Level-2 AOD

The Level-2 SYN AOD product (SY_2_AOD) is produced by a dedicated processor including the whole SYN L1 processing module and a global synergy level 2 processing module retrieving, over land and sea, aerosol optical thickness. The resolution of this product is wider than classic S3 products, as the dataset are provided on a 4.5 km² resolution

The SYN_2_AOD product is identified as a downstream development of the OLCI Level-1 Full Resolution product (OL_1_EFR) and SLSTR Level-1 Radiances and Brightness Temperatures product (SL_1_RBT__):

The OLCI and SLSTR L1B input products need to be co-registered through the first processing module of the SYNERGY Level 1/2 global AOD software, then projected on the same reference grid and stored as intermediate data.

The SYNERGY Level-2 global Aerosol (SY_2_AOD) product contains both retrieved and diagnostic global aerosol parameters in a single measurement dataset at super-pixel resolution for all regions over land and ocean free of snow/ice cover, and excluding high cloud fraction. The retrieved and derived aerosol parameters are:

  • AOD at 440, 550, 670, 985, 1600 and 2250 nm,
  • Error estimates (i.e. standard deviation) in AOD at 440, 550, 670, 985, 1600 and 2250 nm,
  • Single Scattering Albedo (SSA) at 440, 550, 670, 985, 1600 and 2250 nm,
  • Fine-mode aerosol optical depth at 550nm,
  • Aerosol Angstrom parameter between 550 and 865nm,
  • Dust aerosol optical depth at 550nm,
  • Aerosol absorption optical depth at 550nm.

Contextual parameters such as time, quality flags, and geographical position are also included for each super-pixel.

In addition, the products at L2 super-pixels for user interpretation are:

  • Latitude at super-pixel centre
  • Longitude at super-pixel centre,
  • Pixel corner latitude 1 - 4,
  • Pixel corner longitude 1 - 4,
  • Sun zenith angle of nadir centre super-pixel,
  • Satellite zenith angle of nadir centre super-pixel,
  • Relative azimuth angle of nadir centre super-pixel,
  • Acquisition time of nadir centre pixel,
  • Cloud fraction within nadir super-pixel,
  • Cloud fraction within oblique super-pixel,
  • Quality flags to indicate high uncertainty in retrieval of AOD used, e.g. due to glint contamination. Flags give information on:
  • Land / ocean retrieval algorithm
  • Dual view / single view retrieval
  • Glint risk in nadir
  • Glint risk in oblique
  • Surface reflectance retrieved negative
  • AOD zero (<1e4)
  • Fine mode fraction retrieval failed (fraction from climatology)
  • Uncertainty estimation failed

Surface Product for diagnostics:

  • Atmospherically corrected nadir surface directional reflectances at 440, 550, 670, 985, 1600 and 2250 nm at super pixel (4.5km) resolution.
Table 1: Description of the channels considered in the SYN L2 global aerosol product SY_2_AOD
SLSTR channels XXX suffix λ centre (nm) bandwidth (nm)
OLCI Oa3 440 442.5 10
SLSTR S1 550 555 20
SLSTR S2 670 659 20
SLSTR S3 865 865 20
SLSTR S5 1600 1610 60
SLSTR S6 2250 2250 50


Table 2: SYNERGY Global Aerosol Level 2 product physical composition
Product Package Type Description
SY_2_AOD__ Global aerosol parameters
Product Level Diss. Timeliness Product Category Application Domain Spatial Resolution
2 NTC User LND 4.5 km
Product Dissemination Unit Number of Package components Number of Measurement Data Files Number of Annotation Data Files Number of Representation Information Files
Stripe 3 1 0 0


The SY_2_AOD product is projected on super-pixels of ~4.5 km x 4.5 km. This grid is created from the OLCI L2 image grid and each super-pixel is a combination of 15 x 15 300m-OLCI-image pixels. The position and time of the centre pixel of each super-pixel, and information about the position of each corner of the super-pixel, are included in this measurement file. The corners of a super-pixel are defined as follows:

  • Corner_1 = "top-right" corner of the pixel (largest column index/smallest row index in the nadir source image)
  • Corner_2 = "top-left" corner following counterclockwise iteration around the super-pixel (smallest column index/smallest row index)
  • Corner_3 = "bottom-left" corner (smallest column /highest row index)
  • Corner_4 = "bottom-right" corner (highest column /highest row index)

NTC (Non Time Critical) products are typically delivered within few days after data acquisition. This additional delay enables consolidation of more precise auxiliary or ancillary data that might not have been available at acquisition.

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