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.
Data
PARASOL global data for 2008 processed by RemoTAP can be found here:
PARASOL global data for Dec 2007 – Dec 2008 processed by GRASP can be found here:
GRASP
GRASP is a state-of-the-art retrieval algorithm for generating enhanced aerosol and surface products from diverse space-borne and ground-based measurements (Dubovik et al. 2011, 2014, 2021). GRASP retrieval is implemented as a highly advanced statistically optimized fitting of all available observations. For example, GRASP uses an innovative multi-pixel concept in which the fitting is realized simultaneously for a large group of “pixels” (coordinated observations). This principle is especially useful for improving satellite retrievals where it allows to benefit from known a priori limitations on space and time variability of different surface and aerosol parameters. This helps reliable discrimination between aerosol and surface contributions in observation that is generally known as a challenging issue.
The GRASP multi-pixel retrieval concept has already been successfully applied to the observations of different single space-borne instruments: polar-orbiting like POLDER/PARASOL, MERIS, AATSR/ENVISAT, OLCI/Sentinel-3, TROPOMI/S-5p and geostationary, for example, Himawari, satellites. Moreover, the synergetic approaches were successfully approved on the synergy of MERIS and AATSR measurements (ESA CAWA-2 project) as well as on the synergy of the ground-based and satellite (AERONET+OLCI, AERONET+ TROPOMI/Sentinel-5p etc retrieval) measurements (ESA GROSAT project).
GRASP algorithm is planned to be used for aerosol and surface near-real-time characterization from future space-borne polarimetric mission like 3MI, CO2M MAP, GAPMAP etc.
References
RemoTAP
The Remote sensing of Trace gas and Aerosol Products (RemoTAP) algorithm is a flexible algorithm that can be used for retrieval of optical and microphysical aerosol properties from Multi-Angle Polarimeter (MAP) measurements, retrieval of aerosol properties and trace gas columns (e.g. CO2, CH4) from spectroscopic measurements, or for joint retrievals using multiple instruments. RemoTAP is based on iterative fitting a linearized radiative transfer model to the measurements of intensity and polarization of light reflected by the Earth atmosphere and surface. It has large flexibility in the definition of parameters to be retrieved and allows retrievals over land, ocean, and clouds. The RemoTAP software has strong heritage in application to POLDER-3/PARASOL measurements, airborne measurements of the Research Scanning Polarimeter (RSP), SPEX airborne, and airMSPI. RemoTAP is also planned to be used for operational processing of the SpexOne/PACE data. RemoTAP data products from PARASOL have been used for data assimilation / inverse modeling, quantification of the Direct Radiative Effect of Aerosols (DREA), and the Radiative Forcing due to aerosol-cloud interactions (RFaci).
RemoTAP references