Global validation of the Particulate Observing Scanning Polarimeter (POSP) Aerosol Optical Depth products over land

Abstract. A global AOD retrieval product from the Particulate Observing Scanning Polarimeter (POSP) has been proposed. We have validated the AOD from the early stages of on-orbit operation and achieved high accuracy, but we lack an understanding of the retrieval accuracy over longer time scales from a systematic validation and analysis of POSP-retrieved AOD. The objectives of the current study are: (1) To ensure the reliability of POSP AOD products and explore the potential factors influencing their performance; (2) To provide a valuable reference for the enhancement of these products in future developments. To achieve these objectives, POSP AOD products have been validated using Aerosol Robotic Network (AERONET) measurements (over 276 sites) as reference. The results from 19 314 collocations show a high accuracy, with correlation coefficients (R) of 0.914, a root mean square error (RMSE) of 0.085, and the fraction within the expected error (EE) of 78.5 %. In addition, the validation at individual sites indicates that the performance of POSP products is better than that of MODIS (Deep Blue and Dark Target) AOD. Further error analysis indicates that the accuracy of POSP AOD exhibits a clear seasonal variation, being lower in the autumn and winter than in the spring and summer. Additionally, the uncertainty in AOD increases as NDVI decreases. Globally, the spatial variability of the quarterly averaged AOD has been analysed. The results show that the validation metrics of POSP and MODIS AOD are comparable. However, over North Africa and the Arabian Peninsula, POSP AOD is in better agreement with MAIAC AOD, while over other regions, it is in better agreement with DB/DT AOD.