Volcanic plume height during the 2021 Tajogaite Eruption (La Palma) measured by two complementary monitoring methodologies: Implications for satellite-based SO2 mass emission estimates

Volcanic emissions from the Tajogaite volcano, located on the Cumbre Vieja edifice on the island of La Palma (Canary Islands, Spain), caused significant public health and aviation disruptions throughout the volcanic event (19 September – 13 December 2021, officially declared over on 25 December). The Instituto Geográfico Nacional (IGN), the authority responsible for volcano surveillance in Spain, implemented extensive scientific monitoring to track volcanic activity and to provide a robust estimation of the volcanic plume height using a video-surveillance network. In parallel, the State Meteorological Agency of Spain (AEMET), in collaboration with other members of ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) in Spain, in collaboration with other institutions, carried out an unprecedented instrumental deployment to assess the atmospheric composition impacts of this rare event. This effort included a network of aerosol profilers surrounding the volcano. A total of four profiling instruments were installed on La Palma: one MPL-4B lidar and three ceilometers. Additionally, a pre-existing Raman lidar on the island contributed valuable data to this study.

In this study, the eruptive process was characterised in terms of the altitude of the dispersive volcanic plume (hd), measured by both IGN and AEMET-ACTRIS, and the altitude of the eruptive column (hec), measured by IGN. Modulating factors such as seismicity and meteorological conditions were also analysed. The results confirmed the existence of three distinct eruptive phases, encompassing a range of styles from Strombolian explosive to effusive activity.

The consistency between the two independent and complementary datasets (hd,IGN and hd,AEM ET) was assessed throughout the eruption (mean difference of 258.6 m). Furthermore, a comparison of hd,AEM ET with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol layer height product (ALHCALIOP) revealed a systematic underestimation by the satellite product, with a mean difference of 615.0 m (392.2 m if 6 October is excluded from the analysis).

Finally, the impact of using hec in estimating SO2 emissions from the NASA MSV OLSO2L4 satellite-based product was evaluated. When a fixed plume altitude of 8 km was used instead of the observed hec, the total SO2 mass was significantly underestimated by an average of 56.2%, and by up to 84.7% in some cases. These findings underscore the importance of accurately determining the volcanic plume height when deriving SO2 emissions from satellite data.