Since version 3.3, MAJA’s atmospheric correction can rely either on the default ‘continental’ aerosol assumption or on the spatial distribution of actual aerosol species, as provided by the Copernicus Atmosphere Monitoring Service (CAMS). This page summarizes how the CAMS species and their related optical properties have evolved in MAJA.
(netCDF 700Ko)
MAJA’s strength lies in its ability to account for temporal variations in satellite observations. This enables reliable cloud screening and aerosol optical thickness (AOT) estimation. The latter assumes that a change in the reflectance of a given pixel over a short period of time is due to a change in AOT rather than a change in the surface’s reflectance. It is derived using a minimization approach. Once the AOT has been estimated, the relationship between the top of the atmosphere reflectance and the surface reflectance of a given band can be derived from pre-computed look-up tables (LUTs), according to the AOT, solar and viewer angles, surface altitude, and relative humidity.
Such LUTs are calculated using a radiative transfer code for each aerosol species. If the user does not specify the ‘–cams’ option, MAJA will use the ‘Continental’ LUT by default. Otherwise, MAJA searches the CAMS product collection for products at 00:00 and 12:00 UTC that frame the satellite acquisition time. It then derives the relative contribution of each CAMS aerosol species to the total AOT. These contributions are then used to linearly combine the aerosol-specific LUTs, accounting for each aerosol species in the final surface reflectance estimate. For more details, please refer to Rouquié et al. 2017.
The MAJA team is committed to maintaining and updating the entire look-up table collection for each CAMS version upgrade. This accounts for changes in the optical properties of aerosols (refractive index), their size distribution, the growth factor for hydrophilic species and, sometimes, the addition of new species. The table below summarizes these changes.
| CAMS versions Release date MAJA compatible | 46r1 11/07/2019 MAJA 3.3 | 47r1 30/06/2020 MAJA 4.2* | 48r1 17/06/2023 MAJA 4.8 | 49r1 12/11/2024 MAJA 4.10 |
| Ammonium | Hydrophilic. Mono-modal | – | – | – |
| Black Carbon | Mono-modal. Refractive index set to 1.75-0.2i | – | – | – |
| Dust | Mono-modal | – | Updated refractive index and size distribution | Is now hydrophilic |
| Nitrate | Hydrophilic. Mono-modal, with one single fine mode (no coarse nitrate) | – | – | – |
| Organic Matter | Hydrophilic. Mono-modal derived from a combination of WASO/SOOT/INSO | – | New size distribution and growth factors | – |
| Sea Salt | Hydrophilic. Bi-modal. | – | – | – |
| Sulphate | Hydrophilic. Mono-modal. | – | New size distribution and growth factors | New size distribution |
| Secondary Organic | Not until 48r1 | Not until 48r1 | New aerosol species. Hydrophilic. | – |
“-” means unchanged from previous version
* MAJA 4.2 includes a compatibility fix for the CAMS file format
As a user, you need to update both MAJA and its GIPP files in order to account for the evolution of CAMS. Therefore, once you have updated your version of MAJA, it is important that you let StartMAJA download a fresh GIPP collection to a new directory. To do so, simply set “repGipp” to point to an empty folder in your ‘folder.txt’ file, and StartMAJA will download the latest GIPP from our Gitlab repository.
Note that MAJA always remains backward compatible with former CAMS product versions.
If you are interested in the details of the refractive indexes and growth distribution used to compute the MAJA look-up tables, you can download the netCDF file below. This stores each variable alongside the aerosol species, CAMS version, wavelength and relative humidity.
Download MAJA’s aerosols Optical Properties (netCDF 700Ko)
sha1sum : 07b5d6c0f6bc763795932d4f68c14410f24fb2e8
Please note :
- Sulphate, dust, black carbon, ammonium and organic matter aerosols are described with a mono-modal log-normal size distribution
- Sea salt is described with a bi-modal log-normal size distribution
- Nitrate is described in CAMS with tree modes, while MAJA uses only the fine mode properties
- The geometric standard variation of a log-normal size distribution does not vary with the relative humidity
- The refractive indexes do not vary between fine and coarse modes. Fine and coarse modes are only related to aerosol modal radius and size distribution
- Up to version 48r1, dust particles were assumed to be spherical. Since version 49r1, dust are assumed non-spheric in CAMS and ECMWF models. However, we keep the sphericity assumption in the computation of the dust LUT for MAJA, as for the other aerosol species