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For the first time since 1971, the Cumbre Vieja volcano on La Palma (Canary Islands, Spain) started to erupt on 19 September 2021 at 13:10 UTC, more than a month ago. Together with the outflowing lava, the volcano emits ash and several gases, amongst these sulphur dioxide (SO2). Volcanic ash and SO2 are dangerous to public health and aviation (damage to engine due to ash melting and sulphidation). Therefore, tracking and monitoring volcanic plumes and their constituents is very important. For this reason, since 2009 the Support to Aviation Control Service (SACS) system uses satellite observations from UV-vis (UV and visible light) and IR (infrared) hyperspectral sensors to detect ash and SO2 emissions. In case of exceptionally high concentrations of SO2 or ash, the system sends out a warning to stakeholders.

For this particular eruption, no ash notification has currently been sent by SACS. However, since the start of the eruption, daily notifications of exceptional SO2 concentrations have been sent to users by the SACS system. Dedicated webpages are created with key information and links to tailored images (e.g., alert and Google Earth) and data (e.g., SO2 notification). In particular, the alert data file available through the SACS notifications are referred as NCAP, for NetCDF Alert Products (see details in Brenot et al. 2014, 2021).

A key source of information in the context of the Cumbre Vieja eruption is from the Tropospheric Ozone Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5P satellite. TROPOMI provides observations at an unprecedented 3.5 × 5.5 km2 spatial resolution and BIRA-IASB is in charge of the algorithm development to retrieve SO2 columns from TROPOMI (Theys et al., 2017, 2019). In SACS, SO2 is also observed by other sensors, such as the Infrared Atmospheric Sounding Interferometer (IASI) onboard the MetOp satellites.

The transport of the SO2 plume as observed by TROPOMI over northern Africa, southern Europe and the Atlantic Ocean from the 19 September to 20 October 2021, is shown in the animation below.

La Palma animation
Figure 1: TROPOMI SO2 vertical column density and SO2 mass (upper left time-series). The Cumbre Vieja location is shown by a black triangle. SO2 maxima (in Dobson Units) and time tags are indicated. The mean overpass time of TROPOMI sensor over Canary Islands is about 15:00 UTC.

The animation shows that the emitted SO2 has been transported from the surroundings of the Canary Islands to the North of Africa (e.g., on 27 Sept. and 9 Oct.), to the South of Europe (e.g., on 25-26 Sept. and 19-20 Oct.), and along the Atlantic Ocean (e.g., on 30 Sept. and 6 Oct.). From 19 October onwards the plume was transported towards Portugal, Spain, France, UK and the Benelux. The observations on 20 October show a horizontal very thin plume of few hundred kilometers wide (an atmospheric river) which brings SO2 towards Germany, Switzerland, Italy and Poland.

For the time being, it is plausible that the eruption will continue for several weeks or even months. The volcanic emissions of Cumbre Vieja have not stopped since one month with ~50 kt of SO2 emitted per day (inset figure of upper left panel) and a total of ~1.5 Tg.

In the future, SACS will use more sensitive techniques to monitor volcanic emissions. For instance, the BIRA-IASB UV-vis team has recently developed a Covariance-Based Retrieval Algorithm (COBRA) for the retrieval of SO2 from TROPOMI (Theys et al., 2021). As can be seen below, this technique reduces the noise in the data and is very promising as it can detect very low level of SO2. These new observations could help issuing improved early warning messages, constraining dispersion models and be very relevant for volcanologists.

COBRA algorithm
Figure 2: Comparison of SO2 retrievals (column and mass loading) from two algorithms (Operational NRT data vs COBRA algorithm) for the first day of the Cumbre Vieja eruption, the 19th of September 2021. Data from the operational algorithm (NRT data) are shown in figure a), and the new retrievals from the COBRA algorithm are shown in figure b). We can see the clear improvement obtained using our new COBRA algorithm, with lower detection limit and less noise.


This work takes place in the frame of ALARM project, which has received funding from the SESAR Joint Undertaking (JU) under grant agreement No 891467. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the SESAR JU members other than the Union. Currently, we are working on the development and the implementation in SACS of selective detection of SO2 and aerosols (Ash and dust) from SEVIRI.

Read also


  • Dr. Hugues Brenot, UV-Visible DOAS Research group
    E-mail: hugues (dot) brenot (at) aeronomie (dot) be
  • Dr. Nicolas Theys, UV-Visible DOAS Research group
    E-mail: nicolas (dot) theys (at) aeronomie (dot) be


Brenot, H., Theys, N., Clarisse, L., van Geffen, J., van Gent, J., Van Roozendael, M., van der A, R., Hurtmans, D., Coheur, P.-F., Clerbaux, C., Valks, P., Hedelt, P., Prata, F., Rasson, O., Sievers, K., and Zehner, C.: Support to Aviation Control Service (SACS): an online service for near-real-time satellite monitoring of volcanic plumes, Nat. Hazards Earth Syst. Sci., 14, 1099–1123 (2014).

Brenot, H., Theys, N., Clarisse, L., van Gent, J., Hurtmans, D. R., Vandenbussche, S., Papagiannopoulos, N., Mona, L., Virtanen, T., Uppstu, A., Sofiev, M., Bugliaro, L., Vázquez-Navarro, M., Hedelt, P., Parks, M. M., Barsotti, S., Coltelli, M., Moreland, W., Arnold-Arias, D., Hirtl, M., Peltonen, T., Lahtinen, J., Sievers, K., Lipok, F., Rüfenacht, R., Haefele, A., Hervo, M., Wagenaar, S., Som de Cerff, W., de Laat, J., Apituley, A., Stammes, P., Laffineur, Q., Delcloo, A., Lennart, R., Rokitansky, C.-H., Vargas, A., Kerschbaum, M., Resch, C., Zopp, R., Plu, M., Peuch, V.-H., Van Roozendael, M., and Wotawa, G.: EUNADICS early warning system dedicated to support aviation in case of crisis from natural airborne hazard and radionuclide cloud, Nat. Hazards Earth Syst. Sci., accepted (2021).

Theys, N., De Smedt, I., Yu, H., Danckaert, T., van Gent, J., Hörmann, C., Wagner, T., Hedelt, P., Bauer, H., Romahn, F., Pedergnana, M., Loyola, D., and Van Roozendael, M.: Sulfur dioxide retrievals from TROPOMI onboard Sentinel-5 Precursor: algorithm theoretical basis, Atmos. Meas. Tech., 10, 119–153 (2017).
Theys, N., Hedelt, P., De Smedt, I. et al. Global monitoring of volcanic SO2 degassing with unprecedented resolution from TROPOMI onboard Sentinel-5 Precursor. Sci Rep 9, 2643 (2019).

Theys, N., Fioletov, V., Li, C., De Smedt, I., Lerot, C., McLinden, C., Krotkov, N., Griffin, D., Clarisse, L., Hedelt, P., Loyola, D., Wagner, T., Kumar, V., Innes, A., Ribas, R., Hendrick, F., Vlietinck, J., Brenot, H., and Van Roozendael, M.: A Sulfur Dioxide Covariance-Based Retrieval Algorithm (COBRA): application to TROPOMI reveals new emission sources, Atmos. Chem. Phys. Discuss., accepted (2021).

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News image legend 1
This image, captured by the Copernicus Sentinel-2 mission on 30 September, shows the flow of lava from the volcano erupting on the Spanish island of La Palma.
© contains modified Copernicus Sentinel data (2021), processed by ESA, CC BY-SA 3.0 IGO