Extended validation and evaluation of the OLCI–SLSTR SYNERGY aerosol product (SY_2_AOD) on Sentinel-3
Authors: Sogacheva Larisa , Denisselle Matthieu , Kolmonen Pekka , Virtanen Timo H. , North Peter , Henocq Claire , Scifoni Silvia , Dransfeld Steffen 
Published: September 2022

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 aboard the Copernicus Sentinel-3A (S3A) and Sentinel-3B (S3B) satellites. Validation covers period from 14 January 2020 to 30 September 2021. Several approaches, including statistical analysis, time series analysis, and comparison with similar aerosol products from the other spaceborne sensor, the Moderate Resolution Imaging Spectroradiometer (MODIS), were applied for validation and 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 AOD (syAOD) retrieved at
550 nm is good: for S3A and S3B, Pearson correlation
coefficients with the Maritime Aerosol Network (MAN) component of the
AErosol RObotic NETwork (AERONET) are 0.88 and 0.85, respectively; 88.6 % and 89.5 % of pixels fit into the MODIS error envelope (EE) of ±0.05 ± 0.2 × AOD. Over land, correlation coefficients with AERONET AOD (aAOD) are 0.60 and
0.63 for S3A and S3B, respectively; 51.4 % and 57.9 % of pixels fit into MODIS EE. Reduced performance over land is expected since the surface
reflectance and angular distribution of scattering are 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 with the χ2 test indicates
that syAOD prognostic uncertainties (PU) are slightly underestimated (χ2 = 3.1); if outliers are removed, PU describes the syAOD error well
(χ2 = 1.6). The regional analysis of the Angström exponent, which relates to the
aerosol size distribution, shows spatial correlation with expected sources.
For 40 % of the matchups with AERONET in the Northern Hemisphere (NH) and for 60 % of the matchups in the Southern Hemisphere (SH), which fit into the AE size range of [1, 1.8], an offset between SY_2 AE
(syAE) and AERONET AE (aAE) is within ±0.25. General overestimation
of low (< 0.5) syAE and underestimation of high (> 1.8)
syAE results in high (0.94, globally) overall bias. Good agreement (bias < 0.03) was observed between Sy_2 fine-mode AOD (syFMAOD) and AERONET fine-mode AOD (aFMAOD) for
aFMAOD < 1. At aFMAOD > 1, syFMAOD is considerably
underestimated (by 0.3–0.5 in different aFMAOD ranges) in the NH. In the SH, only a few aFMAOD values above 1 are measured. The fine-mode fraction (FMF) in the SY_2 AOD product (syFMF) in the range of [0, 0.7] is
overestimated; the positive offset of 0.3–0.5 for low (< 0.25) FMF
gradually decreases. Differences between the annual and seasonal AOD values from SY_2
and MODIS (mod) Dark Target and Deep Blue products are within 0.02 for the
study area (30∘ S–60∘ N, 80∘ W–45∘ E). The agreement is better over ocean; however, a difference up to 0.6 exists between syFMF and modFMF. Over bright land surface (Saharan desert) the difference in AOD between the two products is
highest (up to 0.11); 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.