Retrieval of Aerosol Optical Thickness with Custom Aerosol Model over Chiba Area from Satellite Images

Abstract:

The retrieval of aerosol optical thickness (AOT) from remotely-sensed data relies on the adopted aerosol model. However, creating the best suitable aerosol model is still a difficult task, and only few studies have been devoted to optimize aerosol models. Facing the unsolved problem, we propose a method for the visibility-derived AOT estimation from daytime satellite images with a custom aerosol model over Chiba area (35.62°N, 140.10°E), which is located in greater Tokyo metropolitan area in Japan. Different from conventionally-used typical aerosol models for the boundary layer (rural, urban, maritime), we create a custom aerosol model by using SKYNET sky-radiometer observation data of aerosol volume size distribution and refractive indices, coupled with spectral response functions (SPFs) of satellite visible bands to alleviate the widely range of path-scattered radiance. We utilize the radiative transfer code 6SV to implement the radiative transfer calculation based on the created custom aerosol model. Both ground-based and satellite-based measurements conducted under clear-sky conditions with minimum aerosol loading are used for the determination of the surface albedo, so that the 6SV simulation yields a well-defined relation between total radiance and surface albedo. We make look-up tables (LUTs) pixel-by-pixel over Chiba area for the custom aerosol model to retrieve the AOT distribution based on the surface albedo. A specific place has the same surface optical properties at the same observation time. Therefore, such a reference of surface albedo generated from clear-sky conditions, in turn, can be employed to retrieve the spatial distribution of AOT on both clear and relatively turbid days. The absolute value for the errors of AOT retrieved using the custom aerosol model is found to be 4% lower than conventionally-used typical aerosol models, indicating that our method yields substantially better performance. It is demonstrated that custom aerosol models created by the method proposed in this study can be used to establish a good reference for the analysis of various visible satellite images such as Himawari-8/AHI or Landsat-8/OLI to evaluate the AOT distribution over the Chiba area and potentially over similar urban areas.