A self-generating approach for explicit mechanisms. B. Aumont, M. Camredon, J.
Lee-Taylor, S. Madronich. International Conference on atmospheric chemical ...
International Conference on atmospheric chemical mechanisms
A self-generating approach for explicit mechanisms B. Aumont, M. Camredon, J. Lee-Taylor, S. Madronich
December 6-8, 2006
Why should we develop explicit organic chemical schemes ? VOC oxidation produces a large number of intermediates. These intermediates play a key role in the HOx and NOx budget and therefore in the tropospheric O3 formation. Secondary VOC are usually multifunctional species, i.e. having high water solubility and/or low vapor pressure. These organic intermediates may : • modify the chemical and physical properties of aerosol by Secondary Organic Aerosols production • modify cloud chemistry (e.g. acid formation) and microphysics (e.g. surface tension effect, properties of aerosol acting as CCN…)
NO →NO2 conversion HOx sink NOx sink
Explicit scheme = reference scheme to test the relevance of simplified (reduced) schemes used in 3D models. Objective tools to explore our ignorance through comparisons with observations
From the parent species to the first generation
From the first to the second generation ONO2
NO →NO2 conversion HOx sink NOx sink
From the parent species to the first generation
NO →NO2 conversion HOx sink NOx sink
From the first to the second generation
How big are explicit chemical schemes ? 107 106
n-alkanes i-alkanes 1-alkenes Isoprene
f o r be m Nu
105
ac e r
ns o ti
of r be m Nu
104
A fully explicit description leads to schemes dealing with an exorbitant number of species
s
pe
s e i c
Two problems : Explicit schemes are too large to be reasonably written by hand
103 Aumont et al., ACP, 2005
102
3
4
5
6
7
8
The quantity of physical and chemical data needed to develop explicit schemes is far in excess of available experimental data.
Carbon number Data processing tools are required to: Assimilate all the data provided by laboratory studies Codify the various estimation methods Generate consistent and comprehensive multiphase oxidation schemes on a systematic basis
The chemical scheme generator µ What we expect from a generator : Automatic creation of fully-explicit schemes on the basis of a predefined protocol. starting point
CH3CH2CH3 CH3CH2CH3 CH3CH2CH(OO.) ………..
CH3CH(OO.)CH3 ……….. ………..
+ OH => CH3CH2CH(OO.) + H2O + OH => CH3CH(OO.)CH3 + H2O ……….. + NO => => ……….. ……….. + NO => => ……….. ending point => CO2 + H2O
k1 k2 … … … …
µ The protocol Define the set of rules that lay out the choice of reaction pathways (and their associated rate constant). 1- When available, kinetic data taken from laboratory measurements are assigned 2- Otherwise, an estimation of the rate constant, stoichiometric coefficients and reaction products is performed using structure/activity relationships.
The chemical scheme generator Generator = Expert system that - assimilates physical and chemical data from laboratory experiments - estimates the missing information based on structure/activity relationships
Input Hydrocarbons
Output Automatic generator for VOC oxidation scheme Machinery :
Laboratory data
Explicit chemical schemes
structure/activity relationships
The protocol currently used : - Conceptually similar to the MCM3 mechanism - Most SAR implemented were borrowed from SAPRC99, MCM and NCAR MM - Described in Aumont et al., ACP, 2005
Low diagram of the generator New reaction products added to the stack VOCn
VOC2 VOC1 Radical ?
no C-H ? OH reaction RH + OH → RO2 >C=C< + OH → >C(OH)C(O2)< O3 reaction >C=C< + O3 → >C=O + Criegee Criegee → products + OH NO3 reaction RCHO + NO3 → R(O)OO >C=C< + NO3 → >C(ONO2)C(O2)< Photolysis RCHO + hν → R + HCO RONO2 + hν → RO + NO2 ROOH + hν → RO + OH
>C=C< ?
RO2 ? Peroxy alkyl chemistry RO2+NO → RO+NO2 RO2+NO → RONO2 RO2+NO3 → RO+NO2+O2 RO2+HO2 → ROOH + O2 RO2+RO2 → products RCO3 ?
