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Photochemical mechanisms are a critical module of air quality models. In the past 20 years, several mechanisms have been developed to study the chemistry of the troposphere. This work compares several state-of-the-science photochemical mechanisms (including LCC, CBM-IV, RADM2, EMEP, RACM, SAPRC99, and CACM which have never been compared before in other studies). Concise descriptions of the chemical schemes are included. The main difference among existing mechanisms is the lumping technique used to classify organic compounds into surrogate groups. First, box model calculations are conducted to highlight the features that lead to differences in the modeled behaviors of photochemical pollutants and their precursors. Results indicate that most chemical schemes yield similar ozone concentrations. Nevertheless, there are significant discrepancies, mainly in predicted concentration of HNO₃ …