Electrification of particles in dust storms: Field measurements during the monsoon period in Niger

On the first fortnight of June 2010, experiments were carried out at a millet field in Niger to address the electrification of soil particles under natural conditions. The experiments were conducted during a period of high wind erosion, resulting from the passage of mesoscale convective systems (MCSs) which generate "walls" of dust passing through the Sahel. Soil particles are lifted from the ground by the stress exerted by the wind, with a threshold for emission that is particle size dependent. These particles then collide with other particles, leading to electrostatic charging of the particles. Soil samples of either positive or negative polarity were collected from the soil surface in 3 situations: 1) during "quiet" periods (far removed from a dust storm), 2) immediately after a dust storm, and 3) 12 h after a dust storm. Our results show that immediately after a dust storm, smaller particles are predominantly charged positive and larger particles are predominantly charged negative; this effect is still evident but smaller in magnitude 12 h after a dust storm. This size dependence for the charge polarity is in apparent contradiction with previous investigations; we believe this contradiction is only apparent, because the very fine particles, which we cannot measure with our technique, are expected to be negatively charged. Our results are rationalized by combining a population balance model for charged species trapped in high energy states with the wind threshold for soil particle motion.