The slant column density SCD_total derived using the DOAS method contains contributions from both the stratosphere and the troposphere. In order to extract the tropospheric information, the stratospheric component has to be subtracted from the slant column, and two methods to accomplish this will be discussed in this section.

Fig 3.4.2.1: Removal of the SC_strat from the SC_total to leave the SC_trop.

Image: AT2-ELS

Before discussing the separation procedures, it is worthwhile comparing briefly the expected global distributions for tropospheric and stratospheric NO₂.