-CHO ?
>C=O ? -ONO2 ? -OOH ? -C(O)OONO2 ?
Thermal decomposition RCO(OONO2) → RCO(OO)+NO2
yes
Peroxy acyl chemistry RCO3+NO → R+CO2+NO2 RCO3+NO2 → RCO(OONO2) RCO3+NO3 → R+CO2+NO2+O2 RCO3+HO2 → RCO(OOH) + O2 RCO3+HO2 → RCO(OH) + O3 RCO3+RO2 → products RO ? Alcoxy chemistry >CH(O⋅)+O2 → >C=O + HO2 >CH(O⋅)C2C(O.)< → >C(O⋅) C2C(OH)< >C(O⋅)-R → >C=O + R
Treat next species in the stack
The chemical scheme generator
Input
Hydrocarbons
Set of rules for simplification Lumping protocol
Output
Automatic generator for VOC oxidation scheme Explicit chemical schemes
Laboratory data
Machinery : structure/activity relationships
Reduced (simplified) chemical schemes
Chemical scheme reduction (Near near) explicit scheme (~70 primary species) Reference scheme 359 660 species 2 270 159 reactions Reduced scheme ~150 species ~500 reactions
Starting scheme
Ending scheme Automatic & systematic reduction
Chemical scheme reduction (Near near) explicit scheme (~70 primary species)
Starting scheme Ending scheme Automatic & systematic reduction
Accuracy of the each reduction procedure can be quantified using the explicit scheme as a reference See Szopa et al., ACP, 2005
Explicit scheme as an « exploratory vehicle » to explore the behavior of organic matter during oxidation
- where does the carbon go ? -
Explicit modeling of octane oxidation Chemical scheme : 1.2×106 species 7.5×106 reactions Simulation conditions : T=298 K [octane]0 = 20 ppb [NOx]0 = 10 ppb
Mixing ration (ppbC)
Modeling VOC oxidation : where does the carbon go ? Total simulated carbon
150
Octane 100
total secondary organics
CO+CO2
50
0
0
2
4
6
8
Time (day)
10
12
How does organic reactivity evolve after the first oxidation steps ? - Contribution to HOx, NOx, Ox budget at regional to global scale ? - How does strong emission sources affect the continental and remote troposphere?
Mixing ratio (ppbC)
Modeling VOC oxidation : where does the carbon go ? Octane 100
Mixing ratio (ppbC)
Chain lengths of secondary organics
total 80
total secondary organics
0
2
4
6
10
12
Number of functional groups borne by C8 species
C8
C8
40
8
Time (day)
80
40
2 1 3
C1 to C7 0
CO+CO2
50
0 120
Total simulated carbon
150
4 0
2
4
6
8
Time (day)
10
12
00
2
4
6
8
Time (day)
10
12
Most secondary organics are water soluble. How does organic matter evolve during cloud events? What is the contribution of aqueous processes in: - The removal of VOC - Acids formation - Organic aerosols formation (after cloud evaporation) - The oxidant budget
2 functional groups : typical species
Number of functional groups borne by C8 species
80
40
O
O
C8
OH
O
ONO2
ONO2
2 1 3 4
OH
O
00
2
4
6
8
Time (day)
10
12
Some secondary organics are non-volatile species. What is the fraction of the parent compound that leads to aerosols ? What is the contribution of organic condensation to: - Hygroscopic behavior (CNN activation) - Size distribution - Optical properties
4 functional groups : typical species Number of functional groups borne by C8 species
80 O
O
O
O
ONO2 O O
O
ONO2
C8
O
OH ONO2 O
40
O
2 1 3 4
OH
OH
00
2
4
6
8
Time (day)
10
12
How far should we go in the description of VOC oxidation ? Number of functional groups borne by C8 species
Mixing ratio (ppbC)
80
C8
SGMM (explicit) MCM
2 40
1 3 0
0
2
4 4
6
Time (day)
8
10
12
How far should we go in the description of VOC oxidation ? Evolution of organic functionalities during octane oxidation
Ri/C =
Number of carbons bearing function i Total number of organic carbons
How far should we go in the description of VOC oxidation ? Evolution of organic functionalities during octane oxidation
Evolution of nitrogen compounds and ozone during octane oxidation
OZONE
Multiphase VOC oxidation : Development of a data processing tools to generate kinetic schemes and thermodynamic equilibrium (LISA/NCAR) gas HC + oxidant ox VOC Water soluble ox VOC semi-volatils
Many questions surround the role of multifunctional organics in the chemistry of multiphase system
The main difficulties : description of the sources and sinks of a myriad of VOC that may partition to condensed phases.
