Reference spectra -- SCIAMACHY data
The slant column retrieval algorithm requires a reference spectrum without
the presence of absorption features of the trace gas to be retrieved, since
the DOAS method is based on the difference in absorption between two
spectra. The reference spectrum can in principle be a solar spectrum, but
since the solar spectra from SCIAMACHY showed problems in the past, an
earthshine spectrum is used as reference spectrum.
For the SO2 slant column retrieval a reference spectrum without any
SO2 absorption must therefore be selected. A good geographic region to do
this is around the equator above the Pacific or Indian ocean, as there are
no sources of SO2 located there. As instrument characteristics may vary over
time, it is necessary to regularly update the reference spectrum with time.
For the retrieval of SACS, a reference spectrum is selected in principle once every month at around the
middle of the month, depending on availability of the data, from a measurement south-west of the southern tip of India, at about 65 degrees East and just south of the equator.
Since the reference spectrum changes every month, the
correction for the offset in the SO2 background
level is determined for each month separately.
In the off-line processing of a given month for the Archive Services, the
reference spectrum and the offset correction are determined from
the data of that month.
Near-real time services
For the near-real time SO2 processing, selecting a reference spectrum from
the current month is not possible, as determining the background offset
correction requires all data of a full month. For that reason the NRT
service uses the reference spectrum from the most recent month for which the
offset has been determined.
At some point in time, when all data of the month is available, the month is
reprocessed for the Archive Service, using an appropriate reference spectrum
and background offset correction.