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| Titel |
A simple and versatile cloud-screening method for MAX-DOAS retrievals |
| VerfasserIn |
C. Gielen, M. Van Roozendael, F. Hendrick, G. Pinardi, T. Vlemmix, V. De Bock, H. De Backer, C. Fayt, C. Hermans, D. Gillotay, P. Wang |
| 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. 10 ; Nr. 7, no. 10 (2014-10-13), S.3509-3527 |
| Datensatznummer |
250115933
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| Publikation (Nr.) |
 copernicus.org/amt-7-3509-2014.pdf |
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| Zusammenfassung |
We present a cloud-screening method based on differential optical absorption
spectroscopy (DOAS) measurements, more specifically using intensity
measurements and O4 differential slant-column densities (DSCDs). Using
the colour index (CI), i.e. the ratio of the radiance at two wavelengths, we
define different sky conditions including clear, thin clouds/polluted,
fully-cloudy, and heavily polluted. We also flag the presence of broken and
scattered clouds. The O4 absorption is a good tracer for cloud-induced
light-path changes and is used to detect clouds and discriminate between
instances of high aerosol optical depth (AOD) and high cloud optical depth
(COD).
We apply our cloud screening to MAX-DOAS (multi-axis DOAS) retrievals at
three different sites with different typical meteorological conditions, more
specifically suburban Beijing (39.75° N, 116.96° E),
Brussels (50.78° N, 4.35° E) and Jungfraujoch
(46.55° N, 7.98° E). We find that our cloud screening
performs well characterizing the different sky conditions. The flags based on
the colour index are able to detect changes in visibility due to aerosols
and/or (scattered) clouds. The O4-based multiple-scattering flag is able
to detect optically thick clouds, and is needed to correctly identify clouds
for sites with extreme aerosol pollution. Removing data taken under cloudy
conditions results in a better agreement, in both correlation and slope,
between the MAX-DOAS AOD retrievals and measurements from other co-located
instruments. |
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