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Developing Secondary OrganicAerosol (SOA) Code
for the MCM
David Johnson
(Mike Jenkin and Steve Utembe)
Department of Environmental Science and Technology, Imperial College London, Silwood Park, Ascot, SL5 7PY
MCM Developer and User Workshop, Leeds,
Thursday/Friday 2/3 December 2004
Introduction
Effects of particulate matter in the atmosphere
Contribution that organic material makes to total particulate burden
SOA from atmospheric oxidation of biogenic volatile organic compounds (VOCs) and anthropogenic aromatic hydrocarbons
Gas-phase degradation mechanisms
Transfer of material from gas- to condensed organic-phase
Formation of SOA and Oxidation Mechanisms
VOC higher polarity, lower volatility products SOA material
Generally only for VOC ≥ C6
aromatic hydrocarbons (e.g. toluene, ethylbenzene, xylenes)terpenic biogenic hydrocarbons (e.g. -pinene, -pinene)
Oxidation mechanisms for large VOCs are very complex
Master Chemical Mechanism (MCM)
e.g. for toluene (methylbenzene) : 268 species, 754 reactions
ca. 120 species with Tb > 450 K
Gas-to-particle partitioning of organic material
Pankow absorption model
Transfer of material represented as a dynamic equilibrium
POH (g) POH (abs)
[POH(abs)]/[POH(g)]
= kin[POH(g)] / kout MO
= Kp MO
Lp
RTK
om
9
p
MW
10501.7
Odum et al. “two-product”-model
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 100 200 300 400
aerosol concentration (M 0) / g m-3
ae
roso
l yie
ld (Y
) / %
i = 1,2
i
iYY
i p
pi
MK
MK
0
0
1
At small M0
Y M0 Kp,i
As M0 Y i
MCM v3.1-“many product”-model
Pankow absorption model
e.g. for toluene, ca. 150 species with Tb > 450 K
Transfer of material represented as a dynamic equilibrium
POH (g) POH (abs)
[POH(abs)]/[POH(g)]
= kin[POH(g)] / kout[OA]
= Kp [OA]
Lp
RTK
om
9
p
MW
10501.7
need to estimate saturated vapour pressures for 150 species
T
T
T
T
R
TSp bbbvapln8.018.1
)(
760ln
L
How to estimate Kp,i for a large number of species?
Use the MCM (Accord) Database and ChemDraw for Excel
1. Convert to unique SMILES strings.
How to estimate Kp for a large number of species?
Use the MCM (Accord) Database and ChemDraw for Excel
2. “Eliminate” radical species (replace “[O]” with “Z”)
How to estimate Kp for a large number of species?
Use the MCM (Accord) Database and ChemDraw for Excel
3. Sub-structure search for oxynitro-compounds (organic nitrates)
Use the MCM (Accord) Database and ChemDraw for Excel
4. Molecular property calculations using Chem Office for Excel
How to estimate Kp for a large number of species?
Use the MCM (Accord) Database and ChemDraw for Excel
4. Molecular property calculations using Chem Office for Excel
How to estimate Kp for a large number of species?
“Deployment” of Kp values
“on-line” vs. “off-line”
kin = 6.2 10-3 s-1
kout = kin / Kp
Average MW ofAbsorbing organic condensed-phasespecies
Smog Chamber Aerosol Data
e.g. for toluene data from EXACT (Effects of the oXidation of Aromatic Compounds in the Troposphere)
0
10
20
30
40
50
60
9 11 13 15 17
hour
ae
roso
l ma
ss / g
m-3
0
150
300
450
[tolu
en
e, 2
x NO
, 2 x N
O2 ] / pp
bv
All partitioning coefficients
27.5
Condensed-Phase Chemistry (Association Reactions)
R R'
OH
+ R"OOH R
OH
R'
OOR
peroxyhemiacetal
Tobias and Ziemann
0
5
10
15
20
25
30
35
40
9 10 11 12 13 14 15hour
Simplified Peroxyhemiacetal Chemistry
ROOH + HC(=O)R’ ROOC(OH)R’H
adduct forming chemistry included
Simplified Peroxyhemiacetal Chemistry
ROOH + HC(=O)R’ ROOC(OH)R’H
0
5
10
15
20
25
30
35
40
9 10 11 12 13 14 15
hour
27.5 (original model)
9.9 (association chemistry)
Effect of NOx on Toluene SOA
0
10
20
30
40
50
60
9 11 13 15 17
hour
ae
roso
l ma
ss / g
m-3
0
150
300
450
[tolu
en
e, 2
x NO
, 2 x N
O2 ] / pp
bv
Effect of NOx on Benzene SOA
0
10
20
30
40
50
60
9 10 11 12 13 14 15 16 17
hour
aero
sol m
ass
/ g
m-3
0
500
1000
1500
2000 [benzene], 40 x [NO
], 40 x [NO
2 ] / ppbv
Effect of NOx on p-Xylene SOA
0
5
10
15
20
25
30
35
40
9 10 11 12 13 14 15 16 17
hour
aero
sol m
ass
/ g
m-3
0
100
200
300
400
500
600
700[p-xylene], 3 x [N
O], 2 x [N
O2 ] / ppbv
Effect of NOx on Mesitylene SOA
0
5
10
15
20
25
30
35
40
9 10 11 12 13 14 15 16
hour
aero
sol m
ass
/ g
m-3
0
50
100
150
200
250
300[1,3,5-T
MB
], [NO
], 0.5 x [NO
2 ] / ppbv
Effect of NOx on Toluene SOA
0
2
4
6
8
10
0 20 40 60 80 100 120
aerosol concentration / g m-3
low-NOx
mid-NOx
high-NOx
Other toluene SOA mass concentration data and the role of NO
0
2
4
6
8
10
12
14
0 50 100 150 200 250
aerosol mass / g m-3
areo
sol y
ield
/ %
RO2. + NO RO. + NO2
HO2. + NO OH + NO2
RO2. + HO2. ROOHROOH adduct
Comparing datasets of toluene SOA yields
point of reference = SOA yield at 50 g m-3 aerosol loading
0
1
2
3
4
5
6
0 5 10 15 20
[toluene]0/[NO]0
aer
oso
l yie
ld /
%
NOx-free, limiting yield
SOA Forming Propensity of other Aromatics
0
1
2
3
4
5
6
7
0 20 40 60 80 100 120
aerosol concentration / g m-3
aero
sol y
ield
/ %
low-yield curve
high-yield curve
1,3,5-trimethylbenzene
toluene
1,2,4-trimethylbenzene
Can we relate variations in SOA yield to differences in gas-phase chemistry?
