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| Titel |
Liquid water absorption and scattering effects in DOAS retrievals over oceans |
| VerfasserIn |
E. Peters, F. Wittrock, A. Richter, L. M. A. Alvarado, V. V. Rozanov, J. P. Burrows |
| 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 ; 7, no. 12 ; Nr. 7, no. 12 (2014-12-05), S.4203-4221 |
| Datensatznummer |
250115980
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| Publikation (Nr.) |
 copernicus.org/amt-7-4203-2014.pdf |
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| Zusammenfassung |
Spectral effects of
liquid water are present in absorption (differential optical absorption
spectroscopy – DOAS) measurements above the ocean and, if insufficiently
removed, may interfere with trace gas absorptions, leading to wrong results.
Currently available literature cross sections of liquid water absorption are
provided in coarser resolution than DOAS applications require, and
vibrational Raman scattering (VRS) is mostly not considered, or is
compensated for using simulated pseudo cross sections from radiative transfer
modeling.
During the ship-based TransBrom campaign across the western Pacific in
October 2009, MAX-DOAS (Multi-AXis differential optical absorption
spectroscopy) measurements of light
penetrating very clear natural waters were performed, achieving average
underwater light paths of up to 50 m. From these measurements, the retrieval
of a correction spectrum (H2Ocorr) is presented,
compensating simultaneously for insufficiencies in the liquid water
absorption cross section and broad-banded VRS structures. Small-banded
structures caused by VRS were found to be very efficiently compensated for by
the intensity offset correction included in the DOAS fit. No interference
between the H2Ocorr spectrum and phytoplankton absorption
was found.
In the MAX-DOAS tropospheric NO2 retrieval, this method was able to
compensate entirely for all liquid water effects that decrease the fit
quality, and performed better than using a liquid water cross section in
combination with a simulated VRS spectrum. The decrease in the residual root mean square
(rms) of the DOAS
fit depends on the measurement's contamination with liquid water structures,
and ranges from ≈ 30% for measurements slightly towards the water
surface to several percent in small angles above the horizon. Furthermore,
the H2Ocorr spectrum was found to prevent misfits of
NO2 slant columns, especially for very low NO2 scenarios, and thus
increases the reliability of the fit. In test fits on OMI satellite data, the
H2Ocorr spectrum was found selectively above ocean surfaces,
where it decreases the rms by up to ≈ 11%. |
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