History of the Master Chemical
Mechanism (MCM) and its
development protocols
Mike Jenkin
Centre for Environmental Policy
m.jenkin@imperial.ac.uk
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
Chemical processing of ozone-precursor emissions
inventory contains
ca. 650 species
VOC
NOX
emissions
Ozone
oxidation
CO2
H2O
nitrate
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
1993-2007: Master Chemical Mechanism
Philosophy


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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.
Mechanism construction methodology
Mechanism construction is broadly a two-stage
process:
1. Development of a “mechanism construction
protocol”
2. Application of the protocol to a series of
emitted (primary) VOC to develop the
mechanism/database
Mechanism development and protocol history
MCM version
emitted VOC
species reactions
protocol
v1 (1996)
102 (+18)
2,400
7,100
(1)
v2 (1999)
123
124
3,800
11,400
(2)
v3 (2002)
126
125
4,400
12,700
(3,4)
v3.1 (2004)
136
135
5,900
13,500
(5)
(1) Jenkin et al. (Atmos. Env. 31, 81, 1997): non-aromatic species
(2) Report on DETR contract, EPG 1/3/70 (1998) : aromatic species
(3) Saunders et al. (ACP, 3, 161, 2003): non-aromatic species
(4) Jenkin et al. (ACP, 3, 181, 2003): aromatic species
(5) Bloss et al. (ACP, 5, 641, 2005): aromatic species
MCM timeline
1996
MCM v1 - 120 VOC; 7100 reactions; 2400 species
1999
MCM v2 - 123 VOC; 11400 reactions; 3800 species
101 non-aromatic anthropogenic species
18 aromatics (provisional chemistry)
1 biogenic species (isoprene)
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
2004
MCM v3.1 - 135 VOC; 13500 reactions; 5900 species
104 non-aromatic anthropogenic species
18 aromatics (first rigorous representation)
3 biogenic species (isoprene: apinene: b-pinene)
114 non-aromatic anthropogenic species
18 aromatics (updated representation)
4 biogenic species (isoprene: apinene: b-pinene: MBO-232)
The Master Chemical Mechanism (1993-2007)
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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)
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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)
MCM construction
methodology
Flow diagram of main features of MCM protocols
Structure of the protocols



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
Approximate hierarchy of
information sources
1) Experimental data
(evaluated)
2) Experimental data (direct)
3) SARs (published)
4) SARs/analogy assumptions
(defined in protocol)
5) Theoretical studies of
specific structures
Simplifications

Initiation criteria

Channel probability

Product degradation

RO2 + RO2/R’O2
Free radical propagated reaction cycle
O3
hu
O2
NO2
VOC
O2
NO
OH
HO2
RO2
RO
O2
O3
NO
NO2
hu
carbonyl
product
O2 reaction,
decomposition or
isomerisation
NO2 + hu → NO + O
O + O2 (+M) → O3 (+M)
Radical termination
HNO3
H2O2
NO2
NO2
VOC
O2
ROOH
HO2
OH
HO2
RO2
RO
HO2
RO2
NO
NO2 NO
NO
NO2
RONO2
ROH + R-HO RO2NO2
carbonyl
product
O2 reaction,
decomposition or
isomerisation
Radical generation (or regeneration) through photolysis
O3
hu
ROOH
H2O2
hu
H2O
NO2
hu
VOC
O2
hu
carbonyls
carbonyls
hu
NO
O2
OH
HO2
RO2
RO
O2
NO
NO2
carbonyl
product
O2 reaction,
decomposition or
isomerisation
organic compound
sunlight
ozone
NO2
RO2
OH
NO
RO
NO
NO2
HO2
first generation products
sunlight
ozone
NO2
RO2
OH
NO
RO
NO
NO2
HO2
second generation products
ozone
sunlight
CO2
orga
aero
CH4
H
C
H
H
H
O3
OH
CH3O2
hu
NO2
NO
NO
CH3O
hu
NO2
HO2
O3
HCHO
OH
HCO
OH-initiated
degradation of
methane (CH4)
hu
NO2
NO
HO2
O3
CO
OH
HOCO
NO
HO2
CO2
hu
NO2
O3
NO2
NO
OH
C2H6
HO2
O2
H
H
C
C2H5O
NO
H2O
NO2
O2
C
H
H
H
C2H5O2
H2O
CH3CHO
O2
H
NO
OH
NO2
HO2
CH3C(O)O2
NO
NO2
OH
NO2
NO
HO2
OH-initiated
degradation of
ethane (C2H6)
HCHO
HCO
O2
H2O
CH3C(O)O
O2
O2
CH3O
CH3O2
CO
NO2
OH
HOCO
NO2
O2
NO
HO2
CO2
NO
CO2
OH
HO
OO
OH
HO
H
H
NO2
NO
H
C
NO2
NO
HO2
C
NO
HO
NO2
O
HO
OO
OH
NO2
HO2
O
OH
OO
NO2
NO
NO
NO
NO2
HO
OH
O
C
H
C
H
O
HO2
O
H
O
NO
O
O
NO
HO2
OH
OO
NO2
HCHO
OH
NO
NO
O
HO
OO
NO2
OH-initiated
degradation of
1,3-butadiene
O
HO
HO2
O
NO2
NO2
OH
HCO
NO2
NO
OO
O
HO
OH
OH
HO2
NO2
NO
NO
O
O
HO
HO2
OH
CO
CO2
HCHO
OH
HOCO
NO
HOCH2CHO
CO2
NO2
NO2
NO
NO
HOCH2CO2
CO2
O
NO
HO2
NO2
HO2
O
OH
HCO
OH
HOCH2CO3
HO2
NO2
NO2
Defining kinetic and mechanistic parameters
NO2
VOC
or
product
O2
NO
OH
HO2
RO2
RO
NO
NO2
carbonyl
product(s)
rxn with O2,
decomposition or
isomerisation
• OH + VOC/organic product
• RO2 + NO, NO2, NO3, HO2, R’O2
• RO
O2 reaction, decomp., isom.
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
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!)
RO radical reactions
reaction with O2
O
R
O
+ O2
+ HO2
R'
R
O
decomposition
R'
O
+ R'
R
R'
R
OH
O
isomerisation
R'
R


