Seasonal characteristics of aerosol radiative effect estimated from ground-based solar radiation measurements in Tsukuba, Japan

The seasonal characteristics of aerosol optical properties and surface direct radiative forcing were evaluated from ground‐based solar radiation measurements under cloudless conditions in Tsukuba, Japan, from April 2004 to March 2008. Optical thickness (τ), single scattering albedo (ω0), and asymmetry factor (g) at visible and near‐infrared wavelengths were estimated from the direct and diffuse spectral irradiances. The radiative forcings in total, visible, and near‐infrared regions were estimated from the measured direct and diffuse broadband irradiances. The value of τ was large from spring to summer and small in winter; ω0 was large in summer and small in winter; and g was large in spring and small in summer, but there is a possibility of underestimating g due to the uncertainty in the estimated size distribution. The radiative forcing (AF24) had almost the same seasonal changes as those of τ. However, the radiative forcing efficiency (AFE24), defined as AF24 per unit τ, was not consistent with ω0 and g. The fractional radiative focing efficiency (FAFE24) was defined as a ratio of AFE24 to the solar irradiance at the top of the atmosphere, and a close correlation was found among ω0, g, and FAFE24. The seasonal radiative effects of aerosols were clarified by FAFE24. These evaluated τ, ω0, g, and AF24 were compared with those at other sites over East Asia. The values of τ, ω0, g, and AF24 in Japan were smaller than those over the continent at all seasons. These differences suggest that the aerosols in Japan have different compositions from those over the continent.