Modeling of the electrical interaction between desert dust particles and the Earth’s atmosphere

In the current work, the two distinct charging mechanisms of spherical dust particles settling in the Earth’s atmosphere, i.e. ion attachment and contact electrification, are studied in parallel. A novel 1D numerical model has been developed that parametrizes the charging processes in the presence of a large scale electric field, under stagnant atmospheric conditions where wind contribution is neglected. The model is able to self-consistently calculate the modification of atmospheric ion densities and the subsequent alteration of the large scale electric field, when dust particles are present and atmospheric ions attach to them, and is further updated to account for the particle charging due to the efficient collisions between particles of sizes ranging from fine (less than 1μm in radius) to giant mode (larger than 50μm in radius). Due to a lack of a rigorous mathematical expression for the quantification of the particle collision process, the mechanical and electrostatic effects are modeled independently, based on works in the past literature, and then are combined by a simple superposition principle. Additionally, the electrostatic effects are studied in the presence of weak external electric fields (less than 1–2 kV/m). Moreover, for the same reasons a simple superposition principle has been applied for the quantification of the ion attachment process. All these assumptions lead to an approximation of the actual mechanisms quantification, but this method can lead to physically reasonable results, highlighting the impact of several processes to the eventual charging of the dust particles. By binning integration of a realistic particle size distribution in the model, the acquired electrical charge on the dust particles is calculated in the range of 0.5 to 2000 elementary charges. The particles become on average negatively charged, but under specific conditions giant mode particles can be locally and temporarily positive, congruently to previously reported laboratory experiments. Moreover, the electrical force that is applied on the particles by each mechanism individually and through the superposition of both, is extracted and compared to the gravitational force acting on each particle. However, the 1D results indicate that the electrical force is not enough to significantly influence the gravitational settling of the charged particles, as it is more than one order of magnitude less than gravity. This designates that although the electrical processes alone are accounted for, significant meteorological processes such as updrafts, wind shears, or horizontal winds need to be included in the model to accurately represent the modulation of particle electrification and so as to represent the impact of electricity on particle dynamics in a more realistic way.