A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 1: Gas phase mechanism development
Introduction
Volatile organic compounds (VOCs) are emitted from both anthropogenic and biogenic sources (e.g. Guenther et al., 1995, Dore et al., 2003) and have a major influence on the chemistry of the lower atmosphere. It is well documented that the atmospheric degradation of VOCs plays a central role in the generation of ozone, O3 (e.g., Leighton, 1961, Finlayson-Pitts and Pitts, 2000, Atkinson, 2000, Jenkin and Clemitshaw, 2000), and leads to the formation of other secondary pollutants, most notably fine particulate matter in the form of secondary organic aerosol (SOA) (e.g., Kanakidou et al., 2005 and references therein). Ozone and fine particles have harmful impacts on human health and on the environment, and influence the Earth's radiation budget: consequently, a sound understanding of the chemistry of VOC degradation, and its adequate representation in atmospheric models, is of central importance to the development of environmental policy on local, regional and global-scale issues.
Emissions speciation data indicate that many hundreds of VOCs are emitted (e.g. Dore et al., 2003), which possess a variety of physico-chemical properties, by virtue of differences in structure and functional group content. Since these factors influence the reactivity (i.e., the rate of oxidation in the atmosphere) and the oxidation pathways available (i.e., the degradation mechanism), it has long been recognised that the propensities of VOCs to form secondary pollutants such as ozone and SOA vary from one compound to another (e.g., Derwent and Jenkin, 1991, Carter, 1994; Grosjean and Seinfeld, 1989). Modelling studies using detailed emitted VOC speciations and comprehensive descriptions of VOC degradation chemistry have thus provided a means of quantifying the roles played by VOCs (both individually and collectively) in atmospheric chemistry, thereby allowing detailed appraisals of the contributions made by individual VOCs, or VOC classes, to ozone and SOA formation (e.g., Johnson et al., 2006a, Johnson et al., 2006b, Derwent et al., 2007a, Derwent et al., 2007b). Those studies made use of the Master Chemical Mechanism, version 3.1 (MCM v3.1), a comprehensive chemical mechanism that describes the detailed degradation of more than 100 emitted VOCs (Jenkin et al., 1997, Jenkin et al., 2003, Saunders et al., 2003, Bloss et al., 2005). Because of its explicit nature, the MCM represents a direct method of utilising and applying the results of studies of elementary chemical processes, and is conceptually simple, since it contains almost no empirical lumping or surrogate species. As a result, however, the mechanism contains many thousands of chemical species and reactions, and it is recognised that the development of less-detailed schemes is essential for many applications where greater computational efficiency is required.
The development of a reduced mechanism to describe the formation of ozone from the oxidation of a similar set of emitted VOCs has previously been described by Jenkin et al. (2002a), the mechanism being known as the Common Representative Intermediates (CRI) mechanism. The construction methodology, which is outlined below, yielded a mechanism of ca. 250 species and 570 reactions (hereafter referred to as CRI v1), with the performance of the whole mechanism (in terms of ozone formation) being optimized in relation to that of the version of the MCM which was current at the time (MCM v2). A recent detailed appraisal of the performance of CRI v1 in relation to that of the latest MCM version (MCM v3.1), has shown that, although CRI v1 generally performs well, it tends to display under-reactivity under highly polluted conditions characteristic of close to emissions sources (Watson, 2007). In addition, the formation of ozone specifically from the oxidation of aromatic hydrocarbons at short timescales is also severely underestimated in comparison with MCM v3.1, improvement of that aspect of the chemistry being one of the major developments in the more recent versions of the MCM (Jenkin et al., 2003, Bloss et al., 2005).
In the present paper, and the companion papers (Watson et al., 2008, Utembe et al., in preparation), the development and application of version 2 of the CRI mechanism (hereafter referred to as CRI v2) is described. The present paper describes the development of the gas phase CRI v2 mechanism itself, and optimisation of its performance in relation to MCM v3.1, to yield a mechanism of intermediate complexity which can degrade methane and 115 emitted non-methane hydrocarbons and oxygenated VOCs. A series of emissions lumping options are considered in Watson et al. (2008), yielding a traceable set of further reduced CRI mechanisms, the smallest of which is considered appropriate for application as a reference mechanism in global chemistry-transport models. Finally, the development and assessment of an associated SOA module, for application with CRI v2 and its reduced variants, is described in Utembe et al. (in preparation).
Section snippets
Quantifying ozone formation from VOC oxidation: the CRI index
The complete oxidation of a given VOC through to CO2 and H2O normally proceeds via a series of intermediate oxidised products. At each stage, the chemistry can be propagated by reactions of free radicals leading to the oxidation of NO to NO2 and resultant formation of O3 as a by-product, following the photolysis of NO2:
The total quantity of O3 potentially generated from the free radical propagated chemistry is therefore dependent on the number of
Overview of construction procedure
CRI v2 was built up on a compound-by-compound basis, with its performance optimised to that of MCM v3.1, using a series of five-day box model simulations. The model represents a well-mixed boundary layer box, 1 km in depth, which receives emissions of NOx, CO, SO2, methane and non-methane VOCs, based on average emission densities in the UK in 2001 (Dore et al., 2003), but with appropriate diurnal and seasonal variations superimposed, as presented by Utembe et al. (2005). The model was run for
Summary and conclusions
A reduced mechanism describing ozone formation from the tropospheric degradation of methane and 115 emitted non-methane hydrocarbons and oxygenated VOCs has been developed, using MCM v3.1 as a reference benchmark. The CRI v2 mechanism was built up on a compound-by-compound basis, with the performance of its chemistry optimised for each compound in turn by comparison with MCM v3.1, using formation of ozone as the primary criterion. The resultant mechanism contains 1183 reactions of 434 chemical
Acknowledgements
The work described in this paper was funded partially by the Department for Environment, Food and Rural Affairs (Defra) under contracts EPG 1/3/200 and AQ0704. Support from the UK Natural Environment Research Council (NERC) is also gratefully acknowledged, via provision of Senior Research Fellowship grant NE/D008794/1 (MEJ), NCAS studentship grant NER/S/R/2004/13091 (LAW) and grant NE/D001846/1 forming part of the QUEST Deglaciation project (SRU).
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