and locations of wake flow downwind of mesquite bushes where there was separation flow or otherwise shelter from the wind.

[1] The problem of dust emissions from playa sources is an important one both in terms of human health and in terms of global dust issues, distribution of loess, and mineral cycling. A refined method of modeling atmospheric dust concentrations due to wind erosion was developed using real-time saltation flux measurements and ambient dust monitoring data at Owens Lake, California. This modeling method may have practical applications for modeling the atmospheric effects of wind erosion in other areas. Windblown dust from the Owens Lake bed often causes violations of federal air quality standards for particulate matter (PM10) that are the highest levels measured in the United States. The goal of this study was to locate dust source areas on the exposed lake bed, estimate their PM10 emissions, and use air pollution modeling techniques to determine which areas caused or contributed to air quality violations. Previous research indicates that the vertical flux of PM10 (Fa) is generally proportional to the total horizontal saltation flux (q) for a given soil texture and surface condition. For this study, hourly PM10 emissions were estimated using Fa = K′ × m15, where m15 is the measured sand flux at 15 cm above the surface, and K′ was derived empirically by comparing air quality model predictions to monitored PM10 concentrations. Hourly sand flux was measured at 135 sites (1 km spacing) on the lake bed, and PM10 was monitored at six off-lake sites for a 30 month period. K′ was found to change spatially and temporally over the sampling period. These changes appeared to be linked to different soil textures and to seasonal surface changes. K′ values compared favorably with other Fa/q values measured at Owens Lake using portable wind tunnel and micrometeorological methods. Hourly trends for the model-predicted PM10 concentrations agreed well with monitored PM10 concentrations. Dust production was estimated at 7.2 × 104 t of PM10 for a 12 month period. A single storm accounted for 9% of the annual dust emissions at 6.5 × 103 t. The modeling results were used to identify 77 km2 of dust-producing areas on the lake bed that will be controlled to attain the federal air quality standard for PM10.

[1] Measurements of sand flux over areas with different vegetation in the Chihuahuan desert show that mean, height-integrated, horizontal flux values for mesquite-dominated sites were higher than those for other kinds of vegetation. Sand transport over mesquite areas displayed seasonal variability for most years. This seasonal variability roughly followed the variability of strong winds. Sand transport rates for collectors within a short distance downwind of mesquite bushes were small compared to those for collectors at the end of streets (elongated patches of bare soil) aligned with wind direction. The increased rate of sand transport (wind erosion) associated with mesquite is important because mesquite-dominated areas are increasing in the northern Chihuahuan desert and are therefore responsible for increasing land degradation (desertification).

International audience; This paper provides an introduction to the special section of the Journal of Geophysical Research on mineral dust. We briefly review the current experimental and theoretical approaches used to quantify the dust radiative impacts, highlight the outstanding issues, and discuss possible strategies to overcome the emerging problems. We also introduce the contributing papers of this special section. Despite the recent notable advances in dust studies, we demonstrate that the radiative effects of dust remain poorly quantified due to both limited data and incomplete understanding of relative physical and chemical processes. The foremost needs are (1) to quantify the spatial and temporal variations of dust burden in the atmosphere and develop a predictive capability for the size‐ and composition‐resolved dust particle distribution; (2) to develop a quantitative description of the processes that control the spatial and temporal variabilities of dust physical and chemical properties and radiative effects; (3) to develop new instrumentation (especially to measure the dust particle size distribution in a wide range from about 0.01 μm to 100 μm, scattering phase function and light absorption by dust particles); and (4) to develop new techniques for interpreting and merging the diverse information from satellite remote sensing, in situ and ground‐based measurements, laboratory studies, and model simulations. Because dust distribution and effects are heterogeneous, both spatially and temporally, a promising strategy to advance our knowledge is to perform comprehensive studies at the targeted regions affected by mineral dust of both natural and anthropogenic origin.

Data from an experiment at Owens Lake provided an opportunity to verify the validity of Raupach's formula that predicts the apparent roughness height at equilibrium with saltation during wind erosion episodes. In addition to that verification, a simplification of Raupach's formula is presented which allows the computation of the wind friction velocity affected by saltating sand grains without the need to measure the threshold wind friction velocity and to do iterative calculations. This method estimates the increase of the wind friction velocity Δu*, above the nonsaltating wind friction velocity caused by the saltating grains: Δu* = 0.3 [U − Ut]2 where U and Ut are the wind speed and the threshold wind speed, respectively, at 10m.

