Microphysical effects of water content and temperature on the triboelectrification of volcanic ash on long timescales

The effects of water and temperature on the triboelectrification of granular materials have been reported by numerous authors, but have not been studied robustly in the context of volcanic plumes. Here, we present the results of a set of experiments designed to elucidate how environmental conditions modulate the triboelectric characteristics of volcanic ash. We find that that small amounts of water can reduce the charge collected by micron-sized ash grains by up to an order of magnitude. Increasing temperature at a constant relative humidity also appears to decrease the amount of charge gained by particles. Analysis of our data shows that if particles undergo low-energy, low-frequency collisions in humid environments under long timescales, charge dissipation dominates over charge accumulation. Thus, our work suggests that triboelectric charging may be an inefficient electrification mechanism outside of the gas-thrust region where collision rates are high and residence times are low.