Absorbing gas corrections
After radiometric calibration, the top of the atmosphere radiances are corrected for minor gas absorption:
Ozone affects the shorter wavelength channel of the POLDER radiometer. The radiances are divided by an a-priori ozone transmission, based on radiative transfer simulation driven by daily TOMS ozone retrievals.
Nitrous Oxide (NO2) affects the blue channel (443 nm) of POLDER. Its absorption is corrected using a monthly climatology based on GOME retrievals.
Water vapor affects slightly the 763, 765 and 865 nm channels of POLDER. Water vapor absorption is corrected using an empirical formula, based on radiative transfer simulations. The formula is based on the sun-target-satellite geometry and the ratio of the 910 and 865 nm measurements. Note that the 910 nm channel is centered on a water vapor absorption band, so that the 910 and 865 nm band ratio is mostly a function of water vapor content.
The 763 and 765 nm channels are both centered on the oxygen A absorption band. The so-called 763 nm channel is much more affected because it is four times narrower. An empirical formula based on the two channels measurements yields an "absorption free" estimate of the radiance.
Stratospheric aerosol correction
Stratospheric aerosol are generated mostly by large volcanic eruptions. The corresponding optical thickness is, a few months after the eruption, well mixed over latitudinal bands. Ground based as well as satellite limb measurements yield the a stratospheric aerosol optical thickness. POLDER processing algorithm are designed to make use of such estimates for a correction of the stratospheric aerosol scattering. The correction is based on an a-priori stratospheric aerosol model.
© CNES Copyrights 2002