Multi-Mission Synergetic Retrieval for Enhanced Aerosol and Surface Characterization: Physical Basis and Concept

Atmospheric aerosols have strong impact on climate, environment, and health. To account correctly for such impact, extended aerosol characterization, including spectral Aerosol Optical Depth (AOD), Angstrom Exponent (AE), spectral Single Scattering Albedo (SSA) etc., are required to be derived globally from space-borne observations. Together with the aerosol, the Earth’s surfaces are an important component of climate system, reflecting and absorbing solar and atmospheric radiation and being sources of emission of different natural aerosol, for example, sea-salt, mineral dust or organic aerosol.

The global information about aerosol and surfaces can be obtained from space-borne measurements only. At present time there are a number of different satellites on Earth orbit dedicated to aerosol and surface studies. Nevertheless, the role of space-borne measurements is essentially limited by satellite swath and instrument information content. In general, no single instrument satisfies all requirements which are necessary for global, high-temporal extended aerosol and surface characterization. One of the promising solutions of this problem originates from the idea of the synergetic aerosol and surface characterization from multi-mission instruments. Since a long time, the realization of this idea has always been related to number of instrumental and algorithmic problems.

In the frame of ESA GROSAT and SYREMIS projects, the synergetic approach was implemented in GRASP algorithm in different synergetic instrument constellations: (i) synergy of satellite and ground-based measurements (GROSAT/GRASP synergy); (ii) synergy of Low Earth polar-Orbiting (LEO+LEO), and (iii) LEO and geostationary (LEO+GEO) satellites. On one hand such synergy constellations extend the spectral range of the measurements. On another hand they provide unprecedented global spatial coverage with several measurements per day which is crucial for global climate studies and air-quality monitoring.

In the GROSAT/GRASP approach, both ground-based (AERONET) and satellite measurements are merged together, and then the synergetic aerosol and surface retrieval is performed on the combined measurements. The main information about aerosol in such synergy comes from AERONET direct sun and diffuse sky-radiance measurements, whereas the information about surface reflection properties originates from satellite observations. The GROSAT/GRASP approach is generalized in such a robust way that it can be applied to any AERONET + Satellite combination. In this regard, it can be used for surface reference generation at any spatial resolution and at any spectral channels measured by satellites in worldwide locations.

The SYREMIS/GRASP LEO+LEO synergy was globally evaluated on Sentinel-5p/TROPOMI, Sentinel-3A, -3B/OLCI instruments. The LEO+GEO synergy was extended with HIMAWARI/AHI sensors. In such synergy the information from the instruments with richest information content transfer to the instruments with lower one. In combination with and proper constraining spectral, spatial and temporal aerosol/surface variability, this results in increased performance of AOD, aerosol size and absorption properties retrieval and more consistent surface BRDF characterization.

In this talk we will discuss physical basis and main principle of passive multi-sensor synergy for advancing aerosol and surface characterisation, which can be applied to diverse synergetic satellite constellations.