Derivation of Atmospheric Reaction Mechanisms for Volatile Organic Compounds by the SAPRC Mechanism Generation System (MechGen)

Abstract. This paper describes the methods that are used in the SAPRC mechanism generation system, MechGen, to estimate rate constants and derive mechanisms for gas-phase reactions of volatile organic compounds (VOCs) in the lower atmosphere. Earlier versions of this system have been used for over 20 years in the development of the SAPRC mechanisms for air quality models and ozone (O₃) reactivity assessments. This is the first complete documentation of MechGen as it currently exists. MechGen can be used to derive explicit mechanisms for most compounds with C, H, O, or N atoms. Included are reactions of stable compounds with hydroxy (OH) and nitrate (NO₃) radicals, O₃, O³P, or by photolysis or unimolecular reactions, and the reactions of the radicals they form in the presence of O₂ and oxides of nitrogen (NO_x) at atmospheric temperatures and pressures. Measured or theoretically calculated rate constants and branching ratios are used when data are available, but in most cases rate constants and branching ratios are estimated using various structure-reactivity or other estimation methods. Types of reactions that are estimated include initial reactions of organics with atmospheric oxidants or by photolysis, and unimolecular and bimolecular reactions of carbon-centered, alkoxy, and peroxy radicals and Criegee and other intermediates that are formed. This manuscript summarizes the capabilities of the MechGen system for deriving chemical mechanisms and the methods, assignments, and estimates used to derive them. Examples of MechGen predictions also are provided. Many of the estimation methods discussed here have not been published previously, and others have not been used previously in developing comprehensive mechanisms. This paper concludes with a discussion of the areas of uncertainty where additional measurements or theoretical estimates are needed and recommendations for future mechanism development and application.