R'
R
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)
acyl-oxy
O
R
R + CO2
O
a-carbonyl-oxy
O
O
R'
R
R'
R
O
O
b-hydroxy-oxy
OH
OH
R'
R
R'
R
O
O
a-alkoxy-oxy
R
O
R'
O
R
O
R'
O
Simplification measure
oxygenated RO radicals –
exclusive decomposition
assumed
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
Organic photolysis processes
Carbonyls
HCHO
 HCO + H
 CO + H2
(J11)
(J12)
4.642 x 10-5
6.853 x 10-5
0.762
0.477
0.353
0.323
CH3CHO
 HCO + CH3
(J13)
7.344 x 10-6
1.202
0.417
C2H5CHO
 HCO + C2H5
(J14)
-5
n-C3H7CHO
1.067
0.358
 HCO + n-C3H7
 CH3CHO + C2H4
(J15)
(J16)
b
2.879 x 10
-5
2.792 x 10
1.675 x 10-5
0.805
0.805
0.338
0.338
i-C3H7CHO
 HCO + i-C3H7
(J17) b
7.914 x 10-5
0.764
0.364
CH2=C(CH3)CHO
 CH3C=CH2 + HCO
 CH2=C(CH3)CO + H
b
(J18)
(J19) b
-5
1.140 x 10
1.140 x 10-5
0.396
0.396
0.298
0.298
CH3C(O)CH3
 CH3CO + CH3
(J21)
7.992 x 10-7
CH3C(O)C2H5
 CH3CO + C2H5
(J22) b
5.804 x 10-6
1.092
0.377
CH3C(O)CH=CH2
 CH3CH=CH2 + CO
 CH3CO + CH=CH2
(J23)
(J24) b
-5
0.395
0.395
0.296
0.296
 CO + CO + H2
 CO + HCHO
 HCO + HCO
(J31)
(J32)
(J33)
CH3C(O)CHO
 CH3CO + HCO
(J34) b
14 parameters also used to
6.845
x 10
0.130
0.201
define
photolysis
rates for
1.032 x 10
0.130
0.201
3.802 x 10
0.644
0.312
several
thousand
other species
1.537 x 10
0.170
0.208
CH3C(O)C(O)CH3
Hydroperoxides
 CH3CO + CH3CO
(J35) b
3.326 x 10-4
0.148
0.215
CH3OOH
Organic nitrates
 CH3O + OH
(J41) b
7.649 x 10-6
0.682
0.279
CH3ONO2
 CH3O + NO2
(J51)
1.588 x 10-6
1.154
0.318
C2H5ONO2
 C2H5O + NO2
(J52)
-6
1.244
0.335
n-C3H7ONO2
 n-C3H7O + NO2
-6
1.196
0.328
i-C3H7ONO2
 i-C3H7O + NO2
-6
1.111
0.316
t-C4H9ONO2
 t-C4H9O + NO2
-5
0.974
0.309
CH3C(O)CH2ONO2
 CH3C(O)CH2O + NO2
 CH3CO + HCHO + NO2
-6
1.015
1.296
0.324
0.322
a-Dicarbonyls
(CHO)2
26 photolysis
processes defined
1.578
0.271
1.836 x 10
1.836 x 10-5
-5
-5
-5
-4
1.907 x 10
(J53)
b
(J54)
b
(J55)
b
b
(J56)
(J57) b
2.485 x 10
4.095 x 10
1.135 x 10
7.549 x 10
3.363 x 10-6
• Laboratory studies
Chamber
data
Website/
database
• Theoretical and semiempirical methods
e.g.
rate coefficients, branching
ratios, absorption spectra,
quantum yields
Detailed gas
phase
mechanism
(i.e. MCM)
Evaluation
Parameterisation
Fundamental
parameters
Scientific
and policy
modelling
Reduced gas
phase
mechanisms
Chemical
mechansims
Mechanism
application