On the effect of changes in wind direction on dust aerosol concentrations in the near-surface layer

The study was carried out using observations in a 5 km long and 200-300 m wide patch of loose sands, located west of the Naryn Khuduk settlement (Russia, 2013-2021). The uniqueness of this area is determined, in particular, by the structure of the Seif dune ridges extending approximately in the latitudinal direction. We used data on concentrations of microparticles (sizes from 0.2 to 5 μm) at two levels (0.5 and 2.0 m) with multichannel registration, on concentrations of microparticles with sizes from 0.4 to 30 μm at 0.2, 0.4, 0.8, 1.6 and 3.2 m, on electric field strength.

The size distribution of microparticles, the concentrations of coarse aerosol fraction [1] are higher when the wind is tangential to the extending of dune than when it is frontal. Concentration values at heights of 20 and 40 cm exceed by several times in profiles built up to a height of 3.2 m for angles of about 10-30º with respect to dune crest compared to other wind directions.

This related to the processes of abrasion of the coarse fraction of microparticles from the newly involved large particles from the zone of the leeward slope. The presence of heavy rolling or stationary particles is confirmed by the occurrence of ripples on the surface.

The connection with the change of wind direction suggests the importance of splashing and abrasion processes when particles fall behind the leeward slope. In this context the influence of an obstacle on air flow with particles suspended in it has been studied for the Lagrangian-Eulerian model by means of the open package OpenFOAM. The particles falling on the surface in the recirculation zone behind the leeward slope created a disturbance of turbulent energy, which contributes to the intensification of the dust aerosol carry out beyond the salting layer.

Microparticles up to 0.5 μm in size, adhere to the surface of saltation. For them, the action of forces of electric nature turns out to be essential [2]. They appear in a free state at the moment of critical charge accumulation on a saltation particle under the influence of electric field created by the flux of large particles moving near the surface. Analytical estimation of the relative change in electric field strength shows a quadratic dependence on the number of generated microparticles.

At wind speeds close to the threshold value and with the wind direction close to tangential with respect to the dune crest line  the electric field strength increases. Concentrations of arid aerosol with sizes 0.2-0.4 μm increase, which is associated with faster charging of saltation particles. This is explained by participation of larger particles in the process with strengthening of tunnel effect of electric charge transfer from larger particles to smaller ones.

The study was supported by the Russian Science Foundation project 20-17-00214.

• Malinovskaya E.A.et.al. Izvestiya, Atmospheric and Oceanic Physics 57(5) 2021
• Malinovskaya E.A.et.al. Doklady Earth Sciences, 502(2) 2022.