Popocatepetl ash charge as function of humidity and temperature

Here, we provide the absolute charge collected by volcanic ash (Popocatepetl) in the range of 250-500 microns through frictional interactions as a function of relative humidity (RH) and temperature. The range of RH is 0-50% (at 25 C) and the range of temperature is -20-40 C (at 30 % RH). Particles were charged triboelectrically and the charge was measuirend on individual particles using a miniature Farday cup. All values are reported in pico-Coulombs.

Methods
Here, we provide the absolute charge collected by volcanic ash (Popocatepetl) in the range of 250-500 microns through frictional interactions as a function of relative humidity (RH) and temperature. The range of RH is 0-50% (at 25 C) and the range of temperature is -20-40 C (at 30 % RH). Particles were charged triboelectrically by tumbling them in a hollow aluminum tube driven by a stepper motor. The interior of the tumbler was coated with particles of identical size and composition as the free grains such that tribocharging arises from particle-particle collisions only (i.e we tried to minimize the effects arising from the interactions between chemically dissimilar materials). For each condition, we cracterized the charge on several hundred particles.

To neutralize any charge that may have been transferred to the particles during handling, the sample in the tube was sprayed with a bipolar ionizing air gun for 1 minute prior to the experiment. Particles were charged by spinning the tube at a rotation rate of 0.5 m/s for a period of 20 minutes. Once this charging period was over, the tumbler was inclined to allow grains to roll out under the action of gravity. Individual grains then fell into a miniaturized Faraday cup. In this manner, we were able to measure the charge on individual grains with an error of 10 fC.  

Additional discussions can be found in:

Electrification of sand on Titan and its influence on sediment transport. JS Méndez Harper, et al (2017). Nature Geoscience 10 (4), 260-265.

Microphysical effects of relative humidity and temperature on the triboelectrification of volcanic ash. J Méndez Harper et al. (2020).  Journal of Geophysical Research: Atmospheres (in Press).