Dale A. Gillette

Relation of vertical flux of particles smaller than 10 μm to total aeolian horizontal mass flux at Owens Lake

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.

Sand flux in the northern Chihuahuan desert, New Mexico, USA, and the influence of mesquite-dominated landscapes

[1]xa0Measurements 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).

Estimation of suspension of alkaline material by dust devils in the United States

Abstract 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.

Large-scale variability of wind erosion mass flux rates at Owens Lake. 1. Vertical profiles of horizontal mass fluxes of wind-eroded particles with diameter greater than 50 μm

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 <100-μm particles.

Optical absorption by aerosol black carbon and dust in a desert region of Central Asia

Abstract 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.

A combined modeling and measurement technique for estimating windblown dust emissions at Owens (dry) Lake, California

[1]xa0The 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.

Emissions of alkaline elements calcium, magnesium, potassium, and sodium from open sources in the contiguous United States

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.

Relationship between the Aerodynamic Roughness Length and the Roughness Density in Cases of Low Roughness Density

This paper presents measurements of roughness length performed in a wind tunnel for low roughness density. The experiments were performed with both compact and porous obstacles (clusters), in order to simulate the behavior of sparsely vegetated surfaces. The experimental results have been used to investigate the relationship between the ratio z0/h and the roughness density, and the influence of an obstacles porosity on this relationship. The experiments performed for four configurations of compact obstacles provide measurements of roughness length z0 for roughness densities λ between 10−3 and 10−2 which are in good agreement with the only data set available until now for this range of low roughness densities. The results obtained with artificial porous obstacles suggests that the aerodynamic behavior of such roughness elements can be represented by the relationship established for compact obstacles, provided a porosity index has been used to determine the efficient roughness density (the fraction of the silhouette area actually sheltered by solid elements) rather than counting the porous object as solid. However, the experiments have been performed with relatively low porosity indices (maximum =u200925%) for which the porosity has a negligible influence. In this range of porosity index, representing the aerodynamic behavior of porous obstacles using the relationship established for compact obstacles, should not lead to a significant error. However, the influence of the porosity may be important for porosity indices larger than 30%.

Size distributions of saltating grains: an important variable in the production of suspended particles

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

Formation of Wind-Erodible Aggregates for Salty Soils and Soils with Less Than 50% Sand Composition in Natural Terrestrial Environments

Of nine possible mechanisms for the formation of non-sandy, wind-erodible aggregates from a more homogeneous soil or sediment, five are found to be widespread in arid and semiarid regions. In approximate order of significance, the most important mechanisms are tension and compression fracturing of shrinkable mud or soil during wet/dry cycles; tension fracturing and molding by compression during freeze/thaw cycles; direct abrasion (corrosion); fracturing and aggregation produced by salt efflorescence; and mechanical disturbance of surface materials by animals. A sixth mechanism (in soils) is by surface films, colloidal matting, or cements. Minor mechanisms are: floatation and lofting of foam and fracturing caused by hydration expansion or other chemical weathering of fine-grained bedrock. The flocculation of small particles in a water suspension or wet mud is a possible minor mechanism not yet observed. Surface soils and sediments with more than 28% clay and more than 2% organic material formed wind-erodible aggregates, but organic-poor materials did not. Calcareous loams, silt loams, silty clay loams, and clay loams formed wind-erodible aggregates, but non-calcareous materials of the same textures did not. Salt efflorescence was locally a major mechanism for production of wind-erodible aggregates. An experiment with expandible (high smectite content) clay shows that wet/dry cycles (such as might result from several summer rain showers on a dry lake bed) can produce wind-erodible aggregates without high temperatures or lengthy droughts and in the absence of salt efflorescence. For example, cold-climate clay dunes (now inactive) fringe ephemeral lakes in deflation basins in Montana at windy semiarid sites with short hot summers and intensely cold winters.