A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 2: Gas phase mechanism reduction

The Common Representative Intermediates mechanism version 2 (CRI v2) is a reduced mechanism of intermediate complexity, traceable to the Master Chemical Mechanism version 3.1 (MCM v3.1), which couples with a detailed speciation of 112 emitted anthropogenic non-methane volatile organic compounds (VOCs). Systematic lumping techniques for the emitted anthropogenic VOC species have been used to reduce CRI v2, creating a series of five reduced variants, the performances of which have been evaluated against that of CRI v2 for a range of ambient conditions. For the initial reduction phases, minor emitted VOCs accounting for increasing proportions of the VOC mass emissions total, up to 20%, were redistributed into appropriate surrogates, and the redundant species and their associated chemistry were removed from the chemical mechanism. The surrogates were selected to maintain the chemical class of the redistributed VOCs within a number of VOC sub-categories, and aimed to preserve the ozone-forming ability of each category using the photochemical ozone creation potential (POCP) index as a criterion. This yielded three reduced mechanisms (CRI v2-R1, CRI v2-R2 and CRI v2-R3) and allowed respective reductions of up to 25% and 32% in the numbers of reactions and species, with no significant degradation in the overall performance of the mechanisms or in the relative contributions of the VOC sub-categories to ozone formation. More severe levels of reduction were also imposed, to limit the number of representative VOCs in each sub-catergory, with the choice of species taking account of their abundance in the detailed speciation, and the simplicity of the associated degradation mechanism. The POCP index was once again used as a guide to help optimise the overall ozone-forming ability of the VOC speciation. This yielded two further mechanisms (CRI v2-R4 and CRI v2-R5) with reductions of up to 53% and 55% in the numbers of reactions and species relative to CRI v2. These mechanisms display a degree of compromise in the ozone-forming ability of the VOC sub-categories, but retain a good level of overall performance. The most reduced variant (CRI v2-R5) uses 19 non-methane VOCs to represent the anthropogenic speciation, and is considered appropriate as a traceable reference mechanism for use in global chemistry-transport models.