Comprehensive Observations and Analysis of Absorptive Aerosols on Typical Days in the Semi-arid Region of the Loess Plateau

The physical properties of absorptive aerosols on typical autumn days, i.e. 3-4, 21, and 28 September 2012, in the semi-arid region of the Loess Plateau are analyzed using a radiative transfer model and comprehensive observation data provided by the SACOL (Semi-arid Climate and Environment Observatory of Lanzhou University). The data include simultaneous observations of sunphotometer, lidar, and microwave radiometer. The relationship between aerosols and meteorology situation is studied. Local dust mixed with anthropogenic aerosols form the major source of aerosols during the period, which have obvious absorptive effects and the aerosol particle sizes are commonly small. However, due to the increased northwesterly winds on the 4 September 2012, there existed more dust transport through long distance, and the aerosol optical depth was the largest and the particle size increased sharply. The reference height was determined to be 7.41 km, 8.47 km, 7.13 km, and 7.66 km, respectively by the gray pertinence method, and the extinction coefficient was also retrieved. The AOD (Aerosol Optical Depth) calculated from the retrievals agreed well with observations from the sunphotometer with a correlation coefficient up to 0.975. The aerosol uplift is mainly attributed to effects of thermal turbulences during the daytime in the study period, when the aerosols were transported upward as the boundary layer developed and reached the maximum height from 1200 LT (Local Time) to 1400 LT every day. Meanwhile, the altitude of aerosols could reach corresponded to level of high heating rate, which at lower levels could be up to 1 K d^-1. The aerosols had negative radiative forcing at the top of the atmosphere and the surface with the daily average values of 12.707 W m^-2 and -25.398 W m^-2, but they had positive radiative forcing in the atmosphere with a daily average value of -12.692 W m^-2. The radiative forcing at the top of the atmosphere is most sensitive to aerosol properties, and the aerosol forcing would be positive at the top of the atmosphere if the aerosols have obvious absorption effects.