Abstract
This study presents a validation of aerosol optical thickness (AOT) and integrated water vapour (IWV) products provided by the European Space Agency (ESA) from multi-spectral imager (MSI) measurements on board the Sentinel-2 satellite (ESA-L2A). For that purpose, data from 94 Aerosol Robotic Network (AERONET) stations over Europe and adjacent regions, covering a wide geographical region with a variety of climate and environmental conditions and during the period between March 2017 and December 2018 have been used. The comparison between ESA-L2A and AERONET shows a better agreement for IWV than the AOT, with normalized root mean square errors (NRMSE) of 5.33% and 9.04%, respectively. This conclusion is also reflected in the values of R2, which are 0.99 and 0.65 for IWV and AOT, respectively. The study period was divided into two sub-periods, before and after 15 January 2018, when the Sentinel-2A spectral response functions of bands 1 and 2 (centered at 443 and 492 nm) were updated by ESA, in order to investigate if the lack of agreement in the AOT values was connected to the use of incorrect spectral response functions. The comparison of ESA-L2A AOT with AERONET measurements showed a better agreement for the second sub-period, with root mean square error (RMSE) values of 0.08 in comparison with 0.14 in the first sub-period. This same conclusion was attained considering mean bias error (MBE) values that decreased from 0.09 to 0.01. The ESA-L2A AOT values estimated with the new spectral response functions were closer to the correspondent reference AERONET values than the ones obtained using the previous spectral response functions. IWV was not affected by this change since the retrieval algorithm does not use bands 1 and 2 of Sentinel-2. Additionally, an analysis of potential uncertainty sources to several factors affecting the AOT comparison is presented and recommendations regarding the use of ESA-L2A AOT dataset are given.
Highlights
Aerosols and water vapour are two of the most important atmospheric constituents of the Earth-atmosphere system because they attenuate solar radiation through scattering and absorption processes, and modulate terrestrial radiation mainly by absorption and emission
The effect of aerosols and water vapour on short wave (SW) irradiance was analysed by Obregón et al [1], who showed that the combined effect of those quantities was not the sum of their individual effects
The aerosol load in the atmospheric column (AOT) and the total water vapour integrated over an atmospheric column (IWV) are key quantities for atmospheric correction of satellite radiation measurements, and their reliability greatly influence the accuracy of the surface reflectance satellite derived products [4]
Summary
Aerosols and water vapour are two of the most important atmospheric constituents of the Earth-atmosphere system because they attenuate solar radiation through scattering and absorption processes, and modulate terrestrial radiation mainly by absorption and emission. AOT and IWV data can be obtained from sunphotometer measurements These are representative of point measurements at the surface, providing local information corresponding to the specific locations where the measurement stations are located and, cannot provide the spatial coverage required for global monitoring. Most AOT and IWV data derived from observations with these instruments have already been validated with data obtained from the Aerosol Robotic Network (AERONET) sunphotometer measurements [4,5,6,7,8,9,10,11,12]. Satellite AOT products that present high spatial resolution are of great interest since they can be used to study local sources of aerosols in urban areas [15] and monitor local air pollution [16].
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