Synergy in GRASP of space multiwavelength multi-angle polarimetry and lidar measurements for vertical profiles of aerosol optical and microphysical properties

During the last years there have been a great advance in the development of satellite missions for Earth Observation. Most of them rely on passive remote sensing measurements, particularly on multiwavelength multi-angular polarimetry measurements (MAP). Upcoming missions such as Sentinel-5 will also deploy MAP and there are even private initiatives to expand space MAP measurements. Although MAP measurements have been proven to be ideal for expanding our knowledge in aerosol optical and microphysical properties (Dubovik et al., 2019), they provided very limited information of the aerosol properties vertically-resolved. On the other hand, multiwavelength lidar measurements are capable of providing accurate aerosol vertical profiles but face with limitations in the retrieval of aerosol optical and microphysical properties because of the limited information content of the stand-alone lidar measurements (Perez-Ramirez et al., 2019). Here we explore the potentiality of inverting aerosol microphysical properties vertically-resolved by combining in the Generalized Retrieval of Atmosphere and Surface Properties (GRASP – Dubovik et al., 2021) space lidar and polarimetry measurements.

We present extensive simulations of aerosol optical and microphysical properties vertical-profiles retrievals combining multiwavelength MAP and lidar measurements in GRASP, with the enhanced capability of differentiating between aerosol fine and coarse mode properties. The retrieval is pushed forward by trying to obtain 22 bins size distribution, similar to those provided by AERONET. In the simulations different mixtures of fine and coarse mode were used, varying refractive indexes from low to high absorption. We have used the HARP-like polarimetry configuration that uses the heritage of POLDER space polarimetry and is deployed in the NASA PACE mission. For lidar measurements, multiwavelength configurations are tested, from single backscattering measurements to adding additional extinction measurements. Our results show full capacity of GRASP to retrieve aerosol properties vertically-resolved differentiating between fine and coarse properties, although the more accurate results are obtained when using all lidar information. However, we found out that optimized retrieval needs of constraining surface properties, particularly because of the impact of BPDF in coarse mode retrieval. Limitations in surface retrievals can be solved with the multi-pixel approach in GRASP when applied to real missions. Finally, we present case-study of synergy retrievals from airborne measurements obtained during NASA field campaigns when AirHARP + lidar flew together. The results of the simulations serve as baseline for future space mission that will combine space lidar + polarimetry or to the synergy of different satellite missions.

References

Dubovik, O., et al., 2019: Polarimetric remote sensing of atmospheric aerosols: instruments, methodologies, results, and perspectives. J. Quant. Spectrosc. Radiat. Transfer, 224, 474-511,

Dubovik, O, et al., 2021: A comprehensive description of multi-term LSM for applying multiple a priori constraints in problems of atmospheric remote sensing: GRASP algorithm, concept and applications. Front. Remote. Sens., 2, 706851.

Perez-Ramirez, D, et al., 2019: Retrievals of aerosol single scattering albedo by multiwavelength lidar measurements: Evaluations with NASA Langley HSRL-2 during DISCOVER-AQ field campaigns. Remote. Sens. Environ., 222, 144-164.