A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO2 radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations. (c) 2022 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Automated summary

The atmospheric chemical mechanism plays a crucial role in airshed models for studying chemical behaviors and impacts. Various mechanisms, such as MCM, CBM, SAPRC, and RACM, have been developed for different research purposes. This article summarizes the development and applications of these mechanisms, compares their compositions and methods, and examines how they treat radicals in box model simulations. CBM can effectively reproduce urban pollution events at a lower cost compared to SAPRC and RACM, while RACM provides a detailed description of radicals and ozone production. SAPRC07 has lower photolysis rates for some oxygenated compounds, leading to an underestimation of radical levels. MCM is an explicit chemical mechanism that describes primary pollutants and oxidation products in detail but is rarely used in regional model simulations due to its large size. Results from box model case studies show that all mechanisms accurately describe the behavior of radicals and the production of ozone. However, CBM and SAPRC tend to underestimate radical levels, resulting in lower ozone production values. MCM and RACM are commonly used in box model studies, while CBM and SAPRC are preferred in regional simulations.