The relationship between the radiance ratio (radiance at wavelength 450 nm to 650 nm) and aerosol optical depth (AOD) is analyzed in this paper by numerical simulation and a "LUT" (look-up table) approach is then presented for the retrieval of AOD from the radiance ratio. In this LUT approach, the AOD retrieval error depends mainly on the assumption of aerosol types. From the preliminary simulation, a typical error of 15%–20% in AOD obtained from the radiance ratio is estimated, due to neglecting changes in the ground albedo and background atmosphere. At its worst, the AOD error reached a maximum of around 50%, which will be refined in the future. In the latter part of the paper, comparisons are made between AOD from the imager and from the CE-318 sun photometer, both located at Xianghe observatory in Hebei Province (39.75° N, 116.96° E). This field experiment shows that AOD from the imager is highly correlated with AOD from the sun photometer, with a correlation coefficient of 0.95 and an average retrieval error of around 7%. A contrast experiment confirms the feasibility of retrieving AOD from all-sky images, but more analysis and future research are required to improve the accuracy.
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