From solar radiation measurements to optical properties: 1998-2008 trends in Japan

Trends from April 1998 to March 2008 in surface global irradiance, aerosol surface direct radiative forcing, and aerosol optical properties (optical thickness, single scattering albedo, and asymmetry factor) were investigated based on measurements of the surface solar radiation under clear skies at Tsukuba, Japan. The optical properties were estimated by optimizing the external mixture of four assumed aerosols (water soluble, soot, sea salt, and dust) to the surface direct and diffuse irradiances in visible and near infrared regions. The surface global irradiance was increased by an increase of single scattering albedo, not by a decrease of optical thickness. Furthermore, the increasing trend of single scattering albedo may be related to the reduced emission of black carbon in East Asia.