Synthetic Validation of a GRASP Inversion Strategy Combining CE376 Lidar and CE318-T Photometer Measurements

The complex interactions of atmospheric aerosols with solar radiation and clouds represent a major source of uncertainty in atmospheric effective radiative forcing. These interactions are driven by aerosol optical and microphysical properties and depend critically on their spatial and vertical distribution. Consequently, obtaining accurate measurements with high spatial and temporal resolution is essential for improving climate models and reducing uncertainty. To this end, remote sensing techniques are employed globally from both space-borne and ground-based platforms. While space-borne instruments provide superior spatial and temporal coverage, ground-based techniques offer more limited spatial extent but higher measurement quality and precision.

However, both techniques measure optical quantities that depend on aerosol properties, which necessitates the use of inversion algorithms to retrieve these underlying characteristics. Among ground-based techniques, sun-sky-lunar photometers and lidars are two of the most prominent instruments, and inversion methods have been developed and applied to each separately. For instance, the AERONET inversion algorithm for photometers employs both aerosol optical depth (AOD) and sky radiance measurements at multiple viewing geometries across four wavelengths to retrieve the aerosol volume size distribution and complex refractive index. For lidars, methods as the Klett-Fernald and Raman enable the retrieval of vertically resolved aerosol properties. Nevertheless, each technique has inherent limitations: while sun photometer inversions can retrieve both optical and microphysical properties, they lack vertical resolution due to the column-integrated nature of their measurements. Conversely, lidar-based methods provide excellent vertical resolution but often rely on assumptions or ancillary data.

Consequently, the combined use of photometers and lidar systems offers the potential to provide complete and robust characterization of vertically resolved aerosol properties. To this end, numerous inversion algorithms have been developed that combine sun-sky-lunar photometers with low-power lidar systems to retrieve both columnar and vertically resolved aerosol optical and microphysical properties. This approach benefits from enhanced spatial and temporal coverage due to the widespread availability of both instrument types. Among these algorithms, GARRLiC (Now integrated into the GRASP algorithm) stands out as a flexible option capable of being applied to a wide range of photometers and lidar configurations, providing both intensive and extensive aerosol properties for two aerosol modes in the atmospheric column and with vertical resolution when the lidar system includes multiple wavelengths or polarization channels.

This study presents a synthetic evaluation of a GRASP-based inversion combining AOD and sky radiance observations (440, 675, 870, and 1020 nm) from a CE318-T sun-sky-lunar photometer with dual-wavelength elastic lidar measurements (532 and 808 nm) from a CE376 micro-pulse lidar to retrieve both columnar and vertically resolved optical and microphysical properties for two aerosol modes (fine and coarse). Our methodology involves generating synthetic observations from selected aerosol properties, adding realistic noise based on reported uncertainties, performing GRASP inversions, and comparing retrieved parameters with input values under diverse aerosol loadings and viewing geometries. This framework provides comprehensive characterization of the algorithm's performance, accuracy, and sensitivity, validating the method for operational application.