Can active sands generate dust particles by wind-induced processes?

Abstract

Mineral dust emission is a major process in determining the global dust cycle. At a global scale there is still uncertainty about the absolute and relative contribution of dust from different source areas and landforms. Dust sources are mainly considered as surfaces containing relatively high percentages of fine particles (e.g. Playas). Yet, active (dune) sands have been identified recently, by remote sensing studies, as dust sources in northern Africa, China, and elsewhere. Previous studies on dust emission from active sands suggested that dust can be generated by different aeolian mechanisms that are related to (i) re-emission of settled dust particles, (ii) clay coating removal, and (iii) abrasion of the sand grains. However, there is only limited information of the relative importance of the different mechanisms for producing this dust under natural aeolian (wind) conditions. This study integrates wind tunnel experiments and high resolution laboratory sand analyses to explore aeolian dust emission from active sands with conditions simulating the natural processes of saltation and explores the role of the different dust emission mechanisms. Sand samples from three sites with different characteristics of grain size, dust content, morphology, and mineralogy were used in the experiments. No dust emission was recorded for shear velocities below the saltation threshold. The initial content of dust-sized particles (>63 μm) in the sand sample was found to influence PM10 emission. PM10 concentrations were increased with the initial content of dust-sized particles in the sand. The experiments identify clay coatings removal as the dominant mechanism over time of dust emission in typical active sand dunes (<2% dust content) with an addition of re-emission of existing dust-sized particles (<63 μm). The results also suggest that aeolian abrasion play only a minor role in PM10 dust generation from active sands, producing mostly relatively course dust-sized particles (>40 μm). The dust emission observed in this study indicates that, in addition to the classic dust sources of non-sandy soils, sand bodies should also be taken into consideration in determining global dust emission.