Long‐term trends of atmospheric absorbing and scattering optical depths over China region estimated from the routine observation data of surface solar irradiances

Abstract

Equivalent optical depths of a standard continental aerosol model are defined as the indexes for atmospheric optical depths of absorption and scattering (ODA and ODS, respectively) under clear‐sky condition and can be calculated with the routinely observed data of broadband surface solar irradiances. The ODS by definition is a proxy of the scattering component of atmospheric aerosol optical depth (AOD) and is shown to be largely free from influences of systematic errors of the radiation observing instruments and uncertainties in atmospheric absorption. The ODA accounts for contributions of aerosols, water vapor, and other gases with profiles deviating from the standard atmosphere model. Applied to the data from 36 observatories of China, from 1957 to 2007, the method provides the long‐term variations of clear‐sky atmospheric absorption and scattering, which are qualitatively compared with other AOD estimations with comparable regional and temporal coverage. The results show the method reasonably separates the absorption and scattering effects and captures the general features of their seasonal variation. The radiation dimming trend over China region during about 1970–1990 is shown to be associated with the increasing ODA and ODS. After 1990, there is not a significant trend in ODS but the ODA has been slightly decreased.