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History of the Master Chemical Mechanism (MCM) and its development protocols Mike Jenkin EPSR Group Department of Environmental Science and Technology.

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Presentation on theme: "History of the Master Chemical Mechanism (MCM) and its development protocols Mike Jenkin EPSR Group Department of Environmental Science and Technology."— Presentation transcript:

1 History of the Master Chemical Mechanism (MCM) and its development protocols Mike Jenkin EPSR Group Department of Environmental Science and Technology

2 1993 – the birth of the MCM University of Leeds  Sam Saunders, Mike Pilling AEA Technology  Mike Jenkin, Colin Johnson UK Meteorological Office  Dick Derwent Work commissioned by the Department of the Environment, DoE (Air Quality Division), to improve the treatment of organic chemistry in ozone policy models

3 Chemical processing of ozone-precursor emissions VOC NO X CO 2 H 2 O nitrate emissions oxidation Ozone inventory contains ca. 650 species

4 Chemistry in DoE ozone models in 1993 Photochemical Trajectory Model chemistry of 95 VOC represented although reasonably detailed, the chemistry did not reflect the current status of kinetic and mechanistic data, e.g. - no formation of organic nitrates from RO 2 + NO - RO 2 + HO 2 reactions not included (except for CH 3 O 2 ) - incomplete degradation of some VOC - many VOC degraded via products known to be wrong (i.e. incorrect RO reactions applied) - very limited representation of photolysis of organics

5 : Master Chemical Mechanism (MCM v1) Philosophy to use information on the kinetics and products of elementary reactions relevant to VOC oxidation to build up an explicit representation of the degradation mechanisms.  the resultant formation of ozone and other gas-phase secondary pollutants apply measured and evaluated parameters (e.g. rate coefficients; branching ratios) from the literature where possible. use analogy and ‘structure-reactivity correlations’ to define the other reactions and parameters. ‘Mechanism Development Protocol’ - Atmospheric Environment, 31, , 1997

6 1996: Master Chemical Mechanism (MCM v1) Degradation of CH 4 and 119 non-methane VOC ca. 2,500 chemical species ca. 7,000 chemical reactions 22 alkanes (C 1 -C 12 ) 16 alkenes (C 2 -C 6 ) 2 dienes (C 4 -C 5 ) 1 alkyne (C 2 ) 18 aromatics (C 6 -C 11 ) 6 aldehydes (C 1 -C 5 ) 10 ketones (C 3 -C 6 ) 17 alcohols (C 1 -C 6 ) 10 ethers (C 2 -C 7 ) 8 esters (C 2 -C 6 ) 3 carboxylic acids (C 1 -C 3 ) 8 halocarbons (C 1 -C 3 ) MCM website launched in March 1997

7 MCM timeline MCM v VOC; 7000 reactions; 2500 species 101 non-aromatic anthropogenic species 18 aromatics (provisional chemistry) 1 biogenic species (isoprene) 1996 MCM v VOC; reactions; 3500 species 103 non-aromatic anthropogenic species 18 aromatics (extended provisional chemistry) 2 biogenic species (isoprene:  pinene) 1999 MCM v VOC; reactions; 4400 species 104 non-aromatic anthropogenic species 18 aromatics (first rigorous representation) 3 biogenic species (isoprene:  pinene:  -pinene) 2002 Contributions to MCM v2 and v3 activities at Leeds: Nic Carslaw, Stephen Pascoe, Volker Wagner

8 2002: Master Chemical Mechanism (MCM v3) 22 alkanes (C 1 -C 12 ) 16 alkenes (C 2 -C 6 ) 2 dienes (C 4 -C 5 ) 2 monoterpenes (C 10 ) 1 alkyne (C 2 ) 18 aromatics (C 6 -C 11 ) 6 aldehydes (C 1 -C 5 ) 10 ketones (C 3 -C 6 ) 17 alcohols (C 1 -C 6 ) 10 ethers (C 2 -C 7 ) 8 esters (C 2 -C 6 ) 3 carboxylic acids (C 1 -C 3 ) 2 other oxygenates (C 3 ) 8 halocarbons (C 1 -C 3 ) Supplementary protocols: Atmospheric Chemistry and Physics, 3, , 2003 (non-aromatic VOC) Atmospheric Chemistry and Physics, 3, , 2003 (aromatic VOC)

