Remote sensing of NO2
3.1.2 Comparison of sunlight and earthshine

An example of the signal measured by the satellite instrument when looking directly into the sun (solar irradiance) and in nadir (earthshine radiance) is shown in the figure. There are some prominent similarities and differences that are briefly discussed below.

Fig 3.1.2.1: 

Image: AT2-ELS

Water vapour absorption Oxygen absorption Oxygen absorption Ozone absorption

Fraunhofer lines

Both spectra show strong absorption features. Most of them are Fraunhofer lines which result from absorption in the solar atmosphere. For the retrieval of the composition of the Earth's atmosphere, they create some problems because they are much stronger than the NO2 signal. Perfect wavelength alignment of the two spectra is needed to avoid interference in the retrieval, and in addition, Fraunhofer lines in scattered light have a slightly different shape than in unattenuated solar light, as described in the section of this module 'Retrieval procedures/Earthshine interactions/Ring effect'.

UV range complexity

Wavelengths below 400 nm are in the ultraviolet range of the spectrum. The spectra in this range are more structured than at longer wavelengths. This is the result of two effects:

  1. There are more Fraunhofer lines in the UV part of the spectrum.
  2. The spectral resolution of the GOME and SCIAMACHY instruments is approximately 0.2 nm below 400 nm and 0.4 nm above that.

Rayleigh scattering

In the solar spectrum, the intensity at 600 nm is much higher than at 350 nm, whereas in the earthshine spectrum they have comparable magnitude. This can be explained by the effect of Rayleigh scattering, which is much more effective in the UV (proportional to λ-4). Thus the UV part appears enhanced in an earthshine measurement, which has a large contribution from scattered light.

Strong atmospheric absorptions

Although the Fraunhofer lines dominate both spectra, some strong absorptions in the Earth's atmosphere are also apparent in the earthshine spectrum only: ozone below about 320 nm, oxygen at 685 and 765 nm and water vapour at 725 nm. (Roll the mouse cursor over these parts of the earthshine spectrum to display the name of the absorbing substance.)

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