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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  28 Apr 2020

28 Apr 2020

Review status
A revised version of this preprint is currently under review for the journal AMT.

Effects of clouds on the UV Absorbing Aerosol Index from TROPOMI

Maurits L. Kooreman, Piet Stammes, Victor Trees, Maarten Sneep, L. Gijsbert Tilstra, Martin de Graaf, Deborah C. Stein Zweers, Ping Wang, Olaf N. E. Tuinder, and J. Pepijn Veefkind Maurits L. Kooreman et al.
  • Royal Netherlands Meteorological Institute, De Bilt, the Netherlands

Abstract. The ultraviolet (UV) Absorbing Aerosol Index (AAI) is widely used as an indicator for the presence of absorbing aerosols in the atmosphere. Here we consider the TROPOMI AAI based on the 340/380 nm wavelength pair. We investigate the effects of clouds on the AAI observed at small and large scales. The large scale effects are studied using an aggregate of TROPOMI measurements over an area mostly devoid of absorbing aerosols (Pacific Ocean). The study reveals that several structural features can be distinguished in the AAI, such as the cloud bow, viewing zenith angle dependence, sunglint, and a previously unexplained increase in AAI values at extreme viewing and solar geometries. We explain these features in terms of the Bidirectional Reflectance Distribution Function (BRDF) of the scene in combination with the different ratio of diffuse and direct illumination of the surface at 340 and 380 nm. To reduce the dependency on the BRDF and homogenize the AAI distribution across the orbit, we present three different AAI retrieval models: the traditional Lambertian Scene Model (LSM), a Lambertian Cloud Model (LCM), and a Scattering Cloud Model (SCM). We perform a model study to assess the propagation of errors in auxiliary databases used in the cloud models. The three models are then applied to the same low-aerosol region. Results show that using the LCM and SCM gives on average a higher AAI than the LSM. Additionally, a more homogeneous distribution is retrieved across the orbit. At the small scale, related to the high spatial resolution of TROPOMI, strong local increases and decreases in AAI are observed in the presence of clouds. This effect was not observed in previous instruments with larger ground pixels such as GOME-2 and OMI. More research is needed to explain these small scale effects.

Maurits L. Kooreman et al.

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Status: final response (author comments only)
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Maurits L. Kooreman et al.

Maurits L. Kooreman et al.


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Latest update: 21 Sep 2020
Publications Copernicus
Short summary
We investigated the influence of clouds on the AAI, which is an indicator of the presence of small particles suspended in the atmosphere. Clouds produce artefacts in the AAI calculation on the individual measurement (7 km) scale, which was not seen with previous instruments, as well as large (1000+ km) scales. To reduce these artefacts, we used three different AAI calculation techniques with varying complexity. We find that the AAI artefacts are reduced when using more complex techniques.
We investigated the influence of clouds on the AAI, which is an indicator of the presence of...