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

Abstract. The global AODs for the one of few single-angle polarimeters currently in orbit, Particulate Observing Scanning Polarimeter (POSP) has been proposed. We have tested the retrievals from the early stages of on-orbit operation and achieved high accuracy, but we still lack an understanding of the retrieval accuracy over longer time scales. The systematic validation and analysis of POSP retrievals are imperative to ensure the reliability of AOD products and to provide scientific references for further enhancement. To this end, POSP AOD products have been compared with Aerosol Robotic Network (AERONET) measurements at over 276 sites. The results from 19314 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) is 78.5 %. The error analysis shows 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. Compared with MODIS AOD products, the accuracy of POSP AOD was higher than that of the Deep Blue product except over barren and forest, and higher than the Dark Target product over all surface types. In addition, the spatial variability characteristics of the global mean AOD has been analyzed. POSP AOD shows an underestimation compared to MODIS BD AOD over North Africa and the Arabian Peninsula, while it overestimates in other regions. However, both POSP AOD and MODIS AOD indicate that high aerosol loading mainly occurs over northern South America, central Africa, northern India, and northeastern China.