SYN Level 2 AOD processing starts, once the whole OLCI and SLSTR datasets have been projected on the OLCI reference grid, with the creation of a super-pixel dataset.

A super-pixel is defined on the OLCI image pixel on a 15x15 pixels basis. All parameters are checked regarding surface classification and cloud detection before being averaging. Then, on this super-pixel dataset, an aerosol retrieval module is performed to retrieved aerosol optical thickness at 550 nm.

This aerosol retrieval is close to the one applied on SYN L2 SDR products (i.e. based on a double optimization of the error model), but with important differences:

• An ocean transfer radiative model has been included, taken into account SLSTR radiances and providing aerosol characteristics over the whole swath
• The inversion process and associated Look-up-tables have been adapted to retrieve continuous aerosols components from 35 aerosols mixtures
• Prior climatology of aerosol properties has been included and uncertainty estimate per retrieval is provided for each pixel

An additional post-processing filtering has been also added to detect missed cloudy pixels. It is based on a simple image processing considering all neighbors of a retrieved super-pixel, i.e. boxes of 9 (3x3) super-pixels. For an individual AOD retrieval to successfully pass this step of filtering the following criteria need to be met:

• at least 3 neighboring super-pixels with successful retrievals are required
• the sample corrected standard deviation of the valid super-pixels in the 3x3 box needs to be smaller than the minimum of 0.15 or 80% of the mean AOD value + 0.04