Volcanoes have terrified and fascinated us equally since the dawn of time. The destruction volcanic eruptions can cause is only matched by the beauty of watching one of the most powerful forces of nature shaping the planet. In terms of impacts on the atmosphere and air quality, volcanoes emit some of the most toxic gases and the largest eruptions can become a game changer for atmospheric chemistry and influence on the climate.
In the background, passive volcanic gas emissions modify atmospheric composition all year round. The Copernicus Atmosphere Monitoring Service (CAMS*) provides data about volcanic sulphur dioxide concentrations and sulphate aerosol particles, regularly monitoring significant eruptions.
Sulphur dioxide (SO2) emissions are sometimes an early indicator of volcanic activity preceding an eruption, and a clear signal of the strength of an eruption once volcanoes start spewing ash, lava and gases. The plumes of SO2 emitted during eruptions can be transported thousands of kilometers around the globe and very high into the atmosphere, sometimes even to the mesosphere, the layer above the stratosphere, at altitudes higher than 50 km. The oxidation of SO2, resulting in the formation of sulphate aerosol is a key player in atmospheric chemistry and climate regulation. Understanding concentration levels and the direction of the transport is essential during volcano crises in order to plan rescue operations and evacuations, as some of the gases are lethal at high concentrations.
Ash monitoring is also key during volcano crises, as volcanic ash can have a high and long-range damage potential for health, property and for civil aviation.
After the immediate emergency management, the atmospheric analysis and reanalysis data are a key asset for scientists studying the effects of volcanic emissions in our atmosphere.
CAMS has unique expertise in tracking atmospheric pollutants through modelling, forecasting and providing reanalysis at the European and global scales, based on ECMWF’s Earth system modelling approach, the Integrated Forecasting System (IFS). CAMS is able to track SO2 and sulphate aerosol particles from volcanoes based on in-situ and satellite measurements providing its global forecasts through the IFS.
Furthermore, CAMS data are available with completely open access and can be used by anyone, from non-expert users via simple tools like charts, to the most advanced users with the open Atmosphere Data Store (ADS).
Two systems with one objective: providing authoritative atmospheric data
CAMS runs two separate systems. The regional air quality system is composed of 11 air quality models from around Europe using a multi-model ensemble approach. It provides a forecast for the coming four days, initialised daily, and assimilates in-situ data for measurement stations on the ground.
The global atmospheric composition system doesn’t have such a fine level of detail as the regional (some 40 kilometers vs. 10 km for the regional ensemble). It is used to initialise 5-day forecasts twice a day based on satellite observations of atmospheric composition including aerosols, ozone, nitrogen dioxide, carbon monoxide and sulphur dioxide and the possible evolution of the atmosphere according to ECMWF’s Earth system model, the IFS.
How does CAMS monitor volcano eruptions?
Besides lava, rock and ashes, volcanoes spew mainly water vapour, carbon dioxide and sulphur dioxide (SO2). These three species count for the vast majority of emissions, the rest being a collection of other gaseous species, depending on the geology and magma type of each volcano.
CAMS focuses on SO2 and aerosols (including ash and sulphate) when it comes to monitoring volcanic activity. SO2 is not exclusive to volcano eruptions, other sources include human fossil fuel combustion and vegetation fires, but volcanic eruptions generate higher SO2 concentrations. Sulphur dioxide is a distinctive gas indicating magmatic activity and, as such, it is a good early sign that magma is getting close to the surface, and is an indicator of volcanic activity that we can monitor from space prior to eruptions. SO2 is also sometimes a proxy when it comes to detecting volcanic plumes, which is vital for aviation safety, as volcanic ash can damage aircraft engines and airframes.
CAMS assimilates total column SO2 observations available from a number of satellite instruments measuring in the ultraviolet and visible parts of the electromagnetic spectrum.
These are the Global Ozone Monitoring Experiment (GOME-2), a spectrometer instrument on the European MetOp satellites, and Tropomi, another spectrometer on the Copernicus Sentinel-5 Precursor (Sentinel-5P) satellite.
