  The plume from the 2011 eruption of Gr\'{i}msv\"{o}tn was highly electrically
charged, as shown by the considerable lightning activity measured by the UK Met
Office's low-frequency lightning detection network. Previous measurements of
volcanic plumes have shown that ash particles are electrically charged up to
hundreds of km away from the vent, which indicates that the ash continues to
charge in the plume [Harrison et al., Env. Res. Lett. 5 024004 (2010),
Hatakeyama J. Met. Soc. Japan 27 372 (1949)]. In this paper we study
triboelectric charging of different size fractions of a sample of volcanic ash
experimentally. Consistently with previous work, we find that the particle size
distribution is a determining factor in the charging. Specifically, our
laboratory experiments demonstrate that the normalised span of the particle
size distribution plays an important role in the magnitude of charging
generated. The influence of the normalised span on plume charging suggests that
all ash plumes are likely to be charged, with implications for remote sensing
and plume lifetime through scavenging effects.