The oxygen isotopic ratios (expressed as parts per thousand relative to mean ocean water, SMOW, delta/sup 18/O) of the quartz from 13 soils undergoing much wind erosion during the study period of 1972-1975 in four southwestern states and from comparison areas were determined. The delta/sup 18/O for quartz from eight Texas (TX) and Arizona (AZ) soils range from 13.0 to 15.9 /sup 0///sub 00/. The quartz of the sands and silts coarser than 20 ..mu..m from three of the soils had delta/sup 18/O values ranging from 13.1 to 15.1 /sup 0///sub 00/, characteristic of an ultimate igneous-metamorphic origin. The delta/sup 18/O values increase greatly with decreasing particle size of quartz from three soils ranging from loamy fine sand to loam to clay in texture. The delta/sup 18/O of the 1-10 ..mu..m quartz fraction (aerosol size) ranged from 19.2 to 20.2 /sup 0///sub 00/ (19.55 +- 0.28 /sup 0///sub 00/; +- sigma) for the thirteen soils most affected by dust storms. The oxygen isotopic ratios of 1-10 ..mu..m quartz from three Hawaiian soils and two sediments from Lake Waiau occurring at 3,970 m altitude on the Mauna Kea summit on the Island of Hawaii give a delta/sup 18/O mean of 18.3more » +- 0.2 /sup 0///sub 00/.« less

A numerical study is presented of the effects of super-μm particles mechanically generated by large fires on coagulation loss of sub-μm aerosols in the smoke plumes. This study shows that inclusion of the source generation of large particles in the model can reduce the contribution to the optical depth of sub-μm particles by a factor of two at visible wavelengths. This assumes a combination of high turbulence and high super-μm concentrations. Since no measurements of super-μm concentrations in large firestorms have been made, a concentration corresponding to values found in medium sized dust devils was assumed. This study also shows that the early-time optical effect of the super-μm particles can be considerable in both visible and infrared wavelengths.

The relationship of the relative size distribution of soil wind erosion aerosols to wind velocity was examined in field measurements and wind tunnel simulations. The ratio of sedimentation velocity to friction velocity ((vertical momentum flux/air density)1/2) at which aerosol particles were significantly affected by settling was larger than 0.12 and smaller than 0.68. The shapes of the size distributions of soil wind erosion aerosols (2 μm < r < 10 μm) were fairly constant with wind speed. This result is evidence that the dominant mechanism of aerosol production by soil erosion is sandblasting of the soil surface.

Simultaneous visibility observations and size-number distribution measurements of airborne soil particles were made during incidents of soil erosion in west Texas. Visibilities were calculated by applying Mie scattering theory to measured size distributions and were compared with observed visibilities. Agreement was found, and similar comparison with artifical modicications to the observed size distributions demonstrated that any major changes in the observed size distributions would result in significant discrepancies between the observed and the calculated visibilities. These comparisons confirm that under our experimental conditions the optically important particles are those in the size range 0.62 < r < 20 μm. The sensitivity of the calculated visibility to modifications in the measured size distribution implies that such comparisons between calculated and observed visibility provide a means of confirming size distribution measurements under a variety of conditions.

The change in the root-mean-square deviation of multiple simulated aerosol size distribution samples as a result of increasing the width of the radial size channel is examined. An optimal size interval spacing is found for data which had a large variance for a size interval spacing of 20 per order of magnitude. Optical extinction crposs-sections are calculated from the size distributions, with the use of Mie's solution for scattering by a spherical particle. The root-mean-square deviation of the calculated optical extinction cross-section is only mildly affected by increasing the width of size intervals up to the optimum limit, because the calculation is an integration over the entire size range. However, since the variance is reduced significantly for the size distributions, comparison of these data is better facilitated.

Simultaneous measurements of visibility and mass concentration of airborne dust have been made in rural areas of west Texas and values of the constants C and γ, relating visibility to the mass concentration by the formula MVγ = C, have been calculated and compared with other measured values. The measured values for C appear to depend on factors such as the presence or absence of local erosion, or the distance from the source of the aerosol. The wide variation in values of C, differing by more than an order of magnitude, indicates that there is no single value of C that is generally applicable for relating the mass concentration of soil-derived aerosols and visibility.

Fast-response light scattering measurements at two heights during a Texas dust storm are combined with horizontal and vertical wind data to derive and compare aerosol flux estimates using three techniques. The major result of this study is that a relative equivalence exists between the fine-particle (0.1 μm < radius < 1 μm) exchange coefficient and the eddy viscosity of the wind. The data also shed some light on the complex dependence of wind speed threshold for suspension and aerosol flux in high winds for different surface conditions and soil types. These results show the value of the experimental technique to studies of toxic particulate suspension and deposition by wind.

A portable open-floored wind tunnel was used to develop threshold wind speeds over two pebble covered desert soils and a sandy agricultural soil.