ox CO2 + H2O Kinetic module Gas phase generator
Method : extension of the self generating approach to model multiphase system
Multiphase VOC oxidation : Development of a data processing tools to generate kinetic schemes and thermodynamic equilibrium (LISA/NCAR) gas HC + oxidant ox VOC Water soluble ox
clouds VOC Water soluble Dynamical module – Gas/droplets mass transfert
VOC semi-volatils aerosols aérosols
ox
SVOC
CO2 + H2O Kinetic module Gas phase Thermo. Module Equilibrium gas/aerosols
VOC semi-volatils
Multiphase VOC oxidation : Development of a data processing tools to generate kinetic schemes and thermodynamic equilibrium (LISA/NCAR) Kinetic module Aqueous phase
gas HC + oxidant ox VOC Water soluble ox
clouds VOC Water soluble Dynamical module – Gas/droplets mass transfert
VOC semi-volatils
ox VOC semi-volatils
aerosols aérosols
ox
SVOC
CO2 + H2O
Polymerisation oxidation
Kinetic module Gas phase Thermo. Module Equilibrium gas/aerosols
non-volatil compounds Kinetic module Aerosol phases
Microphysical Module
Multiphase VOC oxidation : Development of a data processing tools to generate kinetic schemes and thermodynamic equilibrium (LISA/NCAR) Kinetic module Aqueous phase
gas HC + oxidant ox VOC hydrosoluble Water soluble ox
clouds COV hydrosolubles dynamical module – Gas/droplets mass transfert
VOC semi-volatils
ox COV semi-volatils
aerosols aérosols
ox
SVOC SVOC
CO2 + H2O
Polymerisation oxidation
Kinetic module Gas phase Thermo. Module Equilibrium gas/aerosols
non-volatil compounds Kinetic module Aerosol phases
Microphysical Module
Explicit modelling of the gas/particle partitioning
AEROSOL PHASE
GAS PHASE
oxidation
COV +ox
CONDENSATION: absorption COSVi
oxidation
COSVi +ox
CO+CO2
Raoult’s law : Pi = γi xi Pvap i Pvap are estimated for each intermediate using Myrdal & Yalkowski (97) structure/properties relationship (see Camredon et al., Atmos. Env., 2006)
Gradual change of organics during 1-octene oxidation Typical species ONO2
concentration (ppbC)
80
60
O
Total simulated carbon
octene Gaseous secondary organics
OH
OH
ONO2 OH
CO+CO2 O
OH
OH
20
Particulate secondary organics 0 3 4 2 1 0 temps (jours) Simulation conditions : T=298 K, [octene]0 = 10 ppb, [NOx]0 = 1 ppb (constant)
OH
ONO2
40
5
O
ONO2 O
OH
O
ONO2
OH
OH
OH
Evolution of organic functionalities during octene oxidation Gas phase
Aerosol phase
PAN -OH
-OH
-ONO2
-ONO2
-CHO -CO-
Ri/C =
-CO-
Number of carbons bearing function i Total number of organic carbons
ketone
carboxylic acid
PAN
peracid
nitrate
hydroperoxide
aldehyde
alcohol
Future developments … GAS PHASE
CONDENSE PHASE
oxidation
COV polymers
+ox
CONDENSATION: absorption COSVi
COSVi oxidation
polymerization oxidation +ox
+ox
CO+CO2
H2O H2O inorganic
COSVj inorganic
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Explicit Modelling of SOA Precursors in Mexico City J. Lee-Taylor, S. Madronich, G. Tyndall, B. Aumont, M. Camredon
