Threshold for entrainment of mineral aerosols from grain-scale simulations

Abstract Atmospheric mineral dust has a fundamental impact on the climate, the radiation balance, and various other components of the Earth’s system. However, state-of-the-art earth system models rely on empiric parameterization schemes for the vertical dust flux at emission, while values of this flux obtained from field measurements on different soils spread over various orders of magnitude. This spread reflects both the strong influence of soil properties and the difficulty in studying dust emission from experiments alone. Here, we thus conduct discrete element simulations of mineral dust emission, building on a particle-based representation of granular soils in the atmospheric boundary layer. Our model reproduces measurements of the minimal threshold wind shear velocity for direct aerodynamic entrainment in monodisperse beds as a function of particle size, thereby elucidating the influence of interparticle forces and turbulent wind velocity fluctuations. However, we find that a wind-speedup effect induced by grain-scale topography in bidisperse beds of sand and dust grains yields lower-than-predicted thresholds for direct aerodynamic dust entrainment. Our simulations show that dust can be emitted as individual grains, as dust agglomerates, or coated on the surface of sand grains, depending on the particle size - with implication for dust schemes in climate models.