A Common Representative Intermediates (CRI) mechanism for VOC degradation. Part 2: Gas phase mechanism reduction
Introduction
Volatile organic compounds (VOCs) emitted from biogenic and anthropogenic sources contribute to the formation of ozone (O3) and other secondary pollutants in the lower atmosphere (e.g., Leighton, 1961, Finlayson-Pitts and Pitts, 2000, Atkinson, 2000, Jenkin and Clemitshaw, 2000). Due to differences in structure and reactivity, each VOC contributes differently to pollutant formation. Therefore, a thorough description of the chemistry involved in the production of atmospheric pollutants requires very large numbers of species and reactions in order to account for a unique degradation pathway for each VOC. Due to computational limitations, such explicit mechanisms cannot be applied in complex atmospheric models such as three-dimensional dispersion models, so development of simplified mechanisms is essential.
There are a number of different techniques that can be used to reduce the size of explicit chemical mechanisms in order to produce schemes appropriate for use in complex atmospheric models. These usually involve either (i) a systematic reduction in the complexity of the chemistry for the considered suite of VOCs or (ii) the lumping of emissions so that the chemistry for one VOC can be used to represent that of a number of VOCs. The preceding companion paper (Jenkin et al., 2008) considers the first technique, describing a method to reduce the number of degradation steps in the Master Chemical Mechanism (MCM v3.1) in order to produce the Common Representative Intermediates mechanism version 2 (CRI v2). This method was successful in reducing the degradation chemistry for a large number of emitted VOCs while maintaining an accurate description of ozone production, with the resultant CRI v2 mechanism (1183 reactions of 434 chemical species) being smaller than MCM v3.1 by about an order of magnitude. Although CRI v2 is substantially smaller than MCM v3.1, it still contains too many reactions and species for application in models containing more detailed treatments of transport processes. This paper presents a logical method for further reducing CRI v2 through systematic emitted species lumping techniques in order to create a series of reduced schemes, the smallest of which is considered appropriate as a reference mechanism in global chemistry-transport models. This paper focuses on reducing the number of emitted anthropogenic species by redistribution based on their chemical class and photochemical ozone creation potential (POCP), relative to a reference VOC speciation based on that reported by the UK National Atmospheric Emissions Inventory (NAEI).
Section snippets
Overview of methodology
The various stages of mechanism reduction described in this paper were evaluated using the same zero dimensional one-box model applied in the preceding companion paper (Jenkin et al., 2008). 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
Summary and conclusions
Systematic lumping techniques for the detailed speciation of 112 emitted anthropogenic VOCs applied with CRI v2 have been used to generate a series of five reduced variants of the mechanism. Simplification has been achieved by redistributing selected emitted anthropogenic VOCs into appropriate surrogates, and removing redundant species and their associated chemistry from the chemical mechanism.
For the three initial reduction phases, minor emitted VOCs accounting for increasingly greater
Acknowledgements
Support from the UK Natural Environment Research Council (NERC) is gratefully acknowledged, via provision of NCAS studentship grant NER/S/R/2004/13091 (LAW), grant NE/D001846/1 forming part of the QUEST Deglaciation project (SRU) and Senior Research Fellowship grant NE/D008794/1 (MEJ). The work described in this paper was also funded partially by the Department for Environment, Food and Rural Affairs (Defra) under contract AQ0704.
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