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

Donnadieu, F.
Eychenne, J.
Soriaux, C.
Latchimy, T.
Published in Journal of Volcanology and Geothermal Research. 2019, vol. 374, p. 62-79
Abstract Ash particle terminal settling velocity is an important parameter to measure in order to constrain the internal dynamics and dispersion of volcanic ash plumes and clouds that emplace ash fall 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 settling velocity as a function of size. During radar remote sensing measurements of weak volcanic plumes erupted from Stromboli volcano in 2015, an optical disdrometer was used to measure the size and settling velocities of falling ash particles over time, while six ash fallout samples were collected at different distances from the vent. We focus on the implications of the physical parameters of ash for settling velocity measurements and modeling. Two-dimensional sizes and shapes are automatically characterized for a large number of ash particles using an optical morpho-grainsizer MORPHOLOGI G3. Manually sieved ash samples 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 amode 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 ash particles. 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 settling velocities under laboratory conditions with no wind, <3.6 m s−1, are in agreement with the field velocities expected for particleswith sizes <460 μm. The Ganser (1993) empirical model for particle settling velocity is themost consistentwith our disdrometer settling velocity results. Converting disdrometer detected size into circle equivalent diameter shows similar PSDs between disdrometer measurements and G3 analyses. This validates volcanological applications of the disdrometer to monitor volcanic ash sizes and settling velocities 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 ash plumes to infer first-order ash plume concentrations. Detailed PSDs and shape parameters may be used to further refine radar-derived mass loading retrievals of the ash plumes.
Keywords Terminal settling velocityAsh falloutParticle sizeMorphologyDisdrometerStromboli
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FRERET-LORGERIL, Valentin et al. In situ terminal settling velocity measurements at Stromboli volcano: Input from physical characterization of ash. In: Journal of Volcanology and Geothermal Research, 2019, vol. 374, p. 62-79. doi: 10.1016/j.jvolgeores.2019.02.005

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Deposited on : 2019-12-23

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