1. History of the Master Chemical Mechanism (MCM) and its development protocols
Mike Jenkin
Centre for Environmental Policy

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 1999
101 non-aromatic anthropogenic species
18 aromatics (provisional chemistry)
1 biogenic species (isoprene)
1999
MCM v VOC; reactions; 3800 species
103 non-aromatic anthropogenic species
18 aromatics (extended provisional chemistry)
2 biogenic species (isoprene: a-pinene)
2002
MCM v VOC; reactions; 4400 species
104 non-aromatic anthropogenic species
18 aromatics (first rigorous representation)
3 biogenic species (isoprene: a-pinene: b-pinene)
2004
MCM v VOC; 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 reactions OH, 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 cyclehu 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. VOC HNO3 H2O2 ROOH RONO2 RO2NO2 ROH + R-HO Radical termination NO2 NO
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
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
VOC
or product OH
HO2
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. Detailed gas phase mechanism
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