X-Ray micro-tomography unveils the internal features of volcanic ash aggregates.

Volcanic ash aggregation occurs during transport in the atmosphere when individual ash particles collide and stick together. It significantly impacts ash residence time in the atmosphere, with major consequences for hazard assessment and ash dispersal forecasts. Nonetheless, aggregation processes are still not adequately parametrized, mostly due to the low preservation potential of most aggregate types. We present here the first, detailed structural and morphological characterization of the major aggregate types, combining an innovative field collection strategy, which allows for the original aggregate structure to be preserved at deposition, coupled to X-Ray micro-tomography. Resulting observations together with weather information, allowed for the structure of fragile ash clusters and of the elusive cored Ash Pellets (cAP1s) to be fully resolved and their genesis to be better described. The collected dataset represents a fundamental advancement towards a comprehensive characterization of the principal aggregate categories, which is key to accurately interpreting and modelling the process of volcanic ash aggregation and dispersal.