A novel approach to account for the stratospheric NO2 became feasible with the SCIAMACHY instrument, which not only provides nadir data but also stratospheric profiles from limb measurements. As discussed in the section 'Remote sensing/Satellite instruments/Measurement modes', for each nadir state there is a limb measurement performed 7 minutes earlier that can be used to derive a stratospheric profile of NO2.
Fig: Combined limb mode and nadir mode scanning in SCIAMACHY
The limb mode scan produces a stratospheric profile, the nadir mode scan
produces the total slant column.
Image: AT2-ELS
Although the spatial matching of limb and nadir measurement volume is not perfect, the error introduced by the mismatch is small as a result of the smoothness of the stratospheric NO2 field. By integrating the limb profile from the tropopause to the top of the atmosphere, the stratospheric NO2 column can be computed.
This in turn can be converted to a stratospheric slant column SCstrat (the slant column the instrument would see in nadir observation if only the measured stratospheric NO2 were present) to account for the viewing geometry. Subtraction of SCstrat from the total slant column SCtotal yields the tropospheric slant column:
| SCtrop | = | SCtotal - SCstrat |
| = | SCtotal - SCstrat_limb | |
| = | SCtotal - AMFstrat × VCstrat_limb |