Spectrometers are optical instruments that measure the light reflected by the Earth’s surface, dispersed by the molecules of different constituents in the atmosphere.
These devices measure changes in the scattered light at different wavelengths dissipated following interaction with the molecules of different gases, in particular ozone but also NO2 and SO2. Applying our understanding of how different gases interact with light at these wavelengths allows us to estimate their relative abundance from the surface to the top of the atmosphere, known as the total column.
Bringing satellite data to the earthly life
Total column observations provided by satellites don’t include accurate data about the height of volcanic plumes. This information is essential to predict the direction of a plume transport. When the initial altitude data is not accurate, the model can represent transport in the wrong direction, as wind directions vary at different altitudes. Additionally, in a system running operationally and updated twice a day, such as in CAMS, it is not easy to correct input data on the fly.
So, in addition to satellite detections there are several assumptions and mathematical calculations to be made in order to establish concentrations of different gases at different altitudes.
Generally, CAMS uses an assumption of volcanic SO2 injection into an atmospheric layer centered on 550 hectopascals, at approximately 5 kilometers altitude, which is the mid-point of the troposphere and represents an average injection point.
CAMS relies on a number of partners providing satellite observations and data.
EUMETSAT’s Atmospheric Composition Satellite Application Facility (AC-SAF) provides data for several different trace gases, including SO2 from Tropomi and GOME-2. SO2 is derived using retrievals developed by the German Aerospace Centre (DLR) and used in the global NRT CAMS system. Funded by the European Space Agency, the Sentinel-5p+ Innovation SO2 Layer Height project (SO2LH) developed TROPOMI SO2 data that provides additional information about the altitude of the volcanic plumes, making use of DLR’s Full-Physics Inverse Learning Machine (FP-ILM) algorithm. The use of these data is also explored by CAMS.
The information is completed by a dataset of global volcanic outgassing based on satellite and ground-based observations from the Network for Observation of Volcanic and Atmospheric Change (NOVAC) and covering 32 major global volcanoes. Outgassing or passive gas emissions, the volcanic activity that volcanoes can have between eruptions, is an important contributor to the global volcanic SO2 budget. The dataset was created for CAMS by Sweden’s Chalmers University.
“The process of putting together this dataset took several years and involved work from different institutions,” says Santiago Arellano, leading the project at Chalmers University. Initially the teams retrieved more than four decades of SO2 emission data from missions designed to measure ozone and air quality. “Although not designed specifically for this purpose, these missions have been able to provide estimates of the mass, altitude and dispersion of plumes from passive and eruptive volcanic emissions, exploiting the strong absorption bands of SO2 in the UV and the relatively low background concentration of this species in the atmosphere. Explosive eruptions are easier to observe from space because they tend to inject large amounts of gas high up into the atmosphere,” Arellano adds.
The Head of Unit at Geoscience and Remote Sensing from Chalmers also explains the importance of taking outgassing into account when monitoring volcano activity. First, it is an indicator of the different dynamical states of volcanoes. Passive volcano gas emissions have impacts over long distances and their global contribution is by far larger than the gases emitted by erupting volcanoes, so it is essential for models to take these lower intensity but higher volume emissions into account. On the other hand, emissions of explosive volcanoes have more potential impacts on the atmosphere as they can reach higher altitudes and longer distances.
At CAMS, the outgassing dataset and satellite observations provide an analysis, representing the best knowledge of the state of the atmosphere at that moment, used as the start point for running 5-day forecasts twice a day based on ECMWF’s IFS model.
“We have come a long way since we put the first version of the model in place seven years ago,” says Antje Inness, ECMWF Senior Scientist for CAMS, “but the efforts to progress with the challenge of the SO2 plume altitude using a combination of techniques have been proven successful and encouraging for all the team involved,”she adds.
For a model combining satellite observations and a system such as CAMS, extreme events like volcano eruptions pose a challenge when the data is assimilated into the model, because of its unpredictable and sporadic nature.
Taking a relatively recent example, the Mauna Loa volcano in November 2022: on the day of the eruption the global model doesn’t have any volcanic SO2 data as there is no emissions information ingested in the system yet. It’s business as usual, and only after the first satellite SO2 products are assimilated in the IFS, the model starts to include SO2 concentration estimates, and the first forecasts are produced.
