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 Centre for Environmental Policy m.jenkin@imperial.ac.uk

2. 1993 – the conception 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. 1993-2007: Master Chemical Mechanism Philosophy • to use information on the kinetics and products of elementary reactions relevant to VOC oxidation to build up a rigorous 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.

6. Mechanism construction methodology • Mechanism construction is broadly a two-stage process: • Development of a “mechanism construction protocol” • Application of the protocol to a series of emitted (primary) VOC to develop the mechanism/database

7. Mechanism development and protocol history 123 125 135 • Jenkin et al. (Atmos. Env. 31, 81, 1997): non-aromatic species • Report on DETR contract, EPG 1/3/70 (1998) : aromatic species • Saunders et al. (ACP, 3, 161, 2003): non-aromatic species • Jenkin et al. (ACP, 3, 181, 2003): aromatic species • Bloss et al. (ACP, 5, 641, 2005): aromatic species

8. MCM timeline 1996 MCM v1 - 120 VOC; 7100 reactions; 2400 species 101 non-aromatic anthropogenic species 18 aromatics (provisional chemistry) 1 biogenic species (isoprene) 1999 MCM v2 - 123 VOC; 11400 reactions; 3800 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) 2004 MCM v3.1 - 135 VOC; 13500 reactions; 5900 species 114 non-aromatic anthropogenic species 18 aromatics (updated representation) 4 biogenic species (isoprene: a-pinene: b-pinene: MBO-232)

9. The Master Chemical Mechanism (1993-2007) • Degradation of CH4 and 134 non-methane VOC • ca. 5,900 chemical species • ca. 13,500 chemical reactions • 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) • 10 ketones (C3-C6) • 17 alcohols (C1-C6) • 10 ethers (C2-C7) • 8 esters (C2-C6) • 3 carboxylic acids (C1-C3) • 3 other oxygenates (C3-C5) • 17 halocarbons (C1-C3) Species cover ca. 70% of the mass emissions in the UK National Atmospheric Emissions Inventory (anthropogenic) Includes isoprene, a-pinene, b-pinene and MBO-232 (biogenic)

10. MCM construction methodology

11. Flow diagram of main features of MCM protocols

12. Structure of the protocols Approximate hierarchy of information sources Experimental data (evaluated) Experimental data (direct) SARs (published) SARs/analogy assumptions (defined in protocol) Theoretical studies of specific structures • Initiation reactionsOH, NO3, O3, photolysis • Reactions of organic radicals reaction with O2 • Reactions of RO2 intermediates reaction with NO, NO2, NO3, HO2 and R’O2 • Reactions of RO intermediates reaction with O2, decomposition and isomerisation • Reactions of Criegee intermediates excited and stabilised • Removal of Cl atoms • Reactions of degradation products Simplifications • Initiation criteria • Channel probability • Product degradation • RO2 + RO2/R’O2

14. Free radical propagated reaction cycle O3 hu O2 NO2 NO OH HO2 carbonyl product VOC RO2 RO O2 O2 reaction, decomposition or isomerisation NO NO2 O2 O3 NO2 + hu→ NO + O O + O2 (+M) → O3 (+M) hu

15. Radical termination HNO3 H2O2 NO2 NO HO2 NO2 OH HO2 carbonyl product VOC RO2 RO O2 O2 reaction, decomposition or isomerisation HO2 NO NO2 RO2 NO NO2 ROOH RONO2 RO2NO2 ROH + R-HO

16. Radical generation (or regeneration) through photolysis O3 H2O2 carbonyls hu hu ROOH hu H2O NO2 NO O2 hu OH HO2 carbonyl product VOC RO2 RO O2 O2 reaction, decomposition or isomerisation NO NO2 O2 hu carbonyls

18. OH-initiated degradation of methane (CH4)

19. OH-initiated degradation of ethane (C2H6)

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

21. 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.

22. OH radical reactions • Kinetics of OH + VOC/organic products • Rate coefficients have been measured for several hundred organics • Rate coefficients for ca. 4,000 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

23. 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 1000 RO2 radicals in MCM • 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 1000 radicals would require 0.5 million reactions!)

24. RO radical reactions reaction with O2 decomposition isomerisation • There are about 1000 RO radicals in MCM • Relative importance of these modes of reaction largely defined by SAR methods of Carter and Atkinson (1989) and Atkinson (1997)

25. Simplification measure oxygenated RO radicals – exclusive decomposition assumed

26. 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

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

28. Chamber data Website/ database • Laboratory studies • Theoretical and semi-empirical methods • e.g. • rate coefficients, branching ratios, absorption spectra, quantum yields Detailed gas phase mechanism (i.e. MCM) Scientific and policy modelling Evaluation Parameterisation Reduced gas phase mechanisms Fundamental parameters Chemical mechansims Mechanism application