Improved Simulation of Atmospheric Oxidation Capacity Using the MAX1 Chemical Mechanism in North China Plain, China

Atmospheric Oxidation Capacity (AOC) quantifies the ability of atmosphere to oxidize primary species. It plays a crucial role in initiating atmospheric chemical processes and impacts the formation of secondary pollutants, such as ozone (O₃) and secondary aerosols. AOC is fundamentally determined by the concentrations and reactivity of atmospheric oxidants, including O₃, hydroxyl radicals (OH), and nitrate radicals (NO₃). Due to the inherent challenges in direct measurement, AOC is typically inferred through numerical modeling. However, the chemical mechanisms implemented in commonly used 3-D chemical transport models (CTMs) often simplify organic species, leading to underestimations of radical concentrations and AOC.

The Mechanism for Air pollution compleX version 1.0 (MAX1) describing detailed tropospheric chemical processes has been therefore developed to improve the simulation of radicals. MAX1 contains 940 reactions including photolysis, gaseous reactions and heterogeneous reactions of 300 species, which is adequate for both box model and CTM applications. Detailed chemical processes of chlorine chemistry, chemistry of Criegee radicals and heterogeneous uptake of HO₂ and N₂O₅ have been implemented and updated. With this level of explicitness, MAX1 can support investigations on the quantification of secondary pollutant productions and the chemical behavior of the crucial intermedia such as organic peroxy radicals. MAX1 has been validated in box model and regional models. Simulations of MAX1 well captured the variation of O₃ in all cases tested. Meanwhile, significant improvement was made on predictions of radicals compared to other mechanisms, especially under the low NO_x environment.