Direct Effects of Air Humidity on Dust Aerosol Production: Evidences for the Surprising Role of Electrostatic Forces

Abstract

Wind erosion is a common but critical geophysical process, especially in arid and semi-arid regions. It is found that the emission flux of dust with a geometric diameter up to 20 μm (PM 20 ) is approximately 5,000 Tg yr −1 (Kok et al., 2021). Dust aerosols have crucial impacts on human health, weather, climate, and biogeochemical cycles. Dust aerosols inhaled by humans can negatively affect the respiratory system and lungs (Brunekreef & Holgate, 2002;Choudhury et al., 1997). In addition, dust aerosols can affect climate through changing radiative forcing and impacting cloud formation by acting as cloud condensation nuclei or ice nuclei (Miller & Tegen, 1998;Ramanathan et al., 2001). Moreover, the deposition of dust aerosols in ocean and land systems is an important way to transport nutrients, such as iron-containing compounds (Jickells, 2005;Okin et al., 2004).Wind erosion occurs when the wind speed exceeds a threshold velocity and forces soil particles to move. Large soil particles would strike the surface and collide with each other, and then shake off smaller particles which can be lifted up by wind and form dust aerosols (Y. P. Shao, 2008). For soil particles to be lifted up into the air, the forces associated with the wind shear near the surface or the collision forces among moving soil particles must overcome the gravitational force and cohesive forces, including wet-bonding forces, van der Waals forces and electrostatic forces between particle surfaces (Y. P. Shao, 2008). It is also interesting to note that dust aerosols usually carry electrical charges. Soils are generally electrically neutral. However, charges are separated, transferred, or redistributed when soil particles colliding with each other, which results in a net charge transfer between these particles (Harrison et al., 2010;Kok & Renno, 2009;Wei & Gu, 2015). It is observed that soil particles are indeed charged in a wind tunnel (Zheng et al., 2004). And electric field generated by suspended charged dust aerosols could, in turn, affects dust emission under various relative humidity (RH) conditions (Esposito et al., 2016).

Microplastic Emission from Soil-Air Interface

“…This promotes the adsorption of particles onto their surfaces, thereby resulting in a condensation between the particles. This phenomenon has been reported by Ma et al, who refer to it as “wet-bonding forces” . The hydrophobicity of the microplastic surface can retard the formation of liquid bridges.…”

“…Soils are sampled at S1–S4 desert, S5–S6 agricultural land, S7 road, and S8 vegetational land (Figure S2). Their textures are representative of soil types, including sand, loamy sand, loam, silty loam, and silty clay loam . More properties are given in Table S1.…”

“…Their textures are representative of soil types, including sand, loamy sand, loam, silty loam, and silty clay loam. 30 More properties are given in Table S1. Microplastic standards (1, 2.5, 10, 20, 25, 48, 70, 200, and 550 μm diameter PS, PVC, and PE pellets; 40 μm width PP and 35 μm width PS fibers in lengths of 500, 1000, 3000, and 5000 μm) were first added to soils and then were aerosolized in a dust aerosol generator (Figure S3).…”

“…This phenomenon has been reported by Ma et al, who refer to it as "wet-bonding forces". 30 The hydrophobicity of the microplastic surface can retard the formation of liquid bridges. Therefore, it can be anticipated that microplastic particles may retain less adsorbed water, making them more susceptible to being carried by the wind.…”