Nowadays, the Copernicus Sentinel missions together provide a combination of high-resolution observation capabilities in different bands that few years ago were not yet available. If used in a coordinated way, their data can upgrade the scale and spatial and temporal resolution of investigations of small-sized volcanoes, whose eruptions can be violent and potentially hazardous for local communities.
With its peculiar type of activity and limited extent, the Stromboli volcano in southern Italy offers an ideal test case to demonstrate that the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor (Sentinel-5P) satellite has the suitable spatial resolution and sensitivity to carry out local-scale sulfur dioxide (SO2) monitoring of relatively small-size, nearly point-wise volcanic sources and – in combination with Sentinel-2 multispectral observations of the situation on the ground – distinguish periods of different activity intensity.
2. Data and methods
The main input dataset of this study is the whole record of Sentinel-5P TROPOMI Level 2 SO2 geophysical products from UV sensor data acquired over Stromboli, since the beginning of the mission routine operations, after its 6-month-long commissioning phase. In particular, the TROPOMI time series for Stromboli starts on 6 May 2018, and the analyzed data cover three full years of observations, until 31 May 2021.
The latter were processed with purposely adapted Python scripts, according to a methodological workflow encompassing the extraction of total SO2 Vertical Column Density (VCD) at given coordinates (including conditional VCD for three different hypothetical peaks at 0–1, 7 and 15 km), as well as filtering by quality in compliance with the Sentinel-5P Validation Team’s recommendations. The total SO2 VCD time series for the main crater and across different averaging windows (3 x 3, 5 x 5 and 4 x 2) were compared to prove the correctness of the adopted spatial sampling criterion. As described in detail in Cofano et al. (2021), an approach for detecting SO2 VCD peaks at the volcano was trialed, and the detections were compared with the level of SO2 flux measured at ground-based instrumentation. To this scope, bulletin data were obtained from the FLux Automatic MEasurements (FLAME) and ROCcette site (ROC) Stations (Delle Donne et al., 2017; Randazzo et al., 2005). These records permitted to distinguish periods of intense activity from periods with low emissions and tremors, and were used to make a qualitative comparison between the quantity of SO2 emitted on the ground and that observed from satellite.
SO2 time series analysis was then complemented with information provided by contextual Sentinel-2 multispectral (in the visible, near and short-wave infrared) and Suomi NPP VIIRS observations. The aim was to correctly interpret SO2 total VCD peaks when they either (i) coincide with medium to very high SO2 emissions as measured in situ and known from volcanological observatory bulletins, or (ii) occur outside periods of significant emissions despite signs of activity visible in Sentinel-2 data. Finally, SO2 VCD peaks in the time series were further investigated through daily time lapses during the paroxysms in July–August 2019, major explosions in August 2020 and a more recent period of activity in May 2021. Hourly wind records from ECMWF Reanalysis v5 (ERA5) data and World Meteorological Organization (WMO) stations were used to identify local wind direction and SO2 plume drift during the time lapses.
3. Results and discussion
The SO2 time series after Quality Assurance and further filtering includes 589 observations, thus providing robust and sufficiently continuous records of volcano degassing at Stromboli.
Our spatial sampling tests suggest that in the case of volcanoes of “limited” areal extension, i.e., comparable to the pixel size of TROPOMI data, such as Stromboli, it is generally suitable to sample the single pixel and consider its value with respect to that found for larger averaging windows.
The time series of total SO2 column density for the hypothetical profile at the 0–1 km peak altitude was the profile used for reference for weak eruptions and degassing, as in those cases the bulk of the SO2 emitted by Stromboli is expected to be within the first kilometer of the atmosphere (i.e., ground-level plume). During major explosions and paroxysms, higher plume altitudes should be accounted for (e.g. ~9 km observed after the main explosion on 3 July 2019). On days of moderate to very high activity at the volcano, the data showed very clearly a series of significant total VCD at the main crater.
We also found that SO2 total VCD peaks as captured by TROPOMI data can serve as reliable proxies of volcanic activity and, as such, can be used to investigate other volcanoes when information about events is scarce or absent. However, it is not always trivial to associate SO2 total VCD peaks with moderate to very high SO2 emissions recorded by ground-sensor data, and reported in volcanological observatories bulletins. In this regard, the inspection of contextual Sentinel-2 multispectral observations in the visible, near and short-wave infrared (as well as Suomi NPP VIIRS data) provides an effective means to refine the interpretation of SO2 VCD peaks when they occur outside known periods of significant emissions. Although some limitations may occasionally constrain the analysis (i.e., cloud coverage that may hinder the visibility; time lag between the acquisition date of Sentinel-2 vs. TROPOMI data), this multi-sensor data approach adheres to the holistic concept of multi-band and multi-mission observations that are behind the whole Copernicus Earth Observation Programme.
Finally, the multi-temporal analysis of daily time lapses of SO2 VCD during the paroxysms that occurred in July–August 2019, major explosions in August 2020 and a more recent period of activity in May 2021 demonstrated that the proposed approach was successful in showing the SO2 degassing associated with these events, and warning whenever the total SO2 column density values at Stromboli may be overestimated due to clustering with the plume of the Mount Etna volcano. This geographical parameter, alongside wind direction, has to be accounted for whenever the nearly point-wise volcanic source under investigation is located in proximity to other SO2 emission sources, either natural (e.g., Mount Etna) or anthropogenic, that may interfere.
In the current context of an increasing body of literature on the use of Sentinel-5P data for volcanic studies, our presentation aims to deepen the discussion on the practical technical issues involved in the handling and post-processing of these geophysical data that are yet to become of common and standard use across the scientific community interested in volcanological applications, and how they can be used in a synergistic way with Sentinel-2 multispectral imagery to investigate eruptions, paroxysms and other events of potential concern for safety, at small-size Strombolian volcanoes. Based on the evidence gathered at Stromboli from 2018 to 2021, we propose practical recommendations for further implementation in similar volcanic environments. The proposed analysis approach is successful in showing the SO2 degassing associated with these events, and warning whenever the SO2 VCD at Stromboli may be overestimated due to clustering with the plume of the Mount Etna volcano.
Cofano, A., Cigna, F., Amato, L. S., de Cumis, M. S., & Tapete, D. (2021). Exploiting Sentinel-5P TROPOMI and Ground Sensor Data for the Detection of Volcanic SO2 Plumes and Activity in 2018–2021 at Stromboli, Italy. Sensors 2021, Vol. 21, Page 6991, 21(21), 6991. https://doi.org/10.3390/S21216991
Delle Donne, D., Tamburello, G., Aiuppa, A., Bitetto, M., Lacanna, G., D’Aleo, R., & Ripepe, M. (2017). Exploring the explosive-effusive transition using permanent ultraviolet cameras. Journal of Geophysical Research: Solid Earth, 122(6), 4377–4394. https://doi.org/10.1002/2017JB014027
Randazzo, D. A., Caltabiano, T., Salerno, G. G., Murè, F., Bruno, N., Longo, V., Spina, A. la, & Burton, M. R. (2005). Rapporto sullo sviluppo delle reti FLAME Etna e Stromboli, per la misura del flusso SO2, durante il periodo 2005 – 2009. https://www.earth-prints.org/handle/2122/5509