Evaluation of CAMS Reanalysis Aerosol Optical Properties Against POLDER/GRASP Retrievals: Insights into Fine and Coarse Mode Aerosol Characteristics

Discrepancies in aerosol representation in chemistry-transport models (CTMs) arise from uncertainties in emission sources. These are exacerbated by nonlinear physico-chemical processes and transport. Assumptions about optical properties and limitations in satellite retrievals further contribute to the problem. Despite their significance, such disparities remain largely unaddressed. Under the European Commission’s CAMS Model Evolution (CAMEO) project, we conducted an intercomparison of aerosol physical properties between CAMS reanalysis and POLDER/GRASP products. POLDER, a multi-angular polarimeter onboard PARASOL, operated from 2008 to 2013. This study examines adjustments made to the GRASP retrieval algorithm to address these disparities in the CAMS CTM.

Our findings show that CAMS aerosol optical depth (AOD) at multiple wavelengths closely aligns with POLDER/GRASP data when CAMS refractive indices are incorporated into the retrieval algorithm. While CAMS assimilates MODIS 550 nm AOD data, it also produces reliable AODs across other wavelengths. Validation at AERONET stations, focusing on the year 2008, demonstrated reasonable agreement for biomass-burning aerosols, desert dust, and anthropogenic pollutants. However, uncertainties remain in estimating single scattering albedo (SSA). Furthermore, CAMS fine-mode AOD aligns well with POLDER/GRASP retrievals. In contrast, CAMS coarse-mode AOD shows a weak positive linear correlation with POLDER/GRASP. From July to September, CAMS reanalysis exhibits a non-negligible bias at sites dominated by biomass burning and anthropogenic emissions, confirmed by AERONET ground data. These results highlight persistent uncertainties in representing black carbon, brown carbon, and organic matter in both CTMs and satellite retrievals. Over dust-dominated stations, fine-mode dust exhibits light-absorbing properties, while coarse-mode dust shows minimal spectral variation. Such findings, which vary by location and season, provide crucial insights for CTM development.

This work supports future advancements in aerosol modelling and lays the groundwork for exploring the capacities of upcoming multi-angular, multi-viewing polarimetric missions, such as PACE/HARPOL-2, PACE/SPEX, and 3MI. These efforts will improve data assimilation and enhance the accuracy of CAMS reanalysis datasets.