Seasonal Variation of Aerosol Direct Radiative Forcing and Optical Properties Estimated from Ground-Based Solar Radiation Measurements

The surface direct radiative forcing and optical properties of aerosols have been analyzed from a groundbased solar radiation measurement, which was made under clear-sky conditions in Tsukuba, Japan, over two years from April 1997 to March 1999. The global and diffuse irradiances in the total and near-infrared (NIR) solar spectral regions were simultaneously measured by using two sets of the total-band and NIR-band pyranometers, respectively. The visible (VIS)-band irradiances were estimated by taking differences between the totalband and NIR-band irradiances. Spectral aerosol optical thicknesses (AOTs) in the air column were also measured, using a sun photometer. By combining the spectral AOTs and the surface diffuse irradiances, a retrieval algorithm for simultaneously estimating the effective aerosol size distribution and imaginary index of refraction ( mi) was developed. Seasonal variations of the broadband surface radiative forcings and retrieved optical properties of the columnar aerosols have been studied. A close correlation was found among these parameters with similar features of seasonal variations. In winter the columnar aerosols exhibit the minimum surface radiative forcing and a minimum AOT, but the maximum mi value of 0.04. The opposite is true in summer, when the minimum mi value of 0.02 was estimated. The surface radiative forcing in the VIS band was estimated to be almost 4 times larger than in the NIR band. The total-band aerosol forcing efficiency is defined as the change in the surface radiative forcing in the total band due to a unit increase of AOT at 500 nm. This has its largest magnitude of 2 219 Wm 22 in winter and its smallest magnitude of 2 150 Wm 22 in summer. The results suggest that the correlated seasonal variations between the aerosol radiative forcing and the optical properties may result from seasonal changes in the dominant aerosol components.