The lifetime of charged dust in the atmosphere

TL;DR

This study investigates how long electrostatic charge remains on airborne dust particles, revealing that charge decay is mainly influenced by ion capture and environmental conditions, affecting dust transport models.

Contribution

The paper provides experimental data on charge decay times of levitated dust particles and links these findings to atmospheric dust transport modeling.

Findings

Charge decays in over a week in dry air

Decay time decreases to days in humid conditions

UV radiation can alter charge decay depending on polarity

Abstract

Windblown dust plays critical roles in numerous geophysical and biological systems, yet current models fail to explain the transport of coarse-mode particles (>5 $\mu$m) to great distances from their sources. For particles larger than a few microns, electrostatic effects have been invoked to account for longer-than-predicted atmospheric residence times. Although much effort has focused on elucidating the charging processes, comparatively little effort has been expended understanding the stability of charge on particles once electrified. Overall, electrostatic-driven transport requires that charge remain present on particles for days to weeks. Here, we present a set of experiments designed to explore the longevity of electrostatic charge on levitated airborne particles after a single charging event. Using an acoustic levitator, we measured the charge on particles of different material compositions suspended in atmospheric conditions for long periods of time. In dry environments, the total charge on particles decayed in over 1 week. The decay timescale decreased to days in humid environments. These results were independent of particle material and charge polarity. However, exposure to UV radiation could both increase or decrease the decay time depending on polarity. Our work suggests that the rate of charge decay on airborne particles is solely determined by ion capture from the air. Furthermore, using a one-dimensional sedimentation model, we predict that atmospheric dust of order 10 $\mu$m will experience the largest change in residence time due to electrostatic forces.