IASI

COMET has developed methods for measuring SO2 and ash release from active volcanoes using data from the Infrared Atmospheric Sounding Interferometer (IASI).

IASI observations. Credit: ESA

IASI is probably the most advanced instrument carried on the MetOp satellite.  It is providing a wealth of data to further our understanding of atmospheric processes and the interactions between atmospheric chemistry and climate.

The sophisticated IASI instrument is a Fourier Transform Spectrometer based on a Michelson Interferometer coupled to an integrated imaging system that observes and measures infrared radiation emitted from the Earth.

Infrared radiation directed into the IASI instrument. Credit: ESA

The optical interferometry process samples the atmosphere in the infrared band between wavelengths of 3.4 and 15.5 microns. This enables the instrument to establish temperature and water vapour profiles in the troposphere and the lower stratosphere, as well as measuring other compounds including SO2 and volcanic ash.

Data from the whole of the IASI instruments’ lifetimes (2007 to present) have been analysed using algorithms developed by the Earth Observation Data Group at the University of Oxford.

The web page displays volcanic SO2 plumes and has also now been updated with NRT ash flag computation. IASI data are analysed assuming different altitudes for ash (400, 600 and 800 mb) and vertical distribution for SO2 (between 0 and 20 km: 1-2 km, 4-5 km, 9-10 km, 14-15 km, 19-20 km).  All analysed data are available for download, and the data format is netcdf to allow maximum portability.

The data are available in near real time, within ~ 3 hours of the measurement being made by the instrument.

[read more=”Read more on volcanic ash” less=”Read Less”] 

The composition, grain-size and morphology of volcanic ash particles contains important information about the processes of magma ascent and fragmentation in volcanic eruptions. The size and shape of the ash is also critical in influencing its transport through the atmosphere.

While recent ash-rich volcanic eruptions in Iceland (e.g. Eyjafjallajökull, 2010) and Chile (e.g. Chaiten, 2008; Puyehue- Cordon Caulle, 2011) have provided considerable new insights into the details of ash formation, transport and deposition, they have also highlighted the need for new work to improve the quantification of the properties of ash in volcanic plumes from remote-sensing observations.

The SHIVA project is developing a reference database of ash optical properties that will form the basis of work to estimate ash properties from high resolution infrared spectra.

IASI data will be a key part of this, with its emission spectra measurements used to estimate ash composition, altitude, optical depth and effective radius.

The figure below shows the IASI Near Real Time website display image on 28 March 2017, with the plume from the Kambalny eruption in the southern part of the Kamchatka Peninsula, Russia.

Kambalny plume as shown on the webpage on 28th March 2017. Note the button ‘Download source data’ on the bottom left, where it is possible to download the data for SO2 and ash with different vertical profile assumptions

 

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[read more=”Read more on SO2” less=”Read Less”] 

Monitoring SO2 emissions offers insight into volcanic behaviour and is important for understanding the impacts of volcanism on the environment and climate. Despite this, many volcanoes have little or no ground based monitoring, and so satellite remote sensing is valuable for detecting and quantifying emissions.

Ultraviolet satellite based instruments are most commonly used for this purpose; however, there are advantages to using infrared sensors, such as they can monitor during the night and in some cases can simultaneously retrieve the concentration and altitude of the plume.

A number of SO2 retrieval techniques have been developed for IASI, two of which have shown some promise at monitoring low level volcanic emissions.

Here, a ‘fast’ linear retrieval is applied across the globe to detect sources of SO2. These results are dominated by emissions from explosive eruptions, but signals are also evident from smaller eruptions and passive degassing, and from anthropogenic activity.

The figure below shows an example of the elevated levels of SO2 that were frequently identified at volcanoes in Ecuador and Kamchatka, Russia, and so these areas were selected for further study with a ‘full’ iterative retrieval which is capable of quantifying the amount of gas emitted.

Linear retrieval output. (a) Yearly average over Central and South America in 2008. (b) 2008 monthly averages over Ecuador displayed with a colour bar ranging between one standard deviation below the mean to three standard deviations above the mean. (c) Yearly average over Kamchatka in 2010. Note the elevated background signal over the region. This is possibly linked to pollution over China being blown north-east with the prevailing wind. (d) As (b) for Kamchatka in 2010

In both regions, the iterative retrieval captured changing activity which matched reports from the Global Volcanism Program. At Tungurahua, a comparison between the IASI SO2 amount and the quantity of SO2 detected with the Ozone Monitoring Instrument, and ground based flux measurements was possible.

The results were promising with each instrument showing the same trends. These results demonstrate for the first time that IASI can be used to monitor smaller emissions of volcanic SO2 into the troposphere, which if used alongside other datasets could be a valuable contribution to volcanic monitoring.

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