Can Turbulent Mixing Explain the Presence of Coarse Saharan Dust in the Americas?

Mineral desert dust accounts for a substantial fraction of the total atmospheric aerosol loading. Because these particles interact with radiation and clouds, their airborne lifetime has important implications for climate, weather and atmospheric stability. However, large-scale models typically overestimate dust deposition rates, in particular by assuming that particles settle at Stokes' terminal velocity, which is valid for spherical particles in a quiescent environment. As an example, measurements show that coarse dust in the so-called Saharan Air Layer (SAL) remains suspended for over 5 days during its trip from North Africa to the Caribbean, but models fail to reproduce such observations. In this study, we investigate the capability of vertical turbulent mixing to reduce the settling rate of particles immersed in a free-shear fluid layer by using both an analytical model and large-eddy simulations (LES). Parametrizing the eddy fluxes in terms of a constant eddy diffusivity K, the solution to the dust mass conservation equation indicates that the increase due to turbulence in the residence time of a particle with settling speed w_s in a turbulent layer of depth h depends solely on the particle Peclet number Pe = w_sh/K. In the most extreme case, in which turbulence causes instantaneous mixing, the concentration profiles are uniform at all times and particles remain suspended for twice as long as they would in a laminar environment. Our theoretical results have been validated for a wide range of particle sizes with the LES, in which a neutral shear layer contained in between two stratified fluid layers was simulated as a proxy for the SAL. The diagnosed K-profiles are robust over time, and despite their mean magnitude remaining rather invariable with particle size, they become more variable with height with decreasing particle size. This simple model appears to provide reasonable estimates for low-order moments and integral measures such as the size-resolved suspended aerosol mass over time. Furthermore, varying the shear magnitude in the LES suggested that the eddy diffusivity K scales with the vertical velocity rms. Finally, by comparing our theory with Saharan dust concentration measurements on both sides of the Atlantic Ocean obtained during the SALTRACE campaign (Weinzierl et al., 2017), we found that combining the effects of particle asphericity and turbulent mixing on aerosol settling rates greatly helps explain the presence of coarse Saharan dust in the Americas. A more accurate model would likely need to incorporate other processes taking place in reality, such as the interaction between the SAL and the marine boundary layer below it.