Investigating the role of deposition on the size distribution of near-surface dust flux during erosion events

Abstract

Predicting the particle size distribution (PSD) of near-surface turbulent dust flux (Fwc) is a key issue for estimating the size of atmospheric mineral dust. Existing dust emission schemes differ in their parametrization of the emitted dust (Femi) PSD, defining differently the surface inter-particle cohesive force and the influence of wind intensity. Moreover, these schemes have often been validated-fitted against field measurements, assuming PSD similarity between Fwc and Femi. Here, we investigate numerically the main factors influencing Fwc-PSD during erosion events. To this effect, we developed a 1D dust-dispersal model. After evaluating the model against published results, it is shown that Fwc-PSD is influenced by both deposition and Femi-PSD. This latter one is shaped by the inter-particle cohesive bond exponent and the surface dust PSD. A time-to-space conversion of the dust flux variations reveals an increasing enrichment of Fwc in small particles compared to Femi. This enrichment remains lower than a few percent of the total dust flux (in number) for fetch lower than 100 m, but it can rise to more than 10% for fetch longer than 1 km. This fetch dependence of Fwc-PSD is explained by the slow deposition of particles having the lowest deposition velocities. Importantly, this difference between Fwc and Femi PSDs is accentuated with wind intensity, with Femi-PSD dominated by particles with large deposition velocities, and in presence of a large-scale background dust concentration. The role played by the deposition process in shaping the Fwc-PSD should be considered when evaluating dust emission schemes against near-surface field measurements.