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Li, Rui; Hu, Yunjie; Li, Ling; Fu, Hongbo; Chen, Jianmin (2017)
Languages: English
Types: Article
Subjects:
Characteristics of aerosol optical properties, morphologies and their relationship were studied in urban Beijing during the clear, haze and fog episodes, sampled from 24th May to 22nd Jun, 2012. Transmission Electron Microscope (TEM), a Cavity Ring Down Spectrometer (CRDS), a nephelometer and an aethalometer were employed to investigate the corresponding changes of the aerosol properties. Five episodes were categorised according to the meteorological conditions, composition and optical variation. Results show the clear episode (EP-2 and EP-4) featured as the low light extinction with less pollutants, which are mostly externally mixed. Coarse particles were scarcely observed in EP-2 due to the washout of a previous heavy rain. Thus the size distribution in EP-2 was smaller than EP-4, which had some mineral particles introduced from the north. In contrast, industry-induced haze (EP-1) and biomass burning-induced haze (EP-5) were both impacted by the south air mass. Higher AOD (Aerosol Optical Depth) values illustrated heavy loading particle concentrations. Due to the collision, size of most particles was larger with the diameter of 1 μm, resulting in a higher scattering coefficient. However, as the influence of severe crop residue combustion, a large fraction of soot was detected, which sticks to the KCl transformed sulphate or nitrate particles. The light absorption enhancement was contributed by both Black Carbon (BC) acceleration and other light absorbing substances. Comparatively, soot fog period detected in EP-3 was mostly internally mixed with sulphates and nitrates, which revealed themselves after electron exposure. The larger size distribution was likely to be caused by both hygroscopic growth and collision. More internally mixed particles were observed, which favored the light absorption. The comparison of all the episodes provides a deeper insight of how mixing states influence the aerosol extinction properties and also a clue to the air pollution control in the crop burning seasons.

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