Abstract
Abstract. Particle size distribution of dust at emission (dust PSD) is an essential quantity to estimate in dust studies. It has been recognized in earlier research that dust PSD is dependent on soil properties (e.g. whether soil is sand or clay) and friction velocity, u∗, which is a surrogate for surface shear stress and a descriptor for saltation-bombardment intensity. This recognition has been challenged in some recent papers, causing a debate on whether dust PSD is “invariant” and the search for its justification. In this paper, we analyse the dust PSD measured in the Japan Australian Dust Experiment and show that dust PSD is dependent on u∗ and on atmospheric boundary-layer (ABL) stability. By simple theoretical and numerical analysis, we explain the two reasons for the latter dependency, which are both related to enhanced saltation bombardment in convective turbulent flows. First, u∗ is stochastic and its probability distribution profoundly influences the magnitude of the mean saltation flux due to the non-linear relationship between saltation flux and u∗. Second, in unstable conditions, turbulence is usually stronger, which leads to higher saltation-bombardment intensity. This study confirms that dust PSD depends on u∗ and, more precisely, on the probability distribution of u∗, which in turn is dependent on ABL stability; consequently, dust PSD is also dependent on ABL. We also show that the dependency of dust PSD on u∗ and ABL stability is made complicated by soil surface conditions. In general, our analysis reinforces the basic conceptual understanding that dust PSD depends on saltation bombardment and inter-particle cohesion.
Highlights
Gillette (1981) explained that dust emission can be produced by aerodynamic lift and saltation bombardment, but under realistic wind conditions, aerodynamic-lift emission is much weaker than saltation-bombardment emission
By composite analysis for different u∗ and atmospheric boundary-layer (ABL) stabilities, we show that dust PSD depends on u∗, supporting the findings of Alfaro et al (1997) and depends on ABL stability, which is consistent with the findings of Khalfallah et al (2020)
We make the following observations based on the Japan Australian Dust Experiment (JADE) data: (1) dust PSD has rich temporal variations and is not “universal”; (2) dust PSD depends on u∗ and ABL stability; and (3) dust PSD is influenced by soil surface conditions
Summary
Gillette (1981) explained that dust emission can be produced by aerodynamic lift and saltation bombardment, but under realistic wind conditions, aerodynamic-lift emission is much weaker than saltation-bombardment emission. Airborne-dust PSD can be used to approximate emission-dust PSD if it is measured close to the source and the dependency of particle motion in air on particle size can be neglected. In line with Sow et al (2009), Khalfallah et al (2020) reported that emission-flux PSD depends on atmospheric boundarylayer (ABL) stability and attributed this to the dependency of particle diffusivity on particle size They stated that the dependency of emission-dust PSD on u∗, as observed by Alfaro et al (1997), may be of secondary importance in natural conditions compared to its dependency on ABL stability. In contrast to Khalfallah et al (2020), we argue that these dependencies are not mutually exclusive but collectively point to the simple physics that emission-dust PSD is dependent on saltationbombardment intensity and efficiency
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