The SAPRC atmospheric chemical mechanism generation system (MechGen)

Abstract. MechGen is a software system designed to derive gas-phase reaction mechanisms for reactive organic compounds under atmospherically relevant conditions for use in chemical models and for data analysis and interpretation. It has been used to derive versions of the SAPRC mechanisms used in airshed models, with SAPRC-22 being the most recent. MechGen derives fully explicit mechanisms for many types of organic compounds and their oxidation products when they react in the atmosphere in the presence of oxides of nitrogen and other pollutants and then uses these explicit mechanisms to derive reduced or lumped mechanisms more suitable for use in airshed models. This paper gives an overview of the system, describes the procedures it uses to generate explicit and reduced mechanisms, and presents several types of applications. The assignments and estimates used to derive individual chemical reactions and assign rate constants are discussed in a separate companion paper. The system is publicly accessible for generating explicit mechanisms for single compounds and viewing associated documentation using a web-based interface. A separate terminal login is available for deriving mechanisms for multiple compounds, multi-generation mechanisms, and portions of lumped mechanisms for airshed models, as well as for system programming and management. MechGen is designed to accommodate updates to the chemical estimates and assignments that it uses to reflect our evolving knowledge of and ability to estimate atmospheric reactions of organic compounds.