Aerosol Climatology over Pseudo-Invariant Calibration Sites: Application for African and Arabian Desert Sites

Calibration over natural targets is a very powerful way to derive the in-flight calibration of sensors (e.g., PARASOL, Vegetation, Pleiades) or if not, to validate the calibration derived using on-board devices (e.g., MODIS, MERIS). Various type of targets can be considered among oceanic sites (Rayleigh scattering, sunglint), atmospheric targets (deep convective clouds), terrestrial sites (desert, snow), or extra-terrestrial targets (moon, stars). Pseudo-invariant calibration sites (so-called PICS) are used for a very long while for many sensors because of their suitable properties in term of stability, moderated bidirectional variations, accessibility in term of cloudiness. Calibration approaches over PICS directly compare top-of-atmosphere (TOA) radiances measured by two sensors, or try to resolve spectral differences between both sensor responses by considering atmospheric and surface properties through a more or less complex algorithm (e.g. Lachérade et al., 2013). PICS are known to provide very stable surface properties, long-term or seasonal. It is also often assumed the same property for atmosphere as if historical studies have shown limitations. We derived a climatology of aerosol content over the 20 desert sites located in Africa and Arabia. These time series were derived using data from 3 sensors designed for aerosol retrieval: MODIS-Aqua and –Terra aerosol monthly products over land (from GIOVANNI) for the 2000-2014 period, and PARASOL aerosol daily products from for the 2005-2013 period. The aerosol load seasonal cycle is described for all desert sites and in general, a similar behaviour is observed year after year. According MODIS, the aerosol optical thickness at 550nm may vary from 0.2 to 0.6 depending of site and season. A sensible difference is observed with PARASOL, first for the aerosol optical thickness (in general smaller), but also for the seasonal variation that may be opposite to the one derived by MODIS. We interpret this signature as a very strong variation of the aerosol type during the season because PARASOL, which uses the directional and polarized signature of aerosol, is only sensitive to the aerosol fine mode while MODIS, which uses a spectral information up to the SWIR domain, is sensitive to both coarse and fine aerosol modes. A summary of the different behaviours observed for various sites will be presented. Results are confronted to observation from Aeronet for (not exact but) consistent locations. A generally good consistency is observed between datasets. This is an important validation of the conclusion derived from spaceborne sensors. As a conclusion, a single aerosol content cannot be a sufficient hypothesis when cross-calibration sensors, especially when allowing matching from different dates. In order to improve the accuracy of the calibration over desert sites (cross-calibration as well as temporal monitoring) a seasonal cycle of aerosol has to be considered on the data processing