Mechanism for Air pollution compleX version 1.0 (MAX1)

Secondary air pollutants such as ozone (O₃) and secondary organic aerosols (SOA) are not directly emitted but are formed through complex, nonlinear chemical reactions involving nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and other precursors. Their formation and removal are impacted by both meteorological conditions and concentrations of ambient pollutants. Accurately representing these chemical processes is a key scientific challenge for improving air quality modeling and forecasting. However, some existing chemical mechanisms in regional models showed limitations in describing radical chemistry under different emission scenarios, particularly under low-NOₓ conditions due to their simplified representations of HOₓ radical cycling, organic peroxy radical (RO₂) transformations, and heterogeneous chemical processes. To address this critical scientific question, Professor Keding Lu’s research team at the Ozone Laboratory has conducted sustained and systematic mechanism development studies focusing on the chemical evolution of ozone and radical chemistry across a wide range of NOₓ environments.

Recently, Professor Lu’s research group /Our research team? successfully developed a new mechanism -- Mechanism for Air pollution compleX, version 1.0 (MAX1). MAX1 is designed to provide an explicit representation of tropospheric chemistry and incorporates a total of 940 chemical reactions, including photolysis processes, gas-phase reactions, and heterogeneous reactions. The mechanism focus specially on improving the representation of chlorine chemistry, Criegee intermediate chemistry, and heterogeneous uptake of HO₂ and N₂O₅. By systematically integrating these reaction pathways, MAX1 is able to effectively addresses the limitations of existing mechanisms under complex pollution conditions.

To evaluate the performance and applicability of MAX1, our research team conducted comprehensive validations using both box models and chemical transport models (CTMs). The evaluation results demonstrate that, compared with two widely used lumped chemical mechanisms, MAX1 shows substantial improvements in simulating ozone, secondary pollutants, and HOₓ radicals across the full NOₓ concentration range. Notably, under low-NOₓ conditions, MAX1 produces significantly higher OH radical concentrations, markedly alleviating the long-standing systematic underestimation found in existing mechanisms.

Fig 1：Comparison OH, HO₂, and O₃ concentrations simulated from MAX1 and other chemical mechanisms

Further comparisons with MCM v3.3.1, one of the most comprehensive explicit chemical mechanisms, showed that simulations of MAX1 agreed well with results of MCM v3.3.1 with smaller deviations compared to observations. In global-scale, MAX1 accurately reproduces the spatial distributions and temporal variations of ozone and OH radicals over diverse underlying surfaces and emission environments, including land, ocean, rainforest, and urban regions. Importantly, while significantly improving simulation accuracy, MAX1 effectively controls computational complexity, making it suitable not only for high-resolution box model studies, but also computationally feasible for direct implementation in regional and global climate–chemistry models for long-term simulations. This balance between scientific explicitness and computational efficiency provides a new and practical option for atmospheric chemistry research and operational air quality forecasting.

Fig. 2 Comparison of global O₃ and OH simulation results between the MAX1 and CBMZ mechanisms

Owing to its detailed representations of various tropospheric chemical processes, MAX1 is capable for more accurate simulations of ozone and secondary PM₂.₅ formation under varying NOₓ conditions and interpretation of key reaction pathways and yields of intermediates such as organic peroxy radicals. MAX1.0 offers a reliable tool for chemical mechanism intercomparison and detailed simulation of complex atmospheric chemical processes, with strong potential applications in secondary pollutant formation, air quality forecasting and process studies.

The related study, entitled “Development and Evaluation of Mechanism for Air pollution compleX Version 1.0 (MAX1)”, was published online on August 19, 2025, in Advances in Atmospheric Sciences. Dr. Yanhui Liu, a postdoc researcher at the College of Environmental Sciences and Engineering, Peking University, is the first author. Professor Keding Lu and Dr. Houhua Zhou are the corresponding authors. This research was jointly supported by the National Natural Science Foundation of China (Grant Nos. 22325601, 92044302, and 42377105) and the National Major Science and Technology Infrastructure project “Earth System Science Numerical Simulation Facility,” among others.

Article link: https://www.iapjournals.ac.cn/aas/article/doi/10.1007/s00376-025-4456-z