Aerosol impact on the brightening in Japan

Abstract

The long‐term changes in aerosol optical depth (AOD) and single scattering albedo (SSA) at 0.75 μm from the 1970s to the 2000s at 14 sites in Japan were estimated from the direct and diffuse irradiances measured by ground‐based broadband radiometers under clear‐sky conditions. Noticeable changes in AOD and SSA were seen: AOD decreased by 0.02, and SSA increased by 0.21. The ratio of surface solar irradiance to solar irradiance at the top of the atmosphere under clear‐sky conditions, calculated from the estimated AOD and SSA, showed apparent brightening of +0.05, of which +0.01 was attributed to a decrease of AOD and +0.04 to an increase of SSA. The cloud forcing of the surface solar irradiance, estimated as the difference between the surface solar irradiance measured under cloudy‐sky conditions and that under clear‐sky conditions, showed no significant trend. These findings suggest that the brightening in Japan has been caused by changes in aerosol optical properties, especially SSA, rather than by cloud changes.