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DOAS analysis of satellite spectra



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Spectral ranges and fitting results for various DOAS (Differential ) algorithms for GOME spectra developed in our group [Wagner et al., 2002a]. Not displayed are new algorithms for the retrieval of Greenhouse gases from SCIAMCHY near infra-red spectra.


Method

Due to the specific properties of a given molecule its absorption spectrum (or emission spectrum) in the UV/vis spectral range is a characteristic combination of electronic, vibrational and rotational transitions. This is the basis for the selective analysis by spectroscopic techniques.

While in the microwave and infrared region individual lines are resolved by typical instrumentation, typical DOAS instruments are usually not designed to resolve individual lines in the UV/vis spectral range. For example the full width at half maximum (FWHM) of the GOME and SCIAMACHY instruments in the UV range is about 0.2 nm, and thus much larger than the atmospheric widths of the absorbing molecules.

The absorption of radiation by matter is described by the Beer-Lambert law. It determines the relationship between the incident light intensity I0(λ) and the transmitted light intensity I(λ) after traversing the distance (1)

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Here σ(λ,T) is the absorption cross section of the absorbing molecule (depending on wavelength and temperature) and ρ(s) its concentration. The negative logarithm of the ratio of the measured I and I0 is called optical density τ(λ). From τ(λ) the (slant) column density (SCD) of the absorbing species can be calculated: (2)

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The Beer-Lambert law can not directly be applied to atmospheric measurements because of several reasons: ? Besides the absorption of the trace gases also light extinction due to scattering on molecules and aerosols occurs. ? In the atmosphere the absorptions of several species always add up to the total absorption. Thus in most of the cases it is not directly possible to measure only one specific species. ? Usually the initial intensity I0 can not be measured at all or with sufficient accuracy.

These limitations can be solved by applying the method of Differential Optical Absorption Spectroscopy (DOAS) [Platt, 1994]. The DOAS technique relies on the measurement of absorption spectra instead of the light intensity at a single wavelength only. Thus it is possible to separate the absorption structures of several atmospheric species from each other as well as from the extinction due to scattering on molecules and aerosols.

The key principle of DOAS is the separation of the absorption into a part which represents broad spectral features and in another part representing narrow spectral features. Consequently also σ(λ) of an absorbing species is split into a portion σc(λ) which varies only 'slowly' with wavelength λ and into another portion σ(λ) which shows ‘rapid’ variation with λ:

σ(λ) = σc(λ) + σ'(λ)

(3)

In practice the slant column densities SCD’ are determined by applying a non linear least squares fitting algorithm [Gomer et al., 1993; Stutz and Platt, 1996]. First the ratio of the measured spectrum and a spectrum of direct sun light is formed (to remove the solar Fraunhofer lines).

Then the logarithm of this ratio is fitted by the cross sections of the considered trace gases and a polynomial of small degree. The least squares fitting algorithm minimises the sum of the squares of the differences over the selected wavelength range (with λi the centre wavelength of a detector pixel).

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least squares fit over all λl

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Example of the BrO DOAS analysis of a GOME spectrum. Several trace gas cross sections are fitted simultaneously to the measured spectrum. The residual structure is the difference between the measured and modelled spectra and characterises the quality of the DOAS analysis.