On September 22, the Barcelona Raman LIDAR (BRL) Pathfinder installed on the CTAO-North site at the Roque de los Muchachos Observatory on La Palma (Canary Islands, Spain) detected the volcano dust plume while monitoring the atmosphere. These are the first results made public by the BRL team and the most accurate results for the characterization of the atmosphere provided by any current operative LIDAR system on-site.

The observations with the BRL Pathfinder, composed of a powerful laser and a telescope, were carried out between September 18 and 22 2021, covering the eruption of the volcano in Cumbre Vieja (El Paso, southern region of La Palma) on September 19. The data set includes a series of 2000 laser shots into the atmosphere at various zenith angles (from 0o to 60o). From the time required for each shot to travel back to the telescope, it is possible to distinguish the various atmospheric components at each altitude.

Preliminary data reconstruction allows to clearly distinguish two aerosol layers from 1.2 to 1.6 km and from 2.2 to 4.0 km above the CTAO-North site level (3.4 to 3.8 km and 4.4 to 6.2 km above sea level, respectively). The corresponding relation between “backscatter coefficient,” a measure of how light is getting back-scattered during its path through the layer, and the “extinction coefficient,” a measure of how light coming from sky sources is altered as it passes through the different atmosphere layers, indicates the presence of large diameter particles at the lower layer. A similar interpretation could be made when comparing the light extinction at two different wavelengths and deriving the so-called “Ångström coefficient.” This allowed to interpret the lower layer as a dust plume from the Cumbre Vieja volcano, while the upper layer is interpreted as a cloud with typical characteristics for La Palma atmosphere at those altitudes.

These results are an important help for other profiling instruments at the Roque de los Muchachos Observatory, particularly LIDARs, to determine which part of their data can be attributed to the volcano dust plume and which is due to other weather phenomena. It will also help to calibrate dust evolution simulation tools, as those used by the Spanish Meteorological Agency (AEMET, in Spanish), needed to predict with high precision short- and mid-term evolution of the path that the volcanic dust plumes take.

The BRL Pathfinder, built in Barcelona and installed at the CTAO-North site inside the LST-1 construction area in February 2021, is a key instrument to measure the vertical profiles of aerosol within the atmosphere, fundamental for the calibration of the CTAO telescopes. The system is being tested at the CTAO-North site and, after one year, will be returned to Barcelona for updates based on the data collected.

The BRL Pathfinder for the CTAO-North site is a joint project between CTA members from IFAE-BIST (Institute of High Energy Physics – Barcelona Institute of Science and Technology), UAB & IEEC-CERES (Autonomous University of Barcelona & Institute of Space Studies of Catalonia-Center of Space Studies and Research), Center of Astrophysics and Cosmology of the University of Nova Gorica and Department of Physics and Astronomy of the University of Padova. Paolo Calisse, the CTAO-North Site Manager, serves as the on-site project manager for the instrument.

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The CTAO Barcelona Raman Lidar Pathfinder detects the volcano dust plume at La Palma - CTAO