As we have seen earlier in the retrievals section, the earthshine spectrum measured at the satellite contains many features that are not the result of NO₂ absorption.

In order to measure concentrations of NO₂ in the atmosphere we have to filter out the specific absorption information for the gas from the many other interactions that affect the earthshine spectrum measured at the satellite. The concentrations of NO₂ and other trace gases can be extracted using a technique known as Differential Optical Absorption Spectroscopy, or DOAS.

The DOAS procedure also carries out a normalisation of the earthshine spectrum against the irradiance spectrum as well as compensating for Ring effect aberrations.

This section deals with the DOAS procedure in two ways:

• Graphical treatment
  Using spectra and absorption plots this section contains a step-by-step graphical presentation of the stages of the DOAS procedure.
• The DOAS RTE
  The DOAS procedure uses a simplified form of a Radiative Transfer Equation, RTE. This section presents a step-by-step mathematical derivation of the DOAS RTE from a full RTE.