XBAER-derived aerosol optical thickness from OLCI/Sentinel-3 observation
A cloud identification algorithm used for cloud masking, which is based on the spatial variability of reflectances at the top of the atmosphere in visible wavelengths, has been developed for the retrieval of aerosol properties by MODIS. It is shown that the spatial pattern of cloud reflectance, as...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/2511/2018/acp-18-2511-2018.pdf |
| Summary: | A cloud identification algorithm used for cloud masking, which is based on the spatial variability of reflectances at the top of
the atmosphere in visible wavelengths, has been
developed for the retrieval of aerosol properties by MODIS. It is shown that the spatial pattern of cloud reflectance, as observed
from space, is very different from that of aerosols. Clouds show a high spatial variability in the scale of a hundred metres to a
few kilometres, whereas aerosols in general are homogeneous. The concept of spatial variability of reflectances at the top of
the atmosphere is mainly applicable over the ocean, where the surface background is sufficiently homogeneous for the separation
between aerosols and clouds. Aerosol retrievals require a sufficiently accurate cloud identification to be able to mask these ground scenes. However, a conservative mask will
exclude strong aerosol episodes and a less conservative mask could introduce cloud contamination that biases the retrieved
aerosol optical properties (e.g. aerosol optical depth and effective radii). A detailed study on the effect of cloud contamination on
aerosol retrievals has been performed and parameters are established determining the threshold value for the MODIS aerosol cloud mask
(3×3-STD) over the ocean. The 3×3-STD algorithm discussed in this paper is the operational cloud mask used for MODIS
aerosol retrievals over the ocean.<br><br>A prolonged pollution haze event occurred in the northeast part of China during the period 16–21 December 2016. To assess the impact of such
events, the amounts and distribution of aerosol particles, formed in such events, need to be quantified. The newly launched Ocean Land
Colour Instrument (OLCI) onboard Sentinel-3 is the successor of the MEdium Resolution Imaging Spectrometer (MERIS). It provides
measurements of the radiance and reflectance at the top of the atmosphere, which can be used to retrieve the aerosol optical thickness
(AOT) from synoptic to global scales. In this study, the recently developed AOT retrieval algorithm eXtensible Bremen AErosol
Retrieval (XBAER) has been applied to data from the OLCI instrument for the first time to illustrate the feasibility of
applying
XBAER to the data from this new instrument. The first global retrieval results show similar patterns of aerosol optical thickness, AOT, to those from MODIS and MISR aerosol products. The AOT retrieved
from OLCI is validated by comparison with AERONET observations and a correlation coefficient of 0.819 and bias (root mean square) of
0.115 is obtained. The haze episode is well captured by the OLCI-derived AOT product. XBAER is shown to retrieve AOT well from the
observations of MERIS and OLCI. |
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| ISSN: | 1680-7316 1680-7324 |