It is important to monitor emissions from the world’s volcanoes due to the potential harm they may cause to health, vegetation and infrastructure, but also because the emissions from a volcano can tell you something about what is happening inside.  Gases which were dissolved in the magma held inside the volcano can be released quietly into the atmosphere in a process known as volcanic degassing, and when a volcano erupts large quantities of these gases can be released in a short time. The gases include carbon dioxide (CO₂), which is denser than air and if trapped in low-lying areas can be lethal to people and animals, and sulphur dioxide (SO₂) which is irritating to eyes, skin and the respiratory system. In major explosive volcanic eruptions SO₂ can be released in very large quantities and injected into the stratosphere – and this can lead to significant temporarily climate cooling due to the production of sulphate aerosols which reflect incoming sunlight back into space. We can probe volcanic emissions, emitted either passively or during eruptions, using ground-base, airborne or spaceborne remote sensing.  

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The above example shows a relatively recent volcanic eruption which occurred at Kīlauea volcano in Hawaii in May 2018. Hawaii is characterised by mostly non-explosive volcanic activity, and this meant that large amounts of sulphur dioxide were released relatively low to the ground, which inturn generated surface-level volcanic smog (VOG) that caused persistent health problems for downwind populations. The image shows the transport of Kīlaueas SO₂ plume as deduced from the Copernicus Sentinel-5P satellite. These data are overlain with ‘thermal hotspots’ in red derived from data provided by the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument carried by the NASA/NOAA SUOMI NPP satellite. Some of these hotspots are associated with high temperature areas of the volcano, whereas others may be related to vegetation fires ignited by the volcanic activity.

As with landscape fires, the long-range transport of the chemicals in volcanic plumes can have an impact on areas quite distant from the volcanic source, as well as local and regional populations and infrastructures. For explosive volcanic activity in particular, aircraft traffic might be affected by volcanic ash lofted high into the atmosphere and transported long distances. Ash can interfere with aircraft sensors, can affect the visibility through the windshield, and can affect the functioning of the engines, causing stalling or even engine failure. Acid rain may also fall far from a volcano as a result of the emitted sulphur dioxide reacting with other atmospheric constituents.

Another example of satellites monitoring recent volcanic eruptions is when Sentinel-5P and Sentinel-3 both captured the eruption of the Raikoke Volcano in the Kuril Island chain, near the Kamchatka Peninsula in Russia. Whilst ash from the eruption was detected by Sentinel-3, on 21^st June 2019 Sentinel-5P detected the sulphur dioxide coming from the Raikoke eruption. You can see an animation of the sulphur dioxide plume from this eruption below in the Featured imagery, and this data was measured by the TROPOMI instrument carried by Sentinel-5P.  The SO₂ maps reveal the fine details of volcanic plume dispersal in the atmosphere.

Satellites like Sentinel-3 and 5P are crucial for tracking the movement and extent of volcanic clouds and to cross-check predictions from numerical models of the spread of the ash and SO₂. Other satellites that can be used to track the movement of volcanic ash include the geostationary Meteosat satellites, which can monitor ash cloud movement with a very high update frequency of 15-mins. Such an ability plays an important role in tracking the presence and dispersion of ash in European air space in near real-time. Other geostationary satellites perform a similar role over other areas of the planet. The European Metop satellites, carrying a suite of instruments such as IASI, GOME-2 and AVHRR, are able to collect more spatially detailed data about volcanic clouds than can Meteosat, including mapping ash, sulphur dioxide, and ice content, but with less frequency than Meteosat as they only pass over the same area roughly twice a day.