Aerosol impact on the brightening in Japan

The long-term changes in aerosol optical depth (AOD) and single scattering albedo (SSA) at 0.75 mu m 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.