Dependency of Particle Size Distribution at Dust Emission on Friction Velocity and Atmospheric Boundary-Layer Stability

Abstract. Particle size distribution of dust at emission (dust PSD) is an essential quantity to be estimated 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*, a surrogate for surface shear stress and 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 justification. In this paper, we analyze dust PSD measured in the Japan-Australian Dust Experiment and show that dust PSD is dependent on u* and on atmospheric boundary-layer stability. By simple theoretical and numerical analysis, we explain the three reasons for the latter dependency. First, under similar mean wind conditions, the mean of u* is larger for unstable than for stable conditions. Second, 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*. Third, 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 itself is stability dependent. We restate that for a given soil, finer dust is released in case of stronger saltation.