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| Titel |
The impact of vibrational Raman scattering of air on DOAS measurements of atmospheric trace gases |
| VerfasserIn |
J. Lampel, U. Frieß, U. Platt |
| 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 ; 8, no. 9 ; Nr. 8, no. 9 (2015-09-17), S.3767-3787 |
| Datensatznummer |
250116578
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| Publikation (Nr.) |
 copernicus.org/amt-8-3767-2015.pdf |
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| Zusammenfassung |
In remote sensing applications, such as differential optical absorption
spectroscopy (DOAS), atmospheric scattering processes need to be considered.
After inelastic scattering on N2 and O2 molecules, the
scattered photons occur as additional intensity at a different wavelength,
effectively leading to "filling-in" of both solar Fraunhofer lines and
absorptions of atmospheric constituents, if the inelastic scattering happens
after the absorption.
Measured spectra in passive DOAS applications are typically corrected for
rotational Raman scattering (RRS), also called Ring effect, which represents
the main contribution to inelastic scattering. Inelastic scattering can also
occur in liquid water, and its influence on DOAS measurements has been
observed over clear ocean water. In contrast to that, vibrational Raman
scattering (VRS) of N2 and O2 has often been thought to be
negligible, but it also contributes.
Consequences of VRS are red-shifted Fraunhofer structures in scattered light
spectra and filling-in of Fraunhofer lines, additional to RRS. At 393 nm,
the spectral shift is 25 and 40 nm for VRS of O2 and N2,
respectively. We describe how to calculate VRS correction spectra according to the Ring spectrum.
We use the VRS correction spectra in the spectral range of 420–440 nm to
determine the relative magnitude of the cross-sections of VRS of O2
and N2 and RRS of air.
The effect of VRS is shown for the first time in spectral evaluations of
Multi-Axis DOAS data from the SOPRAN M91 campaign and the MAD-CAT MAX-DOAS
intercomparison campaign. The measurements yield in agreement with calculated
scattering cross-sections that the observed VRS(N2) cross-section at
393 nm amounts to 2.3 ± 0.4 % of the cross-section of RRS at 433 nm under
tropospheric conditions. The contribution of VRS(O2) is also found to
be in agreement with calculated scattering cross-sections. It is concluded,
that this phenomenon has to be included in the spectral evaluation of weak
absorbers as it reduces the measurement error significantly and can cause
apparent differential optical depth of up to 3 ×10−4. Its influence
on the spectral retrieval of IO, glyoxal, water vapour and NO2 in the
blue wavelength range is evaluated for M91. For measurements with a large
Ring signal a significant and systematic bias of NO2 dSCDs (differential slant column densities) up to
(−3.8 ± 0.4) × 1014 molec cm−2 is observed if this effect
is not considered. The effect is typically negligible for DOAS fits with an
RMS (root mean square) larger than 4 × 10−4. |
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