Within the project HARPOL (Harmonizing and advancing retrieval approaches for present and future polarimetric space-borne atmospheric missions), a detailed comparison has been 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. The HARPOL study was running between February 2021 and December 2022.

Background

Atmospheric aerosol particles affect Earth’s climate as they scatter and absorb radiation and act as condensation nuclei for cloud droplets and ice crystals. 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)  Multi-Angle Polarimetric (MAP) satellite measurements provide the richest set of information on these aerosol properties.

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 AOS (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. During the HARPOL project we have performed an extensive comparison between GRASP and RemoTAP, applied to PARASOL observations. Based on this comparison both algorithms have been substantially improved.