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

摘要

The seasonal characteristics of aerosol optical properties and surface direct radiativeforcing were evaluated from ground-based solar radiation measurements under cloudlessconditions in Tsukuba,,_Japan, from April 2004 to March 2008. Optical thickness (T),single scattering albedo (w0), and asymmetry factor (g) at visible and near-infraredwavelengths were estimated from the direct and diffuse spectral irradiances. The radiativeforcings in total, visible, and near-infrared regions were estimated from the measureddirect and diffuse broadband irradiances. The value of T was large from spring to summerand small in winter; wo was large in summer and small in winter; and g was large in springand small in summer, but there is a possibility of underestimating g due to the uncertaintyin the estimated size distribution. The radiative forcing (AF24) had almost the sameseasonal changes as those of T. However, the radiative forcing efficiency (AFE24), definedas AF24 per unit T, was not consistent with wo and g. The fractional radiative focingefficiency (FAFE24) was defined as a ratio of AFE24 to the solar irradiance at the top of theatmosphere, and a close correlation was found among wo, g, and FAFE24. The seasonalradiative effects of aerosols were clarified by FAFE24 . These evaluated T, wo, g, andAF24 were compared with those at other sites over East Asia. The values of T,g, andAF24 in Japan were smaller than those over the continent at all seasons. These differencessuggest that the aerosols in Japan have different compositions from those over thecontinent.