9 MCM construction methodology

10 MCM scheme writing framework

11 Free radical propagated oxidation cycle VOC carbonyl product(s) OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 rxn with O 2, decomposition or isomerisation. hh O3O3 O2O2 hh O2O2 O3O3 NO 2 + h  → NO + O O + O 2 (+M) → O 3 (+M)

12 Radical termination VOC carbonyl product(s) OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 HNO 3 NO 2 H2O2H2O2 HO 2 ROOH HO 2 ROH + R -H O RO 2 NO RONO 2 rxn with O 2, decomposition or isomerisation. NO 2 RO 2 NO 2

13 Radical generation (or regeneration) through photolysis VOC carbonyl product(s) OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 H2O2H2O2 ROOH carbonylsRONO 2 rxn with O 2, decomposition or isomerisation. ROOHcarbonyls O3O3 O2O2 O2O2 H2OH2O

14

15 OH-initiated degradation of methane (CH 4 )

16 OH-initiated degradation of ethane (C 2 H 6 )

17 OH-initiated degradation of 1,3-butadiene

18 Defining kinetic and mechanistic parameters VOC or product carbonyl product(s) OHHO 2 RO 2 RO NONO 2 NONO 2 O2O2 rxn with O 2, decomposition or isomerisation OH + VOC/organic product RO 2 + NO, NO 2, NO 3, HO 2, R’O 2 RO O 2 reaction, decomp., isom.

19 OH radical reactions  Kinetics of OH + VOC/organic products Rate coefficients have been measured for several hundred organics Rate coefficients for ca. 2,000 species need to be estimated (e.g. SAR method of Atkinson, 1994; Kwok and Atkinson, 1995)  Product radical distribution of OH + VOC/organic product Mainly inferred from SAR partial rate coefficients Scheme simplification measures applied in some cases -minor channels (<5%) ignored -single representative channel for ≥ C 7 alkanes -so called ‘minor’ products (e.g. RONO 2 ; ROOH) degraded to regenerate existing species

20 RO 2 radical reactions  Kinetics of RO 2 reactions Reactions with NO, NO 2, NO 3, HO 2 and other peroxy radicals (R’O 2 ) are included in MCM There are about 1200 RO 2 radicals in MCM v3 Kinetic data are available for only ca. 20 RO 2 – parameters assigned to majority of reactions by analogy and structure reactivity correlations  Product branching ratios Multiple channels for reactions with NO, HO 2 and R’O 2 Scheme simplification measures applied in some cases -RO 2 from ‘minor’ products react via single channel -RO 2 + R’O 2 reaction are necessarily parameterised (explicit chemistry for 1200 radicals would require 0.7 million reactions!)

21 RO radical reactions reaction with O 2 decomposition isomerisation There are about 1200 RO radicals in MCM v3 Relative importance of these modes of reaction largely defined by SAR methods of Carter and Atkinson (1989) and Atkinson (1997)

22 Simplification measure oxygenated RO radicals – exclusive decomposition assumed

23 VOC/product initiation reactions Reaction with OH – all VOC and oxygenated products Reaction with O 3 – alkenes/dienes and unsaturated products Reaction with NO 3 – alkenes/dienes, aldehydes and cresols Photolysis – carbonyls, RONO 2, ROOH

24 Organic photolysis processes 26 photolysis processes defined 14 parameters also used to define photolysis rates for several thousand other species

25 Laboratory studies Theoretical and semi-empirical methods e.g. rate coefficients, branching ratios, absorption spectra, quantum yields Detailed mechanism construction (MCM) Scientific and policy modelling Fundamental parameters Mechanism development Mechanism application Mechanism reduction Chamber validation


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