Fig 3.3.1.8.1: Differential absorption by subtraction of low-order polynomial.

Image: AT2-ELS

The logarithm of the normalized light spectrum gave the absorption in terms of optical thickness over the wavelength range. The low-order polynomial is then subtracted from this curve to cancel out the broadband effects and produce a curve representing the differential absorption.

The use of the contrast between the rapid rate of change of molecular absorption and the much slower rate of change of other extinction phenomena such as scattering is the key factor in the DOAS technique.