Optimizing GRASP retrieval configurations for space-borne multi-angular polarimetric measurements

Harmonising aerosol approaches between global atmospheric models and satellite remote sensing retrievals is essential for improving the consistency and reliability of aerosol products. Within the CAMEO framework, this study investigates the impact of aerosol chemical composition and size distribution (SD) parameterisation on satellite aerosol retrievals using POLDER/PARASOL polarimetric observations, with validation against AERONET measurements over the full 2008 year. The baseline retrieval configuration consists of five lognormal size bins and a predefined aerosol chemical composition model (Fine mode: Black Carbon, Brown Carbon, Quartz and soluble species and Coarse mode: Iron Oxide, Quartz and soluble species). While this configuration provides robust performance for aerosol optical depth (AOD) and Ångström exponent, limitations remain in reproducing spectral single scattering albedo (SSA) and detailed SD structure.

To address these issues, a series of sensitivity experiments were conducted. These include harmonising the complex refractive index with CAMS aerosol chemistry, incorporating CAMS organic matter in both fine and coarse modes, increasing SD flexibility through an eight-bin representation, and adjusting inversion constraints under different aerosol loading conditions. Results demonstrate that harmonisation between CAMS and satellite retrieval assumptions improves agreement with AERONET, particularly for SSA, highlighting the mutual advantages of harmonisation.

The analysis shows that increasing SD complexity beyond five bins has only a minor impact on retrieved optical properties from PARASOL. This indicates that the information content of PARASOL measurements does not fully support highly complex SD characterisations, and that retrieval model complexity should be adapted to sensor capabilities. Similarly, including coarse-mode dust produces limited changes in optical retrievals, suggesting a reduced sensitivity of PARASOL to coarse aerosol properties.

These findings highlight that future multi-angle polarimetric sensors, such as 3MI, with extended spectral coverage from the visible to shortwave infrared, are expected to provide stronger sensitivity to coarse particles and improved aerosol characterisation. Overall, this study provides evidence that harmonisation between CAMS and remote sensing approaches strengthens the consistency of aerosol retrievals, while emphasizing the need to balance model complexity with observational information content.