Within the project HARPOL (Harmonizing and advancing retrieval approaches for present and future polarimetric space-borne atmospheric missions), a detailed comparison is performed of two algorithms for the retrieval of aerosol properties from Multi-Angle Polarimetric (MAP) measurements: the Generalized Retrieval of Aerosol and Surface Properties (GRASP) algorithm, developed at the Laboratory of Atmospheric Optics (LOA) of the University of Lille and the GRASP-sas company, and the Remote Sensing of Trace gases and Aerosol Properties (RemoTAP) algorithm developed at SRON – Netherlands Institute for Space Research. HARPOL is funded through the ESA program EO Science for Society.
Atmospheric aerosol particles strongly influence climate by scattering and absorbing light (direct forcing) and by changing cloud properties (indirect forcing). The corresponding radiative forcing represents one of the most uncertain radiative forcing terms as reported by the Intergovernmental Panel on Climate Change (IPCC). To improve our understanding of the effect of aerosols on climate and air quality, measurements of aerosol chemical composition, size distribution, optical properties like Aerosol Optical Depth (AOD) and Single Scattering Albedo (SSA), as well as the aerosol height profile are of crucial importance. It has been demonstrated by studies on synthetics measurements, airborne measurements, and space-borne measurements that Multi-Angle Polarimetric (MAP) measurements are needed to provide information about detailed aerosol properties like size distribution, refractive index, SSA, in addition to the AOD.
The only MAP instrument that has provided a multi-year data set (2005-2013) in the past has been the French POLDER-3 instrument on the PARASOL mission. Now space agencies realize the large potential of MAP instrumentation, in the 2020s several of such instruments will be launched, e.g. 3MI on METOP-SG (ESA-2023), SPEXone and HARP-2 on PACE (NASA-2024), and a MAP on the CO2-Monitoring mission (ESA-2025) and Atmos (NASA-2028). To cope with the increased information content on aerosols of MAP instrumentation and to assess the climatic effect of aerosols, new tools for retrieval need to be (further) developed. So far, this development has lagged behind the instrument development, which is the reason for the under-exploitation of the existing POLDER-3/PARASOL data sets.
GRASP and RemoTAP
Currently, GRASP and RemoTAP are the only two algorithms that have demonstrated capability at a global scale to exploit the rich information content of MAP measurements. Both algorithms show good performance against ground based AERONET measurements and already important scientific advancement has been made using the corresponding data products. Nevertheless, when looking at global maps, significant differences are apparent between the two algorithms. In order to improve retrieval products from PARASOL and the upcoming missions containing MAP instrumentation (3MI/METOP-SG, SPEXone/PACE, HARP2/PACE, MAP/CO2M) it is essential to understand the reasons for the differences between the GRASP and RemoTAP algorithms. Therefore, in this project we perform a comparison between the two algorithms. We expect this will lead to optimized algorithm choices for both algorithms leading to better aerosol products and error characterization.