A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 1: Gas phase mechanism development

A reduced mechanism describing ozone formation from the tropospheric degradation of methane and 115 emitted non-methane hydrocarbons and oxygenated volatile organic compounds has been developed, using the Master Chemical Mechanism version 3.1 (MCM v3.1) as a reference benchmark. The Common Representative Intermediates mechanism version 2 (CRI v2) has been built up on a compound-by-compound basis, with the performance of its chemistry optimised for each compound in turn by comparison with that of MCM v3.1, using a series of five-day box model simulations. The resultant mechanism contains 1183 reactions of 434 chemical species, i.e., ca. 10% of the number of reactions and species to degrade the same set of VOCs in MCM v3.1. Similarly to CRI v1 (Jenkin et al., 2002a), a key assumption in the CRI v2 construction methodology is that the potential for ozone formation from a given volatile organic compound (VOC) is related to the number of reactive (i.e., CC and CH) bonds it contains. This index allows a series of generic intermediates to be defined, with each being used as a “common representative” for a large set of species possessing the same index, as formed in detailed mechanisms such as the MCM. The performance of CRI v2 is shown to compare well with that of MCM v3.1 for a wide range of ambient conditions, which consider variations in VOC/NOx emissions ratio over ranges of 32 and 400 for anthropogenic and biogenic VOCs, respectively, in box model simulations; and in simulations of the TORCH 2003 campaign in the southern UK using a photochemical trajectory model, which considers notable ranges in the relative and absolute emissions of NOx and VOCs, and in the relative contributions of anthropogenic and biogenic species to the VOC total. CRI v2 is a reduced mechanism of intermediate complexity, which is traceable to MCM v3.1, and which provides the basis for further systematic reduction. Two companion papers consider further reduction of CRI v2 through emissions lumping and development and assessment of an associated SOA module.