Quantitative Study of Atmospheric Oxidation Capacity

Atmospheric oxidation capacity (AOC) is usually defined as the sum of the rates at which the atmosphere removes trace gaseous constituents through oxidative processes. In the troposphere and near-surface atmosphere, AOC is mainly characterized by the removal or purification of pollutant gases, also known as atmospheric oxidizability. AOC is the essential driving force of tropospheric chemistry, but its quantitative representation remains limited. For researching the “Quantitative Relationship between Regional Atmospheric Oxidizing Capacity and Air Quality and the Principles of Regulation”, supported by the National Key R&D Program of China and other projects, the authors have conducted a series of studies on AOC using the basic theories of atmospheric chemistry and have advanced the quantitative expression of AOC. In this paper, the authors briefly describe these advances, focusing on the “quantitative study of atmospheric oxidation capacity”. First, based on the in-depth knowledge of AOC, the authors constructed the evaluation index of AOC (AOIe) and potential index of AOC (AOIp) using the fundamentals of the thermodynamic and kinetic atmospheric chemistry, respectively, and found that contributions of heterogeneous chemical processes to the AOC should not be neglected when studying the daily variation of the normalized AOIe and AOIp. Moreover, with increasing PM_2.5 pollution, AOIe increased in summer and winter campaigns while AOIp decreased in winter, showing that meteorological conditions had more influence on AOIp variations. Second, the AOC closure research idea was used to explore the “unknown source” of HONO, which is the reservoir molecule of atmospheric OH radicals, and an important heterogeneous source of atmospheric HONO was found in Beijing, explaining the underestimation of the winter AOC by the MCM (Master Chemical Mechanism) mechanism. The AOIp was used to predict the potential pattern of atmospheric ozone pollution in China, showing that the annual mean J(NO₂) value of AOIp_O₃ directly correlates with J(NO₂). Furthermore, O₃ directly correlates with J(NO₂), and the nationwide annual average value of J(NO₂) is 4.39×10⁻³ s⁻¹, with mainly Sichuan, Guizhou, Chongqing, and Hunan exhibiting high values. The combined AOIe and AOIp indices are more accurate, generalizable, and useful than other chemical reaction oxidizability indices. They can evaluate AOC variations during the pollution process and predict the likelihood of urban or regional heavy pollution occurrence as well as its variations and patterns.