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Elucidating the relationship between characteristics of aerosol particles and optical absorption is important to deepen the understanding of atmospheric chemistry. Aerosol particles play significant roles in climate forcing via their opticalabsorption properties. However, the relationship between characteristics ofaerosol particles and optical absorption remains poorly understood. Aerosoloptical properties and morphologies were measured by a transmission electronmicroscope (TEM), cavity ring-down spectrometer (CRDS), a nephelometer and anAethalometer in a urban site of Beijing from 24 May to 22 June. Five episodeswere categorized according to the meteorological conditions and composition.The results showed that the clear episode (EP-2 and EP-4) featured as the lowaerosol optical depth (AOD  =  0.72) and fewer pollutants compared withhaze (1.14) and fog (2.92) episodes and the particles are mostly externallymixed. The high Ångström exponent (> 2.0) suggests thatcoarse particles were scarcely observed in EP-2 due to the washout of aprevious heavy rain, whereas they were widespread in EP-4 (Ångströmexponent  =  0.04), which had some mineral particles introduced from thenorth. In contrast, industry-induced haze (EP-1) and biomass-burning-inducedhaze (EP-5) were both affected by the south air mass. Compared with the EP-2and EP-4, the AOD values and the size distribution of particles during EP-1and EP-5 were much greater because of relatively high particleconcentrations. All of the particles were classified into nine categories,i.e. S-rich, N-rich, mineral, K-rich, soot, tar ball, organic, metal and flyash, on the basis of TEM analysis. In contrast to the EP-1, a large fractionof soot, which sticks to KCl, sulfate or nitrate particles, was detectedduring EP-5. Additionally, evident enhancement of light absorption wasobserved during the EP-5, which was mainly ascribed to both black carbon (BC)acceleration and other absorbing substances. However, soot was found mostlyinternally mixed with sulfate and nitrate during a soot fog episode (EP-3),resulting in evident enhancement of light absorption. The larger sizedistribution was likely to be caused by both hygroscopic growth and collisionbetween particles during the aging. About 28 % of particles wereinternally mixed during the foggy days, which favoured the light absorption.The comparison of all the episodes provides a deeper insight into how mixingstates influence the aerosol extinction properties and also a clue as to howto control air pollution in the crop burning seasons

Real-time aerosol optical properties, morphology and mixing states under clear, haze and fog episodes in the summer of urban Beijing