The TROPOMI NO₂ retrieval algorithm is based on the OMI NO₂ retrieval algorithm (called DOMINO). This algorithm is a three-step DOAS in which the first and third steps derive respectively the slant column and the vertical column. These are discussed here. The second step of the algorithm, presented below, separates the total slant column into stratospheric and tropospheric parts.

Fitting Window

The wavelength range used for the TROPOMI NO₂ retrieval is 405 – 465 nm (spectral band 4). It is the same as for the OMI instrument (DOMINO algorithm).

Separation of Stratospheric and Tropospheric NO₂

The baseline method in the TROPOMI NO₂ algorithm for separating stratospheric and tropospheric contributions to the NO₂ total slant columns is through data assimilation of slant columns in the TM5 chemistry transport model (CTM). In a data assimilation system, meteorological fields are used to drive a CTM, while NO₂ slant column data are assimilated to regularly update the three-dimensional NO₂ distribution of the CTM. The data assimilation ensures that the model simulations of the stratospheric NO₂ column agree closely with the satellite measurements. Once the TROPOMI NO₂ slant columns have been assimilated, the integral from the layer above the tropopause to the upper TM5 layer provides the stratospheric slant column N_s^strat that can be isolated from the total slant column N_s giving the tropospheric slant column N_s^trop.

N_s^trop = N_s - N_s^strat