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. Particle size distribution of dust at emission (dust PSD)is an essential quantity to estimate in dust studies. It has been recognizedin 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 surrogatefor surface shear stress and a descriptor for saltation-bombardment intensity.This recognition has been challenged in some recent papers, causing a debateon whether dust PSD is “invariant” and the search for its justification.In this paper, we analyse the dust PSD measured in the Japan Australian DustExperiment and show that dust PSD is dependent on u∗ and onatmospheric boundary-layer (ABL) stability. By simple theoretical andnumerical analysis, we explain the two reasons for the latter dependency,which are both related to enhanced saltation bombardment in convective turbulentflows. First, u∗ is stochastic and its probability distributionprofoundly influences the magnitude of the mean saltation flux due to thenon-linear relationship between saltation flux and u∗. Second, inunstable conditions, turbulence is usually stronger, which leads to highersaltation-bombardment intensity. This study confirms that dust PSD dependson u∗ and, more precisely, on the probability distribution ofu∗, which in turn is dependent on ABL stability; consequently,dust PSD is also dependent on ABL. We also show that the dependency of dustPSD on u∗ and ABL stability is made complicated by soil surfaceconditions. In general, our analysis reinforces the basic conceptualunderstanding that dust PSD depends on saltation bombardment andinter-particle cohesion.

Dependency of particle size distribution at dust emission on friction velocity and atmospheric boundary-layer stability