Barcelona Raman Lidar operating inside the LST-1 construction area at the CTAO-North site. Credit: Paolo Calisse (CTAO).
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.
