Reply on RC3

Abstract

We present the first extended validation of a new synergy global aerosol product (SY_2_AOD) which is based on synergistic use of data from the Ocean and Land Color Instrument (OLCI), and the Sea and Land Surface Temperature Radiometer (SLSTR) sensors onboard the Copernicus Sentinel-3A (S3A) and Sentinel-3B (S3B) satellites. Several approaches, including statistical analysis, time series analysis, comparison with similar aerosol products from the other spaceborne sensor Moderate Resolution Imaging Spectroradiometer (MODIS), were used for validation end evaluation of S3A and S3B SY_2 aerosol products, including Aerosol Optical Depth (AOD) provided at different wavelengths, AOD pixel level uncertainties, fine mode AOD and Angström exponent. Over ocean, the performance of SY_2 retrieved AOD is good: for S3A and S3B respectively, correlation coefficients with the Maritime Aerosol Network (MAN) component of the AErosol RObotic NETwork (AERONET) are 0.88 and 0.85; 88.6 % and 89.5 % of pixels fit into MODIS error envelope of ±0.05±0.2*AOD. Over land correlation coefficients with AERONET AOD are 0.60 and 0.63 for S3A and S3B respectively; 51.4 % and 57.9 % of pixels fit into MODIS error envelope. Reduced performance over land is expected since the surface reflectance and angular distribution of scattering is higher and more difficult to predict over land than over ocean. The results are affected by a large number of outliers. Evaluation of the per-retrieval uncertainty indicated good correlation with measured error distributions. The retrieval of Angstrom exponent, related to aerosol size distribution, shows good spatial correlation with expected sources but generally overestimates AE for cases where AERONET Angstrom is low, resulting in overall high bias. Differences between the annual and seasonal AOD values from SY_2 and MODIS Dark Target and Deep blue products are within 0.02 for the study area. The agreement is better over ocean. Over bright land surface (Saharan desert) the difference between two products is highest (up to 0.11), and the sign of the difference varies over time and space. For both S3A and S3B AOD products, validation statistics are often slightly better in the Southern hemisphere. In general, the performance of S3B is slightly better.