Particulate brown carbon(BrC) has strong wavelength dependence with absorption increasing sharply from visible to UV ranges and has attracted much attention due to its significant climate effects. The composition, sources, evolution and optical properties of BrC remain highly uncertain, which contributes significantly to the uncertainty in estimating aerosol radiation forcing by climate models. The chemical compositions, sources and formation pathways of particulate BrC in the atmosphere are summarized in this review, which focuses on the relationship between molecular-level compositions, secondary formation mechanisms and light absorption properties. Particulate BrC are classfied into several major compound categories, including methanol soluble organic carbon(MSOC), water soluble fractions WSOC(Water Soluble Organic Carbon) and HULIS-C(Carbon Component of HUmic-like Substances). Nitroaromatic compounds(NACs) and N-heterocyclic compounds are major BrC chromophores. The sources of BrC include primary emissions from inefficient combustion, especially biomass burning, and secondary formation via the oxidation of volatile organic compounds(VOCs) in gas and particle phases. Secondary formation pathways include oxidation of aromatics and reactions between carbonyls and ammonia/amines. Oxidation of anthropogenic aromatic hydrocarbons in the presence of NO_x produces BrC dominated by nitrogen-containing organic compounds(e.g. NACs). Reactions between carbonyls and ammonia/amines generate BrC dominated by N-heterocyclic compounds and oligomers. Precursors and reaction conditions are important factors influencing the compositions and light absorption of secondary BrC. The chromophores would go through decomposition and BrC light absorption would change rapidly during transport in the atmosphere, which is usually named photobleaching. Molecular-level identification of BrC chromophores and further understanding on the secondary formation mechanisms and atmospheric evolution of BrC are required in future studies.