The mass concentration and optical properties of black carbon aerosols over a semi-arid region in the northwest of China

Abstract Black Carbon (BC) is the predominant absorption component of atmospheric aerosols, and it is believed to be the second largest contributor to global warming. Calculating its radiative forcing requires observational data regarding its physical, chemical and optical properties, so observation is the foundation of this research. The Semi–Arid Climate and Environment Observatory of Lanzhou University aims to improve our understanding in this regard by capturing direct evidence of the impact of human activity on the semi–arid climate over the Loess Plateau of Northwestern China. In this paper, the period from November 2010 to February 2011, which is within the heating period, was selected in order to study the optical properties of BC, such as its depolarization ratio, extinction coefficient, optical depth, Angstrom exponent and effective radius. The average BC concentration was 2 334 ± 1 546 ng/m 3 during the observation. The diurnal evolution of BC concentration had two maximums, which appeared at 10:00 and 20:00 (local time), and two minimums, which appeared at 03:00 and 16:00. The average Aerosol Optical Depth (AOD) during the observation was 0.26 ± 0.2, the aerosols existed mostly between the surface of the Earth and a height of 3 km, and the extinction coefficient decreased with height. The average of the depolarization ratio between the surface of the Earth and a height of 3 km, the Angstrom exponent ( α 440/870 nm ) and the effective radius of black carbon aerosols were 0.24, 0.86 ± 0.30 and 0.54 ± 0.17 μm, respectively. The maximum distribution frequency of α 440/870 nm was 27%, with a range of 1.0 to 1.2. The maximum distribution frequency of the effective radius was 28%, with a range of 0.4 μm to 0.5 μm.