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Black carbon is traditionally considered as primary and strong light absorber in the visible wavelength. However, there is an increasing interest in investigating light absorbing property of organic aerosol, which could account for up to 30% and more than 10% of the total light absorption in the UV and mid-visible regions, respectively. These light absorbing organic aerosols are also named as brown carbon and have become the forefront in the field of atmospheric research. Considering the presence of abundant organic carbon in particulate matter in China, properties and sources of light absorbing organic aerosol and their roles on visibility reduction/haze formation and impacts on climate change need to be urgently determined. Here we present our most recent results of studying light absorption properties of water-soluble organic carbon (WSOC) in particulate matter from different cities in China and USA (including Beijing and Guangzhou, China, as well as Atlanta, USA). Besides PM2. 5, size-segregated aerosol samples were also collected by Micro-Orifice Uniform Deposit Impactor (MOUDI) to investigate the influence of particle size on light absorption properties of brown carbon. The results from our work revealed (1) WSOC/OC ratio varied from 48% to 90%, (2) light-absorption characteristics of WSOC in Guangzhou were similar to that in Beijing in terms of light absorption per mass WSOC at 365 nm (MAE365, 0. 85±0. 21 m2/g in Guangzhou and 0. 71±0. 20 m2/g in Beijing) and Angstrom exponent (Ap, a measure of the spectral dependence of aerosol light absorption), but stronger than Atlanta (0. 11-0. 19 m2/g), (3) mass concentrations and MAE365 of WSOC exhibited bimodal distribution with a smaller peak at coarse mode (3. 2 ? m) and a larger peak at fine mode (0. 56-1 ? m), which is also the typical dominant size range during haze period. In summary, WSOC in fine mode (≤2. 5? m) particles was more light absorbing compared to coarse mode (2. 5-10? m) and those in major cities in China exhibited much stronger light absorbing ability compared to Atlanta, an urban city abroad. Therefore, our study provided valuable dataset of brown carbon and suggested that the role of brown carbon on visibility reduction and haze formation in China should not be ignored.