From up above to down below: Comparison of satellite- and ground-based observations of SO2 emissions from the 2021 eruption of Cumbre Vieja, La Palma

Volcanic gas emissions are an integral part of volcano monitoring around the world and can be interpreted to understand the state of a volcano and the evolution of an individual eruption. The low ambient concentrations of SO₂ make it an ideal monitoring candidate.

Throughout the 2021 eruption of Cumbre Vieja, La Palma (Spain), observations of SO₂ emissions were made using ground-based instruments, in transverse mode, static scanners and on-board drones, as well as by numerous satellite instruments. Direct comparison between satellite- and ground-based instruments is always challenging, but the long duration of the eruption and repeated measurements from both data sets made this a good candidate.

Data from the Sentinel-5P instrument TROPOMI was combined with the PlumeTraj back-trajectory analysis toolkit to produce sub-daily SO₂ fluxes that can be directly compared to the ground-based observations as well as other geophysical and geochemical monitoring data.

The volcano produced significant volcanic ash emissions, particularly in the earlier phases of the eruption, which impacted both ground- and satellite-based measurements. This produced underestimations in the SO₂loading where ash was present, impacting the proximal plume more that the distal as the ash settles out with time. This meant that traverse measurements were more impacted, leading to a disparity in the measured fluxes from ground and space. Later, when ash emissions had decreased, the agreement between the two was much improved, with trends closely replicated between the traverse and satellite fluxes.

The initial estimates of the total SO₂ emission from the eruption were 4.1 Mt from TROPOMI and 1.2 Mt from the traverse data.

These measurements formed part of the official monitoring effort, providing insights into the eruption’s evolution and informing the civil defence response throughout the eruption.