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| Titel |
On the impact of Vibrational Raman Scattering of N2/O2 on MAX-DOAS Measurements of atmospheric trace gases |
| VerfasserIn |
Johannes Lampel, Johannes Zielcke, Udo Frieß, Ulrich Platt, Thomas Wagner |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106981
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| Publikation (Nr.) |
 EGU/EGU2015-6667.pdf |
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| Zusammenfassung |
| In remote sensing applications, such as the applications of differential optical absorption
spectroscopy (DOAS), atmospheric scattering processes need to be considered since they can
modify the observed spectra. Inelastic scattering of photons by N2 and O2 molecules can be
observed as additional intensity, effectively leading to filling-in of both, solar Fraunhofer
lines and absorption bands of atmospheric constituents.
The main contribution is due to rotational Raman scattering, which can lead to
changes in observed optical densities of absorption lines up to several percent.
Measured optical densities are typically corrected for this effect (also known as Ring
Effect).
In contrast to that Vibrational Raman scattering of N2 and O2 was often thought to be
negligible, but also contributes to this effect. We present calculations of Vibrational Raman
cross-sections for O2 and N2 for the application in passive DOAS measurements.
Consequences of vibrational Raman scattering are red-shifted Fraunhofer structures, so called
‘Fraunhofer Ghost’ lines (FGL), in scattered light spectra and filling-in of Fraunhofer lines,
additional to rotational Raman scattering.
We also present first unequivocal observations of FGL at optical densities of up
to several 104. From our measurements and calculations of the optical density of
these FGL, we conclude, that this phenomenon has to be included in the spectral
evaluation of weak absorbers. Its relevance is demonstrated in spectral evaluations
of Multi-Axis (MAX)-DOAS data and an agreement with calculated scattering
cross-sections is found. To exclude cross-sensitivities with other absorbers, such as water
vapour, MAX-DOAS data from different latitudes and different instruments were
analysed. We evaluate the influence of the additional intensities due to vibrational
Raman scattering on the spectral retrieval of IO, Glyoxal, H2O and NO2 in the blue
wavelength range. In the case of NO2 the column densities derived from certain
wavelength intervals can underestimate the true tropospheric NO2 concentrations by
several 10 ppt if vibrational Raman scattering is neglected in the evaluation. For all
absorbers in the blue wavelength range, a reduction of the measurement error is
observed, whenever vibrational Raman scattering is explicitly considered, although
the obtained column densities for IO, Glyoxal and H2O are only slightly affected,
with differences of less than 20% and typically below their respective detection
limits. Estimates on the expected optical densities for other spectral regions are
given. |
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