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Organic carbonaceous aerosols were found to be light absorbing in the UV and near visible ranges.With a high fraction of OC in PM2.5 and frequency haze in China,it’s important to study the light extinction caused by OC.In this work,optical properties of water-soluble organic carbon (WSOC) in Beijing and Guangzhou were investigated.Size-segregated aerosol samples were collected by Micro-Orifice Uniform Deposit Impactor (MOUDI) in Beijing,and non-size-segregated samples were collected by high-volume samplers in Guangzhou.All samples were ultrasonic extracted and then measured by the UV-Visible Spectrophotometer.Optical parameters such as light absorption coefficient,mass absorption efficiency (MAE),absorption angstrom exponent (Ap) were calculated based on the light absorption spectra obtained.Back trajectories were displayed using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYPLIST) to investigate relationship between light absorbing properties and air mass sources.The results demonstrated that light absorption by WSOC in Beijing and Guangzhou all exhibited strong wavelength (λ) dependence and varied approximately as λ-7 and λ-6,respectively.Ap value was smaller in the fine mode in general.MAE365 was peaked in the diameter 0.56-1 μm.The bulk mass absorption of WSOC (measured at 365nm) was 1.79±0.24 and 0.71±0.20 m2/g during winter and summer in Beijing,0.85±0.21 m2/g during autumn in Guangzhou,and 0.7±0.07 and 0.31±0.07 m2/g during summer with and without influence of biomass burning,indicating higher mass absorption ability in China than abroad.The large discrepancy between different seasons and places might be attributed to difference in chemical compositions of WSOC and precursors of secondary organic aerosol,with more influence of anthropogenic volatile organic compounds in winter than other seasons,and also might be results from biomass burning influences.This study indicated light absorption by WSOC was ubiquitous in the atmosphere.Light absorption ability might be influenced by air mass sources,particulate sizes,chemical components especially secondary organic aerosols,which will be further investigated in future research.