Application of XBAER aerosol optical depth retrieval algorithm to hyperspectral EnMAP satellite data

Abstract. New high-resolution hyper and multispectral satellite instruments enable the retrieval of aerosol optical depth (AOD) at spatial resolutions of tens of meters. The eXtensible Bremen AErosol Retrieval (XBAER) AOD retrieval algorithm has previously been developed for use with Ocean and Land Colour Instrument (OLCI) and MEdium Resolution Imaging Spectrometer (MERIS) radiance data. With the intention of later modifying XBAER to use the full 30 m spatial resolution data from the Hyper-Spectral Imager (HSI) on board the Environmental Mapping and Analysis Program (EnMAP) satellite, the present study investigates how HSI data compare to OLCI data. For the bands of interest, top of atmosphere reflectances generally compare well (R>0.9), the intercept of the best fit line is less than 0.05 from the origin, and the slope is less than 0.1 from 1. However exceptions exist and these are explained as the result of differences in the spectral response functions of the instruments in the region of the spectrum around the O2 A-Band absorption feature, or as a result of differences in the viewing geometry of the satellites which produces differing bidirectional reflectance distribution function (BRDF) effects. XBAER is then used to retrieve OLCI and HSI surface reflectance (SRF) and AOD. For SRF the comparison between OLCI and HSI yields R=0.953, best fit intercept =0.003 and best fit slope =1.082. The respective comparison for AOD yields R=0.809, best fit intercept =0.153 and best fit slope =0.785. These comparisons are then separated by surface type and insights are gained into the performance of the algorithm. Finally, the unmodified XBAER algorithm is run using the full spatial resolution HSI data. Plumes from biomass-burning are identified in a single scene, and a comparison with AErosol RObotic NETwork (AERONET) AOD is performed for multiple scenes, achieving R=0.631.