Influence of Particle Properties and Grain Size on Aggregation in Dry Particle‐Laden Volcanic Jets

Explosive volcanic eruptions can inject large amounts of ash into the atmosphere. These suspended particles pose hazards to aircraft, so a better understanding of the ash residence time and transport distance is needed. Fine particles combine to form larger aggregates, altering their settling behavior. Ash aggregation is influenced by liquid bonding, electrostatic forces, and preferential concentration driven by turbulence. We examine particle aggregation through controlled, low‐humidity experiments at similar particle‐fluid scaling to conditions near a volcanic vent. Particle‐laden jets used compressed air containing three types of particles at controlled mass loadings: hollow glass spheres (∼10 μm diameter), solid nickel spheres (∼7 μm diameter), and volcanic ash from the 18 May 1980, Mount St. Helens eruption (<2 mm diameter). The jet had Reynolds numbers from 5,000 to 10,000, with Stokes numbers between 0.6 and 24,000 based on convective timescales and length scales. Flow velocity was measured using particle image velocimetry, and particle fallout was collected and imaged on microscope slides. To reduce errors from particle overlap, we introduce a normalization method that compares experimental images with synthetic images of randomly distributed particles. Results show minimal aggregation for a single particle type, but there is more than 10 times enhanced aggregation when hollow glass and nickel particles are combined. This behavior may be attributed to differential charging of the hollow glass and nickel, which promotes electrostatic attraction. Volcanic ash particles showed modest aggregation, but normalized trends were still challenging to quantify because aggregates and large individual particles overlapped in size.