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| Titel |
Extending differential optical absorption spectroscopy for limb measurements in the UV |
| VerfasserIn |
J. Puķīte, S. Kühl, T. Deutschmann, U. Platt, T. Wagner |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 3, no. 3 ; Nr. 3, no. 3 (2010-05-19), S.631-653 |
| Datensatznummer |
250001111
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| Publikation (Nr.) |
 copernicus.org/amt-3-631-2010.pdf |
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| Zusammenfassung |
Methods of UV/VIS absorption spectroscopy to determine the constituents in
the Earth's atmosphere from measurements of scattered light are often based
on the Beer-Lambert law, like e.g. Differential Optical Absorption
Spectroscopy (DOAS). While the Beer-Lambert law is strictly valid for a single light
path only, the relation between the optical depth and the concentration of any
absorber can be approximated as linear also for scattered light observations
at a single wavelength if the absorption is weak. If the light path distribution
is approximated not to vary with wavelength, also linearity between
the optical depth and the product of the cross-section and the concentration
of an absorber can be assumed. These assumptions are widely made for
DOAS applications for scattered light observations.
For medium and strong absorption of scattered light (e.g. along very long light-paths like in
limb geometry) the relation between the optical depth and the concentration
of an absorber is no longer linear. In addition, for broad wavelength intervals
the differences in the travelled light-paths at different wavelengths become
important, especially in the UV, where the probability for scattering
increases strongly with decreasing wavelength.
However, the DOAS method can be extended to cases with medium to strong absorptions
and for broader wavelength intervals by the so called air mass factor
modified (or extended) DOAS and the weighting function modified DOAS. These
approaches take into account the wavelength dependency of the slant column
densities (SCDs), but also require a priori knowledge for the air mass factor
or the weighting function from radiative transfer modelling.
We describe an approach that considers the fitting results obtained from
DOAS, the SCDs, as a function of wavelength and vertical optical depth and
expands this function into a Taylor series of both quantities. The Taylor
coefficients are then applied as additional fitting parameters in the DOAS
analysis. Thus the variability of the SCD in the fit window is determined by
the retrieval itself.
This new approach provides a description of the SCD the exactness of which
depends on the order of the Taylor expansion, and is
independent from any assumptions or a priori knowledge of the considered
absorbers.
In case studies of simulated and measured spectra in the UV range (332–357 nm),
we demonstrate the improvement by this approach for the retrieval of vertical
profiles of BrO from the SCIAMACHY limb observations.
The results for BrO obtained from the simulated
spectra are closer to the true profiles, when applying the new method for the
SCDs of ozone, than when the standard DOAS approach is used.
For the measured spectra the agreement with validation
measurements is also improved significantly, especially for cases with strong
ozone absorption.
While the focus of this article is on the improvement of the BrO profile
retrieval from the SCIAMACHY limb measurements, the novel approach may be
applied to a wide range of DOAS retrievals. |
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