The geometry of the optical path of the light from the Sun to the Earth's surface and back to the satellite is a function of the satellite's position on the dayside of its orbit. This geometry can be represented as two slant columns.

As a further simplification we can neglect those parts of the optical path that are above the stratosphere. These regions of the atmosphere make a negligable contribution to the absorption of the trace gas species we are trying to detect. The spectrum measured at the satellite is therefore taken to apply to a column of atmosphere reaching from the top of the stratosphere to the surface and back. This column is known as the total slant column SC_total, and the concentration of NO₂ it contains is represented by the total Slant Column Density, SCD_total.

Fig 3.4.1.3.1: The total slant column in the stratosphere and the troposphere.

Image: AT2-ELS