Atmospheric humic-like substances (HULIS) are not only an unresolved mixture of macroorganic compounds but also powerful chelating agents in atmospheric particulate matters (PMs);impacting on both the properties of aerosol particles and health effects by generating reactive oxygen species (ROS).Currently,the interests of HUMS are intensively shifting to the investigations of HUMS-metal synergic effects and kinetics modeling studies,as well as the development of HULIS quantification,findings of possible HULIS sources and generation of ROS from HULIS.In light of HULIS studies,we comprehensively review the current knowledge of isolation and physicochemical characterization of HUMS from atmospheric samples as well as HUMS properties (hygroscopic,surface activity,and colloidal) and possible sources of HUMS.This review mainly highlights the generation of reactive oxygen species (ROS) from PMs,HULIS and transition metals,especially iron.This review also summarized the mechanism of ironorganic complexation and recent findings of OH formation from HULIS-metal complexes.This review will be helpful to cany out the modeling studies that conc with HULIS-transition metals and for further studies in the generation of ROS from HULlS-metal complexes.