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Abstract

Ash injection and settling experiments have been carried out using the environmentally controlled recirculating wind tunnel facility at Aarhus University (DK), in order to understand the processes controlling deposition and segregation of ash from volcanic plumes at stratospheric altitudes. Ash particles erupted from the Campi Flegrei volcano (Italy) and smaller than 63 micron were injected in the wind tunnel either from the upwind extremity in the presence of wind (1 m/s) or from the side in the absence of wind. The atmospheric pressure in the wind tunnel was systematically varied to simulate the corresponding elevation in the atmosphere from 10 to 50 km. The vertical and horizontal velocity of the particles was measured, as well as the plume opacity, proxy for particle concentration over time. Settled particles were sampled at different times during the experiments and then analysed for their abundance and size distribution. Both the opacity measurements and the number of particles sampled over time display the decay of particle concentration over time in the suspended plume. The rate of decay is strongly dependent on the atmospheric elevation in a nonlinear way, with modest changes from 10 to 20 km elevation and much larger changes for higher elevations. From these data it is possible to retrieve the settling velocity of volcanic ash particles at a range of elevations that is of interest for both aviation and climate modelling implications.