The lifetime of charge on a lofted particle

The transport of small atmospheric particles across the world’s oceans is crucial for the global biome, for example, dust from the Sahara feeds the Amazon rain forest. This transport can be enhanced by electrostatic forces when individual particles are charged. Yet little is known about the lifetime of charge on such lofted particles. Laboratory measurements are challenging since particles must be levitated and manipulated without contact. Here we use an acoustic radiation trap to levitate and measure the net charge on isolated, millimetric particles for days. Particles are charged with an ionizer, and we adjust the phase of the acoustic field to move the particle through a Faraday cup. In dry, atmospheric environments, we find that the charge decays exponentially with a time constant between 1-10 days. Increasing the humidity leads to a much faster logarithmic decay of charge rather than exponential. This behavior is observed in polystyrene, amaranth, and aerogel particles. By varying the pressure and atmospheric composition we are currently testing two possible mechanisms to explain this behavior: water mobility and hydrophobicity of the surface, and ion recruitment by diffusion to the particle surface.