The atmospheric oxidation capacity (AOC) is the essential driving force of atmospheric chemistry in forming complex air pollution, which determines the removal rate of trace gases and the production rates of secondary pollutants. The pro-cesses and rates of species being oxidized in the atmosphere thus constitute the key factors to quantify the AOC. Currently, the term “oxidation capacity” is defined as the sum of the respective oxidation rates of the molecules (VOCs, CO, CH4, etc.) by the main oxidizing agents (OH, O3, NO3, etc.), and these oxidation processes are described in atmospheric chem-ical reaction mechanisms such as the RACM (Regional Atmospheric Chemistry Mechanism) and the MCM (Master Chem-ical Mechanism). However, these mechanisms cannot explain the rapid secondary aerosol production observed in polluted environments such as winter haze in China, suggesting the existence of missing AOC mechanisms participating in the forma-tion of secondary aerosols. Thus, a deeper understanding of AOC remains a challenging endeavor and is the priority before any efficient pollution mitigation can take place.