Aerosol optical properties and radiative effects in the Yangtze Delta region of China

One year's worth of aerosol and surface irradiance data from September 2005 to August 2006 were obtained at Taihu, the second supersite for the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST‐AIRE). Aerosol optical properties derived from measurements by a Sun photometer were analyzed. The aerosol data were used together with surface irradiance data to quantitatively estimate aerosol effects on surface shortwave radiation (SWR) and photosynthetically active radiation (PAR). The annual mean aerosol optical depth at 500 nm is 0.77, and mean Ångstrom wavelength exponent is 1.17. The annual mean aerosol single scattering albedo and mean aerosol asymmetry factor at 440 nm are 0.90 and 0.72, respectively. Both parameters show a weak seasonal variation, with small values occurring during the winter and larger values during the summer. Clear positive relationships between relative humidity and aerosol properties suggest aerosol hygroscopic growth greatly modifies aerosol properties. The annual mean aerosol direct radiative forcing at the surface (ADRF) is −38.4 W m−2 and −17.8 W m−2 for SWR and PAR, respectively. Because of moderate absorption, the instantaneous ADRF at the top of the atmosphere derived from CERES SSF data is close to zero. Heavy aerosol loading in this region leads to −112.6 W m−2 and −45.5 W m−2 reduction in direct and global SWR, but 67.1 W m−2 more diffuse SWR reaching the surface. With regard to PAR, the annual mean differences in global, direct and diffuse irradiance are −23.1 W m−2, −65.2 W m−2 and 42.1 W m−2 with and without the presence of aerosol, respectively.