Characteristics of Volcanic Ash That Influence Lightning and Cloud Processes

The presence of volcanic ash in the atmosphere can play a significant role in lightning and cloud processes.  Composed of rock, mineral, and glass particles from 2 mm to < 1 μm in diameter, volcanic ash can have a wide variety of chemical compositions and morphologies and may function as ice nuclei and/or act as charge carriers in clouds.    Here we present laboratory studies that address the fundamental physical, chemical, and electrical properties of volcanic ash samples from several locations, including currently active Alaskan volcanoes where volcanic lightning is documented, in addition to ash samples derived from volcanic deposits of various bulk compositions.  The latter set of samples were mechanically milled to homogenize the grain shape and size. We present the role that mineral content, particle shape/size, and grain conductivity may play in both the generation of lightning (specifically volcanic lightning) and the nucleation of ice in the atmosphere.  Ash conductivities were calculated from resistance measurements at controlled humidity and compared to the proportion of different mineral phases, the grain size, and the average concavity index of particles.  Grain sizes were measured using laser diffraction particle size analysis and mineral phase abundances and shape parameters were determined using scanning electron microscopy. X-ray fluorescence was used to determine bulk ash composition. Depositional and immersion-mode ice nucleation experiments were performed using dispersive Raman spectrometry.  Preliminary analyses suggest that compaction, and therefore grain shape and contact points, controls ash conductivity, and not bulk composition, as homogenized samples provide variable conductivity measurements from 1.6 x 10^-3 to 9.9 x 10^-1S/m.  Non-homogenized Alaskan samples are hypothesized to have higher concavity indices and conductivities when compared to the homogenized samples, due to wider variations in grain size and shape, and these data will also be presented. All ash samples efficiently form depositional ice nuclei, but certain mineral phases display higher efficiency of immersion-mode ice nucleation, including K or Na/Ca feldspars.  Based upon the measured physical and chemical characteristics of the samples examined, we assign a relative electrical hazard to particular volcanoes based upon the ability of ash samples to function as charge carriers and ice nuclei in the atmosphere.  These results help to constrain the role that volcanic ash plays in lightning and cloud processes, potentially providing new parameters for atmospheric models and hazard assessment strategies.