Diapositive 1

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International Aerosol Modeling Algorithms
Conference
Explicit modelling of the SOA/VOC/NOx system
B. Aumont, M. Camredon, J. Lee-Taylor, S.
Madronich
December 5-7, 2007
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Why should we develop explicit models for the
SOA/VOC/NOx system ?
Production of multifunctional (i.e. semi
volatile) organic species requires multiple
oxidation steps. Yields of the multifunctional
species depend on atmospheric conditions (NOx
level, humidity, temperature, irradiation )
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Why should we develop explicit models for the
SOA/VOC/NOx system ?
Production of multifunctional (i.e. semi
volatile) organic species requires multiple
oxidation steps. Yields of the multifunctional
species depend on atmospheric conditions (NOx
level, humidity, temperature, irradiation )
Aerosols properties (light absorption,
hygroscopicity, reactivity ) depend on
speciation gt chemical identity of SOA
contributors is required.
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How big are explicit chemical schemes ?
A fully explicit description leads to schemes
dealing with an exorbitant number of species
Aumont et al., ACP, 2005
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Explicit SOA formation model
GAS PHASE
AEROSOL PHASE
COV
ox
oxidation
CONDENSATION absorption
SVOCi
SVOCi
ox
oxidation
COCO2
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1. Gas phase oxidation schemes
GAS PHASE
AEROSOL PHASE
COV
Explicit gas-phase oxidation schemes are written
using a self-generating approach
ox
oxidation
CONDENSATION absorption
SVOCi
SVOCi
ox
oxidation
COCO2
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The chemical scheme generator
? What we expect from a generator
Automatic creation of fully-explicit schemes on
the basis of a predefined protocol.
k1
CH3CH2CH3
Start
OH
CH3CH(OO.)CH3
?
CH3CH2CH3
OH
CH3CH2CH2(OO.)
k2
?
..
...
?
CH3CH(OO.)CH3
NO
..
...
?
..
..
...
?
CH3CH2CH2(OO.)
NO
...
..
?
..
..
?
CO2 H2O
end
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The chemical scheme generator
Expert system that
Generator
Input
Output
Explicit chemical schemes Related data vapor
pressure, Henrys law coef.
Automatic generator for VOC oxidation scheme
Hydrocarbons
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Flow diagram of the generator
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2. Gas/particle partitioning of SVOC
The partitioning of each SVOC is based on an
absorptive model.
GAS PHASE
AEROSOL PHASE
Assume a thermodynamic equilibrium between gas
and particulate phases
COV
ox
oxidation
CONDENSATION absorption
SVOCi
SVOCi
ox
oxidation
Pvap are estimated for each intermediate using
Myrdal Yalkowsky (1997) structure/properties
relationship Camredon et al., Atmos. Env., 2006
COCO2
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2. Gas/particle partitioning of SVOC
The partitioning of each SVOC is based on an
absorptive model.
GAS PHASE
AEROSOL PHASE
Assume a thermodynamic equilibrium between gas
and particulate phases
COV
ox
oxidation
CONDENSATION absorption
SVOCi
SVOCi
ox
oxidation
Pvap are estimated for each intermediate using
Myrdal Yalkowsky (1997) structure/properties
relationship Camredon et al., Atmos. Env., 2006
COCO2
3. System resolution
- Time integration performed using the two-step
solver
- Gas/particule equilibrium solved using an
iterative method
- Thermodynamic equilibrium imposed at each time
step (20 mn)
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Explicit modelling as an exploratory vehicle
to explore the behavior of organic matter during
oxidation - where does the carbon go ? -
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Modeling SOA/VOC/NOx system where does the
carbon go ?
Explicit modeling of octene oxidation
Simulation conditions
T298 K octene0 10 ppb NOx 1 ppb
Gas phase chemical scheme
1.4?106 species
8.9?106 reactions
Gas ? aérosol equilibrium
4.0?105 species
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Modeling SOA/VOC/NOx system where does the
carbon go ?
10 of total carbon
Gaseous oxidation of 1-octene leads to
significant SOA formation
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Modeling SOA/VOC/NOx system where does the
carbon go ?
Gaseous oxidation of 1-octene leads to
significant SOA formation
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Modeling SOA/VOC/NOx system where does the
carbon go ?
Gaseous oxidation of 1-octene leads to
significant SOA formation
Time scales of a few days are required to form
low vapor pressure products
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Sensitivity of the SOA VOC NOx System
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Sensitivity of the SOA/octene/NOx system
Box model simulations
T298 K Humidity 50 Irradiation constant
(?30)
100 ppb
Initial octene load
1 ppb
NOx load (constant value)
50 ppt
100 ppb
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Sensitivity of the SOA/octene/NOx system
Box model simulations
T298 K Humidity 50 Irradiation constant
(?30)
The system is explored on the basis of 45
simulations, conducted with 5 initial octene
concentrations and 9 fixed NOx concentrations
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Sensitivity of the SOA/octene/NOx system SOA
Yield
Aerosol produced (Mo)
Y
SOA Yield
Octene reacted (DHC)
SOA Yield (Y)
Camredon et al., ACP, 2007
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SOA Yield (Y)
Camredon et al., ACP, 2007
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The SOA/octene/NOx system Influence of HC0
Camredon et al., ACP, 2007
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The SOA/octene/NOx system Influence of NOx
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SOA speciation
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SOA speciation organic functionalities as a
function of NOx level
Definition of a substitution index
Number of carbons bearing the functionality k
ROFk/C

Total number of organic carbons
Octene simulation ( HC0 10 ppb)
Camredon et al., ACP, 2007
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SOA speciation organic functionalities as a
function of NOx level
Definition of a substitution index
Number of carbons bearing the functionality k
ROFk/C

Total number of organic carbons
Octene simulation ( HC0 10 ppb)
High contribution of nitrate moieties at high NOx
High contribution of hydroperoxides moieties at
low NOx
Camredon et al., ACP, 2007
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Conclusions
1.
Gaseous oxidation of volatile compounds (here
octene) is simulated to lead to SOA formation at
concentration levels representative of
atmospheric conditions.
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