| Abstrakt: |
Particle aggregates blown along the surface of playas have been linked to the disruption of interparticle bonds, comminution, and dust production. This mechanism was investigated in a set of wind tunnel experiments with the purpose of examining the rate of comminution during transport, role of bed roughness, influence of humidity, system dynamics, and proportionate amount of dust production. The playa sediment selected for testing was obtained from Owens Lake in California, USA. Particle aggregates with diameters between 500 and 710 μm (0.5 ϕ–1.0 ϕ) were isolated by sieving. Small 20 g subsamples were then introduced into the wind tunnel via a drop tube, with some particles captured on a downwind array of sticky glass plates. Aggregate diameter was found to decrease linearly by 30–56 μm per meter of transport (r2 = 0.43) over a total distance of 8.3 m. Enhanced geometric roughness of the bed surface increased this rate, but not dust production and suspension. As compared to an equivalent mass of disaggregated parent material representing the dust emission potential, comminution produced only 0.5%–4% as much suspended PM10. Weak longitudinal flow instabilities involving downwelling and upwelling were found to influence the vertically integrated mass transport rate. This affirms the high sensitivity of dust transport to the structure of the wind field and the need for a paradigm shift in wind tunnel simulations of dust emission to extend measurement of the factors governing dispersion beyond one dimension, and specifically, friction velocity. Plain Language Summary: Desert playas are well known to emit mineral dust which can contribute to poor air quality, limit visibility, and cause disease. Prediction of such emissions from numerical models is dependent on understanding the fundamental processes which govern this geophysical process. Three primary mechanisms are responsible for the transfer of dust from a playa surface to the atmosphere. Two of these (fluid drag and particle ejection) have been well studied, while the lesser known process involves auto‐abrasion of particle clusters or aggregates. As compared to an equivalent mass of silt sized particles blown over a distance of 8 m, auto‐abrasion was observed in our wind tunnel experiments to produce only 0.5%–4% as much dust. Dust production is not equivalent to dust emission, however, as the airflow structure is shown in this study to play an overriding role. Our findings affirm the pivotal importance of encouraging and protecting the formation of particle aggregates in dryland management practices. Key Points: Particle aggregates formed from playa sediment were observed to rapidly fracture during transport by windRelative to the dust emission potential, fractured particle aggregates produced less than 4% as much suspended PM10Dust suspension is extremely sensitive to secondary flow structures within a turbulent wind field [ABSTRACT FROM AUTHOR] |
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