The Earth Probe Total Ozone Mapping Spectrometer (TOMS) detected a sulfur
dioxide cloud associated with the January 2002 eruption of Nyiragongo
as it flew over the region at around 11 a.m. local time (0900 UTC) on January
17. The sensor detected no significant amounts of ash in the eruption
cloud. At the time of the TOMS overpass the cloud extended up to roughly 200 km
(124 miles) northwest of Nyiragongo and was still attached to the volcano. This
observation is consistent with nearly coincident MODIS imagery
which shows an opaque cloud of gas and steam in the same location. The TOMS
measurements show that the amount of sulfur dioxide in the Nyiragongos plume
range from about 10 to 30 kilotons.
Please note that TOMS mass retrievals are dependent on the altitude of the cloud
and may be adjusted as more information becomes available. Since the cloud may
still have been developing at the time of the TOMS overpass, the final sulfur
dioxide burden may have been greater. Wind trajectory data (courtesy of Leslie
Lait, SSAI) suggest that part of the cloud may have reached at least mid- to
upper-tropospheric altitudes of up to 12 km (7 miles), but scientists suspect no
significant stratospheric injection of sulfur dioxide as a result of this eruption
since the gas was not visible over the Democratic Republic of the Congo region
in subsequent TOMS data acquired on January 18.
Production of sulfur dioxide without a significant ash cloud is commonly
observed during effusive eruptions such as the Nyiragongo event. Although dense
low-level ash may be produced during such eruptions, these particulates usually
fall out fairly quickly and elude detection by satellite. The size of the January 17
Nyiragongo cloud and the estimated sulfur dioxide tonnage are fairly modest, and at
least an order of magnitude smaller than values typically measured by TOMS
during eruptions of nearby Nyamuragira during its frequent outbursts (e.g.,
on February 6, 2001). Sulfur dioxide column amounts (measured in Dobson Units) are much higher in the more extensive
Nyamuragira cloud, which contained roughly 420 kilotons of sulfur dioxide. Although
several factors could affect the size of the observed cloud in each casesuch
as the delay between the onset of the eruption and the TOMS overpass, and the
volume of lava emitted and the lava compositionthe TOMS data suggest that
the Nyiragongo magma may have been largely degassed before eruption. One possible
mechanism by which this could be achieved is the cyclic degassing of magma in the
subaerial lava lakes that have been intermittently present in Nyiragongos summit
crater over the past few decades.