The tropospheric degradation of volatile organic compounds: a protocol for mechanism development

Abstract

Kinetic and mechanistic data relevant to the tropospheric oxidation of volatile organic compounds (VOCs) are used to define a series of rules for the construction of detailed degradation schemes for use in numerical models. These rules are intended to apply to the treatment of a wide range of non-aromatic hydrocarbons and oxygenated and chlorinated VOCs, and are currently being used to provide an up-to-date mechanism describing the degradation of a range of VOCs, and the production of secondary oxidants, for use in a model of the boundary layer over Europe. The schemes constructed using this protocol are applicable, however, to a wide range of ambient conditions, and may be employed in models of urban, rural or remote tropospheric environments, or for the simulation of secondary pollutant formation for a range of NO x or VOC emission scenarios. These schemes are believed to be particularly appropriate for comparative assessments of the formation of oxidants, such as ozone, from the degradation of organic compounds. The protocol is divided into a series of subsections dealing with initiation reactions, the reactions of the radical intermediates and the further degradation of first and subsequent generation products. The present work draws heavily on previous reviews and evaluations of data relevant to tropospheric chemistry. Where necessary, however, existing recommendations are adapted, or new rules are defined, to reflect recent improvements in the database, particularly with regard to the treatment of peroxy radical (RO 2) reactions for which there have been major advances, even since comparatively recent reviews. The present protocol aims to take into consideration work available in the open literature up to the end of 1994, and some further studies known by the authors, which were under review at that time. A major disadvantage of explicit chemical mechanisms is the very large number of reactions potentially generated, if a series of rules is rigorously applied. The protocol aims to limit the number of reactions in a degradation scheme by applying a degree of strategic simplification, whilst maintaining the essential features of the chemistry. These simplification measures are described, and their influence is demonstrated and discussed. The resultant mechanisms are believed to provide a suitable starting point for the generation of reduced chemical mechanisms.