Retrieval Strategies

Many trace gas columns are retrieved from SCIAMACHY measurements using the Differential Optical Absorption Spectroscopy (DOAS) method. These retrievals are based on Lambert Beer's law applied to an earthshine measurement using the extraterrestrial solar irradiance as a background spectrum.

Broad band features for example from scattering are filtered out by
subtracting a low order polynomial. From the differential structures the
total absorber amount integrated over the effective light path (slant
column) is determined by least-squares fitting of the measured optical depth
to a linear combination of reference absorption cross-section spectra.

To derive the desired vertical trace gas column, the slant column has to be
divided by an appropriate dimensionless air mass factor (AMF) describing the
enhancement of the absorption due to the slant path viewing geometry and the
solar elevation. This factor is obtained by radiative transfer model (RTM)
simulations. Species which can be retrieved by DOAS are for example O3, NO2,
BrO, OClO, SO2, and HCHO.

The FURM algorithm consists of two parts, a forward model, based on a radiation transfer model (RTM) GOMETRAN (Rozanov, 1997), which calculates for the relevant geometry the radiance for a given state of the atmosphere and an inversion scheme which matches in iterative steps the calculated radiance to the measured radiance by modifying model atmospheric parameters such as the vertical distribution of ozone using appropriate weighting functions as provided by the RTM.