The SO2 transport from the Hawaiian volcano reached the Caribbean via Mexico and the southern United States.
“Volcano eruptions always raise a large interest from the public and the media, and rightly so, this is why we put special attention in producing the best quality predictions and analyses and disseminating the information as much as possible when major volcanic eruptions occur,”
says Mark Parrington, Senior Scientist for CAMS at ECMWF and the main spokesperson of the service to the media.
The long-range effects of volcanic eruptions
Most often, the most hazardous gas concentration levels remain close to the eruption site. These gases are mainly monitored on the ground, in general under the responsibility of civil protection, local or national authorities.
Tracking the ash plume is essential for aviation safety, and tracking SO2 is important to understand air composition and atmospheric chemistry, rather than for air quality stricto-sensu.
It is rare for a long-range SO2 transport to trigger an air quality emergency hundreds or thousands of kilometers away from the eruption site. But SO2 transforms into sulphate aerosol, and it can persist in the atmosphere with implications for atmospheric chemistry and therefore with the potential to play a role in climate and weather conditions.
In the upper layers of the troposphere and in the stratosphere, aerosols have a cooling potential by blocking solar radiation but only a handful of volcano eruptions have been proven to impact global climate in recent history.
Sulphate aerosols in the stratosphere may also play a role in ozone depletion.
When large eruptions inject gases into the stratosphere, the impacts last longer, as there is no rainfall allowing deposition
, and chemical processes are much slower at colder temperatures. Chemical processes are also less understood than in the troposphere.
Despite some recurrent and misleading reports predicting immediate effects of volcanic eruptions on the climate, most volcano eruptions have no major direct consequences on the global climate. Only massive eruptions whose emissions reach the stratosphere, such as the eruption of Mount Pinatubo in 1991, clearly influence the climate system, and yet the attribution of a particular volcano eruption to specific climate effects is a question for research, and not the easiest.
The eruption of the Hunga Tonga – Hunga Ha’apai volcano on 15 January 2022 injected large amounts of water vapour into the stratosphere and beyond. This has since been confirmed by several studies, using different measurement techniques. But the research to determine the precise consequences of the episode in warming the global temperature or weakening the ozone layer is still ongoing.
In summary, volcanoes naturally play a role in the chemical imbalance of the Earth’s atmosphere and do modify the climate, but this should not make us jump to hasty conclusions whenever a large eruption takes place, as cause-to-effect relations are often more complex than it seems within the Earth system.
Testing new SO2 detection approaches
The Raikoke volcano, in Russia’s far East Kuril Islands, woke up on 22 June 2019 sending a thick plume of ash and gases to an altitude of between 13 and 17 kilometers, already in the stratosphere. SO2 plumes covered a large area of the Northern Hemisphere and lasted for a long period of time, making it a perfect case study.
The eruption was a great opportunity for CAMS scientists to test the added value of the new methods to make use of the plume’s altitude. A paper published in 2022 and coordinated by ECMWF’s CAMS Senior Scientist Antje Inness, showed the advantages of using SO2 layer height information from TROPOMI, produced by ESA’s S5p+ S 2 Layer Height Innovation project and based on DLR’s Full-Physics Inverse Learning Machine (FP-ILM) algorithm. Not only did using the additional layer height information improve the vertical location of the SO2 plume in the CAMS system, but it also helped to improve the CAMS model’s SO2 forecast quality.
Cumbre Vieja, a rare opportunity for such an eruption in Europe
The Cumbre Vieja volcano in the Spanish Canary Islands erupted on 19 September 2021 for the first time since 1971. At that time the CAMS SO2 assimilation was more settled. This was a great opportunity to evaluate the model’s performance compared to observations.
Between 19 and 29 September a large SO2 plume travelled across Northern Africa and Europe reaching Scandinavia.
Sulphate aerosol, resulting from the SO2 oxidation, typically starts to appear 2 or 3 days after the observed SO2 from the eruption was first assimilated in the CAMS model.
