A difficulty arises in using equation (1) since no satisfactory method is available for
estimating the value of the aerosol scattering coefficient (δ-δ'). In practice, therefore, observations
are normally made on double pair wavelengths, e.g., the AD wavelengths. Since both the A and
the D wavelength pairs are approximately equally scattered by the atmosphere, the scattering
effect is nearly cancelled so that absorption by ozone becomes by far the major factor affecting the
relative intensities of the double pair wavelengths on which observations are made