History of the Master Chemical Mechanism (MCM) and its development protocols

1. History of the Master Chemical Mechanism (MCM) and its development protocols Mike Jenkin EPSR Group Department of Environmental Science and Technology m.jenkin@imperial.ac.uk

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 inventory contains ca. 650 species Ozone CO2 H2O nitrate VOC NOX oxidation emissions

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 RO2 + NO • RO2 + HO2 reactions not included (except for CH3O2) • 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. 1993-1996: 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, 81-104, 1997

6. 1996: Master Chemical Mechanism (MCM v1) • Degradation of CH4 and 119 non-methane VOC • ca. 2,500 chemical species • ca. 7,000 chemical reactions • 22 alkanes (C1-C12) • 16 alkenes (C2-C6) • 2 dienes (C4-C5) • 1 alkyne (C2) • 18 aromatics (C6-C11) • 6 aldehydes (C1-C5) • 10 ketones (C3-C6) • 17 alcohols (C1-C6) • 10 ethers (C2-C7) • 8 esters (C2-C6) • 3 carboxylic acids (C1-C3) • 8 halocarbons (C1-C3) MCM website launched in March 1997

7. MCM timeline 1996 MCM v1 - 120 VOC; 7000 reactions; 2500 species 101 non-aromatic anthropogenic species 18 aromatics (provisional chemistry) 1 biogenic species (isoprene) 1999 MCM v2 - 123 VOC; 10500 reactions; 3500 species 103 non-aromatic anthropogenic species 18 aromatics (extended provisional chemistry) 2 biogenic species (isoprene: a-pinene) 2002 MCM v3 - 125 VOC; 12700 reactions; 4400 species 104 non-aromatic anthropogenic species 18 aromatics (first rigorous representation) 3 biogenic species (isoprene: a-pinene: b-pinene) Contributions to MCM v2 and v3 activities at Leeds: Nic Carslaw, Stephen Pascoe, Volker Wagner

8. 2002: Master Chemical Mechanism (MCM v3) • 10 ketones (C3-C6) • 17 alcohols (C1-C6) • 10 ethers (C2-C7) • 8 esters (C2-C6) • 3 carboxylic acids (C1-C3) • 2 other oxygenates (C3) • 8 halocarbons (C1-C3) • 22 alkanes (C1-C12) • 16 alkenes (C2-C6) • 2 dienes (C4-C5) • 2 monoterpenes (C10) • 1 alkyne (C2) • 18 aromatics (C6-C11) • 6 aldehydes (C1-C5) • Supplementary protocols: • Atmospheric Chemistry and Physics, 3, 161-180, 2003 (non-aromatic VOC) • Atmospheric Chemistry and Physics, 3, 181-193, 2003 (aromatic VOC)

9. MCM construction methodology

10. MCM scheme writing framework

11. Free radical propagated oxidation cycle O3 hu O2 NO2 NO OH HO2 carbonyl product(s) VOC RO2 RO O2 rxn with O2, decomposition or isomerisation. NO NO2 O2 O3 NO2 + hu→ NO + O O + O2 (+M) → O3 (+M) hu

12. Radical termination HNO3 H2O2 NO2 NO HO2 NO2 OH HO2 carbonyl product(s) VOC RO2 RO O2 rxn with O2, decomposition or isomerisation. HO2 NO NO2 RO2 NO NO2 ROOH RONO2 RO2NO2 ROH + R-HO

13. Radical generation (or regeneration) through photolysis ROOH H2O2 carbonyls O3 O2 NO2 NO H2O OH HO2 carbonyl product(s) VOC RO2 RO O2 rxn with O2, decomposition or isomerisation. O2 NO NO2 carbonyls RONO2 ROOH

15. OH-initiated degradation of methane (CH4)

16. OH-initiated degradation of ethane (C2H6)

17. OH-initiated degradation of 1,3-butadiene

18. Defining kinetic and mechanistic parameters NO2 NO OH HO2 VOC or product carbonyl product(s) RO2 RO O2 rxn with O2, decomposition or isomerisation NO NO2 • OH + VOC/organic product • RO2 + NO, NO2, NO3, HO2, R’O2 • RO O2 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 ≥ C7 alkanes • so called ‘minor’ products (e.g. RONO2; ROOH) degraded to regenerate existing species

20. RO2 radical reactions • Kinetics of RO2 reactions • Reactions with NO, NO2, NO3, HO2 and other peroxy radicals (R’O2) are included in MCM • There are about 1200 RO2 radicals in MCM v3 • Kinetic data are available for only ca. 20 RO2 – parameters assigned to majority of reactions by analogy and structure reactivity correlations • Product branching ratios • Multiple channels for reactions with NO, HO2 and R’O2 • Scheme simplification measures applied in some cases • RO2 from ‘minor’ products react via single channel • RO2 + R’O2 reaction are necessarily parameterised (explicit chemistry for 1200 radicals would require 0.7 million reactions!)

21. RO radical reactions reaction with O2 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 O3 – alkenes/dienes and unsaturated products • Reaction with NO3 – alkenes/dienes, aldehydes and cresols • Photolysis – carbonyls, RONO2, ROOH

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

25. Chamber validation • 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 • Mechanism reduction Fundamental parameters Mechanism development Mechanism application