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Sulphur dioxide (SO2) enters the atmosphere as a results of both natural phenomena and anthropogenic activities, such as fossil fuel combustion, oxidation of organic materials in soils, volcanic eruptions and biomass burning. SO2 contributes to acid rain and it is a key precursor for sulphuric acid aerosol formation. At high concentration, it also adversely affects human health, in particular in combination with fog (smog).

Changes in the abundance of SO2 have an impact on atmospheric chemistry and hence on air quality and on climate. Effects of volcanic eruptions may have an impact on air traffic, as such eruptions are important sources of ash (aerosols) and SO2. Consequently, global observations of SO2 are important for atmospheric and climate research, and for air traffic organisations. Global monitoring of SO2 concentrations is done on the basis of UV-Visible and Infra-Red measurements by satellite based nadir-viewing instruments.

Volcanic emissions and strong pollution events are clearly detected with this approach, which makes it very suitable for use in a Near-Real Time (NRT) service, with an automatic Notification Service for exceptional SO2 concentrations. In addition to that an off-line (reprocessed) archive of data is useful for validation of the Service and the data, and for case studies of past events.

The SO2 Services described here was closely linked to the so-called Support to Aviation Control Service (SACS) of the PROMOTE project. Apart from the SO2 data, SACS also intends to deliver a volcanic ash indicator (VAI) as well as backward and forward trajectories from the location of an SO2 peak value in case of a SO2 notification. The trajectories are meant to indicate the possible origin and future motion of the SO2, and the VAI is meant to provide additional information on possible volcanic eruptions.

The on-line product information

These web pages provide background information on the various aspects of the Volcanic & Air Quality SO2 Service set up to monitor SO2 emissions in NRT and off-line reprocessing, based on UV measurements made by several instruments based on different satellites.

Currently in use are SCIAMACHY (aboard ENVISAT), OMI (aboard EOS-Aura), GOME-2 and IASI (aboard MetOp-1)

The Services are set up using data from SCIAMACHY and so the product information is written from that starting point. Most information is, however, valid for the data of all instruments used -- if this is not the case, it is mentioned explicitly (specially for IASI which is an IR instruments).  

SO2 data products

The technique used to retrieve the SO2 slant column density from GOME-1, SCIAMACHY and GOME-2 data is the Differential Optical Absorption Spectroscopy (DOAS). From the slant column, the vertical or total column can be derived in two ways. One approach is to use an air-mass factor based on realistic SO2 profiles. The other approach is to assimilate the slant column data in the chemistry-transport model TM; this approach is currently not in use. SO2 data based on OMI is derived in a somewhat different way: with a "band residual method" using the residuals of the DOAS-based ozone retrieval.

The near-real time service provides maps of daily orbit data files only, as well as the orbit data files themselves (except for OMI, for which only maps are presented).  

Other data products

The volcanic ash indicator is based on observations of the SEVIRI instrument (aboard the geostationary MSG) and provides data only for the volcanoes in the field of view of SEVIRI (Europe and Africa). A description of this data product can be found ... to follow

The foreward and backward trajectories will be added and described in due time.

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