The general method used for derivation of SO₂ vertical column density is the DOAS method.

Fitting Window

DOAS relies on the assumption of an optically thin atmosphere. For very high SO₂ columns, as regularly encountered in major volcanic eruptions, this assumption is no longer true. Therefore, the envisaged TROPOMI SO2 algorithm will use a multi-window approach:

1. 312-326 nm (spectral bands 2 and 3): classical fitting window, ideal for small column densities. This window is used as a baseline. If non-linear effects due to high SO₂ concentrations are encountered, one of the two following windows will be used instead.
2. 325-335 nm (spectral band 3):  in this window, differential SO₂ spectral features are one order of magnitude smaller than in the classical window. It allows retrieval of moderate SO₂ columns.
3. 360-390 nm (spectral band 3): SO₂ absorption bands are 2-3 orders of magnitude weaker than in the classical window and are best suited for the retrieval of extremely high SO₂ columns.

Offset (Background) Correction

An empirical offset correction (dependent on the fitting window used) is applied to the retrieved slant column density. The latter correction accounts for systematic biases in the SCD and is estimated over clean regions (with presumable zero SO₂ vertical column). This correction ensures geophysical consistency in results.