As mentioned above, in the stratosphere, sulphate aerosol, mostly in the form of sulfuric acid, has the potential to influence the solar radiation budget by blocking solar radiation, and can also trigger ozone depletion. In the troposphere, sulphate aerosol can affect the chemical properties of clouds acting as condensation nuclei and is a main cause of acid rain.
Due to this complex role in atmospheric chemistry, the recent geoengineering proposals to voluntarily inject sulphate aerosols in the stratosphere to counteract global warming, are seen as a dangerous project with many possible unwanted consequences by a large part of the atmospheric science community.
During the Cumbre Vieja episode, the sulphate aerosol and SO2 plumes that were transported across the Atlantic mixed to some degree with concurrent transport of mineral dust from the Sahara, reaching as far as the Caribbean several times during the eruption. There were reports of reduced visibility and air quality in the Caribbean as late as 10 October. Observations from in-situ measurement stations in Puerto Rico confirmed the good performances of the CAMS model for predicting and analysing the transport of the SO2 plume and the sulphate aerosol transport during the episode.
Hunga Tonga – Hunga Ha’apai, a massive SO2 transport across the Indian Ocean
The eruption of the Hunga Tonga – Hunga Ha’apai submarine volcano mentioned above was captured by the satellites creating an unusually strong SO2 and sulphate aerosol signal in the CAMS model over the Pacific. The plume travelled westwards across the Indian Ocean, reaching La Reunion and Madagascar around 20 January, six days after the initial eruption.
The ash column was reported to reach an altitude of 30 km. Volcanic materials were detected at nearly 60 km, making it the highest volcanic plume ever recorded. The Hunga Tonga-Hunga Ha’apai eruption is widely considered to be the strongest volcanic eruption recorded since the eruption of Pinatubo in the Philippines, in 1991.
Europe’s “local volcanoes”
As a European service, CAMS models regularly reflect the frequent eruptions of our most active “local volcano”, Mount Etna in Sicily. One of the last that we monitored closely took place in February 2022, with important SO2 transport recorded for several days and reaching Iran, Iraq and the Arabian Peninsula.
Other neighbouring volcanoes that wake up regularly are Stromboli, and the Icelandic ones. The Fagradalsfjall volcano complex kept everyone on tenterhooks from December 2019, when a series of earthquakes shook the region. The eruption took finally place in March 2021, fortunately without major consequences, beyond a flock of tourists and curious onlookers from around the world.
In May 2021, the CAMS model still recorded a small SO2 plume signal, based on the Sentinel-5P satellite observations.
How CAMS monitors its own data quality
CAMS data is constantly evaluated and monitored independently to ensure its quality. The service provides Evaluation and Quality Assurance (EQA) quarterly reports with statistics and infographics comparing CAMS data against independent observations. Some 65 independent datasets are used to monitor the quality of the global forecast. A validation server gathering the information as it arrives is available to everyone.
The accuracy of the predictions and analyses vary due to a large number of factors. The aerosol tracking during the Cumbre Vieja volcano eruption showed close agreement with the Aeronet observations, both in Europe and the Caribbean.
Atmosphere Monitoring for everyone, free to access
All data produced by CAMS is made available for free and for any purpose. The main entry point to retrieve the CAMS products is the Atmosphere Data Store, with Near Real Time data as well as the analysis and reanalysis products for greenhouse emissions and trace gases.
CAMS data support a large number of services and applications across the world and the data is available for anyone to create new applications, for commercial or research purposes.
For more information on how CAMS air quality data is used, check out the many use cases on the CAMS air quality webpage.
*The Copernicus Atmosphere Monitoring Service (CAMS) is implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF) with EU funding on behalf of the European Commission. Copernicus is the earth observation component of the EU space programme.
Article coordinated by Mark Parrington, holds a PhD in Atmospheric Physics from the University of Oxford and has more than 20 years’ experience in using satellite observations and modelling for atmospheric composition. He is currently Senior Scientist at the Copernicus Atmosphere Monitoring Service (CAMS). The service is implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF) with EU funding on behalf of the European Commission. Copernicus is the earth observation component of the EU space programme.