In the clear atmosphere, most scattering is Rayleigh scattering on air molecules. As explained in the section 'Radiation basics/Radiative transfer/Atmospheric transfer/Scattering/Rayleigh scattering', the Rayleigh scattering cross-section strongly depends on wavelength (~λ^-4). Therefore, the effects of scattering on the airmass factor described in the previous sections depend on wavelength. As illustrated in the figure, the sensitivity in the lower part of the free troposphere increases with wavelength, in agreement with the expected decreasing importance of Rayleigh scattering. The differences will be less pronounced in the presence of aerosols, which act as Mie scatterers having a less steep wavelength dependence.

Fig 3.4.3.4.1: 

Image: AT2-ELS

In conclusion, the tropospheric airmass factor (and in particular its change with altitude) depends on wavelength. This dependency has to be taken into consideration in the analysis. The best sensitivity is obtained at the longest wavelengths while in the UV — at least over dark surfaces — the airmass factor is small for the lowest layers.

• On the one hand this implies that retrieval of tropospheric constituents in the UV is difficult (e.g. for SO₂, which absorbs at wavelengths below 330 nm).
• On the other hand measurements at different wavelengths can — at least in principle — be used to obtain a coarse vertical resolution of the NO₂ column retrieved.