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Eruption of the Grímsvötn volcano on Iceland November 2004

[Image: Matthew J. Roberts]

NOTE:   see the main page on this event for some important remarks regarding the SO2 processing and the maps presented on these pages.

 
Late in the evening of Monday 1 November 2004 the Grímsvötn volcano on Iceland erupted spectacularly. The volcano, which is located under the Vatnajökull ice cap, ejected masses of ash and gases high into the atmosphere, up to 12 km. This forced some airlines to divert their flights, in order to avoid flying through the ash cloud. Ash from the eruption has landed in Norway, Sweden and Finland, the BBC reported.

Like many volcano eruptions, the eruption of Grímsvötn emitted not just ash and water vapour into the atmosphere, but also other gases. One of these gases is sulphur dioxide (SO2). Sulphur dioxide reacts on the surface of a variety of airborne solid particles (aerosols), is soluble in water and can be oxidised within airborne water droplets, producing sulphuric acid. This acidic pollution can be transported by wind over many hundreds of kilometres, and is deposited as acid rain.

The SCIAMACHY instrument, aboard ENVISAT (Environmental Satellite) of the European Space Agency (ESA), measured SO2 concentrations well above the normal background level when the satellite passed over Iceland around 12:14 UTC on 2 November, as the following figure shows.

SO2 slant column density measured by SCIAMACHY on 2 November 2004 when ENVISAT passed over Iceland around 12:16 UTC. The two triangles show the location of the volcanoes Grímsvötn (right) and Hekla (left); the latter erupted most recently in 2000. Only measurements made at Solar Zenith Angles below 85 degrees are shown. The track is not continuous due to the alternating modes SCIAMACHY measures in. The times plotted mark the begin and end of a nadir state in UTC.

Just north of Grímsvötn there is some SO2 visible. Clearly, there is also a large area with SO2 Northeast of Iceland, measured around 12:14 UTC. Measurements at high Solar Zenith Angle (SZA), above 75 degrees, usually have to be treated with care, as the retrieval of data from these measurements is rather difficult. But the fact the the SO2 shows up in a group of ground pixels close together indicates that the SO2 signal is real and a result of the volcano eruption.

This is confirmed by the motion of the air above Grímsvötn due to the winds in the atmosphere, as illustrated in the following figure: from the volcano the air moves first to the Northeast, then bends to Norway and further Southeast.
 

Trajectories of passive tracers in the air starting above the Grímsvötn volcano. The tracers are released at different altitudes (pressure levels) at 12:00 UTC on 2 November 2004. The tracers then move due to the wind fields in the atmosphere, based on data from the ECMWF (European Centre for Medium-range Weather Forecast, Reading, UK). Every 12 hours a >-mark is placed at the trajectory, which shows that air at different altitudes moves at different velocities. For most altitudes, the air from above the volcano crosses the Finland/Russia border within about 24 hours. [Image: Rinus Scheele, KNMI.]

Can the motion of the air from above the volcano across Europe, as depicted in the above figure, be seen in the SO2 data? This question is addressed on a separate page with a time series of the SO2 data. The interpretation given there is preliminary, as the data analysis is still under development.

The following figure shows the SO2 concentration one day earlier. Northeast of Iceland there is are some ground pixels clear SO2 concentration well above the background level, but rather scattered. The eruption of Grímsvötn, however, took place several hours after this measurement, so this SO2 cannot be emitted by the eruption. During the day of 1 November there were several series of earthquakes, which eventually would lead to the eruption (see the Web page on the eruption at the Icelandic Meteorological Office). It is possible that during these earthquakes SO2 was emitted into the air.

SO2 slant column density measured by SCIAMACHY on 1 November 2004 when ENVISAT passed over Iceland around 12:47 UTC.

On the days before 1 November and after 2 November there is (almost) no SO2 visible near Iceland in the SCIAMACHY results.

Related links

BBC Online article about the eruption

The London Volcanic Ash Advisory Centre is responsible for
monitoring and forecasting the movement of volcanic ash over
the United Kingdom, Iceland and the north-eastern part of the
North Atlantic Ocean.

The London VAAC / UK Met Office had a press release on
04 Nov. 2004, which was available at their web site, but
press releases are not kept for forever, unfortunately.

MODIS images of the ash cloud  

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NOTE:   The images at this page were made in January 2008, after a new background correction was implemented (to decrease the large number of artifacts at high SZA) and the captions have been adapted accordingly. With this new background correction, the motion of the SO2 across Europe seems to be more clearly visible. For reference the old page is still available.

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