Remote sensing measurements of fresh volcanic ash during the Mount Etna emission of February 21, 2019

Explosive activity can have a relevant impact in the atmosphere even during weak and continuous volcanic ash emissions. In fact, this type of activity can affect highly populated areas and needs to be investigated in order to reduce potential risks. In this paper, we analyze the volcanic ash emissions that took place on February 21, 2019 from the North East Crater, one of the summit craters of Mount Etna, in Italy. During the activity, a continuous ash emission caused the closure of the International Airport in Catania due to a large quantity of volcanic particles in the atmosphere that were dispersed by winds several kilometers away from the eruptive crater, mainly toward the west, south and south-east directions. This activity was analyzed using a dual depolarization LiDAR and visual and thermal cameras that are part of the instrumental network of the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo. The LiDAR derived aerosol backscattering coefficient and particle linear depolarization ratio profiles, both measured at 355 nm and 532 nm, gave insights on plume dynamics and variations of some features of the particles within the volcanic plume. During this event, we estimated a maximum volcanic plume height of about 3 km above sea level and LiDAR data show two distinct layers in the atmosphere, LiDAR derived aerosol properties were used for a first application of the Volcanic Ash LiDAR Retrieval - Maximum Likelihood (VALR-ML) algorithm on two volcanic ash layers, allowing to obtain a maximum value of volcanic ash concentration of 7.5 ± 3.7 mg/m 3 and 8.1 ± 4.0 mg/m 3 , in the first layer at 355 and 532 nm, respectively; while in the second layer we obtained concentration values of 6.6 ± 3.3 and 8.5 ± 4.2 mg/m 3 at 355 and 532 nm, respectively. Moreover, the plume was composed of very fine ash of about 1 μm dimensions. We found that weak and continuous volcanic ash emissions can reach thresholds that cause troubles to aviation operations. Our work shows how LiDAR systems are able to estimate critical information for aviation safety in the proximity of the airport, such as the altitude, the concentration and the size of emitted ash particles, even during low-intensity explosive activity.