Image via WikipediaMany of the plume related risk management decisions related to the Eyjafjallajökull volcano have been based upon observation and analysis of Aqua and Terra satellite images using MODIS (the Moderate Resolution Imaging Spectroradiometer) and AIRS ( Atmospheric Infrared Sounder) . MODIS captures daily visible and infrared earth imagery and provides daily images of the volcanic plume. AIRS and MODIS contain sulfur dioxide (SO2) absorption channels that enhance volcanic ash detection. ( See http://www.nasa.gov/topics/earth/features/iceland-volcano-plume.html ).
On a related note, here's an interesting link to a volcanic downwind eruption cloud model: http://ready.arl.noaa.gov/hysplitash-bin/dispsrc.pl