The aerosol direct radiative forcing over the Beijing metropolitan area from 2004 to 2011

Ground-based and satellite observations indicated heavy anthropogenic aerosol loading over the Beijing metropolitan area from 2004 to 2011. The aerosol optical properties and radiative forcing had strong seasonal variations and large differences in the city, the suburban and the regional background. The annual means of the aerosol optical depth (AOD) at 550 nm wavelength were 0.40±0.04 in the Beijing City, 0.51±0.05 in the Xianghe suburban and 0.20±0.02 in the background, while the annual means of Ångström exponent were 1.28±0.07, 1.16±0.06 and 0.99±0.05, respectively. The annual means of single scattering albedo (SSA) and aerosol direct radiative forcing at the top of atmosphere (TOA) at 550 nm wavelength were 0.89±0.09 and −0.013±0.002 W/m2 in the Beijing City, 0.87±0.09 and −0.004±0.003 W/m2 in the Xianghe suburban, and 0.88±0.08 and 0.002±0.001 W/m2 in the background. The seasonal maximums of TOA were −0.007±0.001 W/m2, 0.012±0.003 W/m2 and 0.013±0.003 W/m2 in Beijing City, the suburb and the background in spring, respectively. The seasonal minimums of TOA were −0.022±0.003 W/m2, −0.014±0.001 W/m2 and −0.010±0.002 W/m2 in winter. There were no significant weakening trend of the aerosol direct radiative forcing in the large urban area in the period.