Suspended Particulate Matter (SPM) and Turbidity
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As others, SPM and Turbidity products are based on optical satellite data from the Sentinel-2 constellation. 
It is generated in near real-time (NRT) once a week for the entire France and overseas area and derived from the Remote Sensing Reflectances (Rrs) products. 
Pixel-wise SPM and Turbidity products of the project dataset are of two kinds : 
one product derives directly the mass concentration of suspended particulate matter (SPM), the other one,  the Turbidity, its optical counterpart.

Retrieval methodology
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Products are derived from a single Sentinel-2 L2A that has been corrected for atmospheric contribution (see section about :ref:`rrs`). 
Algorithms are then applied to retrieve the estimations of SPM (Nechad et al., 2010) and Turbidity (Dogliotti et al., 2015) from bottom of atmosphere Rrs derived from the atmospheric corrections.


Product limitations
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As Level-2A optical Rrs estimates are the input data used to derive SPM and turbidity products, uncertainties in Rrs are consequently propagated to Level-2B data. 
No data can be produced when clouds are present in the atmosphere. Clouds can be present over the scenes but the masks are not applied directly over the level 2B produced. 
Therefore, the user needs to specify (mask==0) to eliminate land and clouds to obtain usable transparency data.
In very turbid environments, reflectances are expected to saturate and are therefore used sequentially from the short wavelengths to the longer wavelengths. 
Hence, SPM derived from the Red band following a semi-analytical algorithm is supposed to saturate above 100 mg/l and should not be used after 150 mg/l in concentration.
As for Chorophyll-a retrieval, the important contrast between land reflecting high NIR (Near-Infra-Red) signal, and the very low NIR reflected by water for which absorption of NIR by water is very high.