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From Radiation Fields to Atmospheric Concentrations

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CHAPTER 5

5: From Radiation Fields to Atmospheric Concentrations – Introduction into the Retrieval of Geophysical Parameters

The challenging task in spaceborne remote sensing of the atmospheric composition is the quantitative derivation of the constituent distributions – trace gases, aerosols, clouds – from the measured top-of-the-atmosphere spectral radiance or Earth spectral reflectance data. The main source of radiation for passive remote sounding of the atmosphere by SCIAMACHY in the UV-SWIR regions is the sun. The absorption, emission and scattering characteristics of the Earth’s atmosphere can be determined by comparing the radiance reflected from and scattered by the atmosphere to the sensor with the extraterrestrial solar irradiance. The ratio of the radiance to irradiance, the Earth reflectance spectrum, contains the information relevant for determining the atmospheric composition. A quantitative analysis requires:


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spectra of high spectral and radiometric quality,

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an accurate modelling of the radiative transfer of the solar photons through the atmosphere to the sensor,

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techniques to relate the measured top-of-the-atmosphere spectra to the constituent properties (usually referred to as Inversion Methods).


The retrieval of information on atmospheric trace gases relies on the knowledge of the absorption, emission and scattering of electromagnetic radiation in the atmosphere. In the UV, VIS, NIR and SWIR spectral ranges, the radiative transfer through the atmosphere is affected by (see fig. 5-1):


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scattering by air molecules (Rayleigh-, Raman scattering),

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scattering and absorption by aerosol and cloud particles (Mie-scattering),

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absorption and emission by trace gases,

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refraction due to the density gradient in the atmosphere,

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and surface reflection.


These are the processes which must be quantitatively taken into account when retrieving atmospheric geophysical parameters from SCIAMACHY measurements. (fig. 5-1)

 

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fig. 5-1:

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Scheme of the relevant interactions of solar light with the Earth's atmosphere and surface. (Graphics: DLR-IMF)
 

Trace gases usually exhibit characteristic fingerprint spectra in emission or absorption, originating from:


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rotational transitions: primarily observed in the far infrared and microwave spectral regions,

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vibrational-rotational transitions: can be measured in the infrared,

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electronic transitions: mainly detected in the UV, VIS and NIR spectral regions.


SCIAMACHY with its wide spectral coverage from the UV to the SWIR is detecting trace gases mainly via their electronic transition spectra.

 

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