EP TOMS saw the ash cloud from a large eruption of the Soufriere Hills volcano in Montserrat that occurred early on the morning of 26-Dec-1997. An SO2 cloud was also produced. The eruption began between 3 and 4 AM local time. TOMS passed overhead at 11:20 AM. Both the ash and SO2 clouds are shown in the image below, with the ash cloud represented by colors and the SO2 cloud represented by contours (each contour of SO2 represents 5 DU). The location of Montserrat is indicated by a cross.

26-Dec-1997
(This image has been smoothed)

Here's a GOES image of the ash cloud (pulled off of Dennis Chester's excellent GOES web site) taken about 25 minutes after the TOMS overpass (sorry, no continental outlines or grid lines to help give the scale or orientation):

GOES image of eruption

It's interesting to look at what effect the presence of ash has on the ozone amount determined using the operational algorithm. From studies currently being conducted, we know that the algorithm underestimates ozone in the presence of UV-absorbing aerosols and volcanic ash.

The figure above compares the aerosol index and ozone maps, and one does see that the algorithm slightly underestimates ozone in the presence of volcanic ash. A large enhancement in the calculated ozone value is also seen in this image. This enhancement is due to the presence of SO2 in the atmosphere, which causes the operational algorithm to overestimate the amount of ozone.

This is illustrated in the image above, which compares the SOI (which is a measure of the amount of SO2 in the atmosphere) and ozone maps. We see the enhancement in ozone correlates well with the SOI. (The area with no data is due to the fact that we still don't quite have full global coverage with Earth Probe TOMS.)

For more information on the detection of volcanic ash and SO2 using TOMS, see The TOMS Volcanic SO2 and Ash Home Page.

Back to TOMS Aerosol page