Tropospheric Chemistry Modelling

The Master Chemical Mechanism (MCM)

Introduction

Development and construction of a detailed chemical mechanism to describe the complete tropospheric oxidation of 120 volatile organic compounds (VOC) is presented. The VOC which are degraded in this mechanism were selected on the basis of available emissions data(2), and provide approximately 97% mass coverage of the emissions of uniquely identifiable chemical species. The degradation schemes have been constructed using the methodology described in an earlier publication(3). A brief review of the ideas behind the protocol document are given here

The degradation schemes are currently being used to provide an up to date mechanism for 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 NOx 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.

Compilation of the individual VOC degradation schemes has produced the Master Chemical Mechanism (MCM). The organic component of the MCM contains in the region of 7000 reactions and 2500 chemical species. With the exception of 18 aromatic compounds, the chemistry was constructed using the protocol described previously(3). The aromatic chemistry was based on that used in previous work(1,4), but was extended to include the reactions of peroxy radicals with HO2 and NO3, and the permutation reactions of the peroxy radicals. Although there are currently many gaps and uncertainties in the details of the atmospheric degradation mechanisms of aromatic compounds(5), there are several groups active in this field of research(6,7), and new data are constantly emerging.

The difficulties associated with ensuring that there was no species or nomenclature duplication within the compiled mechanism, led to the development of computerised mechanism construction. Details of the format and operation of the computerised system are described herein. Also, due to the large size of the MCM, it is not possible to provide the listing in hard copy. Full facsimile coding is available, together with associated mechanism and species files via e-mail and the Web. The concepts behind the methodology of the mechanism development and construction are such that as new data becomes available the MCM can be updated, utilising the computer aided construction format.

The completed MCM has been coded and fully integrated using Facsimile, within the UK photochemical trajectory model(4), to assess regional scale ozone formation across north west Europe and the British Isles. A Photochemical Ozone Creation Potential (POCP) index is being generated from the model results showing the relative importance of each VOC in ozone formation, on a mass emitted basis.

Related Bibliography

[1] R.G. Derwent and M.E. Jenkin - Hydrocarbons and the long range transport of ozone and PAN across Europe. Atmos. Environ. 25A p1661 (1991)

[2] H.J. Rudd - Emissions of volatile organic compounds from stationary sources in the United Kingdom : speciation. Report AEA/CS/16419033/REMA-029 (1995)

[3] M.E. Jenkin, S.M. Saunders and M.J. Pilling - The tropospheric degradation of volatile organic compounds : a protocol for mechanism development. Atmos. Environ. 31 p31 (1997)

[4] R.G. Derwent, M.E. Jenkin and S.M. Saunders - Photochemical ozone creation potentials for a large number of reactive hydrocarbons under European conditions. Atmos. Environ. 30 p189 (1996)

[5] R. Atkinson - Kinetics and mechanisms of the gas phase reactions of the hydroxyl radical with organic compounds. J. Phys. Chem. Ref. Data Monograph 2 (1994)

[6] G. Le Bras (coordinator) - EUROTRAC Annual Report, Part 8 LACTOZ (1993)

[7] H. Jeffries, Y. Jianzhen and L. Bartolotti - Theoretical and analytical advances in understanding aromatic atmospheric oxidation mechanisms. 210th ACS National Meeting. Paper Number PHYS-15. Chicago, Ill, August 20, (1995)

[8] Synopsys Scientific Systems Ltd. 175 Woodhouse lane, Leeds LS2 3AR, UK.

[9] Microsoft Corporation, USA

[10] A.R. Curtis and W.P. Sweetenham - FACSIMILE release H user's manual. AERE report R11771 (HMSO), London, (1987)

[11] D. Weininger - SMILES, a chemical language and information system. 1. Introduction to methodology and encoding rules. J. Chem. Inf. Comput. Sci. 28 p31 (1988)

[12] Cambridge Scientific Computing, Inc. - 875 Massachusetts avenue, Cambridge, MA 02139, USA.

[13] R. Atkinson - Gas-phase tropospheric chemistry or organic compounds : a review. Atmospheric Environment, 24A p1-41(1990).

[14] J.M. Roberts - The atmospheric chemistry of organic nitrates. Atmospheric Environment, 24A p243-287 (1990).

[15] R.P Wayne, I.Barnes, P.Biggs, J.P.Burrows, C.E. Canos-Mas, J.Hjorth, G. LeBras, G.K. Moortgat, D.Perner, G. Poulet, G.Restelli and H.Sidebottom - The nitrate radical: physics, chemistry and the atmosphere. Atmospheric Environment, 25A p1-206 (1991).

[16] P.D. Lightfoot, R.A. Cox, J.N. Crowley, M.Destriau, G.D. Hayman, M.E. Jenkin, G.K. Moortgat and F. Zabel - Organic peroxy

radicals : kinetics, spectroscopy and tropospheric chemistry. Atmospheric Environment, 26A p1805-1964 (1992).

[17] W.P.L. Carter and R. Atkinson - Atmospheric chemistry of alkanes. J. Atmos. Chem., 3 p377-405 (1985).

[18] R. Atkinson - Kinetics and mechanisms of the gas-phase reactions of the nitrate radical with organic compounds. J. Phys. Chem. Ref. Data, 20, p459 (1991).

[19] R. Atkinson and W.P.L. Carter - Reactions of alkoxy radicals under atmospheric conditions. The relative importance of decomposition versus reaction with O2. J. Atmos. Chem., 13 p195 (1991).

[20] T.J. Wallington, P. Dagaut and M.J. Kurylo - Ultra-violet absorption cross sections and reaction kinetics and mechanisms for peroxy radicals in the gas phase. Chem. Rev., 92, p667-710 (1992).

The MCM can be explored or downloaded in several ways :

Click here for access to the complete MCM archive files.

OR BROWSE the MCM pages for species/compounds of interest

The complete MCM comprises all the linked segments and should be considered in its entirety. The section displayed under a VOC heading DOES NOT give a full mechanism for that VOC. It simply provides a pointer to locate the chemistry associated with a specific VOC. Species not degraded in that section can be located elsewhere in the MCM. For example, the degradation of butane generates n-butanal and the chemistry associated with n-butanal is located on a separate page accessed from the VOC listing under the aldehyde grouping.

The species file gives a complete listing of the code names used in the MCM, together with a linear representation of the associated chemical structure using the SMILES notation. The Smiles string is quite straight forward to interpret, and at present is the only definition available to UNIX based users. However for PC based users, the SMILES string can be interpreted by spawning an external VIEWER using the Chemical MIME data exchange mechanism, which will display the chemical structure on screen. Click here for details on setting up and using the ACCORD Internet Chemistry Viewer if required.

Interpretation of the coded species names is possible by exploring the species file, which is also accessible from specific VOC mechanism pages.

Due to the many gaps and uncertainties in the aromatic chemistry, the level of detail that has been incorporated at this stage does not mirror that of all the other VOC. This is a major objective of the ongoing to work, to provide an up-to-date detailed description of troposheric aromatic degradation.

Definition of generalised rate parameters used in the MCM are given in this summary table

To begin browsing the MCM, first select the compound class of interest from this classification table.