Our focus of interest is the relatively short vertical column of troposphere directly above the ground pixel. The total amount of NO₂ integrated from the surface to the top of the troposphere within this column is known as the Tropospheric Vertical Column, VC_trop. It usually is expressed as NO₂ per unit area (units: molecules per cm²) and is sometimes also called the Tropospheric Vertical Column Density, VCD_trop.

The DOAS technique yields NO₂ densities for the Total Slant Column, SC_total, a long, slanting light path, the geometry of which depends on the solar zenith angle and the line of sight of the spectrometer towards the ground pixel and which varies dramatically during the course of the satellite's dayside orbit. The DOAS value for NO₂ is therefore the Total Slant Column Density, SCD_total.

We need to process the SCD_total to arrive at values for the VC_trop of NO₂.

This section is divided into three parts:

• SCD_total to SCD_trop
  This section gives a step-by-step overview of the tasks involved in getting from the SCD_total to the VCD_trop.
• Stratospheric and tropospheric NO2
  Since the SCD_total passes through the entire atmosphere, not just the troposphere, we need to allow for absorption caused by NO₂ in the slant column in other atmospheric layers — in practice stratospheric NO₂. We can then derive the tropospheric slant column concentration of NO₂, SCD_trop.
• Tropospheric AMF
  Finally, we need to apply an airmass factor, AMF, in order to convert the SCD_trop values to values for the VCD_trop, thus arriving at the goal of our retrieval procedure.