Aerosol optical properties and radiative impacts in the Pearl River Delta region of China during the dry season

Aerosol optical properties and direct radiative effects on surface irradiance were examined using seven years (2006-2012) of Cimel sunphotometer data collected at Panyu-the main atmospheric composition monitoring station in the Pearl River Delta (PRD) region of China. During the dry season (October to February), mean values of the aerosol optical depth (AOD) at 550 nm, the ngstrom exponent, and the single scattering albedo at 440 nm (SSA) were 0.54, 1.33 and 0.87, respectively. About 90% of aerosols were dominated by fine-mode strongly absorbing particles. The size distribution was bimodal, with fine-mode particles dominating. The fine mode showed a peak at a radius of 0.12 mu m in February and October (similar to 0.10 mu m(3)mu m(-2)). The mean diurnal shortwave direct radiative forcing at the surface, inside the atmosphere (F (ATM)), and at the top of the atmosphere, was -33.4 +/- 7.0, 26.1 +/- 5.6 and -7.3 +/- 2.7Wm(-2), respectively. The corresponding mean values of aerosol direct shortwave radiative forcing per AOD were -60.0 +/- 7.8, 47.3 +/- 8.3 and -12.8 +/- 3.1 W m(-2), respectively. Moreover, during the study period, F (ATM) showed a significant decreasing trend (p < 0.01) and SSA increased from 0.87 in 2006 to 0.91 in 2012, suggesting a decreasing trend of absorbing particles being released into the atmosphere. Optical properties and radiative impacts of the absorbing particles can be used to improve the accuracy of inversion algorithms for satellite-based aerosol retrievals in the PRD region and to better constrain the climate effect of aerosols in climate models.