Satellite measurements of sulphur dioxide (SO2) concentrations are used to set up an automated service which issues notifications by e-mail in case of exceptional SO2 concentrations. Such “SO2 events” could signify volcanic activity, as SO2 is one of the major trace gases released during volcanic eruptions. Most eruptions also release ash and in that case SO2 can serve as a marker for the presence of volcanic ash. Volcanic ash, when transported high up in the troposphere, poses a great hazard to aviation: aircraft flying through a volcanic ash cloud may suffer major damage, including engine failure. The SO2 alert service – named the Support to Aviation Control Service (SACS) – thus provides information to the Volcanic Ash Advisory Centres (VAACs), whose official task is to gather information regarding volcanic ash clouds and to assess the possible hazard to aviation. Other users of SACS are volcanological observatories, health care organisations, scientists, etc. Currently SACS uses measurements of the SCIAMACHY, GOME-2 and OMI instruments for SO2 data. This will be extended with an Absorbing Aerosol Index (AAI) from these instruments, as well as SO2 data and an aerosol flag from the IASI instrument. Maps, data and product information can be found at the SACS website: http://sacs.aeronomie.be/