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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 7, issue 10 | Copyright
Atmos. Meas. Tech., 7, 3509-3527, 2014
https://doi.org/10.5194/amt-7-3509-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 13 Oct 2014

Research article | 13 Oct 2014

A simple and versatile cloud-screening method for MAX-DOAS retrievals

C. Gielen1, M. Van Roozendael1, F. Hendrick1, G. Pinardi1, T. Vlemmix2, V. De Bock3, H. De Backer3, C. Fayt1, C. Hermans1, D. Gillotay1, and P. Wang4 C. Gielen et al.
  • 1Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 2Department of Geosciences and Remote Sensing, Delft University of Technology, the Netherlands
  • 3Royal Meteorological Institute of Belgium, Brussels, Belgium
  • 4Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

Abstract. 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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