Charge Decay on Levitated Particles in Atmospheric Conditions

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. We examined particles with vastly different material properties: polystyrene, amaranth, pumice, and aerogel. The 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 conditions, both positive and negative charge decay over 2-10 days. Increasing the humidity leads to large fluctuations in the half life, with the decay following nearly linear, logistic, or exponential behavior, with half lives as small as 0.1 days. Exposing polystyrene particles to UV radiation revealed a strong asymmetry in the electronic bound states: positively charged particles decayed in less than 1 hour, whereas negatively charged particles lasted for several hours.