Lidar measurements of volcanic ash plume during the 12 August 2011 Etna lava fountain

Etna produces powerful lava fountains that forms high eruption columns rising up to about 10 km above sea level. In 2011, volcanic ash dispersed in atmosphere, caused several problems to aviation operation and often the international airport of Catania was forced to close during the activity. In order to reduce the impact of volcanic ash to local population, a better understanding of the activity is necessary. In this work we show measurements carried out by a scanning lidar during the lava fountain of 12 August 2011. The lidar system named "VAMP" (Volcanic Ash Monitoring by Polarisation) uses a frequency doubled Nd:YAG laser source operating at a 532-nm wavelength, with a repetition rate of 1 kHz. The VAMP system was installed at the "M.G. Fracastoro" astrophysical observatory (14.97° E, 37.69° N) of the Ist. Nazionale di Astrofisica in Catania, located at a distance of 7 km from the Etna summit craters. The system was moved in azimuth and elevation in order to retrieve the features of volcanic plumes produced during the lava fountains. Laser pulses were emitted with energy of 0.3 mJ, a duration of 40 ns and a linear polarisation better than 100:1. The system transmits a linearly polarised laser light and parallel and cross-polarised components of the backscattered radiation are collected separately. The spatial resolution is 30 m along the line of sight. Measurements were carried out from 9:00 to 12:00 UTC during the lava fountain activity that started at about 8:30 UTC and lasted up to 12:00 UTC. Depolarization measurements clearly detected a volcanic ash plume from the New South East Crater between 6000 and 7000m far from the lidar station. The retrieved values of volcanic aerosol depolarization ratio, aerosol backscattering and lidar-ratio allowed distinguishing the changes in the properties of volcanic ash plumes during the activity. Column height and volcanic ash spatial distribution were also measured.