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Introduction
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.
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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