This computation involves the Level 2 processing algorithm applied to raw TROPOspheric Monitoring Instrument (TROPOMI) data. The TROPOMI NO2 processing system is based on the DOMINO and QA4ECV processing systems, with improvements related to specific TROPOMI aspects and new scientific insights. The basis for the processing at KNMI is a retrieval-assimilation-modeling system that uses the 3-dimensional global TM5 chemistry transport model as an essential element. The retrieval consists of a three-step procedure, performed on each measured Level-1b spectrum:
1. the retrieval of a total NO2 slant column density (Ns) from the Level-1b radiance and irradiance spectra
measured by TROPOMI using a DOAS (Differential Optical Absorption Spectroscopy) method,
2. the separation of the Ns into a stratospheric and a tropospheric part on the basis of
information coming from a data assimilation system, and
3. the conversion of the tropospheric slant column density into a tropospheric vertical column density and of the stratospheric slant column density into a stratospheric vertical column density, by applying an appropriate AMF based on a look-up table of altitude-dependent AMFs and actual, daily information on the vertical distribution of NO2 from the TM5-MP model on a 1-degree by 1-degree grid. The altitude-dependent AMF depends on the satellite geometry, terrain height, cloud fraction and height and surface albedo.
These steps are described in detail in the ATBD document.