In-situ evidence of volcanic ash aggregation during fallout from combined ground- and UAS-based observations.

This study investigates the sedimentation of volcanic particles from low-altitude (< 2 km a.s.l.), near-daily ash plumes and clouds at Sakurajima volcano (Japan). Plume dynamics were monitored using imagery (visible wavelength) and geophysical (ash discharge rates) data. Ash fallout was characterized by using ground-based (disdrometer, particle electrical charge sensor, and sampling) and drone-mounted (optical particle counter, atmospheric sensor, and sampling) instruments. A comparison of particle size distributions and aggregate proportions between samples collected by drone 500 m above the take-off sites and those collected on the ground shows that aggregation develops rapidly during sedimentation. This process involves collisions between coarse ash (up to 1 mm) and fine ash particles (< 63 µm). Particle binding is promoted by electrostatic attraction (forming particle clusters) or high atmospheric humidity (forming accretionary pellets). These results provide innovative in-situ evidence of ash aggregation, offering new insights into its dynamics in natural settings, crucial for improving volcanic ash dispersion forecasting.