In situ terminal settling velocity measurements at Stromboli volcano: Input from physical characterization of ash

Ashparticle terminal settlingvelocity is an important parameter to measure in order to constrain the internal dynamics and dispersion of volcanic ashplumes and clouds that emplace ashfall deposits from which source eruption conditions are often inferred. Whereas the total Particle Size Distribution (PSD) is the main parameter to constrain terminal velocities, many studies have empirically highlighted the need to consider shape descriptors such as the sphericity to refine ash settlingvelocity as a function of size. During radar remote sensing measurements of weak volcanic plumes erupted from Stromboli volcano in 2015, an optical disdrometerwas used to measure the size and settlingvelocities of falling ashparticles over time, while six ashfallout samples were collected at different distances from the vent. We focus on the implications of the physical parameters of ashfor settlingvelocity measurements and modeling. Two-dimensional sizes and shapes are automatically characterized for a large number of ashparticles using an optical morpho-grainsizer MORPHOLOGI G3. Manually sieved ashsamples show sorted, relatively coarse PSDs spanning a few microns to 2000 μm with modal values between 180 and 355 μm. Although negligible in mass, a population of fine particles below 100 μm form a distinct PSD with a mode around 5–20 μm. All size distributions are offset compared to the indicated sieve limits. Accordingly, we use the diagonal of the upper mesh sizes as the upper sieve limit. Morphologically, particles show decreasing average form factors with increasing circle-equivalent diameter, the latter being equal to 0.92 times the average size between the length and intermediate axes of ashparticles. Average particle densities measured by water pycnometry are 2755 ± 50 kg m−3 and increase slightly from 2645 to 2811 kg m−3 with decreasing particle size. The measured settlingvelocities under laboratory conditions with no wind, <3.6 m s−1, are in agreement with the field velocities expected for particles with sizes <460 μm. The Ganser (1993) empirical model for particle settlingvelocity is the most consistent with our disdrometer settlingvelocity results. Converting disdrometerdetected size into circle equivalent diameter shows similar PSDs between disdrometermeasurements and G3 analyses. This validates volcanologicalapplications of the disdrometerto monitor volcanic ashsizes and settlingvelocities in real-time with ideal field conditions. We discuss ideal conditions and the measurement limitations. In addition to providing sedimentation rates in-situ, calculated reflectivities can be compared with radar reflectivity measurements inside ashplumes to infer first-order ashplume concentrations. Detailed PSDs and shape parameters may be used to further refine radar-derived mass loading retrievals of the ashplumes.