THE EFFECT OF DUST LIFTING PROCESS ON THE ELECTRICAL PROPERTIES OF THE ATMOSPHERE

Airborne dust and aerosol particles affect climate by absorbing and scattering thermal and solar radiation and acting as condensation nuclei for the formation of clouds. For this reason, they strongly influence the thermal... [ view full abstract ]

Airborne dust and aerosol particles affect climate by absorbing and scattering thermal and solar radiation and acting as condensation nuclei for the formation of clouds. For this reason, they strongly influence the thermal structure, balance and circulation of the atmosphere. On Earth and Mars, this ‘climate forcing’ is one of the most uncertain processes in climate change predictions. Moreover, wind-driven movement of sand and dust largely contributes to the reshaping of planetary surfaces through processes like the erosion of rocks, the formation of sand dunes and ripples, and the creation and transport of soil particles. These processes are not confined to Earth, but also occur on Mars, Venus and Titan.
The knowledge of the atmospheric dust properties and of the mechanisms for dust settling and lifting into the atmosphere is important to understand planetary climate and surface evolution.
On Mars the physical processes responsible for dust injection into the atmosphere are still poorly understood, but they likely involve saltation as on Earth. Saltation is a process where large sand grains are forced by the wind to move in ballistic trajectories on the soil surface. During these hops they hit dust particles, that are well bound to the soil due to interparticle cohesive forces, thus transferring to them the required momentum to be entrained into the atmosphere.
It is known that this process is capable of generating strong electric fields in the atmosphere up to 100-150 kV/m. This enhanced electric force acts as a feedback in the dust lifting process, lowering the threshold of the wind friction velocity required to initiate sand saltation. The effect of the electric field is an important aspect of dust lifting process that needs to be well characterized and modelled. Although the literature reports several measurements of E-fields during dust devils events, there is only a limited number of measurements of atmospheric electric properties during dust storms or isolated gusts.
To solve some of the issues raised above we carried out a series of field campaigns in South-eastern Morocco during the 2013 and 2014 dust storm seasons. Here we show that, depending on relative humidity conditions, electric fields contribute to increase the amount of particles emitted into the atmosphere. This means that electrical forces and humidity are critical quantities in dust emission process and should be taken into account in climate and circulation models to obtain a more realistic estimation of dust load in the atmosphere.