Unsaturated aldehydes are reactive in terms of indicated association chemistry
OH
OO
.O
O O
O
O
+ HO2
O O
OO
+ HO2
O O
O O
+ HO2
decomposition
O2
'Peroxide-bicyclic' route
[0.18]
[0.18]
[0.64]
Variations in SOA yield (Aromatics)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.2 0.4 0.6 0.8
primary unsaturated aldehyde yield
aero
sol y
ield
/ %
Extension to the entire MCM
124 parent VOCs (MCM v3)
ca. 12600 chemical reactions
ca. 4500 chemical species
ca. 2000 closed-shell species with Tb(estimated) > 450 K
need to define 2000 new species,2000 phase-equilibria
Which are the most important components of simulated SOA?
Which are the most important SOA precursors?
Partitioning coefficients?
(pseudo-) Lagrangian, well-mixed boundary layer, chemical box model
Background anthropogenic and biogenic emissions throughout (NAEI)
Enhanced anthropogenic emissions for 3 hours
Idealised “trajectory”
Primary emitted OA
Background organic aerosol
Preliminary (Box) Modelling
A
B
Day 0
Day x
hour0 3 X
1
10
Ant
hrop
ogen
ic
Em
issi
on f
acto
r
Preliminary (Box) Model Simulations
Scaling factor = 50
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4 5 6
Day (starting from 8 am)
g
m-3
Background organic aerosol
Secondary organic aerosol
Primary organic aerosol
Preliminary (Box) Model Simulations
Which are the most important SOA precursors?
O2N NO2
OH
O2N NO2
OH
Which are the most prevalent SOA components?
OH
HO
HOO
NO2
ONO2
OO
OO
NO2
OH
Conclusions and Further Work
Gas-aerosol partitioning (equilibrium) coefficients have been estimated for ca. 2000 species within the MCM v3.1.
Validation simulations have been performed using measured SOA data for the photooxidation of (-pinene) benzene, toluene, p-xylene and mesitylene.
These simulations strongly imply the key role of condensed organic-phase association chemistry.
Simulations further suggest the important role of ROOH. effect of NOx concentration.
To do:-
Model-measurement comparisons – scaling factors?; NOx?
Speciation of simulated SOA for low-NOx conditions
Simulated aerosol material dominated by five species:-
OH
O
O
O
OH
OH
O
O
O
O
O
HO
O
OHOH
O OH
O O O
O
O
OH
O
OH
O
In aerosol peroxyhemiacetal formation
ROOH + HC(=O)R’ ROOC(OH)R’H
H abstraction OH addition
.OO
O2
.O
O
+ HO2
O2
NONO
organic nitrate
OH
OO.OH
O2O2
'Phenolic' route
organic nitrate
NO
OH
O
O
OH
O
O
.OO
organic nitrate
NONO
OH
O
O
.O
OH
O.
O
isomerisation
isomerisation
O2
OH
O
O
O
O
O+ HO2
decomposition
O2
'Epoxy-oxy' route
O O
OO
O O
O
O
+ HO2
+ HO2
O O
OO + HO2
O O
O
O+ HO2
O O
OO
+ HO2
decomposition
O2
'Peroxide-bicyclic' route
(0.07)
(0.65)
(0.10)
(0.18)
Peroxyhemiacetal forming chemistry
OH
O
O
O
OH
O
O
OH
O
OH
O
O
O
HO
O
OHOH
Amended Catechol (1,2-dihydroxybenzene) Chemistry
HO
HO
O2N
HO
HO
O
O
OO
HO
O O
HO
HO
HO
HO
NO2
O O
OO
HO
HO OH
HO
0.06 0.800.14
0.20 0.80
Similar toMCM v3.1a
