An Objective Method for Assessment of Langley Calibrations of Sunphotometer

Atmospheric aerosols play a critical role in many aspects of climate, environmental, and human health problems and have attracted increasing attention in recent years. It is of great importance to retrieve aerosol optical depths (AODs) accurately. Calibration of the direct solar radiation data obtained by sun photometers is a key step in obtaining accurate AODs. Unfortunately, there are significant errors in the AODs obtained for heavily polluted regions, such as most parts of China, by the use of incorrect solar calibration constants determined by the widely used Langley method. To overcome this problem, in this paper, we propose a method for performing an objective assessment of Langley calibrations that combines the surface shortwave radiation observed by a pyrheliometer with the SBDART atmospheric radiative transfer model. We verified the feasibility of the proposed method using two and a half years of solar radiation data obtained by the multi-filter rotating shadowband radiometer and the normal incidence pyrheliometer. The results showed that the accuracy of the AOD retrievals was improved and the solar calibration constants could be constrained to a set of stable values.