Estimation of Global PM2.5 from Polarimetric Remote Sensing (POLDER-3/GRASP) and CAMS Reanalysis Data

Ambient fine particulate matter (PM2.5, mean aerodynamic radii less than 2.5µm) is a leading cause of millions of premature deaths annually, linked to lung cancer, pulmonary inflammation, and cardiopulmonary mortality. Therefore, accurate global measurements of PM2.5 are essential for epidemiological studies, designing air quality control strategies, and improving air quality forecasting.

The POLDER-3/GRASP multi-angular polarimetric products enable advanced retrievals of aerosol properties, such as size distribution, refractive index, particle composition, and aerosol layer height (ALH), facilitating the estimation of ground-level PM2.5. However, POLDER has limited sensitivity to ALH and relies on assumed aerosol vertical profiles, which may introduce uncertainties that propagate into PM2.5 estimations. Complementarily, the CAMS reanalysis data provides global coverage with high temporal resolution, offering detailed information on tropospheric aerosol distributions and vertical structures.

In this study, we first validate global ground-level PM2.5 estimations from POLDER/GRASP products using detailed columnar aerosol properties, such as size distribution, refractive index or chemical composition and ALH, against observational data from the US Environmental Protection Agency (EPA). Next, CAMS reanalysis aerosol products are integrated into the POLDER-3/GRASP framework to enhance the estimation accuracy. This integration combines columnar aerosol properties retrieved from POLDER/GRASP at higher spatial resolution with detailed vertical profiles from CAMS, notably ground-level aerosol fractions. Finally, we analyze the improvements achieved by comparing the integrated results with US EPA ground-based measurements. This synergistic approach investigates the potential of combining POLDER/GRASP retrievals with CAMS comprehensive vertical aerosol structures to improve global air quality assessments, paving the way for advanced monitoring capabilities from future multi-angular polarimeter missions, such as PACE/SPEX, PACE/HARPOL-2, and 3MI.